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Effect of desipramine and amphetamine on noradrenergic neurotransmission: electrophysiological studies in the rat brain

Received 27 April 1992. revised MS received 20 June lYY2. accepted 7 July 1992

The prcscnt clcctrophysicJlogical noradrcncrgic cocrulcus

cxpcrimcnts

wcrc undcrtakcn

to invcstigatc the cffcct of dcsipraminc

in the rat central nervous system. The cffcctivcncss

ncurotransmission

and of microiontophoretic

application

of norcpincphrinc

neurons was studied in the dorsal hippocampus.

Dcsipraminc

(NE)

of clcctrical

in suppressing

(0.5 and 5 mg/kg

and d-amphctamfnc stimulation

the firing activity of CA,3 pyramidal

i.v.) and d-amphetamine

(0.25 and 5 mg/kg

dccrcascd the cffcctivcncss of locus cocrulcus sIimulrilicJn and prcJf(JngCd the cffcct of microiontophorctically same pyramidal dcsipraminc

ncumns.

Subsequent

and d-amphctaminc NE.

of locus cocrulcus stimulaticJn. High doses of d-amphetamine pyramidal

neurons

(0.25 mg/kg

the 5 mg/kg

IO mg/kg

antagonist

BMY

cffcctivcncss

antagonist,

whereas

comple:cly

in h-hydroxydopaminc-prctrcatcd

reversed

i.v.)

NE on the

the effects of

and decreased that of microiontopkoretinf dcsipraminc

fS and IQ mg/kg

pyramidal

supprcsscd, as in control

(5 mg/kg

i.v.) on the

i.v.1 decreased the firing activity of

low doses of desipramine

Icsioning of NE projections

on the firing activity of hippocampus

applied

(0.5

mg/kg

with 6-hydroxydopaminc.

neurons was markedly

i.v.) or of the cffcct of

rcduccd,

whcrcas

rats, the firing activity of these neurons.

rats was rcvcrscd by the administration

of the 5-HTJ,

rcccptor

7378. Thcsc data provide cvidcncc that acute administration of dcsipraminc and d-amphctaminc dccrcascs the

of locus cocrulcus

acu~c administration NE and scrotanin

rcspcctively.

cffcct. After

dose of d-amphetamine

This cffcct of d-amphctaminc

stimulation

the cffcct of suhscqucnt administration

by 70 and 9%.

i.v.) wore without

dose of d-amphctaminc

the cumulative

prcvcntcd

an cr,-adrcnoccptor

cffcctivcncss d-amphclaminc

idazoxan

of idazoxan,

of locus cocrulcus

tally applied hippocampus

in addiiion,

i.v. administration

on the cffcctivcncss

nn

of the locus

stimulation

by increasing the activation

;Jf high doses of d-amphctaminc

of terminal

cY,-adrcnoccptor

dccrcascs the firing rate of hippocampus

autorcccptors.

pyramidal

In addition,

neurons by increasing

rclcasc. Norcpincphrinc;

Dcsipraminc;

Amphctaminc;

1. introduction

Dorsal hippocampus;

Locus cocrulcus

uptake biockcr (Giowinski and Axeirod, 1964; Ross and Rcnyi, 1975; Koe, 19761 widely used in clinical practice as an antidcprcssant. It binds with high affin-

fore increase the amount of NE in the synaptic cleft, thereby enhancing the degree of activation of the terminal NE autoreceptor, and possibly that of postsynaptic adrenoceptors as well. Hence, the net effect of dcsipramine and d-amphetamine on NE neurotransmission should be dependent on the degree of activa-

ity to a site located on or near the NE reuptake carrier (Biegon and Rainbow, 1983; Raisman et al., 1982; Rchavi ct al., 1982; Lee and Snyder, 1981). d-Amphct-

tion of these different populations of adrenoceptors. It is well known that both drugs suppress the firing of locus cocruicus NE neurons, which give rise almost

amine has been shown to also block NE reuptakc and to increase the release of NE by a caicium-indepen-

cxclusivciy to the NE innervation of the forebrain (Moore and Bloom, 1979). Despite this suppression of the firing of presynaptic neurons, an increased cxtra-

Dcsipraminc

is a sclcctivc norcpincphrinc

(NE)

rc-

dent mechanism (Carr and Moore, 1969; ZianLc ct al., 1972; Chiueh and Moore, 1974; Raiteri et al., 1974; Heikkiia et al., 1975; Koe. 1976; Arnold et al., 1977; Philips ct al., 1982; L’Hcurcux ct al., 1986). Administration of desipraminc or d-amphetamine should there-

rbrcspondeacc IO: P. Blisr. Ncurohiologkll Psychintly Unit, Dcpartrnenl of Psychialry. McGill University, IO.73 Pine Avenue West. Montrtxl, QIICIWC H3A I Al C’;IIKK~;I. TcI. l.Sl4.3YX 7304. fnX I .s 143)X 4Xfb.

cellular concentration of NE was found in the rat cerebral cortex, by in vivo microdialysis, after the administration

of cithcr of these drugs

(L’Heureux et al.,

1986; Dennis et al., 1987). We have recently reported, using an eiectrophysiological paradigm in the rat, that the suppressant cffcct of endogenous NE, released by electrical stimulation of the locus cocruicus, on the firing activity of dorsal hippocampus CA,

neurons is mcdiatcd by a postsynap-

tic ~?t,-adrenoceptor tCuret and DC Montigny, 1988h). whereas that of exogenous NE applied by microiontaphoresis onto the same neurons is mediated by a tsynaptic a,-adrenoceptor (Curet and De Montigny. 1988a). In addition, it has been demonstrated that the terminal a,-adrenoceptor autoreceptor exerts a potent regulating role on NE transmission in the same experimental paradigm (Curet and De Montigny, 19899). The present studies were undertaken to determine the effects of acute administration of desipramine and d-amphetamine on the effectiveness of NE neurotransmission in the rat hippocampus and the involvement of the terminal ru,-adrenoceptor autoreceptor in these effects. 2, Materials and methods

Male Sprague-Dawley rats (270-300 g) were maintained on a 12: 12 h light-dark cycle with free access to food and water. Two weeks prior to electrophysiological expriments. nine rats received. under chloral hydrare anesthesia (400 mg/kg i.p.1. an intracerebroventricular injection of 6-hydroxydopamine (6-OHDA: 200 pg of free base; Sigma, St. Louis, MO) dissolved in 20 ~1 of 0.9% NaCl and 0.1% of ascorbic acid solution. The serotonin (5-HT) reuptake blocker zimelidine (25 mg/kg i-p.) was administered 1 h prior to the 6-OHDA injection tc protect the S-HT system.

Electrophysiological experiments were performed under chloral hydrate anesthesia (400 mg/kg i.p.1 and supplementary doses were given as needed. The rats were mounted in a stereotaxic apparatus and body temperature was maintained between 36-38°C. Extracellular recordings and microiontophoretic applications were performed with five-barrelled micropipettes preloaded with fiberglass strands and pulled in a conventional manner. The tip was broken back under microscopic control to a diameter of 8-10 pm. The central barrel, used for extracellular unitary recordings was filled with 2 M NaCl solution saturated with Fast Green. One side barrel was filled with 2 M NaCl solution and used for automatic current balancing. The other side barrels, used for microiontophoresis, were filled with the following solutions: acetylcholine hydrochloride (ACh, 20 mM in 0.2 M NaCI. pH 4; Sigma) and NE bitartrate (0.1 M in 0.2 M NaCI, pH 4; Sigma). The impedances were 2-6 MR in the central barrel, 20-40 MR in the balance barrel and 50-100 MR in the other side barrels. ACh and NE were ejected as cations with retaining currents of - 8 to - 6 nA. The microelectrodes were lowered at 4.2 mm anterior to lambda and 4.2 mm lateral to the midline. CA, pyra-

midal neurons of the dorsal hippocampus were recorded extracellularly, at a depth of 3.3-3.9 mm below the cortical surface, over the 200 pm thick stratum pyramidale and identified by the large amplitude (OS-I.2 mV) and long duration (0.8-1.2 ms) of their action potentials and by their characteristic complex spike discharge alternating with simple spike activity (Kandel and Spencer, 1961). These criteria readily distinguish behveen pyramidal neurons and interneurons. Since most hippocampus pyramidal neurons do not discharge spontaneously under chloral hydrate anesthesia, a leak or a small current of ACh t - 1 to 5 nA) was used to maintain a physiological firing rate (S-14 Hz; Rank, 1975). It has been shown previously that the response of CA, pyramidal neurons to microiontophoretic application of NE and to endogenous NE released by electrical stimulation of the NE pathway is identical under ACh and ibotenic acid activation (Curet and de Montigny, 1988b1, thus ruling out an interaction between ACh and NE to account for the results obtained in the present experiments. Dorsal lateral geniculate nucleus neurons were usually recorded 0.5 mm below the CA, pyramidal neurons, that is 3.8-4.4 mm below the cortical surface. These neurons were identified by their brisk response to light. Three aspects of the response of hippocampus pyramidal neurons to the microiontophoretic application of NE were assessed using an IBM computer: (1) the 1 -T,,, value, which is the charge in nC (I nC = 1 nA X 1 s) required to obtain a 50% decrease in firing rate (De Montigny and Aghajanian, ,1977). The I * T,, provides an index of the sensitivity of the postsynaptic neuron, i.e. the more sensitive the neuron, the smaller the 1. T,,, value. (2) The RTs,, value, which is the time required for the firing rate to recover by 50% from the end of microiontophoretic application of NE (De Montigny et al., 1980). The RTs,, value provides an index of the capacity of NE terminals to remove NE from the synaptic cleft by the reuptake carrier, as this value is prolonged by the administration of desipramine or bq the destruction of NE terminals with 6-OHDA (De Montigny et al., 1980; Gravel and De Montigny, 1987). (3) The number of spikes suppressed/nC to evaluate the total suppressant effect of microiontophoretic NE applications. This value is obtained by dividing the total number of spikes suppressed from the beginning of the ejection period up to a recuperation of 90% of the pre-ejection value by the charge used (in nC) to eject NE. The responsiveness of dorsal lateral geniculate neurons to microiontophoretic application of NE was assessed by calculating the number of spikes generated/&, which is obtained by dividing the total number of spikes generated from the beginning of the ejection period up to a recuperation of 150% of the pre-ejection value by the charge used (in nCl to eject NE.

The response of dorsal hippocampus pyramidal .neurons to electrical activation of the ascending NE pathway was studied by implanting a bipolar electrode (NE 100, David Kopf Instruments, Tujunga, CA) with a backward angle of 100 in the locus coeruleus (1.1 mm lateral to the midline, 1.4 mm posterior to lambda, at a depth of 6.5 mm). Pulses of 0.5 ms in duration and of an intensity of 800 EI_Awere delivered with a stimulator (S 88, Grass Instruments, Quincey, MA) through a direct-coupled isolation unit (SIU 478A, Grass Instruments) at a frequency of 1 Hz. In each trial, 150 pulses were delivered, and peristimulus time histograms of hippocampus pyramidal neurons were generated by an IBM computer equipped with a Tecmar interface. The suppressant effect of locus coeruleus stimulation on these neurons was determined by calculating the absolute silence value @IL in ms). The SIL value is defined as the theoretical duration of a total suppression of firing. It represents the number of spikes suppressed by the stimulation as it corresponds to the duration of suppression of firing normalized for the firing activity of the neurons recorded. This latter value was calculated by dividing the number of spikes suppressed by the mean prestimulation frequency of firing of the neuron recorded (Curet and De Montigny 1988b3).To ascertain that the 800 &A intensity used for stimulating the locus coeruleus was supramaximal, the effect of stimulation of the locus coeruleus on hippocampus pyramidal neurons was assessed sequentially with intensities of 200, 400, 600 and 800 PA on the same neurons at a constant frequency of 1 Hz. The effectiveness of locus coeruleus stimulation was increased when the intensity of the stimulation was increased from 200 lo 400 PA but did not increase further at 600 and 800 PA (Curet and de Montigny, 1989). At the end of each experiment, a -29~@A current was passed through the central barrel of the microelectrode for 15 min to deposit Fast Green and a 0.5 mA current was passed through the stimulating electrode for 7 s to produce a lesion for subsequent histological verification of the location of the tips of the recording and stimulating electrodes, respectively. Experiments in which the recording electrode was outside the C-4 3 region and/or in which the stimulating electrode was outside the locus coeruleus were not considered in the results. Dextro-amphetamine sulfate (Smith, Kline and French), desipramine hydrochloride (Merrell DOW Pharmaceuticals) and idazoxan hydrochloride (Reckitt and Colman) were dissolved in saline and administered via a lateral tail vein.

capitation), the following experiments were performed with rat hippocampus slices. After a preincubation period of 3 min at 37°C with Krebs solution alone or in the presence of 1 PM desipramine, hippocampus slices (400 pm in thickness) were incubated at 37°C for 3 min with 0.1 PM [“HINE. The Krebs solution was continuously bubbled with a mixture of 95% O,-5% COZ. Passive diffusion was measured in parallel at 0°C. In each group, three rats were tested and each of these three values were obtained by assessing uptake in two slices in quintuple. At the end of the incubation period, the slices were transferred to Krebs solution at 0°C to stop uptake and were solubihzed. The radioactivity in the slices and in the media was measured by liquid scintillation spectometry. The inhibition of uptake was calculated according to the formula: % inhibition of uptake = [CR, - R,)/(R, - R,,)] x 100, where R, is the ratio tissue/medium for the control slices, R, is the ratio tissue/medium incubated with desipramine, or in slices prepared from desipramine-injetted rats, and R, is the ratio tissue/medium for the control slices at 0°C. 2.3. Statistical analysis In order to determine the effect of systemic administration of d-amphetamine, desipramine and idazoxan on the responsiveness of hippocampus pyramidal neurons to microiontophoretic application of NE, the 1 . T,,, values,the number of spikes suppressed/nC. and RT,,, values were calculated prior to and following the injection of the drug. The responsiveness of dorsal lateral geniculatc nucleus neurons to microiontophoretic application of NE prior to and following desipramine was determined by calculating the number of spikes generated/nC. To determine the effect of systemic administration of d-amphetamine, desipramine and idazoxan on the responsiveness of hippocampus pyramidal neurons to !ocus coeruleus stimulation, SlL values were calculated prior to and following the injection of the drug. All data were generated in pairs and analyzed for statistical significance by using the paired Student’s t-test. The unpaired Student’s t-test was used to compare the effects of drugs in control and 6-OHDAtreated rats.

3. Results 3. I. Effect of systemic adttlinistration of desipranrine on

2.2. /.‘HINE uptake

the responsil~eness of hippocampus pyramidal exogetlous and endogenous NE

neurons to

In order to verify the degree of inhibition of the NE reuptake process by iiv. administered desipramine (0.5 mg/kg iv. under chloral anesthesia 5 min before de-

In order to assess the effect of desipraminc on the response of hippocampus pyramidal neurons to exogcnous and endogenous NE, SIL values and number of

DESlPRANIlNE 0.5 mg/kg. ix. NE

2

mn

IDAZOXAN 0.5 mglkg. i.v.

a

Fig. 1. integrated trrmg rate histogram (A) and peristimulus time histograms (B, C, Dk illustrating the effect of a systemic injection of dtsipramint: and of a subsequent injsction of idazoxan on the response of CA, hippocampuspyramidal neurclnsto NE applied by micnkntc~phwesis(A) and IO electrical stimulation of the locus corruleus. Horizontal hars above the integrz!ed firing rate histogram indicate durrlticmof ej~ctkmsfor which currents are given in nA. Each peristimulus time histogram was constructed from IS0 stimuli of 0.5 ms delivered at I Hz at time I) with an intensity of Tut p A. Bin width is 2 ms.

spikes suppresscd/nC were determined prior to and following drug injection. At a dose of 0.5 mg/kg, i.v., desipramine did not alter the firing rate of CA, neununs. activated with ACh. but decreased by 84% the effectiveness of locus coeruleus stimulation and increased by 40% the effectiveness of NE applied microiontophorctically to the same neurons in six rats (figs. I. 2. 3A). Subsequent administration 0;’ lnc cyzadrenoceptor antagonist idazoxan W5 mg/kg . -I.: Freedman and AghaJanian. 1984) completely rcverscrl the effect of desipramine (0.5 mg/kg iv.1 on IOCUS coeruleus stimulation and. in agreement with a previous report (Curet and De Montigny, lY88a). decreased the effect of exogenous NE by 57% (fig. 2). In addition, in another series of experiments, a high dose of desiprami?le (5 mg/kg iv.1 decreased the effect of stimulation (SII, value prior to desipraminc: 40 ~fr:II ms; following desipramine: 6 f 2 ms; n = 6: P < 0.01) and increased the effectiveness of NE applied microiontophoreticaily pressed/nC desipramine:

on

the

same

neurons

(spikes

sup-

prior to desipramine: 10 f 2: following I9 + 6; n = h; P < Q.OI).

Desipramine ((I.5 snd 5 rng/kg i.v.1 failed tn modify the ACh-induced firing activity of CA, neurons and the I - T5,, values, whereas it incrcascd RTs,, values by 75 and 157% in two separate series of experiments

a

PRIOR TO DRUG INJECTION FOLLOWING DESIPRAMINE

a

FOLLOWING IDAZOXAN

(0.5 mg/kg. I.v.)

(0 5 mglkg. I.v.)

Fig. 2. Effwr. nf desipramk_p and of :I S~J!W~WR~ injection of idazoxan on the effectiveness of electrical stimulation of the locus corruleus (A) and on the effectiveness of NE applied by mjcrrliontophoresis U3f in suppressing the firing activity of the srme CA, dorsal hippocampus pyramidal neurons. The SIL value represents the duration of the suppression of firing relative to the firing activity of the neuron recorded (see text for details). Data were obtained from six r&s (one neuron per rat). * P < (MS; * * P < 0.01, using the paired Student’s t-test.

63 TABLE

I

Effect of desipramine on the initial responsiveness (I.T,,, fS.E.M.) and on the recovery time (RTse + S.E.M.) after microiontophoretic application of NE.

Before Fnllowing desipramine (0.5 mg/kg i.v.) Before Following dcsipramine (5 mg/kg i.v.1

na

~-%I

R%,

tnC)

(s)

87rfr14 86 f 11 70+25 76231

28*4 49+ 7 h 37+7 933_c9’

6 6 6 6

’ Number of neurons tested. ’ P < 0.05, ’ P < 0.01 (paired Student’s t-test).

(table 1). In yet another series of experiments, intravenous administration of three consecutive doses of 2 mg/kg of desipramine failed to decrease significantly the firing rate of hippocampus pyramidal neurons (-- 12 f 3%; n = 61, but increased in a dose-dependent manner the duration of the suppressant effect of micro~ontophoreticaily applied NE (data not shown) as previously reported (Lacroix et a!., 19911. These results indicate that the increase in the number of spikes suppressed/nC following desipramine was entirely attributable to a prolongation of the effect of the microiontophoretically applied NE, the initial responsiveness of the neuron being unchanged. To verify further that the decrementa effect of

A

DESIPRAMINE Q-5 m&kg, i-v.

2 mg/kg,

desipramine on the effectiveness of locus coeruleus stimulation on the firing activity of dorsal hippocampus pyramidal neurons was mediated by an increased activation of the terminal a,-autoreceptor, desipramine was administered fobowing systemic administration of the a,-adrenoceptor antagonist idazoxan in another series of experiments. As has been reported (De Montigny and Curet, 1988a, bl, idazoxan (1 mg/kg i.v.1 by itself increased the effectiveness of locus coeruleus stimulation by 60% and decreased that of microiontophoreticafty applied NE by 73% (fig. 41. Under these conditions, subsequent administration of desipramine (0.5 mg/kg i.v.1 failed to decrease the effectiveness of stimulation. Supplementary doses of desipramine up to a cumulative dose of 1.5 mg/kg i.v. did not modify the effectiveness of st~ulation (data not shown). 3.3 Inhibition of neuronal uptake of I”H]lVE by def@liHiiiiiie

Consistent with previous results obtained with the same meth~olo~ (Galzin et af., 19841, 1 FM desipramine produced a 66 + 3% inhibition of f3H]NE uptake in rat hippocampus slices (n = 3 rats) when compared to control slices in Krebs alone. An equivalent inhibition of [“H]NE uptake (55 & 5%; n = 3 rats1 was obtained when 0.5 mg/kg of dcsipramine was injected i.v. 5 min prior to decapitation.

ix

NE I

lli

g

m

AMPHETAMINE 0.25 mglkg, i.v.

B

5 mgfkg, i.v.

NE X

m

m

PI

I

I

Fig. 3. Integrated firing rate histograms illustrating the effects of successivesystemic injections of desipramine (A) and of amphetamine (B) en the res~nsiveness of CA, hippocampus pyramidal neurons to mic~iontophor~ti~lly applied NE. Time hasc applied to both traces.

0

PRIOR TO DRUG INJECTION

m

FOLLOWNG

IDAZOXAN

a

FOk.LOWlNG

DESIPRAMINE

11 mg

kg. IV,

t0 5 mg kg IV I

Fig. 1. Effect of idazoxan and of subsequent injection of desipramine on SIL values of CA j hippocampus pyramidal neurons ehtained by stimulatwn of the locus coeruleus and on the responsivenessof thr same neurons to the microiontophoretic application of NE. Data vvcreobtained born sir rats lonr neuron per rat). * P < 0.05. * * P < 0.01, using the paired Student‘s t-test.

33 Effect of systemically admiruktered desipramine OH the responsiveness of dorsal lateral geniculate nrlc1erc.s neurons to exogenous NE

Since desipramine displays some affinity for a,adrenoceptor binding sites (UPrichard et al..1978), the possibility that it might suppress the firing of pyramidal neurons induced by locus coeruleus stimulation by bIocking a,-adrenoceptors had to be ruled out. To this end, the effect of a small dose of desipramine was studied on the excitation of dorsal lateral geniculate nucleus neurons induced by NE, which is mediated by a,-adrenoceptors (Rogawski and Ag,hajanian, 1980). Desipramine (0.5 mg/kg i.v.) did not decrease, but rather increased, the effectiveness of microiontophoretica!iy applied NE (fig. 5) (number of spikes generated/nC prior to desipramine: 1.3 +-0.2; following desipramine: 1.9 + 0.5; n = 6; P < 0.05). 3.4 Effect of systemically administered the responsir*eness of hippocampus exogenous and endogenorrs N.5

amphetamine on pyramidal neurons to

In order to assess the effect of acute administration of d-amphetamine on the effectiveness of exogenous and endogenous NE, the number of spikes suppressed/nC and the SIL values were calculated prior to and following systemic injection of the drug. !n a first series of experiments, d-amphetamine (0.25 mg/kg i.v.) decreased the effectiveness of locus coeruleus stimulation by 82% and increased that of microion-

tophoretically applied NE by 52% (figs. 38, 6). A subsequent injection of idazoxan (0.5 mg/kg i.v.) completely restored the suppressant effect of locus coeruleus stimulation (fig. 8) and decreased the effectiveness of microiontophoretically applied NE by 75%. In a second series of experiments, two successive doses (0.25 and 5 mg/kg i.v.1 were administered. In this series, the initial 0.25 mg/kg dose produced effects comparable to those obscrvcd in the first series (fig. 7). The subsequent 5 mg/kg dose further reduced the effectiveness of locus cocruleus stimulation and further increased the efficacy of microiontophoretically applied NE. However, as illustrated in fig. 8, the effect of the 5 mg/kg dose of d-amphetamine on locus coeruleus stimulation was not completely reversed by a cumulative dose of 2.5 mg/kg of idazoxan, whereas the effect of the 0.25 mg/kg dose of d-amphetamine was reversed by a 0.5 mg/kg dose of idazoxan. Amphetamine did not change the initial responsiveness of hippocampus pyramidal neurons to microiontophoretically applied NE, as indicated by the unchanged I * Ts,, values, but prolonged its effect as reflected by increased the RTs,, values (table 2). These data demonstrate that the increase in the number of spikes suppressed/nC following d-amphetamine administration was entirely attributable to an inceease in the period of recuperation foiiowing microiontophoretic application of NE, most likely resulting from NE reuptake blockade, without altering the responsiveness of the postsynaptic neurons. A low dose of d-amphetamine fO:5 mg/kg iv,? did not significantly alter the firing rate of hippocampus pyramidal neurons f - 17 + 6%; n = 61, whereas two subsequent i.v. doses of 5 mg/kg of d-amphetamine induced a decrease of 70 and 98%, respectively (figs. 9, 10). The effect of a 5 mg/kg dose of d-amphetamine could not be reversed by the subsequent administration of either the a,-adrenoceptor antagonist prazosin (0.25 mg/kg i.v.; n = 5) (fig. 9) or idazoxan (1 mg/kg i.v.; n = 2) (data not shown).

NE 6 701

DES&AMINE 0.5 mg/kg, i.v.

2 inin

Fig. 5. Integrated firing rate histogram illustrating the effect of desipramine on the excitation induced hy NE applied microiontophoretically to a dorsolateral geniculate neuron.

b5

A

d-AMPHETAMINE 0.25 mgfkg, i-v. NE 3 m

ID

II

200-t

sa .,I

B

-20

0

20

60

100

140

40

ir ;o

Qo

TIME (mst

’ 100

140

TIME Ims)

Fig. 6. Integrated

firing rate histogram (A) and peristimulus time histograms (B. C) illustrating the effect of a systemic injection of hippocampus pyramidal neurons to NE applied by microiontophoresis and to locus coeruleus stimulation. Each peristimulus was constructed from 1.50stimuli of 0.5 ms delivered at 1 Hz at time 0 with an intensity of X00 &A. Bin width is 2 ms. d-amphetamine

on the response of CA,

In order to determine the involvement of catechciiaminergic systems in the decreasing effect of a high dose of d-amphetamine on the firing activity of

0

t 0

PRIOR TO DRUG INJECTION FOLLOWING

AMPHETAMINE

m

FOLLOWING

AMPHETAMINE

m

FOLLOWING

IDAZOXAN

(25

(0.25 mg/kg, I.v.)

I

0.5

t

,

6

t

1.D

1.5

2.0

2.5

CUMULATIVE DOSE OF IDAZOXAN (mg/kg, I.v.)

(5 mg/kg. ix)

-7

mg/kg. i.v.)

Fig. 7. Effects of two successive doses of amphetamine and of subsequent administration of idazoxan on the effectiveness of stimulation of the locus coeruleus (A) and on microiontophoretic application of NE (B) in suppressing the firing activity of the same hippocampus CA, pyramidal neurons. Data were obtained from six rats (one neuron per rat). * P < 0.o.C.** P < 0.01, using paired Student’s t-test.

-AMPHETAMINE o-----o AMPHETAMINE

(0.25 mg/kg. is.) (5 mgikg. I.v.)

Fig. 8. Differential efficacy of the cu2-adrenoceptorantagouist idazoxan in restoring the suppression of firing activity of CA, dorsal hippocampus pyramidal neurons produced by stimulation of the locus coertleus of rats treated with a low or a high dose of amphetamine. Data were obtained from six rats in each group (one neuron per rat). * P < 0.05, ** P