Npuroscimce, Vol. 3. pp. 755-760. Pergamon
0
0306-4522/78/08Ol-O755SO2.C0/
PressLtd.1978. Printed inGreatBritain.
IBRO
PERIODIC BEHAVIORAL CHANGES DURING HIPPOCAMPAL THETA RHYTHM ELICITED BY SEPTAL STIMULATION IN RATS W. WETZEL, T. OTT and H. MATTHIES Institute
Abstract-The of the dorsal
of Pharmacology and Toxicology, Medical Academy, 301 Magdeburg, German Democratic Republic
influence of an electrical stimulation of the medial septum on the electroencephalogram
hippocampus and behavior was studied in freely moving rats. A short stimulation at 4-12 Hz frequency always induced orienting behavior (searching, rearing, sniffing, exploration) as soon as hippocampal rhythmic slow activity, or theta rhythm is elicited. An increase in stimulation intensity or in pulse duration led to an increased occurrence of theta rhythm and likewise, to an increase of orienting behavior. Both the theta rhythm and orienting behavior were dependent on the stimulation frequency. During long-lasting septal stimulation at 7 Hz, which elicited a continuous hippocampal theta rhythm, periodic behavioral changes were observed, consisting of orienting behavior alternating with grooming behavior (cleaning, washing, licking, scratching). These rhythmic periods lasted for 3-5 min. The significance of the rhythmic behavioral changes are discussed in relation to the activity of the cholinergic septohippocampal system and to rapid-eye-movement sleep and memory consolidation.
ELECTRICALstimulation
of the medial septum is capable of eliciting changes in the electroencephalogram (EEG) activity of the hippocampus (BRCJCKE,PETSCHE, PILLAT &
DEISENHAMMER, 1959; VON
EULER &
GREEN, 1960; GRAY & BALL, 1970). We
used this method to study the influence of an electrical stimulation of the septohippocampal input on the consolidation of a conditioned reaction (WETZEL, OTT & MATTHIES, 1977h) as well as on the amino acid incorporation into hippocampal proteins (JORK, L&SNER & MATTHIES, 1978). The septal stimulation activity,
EEG
called
theta
(BRUCKE et a/.,
can rhythm,
elicit in
a rhythmic the
slow
hippocampal
1959; GRAY & BALL, 1970;
LANDFIELD, 1976; WETZEL, OTT & MATTHIES, 1977~).
Numerous investigations have been made into the hippocampal theta rhythm and different conclusions were drawn regarding its possible physiological significance (for review, see BLACK, 1976; KLEMM, 1976). But there are only limited studies concerning the relationship between septally induced theta rhythm and behavior in freely moving animals. For this reason, it was decided to extend previous experiments (WETZEL et al., 1977a). In the present work, the influence of various parameters of medial septal stimulation on hippocampal EEG and behavior in freely moving rats has been studied. Particularly, our attention was focused on the effect of long-lasting periods of stimulation as used in memory consolidation studies (WETZEL et al., 19776) and amino acid incorporation experiments (JORK et al.. 1978).
Abbreriations: rapid-eye-movement; in the hippocampal
EEG, electroencephalogram; REM, theta rhythm, rhythmic slow activity EEG. 755
EXPERIMENTAL
PROCEDURES
The experiments were performed on 30 adult male Wistar rats from our own breeding stock, weighing about 200g. The animals were kept individually under standard laboratory conditions, with free access to food and water. One week before the experiments, bipolar electrodes consisting of Teflon-insulated steel wire were implanted under hexobarbital anaesthesia using stereotaxic coordinates according to SKINNER(1971).The stimulation electrodes with resistances of about 50 kR were implanted into the medial septum using the following coordinates: 1.7 mm AP from bregma, 1.6 mm lateral, and 6.8 mm deep, at an angle of 15” with the median plane. The recording electrodes were implanted into the right dorsal hippocampus using the following coordinates: -3.5 mm AP, 3.1 mm lateral, and 3.1 mm deep. Stimulation (METRA Precision Stimulator, Radebeul, GDR) and EEG recording (8-channel Electroencephalograph, ZwBnitz, GDR) were performed on freely moving rats. To eliminate the influence of external disturbances, the behavior of animals was observed by a television camera. The behavior was continuously noted on the EEG paper during the whole stimulation experiment. The septal stimulation was done by means of square pulses of different duration (0.1-1.0 ms) and different frequency (l-50 Hz). The stimulation intensity ranged from 0.5 to 5.0 V, resulting in current values of about l@lOOpA. Furthermore, the influence of the duration of stimulation on hippocampal EEG and behavior was studied. Experiments with long-lasting septal stimulation were not repeated in the same rat. The exact localization of the electrodes in the septum and the hippocampus was histologically verified. EEG records were evaluated by visual means. The amount of theta rhythm was indicated as per cent theta rhythm per minute. Similarly, the presence of different types of behavior (orienting, grooming, other) was expressed as “,;/min. For calculation of theta rhythm amplitude the highest amplitude of stimulation-induced theta rhythm waves was measured. In the figures, the
W. W~.TZLIL.T. OTT and H.
756
MATTHRS
results for lnd~vidual animals are shown, m order to avoid a loss of information by averaging the data of a group of rats. Nevertheless, each example described is representative of a number of animals. RESULTS Septal stimulation
of short duration
Initially, we examined the influence of various stimulation parameters on hippocampal EEG and behavior applying septal stimulation periods of short duration (lG-30~). In each experiment only one stimulation parameter was varied. while the other parameters were held constant. The initial results of these studies have been reported in a short communication (WmzEL et al.. 1977~1). The increase in stimulation intensity under conditions of constant pulse duration (OS ms) and constant stimulation frequency (7 Hz) induced an increase in the amount of theta rhythm in the hippocampal EEG as well as an increase of orienting behavior (searching, rearing, sniffing, exploration) up to 100~~;l.The stimulation intensity required for the occurrence of these effects (OSSV) was in a range below the hippocampal seizure threshold. The application of a subthreshold intensity for eliciting theta rhythm was followed by a desynchronization in hippocampal EEG, especially when high voltage slow wave activity was present. With small increases in intensity, theta rhythm occurred first at the end of the 0.5-min stimulation period. Then, with further increasing stimulation intensity. the latency between the onset of stimulation and the occurrence of theta rhythm decreased towards zero. The latcncies between the onset of stimulation and theta rhythm occurrence as well as between the onset of stimulation and occurrence of orienting behavior were found to be equal. i.e. theta rhythm and orienting behavior always started at the same moment. Under conditions of a constant stimulation intensity and stimulation frequency (7 Hz), an increase in the duration of individual pulses induced an increase in theta rhythm and orienting behavior up to 100”;,. An example is given in Fig. 1 illustrating the effect of square wave pulses with increasing duration from 0.1 to 1.0 ms. The figure also shows the enhanced amount of rearing related to the whole orienting behavior up to IOO”,,,.Every stimulation period lasted 15s. Further experiments revealed that the threshold intensity required for septal elicitation of theta rhythm could not be decreased further by an additional increase in the pulse duration exceeding 1 ms. In the frequency range between 3-4 Hz and 12-14 Hz a stimulus-synchronous hippocdmpal theta rhythm was induced by stimulation at different frequencies. If for each stimulation frequency the threshold intensity for the elicitation of theta rhythm was determined. this threshold intensity revealed a minimum in the 6 7 Hz band, i.e. in the frequency in which physiological theta rhythm mostly occurred. Further decrease in the threshold intensity for theta
Pulse
duration,
ms
FIG. I. Dependence of elicited hippocampal theta rhythm and orienting behavior of a rat on pulse duration during medial soptal stimulation using constant stimulation intensity and stimulation frequency (7 Hz); duration of stimulation : i 5 s. Similar results were obtained in 2 other rats. Ordinate: ‘lo theta rhythm (--I; “/, orienting behavior (- - 1: I’,, rearing -0).
rhythm became evident using frequencies of 10-15 Hz and higher. A similar relationship was found between the stimulation frequency and the threshold intensity required for eliciting orienting behavior. The use of an intermediate intensity of stimulation up to a frequency of 12 Hz caused an increase in amplitude of the elicited theta waves (Fig. 2). However, a further increase in stimulation frequency led to a decrease of theta rhythm amplitude accompanied by the occurrence of hippocampal EEG waves which were not synchronous with the stimulation frequency. That means, during application of a stimulation frequency of 12 Hz, theta waves of 6 Hz were evident, during 14 Hz stimulation waves of 7 Hz, etc. Likewise, the dependence of rearing behavior on stimulation frequency paralleled the changes in theta rhythm amplitude (Fig. 2). Continuous
long-lasting
svptul stimulation
In further experiments the septal stimulation was applied continuously over longer time-periods ranging from several minutes to half an hour. A stimulation intensity capable of eliciting hippocampal theta rhythm was chosen, and pulse duration (0.5 ms) and stimulation frequency (7 Hz) were held constant. In a first series septal stimulation of 5 min duration was performed. Under these conditions theta rhythm occurred in the hippocampal EEG, comprising between SO and iOO”,,.just as it did after short-lasting stimulation. However, after several minutes of continuous stimulation the behavior of animals showed changes which did not occur during short-term stimu-
Stimulation-induced
theta rhythm and behavior 100
06
0.4
I!
% In
z
.
50
.
I(
\, /
\,
Oi
L I
0
I 2
I
I
I
4
i / ;’
,I
\ I
v
I
I
6
8
/
1 IO
min
(
FIG. 3. Hippocampal theta rhythm and behavior in a rat during septal stimulation of IO min duration using constant stimulation intensity, stimulation frequency (7 HZ) and pulse duration (0.5 ms). Similar results were obtained in 9 other rats. Ordinate: % theta rhythm/min (---); 7; orienting behaviorimin (---); “/, orienting plus 9/, grooming/min (===).
C
Stimulation frequency,
Hz
FIG. 2. Dependence of theta rhythm wave amplitude and rearing behavior of a rat on stimulation frequency during septal stimulation using constant intensity and constant pulse duration (0.5 ms); duration of stimulation: 15 s. The hatched field shows the range of stimulation frequency in which stimulus-nonsynchronous waves (6 Hz, 7 Hz) occurred in the hippocampal EEG. Similar results were obtained in 3 other rats. Ordinate: amplitude of elicited theta rhythm waves in mV (A); rearing behavior in s (0).
orienting behavior was accompanied by an increase of automatic behavior, p~ticularly looming (cleaning, washing, licking, scratching). The sum of orienting behavior and grooming per minute was approximately equivalent to the amount of theta rhythm in the hippocampal EEG throughout the stimulation period. The results obtained from a septal stimulation of 10min duration are shown in Fig. 3. During the first 3 min of stimulation the values of theta rhythm/min are similar to those of orienting behavior/min. Afterwards, the orienting behavior is decreasing nearly to zero, while the sum of orienting behavior plus groom-
lation. The values obtained for hippocampal theta rhythm and orienting behavior were similar only during the first 3 min of stimulation. However, during the following few minutes of stimulation the orienting behavior was reduced, while the elicited hippocampal theta rhythm remained nearly constant. Thus, during long-lasting septal stimulation the reduced amount of
0
m
Orienting
m
Grooming
0
Other
5
behavior
I6
16
20
i5
30
min
FIG. 4. Behavioral changes in a rat during septal stimulation of 30 min duration using constant stimulation intensity, stimulation frequency (7 Hz) and pulse duration (0.5 ms). Similar results were obtained in 3 other rats. Black field: Yd orienting behavior/min; shaded field: 0/0 grooming behavior/min; 0000000% hippocampal theta rhythm/min.
7%
W. WETZ~L.T. OTT and Ii. MATTHUS
ing is approximately equivalent to the course of theta rhythm/min. After further continuous stimulation the orienting behavior~min returns to the original level. Figure 4 illustrates the time course of orienting behavior and grooming during continuous septal stimulation of 30 min duration. The amount of elicited theta rhythm/min occurred at an approximately constant level between 70 and 1009,, throughout the stimulation period. The time course of behavioral changes showed a periodically occurring increase and decrease in orienting behavior, the length of the periods ranging between 3 and 5 min. When the stimulation lasted about IOmin, the behavior of the animals was attributed to orienting behavior and grooming, the sum of both being for each minute nearly IOO”,,. Afterwards additional behavioral reactions occurred such as non-oriented movements and short periods of rest, whereas the amount of orienting behavior, especially rearing, was slightly reduced. and grooming remained nearly constant.
DISCUSSION
also increased, another correlation exists between stimulation frequency and stimulation effect. Good theta rhythm elicitation at stimulation frequencies within the physiological theta rhythm range was already reported by GRAY & BALL (1970). Using stimulation frequencies from 4 to 12 Hz, eliciting hippocampal theta rhythm of equal frequencies. we always observed the occurrence of orienting behavior during such stimulation. The finding that theta rhythm was elicited in awake animals during immobility, as reported by KRAMIS, VAN~ERW~LF & BLAND (1975) using tegmental stimulation, was not observed under our experimental conditions. In animals anaesthetized with hexobarbital we registered a shift of the frequency range for theta rhythm elicitation to tower frequencies: in these animals, in contrast to untreated rats, theta rhythm was easily elicited even under conditions of behavioral immobility‘ Using a stimulation frequency of 12 Hz a reversal point was found related to frequency and amplitude of the elicited theta rhythm as well as to the induced behavioral reaction (Fig. 2). which point is obviously determined by the upper limit of the physiological theta rhythm frequency.
The present experiments showed that electrical stimulation of the medial septum in freely moving Long-lusting continuous .stimulution of the septum rats can elicit the occurrence of regular hippocampal Our experiments with long-lasting continuous theta rhythm accompanied by certain behavioral stimulation revealed that several minutes after septal changes. Short-term stimulation at 4-12 Hz frequency always induced a typical orienting behavior. stimulation a dissociation between elicited theta Stimulation with varying stimulus parameters led rhythm and orienting behavior occurred: such an effect was not observed during short-lasting stimuin each case to the appearance of orienting behavior, lation. This dissociation was characterized by periodic at the time when theta rhythm occurred in the hippocampal EEG. This effect of medial septal stimulation changes of orienting behavior and grooming, while the amount of elicited theta rhythm remained nearly differed from the action of stimulation performed in constant. The observed rhythmic changes in behavior other brain regions showing, for example, in the suggest the existence of a stable endogenous rhythm hypothalamus the occurrence of different behavioral manifested under the conditions of long-lasting septal reactions dependent on the use of different stimuRhythmic oscillations with similar length lation parameters (MOGENSON, 1971: JOHANSON, stimulation. of periods have already been described in other bioKALIMO, NISKANEN & RUUSUNEN. 1974). However, the logical phenomena such as reaction time of condiintensity of elicited orienting behavior revealed stimutioned reactions (HECHT, HECHT, POPPEI, TREPTOW, lation-dependent changes, for instance, the amount of HARTRODT& ~H(~I~~WSKI, 1975). responses to exterrearing behavior, which is represented in Figs 1 and nal stimuli (HEMMER & K~~HLER. 1975), changes of 2. In contrast to the stimulation of other brain areas. psychophysiological parameters (SIN& AMENDA & the hippocdmpal EEG exhibited no habituation of FITZKE, 1975). LAT (1973: 1976) reported an extensive the septal stimulation effect even after repeated or analysis of periodic changes in the behavior of rats tong-tasting st~mutation. This might be due to the fact in a new environment. In other experiments concernthat in the medial septal stimulation a monosynaptic ing conditioned behavior (shuttle-box avoidance) as connection leading to the hippocampus is stimulated, well as uncondition~ behavior (drinking after water whereas for some o&her stimulation sites, the effects deprivation) we found periodically occurring changes of polysynaptic stimulation might be of more signifiin behavior with a period length of approx. 3 min. cance. In our experiments, even after long-lasting septal stimulation, a constant occurrence of hippocampai theta rhythm and periodically occurring orienting beIn view of these considerations. it is of particular havior was observed. Only certain behavioral cominterest that the phases of sleep also show a periodiponents were gradually reduced to a moderate city, since it is known that rapid-eye-movement degree; this fact might be attributed to development (REM) sleep is accompanied by a marked hippocamof motor exhaustion rather than habituation. pal theta rhythm and further, that generation of theta Whereas with increasing intensity of septal stimurhythm during REM sleep, orienting movements and lation and with enhancing pulse duration the elicited sensory stimulation possibly share the same underlyhippocampal theta rhythm and orienting behavior
Stimulation-induced ing m~h~ism is noteworthy 152Smin
(WINSON, 1976). In this connection that in the rat a REM
759
theta rhythm and behavior it
sleep phase lasts
(WEISS, BOHDANECK~ & RFKCJVA, 1964;
WAUPT, 1973; GAILLARD & TUGLULAR, 19761,
i.e. approximately half of the duration of one period (3-5min) of the behavioral changes as observed in our theta rhythm stimulation experiments. This seems to support the assumption that the REM periodicity in sleep as well as the observed ~riodicities in awake animals might be governed by common mechanisms
SC~TTI DE CAROLIS, 1970; SITARAM, WYATT, DAwsdN & GILLIN, 1976). Recently, correlations to exist between the one
hand,
REM and
memory
consolidation,
other hand: REM sleep enhancement the consolidation occurred
phase,
after REM
were reported
sleep and theta rhythm,
and
occurred
retention
sleep deprivation
on
on the
during
impairment (LECONTE &
HENNEVIN, 1973;
FISIIBEIN, KASTANIOTIS& CHATTMAN, 1974; PEARLMAN & BECKER,1973); furthermore retention facilitation by pharmacological or electrical elicitation of theta rhythm after training has been de(KLEITMAN, 1963). scribed (GRECKSCH,OTT & MATTHIES,1978; LANDA cholinergic activation appears to play an essenFIELD,1976; WETZELef al., 197733.In accordance with tial role both in (i) the elicitation of theta rhythm in the awake animaI-as shown by studies using sys- these finding, SKINNER, OVERS~EET & ORBACH temic (TORII & WIKLER, 1966; TEITE~BAUM, LEE 6% (1976) showed that the memory-disruptive effect of REM sleep deprivation was abolished by a post-trial JOHANNESSEN, 1975) and intrahippocampal appliapplication of physostigmine. cation (OTT, MALISCH & KRUG, 1977) of choiinergic substances pocampal
or stimulation
of the cholinergic
input (BR~~CKEet al., 1959; GRAY
septohip& BALL,
1970; LANDFIELD, 1976; WETZEL et al., 1977a)--and in
(ii) eliciting of REM sleep (KARCZMAR, L~NCO &
Acknawiedge~lents-This
work was supported by the GDR
Ministry of Science and Technology. The authors wish to thank Mrs I. BOLVTfor skilful technical assistance in these experiments.
BLACKA. H. (1976).Hippocampal electrical activity and behavior. In The Nippocampus (eds ISAACSON R. L. & ~IBBAM K. H.). Vol. 2, pp. 129-169. Plenum Press, New York. BRXICKEF., PIZSCHE H., PILLAT B. & ~EIS~HAMMER E. (1959). Ein Schrittmacher in der medialen Septumregion Kaninchengehirns. Purgers Arch. ges. Physial. X9, 13.5-140.
des
FISHBEINW,, KASTANIOTIS C. & CHATTMAND. (1974). Paradoxical sleep: Prolonged augmentation following learning. Brain Res. 79, 61-75. GAILLARD J. M. & TUGLULARI, (1976). The orthodox-paradoxical sleep cycle in the rat. ~xperien~ja 32, 718-719. GRAY J. A. & BALLG. G. (1970). Frequency-specific relation between hippocampal theta rhythm, behavior, and amobar-
bital action. Science, N.k: 168, 1246-1248. GRECKSCHG., OTT T. & MATTHIESIi. (1978). Influence of post-training intrahippocampally applied oxotremorine on the consolidation of a brightness discrimination. Fharmacal. Biochem. B&au. 8, 215-2 18. HA~~IPT R. (1973). The sleep-wake behavior of rats. In Sleep: Physiology, Bi~c~ei~jsrr)~,Pharmacology. Clinical Implications, (1st Europ. Congr. Sleep Res. Base1 1972), pp. 285-289. Karger, Basel. HECHTK., HECHTT., POPPEIM., TREP~W K., HARTRODTW. & CHOINOWSKI S. (1975). iiber die Wirkung von Angiotensin II auf bedingt-reflektorische Reaktionsmuster neurotischer Albinoratten. Acta biol. med. germ. 34, 1453-1464. HEMMER H. & KBHLERS. (1975). Rhythmische jinderungen der Reaktion auf verschiedene olfaktorische Reize beim Krallenfiosch (Xenopus lueuis). Experientia 31, 449-450. JOHAN~~NG. G., KALIMOR., NISKANENH. & RUUSUNENS. (1974). Effects of stimulation parameters on behavior elicited by stimulation of the hypothalamic defense area. J. camp. physiai. Pspchuf. 87, 1100..1108. JORK R., L&SN~ER B. & MATTHIESH. (19’78).Incorporation of [3H]Ieucine and C3H]fucose into total proteins of different hipp~ampal sectors after electrically and pharmacolo~ca~ly elicited theta rhythm in the rat. To be published. KARCZMARA. G., LONGOV. G. & SCOTTIDE CAROLISA. (1970). A pharmacological model of paradoxical sleep: the role of cholinergic and monoamine systems. Physiol. Behav. 5, 175-182. KLEITMANN. (1963). Sleep nnd Wakefulness. The University of Chicago Press, Chicago. KLEMMW. R. (1976). Physiological and behavioral significance of hippocampal rhythmic slow activity (‘theta rhythm’}. In Progress in ~e~robja~ogy, (eds KERKUTG. A. & PHILUS J. W.). Vol. 6. Pergamon Press, Oxford. KRAMISR,, VANDERWOLF C. H. & BLAND B. H. (1975). Two types of hippocampal rhythmic slow activity in both the rabbit and the rat: Relations to behavior and effects to atropine, diethyl ether, urethane, and pentobarbital. Expl Neural. 49, 58-85. LANDFIELDP. W. (1976). Synchronous EEG rhythms: Their nature, and their possible functions in memory, information transmission and behavior. In ~o~ecu~ur and Fanctiona~ ~eura~iaiogy (ed GISPEN W. H.), pp. 389-424. Elsevier, Amsterdam. LAr J. (1973). The analysis of habituation. Acta Neurohioi. Exp. 33, 771-789. Lli~ J. (1976). The theoretical curve of learning and of arousal. Actiu. New. Step. 18, 3643, LECONTEP. & ~E~N~VI~ E. (1973). Temporal characteristics of the au~meutation of paradoxicai sleep following learning in the rat. Phgsiol. Behau. 11, 677-686.
W. WETZEL, T. OTT and H. MATTHIES
760
MOGENSON G. J. (1971). Feeding and drinking elicited by low and high frequencies of hypothalamic stimulation. Brain Rex 33, 127-137. OTT T., MALISCH R. & KRUG M. (1977). Pharmacological analysis of hippocampal ‘theta’ rhythm. ~crcr Neurobiol. Exp. 36, 720. PEARLMAN CH. & BECKER M. (1973). Brief post-trial REM sleep deprivation impairs discrimination learning in rats. Physiol. Psychol. 1, 373-376. SINK R., AMENDA G. & FITZKE S. (1975). Kopplungscharakteristik minutenrhythmischer Modulationsschwingungen physiologischer Funktionen unter Diazepam. Actu hid. med. germ. 34, 275-301. SITARAM M., WYATT R. J.. DAWSON S. & GILLIN J. C. (1976). REM sleep induction by physostigmine infusion during sleep. Science, N.K 191, 1281-1283. SKINNER J. E. (1971). Neuroscience: A Lahorutory Munuul. W. B. Saunders, Philadelphia. SKINNER D. M., OVERSTREET D. H. & ORBACH J. (1976). Reversal of the memory-disruptive effects of REM sleep deprivation by physostigmine. Behave. Biol. 18, 189-198. TEITELBAUM H., LEE J. F. & JOHANNESSENJ. N. (1975). Behaviorally evoked hippocampal theta waves: a cholinergic response. Science, N. Y 188, 1114 1116. TORII S. & WIKLER A. (1966). Effects of atropine on electrical activity of hippocampus and cerebral cortex in cat. Psychopharmacologia 9, 189-204. VON EULER C. & GREEN J. D. (1960).
in rabbit.
Actu physio/.
WEISS T., BOHDANECK$
stand. Z. &
Excitation,
inhibition
and
rhythmical
activity
in hippocampal
pyramidal
cells
48, 1 i&125.
FIFKOV,~
E. (1964). Influence
5, 126-135. WETZEL W., OTT T. & MATTH[ES H. (19770).
Hippocampal
medial septal stimulation in rats. Behac. Biol. 19, 534542. WETZ~L W., OTT T. & MA~THIES H. (1977h). Post-training
of atropine rhythmic hippocampal
on sleep Cycle in rats. slow activity rhythmic
P.Y,Vchoph~rm~co&iQ
(‘theta’) and behavior
elicited
by
slow
elicited
by
stimulation improves memory consolidation in rats. Brhar. Biol. 21. 32-40. WINSON J. (1976). Hippocampal theta rhythmII. Depth profiles in the freely moving
activity
(‘theta’)
septal
(Accepted
4 April
1978)
rabbit.
Brain
Rex
103. 71~ 79.
0
0306-4522/78/08Ol-O755SO2.C0/
PressLtd.1978. Printed inGreatBritain.
IBRO
PERIODIC BEHAVIORAL CHANGES DURING HIPPOCAMPAL THETA RHYTHM ELICITED BY SEPTAL STIMULATION IN RATS W. WETZEL, T. OTT and H. MATTHIES Institute
Abstract-The of the dorsal
of Pharmacology and Toxicology, Medical Academy, 301 Magdeburg, German Democratic Republic
influence of an electrical stimulation of the medial septum on the electroencephalogram
hippocampus and behavior was studied in freely moving rats. A short stimulation at 4-12 Hz frequency always induced orienting behavior (searching, rearing, sniffing, exploration) as soon as hippocampal rhythmic slow activity, or theta rhythm is elicited. An increase in stimulation intensity or in pulse duration led to an increased occurrence of theta rhythm and likewise, to an increase of orienting behavior. Both the theta rhythm and orienting behavior were dependent on the stimulation frequency. During long-lasting septal stimulation at 7 Hz, which elicited a continuous hippocampal theta rhythm, periodic behavioral changes were observed, consisting of orienting behavior alternating with grooming behavior (cleaning, washing, licking, scratching). These rhythmic periods lasted for 3-5 min. The significance of the rhythmic behavioral changes are discussed in relation to the activity of the cholinergic septohippocampal system and to rapid-eye-movement sleep and memory consolidation.
ELECTRICALstimulation
of the medial septum is capable of eliciting changes in the electroencephalogram (EEG) activity of the hippocampus (BRCJCKE,PETSCHE, PILLAT &
DEISENHAMMER, 1959; VON
EULER &
GREEN, 1960; GRAY & BALL, 1970). We
used this method to study the influence of an electrical stimulation of the septohippocampal input on the consolidation of a conditioned reaction (WETZEL, OTT & MATTHIES, 1977h) as well as on the amino acid incorporation into hippocampal proteins (JORK, L&SNER & MATTHIES, 1978). The septal stimulation activity,
EEG
called
theta
(BRUCKE et a/.,
can rhythm,
elicit in
a rhythmic the
slow
hippocampal
1959; GRAY & BALL, 1970;
LANDFIELD, 1976; WETZEL, OTT & MATTHIES, 1977~).
Numerous investigations have been made into the hippocampal theta rhythm and different conclusions were drawn regarding its possible physiological significance (for review, see BLACK, 1976; KLEMM, 1976). But there are only limited studies concerning the relationship between septally induced theta rhythm and behavior in freely moving animals. For this reason, it was decided to extend previous experiments (WETZEL et al., 1977a). In the present work, the influence of various parameters of medial septal stimulation on hippocampal EEG and behavior in freely moving rats has been studied. Particularly, our attention was focused on the effect of long-lasting periods of stimulation as used in memory consolidation studies (WETZEL et al., 19776) and amino acid incorporation experiments (JORK et al.. 1978).
Abbreriations: rapid-eye-movement; in the hippocampal
EEG, electroencephalogram; REM, theta rhythm, rhythmic slow activity EEG. 755
EXPERIMENTAL
PROCEDURES
The experiments were performed on 30 adult male Wistar rats from our own breeding stock, weighing about 200g. The animals were kept individually under standard laboratory conditions, with free access to food and water. One week before the experiments, bipolar electrodes consisting of Teflon-insulated steel wire were implanted under hexobarbital anaesthesia using stereotaxic coordinates according to SKINNER(1971).The stimulation electrodes with resistances of about 50 kR were implanted into the medial septum using the following coordinates: 1.7 mm AP from bregma, 1.6 mm lateral, and 6.8 mm deep, at an angle of 15” with the median plane. The recording electrodes were implanted into the right dorsal hippocampus using the following coordinates: -3.5 mm AP, 3.1 mm lateral, and 3.1 mm deep. Stimulation (METRA Precision Stimulator, Radebeul, GDR) and EEG recording (8-channel Electroencephalograph, ZwBnitz, GDR) were performed on freely moving rats. To eliminate the influence of external disturbances, the behavior of animals was observed by a television camera. The behavior was continuously noted on the EEG paper during the whole stimulation experiment. The septal stimulation was done by means of square pulses of different duration (0.1-1.0 ms) and different frequency (l-50 Hz). The stimulation intensity ranged from 0.5 to 5.0 V, resulting in current values of about l@lOOpA. Furthermore, the influence of the duration of stimulation on hippocampal EEG and behavior was studied. Experiments with long-lasting septal stimulation were not repeated in the same rat. The exact localization of the electrodes in the septum and the hippocampus was histologically verified. EEG records were evaluated by visual means. The amount of theta rhythm was indicated as per cent theta rhythm per minute. Similarly, the presence of different types of behavior (orienting, grooming, other) was expressed as “,;/min. For calculation of theta rhythm amplitude the highest amplitude of stimulation-induced theta rhythm waves was measured. In the figures, the
W. W~.TZLIL.T. OTT and H.
756
MATTHRS
results for lnd~vidual animals are shown, m order to avoid a loss of information by averaging the data of a group of rats. Nevertheless, each example described is representative of a number of animals. RESULTS Septal stimulation
of short duration
Initially, we examined the influence of various stimulation parameters on hippocampal EEG and behavior applying septal stimulation periods of short duration (lG-30~). In each experiment only one stimulation parameter was varied. while the other parameters were held constant. The initial results of these studies have been reported in a short communication (WmzEL et al.. 1977~1). The increase in stimulation intensity under conditions of constant pulse duration (OS ms) and constant stimulation frequency (7 Hz) induced an increase in the amount of theta rhythm in the hippocampal EEG as well as an increase of orienting behavior (searching, rearing, sniffing, exploration) up to 100~~;l.The stimulation intensity required for the occurrence of these effects (OSSV) was in a range below the hippocampal seizure threshold. The application of a subthreshold intensity for eliciting theta rhythm was followed by a desynchronization in hippocampal EEG, especially when high voltage slow wave activity was present. With small increases in intensity, theta rhythm occurred first at the end of the 0.5-min stimulation period. Then, with further increasing stimulation intensity. the latency between the onset of stimulation and the occurrence of theta rhythm decreased towards zero. The latcncies between the onset of stimulation and theta rhythm occurrence as well as between the onset of stimulation and occurrence of orienting behavior were found to be equal. i.e. theta rhythm and orienting behavior always started at the same moment. Under conditions of a constant stimulation intensity and stimulation frequency (7 Hz), an increase in the duration of individual pulses induced an increase in theta rhythm and orienting behavior up to 100”;,. An example is given in Fig. 1 illustrating the effect of square wave pulses with increasing duration from 0.1 to 1.0 ms. The figure also shows the enhanced amount of rearing related to the whole orienting behavior up to IOO”,,,.Every stimulation period lasted 15s. Further experiments revealed that the threshold intensity required for septal elicitation of theta rhythm could not be decreased further by an additional increase in the pulse duration exceeding 1 ms. In the frequency range between 3-4 Hz and 12-14 Hz a stimulus-synchronous hippocdmpal theta rhythm was induced by stimulation at different frequencies. If for each stimulation frequency the threshold intensity for the elicitation of theta rhythm was determined. this threshold intensity revealed a minimum in the 6 7 Hz band, i.e. in the frequency in which physiological theta rhythm mostly occurred. Further decrease in the threshold intensity for theta
Pulse
duration,
ms
FIG. I. Dependence of elicited hippocampal theta rhythm and orienting behavior of a rat on pulse duration during medial soptal stimulation using constant stimulation intensity and stimulation frequency (7 Hz); duration of stimulation : i 5 s. Similar results were obtained in 2 other rats. Ordinate: ‘lo theta rhythm (--I; “/, orienting behavior (- - 1: I’,, rearing -0).
rhythm became evident using frequencies of 10-15 Hz and higher. A similar relationship was found between the stimulation frequency and the threshold intensity required for eliciting orienting behavior. The use of an intermediate intensity of stimulation up to a frequency of 12 Hz caused an increase in amplitude of the elicited theta waves (Fig. 2). However, a further increase in stimulation frequency led to a decrease of theta rhythm amplitude accompanied by the occurrence of hippocampal EEG waves which were not synchronous with the stimulation frequency. That means, during application of a stimulation frequency of 12 Hz, theta waves of 6 Hz were evident, during 14 Hz stimulation waves of 7 Hz, etc. Likewise, the dependence of rearing behavior on stimulation frequency paralleled the changes in theta rhythm amplitude (Fig. 2). Continuous
long-lasting
svptul stimulation
In further experiments the septal stimulation was applied continuously over longer time-periods ranging from several minutes to half an hour. A stimulation intensity capable of eliciting hippocampal theta rhythm was chosen, and pulse duration (0.5 ms) and stimulation frequency (7 Hz) were held constant. In a first series septal stimulation of 5 min duration was performed. Under these conditions theta rhythm occurred in the hippocampal EEG, comprising between SO and iOO”,,.just as it did after short-lasting stimulation. However, after several minutes of continuous stimulation the behavior of animals showed changes which did not occur during short-term stimu-
Stimulation-induced
theta rhythm and behavior 100
06
0.4
I!
% In
z
.
50
.
I(
\, /
\,
Oi
L I
0
I 2
I
I
I
4
i / ;’
,I
\ I
v
I
I
6
8
/
1 IO
min
(
FIG. 3. Hippocampal theta rhythm and behavior in a rat during septal stimulation of IO min duration using constant stimulation intensity, stimulation frequency (7 HZ) and pulse duration (0.5 ms). Similar results were obtained in 9 other rats. Ordinate: % theta rhythm/min (---); 7; orienting behaviorimin (---); “/, orienting plus 9/, grooming/min (===).
C
Stimulation frequency,
Hz
FIG. 2. Dependence of theta rhythm wave amplitude and rearing behavior of a rat on stimulation frequency during septal stimulation using constant intensity and constant pulse duration (0.5 ms); duration of stimulation: 15 s. The hatched field shows the range of stimulation frequency in which stimulus-nonsynchronous waves (6 Hz, 7 Hz) occurred in the hippocampal EEG. Similar results were obtained in 3 other rats. Ordinate: amplitude of elicited theta rhythm waves in mV (A); rearing behavior in s (0).
orienting behavior was accompanied by an increase of automatic behavior, p~ticularly looming (cleaning, washing, licking, scratching). The sum of orienting behavior and grooming per minute was approximately equivalent to the amount of theta rhythm in the hippocampal EEG throughout the stimulation period. The results obtained from a septal stimulation of 10min duration are shown in Fig. 3. During the first 3 min of stimulation the values of theta rhythm/min are similar to those of orienting behavior/min. Afterwards, the orienting behavior is decreasing nearly to zero, while the sum of orienting behavior plus groom-
lation. The values obtained for hippocampal theta rhythm and orienting behavior were similar only during the first 3 min of stimulation. However, during the following few minutes of stimulation the orienting behavior was reduced, while the elicited hippocampal theta rhythm remained nearly constant. Thus, during long-lasting septal stimulation the reduced amount of
0
m
Orienting
m
Grooming
0
Other
5
behavior
I6
16
20
i5
30
min
FIG. 4. Behavioral changes in a rat during septal stimulation of 30 min duration using constant stimulation intensity, stimulation frequency (7 Hz) and pulse duration (0.5 ms). Similar results were obtained in 3 other rats. Black field: Yd orienting behavior/min; shaded field: 0/0 grooming behavior/min; 0000000% hippocampal theta rhythm/min.
7%
W. WETZ~L.T. OTT and Ii. MATTHUS
ing is approximately equivalent to the course of theta rhythm/min. After further continuous stimulation the orienting behavior~min returns to the original level. Figure 4 illustrates the time course of orienting behavior and grooming during continuous septal stimulation of 30 min duration. The amount of elicited theta rhythm/min occurred at an approximately constant level between 70 and 1009,, throughout the stimulation period. The time course of behavioral changes showed a periodically occurring increase and decrease in orienting behavior, the length of the periods ranging between 3 and 5 min. When the stimulation lasted about IOmin, the behavior of the animals was attributed to orienting behavior and grooming, the sum of both being for each minute nearly IOO”,,. Afterwards additional behavioral reactions occurred such as non-oriented movements and short periods of rest, whereas the amount of orienting behavior, especially rearing, was slightly reduced. and grooming remained nearly constant.
DISCUSSION
also increased, another correlation exists between stimulation frequency and stimulation effect. Good theta rhythm elicitation at stimulation frequencies within the physiological theta rhythm range was already reported by GRAY & BALL (1970). Using stimulation frequencies from 4 to 12 Hz, eliciting hippocampal theta rhythm of equal frequencies. we always observed the occurrence of orienting behavior during such stimulation. The finding that theta rhythm was elicited in awake animals during immobility, as reported by KRAMIS, VAN~ERW~LF & BLAND (1975) using tegmental stimulation, was not observed under our experimental conditions. In animals anaesthetized with hexobarbital we registered a shift of the frequency range for theta rhythm elicitation to tower frequencies: in these animals, in contrast to untreated rats, theta rhythm was easily elicited even under conditions of behavioral immobility‘ Using a stimulation frequency of 12 Hz a reversal point was found related to frequency and amplitude of the elicited theta rhythm as well as to the induced behavioral reaction (Fig. 2). which point is obviously determined by the upper limit of the physiological theta rhythm frequency.
The present experiments showed that electrical stimulation of the medial septum in freely moving Long-lusting continuous .stimulution of the septum rats can elicit the occurrence of regular hippocampal Our experiments with long-lasting continuous theta rhythm accompanied by certain behavioral stimulation revealed that several minutes after septal changes. Short-term stimulation at 4-12 Hz frequency always induced a typical orienting behavior. stimulation a dissociation between elicited theta Stimulation with varying stimulus parameters led rhythm and orienting behavior occurred: such an effect was not observed during short-lasting stimuin each case to the appearance of orienting behavior, lation. This dissociation was characterized by periodic at the time when theta rhythm occurred in the hippocampal EEG. This effect of medial septal stimulation changes of orienting behavior and grooming, while the amount of elicited theta rhythm remained nearly differed from the action of stimulation performed in constant. The observed rhythmic changes in behavior other brain regions showing, for example, in the suggest the existence of a stable endogenous rhythm hypothalamus the occurrence of different behavioral manifested under the conditions of long-lasting septal reactions dependent on the use of different stimuRhythmic oscillations with similar length lation parameters (MOGENSON, 1971: JOHANSON, stimulation. of periods have already been described in other bioKALIMO, NISKANEN & RUUSUNEN. 1974). However, the logical phenomena such as reaction time of condiintensity of elicited orienting behavior revealed stimutioned reactions (HECHT, HECHT, POPPEI, TREPTOW, lation-dependent changes, for instance, the amount of HARTRODT& ~H(~I~~WSKI, 1975). responses to exterrearing behavior, which is represented in Figs 1 and nal stimuli (HEMMER & K~~HLER. 1975), changes of 2. In contrast to the stimulation of other brain areas. psychophysiological parameters (SIN& AMENDA & the hippocdmpal EEG exhibited no habituation of FITZKE, 1975). LAT (1973: 1976) reported an extensive the septal stimulation effect even after repeated or analysis of periodic changes in the behavior of rats tong-tasting st~mutation. This might be due to the fact in a new environment. In other experiments concernthat in the medial septal stimulation a monosynaptic ing conditioned behavior (shuttle-box avoidance) as connection leading to the hippocampus is stimulated, well as uncondition~ behavior (drinking after water whereas for some o&her stimulation sites, the effects deprivation) we found periodically occurring changes of polysynaptic stimulation might be of more signifiin behavior with a period length of approx. 3 min. cance. In our experiments, even after long-lasting septal stimulation, a constant occurrence of hippocampai theta rhythm and periodically occurring orienting beIn view of these considerations. it is of particular havior was observed. Only certain behavioral cominterest that the phases of sleep also show a periodiponents were gradually reduced to a moderate city, since it is known that rapid-eye-movement degree; this fact might be attributed to development (REM) sleep is accompanied by a marked hippocamof motor exhaustion rather than habituation. pal theta rhythm and further, that generation of theta Whereas with increasing intensity of septal stimurhythm during REM sleep, orienting movements and lation and with enhancing pulse duration the elicited sensory stimulation possibly share the same underlyhippocampal theta rhythm and orienting behavior
Stimulation-induced ing m~h~ism is noteworthy 152Smin
(WINSON, 1976). In this connection that in the rat a REM
759
theta rhythm and behavior it
sleep phase lasts
(WEISS, BOHDANECK~ & RFKCJVA, 1964;
WAUPT, 1973; GAILLARD & TUGLULAR, 19761,
i.e. approximately half of the duration of one period (3-5min) of the behavioral changes as observed in our theta rhythm stimulation experiments. This seems to support the assumption that the REM periodicity in sleep as well as the observed ~riodicities in awake animals might be governed by common mechanisms
SC~TTI DE CAROLIS, 1970; SITARAM, WYATT, DAwsdN & GILLIN, 1976). Recently, correlations to exist between the one
hand,
REM and
memory
consolidation,
other hand: REM sleep enhancement the consolidation occurred
phase,
after REM
were reported
sleep and theta rhythm,
and
occurred
retention
sleep deprivation
on
on the
during
impairment (LECONTE &
HENNEVIN, 1973;
FISIIBEIN, KASTANIOTIS& CHATTMAN, 1974; PEARLMAN & BECKER,1973); furthermore retention facilitation by pharmacological or electrical elicitation of theta rhythm after training has been de(KLEITMAN, 1963). scribed (GRECKSCH,OTT & MATTHIES,1978; LANDA cholinergic activation appears to play an essenFIELD,1976; WETZELef al., 197733.In accordance with tial role both in (i) the elicitation of theta rhythm in the awake animaI-as shown by studies using sys- these finding, SKINNER, OVERS~EET & ORBACH temic (TORII & WIKLER, 1966; TEITE~BAUM, LEE 6% (1976) showed that the memory-disruptive effect of REM sleep deprivation was abolished by a post-trial JOHANNESSEN, 1975) and intrahippocampal appliapplication of physostigmine. cation (OTT, MALISCH & KRUG, 1977) of choiinergic substances pocampal
or stimulation
of the cholinergic
input (BR~~CKEet al., 1959; GRAY
septohip& BALL,
1970; LANDFIELD, 1976; WETZEL et al., 1977a)--and in
(ii) eliciting of REM sleep (KARCZMAR, L~NCO &
Acknawiedge~lents-This
work was supported by the GDR
Ministry of Science and Technology. The authors wish to thank Mrs I. BOLVTfor skilful technical assistance in these experiments.
BLACKA. H. (1976).Hippocampal electrical activity and behavior. In The Nippocampus (eds ISAACSON R. L. & ~IBBAM K. H.). Vol. 2, pp. 129-169. Plenum Press, New York. BRXICKEF., PIZSCHE H., PILLAT B. & ~EIS~HAMMER E. (1959). Ein Schrittmacher in der medialen Septumregion Kaninchengehirns. Purgers Arch. ges. Physial. X9, 13.5-140.
des
FISHBEINW,, KASTANIOTIS C. & CHATTMAND. (1974). Paradoxical sleep: Prolonged augmentation following learning. Brain Res. 79, 61-75. GAILLARD J. M. & TUGLULARI, (1976). The orthodox-paradoxical sleep cycle in the rat. ~xperien~ja 32, 718-719. GRAY J. A. & BALLG. G. (1970). Frequency-specific relation between hippocampal theta rhythm, behavior, and amobar-
bital action. Science, N.k: 168, 1246-1248. GRECKSCHG., OTT T. & MATTHIESIi. (1978). Influence of post-training intrahippocampally applied oxotremorine on the consolidation of a brightness discrimination. Fharmacal. Biochem. B&au. 8, 215-2 18. HA~~IPT R. (1973). The sleep-wake behavior of rats. In Sleep: Physiology, Bi~c~ei~jsrr)~,Pharmacology. Clinical Implications, (1st Europ. Congr. Sleep Res. Base1 1972), pp. 285-289. Karger, Basel. HECHTK., HECHTT., POPPEIM., TREP~W K., HARTRODTW. & CHOINOWSKI S. (1975). iiber die Wirkung von Angiotensin II auf bedingt-reflektorische Reaktionsmuster neurotischer Albinoratten. Acta biol. med. germ. 34, 1453-1464. HEMMER H. & KBHLERS. (1975). Rhythmische jinderungen der Reaktion auf verschiedene olfaktorische Reize beim Krallenfiosch (Xenopus lueuis). Experientia 31, 449-450. JOHAN~~NG. G., KALIMOR., NISKANENH. & RUUSUNENS. (1974). Effects of stimulation parameters on behavior elicited by stimulation of the hypothalamic defense area. J. camp. physiai. Pspchuf. 87, 1100..1108. JORK R., L&SN~ER B. & MATTHIESH. (19’78).Incorporation of [3H]Ieucine and C3H]fucose into total proteins of different hipp~ampal sectors after electrically and pharmacolo~ca~ly elicited theta rhythm in the rat. To be published. KARCZMARA. G., LONGOV. G. & SCOTTIDE CAROLISA. (1970). A pharmacological model of paradoxical sleep: the role of cholinergic and monoamine systems. Physiol. Behav. 5, 175-182. KLEITMANN. (1963). Sleep nnd Wakefulness. The University of Chicago Press, Chicago. KLEMMW. R. (1976). Physiological and behavioral significance of hippocampal rhythmic slow activity (‘theta rhythm’}. In Progress in ~e~robja~ogy, (eds KERKUTG. A. & PHILUS J. W.). Vol. 6. Pergamon Press, Oxford. KRAMISR,, VANDERWOLF C. H. & BLAND B. H. (1975). Two types of hippocampal rhythmic slow activity in both the rabbit and the rat: Relations to behavior and effects to atropine, diethyl ether, urethane, and pentobarbital. Expl Neural. 49, 58-85. LANDFIELDP. W. (1976). Synchronous EEG rhythms: Their nature, and their possible functions in memory, information transmission and behavior. In ~o~ecu~ur and Fanctiona~ ~eura~iaiogy (ed GISPEN W. H.), pp. 389-424. Elsevier, Amsterdam. LAr J. (1973). The analysis of habituation. Acta Neurohioi. Exp. 33, 771-789. Lli~ J. (1976). The theoretical curve of learning and of arousal. Actiu. New. Step. 18, 3643, LECONTEP. & ~E~N~VI~ E. (1973). Temporal characteristics of the au~meutation of paradoxicai sleep following learning in the rat. Phgsiol. Behau. 11, 677-686.
W. WETZEL, T. OTT and H. MATTHIES
760
MOGENSON G. J. (1971). Feeding and drinking elicited by low and high frequencies of hypothalamic stimulation. Brain Rex 33, 127-137. OTT T., MALISCH R. & KRUG M. (1977). Pharmacological analysis of hippocampal ‘theta’ rhythm. ~crcr Neurobiol. Exp. 36, 720. PEARLMAN CH. & BECKER M. (1973). Brief post-trial REM sleep deprivation impairs discrimination learning in rats. Physiol. Psychol. 1, 373-376. SINK R., AMENDA G. & FITZKE S. (1975). Kopplungscharakteristik minutenrhythmischer Modulationsschwingungen physiologischer Funktionen unter Diazepam. Actu hid. med. germ. 34, 275-301. SITARAM M., WYATT R. J.. DAWSON S. & GILLIN J. C. (1976). REM sleep induction by physostigmine infusion during sleep. Science, N.K 191, 1281-1283. SKINNER J. E. (1971). Neuroscience: A Lahorutory Munuul. W. B. Saunders, Philadelphia. SKINNER D. M., OVERSTREET D. H. & ORBACH J. (1976). Reversal of the memory-disruptive effects of REM sleep deprivation by physostigmine. Behave. Biol. 18, 189-198. TEITELBAUM H., LEE J. F. & JOHANNESSENJ. N. (1975). Behaviorally evoked hippocampal theta waves: a cholinergic response. Science, N. Y 188, 1114 1116. TORII S. & WIKLER A. (1966). Effects of atropine on electrical activity of hippocampus and cerebral cortex in cat. Psychopharmacologia 9, 189-204. VON EULER C. & GREEN J. D. (1960).
in rabbit.
Actu physio/.
WEISS T., BOHDANECK$
stand. Z. &
Excitation,
inhibition
and
rhythmical
activity
in hippocampal
pyramidal
cells
48, 1 i&125.
FIFKOV,~
E. (1964). Influence
5, 126-135. WETZEL W., OTT T. & MATTH[ES H. (19770).
Hippocampal
medial septal stimulation in rats. Behac. Biol. 19, 534542. WETZ~L W., OTT T. & MA~THIES H. (1977h). Post-training
of atropine rhythmic hippocampal
on sleep Cycle in rats. slow activity rhythmic
P.Y,Vchoph~rm~co&iQ
(‘theta’) and behavior
elicited
by
slow
elicited
by
stimulation improves memory consolidation in rats. Brhar. Biol. 21. 32-40. WINSON J. (1976). Hippocampal theta rhythmII. Depth profiles in the freely moving
activity
(‘theta’)
septal
(Accepted
4 April
1978)
rabbit.
Brain
Rex
103. 71~ 79.
