Dopamine receptor elevation by cholecystokinin

High concentrations of cholecystokinin (CCK) in the striatum and limbic areas of the brain suggest that this peptide may influence dopaminergic transmission. Thus, the effect of CCK on dopamine D₂ receptors in the striatum and nucleus accumbens of the rat brain both in vitro and in vivo (central and peripheral administration) was studied by determining the binding of ³H-spiperone. The density (B_max) of D₂ receptors was elevated (a) by 20% in the accumbens upon in vitro co-incubation with 10⁻⁶ M CCK. (A non-significant drop of 10% occurred in the striatum); (b) by about 40% in the accumbens and 25% in the striatum after continuous intraventricular infusion of CCK for 24 hr. The increase in receptor density in the accumbens was maintained for 14 days and in both tissues was specific to CCK (neurotensin infusion did not alter ³H-spiperone binding); (c) by 20% in the accumbens and 15% in the striatum 3 hr after a single IP injection of 50 μg/kg CCK or caerulein, and maintained up to 14 days later. These results suggest that CCK elevates dopamine D₂ receptors in the accumbens and striatum and may be a physiological modulator of the dopaminergic system.     

Synthesis of 2-(2,3-dimethoxyphenyl)-4-(aminomethyl)imidazole analogues and their binding affinities for dopamine D(2) and D(3) receptors.

A series of 2-(2,3-dimethoxyphenyl)-4-(aminomethyl)imidazole derivatives was prepared and their affinity for dopamine D₂ and D₃ receptors was measured using in vitro binding assays. Several oxadiazole analogues were also prepared and tested for their affinity for dopamine D₂ and D₃ receptors. The results of receptor binding studies indicated that the incorporation of an imidazole moiety between the phenyl ring and the basic nitrogen did not significantly increase the selectivity for dopamine D₃ receptors, whereas the incorporation of an oxadiazole at the same region resulted in a total loss of affinity for both dopamine receptor subtype binding sites. The most selective compound in this series is 2-(5-bromo-2,3-dimethoxyphenyl)-4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolinomethyl)imidazole (5i), which has a D₃ receptor affinity of 21 nM and a 7-fold selectivity for D₃ versus D₂ receptors. The binding affinity for σ₁ and σ₂ receptors was also measured, and the results showed that several analogues were selective σ₁ receptor ligands.     

Regulation of Extracellular Dopamine by the Norepinephrine Transporter

Abstract: There is growing evidence of an interaction between dopamine and norepinephrine. To test the hypothesis that norepinephrine terminals are involved in the uptake and removal of dopamine from the extracellular space, the norepinephrine uptake blocker desmethylimipramine (DMI) was infused locally while the extracellular concentrations of dopamine were simultaneously monitored. DMI increased the extracellular concentrations of dopamine in the medial prefrontal cortex and nucleus accumbens shell but had no effect in the striatum. The combined systemic administration of haloperidol and the local infusion of DMI produced an augmented increase in extracellular dopamine in the cortex compared with the increase produced by either drug alone. This synergistic increase in dopamine overflow is likely due to the combination of impulse-mediated dopamine release produced by haloperidol and blockade of the norepinephrine transporter. No such synergistic effects were observed in the nucleus accumbens and striatum. Local perfusion of the α₂-antagonist idazoxan also increased the extracellular concentrations of dopamine in the cortex. Although the stimulation of extracellular dopamine by idazoxan and DMI could be due to the increased extracellular concentrations of norepinephrine produced by these drugs, an increase in dopamine also was observed in lesioned rats that were depleted of norepinephrine and challenged with haloperidol. This contrasted with the lack of an effect of haloperidol on cortical dopamine in unlesioned controls. These results suggest that norepinephrine terminals regulate extracellular dopamine concentrations in the medial prefrontal cortex and to a lesser extent in the nucleus accumbens shell through the uptake of dopamine by the norepinephrine transporter.     

Kinetic evidence for isomerization of the dopamine receptor-raclopride complex

The binding kinetics of the specific dopamine D₂ antagonist [³H]raclopride to dopamine D₂ receptors in rat neostriatum were studied. The pseudo-first-order rate constants of [³H]raclopride binding with these membranes revealed a hyperbolic dependence upon the antagonist concentration, indicating that the reaction had at least two consecutive and kinetically distinguished steps. The first step was fast binding equilibrium, characterized by the dissociation constant K_A = 12 ± 2 nM. The following step corresponded to a slow isomerization of the receptor-antagonist complex, characterized by the isomerization equilibrium constant K_i = 0.11. The dissociation constant K_d = 1.3 nM, calculated from these kinetic data, was similar to K_d = 2.4 nM, determined from equilibrium binding isotherm for the radioligand. Implications of the complex reaction mechanism on dopamine D₂ receptor assay by [³H]raclopride were discussed.     

Activation of post-synaptic dopamine D₁ receptors promotes the release of tissue plasminogen activator in the nucleus accumbens via PKA signaling

We have previously demonstrated that tissue plasminogen activator (tPA) plays an important role through the conversion of plasminogen to plasmin in the release of dopamine in the nucleus accumbens (NAc) evoked by depolarization or the systemic administration of drugs of abuse such as morphine and nicotine. In the present study, we examined the mechanisms by which drugs of abuse increase extracellular tPA activity in the NAc in vivo using in situ zymography. The dopamine D₁ receptor (D₁R) agonist SKF38393, but not D₂ receptor agonist quinpirole, significantly increased extracellular tPA activity in the NAc. The effect of SKF38393 was blocked by pre-treatment with the dopamine D₁R antagonist SCH23390. Microinjection of Rp-cAMPs, a protein kinase A inhibitor, into the NAc completely blocked the effect of SKF38393. Systemic administration of morphine and methamphetamine increased extracellular tPA activity in the NAc, and these effects were completely blocked by pre-treatment with SCH23390 and raclopride. The results suggest that activation of post-synaptic dopamine D₁Rs in the NAc leads to an increase in extracellular tPA activity via protein kinase A signaling. Furthermore, dopamine D₂ receptors are also involved in the release of tPA induced by morphine and methamphetamine.     

Neurochemical Evidence that Postsynaptic Nucleus Accumbens D3 Receptor Stimulation Enhances Cocaine Reinforcement

Abstract: The mechanism by which two D₃ receptor-preferring agonists, 7-hydroxydipropylaminotetralin (7-OH-DPAT) and quinelorane, modulate cocaine reinforcement was examined by monitoring nucleus accumbens dopamine levels with in vivo microdialysis while rats intravenously self-administered the following four different drug solutions consecutively: (1) cocaine; (2) a combination of cocaine plus a low dose of either agonist; (3) either agonist alone; and finally, (4) a physiological saline solution. Both 7-OH-DPAT (4 µg/infusion) and quinelorane (0.25 µg/infusion) decreased cocaine (0.25 mg/infusion) intake in a manner indicating an enhancement of cocaine reinforcement and simultaneously decreased the cocaine-induced elevations in nucleus accumbens dopamine levels by >50%. Subsequent self-administration of either 7-OH-DPAT (4 µg/infusion) or quinelorane (0.25 µg/infusion) alone resulted in significant, but stable, increases in drug intake, with a concurrent decrease in nucleus accumbens dopamine levels to ∼50% below nondrug baseline levels. These findings indicate that postsynaptic D₃ receptor stimulation in the nucleus accumbens enhances the reinforcing properties of cocaine. In a second experiment, local application of 7-OH-DPAT via reverse dialysis (30 and 100 n M perfusate concentrations) dose-dependently decreased nucleus accumbens dopamine efflux to 76 ± 3.9 and 61 ± 6.3% of baseline, respectively, whereas there was no effect of this agonist on dopamine efflux in the ipsilateral striatum of these same animals. Coperfusion with the D₃ receptor-preferring antagonist nafadotride dose-dependently blocked the effect of 7-OH-DPAT on nucleus accumbens dopamine efflux. These results suggest that, at low concentrations, 7-OH-DPAT selectively activates D₃ receptors in vivo.     

INTRASTRIATAL ADMINISTRATION OF AN OLIGODEOXYNUCLEOTIDE ANTISENSE TO THE D2 DOPAMINE RECEPTOR mRNA INHIBITS D2 DOPAMINE RECEPTOR-MEDIATED BEHAVIOR AND D2 DOPAMINE RECEPTORS IN NORMAL MICE AND IN MICE LESIONED WITH 6-HYDROXYDOPAMINE

Previous studies have shown that the intracerebroventricular injection of antisense oligodeoxynucleotides targeted to the mRNAs encoding the different subtypes of dopamine receptors inhibited behaviors mediated by these receptors. The present studies were designed to determine whether such antisense oligodeoxynucleotides could produce similar effects when injected into a discrete brain area. A D₂ dopamine receptor antisense oligodeoxynucleotide (D₂ antisense) was repeatedly injected into one corpus striatum of either normal mice or mice with unilateral lesions of the striatum induced by 6-hydroxydopamine. In the latter, intrastriatal injection of D₂ antisense blocked the contralateral rotational behavior induced by the parenteral administration of the D₂ dopamine receptor agonist quinpirole. The inhibitory effect of D₂ antisense was dose- and time-related and was reversed upon cessation of D₂ antisense treatment. This inhibitory effect was also selective in that D₂ antisense treatment inhibited the rotational behavior induced by quinpirole but not that induced by the D₁ dopamine receptor agonist SKF 38393 or by the muscarinic cholinergic agonist oxotremorine. Following repeated intrastriatal injections of D₂ antisense into normal mice, parenteral administration of quinpirole caused rotational behavior ipsilateral to the side in which the D₂ antisense was injected. No such rotational behavior was seen when similarly treated mice were challenged with SKF 38393 or oxotremorine. The quinpirole-induced rotational behavior in mice given intrastriatal injections of D₂ antisense disappeared upon cessation of D₂ antisense treatment. Repeated intrastriatal administration of D₂ antisense also caused a significant reduction in the levels of D₂, but not D₁, dopamine receptors in striatum, as determined by receptor autoradiography. The levels of D₂ dopamine receptors returned to normal upon cessation of D₂ antisense treatment. Intrastriatal administration of an oligodeoxynucleotide with randomly placed nucleotides failed to alter the rotational response to quinpirole in either 6-hydroxydopamine-lesioned or normal mice and failed to alter the levels of D₂ dopamine receptors in striatum. These results show that selective inhibition of behavioral responses mediated by D₂ dopamine receptors can be achieved by the direct injection of a D₂ antisense oligodeoxynucleotide into a discrete brain area. Copyright © 1996 Elsevier Science Ltd     

Effects of endogenous glutamate on extracellular concentrations of taurine in striatum and nucleus accumbens of the awake rat: Involvement of NMDA and AMPA/kainate receptors

Summary. Using microdialysis, the effects of endogenous glutamate on extracellular concentrations of taurine in striatum and nucleus accumbens of the awake rat were investigated. The glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) was used to increase the extracellular concentration of glutamate. PDC (1, 2 and 4 mM) produced a dose-related increase of extracellular concentrations of glutamate and taurine in striatum and nucleus accumbens. Increases of extracellular taurine were significantly correlated with increases of extracellular glutamate, but not with PDC doses, which suggests that endogenous glutamate produced the observed increases of extracellular taurine in striatum and nucleus accumbens. The role of ionotropic glutamate receptors on the increases of taurine was also studied. In striatum, perfusion of the antagonists of NMDA and AMPA/kainate glutamate receptors attenuated the increases of extracellular taurine. AMPA/kainate, but not NMDA receptors, also reduced the increases of extracellular taurine in nucleus accumbens. These results suggest that glutamate-taurine interactions exist in striatum and nucleus accumbens of the awake rat. Received March 5, 1999/Accepted September 22, 1999     

Endogenous interaction of glutamate and dopamine in the basal ganglia of the awake rat during aging

The interaction of glutamate and dopamine in the basal ganglia of fully conscious rat during the normal process of aging is reviewed. Using a novel approach, that of blocking the reuptake of glutamate, the effects of increasing concentrations of endogenous glutamate on the extracellular concentrations of dopamine in striatum and nucleus accumbens in the young rat were investigated. It was found that increasing concentrations of glutamate correlated significantly with increasing concentrations of dopamine in striatum and nucleus accumbens. Moreover the increase of dopamine in both structures was significantly reduced after blockade of NMDA and AMPA/kainate glutamate receptors, suggesting that the increase of dopamine was mediated by glutamate. The interaction glutamate/dopamine expressed by its ratio showed a significant age-related decrease in nucleus accumbens but not in striatum, so that to a given amount of glutamate less increase of dopamine is produced. It is suggested that the interaction glutamate-dopamine represents a balanced input to the GABA neuron in the basal ganglia and that during aging this balance is disrupted. In addition, we also speculate on the significance of this glutamate-dopamine disruption in relation to the changes in motor behavior found with age.     

Comparison in changes of glutamate level in the rat nucleus accumbens induced by D1- and D2-dopamine receptors during feeding

The influence of dopamine D1- and D2-like receptors blockage on glutamate level in the n. accumbens of Sprague-Dawly rats during feeding was investigated by in vivo microdialysis combined with HPLC-EC analysis. Food intake resulted in a decrease in extracellular glutamate level. Infusion of D1-like dopamine receptor-blocker (SCH-23390, 0.01 mM) into the n. accumbens did not change this effect. Infusion of D2-like dopamine receptor-blocker (raclopride, 0.1 mM) into the n. accumbens caused an increase in extracellular glutamate level during feeding. The findings suggest, that decrease in extracellular glutamate level in n. accumbens is caused by dopamine D2-like, but not D1-like receptors activation.     

BIMG 80, a Novel Potential Antipsychotic Drug: Evidence for Multireceptor Actions and Preferential Release of Dopamine in Prefrontal Cortex

Abstract: In radioligand binding studies, BIMG 80, a new putative antipsychotic, displayed good affinity at certain serotonin (5-HT_1A, 5-HT_2A, 5-HT₆), dopamine (D₁, D_2L, D₄), and noradrenergic (α₁) receptors. The effect of acute subcutaneous BIMG 80, clozapine, haloperidol, risperidone, amperozide, olanzapine, and Seroquel was then investigated on dopamine release in medial prefrontal cortex, nucleus accumbens, and striatum in freely moving rats using the microdialysis technique. Four different neurochemical profiles resulted from the studies: (a) Systemic administration of BIMG 80, clozapine, and amperozide produced greater percent increases in dopamine efflux in medial prefrontal cortex than in the striatum or the nucleus accumbens. (b) Haloperidol induced a similar increase in dopamine concentrations in the striatum and nucleus accumbens with no effect in the medial prefrontal cortex. (c) Risperidone and olanzapine stimulated dopamine release to a similar extent in all brain regions investigated. (d) Seroquel failed to change significantly dopamine output both in the medial prefrontal cortex and in the striatum. Because an increase in dopamine release in the medial prefrontal cortex may be predictive of effectiveness in treating negative symptoms and in the striatum may be predictive of induction of extrapyramidal side effects, BIMG 80 appears to be a potential antipsychotic compound active on negative symptoms of schizophrenia with a low incidence of extrapyramidal side effects.     

Eating-Induced Dopamine Release from Mesolimbic Neurons Is Mediated by NMDA Receptors in the Ventral Tegmental Area: A Dual-Probe Microdialysis Study

Abstract: This study was aimed at identifying the neuronal pathways that mediate the eating-induced increase in the release of dopamine in the nucleus accumbens of the rat brain. For that purpose, a microdialysis probe was implanted in the ventral tegmental area and a second probe was placed in the ipsilateral nucleus accumbens. Receptor-specific compounds acting on GABA_A (40 µ M muscimol; 50 µ M bicuculline), GABA_B (50 µ M baclofen), acetylcholine (50 µ M carbachol), NMDA [30 µ M (±)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP)], and non-NMDA [300 µ M 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)] receptors were infused into the ventral tegmental area by retrograde dialysis, whereas extracellular dopamine was recorded in the ipsilateral nucleus accumbens. Intrategmental infusion of muscimol or baclofen decreased extracellular dopamine in the ipsilateral nucleus accumbens; CPP and CNQX were without effect, and bicuculline and carbachol increased dopamine release. During infusion of the various compounds, food-deprived rats were allowed to eat for 10 min. The infusions of muscimol, bicuculline, baclofen, carbachol, and CNQX did not prevent the eating-induced increase in extracellular dopamine in the nucleus accumbens. However, during intrategmental infusion of CPP, the eating-induced increase in extracellular dopamine in the nucleus accumbens was suppressed. These results indicate that a glutamatergic projection to the ventral tegmental area mediates, via an NMDA receptor, the eating-induced increase in dopamine release from mesolimbic dopamine neurons.     

Implication of Rho-associated kinase in the elevation of extracellular dopamine levels and its related behaviors induced by methamphetamine in rats

A growing body of evidence suggests that several protein kinases are involved in the expression of pharmacological actions induced by a psychostimulant methamphetamine. The present study was designed to investigate the role of the Rho/Rho-associated kinase (ROCK)-dependent pathway in the expression of the increase in extracellular levels of dopamine in the nucleus accumbens and its related behaviors induced by methamphetamine in rats. Methamphetamine (1 mg/kg, subcutaneously) produced a substantial increase in extracellular levels of dopamine in the nucleus accumbens, with a progressive augmentation of dopamine-related behaviors including rearing and sniffing. Methamphetamine also induced the decrease in levels of its major metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA). Both the increase in extracellular levels of dopamine and the induction of dopamine-related behaviors by methamphetamine were significantly suppressed by pretreatment with an intranucleus accumbens injection of a selective ROCK inhibitor Y-27632. In contrast, Y-27632 had no effect on the decrease in levels of DOPAC and HVA induced by methamphetamine. Under these conditions, there were no changes in protein levels of membrane-bound RhoA in the nucleus accumbens following methamphetamine treatment. It is of interest to note that the microinjection of Y-27632 into the nucleus accumbens failed to suppress the increases in extracellular levels of dopamine, DOPAC, and HVA in the nucleus accumbens induced by subcutaneous injection of a prototype of micro -opioid receptor agonist morphine (10 mg/kg). Furthermore, perfusion of a selective blocker of voltage-dependent Na+ channels, tetrodotoxin (TTx) into the rat nucleus accumbens did not affect the increase in extracellular levels of dopamine in the rat nucleus accumbens by methamphetamine, whereas the morphine-induced dopamine elevation was eliminated by this application of TTx. The extracellular level of dopamine in the nucleus accumbens was also increased by perfusion of a selective dopamine re-uptake inhibitor 1-[2-[bis(4-fluorophenyl)methoxy]-4-(3-phenylpropyl)piperazine (GBR-12909) in the nucleus accumbens. This effect was not affected by pretreatment with intranucleus accumbens injection of Y-27632. These findings provide first evidence that Rho/ROCK pathway in the nucleus accumbens may contribute to the increase in extracellular levels of dopamine in the nucleus accumbens evoked by a single subcutaneous injection of methamphetamine. In contrast, this pathway is not essential for the increased level of dopamine in this region induced by morphine, providing further evidence for the different mechanisms of dopamine release by methamphetamine and morphine in rats.     

Effects of Aging on the Interaction Between Glutamate, Dopamine, and GABA in Striatum and Nucleus Accumbens of the Awake Rat

The aim of the present study was to investigate, using microdialysis, the effects of aging on the glutamate/dopamine/GABA interaction in striatum and nucleus accumbens of the awake rat. For that, the effects of an increase of the endogenous concentration of glutamate on the extracellular concentration of dopamine and GABA in striatum and nucleus accumbens of young (2-4 months), middle-aged (12-14 months), aged (27-33 months), and very aged (37 months) male Wistar rats were studied. Endogenous extracellular glutamate was selectively increased by perfusing the glutamate uptake inhibitor L-trans-pyrrolidine-3,4-dicarboxylic acid (PDC) through the microdialysis probe. In young rats, PDC (1, 2, and 4 mM) produced a dose-related increase of dialysate concentrations of glutamate in both striatum and nucleus accumbens. PDC also increased dialysate dopamine and GABA in both structures. These increases were significantly correlated with the increases of glutamate but not with the PDC dose used, which strongly suggests that the increases of dopamine and GABA were produced by glutamate. In striatum, there were no significant differences in the dopamine/glutamate and GABA/glutamate correlations between young and aged rats. This means that the effects of glutamate on dopamine and GABA do not change during aging. On the contrary, in the nucleus accumbens of aged rats, the increases of dopamine, when correlated with the increases of glutamate, were significantly lower than in young rats. Moreover, the ratio of dopamine to glutamate increases at maximal increases of glutamate was negatively correlated with aging. On the contrary, the ratio of GABA to glutamate increases in nucleus accumbens was positively correlated with aging, which suggests that the effects of endogenous glutamate on GABA tend to be higher in the nucleus accumbens of aged rats. The findings of this study suggest that aging changes the interaction between endogenous glutamate, dopamine, and GABA in nucleus accumbens, but not in striatum, of the awake rat.     

Cocaine-like neurochemical effects of antihistaminic medications

The pattern of activation of dopamine (DA) neurotransmission in the nucleus accumbens (NAc) of rats produced by H₁ histamine antagonists which have behavioral effects like those of psychostimulant drugs was examined. Diphenhydramine and (+)-chlorpheniramine were compared with triprolidine, a potent and selective H₁ antagonist and (−)-chlorpheniramine which is less active than its enantiomer at H₁ receptors. Affinities of the drugs to DA, serotonin, and norepinephrine transporters at H₁ receptors and potencies for DA uptake inhibition in striatal synaptosomes were determined to assess mechanisms by which the compounds increased DA levels. Intravenous diphenhydramine (1.0–3.0 mg/kg) (+)- and (−)-chlorpheniramine (1.0–5.6 mg/kg) but not triprolidine (1.0–3.0 mg/kg) elicited a cocaine-like pattern of stimulation of DA transmission with larger effects in the NAc shell than core. The absence of stereospecific effects with chlorpheniramine enantiomers along with the lack of an effect with triprolidine suggest that the effects on DA transmission were not related to H₁ receptor antagonism. Although in vivo potencies were not directly related to DA transporter affinities, it is hypothesized that actions at that site modulated by other actions, possibly those at the serotonin transporter, are primarily responsible for the neurochemical actions of the drugs on DA neurotransmission and might underlie the occasional misuse of these medications.     

Adenosine and dopamine receptor antagonist binding in the rat ventral and dorsal striatum: lack of changes after a neonatal bilateral lesion of the ventral hippocampus.

There is experimental evidence from radioligand binding experiments for the existence of strong antagonistic interactions between different subtypes of adenosine and dopamine receptors in the striatum, mainly between adenosine A1 and dopamine D1 and between adenosine A2A and dopamine D2 receptors. These interactions seem to be more powerful in the ventral compared to the dorsal striatum, which might have some implications for the treatment of schizophrenia. The binding characteristics of different dopamine and adenosine receptor subtypes were analysed in the different striatal compartments (dorsolateral striatum and shell and core of the nucleus accumbens), by performing saturation experiments with the dopamine D1 receptor antagonist [125I]SCH-23982, the dopamine D2-3 receptor antagonist [3H]raclopride, the adenosine A1 receptor antagonist [3H]DPCPX and the adenosine A2A receptor antagonist [3H]SCH 58261. The experiments were also performed in rats with a neonatal bilateral lesion of the ventral hippocampus (VH), a possible animal model of schizophrenia. Both dopamine D2-3 and adenosine A2A receptors follow a similar pattern, with a lower density of receptors (40%) in the shell of the nucleus accumbens compared with the dorsolateral caudate-putamen. A lower density of adenosine A1 receptors (20%) was also found in the shell of the nucleus accumbens compared with the caudate-putamen. On the other hand, dopamine D1 receptors showed a similar density in the different striatal compartments. Therefore, differences in receptor densities cannot explain the stronger interactions between adenosine and dopamine receptors found in the ventral, compared to the dorsal striatum. No statistical differences in the binding characteristics of any of the different adenosine and dopamine receptor antagonists used were found between sham-operated and VH-lesioned rats.     

Differential Effects of δ- and μ-Opioid Receptor Antagonists on the Amphetamine-Induced Increase in Extracellular Dopamine in Striatum and Nucleus Accumbens

Abstract: The specific opioid receptor antagonist naloxone attenuates the behavioral and neurochemical effects of amphetamine. Furthermore, the amphetamine-induced increase in locomotor activity is attenuated by intracisternally administered naltrindole, a selective δ-opioid receptor antagonist, but not by the irreversible μ-opioid receptor antagonist β-funaltrexamine. Therefore, this research was designed to determine if naltrindole would attenuate the neurochemical response to amphetamine as it did the behavioral response. In vivo microdialysis was used to monitor the change in extracellular concentrations of dopamine in awake rats. Naltrindole (3.0, 10, or 30 µg) or vehicle was given 15 min before and β-funaltrexamine (10 µg) or vehicle 24 h before the start of cumulative dosing, intracisternally in a 10-µl volume, while the rats were lightly anesthetized with methoxyflurane. Cumulative doses of subcutaneous d-amphetamine (0.0, 0.1, 0.4, 1.6, and 6.4 mg/kg) followed pretreatment injections at 30-min intervals. Dialysate samples were collected every 10 min from either the striatum or nucleus accumbens and analyzed for dopamine content by HPLC. Amphetamine dose-dependently increased dopamine content in both the striatum and nucleus accumbens, as reported previously. Naltrindole (3.0, 10, and 30 µg) significantly reduced the dopamine response to amphetamine in the striatum. In contrast, 30 µg of naltrindole did not modify the dopamine response to amphetamine in the nucleus accumbens. On the other hand, β-funaltrexamine (10 µg) had no effect in the striatum but significantly attenuated the amphetamine-induced increase in extracellular dopamine content in the nucleus accumbens. These data suggest that δ-opioid receptors play a relatively larger role than μ-opioid receptors in mediating the amphetamine-induced increase in extracellular dopamine content in the striatum, whereas μ-opioid receptors play a larger role in mediating these effects in the nucleus accumbens.     

Cumulative effect of norepinephrine and dopamine carrier blockade on extracellular dopamine increase in the nucleus accumbens shell, bed nucleus of stria terminalis and prefrontal cortex

We investigated, by microdialysis in various brain areas, the possibility that dopamine could be captured by the norepinephrine transporter when the dopamine transporter is pharmacologically blocked. Administration of reboxetine, a selective blocker of the norepinephrine transporter, 20 min after the administration of GBR 12909, a selective blocker of the dopamine transporter, produced an increase of dopamine output in the nucleus accumbes shell (+408% above basal) greater than that obtained by GBR 12909 alone (+308% above basal). On the contrary, reboxetine did not increase further the dopamine output produced by GBR 12909 in the nucleus accumbens core or in the dorsal caudate, areas lacking a consistent noradrenergic innervations. A cumulative effect of dopamine and norepinephrine transporter blockade on the output of dopamine in dialysates was also observed in the bed nucleus of stria terminalis and in the prefrontal cortex. This study shows that dopamine extracellular concentration can be elevated by norepinephrine transporter blockade, even in areas where the dopamine transporter is predominant, when the latter is pharmacologically blocked. This phenomenon may have relevance in psychostimulant dependence as well as in antidepressant pharmacology.     

Receptors coupled to adenylate cyclase in isolated rabbit retina

Intact pieces or homogenates of rabbit retina were exposed to various established or putative retinal neurotransmitters for the study of receptors (or receptor-subtypes) linked to the production of cAMP. Since a dopamine-sensitive adenylate cyclase has been previously characterized in the retina of several species, the novel D₁-agonist SKF 38393 was applied under similar experimental conditions. This compound was found to be more potent (although less efficacious) than dopamine, confirming the existence of a population of D₁-receptors. On the other hand, the novel D₁-antagonist SCH 23390 was able to inhibit the stimulating effects of dopamine and of SKF 38393 in a concentration-dependent manner. Attempts to detect D₂-receptors (negatively coupled with adenylate cyclase) were not conclusive, when a selective D₂-agonist (quinpirole) was applied in the absence or presence of a D₂-antagonist (sulpiride).

A stimulation of cAMP production (mediated by A₂-receptors) was also detected in response to adenosine or adenosine-analogs, which was blocked by IBMX in a concentration-dependent manner. The effects of adenosine were potentiated in the presence of dipyridamole, an adenosine uptake inhibitor. Compared to the effects of dopamine and adenosine, the stimulation induced by VIP, a retinal neuropeptide, was found to be much more pronounced.

These results indicate that retinal cAMP can be generated by three different neurotransmitters in an independent way via the stimulation of specific receptors.