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Footshock and conditioned stress increase 3, 4-dihydroxyphenylacetic acid (DOPAC) in the ventral tegmental area but not substantia nigra
Brain Research, 333 (1985) 143- 146 Elsevier
143
BRE 20739
Footshock and conditioned stress increase 3,4-dihydroxyphenylacetic acid (DOPAC) in the ventral tegmental area but not substantia nigra ARIEL Y. DEUTCH*, SEE-YING TAM and ROBERT H. ROTH Departments of Pharmacology and Psychiatry, Yale University School of Medicine, New Haven, CT 06510 (U. S. A. ) (Accepted November 20th, 1984) Key words: ventral tegmental area - - prefrontal cortex - - dopamine - - 3,4-dihydroxyphenylacetic acid (DOPAC) - - stress
The effects of stress on dopamine (DA) metabolism in the mesencephalic DA cell body areas and DA terminal field regions were examined. Both mild footshock stress and exposure to a neutral stimulus previously paired with footshock resulted in a selective increase in the levels of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the prefrontal cortex as has been previously reported. Footshock stress also resulted in a slight but significant increase in DOPAC levels in the olfactory tubercles. DOPAC levels were selectively increased in the A10 cell body area (ventral tegmental area) but not A9 region (substantia nigra) by both footshock and the conditioned stress paradigm. These data indicate that the cell bodies of origin of the mesocortical dopaminergic system are activated by stress in contrast to those DA neurons innervating the striatum. It appears that mesocortical dopaminergic neurons exhibit different regulatory features than mesolimbic or nigrostriatal neurons,
A variety of n e u r o t r a n s m i t t e r systems appear to respond to stress, including d o p a m i n e ( D A ) 12,16,33,
The mesoprefrontal cortical dopaminergic system arises from the A10 D A cell in the ventral tegmental
norepinephrine (NE)IS, 25, 7-aminobutyric acid (GABA)17, 5-hydroxytryptamine (5-HT) 10, opioid
area ( V T A ) ; other D A n e u r o n s within A10 project to other cortical and mesolimbic terminal fields, in-
peptides24 and substance p4,21. However, mild stress
cluding the nucleus accumbens and olfactory tubercles 3t. The A9 cell group in the pars compacta of the
exposure appears to selectively activate the mesocortical dopaminergic system12,16A9,22,33. Mild footshock stress 12,33, exposure to a novel e n v i r o n m e n t31 and conditioned fear ~6 all result in increased levels of the D A metabolite 3,4-dihydroxyphenylacetic acid ( D O P A C ) in the prefrontal cortex (PFC). Such mild stress exposure does not effect biochemically measurable D O P A C increases in the striatum 33 nor are consistent changes observed in the mesolimbic terminal fields 12,16,19,33. Exposure to mild stress does not appear to alter N E levels 33 or N E metabolism 27. Furthermore, in animals conditioned to mild stress (exposure to the neutral stimulus previously paired with footshock) changes in D O P A C levels are observed only in the PFC, although some brain areas such as the amygdala exhibited increase D A metabolite levels following exposures to the footshock stress itself16.
substantia nigra (SN) projects to the striatum (CP) 3t. In light of the selective effects of stress exposure upon the mesoprefrontal, but not mesolimbic or nigrostriatal D A systems, we have examined the effects of both mild footshock stress and conditioned fear upon the A10 ( V T A ) and A9 (SN) cell group regions. Adult male Charles River rats were used in all experiments. Rats were subjected to mild footshock stress (0.2 m A , as calibrated by Davis and Astrachan11, 160 ms duration with a 320 ms interval) for 20 min and sacrificed by decapitation. Control rats were placed in the shock box, but the shock generator was disconnected such that only the auditory click of the relays was operative. Two additional groups of rats were exposed to either the shock or the control (sham) condition for one day (after 3 daily acclimati-
Correspondence: A. Y. Deutch. Present address: Department of Pharmacology, Yale University School of Medicine, New Havcn, CT 06510, U.S.A. 0006-8993/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)
144 zation sessions in the shock box), and on the test day received footshock or the auditory signal previously paired with the shock. Thus, 4 groups of 8 animals each were used: sham-sham, sham-shock, shocksham (conditioned) or shock-shock. Following decapitation, the brains were removed from the cranial vault, various areas dissected from either 1 or 2 mm thick coronal slices (as illustrated in Fig. 1) and frozen at -70 °C until assayed for D O P A C levels by high-performance liquid chromatography with electrochemical detection27. Data were analyzed by means of a two-factor analysis of variance with post hoc Dunnett's test. The effects of mild footshock stress on D A metabolism within the various terminal field areas was consistent with previous reports: a large increase in D O P A C levels in the PFC was observed as well as a much smaller, but significant, increase in metabolite concentration in the olfactory tubercle. No changes in D O P A C levels in the striatum, cingulate cortex or nucleus accumbens were observed. In animals receiving footshock on both days, an identical pattern of D O P A C increases in terminal field areas was obA 9820 A 11200
Prefrontal Cortex
A 8380
Cingulate Cortex A 2420
A 1420 ~.,
VTA/
Fig. 1, Schematic representation of the dissection boundaries of the brain sites examined in the present study. CIN, cingulate cortex; CP, striatum; NAS, nucleus accumbens septi; PFC, prefrontal cortex; SN, substantia nigra; TUO, olfactory tubercle; VTA, ventral tegmental area.
served. Those animals in which the auditory signal of the relay clicks had been paired on the previous day with shock (conditioned group) exhibited an increase in D O P A C levels only in the PFC; no significant elevation in D O P A C in the olfactory tubercles of conditioned animals was observed. The effects of both footshock and conditioned lear on D O P A C levels within the VTA and SN paralleled those changes observed in the terminal fields. No changes in levels of the metabolite in the SN were observed in either shock condition or the conditioned stress group. However, D O P A C levels within the VTA were significantly elevated in both groups of animals receiving footshock as well as the group exposed to the neutral tone previously paired with footshock. These data demonstrate that the stress-induced changes previously observed in the dopaminergic terminal fields are paralleled by selective changes in the D A cell body regions. Thus, D O P A C levels are increased only in the DA cell body region which is the source of the dopaminergic innervation of the terminal regions which exhibit stress-induced D O P A C changes. Conversely, no change in metabolite levels is observed in the A9 cell body region, the source of the dopaminergic innervation of the striatum. The selective increase in D O P A C levels in the VTA induced by stress is presumably attributable to dendritic release of D A from the A10 neurons. Dendritic release of DA from neurons in the VTA and SN has been demonstrated using both push-pull cannulation methods and superfusion techniquesS,~.8,14,z), and dendrodendritic synapses have been reported in the SN 36. The functional significance of dendritic release of D A (in the presence of appropriate D A receptors) appears to be to autoregulate DA neuronal firingT.S,15.22. In situations in which neuronal firing is enhanced one would therefore expect an increase in both VTA and PFC D O P A C levels in response to stress, but not in other terminal field regions. However, both the VTA and PFC exhibit increased (rather than decreased) levels of D O P A C in response to stress. These data suggest that those dopaminergic neurons projecting to the PFC exhibit different regulatory mechanisms than other DA neurons~ Recent biochemical and electrophysiotogical data indicate that most prefrontal cortical dopaminergic neurons originating in the VTA lack both terminal and soma-
145 crease in V T A D O P A C which occurs in response to
F~LSHAM-SHAM ~SHOCK-SHAM ~3SHAM-SHOCK ~SHOCK-SHOCK
175
160
footshock or conditioned fear apparently reflects sustained activation of only a subpopulation of V T A do-
145
paminergic neurons, the size of which is in accord with the size of the dopaminergic mesoprefrontal neuronal population.
130 +1_ Ib5 o 'E
Recent reports indicate that the firing rates of pre~
B5
0 ~
7O
o
sumed dopaminergic n e u r o n s in the V T A of the freely moving cat are increased by stress26, 34. The current
55
study presents biochemical evidence of increased ac-
40
tivity of V T A D A n e u r o n s in response to stress. The
25
PFC
VTA
TUO
NAS
CIN
SN
CP
Fig. 2. Stress-induced changes in regional DOPAC levels m footshock (sham-shock and shock-shock) and conditioned stress (shock-sham) groups, relative to control animals (sham-sham). Abbreviations are as shown in Fig. 1. * P ~< 0.05.
todendritic autoreceptors 9,29,3s. It therefore appears that despite continued dendritic as well as axonal D A release in response to stress, a subpopulation of dopaminergic A10 n e u r o n s which lacks autoreceptors remains active, in contrast to other dopaminergic neurons within the VTA. This interpretation is consistent with the difference in magnitude of the D O P A C response between the PFC and V T A : the increase in metabolite levels in the cortical area is more than twice observed in the VTA. Recent data presented by Swanson3~ indicate that from within the region dissected as V T A in the present study (including the V T A , central linear and rostral linear nuclei) approximately 35% of the n e u r o n s project to the pre-
changes in D O P A C levels elicited by conditioned stress are regionally specific: increased metabolite levels are found in mesoprefrontal but not in nigrostriatal or mesolimbic sites. The magnitude of the increase in D O P A C levels within the V T A suggests that a subpopulation of these D A neurons is in a continued state of activation induced by stress; other dopaminergic neurons may be transiently activated, but such activation does not appear to be m a i n t a i n e d 37. The mesocortical and mesolimbic D A systems have been suggested to be involved in schizophrenia, primarily on the basis of data indicating that the site of action of the neuroleptic agents may be at these areas 1,13.23.3°. In particular, mesoprefrontal dopaminergic neurons do not appear to develop tolerance to the biochemical actions of the neuroleptics2,3-27 The mechanism whereby stress exacerbates the schizophrenic process may be related in part to an activation of A10 mesocortical neurons. We would like to thank Maria Varadi for expert
genual frontal cortex; of these n e u r o n s only onethird to one-half are dopaminergic. The observed in-
technical assistance and Sue Mulready for attentive preparation of the manuscript. Supported by MH-14092 and the State of Connecticut.
1 Anden, N.-E., Antipsychotic drugs and catecholamine synapses, J. Psychiat. Res., 11 (1974) 97-104. 2 Bacopoulos, N. C., Spokes, E. G., Bird, E. D. and Roth, R. H., Antipsychotic drug action in schizophrenic patients: effect on cortical dopamine metabolism after long-term treatment, Science, 205 (1979) 1405-1407. 3 Bacopoulos, N. C., Redmond, D. E., Baulu, J. and Roth, R. H., Chronic haloperidol or fluphenazine: effects on dopamine metabolism on brain, cerebrospinal fluid, and plasma of Cercopithacus aethiops (vervet monkey), J. Pharmacol. exp. Ther., 212 (1980) 1-5. 4 Bannon, M. J., Elliott, P. J., Alpert, J. E., Goedert, M., Iversen, S. D. and Iversen, L. L., Role of endogenous substance P in stress-induced activation of mesocortical dopa-
mine neurons, Nature (Lond.), 306 (1983) 791-797. 5 Beart, P. M. and McDonald, D., Neurochemical studies of the mesolimbic dopaminergic pathway: somatodendritic mechanisms and GABAergic neurons in the rat ventral tegmentum, J. Neurochem., 34 (1980) 1622-1629. 6 Beart, P. M., McDonald, D. and Gundlach, A. L., Mesolimbic dopaminergic neurones and somatodendritic mechanisms, Neurosci. Lett., 15 (1979) 165-170. 7 Bunney, B. S., Aghajanian, G. K. and Roth, R. H., Comparison of effects of L-DOPA, amphetamine and apomorphine on firing rate of rat dopaminergic neurons, Nature (Lond.), 245 (1973) 123-125. 8 Cheramy, A., Leviel, V. and Glowinski, J., Dendritic release of dopamine in the substantia nigra, Nature (Lond.),
146 289 (1981) 537-542. 9 Chiodo, L. A., Bannon, M. J., Grace, A. A., Roth, R. H. and Bunney, B. S., Evidence for the absence of impulseregulating somatodendritic and synthesis-modulating nerve terminal autoreceptors on subpopulations of mesocortical dopamine neurons, Neuroscience, 12 (1984) 1-16. 10 Culman, J., Kvetnansky, R., Torda, T. and Murgas, K., Serotonin concentration in individual hypothalamic nuclei of rats exposed to acute immobilization stress, Neuroscience, 5 (1980) 1503-1506. 11 Davis, M. and Astrachan, D. I., Conditioned fear and startle magnitude: effects of different footshock or backshock intensities used in training, J. exp. Psychol. : Anita. Behav. Proe., 4 (1978) 195-203. 12 Fadda, F., Argiolis, A., Melis, M. R., Tissari, A. H., Onali, P. C. and Gessa, G. L., Stress-induced increase in 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the cerebral cortex and in n. accumbens: reversal by diazepam, L@ Sci., 23 (1978) 2219-2224. 13 Fuxe, K., Nystr0m, M., Tovi, M., Smith, R. and Ogren, S.-O., Central catecholamine neurons, behavior and neuroleptic drugs: an analysis to understanding the involvement of catecholamines in schizophrenia, J. psychiat. Res:, 11 (1974) 151-161. 14 Geffen, L. R., Jessell, T. H., Cuello, A. C. and Iversen, L. L., Release of dopamine from dendrites in rat substantia nigra, Nature (Lond.), 260 (1976) 258-260. 15 Groves, P. M., Wilson, C. J., Young, S. J. and Rebec, G. V., Self-inhibition by dopaminergic neurons, Science, 190 (1975) 522-529. 16 Herman, J. P., Guillonneau, D., Dantzer, R., Scatton, B., Semerdjian-Rouquier, L. and LeMoal, M., Differential effects of inescapable footshocks and of stimuli previously paired with inescapable footshocks on dopamine turnover in cortical and limbic areas of the rat, Life Sci., 30 (1982) 2207-2214. 17 Kuriyama, K., Kamori, K. and Yoneda, Y., Functional alterations in central G A B A neurons induced by stress. In L. Hertz and E. Kuamme (Eds.), Glutamine, Glutamate and GABA in the Central Nervous System, Alan R. Liss, New York, 1983, pp. 559-569. 18 Kvetnensky, R., Palkovits, M., Mitro, A., Torda, T. and Mikwaj, L., Catecholamines in individual hypothalamic nuclei of acutely and repeatedly stressed rats, Neuroendocrinotogy, 23 (1977) 257-267. 19 Lavielle, S,, Tassin, J. P., Thierry, A.-M., Blanc, G., Herve, D., Bathelemy, C. and Glowinski, J., Blockade by benzodiazepines of the selective high increase in dopamine turnover induced by stress in mesoeortical dopaminergic neurons of the rat, Brain Research, 168 (1978) 585-594. 20 Leviel, V., Cheramy, A. and Glowinski, J., Role of the dendritic release of dopamine in the reciprocal control of the two nigro-striatal dopaminergic pathways, Nature (Lond.), 280 (1979) 236-239. 21 Lisoprawski, A., Blanc, G. and Glowinski, J., Activation by stress of the habenulo-interpeduncular substance P neurons in the rat, Neurosci. Lett., 25 (1981) 47-51.
22 Llinas, R., Greenfield, S. A. and Jahnsen, tf., t/lcctroph b iology of pars compacta cells in the in vitro substantia nigra a possible mechanism for dendritic release, Brain Research, 294 (1984) 127-132. 23 Matthysse, S., Dopamine and the pharmacology of schizophrenia: the state of the evidence, J. psychiat Re.s. t1 (1974) 107-117. 24 Miller, J. D., Speciale, S. G., McMillen, B. A. and German, D. C., Naloxone antagonism of stress-induced augmentation of frontal cortex dopamine metabolism, Europ J. Pharmacol., 98 (1984) 437-439. 25 Nakagawa, R., Tanaka, M., Kohno, Y., Noda, Y. and Nagasaki, N., Regional responses of rat brain noradrenergic neurones to acute intense stress, PharmacoL Biochem. Behay., 14 (1981) 729-732. 26 Preussler, D. W. and Trulson. M. E.. Activity of dopaminecontaining ventral tegmental area neurons in freely moving cats, Soc. Neurosci. Abstr.. 8 (1982) 896. 27 Reinhard, J. R., Jr,, Bannon. M. J. and Roth. R. H.. Acceleration by stress of dopamine synthesis and metabolism m prefrontal cortex: antagonism by diazepam. NaunynSchmiedeberg's Arch. Pharmacol.. 308 (1982) 374-377. 28 Roth, R. H., Bacopoulous. N. G., Bustos, (i and Redmond, D. E., Jr., Antipsychotic drugs: differential effects on dopamine neurons in basal ganglia and mesocortex lollowing chronic administration in human and nonhuman primates, Advanc. Biochem. PsychopharmacoL. 24 (1980) 513-520. 29 Shepard, P. D. and German. D. C., A subpopulation olmesocortical dopamine neurons possesses autoreceptors. Eutop. J. Pharmacol., 98 (1984) 455-566. 30 Stevens, J., An anatomy of schizophrenia?, Arch. gen. Psvchiat., 29 (1973) 177-189. 31 Swanson, L. W., The projections of the ventral tegmental area and adjacent regions: a combined fluorescen! retrograde tracer and immunofluorescence study m the rat. Brain Res. Bull., 9 (1982) 321-353 32 Tassin, J. P., Herve, D.. Blanc. G and Glowinski. J,, Differential effects of a two minute open-field session on dopamine utilization in the frontal cortices of BALBIC and C57 BL/6 mice, Neurosci. Lett., 17 (1980) 67-71. 33 Thierry, A. M., Tassin, J. P., Blanc, G. and GIowinski, J., Selective activation of the mesocortical DA system by stress, Nature (Lond.), 263 (1976) 242-244. 34 Trulson, M. E. and Preusster, D. W., Dopamine-containing ventral tegmentat neurons in freely moving cats: activity during the sleep-waking cycle and effects of stress, Exp. Neurol., 83 (1984)367-377. 35 White, F. J. and Wang, R. Y., Differential effects of classical and atypical antipsychotic drugs on A9 and A10 dopamine neurons, Science, 221 (1983) 1054-1056. 36 Wilson, C. J., Groves, P. M. and Fifkova, E., Monoaminergic synapses, including dendrodendritic synapses in the rat substantia nigra, Exp. Brain Res., 30 (1977) 101-174. 37 Zigmond, M. J. and Stricker, E. M,, Parkinson's disease: studies with an animal model, Life Sci., 35 (1984) 5-18.
143
BRE 20739
Footshock and conditioned stress increase 3,4-dihydroxyphenylacetic acid (DOPAC) in the ventral tegmental area but not substantia nigra ARIEL Y. DEUTCH*, SEE-YING TAM and ROBERT H. ROTH Departments of Pharmacology and Psychiatry, Yale University School of Medicine, New Haven, CT 06510 (U. S. A. ) (Accepted November 20th, 1984) Key words: ventral tegmental area - - prefrontal cortex - - dopamine - - 3,4-dihydroxyphenylacetic acid (DOPAC) - - stress
The effects of stress on dopamine (DA) metabolism in the mesencephalic DA cell body areas and DA terminal field regions were examined. Both mild footshock stress and exposure to a neutral stimulus previously paired with footshock resulted in a selective increase in the levels of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the prefrontal cortex as has been previously reported. Footshock stress also resulted in a slight but significant increase in DOPAC levels in the olfactory tubercles. DOPAC levels were selectively increased in the A10 cell body area (ventral tegmental area) but not A9 region (substantia nigra) by both footshock and the conditioned stress paradigm. These data indicate that the cell bodies of origin of the mesocortical dopaminergic system are activated by stress in contrast to those DA neurons innervating the striatum. It appears that mesocortical dopaminergic neurons exhibit different regulatory features than mesolimbic or nigrostriatal neurons,
A variety of n e u r o t r a n s m i t t e r systems appear to respond to stress, including d o p a m i n e ( D A ) 12,16,33,
The mesoprefrontal cortical dopaminergic system arises from the A10 D A cell in the ventral tegmental
norepinephrine (NE)IS, 25, 7-aminobutyric acid (GABA)17, 5-hydroxytryptamine (5-HT) 10, opioid
area ( V T A ) ; other D A n e u r o n s within A10 project to other cortical and mesolimbic terminal fields, in-
peptides24 and substance p4,21. However, mild stress
cluding the nucleus accumbens and olfactory tubercles 3t. The A9 cell group in the pars compacta of the
exposure appears to selectively activate the mesocortical dopaminergic system12,16A9,22,33. Mild footshock stress 12,33, exposure to a novel e n v i r o n m e n t31 and conditioned fear ~6 all result in increased levels of the D A metabolite 3,4-dihydroxyphenylacetic acid ( D O P A C ) in the prefrontal cortex (PFC). Such mild stress exposure does not effect biochemically measurable D O P A C increases in the striatum 33 nor are consistent changes observed in the mesolimbic terminal fields 12,16,19,33. Exposure to mild stress does not appear to alter N E levels 33 or N E metabolism 27. Furthermore, in animals conditioned to mild stress (exposure to the neutral stimulus previously paired with footshock) changes in D O P A C levels are observed only in the PFC, although some brain areas such as the amygdala exhibited increase D A metabolite levels following exposures to the footshock stress itself16.
substantia nigra (SN) projects to the striatum (CP) 3t. In light of the selective effects of stress exposure upon the mesoprefrontal, but not mesolimbic or nigrostriatal D A systems, we have examined the effects of both mild footshock stress and conditioned fear upon the A10 ( V T A ) and A9 (SN) cell group regions. Adult male Charles River rats were used in all experiments. Rats were subjected to mild footshock stress (0.2 m A , as calibrated by Davis and Astrachan11, 160 ms duration with a 320 ms interval) for 20 min and sacrificed by decapitation. Control rats were placed in the shock box, but the shock generator was disconnected such that only the auditory click of the relays was operative. Two additional groups of rats were exposed to either the shock or the control (sham) condition for one day (after 3 daily acclimati-
Correspondence: A. Y. Deutch. Present address: Department of Pharmacology, Yale University School of Medicine, New Havcn, CT 06510, U.S.A. 0006-8993/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)
144 zation sessions in the shock box), and on the test day received footshock or the auditory signal previously paired with the shock. Thus, 4 groups of 8 animals each were used: sham-sham, sham-shock, shocksham (conditioned) or shock-shock. Following decapitation, the brains were removed from the cranial vault, various areas dissected from either 1 or 2 mm thick coronal slices (as illustrated in Fig. 1) and frozen at -70 °C until assayed for D O P A C levels by high-performance liquid chromatography with electrochemical detection27. Data were analyzed by means of a two-factor analysis of variance with post hoc Dunnett's test. The effects of mild footshock stress on D A metabolism within the various terminal field areas was consistent with previous reports: a large increase in D O P A C levels in the PFC was observed as well as a much smaller, but significant, increase in metabolite concentration in the olfactory tubercle. No changes in D O P A C levels in the striatum, cingulate cortex or nucleus accumbens were observed. In animals receiving footshock on both days, an identical pattern of D O P A C increases in terminal field areas was obA 9820 A 11200
Prefrontal Cortex
A 8380
Cingulate Cortex A 2420
A 1420 ~.,
VTA/
Fig. 1, Schematic representation of the dissection boundaries of the brain sites examined in the present study. CIN, cingulate cortex; CP, striatum; NAS, nucleus accumbens septi; PFC, prefrontal cortex; SN, substantia nigra; TUO, olfactory tubercle; VTA, ventral tegmental area.
served. Those animals in which the auditory signal of the relay clicks had been paired on the previous day with shock (conditioned group) exhibited an increase in D O P A C levels only in the PFC; no significant elevation in D O P A C in the olfactory tubercles of conditioned animals was observed. The effects of both footshock and conditioned lear on D O P A C levels within the VTA and SN paralleled those changes observed in the terminal fields. No changes in levels of the metabolite in the SN were observed in either shock condition or the conditioned stress group. However, D O P A C levels within the VTA were significantly elevated in both groups of animals receiving footshock as well as the group exposed to the neutral tone previously paired with footshock. These data demonstrate that the stress-induced changes previously observed in the dopaminergic terminal fields are paralleled by selective changes in the D A cell body regions. Thus, D O P A C levels are increased only in the DA cell body region which is the source of the dopaminergic innervation of the terminal regions which exhibit stress-induced D O P A C changes. Conversely, no change in metabolite levels is observed in the A9 cell body region, the source of the dopaminergic innervation of the striatum. The selective increase in D O P A C levels in the VTA induced by stress is presumably attributable to dendritic release of D A from the A10 neurons. Dendritic release of DA from neurons in the VTA and SN has been demonstrated using both push-pull cannulation methods and superfusion techniquesS,~.8,14,z), and dendrodendritic synapses have been reported in the SN 36. The functional significance of dendritic release of D A (in the presence of appropriate D A receptors) appears to be to autoregulate DA neuronal firingT.S,15.22. In situations in which neuronal firing is enhanced one would therefore expect an increase in both VTA and PFC D O P A C levels in response to stress, but not in other terminal field regions. However, both the VTA and PFC exhibit increased (rather than decreased) levels of D O P A C in response to stress. These data suggest that those dopaminergic neurons projecting to the PFC exhibit different regulatory mechanisms than other DA neurons~ Recent biochemical and electrophysiotogical data indicate that most prefrontal cortical dopaminergic neurons originating in the VTA lack both terminal and soma-
145 crease in V T A D O P A C which occurs in response to
F~LSHAM-SHAM ~SHOCK-SHAM ~3SHAM-SHOCK ~SHOCK-SHOCK
175
160
footshock or conditioned fear apparently reflects sustained activation of only a subpopulation of V T A do-
145
paminergic neurons, the size of which is in accord with the size of the dopaminergic mesoprefrontal neuronal population.
130 +1_ Ib5 o 'E
Recent reports indicate that the firing rates of pre~
B5
0 ~
7O
o
sumed dopaminergic n e u r o n s in the V T A of the freely moving cat are increased by stress26, 34. The current
55
study presents biochemical evidence of increased ac-
40
tivity of V T A D A n e u r o n s in response to stress. The
25
PFC
VTA
TUO
NAS
CIN
SN
CP
Fig. 2. Stress-induced changes in regional DOPAC levels m footshock (sham-shock and shock-shock) and conditioned stress (shock-sham) groups, relative to control animals (sham-sham). Abbreviations are as shown in Fig. 1. * P ~< 0.05.
todendritic autoreceptors 9,29,3s. It therefore appears that despite continued dendritic as well as axonal D A release in response to stress, a subpopulation of dopaminergic A10 n e u r o n s which lacks autoreceptors remains active, in contrast to other dopaminergic neurons within the VTA. This interpretation is consistent with the difference in magnitude of the D O P A C response between the PFC and V T A : the increase in metabolite levels in the cortical area is more than twice observed in the VTA. Recent data presented by Swanson3~ indicate that from within the region dissected as V T A in the present study (including the V T A , central linear and rostral linear nuclei) approximately 35% of the n e u r o n s project to the pre-
changes in D O P A C levels elicited by conditioned stress are regionally specific: increased metabolite levels are found in mesoprefrontal but not in nigrostriatal or mesolimbic sites. The magnitude of the increase in D O P A C levels within the V T A suggests that a subpopulation of these D A neurons is in a continued state of activation induced by stress; other dopaminergic neurons may be transiently activated, but such activation does not appear to be m a i n t a i n e d 37. The mesocortical and mesolimbic D A systems have been suggested to be involved in schizophrenia, primarily on the basis of data indicating that the site of action of the neuroleptic agents may be at these areas 1,13.23.3°. In particular, mesoprefrontal dopaminergic neurons do not appear to develop tolerance to the biochemical actions of the neuroleptics2,3-27 The mechanism whereby stress exacerbates the schizophrenic process may be related in part to an activation of A10 mesocortical neurons. We would like to thank Maria Varadi for expert
genual frontal cortex; of these n e u r o n s only onethird to one-half are dopaminergic. The observed in-
technical assistance and Sue Mulready for attentive preparation of the manuscript. Supported by MH-14092 and the State of Connecticut.
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