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Frequency specific effects of dopamine in the nucleus accumbens
Neuroscience Letters, 22 (1981) 295-301
295
© Elsevier/North-Holland Scientific Publishers Ltd.
FREQUENCY SPECIFIC EFFECTS OF DOPAMINE IN THE NUCLEUS ACCUMBENS
J.F. DeFRANCE, R.W. SIKES, G.C. PALMER and R.B. CHRONISTER
Department of Neurobiology and Anatomy, University of Texas Medical School, Houston, TX 77025 (U.S.A.) and
Departments of Anatomy and Pharmacology, University of South Alabama, Mobile, AL 36688 (U.S.A .) (Reiceived July 7th, 1980; Revised version received December 3rd, 1980; Accepted December 8th, 1980)
The action of dopamine was evaluated in the nucleus accumbens of acutely prepared rabbits. It was found that the effect of iontophoretically applied dopamine depended upon the frequency of stimulation of an afferent pathway; in this case the ipsilateral fimbria. Dopamine had a marked suppressive effect on field responses evoked by fimbria stimulation at 0.5 Hz, but not those responses evoked at 6.0 Hz. Dopamine was also effective in activating adenylate cyclase. Both the physiological and the biochemical effects of dopamine could be blocked by appropriate antagonists, suggesting that the phenomena observed were receptor mediated. It is suggested that dopamine serves to enhance information arriving from the hippocampal formation within the theta range by the suppression of competing non-theta activity.
Significant controversy has arisen in the past several years concerning the function of dopamine (DA) in the central nervous system. Some investigators [2, 15] have found DA to be excitatory, while others [7, 16, 19, 21] have reported it to be predominantly inhibitory on cell firing. To compromise this apparent contradiction, it has been suggested that DA functions in either way, depending upon the nature of the specific DA receptor activated [8]. An extension of this notion is that one mode of action would be expressed through activation of adenylate cyclase mechanisms while the other mode of action would be through a mechanism uncoupled to adenylate cyclase [4, 14]. The purpose of the present report is to show some attributes of DA function in the nucleus accumbens that add additional complexities to any model of DA function. Electrophysiological observations were taken from 18 adult, male, New Zealand rabbits, anesthetized with urethane (1.0-1.6 g/kg). After an extensive craniotomy, the cortex and corpus callosum were removed by suction, thereby exposing the septal nuclei and hippocampus. Micropipettes were used for both constant current stimulation and recording. Stimulation micropipettes (1-5 Mfl) were placed in the
296
lateral margin of the ipsilateral fimbria (IFim) to activate afferents from the ventral subiculum [12, 18]. Multibarrel micropipettes were used for recording field potentials in the nucleus accumbens following IFim stimulation, and for the iontophoresis of DA (10-50 mM, p H 6.5). The multibarrel array always included a recording and a current summing barrel containing 2 M NaC1 and Fast Green [20] or Pontamine sky blue [12] for electrode tip localization. To test for the specificity of the DA effect, haloperidol (Haldol, 0.15-0.5 m g / k g ) was injected intravenously. In order to study the effects of DA and haloperidol on field responses elicited at different frequencies of stimulation, the presentation of stimuli was precisely tailored. Each experiment of the series consisted of a number of trials. Each trial consisted of 8 stimuli at 0.5 Hz, followed by 8 tetanic stimuli at 6.0 Hz. The initial 8 stimuli at 0.5 Hz established the baseline response value. The intertrial interval was 30 sec. The physiological data were analyzed as a 2 x 2 factorial design. The two factors in the experiment, each evaluated at two predetermined levels, were frequency of stimulation (0.5 and 6.0 Hz) and DA administration (prior to and during). The values of each combination of factor and level were the mean of the m a x i m u m negative component f r o m four consecutive trials. The experiment was replicated in block fashion, and the variation due to replication, which was significant at the c~ = 0.01 level, was mathematically removed. To determine the potential of DA to stimulate adenylate cyclase activity, the nucleus accumbens was removed and homogenized in cold glycylglycine buffer (2 mM + 1 mM MgSO4 + 0.1 m M E G T A , p H 7.4). The sulcus limitans [13] was used to identify the proper anterior-posterior plane and the dorsal extent of the nucleus. A line drawn vertically from the anterior horn of the lateral ventricle downward was taken as the lateral border of the nucleus. The nucleus of the diagonal band of Broca and the olfactory tubercle were taken as the medial and ventral boundaries, respectively. The assay was run exactly as described previously [16], except that 0.2 mM each of papaverine and isobutylmethylxanthine were used as phosphodiesterase inhibitors. All experimental conditions were assayed in duplicate and adenylate cyclase activity was expressed as/~mol of cyclic A M P f o r m e d / m i n / m g protein. The antagonist fluphenazine was used to test the specificity of the DA effect. A single stimulus presented to the lateral margins of the IFim evoked a field response in the caudal half of the nucleus accumbens which appears as a composite negativity (i.e. population EPSP and population spike) followed by a bicucullinesensitive positivity [9-11]. When using submaximal stimulation to evoke the field response, the N component can be taken to closely approximate the number of contributing cellular firings [9]. This is similar to the hippocampal population spike analysis of Anderson et al. [1]. The factor analysis o f 18 experimental series showed that the drug (c~ < 0.01) and frequency (c~ < 0.01) factors were significant. There was a significant interaction
297
between these factors (~ <0.01), indicating a differential effect of DA across frequencies• To better illustrate the DA effect, the results from a single experimental series is shown in Fig. 1.
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TRIAL Fig. 1. Effects o f dopamine (DA) on field responses elicited by two frequencies of ipsilateral fimbria stimulation. A: records, each an average o f 8, for trials 4, 8 and 14 at 0.5 Hz and 6.0 Hz stimulation. N indicates the negative component and P the positive component. D A was ejected between Trials 5 and 10. B: plot of peak amplitudes o f the N and P components. Average responses at 0.5 Hz are shown as continuous lines in both the upper (N) and lower (P) plots. Averaged responses at 6.0 Hz are shown as discontinuous lines.
Fig. 1 shows the typical effects of DA in the nucleus accumbens on field responses elicited at a low frequency of stimulation (0.5 Hz) vesus a higher rate of stimulation (6.0 Hz). The rate of 6.0 Hz was chosen because it is in the mid-theta range. Fig. 1A shows records of averaged ( × 8) responses at 0.5 Hz and at 6.0 Hz for selected trials. An average of 8 responses was sufficient to produce records with a good signal-to-noise ratio, but still allowed the individual trials to be brief enough to get good resolution on the time-to-effect and time-to-recovery. Trial 5 is the final preejection control. Trial 8 is the fourth of the 6 trials taken during ejection, and Trial 14 is the final post-ejection control. The plot of the peak amplitudes for the negative (N) and positive (P) components, at both frequencies, is shown in Fig. lB. The average of the N component of the control series at 0.5 Hz was taken as the baseline value, against which all other values are normalized• In the control series (Trials
298
1-4), the responses to 0.5 Hz were stable; that is, variation was < 5% about the average of the pretetanic responses of the first 4 trials. With the delivery of DA ( + 40 nA) in Trials 5-10, the N amplitude decreased to 8 0 - 8 5 % of control values. Inspection of individual records showed that the response change was not immediate, but changed only after several seconds of ejection, even though the first averaged field response showed the decrease. With the termination of the ejection current, the N component regained its amplitude and showed complete recovery in Trial 13 (90 sec after the termination of the ejection). In contrast to the suppressive effect at 0.5 Hz, DA has essentially no effect on the N component when elicited at a stimulus frequency of 6.0 Hz. In this example, there is negligible frequency potentiation at 6.0 Hz in the pre-ejection controls. In Fig. IA, compare the responses in Trials 4, 8 and 14 (6.0 Hz). In contrast to those responses at 0.5 Hz, responses at 6.0 Hz show no effect of the DA delivery. It is important to keep in mind that each trial consisted of both 0.5 Hz and 6.0 Hz stimulation, so the results are not influenced greatly by local changes in extracellular DA concentration. Therefore, DA has differential consequences on the N component depending upon the frequency of incoming excitation. The effect on the P component, which represents synaptic currents for IPSPs [10] and which is bicuculline sensitive [9], shows no such differential effect. The P component shows a DA-related decrease at both 0.5 Hz and 6.0 Hz, suggesting that
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Fig. 2. Effects o f haloperidol administration (0.4 m g / k g ) on field responses elicited by ipsilateral fimbria stimulation at 0.5 Hz and 6.0 Hz. A: records, an average of 8, at 0.5 Hz and 6.0 Hz prior to haloperidol administration. B: averaged records at 0.5 Hz and 6.0 Hz following haloperidol administration. C: plot of peak amplitudes of the negative component. Trial 5 was then 15 min after injection; after which the intertrial interval was 2 min.
299
suppression of the N component at 0.5 Hz is not linked to a modulation of the influence of the P component. To test for the specificity of the DA effect, the DA antagonist haloperidol (HAL) was injected intravenously. Haloperidol Was, indeed, effective in preventing the DA effect. However, HAL had an effect in the absence of exogeneous DA administration. A factor analysis of 12 experimental series showed a significant (a < 0.05) drug effect, although there was no significnt frequency effect of the drug. The data shown in Fig. 2 illustrate this point. Fig. 2A shows averaged records from responses at 0.5 Hz and 6.0 Hz prior to HAL administration (0.4 mg/kg) (Trial 4). Fig. 2B shows a similar tandem of responses after HAL (Trial 6). The plot of the peak amplitudes is in Fig. 2C. Again, the responses prior to HAL (Trials 1-4) at 0.5 Hz serve to establish the baseline values. Whereas DA had its suppressive effect preferentially on responses at 0.5 Hz, HAL had the opposite effect. After HAL, the N components at 0.5 Hz became slightly enhanced, to approximately 112°70 of control values, implying that there is a tonic inhibitory DA influence even under these experimental conditions. Notice, also, that HAL has no effect on the P component, suggesting that the HAL-induced response augmentation is independent of bicuculline-sensitive inhibitory mechanisms. Concentrations of 0. l-100 #M DA were tested for their ability to elicit adenylate
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Fig. 3. D o s e - r e s p o n s e curve for dopamine stimulation of nucleus accumbens adenylate cyclase. Values are the m e a n _+ S.E.M. percent enzyme activation over basal activity (307 +_ 13.4 pmol of cyclic A M P / m i n / m g protein). D o s e - r e s p o n s e curve for fluphenazine inhibition of dopamine (100 /~M)activated adenylate cyclase. Values are the mean _+ S.E.M. percent inhibition of respective dopamine actions, n = 4 to 7 separate experiments per data point.
300 cyclase s t i m u l a t i o n in h o m o g e n a t e s (Fig. 3). M a x i m a l e n z y m e a c t i v a t i o n (47%) was o b s e r v e d at 100 #M D A c o n c e n t r a t i o n a n d the EDs0 v a l u e was 4.5 #M. F l u p h e n a z i n e effectively b l o c k e d the D A (100 #M) a c t i v a t i o n o f a d e n y l a t e cyclase. T h e m a x i m a l i n h i b i t i o n o c c u r r e d at 10 t~M, the C50 value ( a m o u n t t h a t i n h i b i t e d e n z y m e activity by 50%) was 0.4 ~M a n d the Ki (enzyme i n h i b i t o r y d i s s o c i a t i o n c o n s t a n t ) was 0.02 /~M. T h e results o f this s t u d y indicate t h a t D A ' s suppressive effect is specific for lower rates o f i n c o m i n g signals a n d that D A is c a p a b l e o f s t i m u l a t i n g a d e n y l a t e cyclase activity. This latter f i n d i n g generalizes the w o r k o f C l e m e n t - C o r m i e r et al. [6] to o u r e x p e r i m e n t a l m o d e l , the r a b b i t . T h e a c t i v a t i o n o f a d e n y l a t e cyclase m e c h a n i s m s s h o u l d be expected to increase cyclic A M P levels. Cyclic A M P has been s h o w n to be suppressive o f n e u r o n a l firing in the nucleus a c c u m b e n s [3]. It is possible, t h e r e f o r e , that the s u p p r e s s i o n o f responses elicited by low rates o f s t i m u l a t i o n is m e d i a t e d b y cyclic A M P . S u p p o r t e d by N I M H 1R03 MH32418-01 the Scottish Rite S c h i z o p h r e n i a F o u n d a t i o n , a n d the E p i l e p s y F o u n d a t i o n o f A m e r i c a . T h e fine technical assistance o f J o a n e D a l t o n a n d S.J. P a l m e r is cheerfully a c k n o w l e d g e d . T h e a u t h o r s also wish to t h a n k Dr. S.T. Kitai for his critical review o f the m a n u s c r i p t . F l u p h e n a z i n e was a gift f r o m the S q u i b b Institute for M e d i c a l Research.
1 Anderson, P., Bliss, T.V.P. and Skrede, K.K., Unit analysis of hippocampal population spikes, Exp. Brain Res., 13 (1971) 208-221. 2 Assaf, S.Y. and Miller, J.E., Excitatory action of the mesolimbic dopamine system on septal neurons, Brain Res., 129 (1977) 353-360. 3 Bunney, B.S. and Aghajanian, G.K., Electrophysiological effects of amphetamine in dopaminergic neurons. In E. Usdin and S.H. Snyder (Eds.), Frontiers in Catecholamine Research, Pergamon Press, New York, 1973, pp. 957-962. 4 Chronister, R.B. and DeFrance, J.F., Functional organization of monoamines. In G. Palmer (Ed.), Psychopharmacology of Central Nervous System Disorders, in press. 5 Chronister, R.B., Palmer, G.C., DeFrance, J.F., Hubbard, J.I. and Sikes, R.W., Histamine: correlative studies in nucleus accumbens, submitted. 6 Clement-Cormier, Y.C., Kebabian, J.W., Petzold, G.L. and Greengard, P., Dopamine-sensitive adenylate cyclase in mammalian brain: a possible site of action of antipsychotic drugs, Proc. nat. Acad. Sci (Wash.), 71 (1974) 1113 1117. 7 Connor, J.D., Caudate nucleus neurons; correlation of the effects of substantia nigra stimulation with iontophoretic dopamine, J. Physiol. (Lond.), 208 (1970) 691-703. 8 Cools, A.R., Two functionally and pharmacologicallydistinct dopamine receptors in the rat brain. In E. Costa and G.L. Gessa (Eds.), Advances in Psychopharmacology, Vol. 16, Raven, New York, 1977, pp. 215-225. 9 DeFrance, J.F., Component analysis of field responses in nucleus accumbens, in preparation. 10 DeFrance, J.F. and Yoshihara, H., Fimbria input to the nucleus accumbens septi, Brain Res., 90 (1975) 158 163. 11 DeFrance, J.F., Marchand, J.E., Hubbard, J.l. and Sikes, R.W., Correlative physiological and morphological analysis of the fimbria projection to the nucleus accumbens, in preparation.
301 12 Hellon, R.F., The marking of electrode tip position in nervous tissue, J. Physiol. (Lond.), 214 (1971) 12P. 13 Johnston, J.B., The morphology of the septum, hippocampus, and pallial commissures in reptiles and mammals, J. comp. Neurol., 23 (1973) 371-478. 14 Kababian, J.W. and Calne, D.B., Multiple receptors for dopamine, Nature (Lond.), 277 (1979) 93 -96. 15 Kitai, S.T., Sugimori, M. and Kocis, J., Excitatory nature of dopamine in the nigro-caudate pathway, Exp. Brain Res., 24 (1976) 351-363. 16 McLennan, H. and York, D.H., The action of dopamine on neurons of the caudate nucleus, J. Physiol. (Lond.), 189 (1967) 393-402. 17 Palmer, G.C., Interactions of antiepileptic drugs on adenylate cyclase and phosphodiesterases in rat and mouse cerebrum, Exp. Neurol., 63 (1979) 322-335. 18 Siegel, A. and Tassoni, J.P., Differential efferent projections from the ventral and dorsal hippocampus of the cat, Brain Behav. Evol., 4 (1971) 185-200. 19 Siggins, G.R., Hoffer, B.J. and Ungerstedt, U., Electrophysiological evidence for involvement of cyclic adenosine monophosphate in dopamine responses of caudate neurons, Life Sci., 15 (1974) 779-792. 20 Thomas, R.C. and Wilson, V.J., Precise localization of Renshaw cells with a new marking technique, Nature (Lond.), 206 (1965) 211-213. 21 York, D.H., The inhibitory actions of dopamine on neurons of the caudate nucleus, Brain Res., 5 0967) 263-266.
295
© Elsevier/North-Holland Scientific Publishers Ltd.
FREQUENCY SPECIFIC EFFECTS OF DOPAMINE IN THE NUCLEUS ACCUMBENS
J.F. DeFRANCE, R.W. SIKES, G.C. PALMER and R.B. CHRONISTER
Department of Neurobiology and Anatomy, University of Texas Medical School, Houston, TX 77025 (U.S.A.) and
Departments of Anatomy and Pharmacology, University of South Alabama, Mobile, AL 36688 (U.S.A .) (Reiceived July 7th, 1980; Revised version received December 3rd, 1980; Accepted December 8th, 1980)
The action of dopamine was evaluated in the nucleus accumbens of acutely prepared rabbits. It was found that the effect of iontophoretically applied dopamine depended upon the frequency of stimulation of an afferent pathway; in this case the ipsilateral fimbria. Dopamine had a marked suppressive effect on field responses evoked by fimbria stimulation at 0.5 Hz, but not those responses evoked at 6.0 Hz. Dopamine was also effective in activating adenylate cyclase. Both the physiological and the biochemical effects of dopamine could be blocked by appropriate antagonists, suggesting that the phenomena observed were receptor mediated. It is suggested that dopamine serves to enhance information arriving from the hippocampal formation within the theta range by the suppression of competing non-theta activity.
Significant controversy has arisen in the past several years concerning the function of dopamine (DA) in the central nervous system. Some investigators [2, 15] have found DA to be excitatory, while others [7, 16, 19, 21] have reported it to be predominantly inhibitory on cell firing. To compromise this apparent contradiction, it has been suggested that DA functions in either way, depending upon the nature of the specific DA receptor activated [8]. An extension of this notion is that one mode of action would be expressed through activation of adenylate cyclase mechanisms while the other mode of action would be through a mechanism uncoupled to adenylate cyclase [4, 14]. The purpose of the present report is to show some attributes of DA function in the nucleus accumbens that add additional complexities to any model of DA function. Electrophysiological observations were taken from 18 adult, male, New Zealand rabbits, anesthetized with urethane (1.0-1.6 g/kg). After an extensive craniotomy, the cortex and corpus callosum were removed by suction, thereby exposing the septal nuclei and hippocampus. Micropipettes were used for both constant current stimulation and recording. Stimulation micropipettes (1-5 Mfl) were placed in the
296
lateral margin of the ipsilateral fimbria (IFim) to activate afferents from the ventral subiculum [12, 18]. Multibarrel micropipettes were used for recording field potentials in the nucleus accumbens following IFim stimulation, and for the iontophoresis of DA (10-50 mM, p H 6.5). The multibarrel array always included a recording and a current summing barrel containing 2 M NaC1 and Fast Green [20] or Pontamine sky blue [12] for electrode tip localization. To test for the specificity of the DA effect, haloperidol (Haldol, 0.15-0.5 m g / k g ) was injected intravenously. In order to study the effects of DA and haloperidol on field responses elicited at different frequencies of stimulation, the presentation of stimuli was precisely tailored. Each experiment of the series consisted of a number of trials. Each trial consisted of 8 stimuli at 0.5 Hz, followed by 8 tetanic stimuli at 6.0 Hz. The initial 8 stimuli at 0.5 Hz established the baseline response value. The intertrial interval was 30 sec. The physiological data were analyzed as a 2 x 2 factorial design. The two factors in the experiment, each evaluated at two predetermined levels, were frequency of stimulation (0.5 and 6.0 Hz) and DA administration (prior to and during). The values of each combination of factor and level were the mean of the m a x i m u m negative component f r o m four consecutive trials. The experiment was replicated in block fashion, and the variation due to replication, which was significant at the c~ = 0.01 level, was mathematically removed. To determine the potential of DA to stimulate adenylate cyclase activity, the nucleus accumbens was removed and homogenized in cold glycylglycine buffer (2 mM + 1 mM MgSO4 + 0.1 m M E G T A , p H 7.4). The sulcus limitans [13] was used to identify the proper anterior-posterior plane and the dorsal extent of the nucleus. A line drawn vertically from the anterior horn of the lateral ventricle downward was taken as the lateral border of the nucleus. The nucleus of the diagonal band of Broca and the olfactory tubercle were taken as the medial and ventral boundaries, respectively. The assay was run exactly as described previously [16], except that 0.2 mM each of papaverine and isobutylmethylxanthine were used as phosphodiesterase inhibitors. All experimental conditions were assayed in duplicate and adenylate cyclase activity was expressed as/~mol of cyclic A M P f o r m e d / m i n / m g protein. The antagonist fluphenazine was used to test the specificity of the DA effect. A single stimulus presented to the lateral margins of the IFim evoked a field response in the caudal half of the nucleus accumbens which appears as a composite negativity (i.e. population EPSP and population spike) followed by a bicucullinesensitive positivity [9-11]. When using submaximal stimulation to evoke the field response, the N component can be taken to closely approximate the number of contributing cellular firings [9]. This is similar to the hippocampal population spike analysis of Anderson et al. [1]. The factor analysis o f 18 experimental series showed that the drug (c~ < 0.01) and frequency (c~ < 0.01) factors were significant. There was a significant interaction
297
between these factors (~ <0.01), indicating a differential effect of DA across frequencies• To better illustrate the DA effect, the results from a single experimental series is shown in Fig. 1.
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TRIAL Fig. 1. Effects o f dopamine (DA) on field responses elicited by two frequencies of ipsilateral fimbria stimulation. A: records, each an average o f 8, for trials 4, 8 and 14 at 0.5 Hz and 6.0 Hz stimulation. N indicates the negative component and P the positive component. D A was ejected between Trials 5 and 10. B: plot of peak amplitudes o f the N and P components. Average responses at 0.5 Hz are shown as continuous lines in both the upper (N) and lower (P) plots. Averaged responses at 6.0 Hz are shown as discontinuous lines.
Fig. 1 shows the typical effects of DA in the nucleus accumbens on field responses elicited at a low frequency of stimulation (0.5 Hz) vesus a higher rate of stimulation (6.0 Hz). The rate of 6.0 Hz was chosen because it is in the mid-theta range. Fig. 1A shows records of averaged ( × 8) responses at 0.5 Hz and at 6.0 Hz for selected trials. An average of 8 responses was sufficient to produce records with a good signal-to-noise ratio, but still allowed the individual trials to be brief enough to get good resolution on the time-to-effect and time-to-recovery. Trial 5 is the final preejection control. Trial 8 is the fourth of the 6 trials taken during ejection, and Trial 14 is the final post-ejection control. The plot of the peak amplitudes for the negative (N) and positive (P) components, at both frequencies, is shown in Fig. lB. The average of the N component of the control series at 0.5 Hz was taken as the baseline value, against which all other values are normalized• In the control series (Trials
298
1-4), the responses to 0.5 Hz were stable; that is, variation was < 5% about the average of the pretetanic responses of the first 4 trials. With the delivery of DA ( + 40 nA) in Trials 5-10, the N amplitude decreased to 8 0 - 8 5 % of control values. Inspection of individual records showed that the response change was not immediate, but changed only after several seconds of ejection, even though the first averaged field response showed the decrease. With the termination of the ejection current, the N component regained its amplitude and showed complete recovery in Trial 13 (90 sec after the termination of the ejection). In contrast to the suppressive effect at 0.5 Hz, DA has essentially no effect on the N component when elicited at a stimulus frequency of 6.0 Hz. In this example, there is negligible frequency potentiation at 6.0 Hz in the pre-ejection controls. In Fig. IA, compare the responses in Trials 4, 8 and 14 (6.0 Hz). In contrast to those responses at 0.5 Hz, responses at 6.0 Hz show no effect of the DA delivery. It is important to keep in mind that each trial consisted of both 0.5 Hz and 6.0 Hz stimulation, so the results are not influenced greatly by local changes in extracellular DA concentration. Therefore, DA has differential consequences on the N component depending upon the frequency of incoming excitation. The effect on the P component, which represents synaptic currents for IPSPs [10] and which is bicuculline sensitive [9], shows no such differential effect. The P component shows a DA-related decrease at both 0.5 Hz and 6.0 Hz, suggesting that
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Fig. 2. Effects o f haloperidol administration (0.4 m g / k g ) on field responses elicited by ipsilateral fimbria stimulation at 0.5 Hz and 6.0 Hz. A: records, an average of 8, at 0.5 Hz and 6.0 Hz prior to haloperidol administration. B: averaged records at 0.5 Hz and 6.0 Hz following haloperidol administration. C: plot of peak amplitudes of the negative component. Trial 5 was then 15 min after injection; after which the intertrial interval was 2 min.
299
suppression of the N component at 0.5 Hz is not linked to a modulation of the influence of the P component. To test for the specificity of the DA effect, the DA antagonist haloperidol (HAL) was injected intravenously. Haloperidol Was, indeed, effective in preventing the DA effect. However, HAL had an effect in the absence of exogeneous DA administration. A factor analysis of 12 experimental series showed a significant (a < 0.05) drug effect, although there was no significnt frequency effect of the drug. The data shown in Fig. 2 illustrate this point. Fig. 2A shows averaged records from responses at 0.5 Hz and 6.0 Hz prior to HAL administration (0.4 mg/kg) (Trial 4). Fig. 2B shows a similar tandem of responses after HAL (Trial 6). The plot of the peak amplitudes is in Fig. 2C. Again, the responses prior to HAL (Trials 1-4) at 0.5 Hz serve to establish the baseline values. Whereas DA had its suppressive effect preferentially on responses at 0.5 Hz, HAL had the opposite effect. After HAL, the N components at 0.5 Hz became slightly enhanced, to approximately 112°70 of control values, implying that there is a tonic inhibitory DA influence even under these experimental conditions. Notice, also, that HAL has no effect on the P component, suggesting that the HAL-induced response augmentation is independent of bicuculline-sensitive inhibitory mechanisms. Concentrations of 0. l-100 #M DA were tested for their ability to elicit adenylate
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CONCENTRATION (M. in - I / 2 log doses)
Fig. 3. D o s e - r e s p o n s e curve for dopamine stimulation of nucleus accumbens adenylate cyclase. Values are the m e a n _+ S.E.M. percent enzyme activation over basal activity (307 +_ 13.4 pmol of cyclic A M P / m i n / m g protein). D o s e - r e s p o n s e curve for fluphenazine inhibition of dopamine (100 /~M)activated adenylate cyclase. Values are the mean _+ S.E.M. percent inhibition of respective dopamine actions, n = 4 to 7 separate experiments per data point.
300 cyclase s t i m u l a t i o n in h o m o g e n a t e s (Fig. 3). M a x i m a l e n z y m e a c t i v a t i o n (47%) was o b s e r v e d at 100 #M D A c o n c e n t r a t i o n a n d the EDs0 v a l u e was 4.5 #M. F l u p h e n a z i n e effectively b l o c k e d the D A (100 #M) a c t i v a t i o n o f a d e n y l a t e cyclase. T h e m a x i m a l i n h i b i t i o n o c c u r r e d at 10 t~M, the C50 value ( a m o u n t t h a t i n h i b i t e d e n z y m e activity by 50%) was 0.4 ~M a n d the Ki (enzyme i n h i b i t o r y d i s s o c i a t i o n c o n s t a n t ) was 0.02 /~M. T h e results o f this s t u d y indicate t h a t D A ' s suppressive effect is specific for lower rates o f i n c o m i n g signals a n d that D A is c a p a b l e o f s t i m u l a t i n g a d e n y l a t e cyclase activity. This latter f i n d i n g generalizes the w o r k o f C l e m e n t - C o r m i e r et al. [6] to o u r e x p e r i m e n t a l m o d e l , the r a b b i t . T h e a c t i v a t i o n o f a d e n y l a t e cyclase m e c h a n i s m s s h o u l d be expected to increase cyclic A M P levels. Cyclic A M P has been s h o w n to be suppressive o f n e u r o n a l firing in the nucleus a c c u m b e n s [3]. It is possible, t h e r e f o r e , that the s u p p r e s s i o n o f responses elicited by low rates o f s t i m u l a t i o n is m e d i a t e d b y cyclic A M P . S u p p o r t e d by N I M H 1R03 MH32418-01 the Scottish Rite S c h i z o p h r e n i a F o u n d a t i o n , a n d the E p i l e p s y F o u n d a t i o n o f A m e r i c a . T h e fine technical assistance o f J o a n e D a l t o n a n d S.J. P a l m e r is cheerfully a c k n o w l e d g e d . T h e a u t h o r s also wish to t h a n k Dr. S.T. Kitai for his critical review o f the m a n u s c r i p t . F l u p h e n a z i n e was a gift f r o m the S q u i b b Institute for M e d i c a l Research.
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