Loading...
(1/1985) Cellular sites for dynorphin activation of kappa-opioid receptors in the rat nucleus accumbens shell.

The nucleus accumbens (Acb) is prominently involved in the aversive behavioral aspects of kappa-opioid receptor (KOR) agonists, including its endogenous ligand dynorphin (Dyn). We examined the ultrastructural immunoperoxidase localization of KOR and immunogold labeling of Dyn to determine the major cellular sites for KOR activation in this region. Of 851 KOR-labeled structures sampled from a total area of 10,457 microm2, 63% were small axons and morphologically heterogenous axon terminals, 31% of which apposed Dyn-labeled terminals or also contained Dyn. Sixty-eight percent of the KOR-containing axon terminals formed punctate-symmetric or appositional contacts with unlabeled dendrites and spines, many of which received convergent input from terminals that formed asymmetric synapses. Excitatory-type terminals that formed asymmetric synapses with dendritic spines comprised 21% of the KOR-immunoreactive profiles. Dendritic spines within the neuropil were the major nonaxonal structures that contained KOR immunoreactivity. These spines also received excitatory-type synapses from unlabeled terminals and were apposed by Dyn-containing terminals. These results provide ultrastructural evidence that in the Acb shell (AcbSh), KOR agonists play a primary role in regulating the presynaptic release of Dyn and other neuromodulators that influence the output of spiny neurons via changes in the presynaptic release of or the postsynaptic responses to excitatory amino acids. The cellular distribution of KOR complements those described previously for the reward-associated mu- and delta-opioid receptors in the Acb shell.  (+info)

(2/1985) Effects of stimulants of abuse on extrapyramidal and limbic neuropeptide Y systems.

Neuropeptide Y (NPY), an apparent neuromodulating neuropeptide, has been linked to dopamine systems and dopamine-related psychotic disorders. Because of this association, we determined and compared the effects of psychotomimetic drugs on extrapyramidal and limbic NPY systems. We observed that phencyclidine, methamphetamine (METH), (+)methylenedioxymethamphetamine (MDMA), and cocaine, but not (-)MDMA, similarly reduced the striatal content of NPY-like immunoreactivity from 54% (phencyclidine) to 74% [(+) MDMA] of control. The effects of METH on NPY levels in the nucleus accumbens, caudate nucleus, globus pallidus, and substantia nigra were characterized in greater detail. We observed that METH decreased NPY levels in specific regions of the nucleus accumbens and the caudate, but had no effect on NPY in the globus pallidus or the substantia nigra. The dopamine D1 receptor antagonist SCH-23390 blocked these effects of METH, suggesting that NPY levels throughout the nucleus accumbens and the caudate are regulated through D1 pathways. The D2 receptor antagonist eticlopride did not appear to alter the METH effect, but this was difficult to determine because eticlopride decreased NPY levels by itself. A single dose of METH was sufficient to lower NPY levels, in some, but not all, regions examined. The effects on NPY levels after multiple METH administrations were substantially greater and persisted up to 48 h after treatment; this suggests that synthesis of this neuropeptide may be suppressed even after the drug is gone. These findings suggest that NPY systems may contribute to the D1 receptor-mediated effects of the psychostimulants.  (+info)

(3/1985) Ethanol exposure differentially alters central monoamine neurotransmission in alcohol-preferring versus -nonpreferring rats.

Individual differences in ethanol preference may be linked to differences in the functional activity of forebrain monoamine systems or their sensitivity to modification by ethanol. To test this hypothesis, basal extracellular concentrations of dopamine (DA) and serotonin (5-HT) in the nucleus accumbens as well as the effects of repeated ethanol pretreatment on the basal release of these transmitters were examined in alcohol-preferring (P), alcohol-nonpreferring (NP), and genetically heterogeneous Wistar rats. All animals received i.p. injections of ethanol (1.0 g/kg) or saline for 5 consecutive days. Fifteen hours after the final pretreatment, basal extracellular concentrations and "in vivo extraction fraction" values for DA and 5-HT were determined by no-net-flux in vivo microdialysis. In ethanol-naive rats, significant line differences were observed with high basal 5-HT release in P rats, low 5-HT release in NP rats, and intermediate 5-HT levels in Wistar rats. No differences among groups were noted in basal DA release. Ethanol pretreatment decreased basal extracellular 5-HT levels in P rats whereas increasing 5-HT efflux was seen in the Wistar and NP lines. In addition, ethanol pretreatment increased extracellular DA concentrations in Wistar and P rats, but not in NP rats. The results confirm a relationship between the functional status of forebrain DA and 5-HT systems and ethanol preference or aversion. Moreover, the data suggest that ethanol exposure can alter basal DA and 5-HT in the nucleus accumbens and that vulnerability to ethanol-induced changes in monoamine neurotransmission may be a factor in genetically determined ethanol preference.  (+info)

(4/1985) Age-related reductions in [3H]WIN 35,428 binding to the dopamine transporter in nigrostriatal and mesolimbic brain regions of the fischer 344 rat.

In the present study, we used the potent cocaine analog [3H]WIN 35, 428 to map and quantify binding to the dopamine transporter (DAT) within the dorsal striatum, nucleus accumbens, substantia nigra, and ventral tegmental area in young (6-month-old), middle-aged (12-month-old), and aged (18- and 24-month-old) Fischer 344 rats. Quantitative autoradiographic analysis of indirect [3H]WIN 35,428 saturation curves revealed two-site binding for all four brain regions in every age group. The percentage of binding to the high- or low-affinity sites did not differ with age or region and was approximately 50%. However, significant age-related decreases in the overall density (Bmax) of [3H]WIN 35,428-binding sites were observed in the striatum, nucleus accumbens, substantia nigra, and ventral tegmental area. The Bmax within all brain regions declined by more than 15% every 6 months, with the Bmax in the aged (24-month-old) group being approximately half that measured in the young adult (6-month-old) group. Competition experiments indicated that nomifensine also exhibited two-site binding to the DAT in Fischer 344 rats. No consistent age-related differences in binding affinities were noted with either [3H]WIN 35,428 or nomifensine. Taken together, these results support the hypothesis that functional DATs within the nigrostriatal and mesolimbic systems are down-regulated with age, without changing their affinity for ligands.  (+info)

(5/1985) Effects of mCPP on the extracellular concentrations of serotonin and dopamine in rat brain.

Intravenous administration of m-chloro-phenylpiperazine (mCPP) (0.25 or 2.5 mg/kg) induced a marked and dose-related increase in extracellular concentrations of serotonin in hippocampus (300-1,400% of baseline) as measured using in vivo microdialysis in awake male Wistar rats of the spontaneously hypertensive (SH) strain. Indicating that the effect of mCPP was caused by a reversal of the serotonin transporter, it was antagonized by pretreatment with the serotonin re-uptake inhibitor citalopram (10 mg/kg) but was unaffected by local administration of the sodium channel blocker tetrodotoxin (TTX; 1 microns). mCPP was also shown to induce an increase in extracellular concentrations of dopamine in the nucleus accumbens and the striatum of SH rats and in the nucleus accumbens of rats of the Sprague-Dawley (SD) strain; this effect of mCPP was, however, much weaker (125-170% of baseline) than the effect on serotonin; moreover, it seems to be TTX-sensitive. In anesthetized SD rats, mCPP induced a moderate reduction of nigral dopamine cell firing rate; supporting the assumption that this effect is secondary to the observed increase in dopamine release, it was blocked by pretreatment either with the dopamine synthesis inhibitor alpha-methyl-para-tyrosine or with the dopamine D2 receptor antagonist haloperidol. In conclusion, the results suggest that mCPP induces a marked, TTX-insensitive increase in serotonin release in rat brain, but only a modest and TTX-sensitive increase in the extracellular levels of dopamine.  (+info)

(6/1985) Dissociation in effects of lesions of the nucleus accumbens core and shell on appetitive pavlovian approach behavior and the potentiation of conditioned reinforcement and locomotor activity by D-amphetamine.

Dopamine release within the nucleus accumbens (NAcc) has been associated with both the rewarding and locomotor-stimulant effects of abused drugs. The functions of the NAcc core and shell were investigated in mediating amphetamine-potentiated conditioned reinforcement and locomotion. Rats were initially trained to associate a neutral stimulus (Pavlovian CS) with food reinforcement (US). After excitotoxic lesions that selectively destroyed either the NAcc core or shell, animals underwent additional CS-US training sessions and then were tested for the acquisition of a new instrumental response that produced the CS acting as a conditioned reinforcer (CR). Animals were infused intra-NAcc with D-amphetamine (0, 1, 3, 10, or 20 microg) before each session. Shell lesions affected neither Pavlovian nor instrumental conditioning but completely abolished the potentiative effect of intra-NAcc amphetamine on responding with CR. Core-lesioned animals were impaired during the Pavlovian retraining sessions but showed no deficit in the acquisition of responding with CR. However, the selectivity in stimulant-induced potentiation of the CR lever was reduced, as intra-NAcc amphetamine infusions dose-dependently increased responding on both the CR lever and a nonreinforced (control) lever. Shell lesions produced hypoactivity and attenuated amphetamine-induced activity. In contrast, core lesions resulted in hyperactivity and enhanced the locomotor-stimulating effect of amphetamine. These results indicate a functional dissociation of subregions of the NAcc; the shell is a critical site for stimulant effects underlying the enhancement of responding with CR and locomotion after intra-NAcc injections of amphetamine, whereas the core is implicated in mechanisms underlying the expression of CS-US associations.  (+info)

(7/1985) Behavioral and neurochemical effects of the dopamine transporter ligand 4-chlorobenztropine alone and in combination with cocaine in vivo.

The current studies evaluated the novel diphenylmethoxytropane analog 4-chlorobenztropine (4-Cl-BZT), cocaine, and combinations of the two drugs for their abilities to stimulate locomotor activity, produce cocaine-like discriminative stimulus effects, and elevate extracellular dopamine (DA) in the nucleus accumbens (NAc) as measured by in vivo microdialysis. Peripherally administered cocaine was approximately twice as efficacious as 4-Cl-BZT as a locomotor stimulant and was behaviorally active at a lower dose than was 4-Cl-BZT. Cocaine also was more efficacious than 4-Cl-BZT in producing discriminative-stimulus effects in rats trained to discriminate i.p. injections of 10 mg/kg cocaine from saline. The time course of behavioral activation differed markedly between the two drugs, with much shorter onset and duration of locomotor stimulant effects for cocaine relative to 4-Cl-BZT. Similarly, i.p. cocaine (10 and 40 mg/kg) induced a pronounced, rapid, and short-lived increase in DA in the NAc, whereas i.p. 4-Cl-BZT was effective only at the higher dose and produced a more gradual, modest, and sustained (>/=2 h) elevation in accumbens DA. In contrast to i.p. administration, local infusion of 4-Cl-BZT (1-100 microM) into the NAc through the microdialysis probe elevated extracellular DA to a much greater extent than did local cocaine (nearly 2000% of baseline maximally for 4-Cl-BZT versus 400% of baseline for cocaine) and displayed a much longer duration of action than cocaine. However, when microinjected bilaterally into the NAc at 30 or 300 nmol/side, cocaine remained a more efficacious locomotor stimulant than 4-Cl-BZT. Finally, pretreatment with i.p. 4-Cl-BZT dose dependently enhanced the locomotor stimulant, discriminative stimulus effects, and NAc DA response to a subsequent low-dose i.p. cocaine challenge. The diphenylmethoxytropane analog also facilitated the emergence of stereotyped behavior and convulsions induced by high-dose cocaine. The current results demonstrate that DA transporter ligands that do not share the neurochemical and behavioral profiles of cocaine nevertheless may enhance the effects of cocaine in vivo.  (+info)

(8/1985) The regulation of dopamine transmission by metabotropic glutamate receptors.

Receptor subtype nonselective metabotropic glutamate receptor (mGluR) agonists have been shown to regulate the release of dopamine. The eight mGluR subtypes have been pharmacologically categorized into three groups, and the present study used in vivo microdialysis to examine the capacity of mGluR subgroup-selective drugs to modulate the extracellular levels of dopamine in the nucleus accumbens. By administering the drugs in the dialysis buffer, it was found that the group 3 mGluR agonist L-amino-4-phosphonobutyrate produced a dose-dependent reduction in extracellular dopamine, whereas the group 1 agonist 3,5-dihydroxyphenylglycine was ineffective. The group 2 agonist (2S,1'R,2'R,3'R)-2-(2, 3-dicarboxycyclopropyl)glycine produced a reduction that was biphasic with respect to dose. The group 2/3 antagonist alpha-methyl-4-phosphnophenylglycine elicited a dose-dependent increase in extracellular dopamine that was antagonized by coperfusion with either the L-type calcium channel blocker diltiazem or the group 3 agonist L-amino-4-phosphonobutyrate. These data demonstrate that group 3 and to a lesser extent group 2 mGluR may presynaptically regulate dopamine release or reuptake. Moreover, there exists significant in vivo glutamatergic tone on group 2/3 mGluRs to suppress extracellular dopamine levels.  (+info)