Granulocyte Colony Stimulating Factor Enhances Reward Learning through Potentiation of Mesolimbic Dopamine System Function

Deficits in motivation and cognition are hallmark symptoms of multiple psychiatric diseases. These symptoms are disruptive to quality of life and often do not improve with available medications. In recent years there has been increased interest in the role of the immune system in neuropsychiatric illness, but to date no immune-related treatment strategies have come to fruition. The cytokine granulocyte-colony stimulating factor (G-CSF) is known to have trophic and neuroprotective properties in the brain, and we recently identified it as a modulator of neuronal and behavioral plasticity. By combining operant tasks that assess discrete aspects of motivated behavior and decision-making in male mice and rats with subsecond dopamine monitoring via fast-scan cyclic voltammetry, we defined the role of G-CSF in these processes as well as the neural mechanism by which it modulates dopamine function to exert these effects. G-CSF enhanced motivation for sucrose as well as cognitive flexibility as measured by reversal learning. These behavioral outcomes were driven by mesolimbic dopamine system plasticity, as systemically administered G-CSF increased evoked dopamine release in the nucleus accumbens independent of clearance mechanisms. Importantly, sustained increases in G-CSF were required for these effects as acute exposure did not enhance behavioral outcomes and decreased dopamine release. These effects seem to be a result of the ability of G-CSF to alter local inflammatory signaling cascades, particularly tumor necrosis factor α. Together, these data show G-CSF as a potent modulator of the mesolimbic dopamine circuit and its ability to appropriately attend to salient stimuli.SIGNIFICANCE STATEMENT Emerging evidence has highlighted the importance of the immune system in psychiatric diseases states. However, the effects of peripheral cytokines on motivation and cognitive function are largely unknown. Here, we report that granulocyte-colony stimulating factor (G-CSF), a pleiotropic cytokine with known trophic and neuroprotective properties in the brain, acts directly on dopaminergic circuits to enhance their function. These changes in dopaminergic dynamics enhance reward learning and motivation for natural stimuli. Together, these results suggest that targeting immune factors may provide a new avenue for therapeutic intervention in the multiple psychiatric disorders that are characterized by motivational and cognitive deficits.

Microglial dopamine receptor elimination defines sex-specific nucleus accumbens development and social behavior in adolescent rats

Adolescence is a developmental period in which the mesolimbic dopaminergic "reward" circuitry of the brain, including the nucleus accumbens (NAc), undergoes significant plasticity. Dopamine D1 receptors (D1rs) in the NAc are critical for social behavior, but how these receptors are regulated during adolescence is not well understood. In this report, we demonstrate that microglia and complement-mediated phagocytic activity shapes NAc development by eliminating D1rs in male, but not female rats, during adolescence. Moreover, immune-mediated elimination of D1rs is required for natural developmental changes in male social play behavior. These data demonstrate for the first time that microglia and complement-mediated immune signaling (i) participate in adolescent brain development in a sex-specific manner, and (ii) are causally implicated in developmental changes in behavior. These data have broad implications for understanding the adolescent critical period of development, the molecular mechanisms underlying social behavior, and sex differences in brain structure and function.

Chronic ethanol consumption: role of TLR3/TRIF-dependent signaling

Chronic ethanol consumption stimulates neuroimmune signaling in the brain, and Toll-like receptor (TLR) activation plays a key role in ethanol-induced inflammation. However, it is unknown which of the TLR signaling pathways, the myeloid differentiation primary response gene 88 (MyD88) dependent or the TIR-domain-containing adapter-inducing interferon-β (TRIF) dependent, is activated in response to chronic ethanol. We used voluntary (every-other-day) chronic ethanol consumption in adult C57BL/6J mice and measured expression of TLRs and their signaling molecules immediately following consumption and 24 hours after removing alcohol. We focused on the prefrontal cortex where neuroimmune changes are the most robust and also investigated the nucleus accumbens and amygdala. Tlr mRNA and components of the TRIF-dependent pathway (mRNA and protein) were increased in the prefrontal cortex 24 hours after ethanol and Cxcl10 expression increased 0 hour after ethanol. Expression of Tlr3 and TRIF-related components increased in the nucleus accumbens, but slightly decreased in the amygdala. In addition, we demonstrate that the IKKε/TBK1 inhibitor Amlexanox decreases immune activation of TRIF-dependent pathway in the brain and reduces ethanol consumption, suggesting the TRIF-dependent pathway regulates drinking. Our results support the importance of TLR3 and the TRIF-dependent pathway in ethanol-induced neuroimmune signaling and suggest that this pathway could be a target in the treatment of alcohol use disorders.

[Effects of repeated electroacupuncture on gene expression of cannabinoid receptor-1 and dopamine 1 receptor in nucleus accumbens-caudate nucleus region in inflammatory-pain rats]

OBJECTIVE

To observe the effect of repeated electroacupuncture (EA) on the expression of cannabinoid receptor-1 (CB 1) mrRNA and dopamine 1 receptor (D 1) mRNA in Nucleus Accumbens (NAC)-Caudate Nucleus (CN) region in inflammatory-pain rats, so as to study its underlying mechanism in analgesia.

METHODS

A total of 30 SD rats were randomized into normal control, model, EA, EA+ AM 251 and WIN 55212-2 groups, with 6 cases in each group. EA (2 Hz/100 Hz, 1 -3 mA) was applied to "Zusanli"(ST 36) and "Kunlun"(BL 60) for 30 min, once every other day, and 4 sessions all together. Arthritis model was established by injection of Freund's complete adjuvant 0.05 mL in the rat's left ankle. Thermal pain threshold (paw withdrawal latency, PWL) was detected before and after modeling and after repeated EA and/or intraperitoneal injection of AM 251(an inverse antagonist at the CB 1 cannabinoid receptor, 0. 1 mg/100 g) and WIN 55212-2 (a potent cannabinoid receptor agonist, 0. 2 mg/100 g). The expression of CB 1 receptor mRNA and D 1 receptor mRNA in the NAC-CN region was measured by real time fluorescence quantitative-polymerase chain reaction.

RESULTS

Compared with the control group, the pain threshold values of the model group was decreased significantly (P<0.01). In comparison with the model group, the pain threshold values of the EA group and WIN 55212-2 group were increased considerably on day 10 (P<0. 01). No significant differences were found between the EA+ AM 251 and model groups and between the EA and WIN 55212-2 groups in PWL after the treatment (P>0.05). Compared with the control group, both CB 1 R mRNA and D 1 R mRNA expression levels in the model group were increased slightly, while in comparison with the model group and EA+ AM 251 group, CB 1 R mRNA and D 1 R mRNA expression levels in the EA group and WIN 55212-2 group were upregulated obviously. No significant differences were found between the EA + AM 251 and model groups and between the EA and WIN 55212-2 groups in CB 1 R mRNA and D 1 R mRNA expression levels.

EVIDENCE FOR A ROLE OF DOPAMINERGIC MECHANISMS IN THE IMMUNOSTIMULATING EFFECT OF µ-OPIOID RECEPTOR AGONIST DAGO

The present study demonstrates that: (1) activation of micro -opioid receptors by systemic administration of a highly selective agonist DAGO (100 microg/kg) results in a significant increase in the number of plaque- and rosette-forming cells in the spleen of CBA mice as well as Wistar rats on the 5th day following sheep red blood cells (5 x 10(8)) immunization, (2) the immunostimulatory effect of DAGO is mediated by central mechanisms including the hypothalamus-hypophysis complex; (3) the postsynaptic dopamine (DA) receptors of D2 type are involved in the DAGO-induced immunostimulation since the combined treatment of animals with haloperidol (2 mg/kg), a blocker of DA D2 receptors, and DAGO abolished this effect; (4) the nuclei caudatus and accumbens of the nigrostriatal and mesolimbic DAergic systems, respectively, are implicated in the immune response stimulation caused by DAGO.

Involvement of dopamine D1 and D2 receptors in the rat nucleus accumbens in immunostimulation

Analysis of the nature of changes in the immune response in operated Wistar rats showed that electrolytic lesioning of the nucleus accumbens, the site of the greatest density of dopamine D1 and D2 receptors, led to suppression of the immune response in animals immunized with sheep erythrocytes. Administration of SKF 38393 (20 mg/kg) and quinpirol (1 mg/kg), selective agonists of dopamine D1 and D2 receptors respectively, to sham-operated rats induced significant increases in immune responses. However, no immunostimulation was seen on administration of the selective dopamine D2 agonist quinpirol to animals with lesions to the nucleus accumbens as compared with controls. At the same time, treatment of animals with nucleus accumbens lesions using the dopamine D1 receptor agonist SKF 38393 had no effect on the immune response as compared with that in sham-operated animals given the D1 receptor agonist. These data provide evidence that dopamine D2 receptors in the nucleus accumbens have a role in the mechanisms of immunostimulation, though D2 receptors in other brain structures may also make some contribution to this process; D1 receptors in the nucleus accumbens make no significant contribution to controlling the immune response.

Neuropeptide Y Y1 receptors mediate morphine-induced reductions of natural killer cell activity

Morphine suppresses a number of immune parameters, such as natural killer (NK) cell activity and lymphocyte proliferation, by acting through mu-opioid receptors in the central nervous system. Prior studies have implicated the sympathetic nervous system in mediating the immunomodulatory effects of acute morphine treatment. However, the peripheral mechanism whereby morphine inhibits NK cell activity is not fully understood. The aim of the present study was to investigate the role of the sympathetic transmitter neuropeptide Y (NPY) in mediating morphine-induced immune alterations. The results showed that administration of the selective NPY Y1 receptor antagonist BIBP3226 blocked morphine's effect on splenic NK activity but did not attenuate the suppression splenocyte proliferative responses to Con-A or LPS. Furthermore, intravenous NPY administration produced a dose-dependent inhibition of splenic NK activity but did not suppress lymphocyte proliferation. Recent studies from our laboratory have demonstrated that morphine modulates NK activity through a central mechanism that requires the activation of dopamine D1 receptors in the nucleus accumbens. Results from the present study showed that microinjection of the D1 receptor agonist SKF-38393 into the nucleus accumbens shell induced a suppression of NK activity that was reversed by BIBP3226. Collectively, these findings demonstrate that NPY Y1 receptors mediate morphine's suppressive effect on NK activity and further suggest that opioid-induced increases in nucleus accumbens D1 receptor activation inhibit splenic NK activity via NPY released from the sympathetic nervous system.

Suppression of natural killer cell activity by morphine is mediated by the nucleus accumbens shell

Despite a wealth of data indicating that morphine modulates immune status by acting at mu-opioid receptors in the brain, there is little known about how the opioid system interacts with other neurotransmitter systems to modulate specific immune parameters. The aim of the present study was to investigate whether dopaminergic projections to the nucleus accumbens are involved in morphine-induced suppression of splenic natural killer (NK) cell activity. The results indicate that administration of the dopamine D1 antagonist SCH-23390 into the nucleus accumbens shell, but not core, blocked morphine's suppressive effect on NK activity in male Lewis rats. In support of these findings, the effect of morphine was also prevented by intra-accumbens microinfusions of the dopaminergic immunotoxin anti-DAT-saporin. Additionally, administration of the D1 agonist SKF-38393 into the nucleus accumbens shell produced reductions in splenic NK activity comparable to morphine, suggesting a critical role for D1 receptors in the modulation of NK activity. Collectively, these findings demonstrate that dopaminergic inputs to the nucleus accumbens are critically involved in opioid-induced immunosuppression and suggest that opioid-induced increases in D1 receptor activation may have adverse consequences on immune status.

[The role of D1 and D2 dopamine receptors of the rat nucleus accumbens in immunostimulation]

The analysis of the immune response changes in Wistar rats has shown that bilateral electrolytic lesions of the nucleus accumbens characterized by a high density of D1 an D2 dopamine (DA) receptors resulted in a decrease of the immune response to SRBC. Administration of selective agonists of D1 and D2 DA receptors to sham-operated animal: 20 mg/kg of SKF 38393 or 1.0 mg/kg of quinpirol, respectively, produced significant enhancement of plaque- and rosette-formation. However, the immune response level in the damaged rats did not increase following quinpirol administration, but was maintained at control values, rather. At the same time, activation of D1 DA receptors in rats with destructed nucleus accumbens did not affect the immune response level as compared to that of sham-operated animals receiving SKF 38393. The data obtained give evidence of involvement of D2 DA receptors of the nucleus accumbens in immunomodulation, although D2 DA receptors of other brain structures may also contribute to this process. D1 DA receptors of this localization seem not to play any important role in the immune response control.

Attenuation of cocaine-induced conditioned place preference by Polygala tenuifolia root extract

A recent investigation indicated that Polygala tenuifolia Willdenow extract (PTE) possesses a potential antipsychotic effect. In this study, we examined the effects of PTE on the cocaine-induced changes in locomotor activity, conditioned place preference (CPP), fos-related antigen-immunoreactivity (FRA-IR), and activator protein (AP)-1 DNA binding activity. Cocaine-induced behavioral effects (hyperlocomotion and CPP) occurred in parallel with increases in FRA-IR and AP-1 DNA binding activity in the nucleus accumbens. These responses induced by cocaine were consistently attenuated by concurrent treatment with PTE (25 mg or 50 mg/kg/day, i.p. x 7). The adenosine A2A receptor antagonist, 1,3,7-trimethyl-8-(3-chlorostyrl)xanthine (0.5 or 1.0 mg/kg, i.p.), reversed the PTE-mediated pharmacological action in a dose related manner; neither the adenosine A(1) receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (0.5 or 1.0 mg/kg, i.p.) nor the A2B receptor antagonist, alloxazine (1.5 or 3.0 mg/kg, i.p.) significantly affected this pharmacological action. Our results suggest that PTE prevents cocaine-induced behavioral effects, at least in part, via the activation of the adenosine A2A receptor.

Haloperidol reduces IgG immunoreactivity in the rat brain

Immunohistochemical staining with non-specific IgG reliably labels a subset of neurons in both rat and human brain, overlapping the distribution of cortico-limbic D2 receptors (D2R). We used haloperidol to up-regulate rat D2R to observe any associated changes in IgG immunostaining. Rats were treated with haloperidol or vehicle for 30 d. Some rats were assessed for D2R up-regulation with apomorphine. The remaining rats were processed immunohistochemically and IgG-stained cells counted and statistically analysed in three cortico-limbic areas. Other brain regions were surveyed qualitatively. Positive (anti-glial fibrillary acidic protein or anti-tyrosine hydroxylase staining) and negative (antisera omitted) controls were performed on adjacent sections. Haloperidol dramatically the IgG staining areas quantified with all surveyed regions significantly decreased. Positive control staining was robust, ruling out generalized immunoreactivity reduction. These observations raise the possibility of an immunological effect of haloperidol in the brain. The identification and function of these IgG-labelled sites may have useful implications for psychosis.

Involvement of the nucleus accumbens in stimulation of the immune response in rats after activation of opioid mu receptors with DAGO

The involvement of the nucleus accumbens in neuroimmunostimulation was demonstrated during activation of opioid mu receptors with the selective agonist DAGO (100 microg/kg); single doses of this agent to sham-operated (control) Wistar rats induced significant increases in the numbers of direct IgM antibody-forming and total rosette-forming cells after immunization with sheep erythrocytes. Bilateral electrolytic lesioning of the nucleus accumbens in rats led to sharp decreases in the intensity of immune responses; there was no immunostimulation after administration of DAGO to these animals. These data provide evidence for the involvement of the nucleus accumbens in the process of immunomodulation and for the importance of opioid mu receptors in the nucleus accumbens in the stimulation of immunogenesis.

Double immunocytochemistry of vasoactive intestinal peptide and cGnRH-I in male quail: photoperiodic effects

Reproduction in Japanese quail is primarily regulated by photoperiod. Vasoactive intestinal peptide (VIP) has been suggested as a transducer of environmental information, especially photoperiodic cues, to the hypothalamo-pituitary-gonadal axis. To investigate the possible interaction of VIP and the reproductive (gonadotropin-releasing hormone, GnRH) system, double-immunocytochemical staining for VIP and cGnRH-I was conducted in sexually mature male quail held under a long-day photoperiod (16L:8D; LD) and in sexually quiescent males held under a short-day photoperiod (8L:16D; SD). VIP-immunoreactive (ir) cells were found primarily in three locations: lateral septal organ (LSO) in nucleus accumbens (Ac), ventral hypothalamus, and infundibular area. VIP-ir cells in LSO displayed characteristics typical of cerebrospinal fluid (CSF)-contacting cells, and co-existed with cGnRH-I-ir cells and beaded fibers. In contrast, VIP-ir cells in the infundibular area did not co-exist with cGnRH-I-ir structures. The number of visible VIP-ir cells in the infundibular area of SD males was significantly lower than that of LD males, while the number of visible VIP-ir cells in Ac/LSO was not altered by photoperiod. A cluster of cGnRH-I-ir cells in the caudalmost septal area was heavily innervated by VIP-ir fibers, which appeared to contact cGnRH-I-ir cells directly at this location. Both VIP- and cGnRH-I-ir fibers heavily innervated the external layer of the median eminence (ME). These data suggest that Ac/LSO, the caudalmost septal area, and ME are possible sites of interaction between the VIP and the GnRH systems.

Clozapine decreases neuropeptide Y-like immunoreactivity and neuropeptide Y mRNA levels in rat nucleus accumbens

The aim of this study was to evaluate the effect of acute, subchronic (14 days) and chronic (28 days) intraperitoneal (i.p.) administration of clozapine (10 or 25 mg/kg) on neuropeptide Y (NPY) system activity in the nucleus accumbens of the rat. NPY-like immunoreactivity (NPY-LI) decreased 24 h after subchronic clozapine while NPY mRNA after both acute and subchronic clozapine treatment. NPY-LI levels were also reduced 8 days after cessation of chronic lower-dose treatment. Subchronic (14 days) administration of the 5-HT2A antagonist ketanserin (1 mg/kg i.p.) or the dopamine D2/D3 antagonist (+/-) sulpiride (100 mg/kg i.p.) reduced NPY-LI levels, whereas the dopamine D1-like antagonist SCH 23390 (0.5 mg/kg i.p.), dopamine D4 antagonist L-745,870 (1 mg/kg per os), and alpha1-adrenergic antagonist prazosin (0.2 mg/kg i.p.) had no effect. There were no significant differences between the ketanserin-induced decrease in NPY-LI levels and the effects of the following two-drug combinations: ketanserin and SCH 23390, ketanserin and L-745,870, and ketanserin and prazosin. The study has shown that clozapine reduces NPY system activity in the rat nucleus accumbens. It seems that the action of clozapine is partly mediated by blockade of 5-HT2A and D2/D3 dopaminergic receptors.

Pharmacological studies of the regulation of chronic FOS-related antigen induction by cocaine in the striatum and nucleus accumbens

Previous work has demonstrated that chronic administration of cocaine induces apparently novel Fos-like transcription factors, termed chronic Fras (Fos-related antigens), in the rat striatum and nucleus accumbens. Induction of these proteins is associated with prolonged increases in AP-1 DNA binding activity that parallel the long half-life of the chronic Fras in brain. The goal of the present study was to characterize pharmacologically the regulation of chronic Fra induction by cocaine. Chronic Fra induction was examined with respect to the cocaine dose, time course and administration intervals used. Cocaine was found to induce the chronic Fras over widely differing treatment regimens in the striatum and nucleus accumbens, although clear differences between the two brain regions were observed. In general, maximal induction occurred with moderate treatment conditions, with more or less intensive treatments resulting in lower levels of chronic Fras. The pharmacological mechanisms underlying cocaine induction of the chronic Fras were also investigated. Pretreatment with a D1 receptor antagonist, which did not affect chronic Fra levels by itself, attenuated cocaine induction of the chronic Fras in striatum and nucleus accumbens. In contrast, treatment with a D2 receptor antagonist alone greatly induced chronic Fra levels, with no further increase seen in response to combined treatment with cocaine. Combined treatment with D1 and D2 receptor agonists, or with amphetamine, led to a strong induction of chronic Fras. Similarly, repeated treatment with a specific dopamine transporter inhibitor increased chronic Fra levels, whereas treatment with a specific serotonin or norepinephrine transporter inhibitor failed to produce this effect. These results support an important role for dopaminergic neurotransmission in the induction of chronic Fras by cocaine. Taken together, the results of the present study provide a more complete understanding of the pharmacological properties underlying cocaine regulation of the chronic Fras, which will assist in identifying the functional role played by these proteins in cocaine action.

The effect of clozapine on Fos protein immunoreactivity in the rat forebrain is not mimicked by the addition of alpha 1-adrenergic or 5HT2 receptor blockade to haloperidol

The involvement of alpha 1-adrenergic and 5HT2-receptor blockade in the induction of Fos protein produced by the 'atypical' neuroleptic clozapine was investigated in the rat forebrain. The Fos protein immunohistochemical technique has been used to identify the anatomical substrate underlying the effects of typical and atypical neuroleptics. Clozapine (20 mg/kg) induced a significantly higher Fos protein immunoreactivity response in the medial prefrontal cortex and a significantly lower response in the dorsolateral striatum compared to the effect of haloperidol (1 mg/kg). The alpha 1-adrenergic antagonist prazosin (0.3 and 1.0 mg/kg) and the 5HT2 antagonist ritanserin (1 and 3 mg/kg) did not increase Fos protein immunoreactivity by themselves and did not mimic the clozapine response when co-administered with haloperidol (1 mg/kg). Consequently, this study suggests that neither alpha 1-adrenergic receptor blockade nor the 5HT2-receptor blockade accounts for the unique Fos protein expression pattern produced by clozapine.

D1 and D2 dopamine receptors differentially increase Fos-like immunoreactivity in accumbal projections to the ventral pallidum and midbrain

Alterations in dopaminergic neurotransmission have profound effects on neuronal expression of the putative activity marker, Fos, in both the dorsal and ventral striatum. Stimulants such as D-amphetamine and cocaine increase Fos-like immunoreactivity by enhancing the activation of D1 dopamine receptors. In contrast, neuroleptics such as haloperidol and raclopride increase Fos-like immunoreactivity by blocking striatal D2 dopamine receptors. In the dorsal striatum, D1 receptor stimulation elevates Fos-like immunoreactivity predominantly in neurons projecting to the midbrain (substantia nigra), whereas D2 receptor antagonism enhances Fos-like immunoreactivity principally in neurons projecting to the pallidum (globus pallidus). These findings are consistent with the proposal that D1 receptors are located chiefly on striatonigral neurons, whereas D2 receptors reside mainly on striatopallidal neurons. Since the nucleus accumbens (largest component of the ventral striatum) also sends projections to the midbrain (ventral tegmental area and substantia nigra) and pallidum (ventral pallidum), the present study utilized retrograde tract-tracing techniques to determine if there was a similar segregation of D1 agonist- and D2 antagonist-induced Fos-like immunoreactivity in these accumbal projections. In addition, we examined whether these relationships were the same in the core and shell regions of the nucleus accumbens. Like the dorsal striatum, D1 agonists (D-amphetamine and CY 208-243), but not D2 antagonists (haloperidol and clozapine), increased Fos-like immunoreactivity in accumbal neurons projecting to the midbrain (ventral tegmental area and substantia nigra). Also like the dorsal striatum, D2 antagonist-induced Fos-like immunoreactivity was located preferentially in accumbal neurons projecting to the pallidum (ventral pallidum). However, unlike the dorsal striatum, where the vast majority of neurons which display D1 agonist-induced Fos-like immunoreactivity project to the midbrain, nearly 50% of those neurons in the nucleus accumbens which were Fos-immunoreactive after D-amphetamine or CY 208-243 projected to the ventral pallidum. Thus, a similar number of accumbal neurons which expressed D1 agonist-induced Fos-like immunoreactivity were retrogradely labelled from the midbrain and ventral pallidum. Accumbal projections to the midbrain and ventral pallidum were retrogradely labelled with different retrograde tracers in order to determine the degree of collateralization between these pathways. Approximately 20% of retrogradely labelled neurons displayed both tracers, indicating that collateralization and damage to fibres of passage could not account for all of those cases in which D1 agonist-induced Fos-like immunoreactivity was detected in accumbal neurons projecting to the ventral pallidum.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural study of CCK and tyrosine hydroxylase immunoreactivity in the rat nucleus accumbens

The cholecystokinin (CCK)- and tyrosine hydroxylase (TH)-like immunoreactive (LI) axons and boutons were studied in the caudal and medial parts of the rat nucleus accumbens (NAC), using the indirect immunoperoxidase technique, at the electron microscopic level. Both CCK- and TH-LI boutons contained clear synaptic vesicles and large granular vesicles of similar size, but the CCK-LI boutons contained more large granular vesicles than TH-LI boutons. The CCK-LI and TH-LI boutons were heterogeneous. This finding might be related to the various immunoreactive neuronal types innervating the caudomedial NAC. However, the CCK-LI boutons (containing mostly small, round, clear synaptic vesicles) formed mainly asymmetrical synaptic contacts with dendritic spines whereas the TH-LI boutons (containing medium-sized as well as small, round, clear synaptic vesicles) formed mostly symmetrical synaptic contacts with dendritic shafts.

Ultrastructural characteristics of enkephalin-immunoreactive boutons and their postsynaptic targets in the shell and core of the nucleus accumbens of the rat

The present study compared the ultrastructural morphology of enkephalin-immunoreactive boutons and their postsynaptic targets in different territories of the nucleus accumbens in the rat. The synaptic bouton profiles were identified by antibodies directed against [leu5]enkephalin. Ninety-five percent of the synaptic contacts were symmetric in configuration and the remaining 5% were asymmetric. Axosomatic contacts comprised 6% of all enkephalin-immunoreactive junctions and were distributed equally in all parts of the nucleus. Most (76%) synaptic terminals contacted dendrites but they contacted proportionally fewer dendrites in the shell (71%) than in the core (78%). Moreover, enkephalin-immunoreactive synaptic boutons in the shell (19%) and caudal enkephalin-rich areas (17%) of the core contacted twice as many spines than in the remaining parts of the core (8.5%). In the core, long pallidum-like dendrites were occasionally found ensheathed in enkephalin-immunoreactive terminal boutons. We conclude that the differential arrangement of enkephalinergic contacts in the shell and core could have important functional consequences, especially when considered in relation to other known morphological and neurochemical differences between these regions.