Effects of 6-hydroxydopamine lesions of the prefrontal cortex on tyrosine hydroxylase activity in mesolimbic and nigrostriatal dopamine systems

0306-4522/92 $5.00+0.00

Neuroscience Vol. 48,No. 4,pp. 831-839,1992 Printedin Great Britain

Pcrgamon Press Ltd 0 1992IBRO

EFFECTS OF 6-HYDROXYDOPAMINE LESIONS OF THE PREFRONTAL CORTEX ON TYROSINE HYDROXYLASE ACTIVITY IN MESOLIMBIC AND NIGROSTRIATAL DOPAMINE SYSTEMS D. L. ROSIN,* W. A. CLARK,* M. GOLDSTEIN,~ R. H. ROTH* and A. Y. DEUTCH*$ *Departments tDepartment

of Psychiatry and Pharmacology, Yale University School of Medicine, New Haven, CT 06510, U.S.A. of Psychiatry, New York University School of Medicine, New York, NY 10016, U.S.A.

Abstract-The effects of prefrontal cortical dopamine depletion on subcortical dopamine function in the rat were examined 6-Hydroxydopamine lesions of the dopaminergic innervation of the prefrontal cortex did not alter concentrations of dopamine or its metabolite 3,4-dihydroxyphenylacetic acid in either the striatum or nucleus accumbens. Similarly, the activity of the catecholamine biosynthetic enzyme tyrosine hydroxylase in the striatal complex was not changed in animals with prefrontal cortical lesions. Animals sustaining neurotoxic lesions of the prefrontal cortex were challenged with haloperidol in order to activate submaximally tyrosine hydroxylase activity. The magnitude of the halopetidol-induced increase in enzyme activity in the nucleus accumbens was significantly greater in lesioned subjects than in control animals. These data suggest that lesions of the prefrontal cortical dopamine innervation do not result in significant alterations in basal dopaminergic function in the striatal complex. However, lesions of the dopaminergic innervation of the prefrontal cortex significantly increase the responsiveness of mesolimbic dopamine afferents to pharmacological challenge.

A growing body of literature suggests that the mesocortical, mesolimbic, and nigrostriatal dopamine (DA) systems may be anatomically and functionally interrelated.g*‘3*‘5*3M,55In particular, a number of investigations have suggested that the medial prefrontal cortex (PFC) regulates subcortical dopaminergic function. Lesions of the frontal cortex have long been known to increase spontaneous locomotor activity, and have been shown to enhance the locomotorstimulant properties of amphetamine;2* both spontaneous and amphetamine-elicited locomotor activity are thought to be subserved by dopaminergic mechanisms in the nucleus accumbens (NAS).35 Furthermore, lesions of the ventral tegmental area (VTA), from which the PFC DA innervation emanates,“*” result in hyperactivity; the magnitude of lesioninduced hyperactivity is correlated with the degree of frontal cortical DA depletion.58 These results may suggest that prefrontal cortical DA exerts an inhibitory influence over DA transmission in the NAS. $To whom correspondence should be addressed at: Department of Psychiatry, Yale University School of Medicine, Connecticut Mental Health Center, 34 Park Street, New Haven, CT 06508, U.S.A. Abbreviations: AAAD, l-aromatic amino acid decarboxylase; BH,, [6R]-5,6,7,8-tetrahydro-r.-biopterin; DA, dopamine; DMI, desmethylimipramine; DOPAC, 3,4dihydroxyphenylacetic acid; MOPS, 3-[N-morpholinolpropanesulfonic acid; NAS, nucleus accumbens; NE, norepinephrine; 6-OHDA, 6-hydroxydopamine; PFC, medial prefrontal cortex; SN, substantia nigra; TH, tyrosine hydroxylase; TH-LI, TH-like immunoreactivity; VIA, ventral tegmental area.

However, such conclusions are confounded by the possibility that VTA lesions may also destroy other catecholaminergic axons which course through the midbrain as they ascend to forebrain sites. Selective 6-hydroxydopamine (6-OHDA) lesions of the PFC DA innervation allow a more direct assessment of the role of PFC DA afferents in regulation of subcortical DA systems. Electrophysiological studies suggest that DA in the PFC exerts a tonic inhibitory influence over cortical projection neurons. g~53*6’ Frontal cortical projections to both the dorsal striatum and ventral striatum (including the NAS; see Ref. 26) are topographically organized,‘@,” and use an excitatory amino acid (such as glutamate) as a neurotransmitter.36,43.M Glutamate has been reported to evoke striatal DA release via a presynaptic, impulse-independent mechanism.7,3’*52 Thus, the requisite anatomical connections and pharmacology through which prefrontal cortical DA can regulate subcortical DA systems appear to be present. For example, depletion of prefrontal cortical DA may remove glutamatergic corticostriatal neurons from tonic DA inhibition, and the resultant increase in striatal glutamate release in turn evokes striatal DA release. Accordingly, a number of groups have examined biochemical indices of subcortical DA function following PFC DA depletion. Pycock, Carter, and associates6*4gss0reported that 6-OHDA lesions of the PFC increased DA levels and DA turnover in the striatum; certain aspects of their findings have been confirmed.25~37~42 However, other groups have 831

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D. L. ROSIN1’1ui

been unable to confirm either the biochemical or behavioral findings of Pycock and associates.‘4.33,47 Moreover, Pycock and colleagues did not observe consistent effects of PFC DA deafferentation on subcortical DA function.6.49~50More recently, we have reported that depletion of DA in the PFC does not alter basal DA metabolism, but does enhance the responsiveness of mesolimbic DA afferents to an environmental challenge, mild foot-shock stressI In light of the conflicting reports concerning the sequelae of PFC DA depletion on measures of subcortical DA function, and the significance of a role of the PFC DA system in regulating subcortical DA function, we have examined the effects of PFC DA depletion on tyrosine hydroxylase (TH) activity in mesolimbic and striatal sites. In addition, we have examined the biochemical responsiveness of subcortical DA systems to a pharmacological challenge, acute haloperidol administration, in animals with PFC DA depletions.

“1

EXPERIMENTAL PROCEDURES

Animals and surgery

Adult male Sprague-Dawley rats (Charles River Laboratories) (250-275 g body weight) were pretreated with desmethylimipramine (DMI; 25 mgjkg, i.p.) 25 min prior to chloral hydrate/pentobarbital (Equithesin) administration. Animals were placed in a stereotaxic frame and either 2.0 ~1 6-OHDA (4.Opg free base/p1 0.02% ascorbatesaline vehicle) or the vehicle was bilaterally infused into the PFC at a rate of 0.2$/min, as previously described.14 The 30-g injector cannulae were left in place for an additional 5 min before removal. The animals were returned to their home cages following recovery from anesthesia. Drug treatment

Animals were sacrificed by decapitation two weeks after surgery. In the experiment assessing changes in TH activity in response to PFC DA depletion, half of the lesioned animals and half of the vehicle-infused control subjects received haloperidol injections (100 pg/kg i.p.), and were returned to their home cages for the 2-h interval until being killed by decapitation. The dose of haloperidol was chosen on the basis of pilot experiments indicating a sub-maximal activation of TH in response to this dose of haloperidol. Remaining animals received vehicle injections. Tissue dissection

The PFC, NAS, and striatum were dissected as previously described.‘2 However, in the experiment in which TH activity was assessed, the striatal dissection was modified to remove separately the medial periventricular striatum (i.e. the medial PFC recipient zone of the striatum;‘,“,” see Fig. 1). Lateral striatal tissue was punch-dissected (1.6 mm diameter) from the same slice; this striatal sector was examined since it receives only a very sparse input from the medial aspects of the PFC. The VTA and substantia nigra (SN) were dissected as previously described.iz Tissue samples were stored at -70°C until assayed. Biochemical analyses

The magnitude of PFC DA depletion was assessed by measuring PFC DA concentrations. Tissue concentrations of norepinephrine (NE), DA, and 3,4_dihydroxyphenylacetic acid (DOPAC) were determined by HPLC with electrochemical detection.45

Fig. 1. Schematic illustration of dissection of the NAS (A) and the striatum (B). Two striatal areas were dissected: the medial periventricular striatum, which receives afferents from the PFC [the PFC-recipient zone (CP,)], and a more lateral non-recipient sector (CP,,) which receives a sparse input from the PFC. TH activity in soluble tissue extracts was measured using a modification of previously published methods.Y,6’ Activity was determined by measuring the evolution of “CO1 from L-[ I-“Cltyrosine in the presence of excess l-aromatic amino acid decarboxylase (AAAD). A lO+l aliquot of tissue supematant was incubated at 37°C with 15 ~1 reaction mixture, final concentration: 100 mM 3-[N-morpholinojpropanesulfonic acid (MOPS) buffer, pH 7.0. 0.5% Triton X-100, 0.1 mM ferrous ammonium sulfate, 5.0 mM dithiothreitol, I .OmM ascorbate. 100 p M tyrosine. and containing 3780 U catalase and 0.1 PCi L-[I-“Qtyrosine (Amersham Corp.). Samples were assayed with either a sub-saturating (0.1 mM) or saturating (I .OmM) concentration of the natural pterin co-factor [6R]-5,6,7&tetrahydro+biopterin dihydrochloride (BH,; Dr 8. Schirks Laboratories, Jona, Switzerland). After IO min IO ~1 ofa second reaction mixture (final concentration: 25 PM pyridoxal 5’-phosphate, and 100 PM 3-iodo-L-tyrosine in 100 mM MOPS buffer, pH 7.0, containing 2.0 ~1 AAAD, prepared from hog kidney? was added, samples were gassed with nitrogen, capped, and incubated at 30°C for 30 min. The reaction was terminated by the addition of 50 pl of 10% (w/v) trichloroacetic acid. Samples remained at WC for an additional 30 min to ensurc complete liberation from the solution and trapping of “CO, on methylbenzethonium hydroxide-soaked paper wicks, and radioactivity was counted. The use of this microassay for TH activity allowed us to measure enzyme activity in tissue samples of less than 0.5 mg wet weight. Unless otherwise noted. reagents were obtained from Sigma Chemical Co. Histology

Six animals received 6-OHDA infusions into the PFC. and were sacrificed two weeks later by Equithesin overdose. Animals were transcardially perfused with 4%

Frontal cortical regulation of accumbens dopamine paraformaldehyde in 0.1 M sodium phosphate buffer, pH 7.4. Frozen sections were cut through the PFC, stained with Neutral Red, and examined for evidence of nonspecific damage induced by the lesion procedure. Adjacent sections were also processed for TH immunohistochemistry in order to delineate the lesion extent. The examination of TH-like immunoreactive (TH-LI) fibers in the PFC was performed using an immunoperoxidase method” with a well-characterized TH antiserum!’ Data analysis In the experiment assessing lesion-induced changes in DA turnover, data were analysed by analyses of variance; subsequent post hoc comparisons of the means were performed when indicated using the Newman-Keuls test. In the case of the in vitro TH activity experiments, data were pooled from two separate experiments, and analysed using the Wilcoxon sign test. The extent of DA and NE depletions in the experiments was compared by the Mann-Whitney U-test. RESULTS

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extensive loss of TH-LI fibers in the anteromedial PFC [including the deep layers of the prelimbic (area 32), anterior cingulate (area 24b), and medial precentral cot&es] was observed (Fig. 3). The long TH-LI fibers coursing in the superficial layers and the TH-LI fibers running perpendicular to the pial surface, thought to correspond to noradrenergic fibers, were partially spared (see Fig. 4), again suggesting that the 6-OHDA infusions did not induce nonspecific damage, but instead selectively targeted certain catecholaminergic fibers. The lesion, as assessed by loss of TH-immunoreactive fibers, did not extend ventrally and caudally: the dorsal but not ventral infralimbic region of the PFC was involved, and some sparing of TH-LI fibers in the caudal prelimbic cortex was seen (see Fig. 3). In addition, the TH-LI fiber plexus in the dorsolateral aspects of the medial precentral

Lesion assessment

The degree of DA depletion induced by the 6OHDA lesions was determined by measuring monoamine levels in the PFC. In the initial experiment which examined changes in DA utilization following lesions, DA concentrations were reduced to 23% of control values (see Fig. 2); levels of NE were also significantly decreased (57% of control values). DA depletion in the experiment examining in vitro TH activity was somewhat more extensive, with DA and NE levels in the PFC at 9 and 35% of control, respectively (see Fig. 2). The degree of catecholamine

depletions was not statistically different across the two experiments. In neither experiment did the 6OHDA infusions result in DA depletion in the NAS or striatum, consistent with our previous report.” In those animals in which histological examination of the lesion placement was made, nonspecific damage to the PFC was minimal or not present. An EXPERIMENT I zi i?

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60

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EXPERIMENT II

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Fig. 2. Catecholamine depletion in the PFC following 6-OHDA lesions. DA and NE depletions are expressed as percentage of vehicle-infused controls. The two sets of data are from the two different experiments reported, the first (1) examining effects of PFC 6-OHDA lesions on subcortical DA utilization (a = 12 sham, n = 10 lesion), and the second (II) examining the consequences of the lesions on subcortical TH activity (n = 12 for both groups). The Percentage depletion of DA and NE across the two experiments did not differ significantly. *P < 0.01.

Fig. 3. Schematic illustration of the loss of the dopaminergic innervation of the PFC induced by 6-OHDA infusions. The dopaminergic innervation of the medial PFC is denicted bv the stippled area in the hemisection on the right. The extent of a typical 6-OHDA lesion of the PFC. as reflected bv a decrease in the density of presumed DA’fibers, is depicied by the blackened area in the left side of the figure.

Fig. 4. Photomicrograph illustrating the loss of the TH-immunoreactive fibers in the medial PFC (prelimbic cortex). A dense plexus of TH-LI fibers can be observed on the intact (right) side. In contrast, the density of TH-LI on the side of the lesion is greatly reduced. and those fibers remaining at this time (two weeks post-operatively) appear to be undergoing degeneration, with swollen and distorted processes (arrows). The TH-LI fibers in the superficial layers (arrowhead), running parallel to the pial surface, appearnormal.

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Frontal cortical regulation of accumbens dopamine

Table 1. Basal tyrosine hydroxylase activity (mean & S.E.M. pmol 14CO&rin per mg protein) in selected brain areas (n = 10 for each area), assayed in the presence of 0.1 or 1.OmM natural pterin co-factor

(shoulder) cortex of the PFC appeared grossly normal. The histological assessment did not reveal any overt loss of TH-LI axons in the NAS or the striatum.

Co-factor concentration 1.0 mM BH, 0.1 mM BH,

Lesion effects on tyrosine hydroxylase activity

6-OHDA lesions of the PFC did not alter DA utilization, as reflected by changes in DOPAC/DA, in the striatum or NAS, although there was a slight trend toward an increase in DA utilization in the striatum (data not shown). TH activity in the striatal or ventral mesencephalic sites examined was not altered by 6-OHDA lesions of the PFC (see Table 1, Fig. 5). Haloperidol administration increased TH

3.79 + 3.40 + 5.24 * 13.16 k 12.66 +

CP, CP, NAS SN VT-A

0.17 0.17 0.37 1.85 2.76

29.29 + 20.90 * 24.50 + 59.51 f 49.68 f

3.20 2.10 2.30 6.83 8.45

The regions assayed include: CP,, striatal recipient zone of medial prefrontal cortical afferents (see text); CP,, striatal non-recipient area of prefrontal cortical afferents.

0.1 mM BH 1 9

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Fig. 5. The effect of 6-OHDA lesions of the PFC (6-OHDA) on TH activity in the dorsal striatum (CP, and CP,; see Fig. 1) and NAS. TH activity was measured in the presence of 0.1 or 1.0 mM pterin co-factor at pH 7.0. Basal TH activity for the areas examined are shown in Table 1. The data are depicted for vehicle-infused animals treated with either saline (SHAM, n = 10) or 0.1 mg/kg haloperidol (SHAMHAL, n = 6), and for animals with PFC 6-OHDA lesions receiving either saline (6-OHDA, n = 8) or haloperidol challenge (6-OHDA-HAL, n = 5). 6-OHDA lesions of the PFC did not significantly change basal TH activity in the areas examined. Haloperidol challenge markedly increased enzyme activity in both the PFC recipient zone of the striatum (CP,) and the non-recipient sector of the striatum (CP,) as well as in the NAS. The magnitude of the increase in enzyme activity (at 0.1 mM co-factor concentration) elicited by haloperidol challenge was significantly greater in the NAS of lesioned rats. lP c 0.005, relative to SHAM (vehicle-infused, non-haloperidol challenged); **P c 0.001, relative to SHAM; ***P < 0.01, relative to SHAM; +P c 0.05, relative to SHAM-HAL (vehicle-infused, haloperidol challenged).

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D. L. ROSIN ei al

activity in the striatum and NAS in both control and lesioned subjects (see Fig. 5) as well as in the SN and VTA. The magnitude of the haloperidol-elicited increase in TH activity was significantly greater in the NAS of lesioned animals than in control subjects (Fig. 5). PFC DA depletion did not modify the haloperidol-elicited increase in TH activity in the SN or VTA (data not shown).

striatal DOPAC/DA (an index of DA utilization) approached but did not achieve a statistically signihcant increase. Striatal TH activity also tended to increase, albeit not to a statistically significant degree. in PFC-lesioned animals. Such a tendency toward an increase in various parameters of DA function after PFC lesions has also been observed by other labs (c‘. Kilts; J. P. Tassin; personal communications): these data may suggest that the effects of PFC DA denervation are restricted to certain striatal sectors, rathei DlSCUSSlON than the entire striatum. We therefore examined TH activity in the NAS and in two dorsal striatal regions: Lesions of the DA innervation of the PFC did not the medial periventricular striatum (i.e. the zone of alter basal subcortical DA function, as reflected by the striatum which receives PFC afferents), and a changes in DA turnover or TH activity. The lack of lateral striatal region (i.e. an area of the striatum not change in basal subcortical DA function therefore invested by PFC afferents). There was no significant prompted examination of the reactivity, rather than effect of PFC 6-OHDA lesions on basal TH activity activity, of the NAS and striatal DA systems followin either of these striatal sites. ing PFC lesions. 6-OHDA lesions of the PFC enA second feature of corticostriatal anatomy may hanced the responsiveness of TH activity in the NAS also be important. PFC efferents terminate in a to pharmacological challenge, suggesting that DA non-homogeneous distribution characterized by disdepletion of the PFC trans-synaptically alters certain tinct patches within the striatum.23.s’ Initial studies aspects of subcortical DA function. suggested that PFC axons innervating the striatum were in register with the striatal patch compartment,“’ Comparison with previous reports co-extensive with the islandic DA innervation system;22,24in contrast, striatal afferents from cortical The lack of significant change in basal subcortical sites other than the PFC were reported to preferenDA activity in animals with 6-OHDA lesions of the tially terminate in register with the diffuse (matrix) PFC is consistent with several recent findings*4~29~33~47 DA innervation.16 More recently, it has been and mirror the unpublished findings of a number of suggested that projections from deep layers of the groups (C. Chiueh; C. Kilts; T. Robbins; J. P. Tassin; PFC (deep layers V and VI) project to the patch personal communications). However, the lack of compartment, whereas neurons in superficial layer V effects on basal subcortical DA function is at variance innervate the matrix.” The two striatal DA systems The mechanisms with certain other reports. 6.37,42,49,50 exhibit different DA turnover rates and are differenwhich contribute to such discrepancies are not tially responsive to certain DA agonists.“,‘” It is known. possible that the effects of PFC DA depletion on Certain critical variables which may account striatal DA function are restricted to a single striatal for the different findings can be at least partially DA system (e.g. the islandic innervation) and that addressed. The sex, age, and strain (but not source) of rats used, as well as housing conditions (group vs any effects of PFC lesions on DA activity within the striatum are therefore diluted when examining both individual) appear to be at least somewhat comparstriatal DA systems. Similarly, recent anatomical able in some of the studies. Similarly, the findings indicate that there are two distinct comparttime between surgery and sacrifice is comparable in ments within the NAS, termed the core and shell? a number of studies; none the less, it is possible that different cortical regions differentially invest these the basal function of the NAS does vary with differaccumbal sectors.’ We have recently demonstrated ent post-operative intervals. that the DA innervations of the NAS core and shell Another factor which may be relevant is the can be functionally dissociated.” It is therefore possdegree of sparing of the PFC NE innervation followible that the effects of PFC DA depletions on basal ing 6-OHDA lesions. Regulatory controls over cortiDA function in the NAS may be present in only one cofugal projection neurons involve contributions of the two accumbal sectors. from, and interactions of, DA and NE.4,5,59160 DMI pre-treatment has generally been used in order to Increased responsiveness of mesolimbic dopamine minimize 6-OHDA-induced depletions of cortical neurons to pharmacological challenge after medial NE. While DMI pretreatment afforded some protecprefrontal cortex dopamine lesions tion of cortical NE axons, we still observed a signifiNo significant changes in subcortical DA function cant decline in PFC NE concentrations. However, the in response to PFC DA depletion were noted in the magnitude of NE depletion in our study was similar to that noted in most reports demonstrating an effect present study; however, lesion-induced changes in the reactivity of the subcortical DA systems were obof PFC lesions on subcortical DA activity. served. Examination of the magnitude of the halopePFC lesions did not significantly change subridol-elicited increase in TH activity revealed that the cortical DA turnover; however, in lesioned animals

Frontal cortical regulation of accumbens dopamine DA innervation of the NAS was hyper-responsive to pharmacological challenge in animals with PFC DA depletion. Thus, the degree of haloperidol-elicited increase in TH activity in lesioned animals was significantly greater than that Seen in vehicle-infused subjects when assayed under subsaturating (0.1 mM) concentration of the pterin co-factor. When assayed at saturating (1 .OmM) co-factor concentration, the magnitude of the increase in TH activity approached but did not achieve statistical significance; this probably reflects the difficulty in observing significant increases in enzyme activity along the asymptotic portion of the enzyme activity curve. These data suggest that the NAS DA innervation is functionally hyper-responsive to pharmacological perturbation. We have recently observed that 6OHDA lesions of the PFC render the DA innervation of the NAS hyper-responsive to mild foot-shock stress “Jo thus, foot-shock currents that do not elicit an i&ease in DA release in the NAS of non-lesioned animalsI do evoke a significant increase in DA release in PFC-lesioned subjects.14 The discrepant findings concerning the effects of PFC lesions on subcortical DA systems may therefore be partially attributable to different degrees of stress exposure in the various studies presented in the literature. For example, if animals are transported on the day of sacrifice from the vivarium to a laboratory room, the stress of the move may be of sufficient magnitude as to induce an increase. in DA metabolism in the NAS. Our previous findings of an increased responsiveness of mesolimbic DA neurons to stress after PFC DA deafferentation, in conjunction with the present findings, suggest that the mesoaccumbens DA system is hyper-responsive to both pharmacological and environmental perturbation. The present results are at odds with certain recent reports which suggest that axon-sparing lesions of cells in the PFC (such as accomplished by ibotenic acid) result in a transient enhancement of basal subcortical DA transmissiongo and enhancement of stress-induced function.29 It is difficult to explain the mechanism which might underlie the latter findings, since removal of corticostriatal glutamatergic projections might be expected to dampen, rather than enhance, subcortical DA function. It is worth noting that there is considerable disagreement that plagues not only studies of the effects of cortical DA depletion, but also the effects of intrinsic lesions of the cortex: most authors have reported no increase in DA utilization after such lesions.8*32 We have also observed no effect of ibotenic acid lesions of the PFC on basal DA turnover in the NAS, but have

837

found that such lesions suppress the ability of the NAS DA innervation to respond to (moderate intensity) stress.’ Clinical implications

A current hypothesis concerning the pathogenesis of schizophrenia posits a developmentally specific event which results in the dysfunction of the primate dorsolateral prefrontal cortex; specifically suggested is a DA deafferentation of this cortical region.” Central to this hypothesis is the concept of an interaction between the prefrontal cortical DA system and subcortical DA systems, such that DA depletion of the PFC would result in an increase in dopaminergic activity in subcortical regions such as the striatum and NAS. The possible involvement of other cortical areas in schizophrenia may result in similar types of subcortical alterations but they occur in different cortical afferent zones within the striatal complex. The present data suggest that although basal dopaminergic activity in rat subcortical areas may not be augmented by DA depletion of the PFC, such PFC lesions do increase the reactivity of the NAS DA innervation to pharmacological challenge. These data are also consistent with our recent observations that PFC DA denervation renders the NAS DA innervation hyper-responsive to an environmental perturbation, foot-shock stress.14 Stress has been reported to precipitate psychotic episodes in schizoIf certain of the schizophrenic phrenic patients. 3,46,62 reactions are marked by DA depletion of the PFC, it is conceivable that stress-induced changes in the psychotic process may be related to a hyper-reactivity of the mesolimbic DA innervation secondary to PFC DA deafferentation. Moreover, if there is a DA deafferentation of the PFC in certain populations of schizophrenic patients, notably those with prominent negative symptoms, the response characteristics of such patients to neuroleptic treatment may differ from that of schizophrenic patients with positive symptoms and a functionally intact cortical dopaminergic innervation.9 Acknowledgements-We appreciate the assistance of Rev. Laurie Tubbs in performing catecholamine analyses. We are also indebted to Drs Mike Chiueh, George Jaskiw, Clinton Kilts, Trevor Robbins, Jean-PO1 Tassin, and Daniel Weinberger, who openly discussed with us their unpublished data. These studies were supported in part by NIMH grants MH-45124 (A.Y.D.), MH-14092 (R.H.R.), and MH-43230 (M.G.), grants from the Scottish Rite Schizophrenia Research Program (A.Y.D.) and the Pharmaceutical Manufacturers Association (D.L.R.), and support from the Abraham Ribicoff Research Facilities of the Connecticut Mental Health Center.

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302, 437446. (Accepted 28 November 1991)