Physiology & Behavior, Vol. 16, pp. 257-266. Pergamon Press and Brain Research Publ., 1976. Printed in the U.S.A.
Endogenous Circadian Rhythms in Rats Recovered From Lateral Hypothalamic Lesions NEIL ROWLAND 1
Psychology Department, University o f Pittsburgh, Pa 15260. (Received 6 June 1975) ROWLAND, N. Endogenous circadian rhythms in rats recovered from lateral hypothalamic lesions. PHYSIOL. BEHAV. 16(3) 257-266, 1976. - Compared with intact controls, rats which have recovered spontaneous ingestion after lateral hypothalamic lesions show exaggerated nocturnal rhythms. Food intake was low and water intake negligible in the light part of a LD 12:12 cycle. The endogenous rhythms persisted in constant dark, and were clearly free running in constant light with periods of 25-26 hr. Animals with relatively asymmetric damage showed strong LL suppression of intake on the first day; animals with symmetric damage showed no LL suppression and the strongest free running rhythms; rats with the largest lesions showed a permanent LL suppression of fluid intake from Day 3 onward. The LL rhythms were rapidly reentrained with a restored LD cycle, with phase shifts of over 90°]24 hr. Recovered laterals which were previously blinded by orbital enucleation showed free running rhythms with periods close to 24 hr. The division of the 24 hr period into active (a) and quiescent (o) phases is much more sharply defined after the lesion. This is especially marked for drinking; feeding and activity rhythms are very similar. It is suggested that the recovered lateral is dependent upon endogenous oscillator(s) subserving motivated behaviors, and implications for the recovery of function and residual impairments are discussed. Circadian rhythms
Drinking
Lateral hypothalamus
Lesions
EXTENSIVE lesions of the lateral hypothalamus (LHA) cause severe behavioral disruptions which include sensory inattention and failure to eat or drink. Many rats bearing these lesions may be maintained by tube feeding, and show a progressive sequence of recovery of the lost behaviors [6, 14, 33]. The recovered lateral describes the rat which has reached a stage of self-maintenance on dry food and water, but it is hardly surprising that these animals still show persistent behavioral disruptions [6]. They show abnormal patterns of ad lib ingestion [ 11,12] and failures to respond to acute homeostatic challenges with rapid regulatory behavioral adaptations [ 6,13 ]. Such findings were originally taken as evidence that ingestive behavior in the recovered lateral was under novel controls, and that the recovery sequence at least partly reflected the time necessary for the institution of these new control mechanisms [6, 13, 32]. However, recent findings have suggested that the failures of these rats to respond to acute challenges may reflect a decreased capacity for function in the damaged brain systems. The behavioral deficits are thus viewed as a general or nonspecific nature rather than a result of damage to ingestion-specific circuits [30]. For example, the recovered lateral which classically does not drink in response to hypertonic NaC1 challenge [6] in fact drinks quite well if the tests are prolonged [29]. The drinking which is observed is nocturnal [23]. These, and other demonstrations that recovered laterals appear to be sensitive to their internal hydrational status [22,24] have led me to reexamine the ad lib ingestive patterns of these animals.
Kissileff's comprehensive analysis revealed that the LHA lesioned rat recovered to eating dry pellets and drinking water was both a nibbler and a prandial drinker [11, 12, 13]. Meals of dry food are prolonged affairs of several hours' duration (nibbling); after every few mouthfuls feeding is interrupted for a small draft of water (prandial drinking). Both of these behaviors are seen in the neurologically intact (NI) surgically desalivate rat [ 11,13 ]. Kissileff also noted that the ingestive behavior of LHA lesioned animals occurred almost exclusively during the dark part of the light-dark (LD) cycle, a finding in agreement with reports in which L and D intakes were measured separately [10, 26, 33]. The slow rates of ingestion of recovered laterals thus implies that almost the entire nighttime must be spent in ingestive behavior. The NI rat is a nocturnal animal with some 7 0 - 9 0 % of ingestive behavior during the night of a 12:12 LD cycle [34]. The night is, however, broken into discrete meals followed .by long periods of no ingestion [ 11 ], more or less well defined activity cycles [21] and presumably some resting periods. The available evidence suggests that the recovered lateral does not show such well defined sequences of motivated behavior. The present study is addressed to the issue of disrupted behavioral rhythms in these rats, and some new facets of the lateral hypothalamic syndrome have been revealed. Standard experimental probes of circadian rhythms were applied, mainly to drinking behavior, and led to the conclusion that LHA lesioned rats have exceptionally strong or well defined circadian rhythms. The nocturnal habits of these rats may be understood as the active period
I am grateful to Alan E. Fisher for provision of facilities and financial support (USPHS-MH 1951) for this research. 257
258
ROWLAND
(a) o f such a r h y t h m which is phase synchronized with the darkness. GENERAL METHOD
Animals and Procedures Male adult albino rats o f the Sprague Dawley strain (Zivic Miller, Allison Pk,PA, or lab raised derivatives thereof) were used. Powdered Purina rat chow was available ad lib f r o m glass jars (6 cm high, 4 cm dia.) placed on the cage floor. Both normal and lesioned animals fed from these jars with negligible spillage. Unless stated, tap water was the drinking fluid available from a graduated cylinder with metal drinking spout. The orifice was at least 0.35 crn dia., since LHA lesioned rats have difficulty drinking from small surfaces. The animals were allowed at least 1 week to adapt to the laboratory 12:12 lighting cycle (lights on 0800-2000 hr local time) provided by overhead fluorescent fixtures. Most experiments were c o n d u c t e d in a r o o m housing NI conspecifics hence giving an u n c o n t r o l l e d nocturnal noise cue. Rats were individually housed either in regular wire mesh rack cages, in cages with a cylindrical Plexiglas wall (30 cm dia.), or in 45 cm dia. cages with one half of the cylindrical wall of Plexiglas. The drinking spouts protruded a p p r o x i m a t e l y 0.5 cm through the transparent wails, or through the front wall o f the mesh cages, at a height of 3 cm f r o m the floor. The daytime light intensity, measured by reflectance, was similar (2 ft-lamberts) at the drinking spout in all of the cages. F o o d and water intakes were recorded every day at 0 8 0 0 and 2000 hr, and b o d y w e i g h t was periodically checked for stability. In a d d i t i o n to these measures which provided, respectively, L and D intakes for rats on a LD cycle continuous drinking records were obtained for m o s t of the animals. Grason-Stadler drinkometers were attached to the drinking spouts (using the cage floor as ground, hence the animal c o m p l e t e d the circuit w h e n drinking) and the outputs recorded on a 20 channel Esterline Angus event recorder (chart speed 7.6 cm/hr). C o n t e m p o r a n e o u s feeding patterns were recorded in some o f the rats. The feeding d e t e c t o r consisted o f the plastic screw top lid to a food jar from which a 150 ° sector was removed. A microswitch was m o u n t e d on the lid such that the lever on the switch was parallel to and near one side o f the 150 ° hole. When screwed o n t o the jar, the feeding rat regularly knocks the lever aside thus depressing the microswitch; this e c o n o m i c device functions e x t r e m e l y reliably. (A second plastic top with sector removed is taped on top of the first, thus covering the switch and wires, making it rat-proof). Most rats are c o m p l e t e l y u n p e r t u r b e d by this device and continue to feed in the same way as from o p e n jars. This device allows the estimation o f free feeding o f dry food w h e n the m o u t h f u l is the o n l y natural q u a n t u m , not true for pellet feeding situations (see [12]). The feeder was attached to the wall of a 45 cm diameter cage, opposite the drinking spout. Thus in order to m o v e from feeder to drinking spout the rat crossed the cage. In so doing, a p h o t o b e a m was cut and detected by a photocell connected to the event recorder. In such a way some measure of l o c o m o t o r activity was obtained. The 3 records, feeding, drinking and activity were obtained on adjacent traces on the chart paper.
Surgical Procedure and Recovery Care Stereotaxic
(Kopf)
placement
of
LHA
lesions was
p e r f o r m e d using ether anaesthesia. Insulated (Krylon R) 000 steel insect pin electrodes were lowered to A: 5 . 5 - 6 . 0 , L: -+2.0, D: 7 . 5 - 8 . 0 m m from dura, with the t o o t h b a r set at the level of the earbars. Anodal current of 1.5 m A was passed for 1 0 - 2 5 sec using a rectal cathode. Only rats which were totally aphagic and adipsic for at least 2 days are included in the present study. Special care, including tube feeding (1:1 diluted Borden's sweetened condensed milk) and palatable foods (wet mash, or more usually baby cereal m i x e d with sweet milk) were administered for as long as necessary in the postoperative period, and the recovery times are shown in Table 1. When the present study was c o m m e n c e d all rats were ingesting dry chow and water in a m o u n t s sufficient to maintain stable bodyweight (typically k n o w n as Stage 4 [6,33] ). A few rats were still in Stage 3, drinking saccharin sweetened water, and these rats never progressed to Stage 4 subsequently. TABLE1 RECOVERY TIMES FOLLOWING LHA LESIONS AND EXPERIMENTAL RECORDS
Rat Sighted ADL l ACL 7 ACL 3 ACON 1 ACON 3 ACL 2 ACL l K 51 K 50 ACL 5 ACON 5 ACL 4
Total Aphagia (days)
Time to Ingest Water (days)
l0 3 2 2 3 8 5 7 7 8 4 7
20 12 2 3 12 13 18 13 23 >215 >250 >250
7 5 2 8 3 10 6 5 3 5 8
15 12 3 >60 10 37 >100 >130 12 >62 >106
Experiments Served in
DD DD LL (B) DD, LL (B) DD, LL (B) DD, LL (B) DD, LL (A) LL (A) LL (C) LL (C), DD LL (C) LL (C)
Blinded ENU 3 5 6 8 11 12 13 14 16 18 19
r pre ---24.00 24.00 24.15 24.24 24.12 24.00 24.23 24.00
r post 24.05 24.09 24.07 -24.02 24.15 24.33 -23.47 24.19 24.18
DD = experiment 1, constant dark; LL = experiment 2, constant light; ( ) refers to the group of the animal. r pre, r post = pre- and postlesion free running periods of drinking in enucleated animals, (hr. rain). Times to ingest water (i.e. to Stage 4) marked > indicate animal was drinking saccharin water at time of death or sacrifice.
Histological Verification When the rats were sacrificed, often m a n y m o n t h s after the present experiments, Formalin perfused and fixed brains were examined. Frozen 40 u sections o f a coronal plane were m o u n t e d and stained for cell (cresyl violet) or
CIRCADIAN RHYTHMS IN RECOVERED LATERALS
259
fiber (Weil) coloration. Lesions were reconstructed using a microprojector and standard atlas plates. Data Presen tan'on
LD-1
Many of the results are given as representative individual records. Original Esterline Angus traces were appropriately cut and mounted such that 24 hr records for an individual were vertically arranged. In the convention used in this paper it should be noted that time runs from right to left of the figures. For the purposes of rhythm analysis visual inspection was adequate for the relatively unsophisticated treatment. The figures were reconstructed according to Quay [20]. If drinking occurred within a 10 min time bin, the corresponding 10 min block was shaded on graph paper, and so on for the whole record. Thus, if a rat drank once every 10 min for 3 hr that whole 3 hr block would appear shaded, and would be identical with that for a rat which drank every minute for 3 hr. The original records of feeding, drinking and activity showed that intervals between drinks of up to 10 min were invariably filled with intermittent feeding and locomotion. Thus the qualitative drinking patterns as reconstructed are probably more accurately termed behavioral arousal records. No attempt was made to analyse the data over shorter intervals of time, nor to make a quantitative mathematical analysis of the rhythms; all of the essential findings are evident from the figures. EXPERIMENT 1 NI rats ingest about 80% of their daily water by night [34], while recovered laterals are more than 95% nocturnal drinkers [10]. This low, often zero daytime water intake might be due to an exaggerated sensitivity of the lesioned rats to light, with consequent light suppression of drinking. A continuously dark (DD) environment would remove this constraint, and drinking might occur throughout the 24 hr. Method and Results
Six rats recovered to Stage 4 (drinking water) after LHA lesions were used in this experiment. One of the rats was adrenalectomized and corticosteroid replaced (monthly long-acting Percorten ®, 25 mg/kg). After 3 × 24 hr baseline measures in LD, the lights were turned off in the middle of the 46th postlesion day. The DD regimen then remained in force for 17 x 24 hr after which time the original LD cycle was restored. During the DD phase a dim red light was used for taking measurements; additional leakage of white light was unavoidable whenever the door was opened (at irregular times, in general). The transition from LD to DD had no effect upon the total 24 hr intake of recovered laterals (Fig. 1), and there was no great change in the drinking patterns (Fig. 2). Drinking remained principally during the former night hours, and only after several days was there any tendency for the rhythms to become less sharply defined (Fig. 2). As a consequence the water intakes between 0800 and 2000 hr, negligible in LD, reached 15% of the 24 hr total by Days 7 and 8 in DD. There was no apparent change between Days 8 and 16 in DD (Fig. 1). Feeding rhythms were slightly less exaggerated in LD, and drifted in DD in much the same degree as drinking (Fig. 1). These changes are all significant (p
Endogenous Circadian Rhythms in Rats Recovered From Lateral Hypothalamic Lesions NEIL ROWLAND 1
Psychology Department, University o f Pittsburgh, Pa 15260. (Received 6 June 1975) ROWLAND, N. Endogenous circadian rhythms in rats recovered from lateral hypothalamic lesions. PHYSIOL. BEHAV. 16(3) 257-266, 1976. - Compared with intact controls, rats which have recovered spontaneous ingestion after lateral hypothalamic lesions show exaggerated nocturnal rhythms. Food intake was low and water intake negligible in the light part of a LD 12:12 cycle. The endogenous rhythms persisted in constant dark, and were clearly free running in constant light with periods of 25-26 hr. Animals with relatively asymmetric damage showed strong LL suppression of intake on the first day; animals with symmetric damage showed no LL suppression and the strongest free running rhythms; rats with the largest lesions showed a permanent LL suppression of fluid intake from Day 3 onward. The LL rhythms were rapidly reentrained with a restored LD cycle, with phase shifts of over 90°]24 hr. Recovered laterals which were previously blinded by orbital enucleation showed free running rhythms with periods close to 24 hr. The division of the 24 hr period into active (a) and quiescent (o) phases is much more sharply defined after the lesion. This is especially marked for drinking; feeding and activity rhythms are very similar. It is suggested that the recovered lateral is dependent upon endogenous oscillator(s) subserving motivated behaviors, and implications for the recovery of function and residual impairments are discussed. Circadian rhythms
Drinking
Lateral hypothalamus
Lesions
EXTENSIVE lesions of the lateral hypothalamus (LHA) cause severe behavioral disruptions which include sensory inattention and failure to eat or drink. Many rats bearing these lesions may be maintained by tube feeding, and show a progressive sequence of recovery of the lost behaviors [6, 14, 33]. The recovered lateral describes the rat which has reached a stage of self-maintenance on dry food and water, but it is hardly surprising that these animals still show persistent behavioral disruptions [6]. They show abnormal patterns of ad lib ingestion [ 11,12] and failures to respond to acute homeostatic challenges with rapid regulatory behavioral adaptations [ 6,13 ]. Such findings were originally taken as evidence that ingestive behavior in the recovered lateral was under novel controls, and that the recovery sequence at least partly reflected the time necessary for the institution of these new control mechanisms [6, 13, 32]. However, recent findings have suggested that the failures of these rats to respond to acute challenges may reflect a decreased capacity for function in the damaged brain systems. The behavioral deficits are thus viewed as a general or nonspecific nature rather than a result of damage to ingestion-specific circuits [30]. For example, the recovered lateral which classically does not drink in response to hypertonic NaC1 challenge [6] in fact drinks quite well if the tests are prolonged [29]. The drinking which is observed is nocturnal [23]. These, and other demonstrations that recovered laterals appear to be sensitive to their internal hydrational status [22,24] have led me to reexamine the ad lib ingestive patterns of these animals.
Kissileff's comprehensive analysis revealed that the LHA lesioned rat recovered to eating dry pellets and drinking water was both a nibbler and a prandial drinker [11, 12, 13]. Meals of dry food are prolonged affairs of several hours' duration (nibbling); after every few mouthfuls feeding is interrupted for a small draft of water (prandial drinking). Both of these behaviors are seen in the neurologically intact (NI) surgically desalivate rat [ 11,13 ]. Kissileff also noted that the ingestive behavior of LHA lesioned animals occurred almost exclusively during the dark part of the light-dark (LD) cycle, a finding in agreement with reports in which L and D intakes were measured separately [10, 26, 33]. The slow rates of ingestion of recovered laterals thus implies that almost the entire nighttime must be spent in ingestive behavior. The NI rat is a nocturnal animal with some 7 0 - 9 0 % of ingestive behavior during the night of a 12:12 LD cycle [34]. The night is, however, broken into discrete meals followed .by long periods of no ingestion [ 11 ], more or less well defined activity cycles [21] and presumably some resting periods. The available evidence suggests that the recovered lateral does not show such well defined sequences of motivated behavior. The present study is addressed to the issue of disrupted behavioral rhythms in these rats, and some new facets of the lateral hypothalamic syndrome have been revealed. Standard experimental probes of circadian rhythms were applied, mainly to drinking behavior, and led to the conclusion that LHA lesioned rats have exceptionally strong or well defined circadian rhythms. The nocturnal habits of these rats may be understood as the active period
I am grateful to Alan E. Fisher for provision of facilities and financial support (USPHS-MH 1951) for this research. 257
258
ROWLAND
(a) o f such a r h y t h m which is phase synchronized with the darkness. GENERAL METHOD
Animals and Procedures Male adult albino rats o f the Sprague Dawley strain (Zivic Miller, Allison Pk,PA, or lab raised derivatives thereof) were used. Powdered Purina rat chow was available ad lib f r o m glass jars (6 cm high, 4 cm dia.) placed on the cage floor. Both normal and lesioned animals fed from these jars with negligible spillage. Unless stated, tap water was the drinking fluid available from a graduated cylinder with metal drinking spout. The orifice was at least 0.35 crn dia., since LHA lesioned rats have difficulty drinking from small surfaces. The animals were allowed at least 1 week to adapt to the laboratory 12:12 lighting cycle (lights on 0800-2000 hr local time) provided by overhead fluorescent fixtures. Most experiments were c o n d u c t e d in a r o o m housing NI conspecifics hence giving an u n c o n t r o l l e d nocturnal noise cue. Rats were individually housed either in regular wire mesh rack cages, in cages with a cylindrical Plexiglas wall (30 cm dia.), or in 45 cm dia. cages with one half of the cylindrical wall of Plexiglas. The drinking spouts protruded a p p r o x i m a t e l y 0.5 cm through the transparent wails, or through the front wall o f the mesh cages, at a height of 3 cm f r o m the floor. The daytime light intensity, measured by reflectance, was similar (2 ft-lamberts) at the drinking spout in all of the cages. F o o d and water intakes were recorded every day at 0 8 0 0 and 2000 hr, and b o d y w e i g h t was periodically checked for stability. In a d d i t i o n to these measures which provided, respectively, L and D intakes for rats on a LD cycle continuous drinking records were obtained for m o s t of the animals. Grason-Stadler drinkometers were attached to the drinking spouts (using the cage floor as ground, hence the animal c o m p l e t e d the circuit w h e n drinking) and the outputs recorded on a 20 channel Esterline Angus event recorder (chart speed 7.6 cm/hr). C o n t e m p o r a n e o u s feeding patterns were recorded in some o f the rats. The feeding d e t e c t o r consisted o f the plastic screw top lid to a food jar from which a 150 ° sector was removed. A microswitch was m o u n t e d on the lid such that the lever on the switch was parallel to and near one side o f the 150 ° hole. When screwed o n t o the jar, the feeding rat regularly knocks the lever aside thus depressing the microswitch; this e c o n o m i c device functions e x t r e m e l y reliably. (A second plastic top with sector removed is taped on top of the first, thus covering the switch and wires, making it rat-proof). Most rats are c o m p l e t e l y u n p e r t u r b e d by this device and continue to feed in the same way as from o p e n jars. This device allows the estimation o f free feeding o f dry food w h e n the m o u t h f u l is the o n l y natural q u a n t u m , not true for pellet feeding situations (see [12]). The feeder was attached to the wall of a 45 cm diameter cage, opposite the drinking spout. Thus in order to m o v e from feeder to drinking spout the rat crossed the cage. In so doing, a p h o t o b e a m was cut and detected by a photocell connected to the event recorder. In such a way some measure of l o c o m o t o r activity was obtained. The 3 records, feeding, drinking and activity were obtained on adjacent traces on the chart paper.
Surgical Procedure and Recovery Care Stereotaxic
(Kopf)
placement
of
LHA
lesions was
p e r f o r m e d using ether anaesthesia. Insulated (Krylon R) 000 steel insect pin electrodes were lowered to A: 5 . 5 - 6 . 0 , L: -+2.0, D: 7 . 5 - 8 . 0 m m from dura, with the t o o t h b a r set at the level of the earbars. Anodal current of 1.5 m A was passed for 1 0 - 2 5 sec using a rectal cathode. Only rats which were totally aphagic and adipsic for at least 2 days are included in the present study. Special care, including tube feeding (1:1 diluted Borden's sweetened condensed milk) and palatable foods (wet mash, or more usually baby cereal m i x e d with sweet milk) were administered for as long as necessary in the postoperative period, and the recovery times are shown in Table 1. When the present study was c o m m e n c e d all rats were ingesting dry chow and water in a m o u n t s sufficient to maintain stable bodyweight (typically k n o w n as Stage 4 [6,33] ). A few rats were still in Stage 3, drinking saccharin sweetened water, and these rats never progressed to Stage 4 subsequently. TABLE1 RECOVERY TIMES FOLLOWING LHA LESIONS AND EXPERIMENTAL RECORDS
Rat Sighted ADL l ACL 7 ACL 3 ACON 1 ACON 3 ACL 2 ACL l K 51 K 50 ACL 5 ACON 5 ACL 4
Total Aphagia (days)
Time to Ingest Water (days)
l0 3 2 2 3 8 5 7 7 8 4 7
20 12 2 3 12 13 18 13 23 >215 >250 >250
7 5 2 8 3 10 6 5 3 5 8
15 12 3 >60 10 37 >100 >130 12 >62 >106
Experiments Served in
DD DD LL (B) DD, LL (B) DD, LL (B) DD, LL (B) DD, LL (A) LL (A) LL (C) LL (C), DD LL (C) LL (C)
Blinded ENU 3 5 6 8 11 12 13 14 16 18 19
r pre ---24.00 24.00 24.15 24.24 24.12 24.00 24.23 24.00
r post 24.05 24.09 24.07 -24.02 24.15 24.33 -23.47 24.19 24.18
DD = experiment 1, constant dark; LL = experiment 2, constant light; ( ) refers to the group of the animal. r pre, r post = pre- and postlesion free running periods of drinking in enucleated animals, (hr. rain). Times to ingest water (i.e. to Stage 4) marked > indicate animal was drinking saccharin water at time of death or sacrifice.
Histological Verification When the rats were sacrificed, often m a n y m o n t h s after the present experiments, Formalin perfused and fixed brains were examined. Frozen 40 u sections o f a coronal plane were m o u n t e d and stained for cell (cresyl violet) or
CIRCADIAN RHYTHMS IN RECOVERED LATERALS
259
fiber (Weil) coloration. Lesions were reconstructed using a microprojector and standard atlas plates. Data Presen tan'on
LD-1
Many of the results are given as representative individual records. Original Esterline Angus traces were appropriately cut and mounted such that 24 hr records for an individual were vertically arranged. In the convention used in this paper it should be noted that time runs from right to left of the figures. For the purposes of rhythm analysis visual inspection was adequate for the relatively unsophisticated treatment. The figures were reconstructed according to Quay [20]. If drinking occurred within a 10 min time bin, the corresponding 10 min block was shaded on graph paper, and so on for the whole record. Thus, if a rat drank once every 10 min for 3 hr that whole 3 hr block would appear shaded, and would be identical with that for a rat which drank every minute for 3 hr. The original records of feeding, drinking and activity showed that intervals between drinks of up to 10 min were invariably filled with intermittent feeding and locomotion. Thus the qualitative drinking patterns as reconstructed are probably more accurately termed behavioral arousal records. No attempt was made to analyse the data over shorter intervals of time, nor to make a quantitative mathematical analysis of the rhythms; all of the essential findings are evident from the figures. EXPERIMENT 1 NI rats ingest about 80% of their daily water by night [34], while recovered laterals are more than 95% nocturnal drinkers [10]. This low, often zero daytime water intake might be due to an exaggerated sensitivity of the lesioned rats to light, with consequent light suppression of drinking. A continuously dark (DD) environment would remove this constraint, and drinking might occur throughout the 24 hr. Method and Results
Six rats recovered to Stage 4 (drinking water) after LHA lesions were used in this experiment. One of the rats was adrenalectomized and corticosteroid replaced (monthly long-acting Percorten ®, 25 mg/kg). After 3 × 24 hr baseline measures in LD, the lights were turned off in the middle of the 46th postlesion day. The DD regimen then remained in force for 17 x 24 hr after which time the original LD cycle was restored. During the DD phase a dim red light was used for taking measurements; additional leakage of white light was unavoidable whenever the door was opened (at irregular times, in general). The transition from LD to DD had no effect upon the total 24 hr intake of recovered laterals (Fig. 1), and there was no great change in the drinking patterns (Fig. 2). Drinking remained principally during the former night hours, and only after several days was there any tendency for the rhythms to become less sharply defined (Fig. 2). As a consequence the water intakes between 0800 and 2000 hr, negligible in LD, reached 15% of the 24 hr total by Days 7 and 8 in DD. There was no apparent change between Days 8 and 16 in DD (Fig. 1). Feeding rhythms were slightly less exaggerated in LD, and drifted in DD in much the same degree as drinking (Fig. 1). These changes are all significant (p
