Presynaptic α2A-adrenoceptors inhibit the release of endogenous dopamine in rabbit caudate nucleus slices

₂-Adrenoceptors modulating the release of dopamine were identified and characterized in slices of the head of the rabbit caudate nucleus. Release of endogenous dopamine was measured by fast cyclic voltammetry as the increase in the extracellular concentration of dopamine elicited by electrical stimulation. The electrochemical signal was identified as dopamine by means of the oxidation potential, the voltammogram and the fact that the signal was not changed by desipramine, which inhibits the high affinity uptake of noradrenaline, but was greatly increased by nomifensine, which in addition inhibits the high affinity uptake of dopamine.Stimulation by 6 pulses/100 Hz increased the extracellular concentration of dopamine by about 85 nM. The selective ₂-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced this release with an EC₅₀ of 173 nM and by maximally 75%. The ₂-adrenoceptor agonists clonidine and oxymetazoline only tended to cause a decrease. Six drugs, including oxymetazoline, were tested as antagonists against UK 14,304. Their order of antagonist potency (pK_D values in brackets) was rauwolscine (8.0) > oxymetazoline (7.5) > 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101; 7.3) > phentolamine (7.1) > corynanthine (5.1) prazosin (< 6). Given alone, the antagonists did not change the release of dopamine elicited by 6 pulses/100 Hz, and the same was true for the dopamine receptor antagonist sulpiride. When caudate slices were stimulated by 10 pulses/1 Hz, sulpiride increased the release of dopamine. Desipramine and rauwolscine, in contrast, again caused no change.It is concluded that dopaminergic axons in the rabbit caudate nucleus possess release-inhibiting ₂-adrenoceptors. The antagonist affinities indicate that they belong to the _2A subtype. In this, they agree with all presynaptic ₂-autoreceptors studied so far in rabbits as well as with the ₂-heteroreceptors modulating the release of serotonin in rabbit brain cortex, suggesting that at least the majority of presynaptic ₂-adrenoceptors in the rabbit are _2A. The agonist sensitivity of the caudate presynaptic ₂-adrenoceptors is low in comparison with cerebrocortical presynaptic ₂-autoreceptors, possibly due to absence of a receptor reserve. Correspondence to: N. Limberger at the above address     

κ1- and κ2-opioid receptors mediating presynaptic inhibition of dopamine and acetylcholine release in rat neostriatum

1. The effects of selective opioid receptor agonists and antagonists on N-methyl-D-aspartate (NMDA, 10 μM)-induced release of [³H]-dopamine and [¹⁴C]-acetylcholine (ACh) from superfused neostriatal slices were studied to investigate the possible occurrence of functional κ-opioid receptor subtypes in rat brain.
2. The κ receptor agonists (−)-ethylketocyclazocine ((−)-EKC), {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593 and the endogenous opioid peptide dynorphin A_1–13 caused a naloxone-reversible inhibition of NMDA-induced [³H]-dopamine release, with pD₂ values of about 9, 8.5 and 8.2, respectively, whereas both the μ agonist Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO) and theδ agonist D-Pen²-D-Pen⁵-enkephalin (DPDPE) were ineffective in this respect. The inhibitory effect of submaximally effective concentrations of dynorphin A_1–13, {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593 and (−)-EKC on NMDA-induced [³H]-dopamine release were not changed by the δ₁/δ₂-opioid receptor antagonist naltrindole (up to a concentration of 1 μM), but reversed by the κ receptor antagonist nor-binaltorphimine (nor-BNI), with an IC₅₀ as low as 0.02 nM, indicating the involvement of {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593-sensitive κ₁-opioid receptors.
3. NMDA-induced [¹⁴C]-ACh release was reduced in a naloxone-reversible manner by DPDPE (pD₂ about 7.2), dynorphin A_1–13 (pD₂ 6.7) and EKC (pD₂ 6.2), but not by {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593 and DAMGO. The inhibitory effect of a submaximally effective concentration of DPDPE, unlike those of dynorphin A_1–13 and (−)-EKC, on NMDA-induced [¹⁴C]-ACh release was antagonized by naltrindole with an IC₅₀ of 1 nM, indicating the involvement of δ-opioid receptors in the inhibitory effect of DPDPE. On the other hand, the inhibitory effects of dynorphin A_1–13 and (−)-EKC on [¹⁴C]-ACh release were readily antagonized by nor-BNI with an IC₅₀ of about 3 nM. A 100 fold higher concentration of nor-BNI also antagonized the inhibitory effect of DPDPE, indicating the involvement of {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593-insensitive κ₂-opioid receptors in the inhibitory effects of dynorphin A_1–13 and (−)-EKC.
4. Although naloxone benzoylhydrazone (NalBzoH), displaying high affinity towards the putative κ₃-opioid receptor, antagonized the inhibitory effects of dynorphin A_1–13 and (−)-EKC on [³H]-dopamine and [¹⁴C]-ACh release as well as that of {"type":"entrez-nucleotide","attrs":{"text":"U69593","term_id":"4205069","term_text":"U69593"}}U69593 on [³H]-dopamine release, it displayed a low apparent affinity (IC₅₀ about 100 nM) in each case.
5. In conclusion, whereas activation of κ₁-opioid receptors causes presynaptic inhibition of NMDA-induced dopamine release, κ₂ receptor activation results in inhibition of ACh release in rat neostriatum. As such, this study is the first to provide unequivocal in vitro evidence for the existence of functionally distinct κ-opioid receptor subtypes in the brain.

     

In vitro exposure to nicotine induces endocytosis of presynaptic AMPA receptors modulating dopamine release in rat nucleus accumbens nerve terminals

Here we provide functional and immunocytochemical evidence supporting the presence on Nucleus Accumbens (NAc) dopaminergic terminals of cyclothiazide-sensitive, alfa-amino-3-hydroxy-5-methyl-4-isoxazolone propionate (AMPA) receptors, which activation causes Ca²⁺-dependent [³H]dopamine ([³H]DA) exocytosis. These AMPA receptors cross-talk with co-localized nicotinic receptors (nAChRs), as suggested by the finding that in vitro short-term pre-exposure of synaptosomes to 30 μM nicotine caused a significant reduction of both the 30 μM nicotine and the 100 μM AMPA-evoked [³H]DA overflow. Entrapping pep2-SVKI, a peptide known to compete for the binding of GluA2 subunit to scaffolding proteins involved in AMPA receptor endocytosis, in NAC synaptosomes prevented the nicotine-induced reduction of AMPA-mediated [³H]DA exocytosis, while pep2-SVKE, used as negative control, was inefficacious. Immunocytochemical studies showed that a significant percentage of NAc terminals were dopaminergic and that most of these terminals also posses GluA2 receptor subunits. Western blot analysis of GluA2 immunoreactivity showed that presynaptic GluA2 proteins in NAc terminals were reduced in nicotine-pretreated synaptosomes when compared to the control. The nACh-AMPA receptor–receptor interaction was not limited to dopaminergic terminals since nicotine pre-exposure also affected the presynaptic AMPA receptors controlling hippocampal noradrenaline release, but not the presynaptic AMPA receptors controlling GABA and acetylcholine release. These observations could be relevant to the comprehension of the molecular mechanisms at the basis of nicotine rewarding.     

Investigations of the roles of dihydropyridine and ω-conotoxin-sensitive calcium channels in mediating depolarisation-evoked endogenous dopamine release from striatal slices

Summary The relative roles of L- and N-type voltage-sensitive calcium channels (VSCC) in mediating endogenous dopamine release have been investigated by examining the effects of the dihydropyridine (DHP) agonist BAY K 8644 and the antagonist PN 200-110, as well as the VSCC-blocking peptide -conotoxin GVIA, on depolarisationevoked dopamine release from superfused rat striatal slices. Dopamine release evoked by electrical field stimulation was virtually unaffected by either of the DHP drugs, but release evoked by raising the K⁺ concentration to 25 mmol/l was significantly increased by BAY K 8644 and reduced stereospeciflcally by PN 200-110. Quantitative differences between electrically-evoked and K⁺-evoked dopamine release with respect to their dependence on extracellular calcium concentration were also observed, with electrically-evoked release requiring higher calcium concentrations. The adenylate cyclase activator forskolin itself increased dopamine release, but did not appear to influence the effectiveness of either DHP drug in altering dopamine release. In contrast to the relatively small effects of the DHP drugs, -conotoxin produced a major reduction in electrically-evoked dopamine release as well as a substantial decrease in K⁺-evoked release. Since -conotoxin is thought to block both L- and N-type neuronal VSCC whereas DHP drugs affect only L-type VSCC, these findings suggest that electrically-evoked dopamine release is mediated mainly by calcium influx through N-type VSCC, accounting for the reported lack of effect of many organic calcium antagonists on this process. In contrast, K⁺-evoked dopamine release appears to involve both L- and N-type VSCC, and can occur at lower extracellular calcium concentrations.Abbreviations DHP 1,4-dihydropyridine - HPLC high-performance liquid chromatography - VSCC voltage-sensitive calcium channels Send offprint requests to H. Herdon at the above address     

β-Endorphin-induced locomotor stimulation and reinforcement are associated with an increase in dopamine release in the nucleus accumbens

In vivo microdialysis was used to compare the effects of β-endorphin upon dopamine (DA) release in the nucleus accumbens (NAC) of anesthetized versus freely moving rats, and to examine the role of the the mesolimbic DA system in mediating both the motoric and secondary reinforcing effects of this peptide. Microdialysis probes were inserted into the NAC and perfusates were analyzed for DA and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), using a reversed phase HPLC system with electrochemical detection for separation and quantification. Intracerebroventricular (ICV) administration of β-endorphin (2.5 and 5.0 µg) increased DA release and metabolites in both freely moving and anesthetized rats. This effect was of greater magnitude and duration in freely moving rats and was accompanied by stimulation of locomotor activity. The 5 µg dose also functioned as a secondary reinforcer in a conditioned place preference paradigm. A higher dose of β-endorphin (7.5 µg) stimulated DA release and metabolites in anesthetized rats but failed to affect these parameters in freely moving rats. At this dose, catalepsy and a loss of the reinforcing effects of this peptide were observed. These data demonstrate marked differences in the effects of β-endorphin upon DA release in the awake versus anesthetized rat. Further, the finding that the reinforcing and locomotor stimulating effects of β-endorphin only occur at those doses which stimulate DA release suggest that this action is critical for the expression of both behavioral effects.     

Association between depressive behavior and absence of serotonin–dopamine interaction in the nucleus accumbens

RATIONALE: Current hypotheses on the etiology of depression attribute the disorder to alterations in serotonin and norepinephrine neurotransmission. However, the relationship between these alterations and depressive behavior is poorly understood. Conversely, an interaction between the serotonergic and dopaminergic systems in the nucleus accumbens has been established. Since motivation and hedonia have been associated with dopamine release in the nucleus accumbens, we decided to test its modulation by serotonin in relation to depressive-like behavior. OBJECTIVES AND METHODS: The extracellular dopamine levels in the nucleus accumbens were studied in vivo in Flinders Sensitive Line (FSL, a rat model of depressive behavior) and control rats, before and after antidepressant treatment. Rats were chronically treated with the antidepressants desipramine (5 mg/kg/day) and paroxetine (7.5 mg/kg/day) for 18 consecutive days. As a measure of depressive behavior we used a modified swim test. The release of dopamine in response to local serotonin application was monitored using the microdialysis technique. RESULTS: Serotonin (0.5 microM) facilitated dopamine release in the nucleus accumbens of control rats. In FSL rats, basal extracellular dopamine levels in the nucleus accumbens were 40% lower than in control rats and did not increase in response to serotonin stimulation. However, chronic antidepressant treatment of the FSL rats normalized the serotonin-dopamine interaction as well as their behavioral deficiencies. CONCLUSIONS: The inability of serotonin to stimulate dopamine release in the nucleus accumbens, thereby leading to anhedonia and lack of motivation, may therefore be an essential factor in the onset of depression and a target for modulation by antidepressant drugs.     

Novel δ1-receptor agonist KNT-127 increases the release of dopamine and L-glutamate in the striatum, nucleus accumbens and median pre-frontal cortex

The effects of systemic δ1-agonist on neurotransmission remains obscure, since no selective δ1-agonist exists that can penetrate the blood-brain barrier. Recently, we succeeded in synthesizing a putative δ1-receptor agonist, KNT-127, which has been demonstrated the effectiveness of systemic administration against anxiety and depressive-like behavior. To clarify the functional selectivity of KNT-127 and neurotransmission regulating system of δ1-receptor, the present study investigated the interaction between KNT-127 and δ-receptor antagonists on the release of dopamine, L-glutamate and GABA in nucleus accumbens (NAc), striatum and median pre-frontal cortex (mPFC) using multi-probe microdialysis. Intraperitoneal administration of KNT-127 increased the release of dopamine and L-glutamate in three regions, but decreased and increased GABA releases in respective NAc and mPFC without affecting that in striatum. The effects of KNT-127 in the three regions were abrogated by δ1-antagonist but not by δ2-antagonist. MK801 inhibited KNT-127-induced dopamine release in striatum and NAc, but enhanced that in mPFC, inhibited KNT-127-induced mPFC GABA release without affecting KNT-127-induced GABA reduction in NAc. Muscimol enhanced KNT-127-induced dopamine release in mPFC. Sulpiride inhibited KNT-127-induced reduction of GABA release in NAc. The results indicated that KNT-127 is a selective δ1-agonist, and suggested that δ1-receptor directly activates the release of dopamine and L-glutamate in the striatum, NAc and mPFC, but not that of GABA in the three regions. δ1-receptor indirectly inhibited GABA release in NAc via activated dopaminergic transmission, while δ1-receptor indirectly enhanced GABA release in mPFC via activated glutamatergic transmission.     

A microdialysis profile of β-endorphin and catecholamines in the rat nucleus accumbens following alcohol administration

Rationale Alcohol stimulates the release of dopamine in the nucleus accumbens (NACB) of rats, mice and humans. There is evidence to suggest that the activation of beta-endorphin (β-EP) in the mesolimbic pathway by alcohol and other drugs of abuse may be associated with the rise in dopamine levels in the NACB. Objectives The present studies investigate whether the release of β-EP in the NACB is (1) dependent on the dose of alcohol that is administered, and (2) associated with changes in the extracellular concentrations of the catecholamines dopamine and norepinephrine, and the dopamine metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the NACB. Methods Male Sprague-Dawley rats were implanted with a microdialysis probe positioned in the shell region of the NACB. Artificial cerebrospinal fluid was pumped at a rate of 2.3 μl/min in awake and freely moving animals and the dialysate was collected at 30-min intervals. After a baseline period, rats were injected intraperitoneally with either physiological saline or one of three doses of alcohol: 0.8, 1.6, or 2.4 g ethanol/kg body weight. The dialysates collected were analyzed with radioimmunoassay, to estimate the content of β-EP; and high performance liquid chromatography, to estimate the content of dopamine, norepinephrine, DOPAC and HVA. Results Alcohol induced a dose-dependent increase in the extracellular levels of β-EP and dopamine. However, elevations in the extracellular levels of norepinephrine, DOPAC and HVA did not reach significance. The largest increase in β-EP and dopamine was observed with the 2.4 g/kg dose. Conclusion The alcohol-induced release of β-EP and dopamine in the NACB is dose-dependent, where the highest dose resulted in more pronounced concentrations in the dialysate. Furthermore, the increase in the extracellular levels of dopamine appeared to occur at an earlier time point following alcohol administration, than for β-EP. These results suggest that alcohol stimulates dopamine and β-EP in the NACB, but probably does so via independent mechanisms.     

The novel α7 nicotinic acetylcholine receptor agonist EVP-6124 enhances dopamine,acetylcholine, and glutamate efflux in rat cortex and nucleus accumbens

Background

Alpha7 and α4β2 nicotinic acetylcholine receptor (nAChR) agonists have been shown to improve cognition in various animal models of cognitive impairment and are of interest as treatments for schizophrenia, Alzheimer’s disease, and other cognitive disorders. Increased release of dopamine (DA), acetylcholine (ACh), glutamate (Glu), and γ-aminobutyric acid (GABA) in cerebral cortex, hippocampus, and nucleus accumbens (NAC) has been suggested to contribute to their beneficial effects on cognition.

Results

Using in vivo microdialysis, we found that EVP-6124 [(R)-7-chloro-N-quinuclidin-3-yl)benzo[b]thiophene-2-carboxamide], a high-affinity α7 nAChR partial agonist, at 0.1 mg/kg, s.c., increased DA efflux in the medial prefrontal cortex (mPFC) and NAC. EVP-6124, 0.1 and 0.3 mg/kg, also increased efflux of ACh in the mPFC but not in the NAC. Similarly, EVP-6124, 0.1 mg/kg, but not 0.03 and 0.3 mg/kg, significantly increased mPFC Glu efflux. Thus, EVP-6124 produced an inverted U-shaped curve for DA and Glu release, as previously reported for other α7 nAChR agonists. The three doses of EVP-6124 did not produce a significant effect on GABA efflux in either region. Pretreatment with the selective α7 nAChR antagonist, methyllycaconitine (MLA, 1.0 mg/kg), significantly blocked cortical DA and Glu efflux induced by EVP-6124 (0.1 mg/kg), suggesting that the effects of EVP-6124 on these neurotransmitters were due to α7 nAChR agonism. MLA only partially blocked the effects of EVP-6124 on ACh efflux in the mPFC.

Conclusion

These results suggest increased cortical DA, ACh, and Glu release, which may contribute to the ability of the α7 nAChR agonist, EVP-6124, to treat cognitive impairment and possibly other dimensions of psychopathology.     

Endogenous κ-opioid systems in opiate withdrawal: role in aversion and accompanying changes in mesolimbic dopamine release

The -opioid receptor antagonist nor-binaltor-phimine (nor-BNI) was recently shown to potentiate certain overt withdrawal signs in morphine-dependent rats. The present study sought to further assess this phenomenon by examining the influence of nor-BNI treatment upon the conditioned place aversion associated with the naloxone-precipitated withdrawal syndrome. In addition, in vivo microdialysis studies were conducted in morphine-dependent rats to determine whether nor-BNI treatment can modify withdrawal-induced changes in basal dopamine (DA) release within the mesolimbic system. Rats were pretreated with either saline or a single dose of nor-BNI and then received ascending doses of morphine for 10 days. A withdrawal syndrome was then precipitated by the administration of naloxone (1 mg/kg SC). In rats which received chronic morphine injections, administration of naloxone produced a characteristic withdrawal syndrome and a marked aversion for an environment previously associated with naloxone-precipitated withdrawal. Nor-BNI treatment potentiated most overt signs of physical dependence. This treatment also resulted in a greater withdrawal-induced place aversion. Morphine-dependent rats exhibited a marked reduction in basal mesolimbic DA release. An even greater decrease in basal DA release was observed in nor-BNI treated rats. These results suggest that endogenous -systems are important in the modulation of mesolimbic DA release and the accompanying place aversion which occurs during opiate withdrawal.     

Alterations in AMPA receptor subunits and TARPs in the rat nucleus accumbens related to the formation of Ca²⁺-permeable AMPA receptors during the incubation of cocaine craving

Cue-induced cocaine seeking intensifies or incubates after withdrawal from extended access cocaine self-administration, a phenomenon termed incubation of cocaine craving. The expression of incubated craving is mediated by Ca²⁺-permeable AMPA receptors (CP-AMPARs) in the nucleus accumbens (NAc). Thus, CP-AMPARs are a potential target for therapeutic intervention, making it important to understand mechanisms that govern their accumulation. Here we used subcellular fractionation and biotinylation of NAc tissue to examine the abundance and distribution of AMPAR subunits, and GluA1 phosphorylation, in the incubation model. We also studied two transmembrane AMPA receptor regulatory proteins (TARPs), γ-2 and γ-4. Our results, together with earlier findings, suggest that some of the new CP-AMPARs are synaptic. These are probably associated with γ-2, but they are loosely tethered to the PSD. Levels of GluA1 phosphorylated at serine 845 (pS845 GluA1) were significantly increased in biotinylated tissue and in an extrasynaptic membrane-enriched fraction. These results suggest that increased synaptic levels of CP-AMPARs may result in part from an increase in pS845 GluA1 in extrasynaptic membranes, given that S845 phosphorylation primes GluA1-containing AMPARs for synaptic insertion and extrasynaptic AMPARs supply the synapse. Some of the new extrasynaptic CP-AMPARs are likely associated with γ-4, rather than γ-2. The maintenance of CP-AMPARs in NAc synapses during withdrawal is accompanied by activation of CaMKII and ERK2 but not CaMKI. Overall, AMPAR plasticity in the incubation model shares some features with better described forms of synaptic plasticity, although the timing of the phenomenon and the persistence of related neuroadaptations are significantly different.This article is part of a Special Issue entitled ‘Synaptic Plasticity and Addiction’.     

Involvement of adenylate cyclase and protein kinase C in the α2-adrenoceptor-mediated inhibition of noradrenaline and 5-hydroxytryptamine release in rat hypothalamic slices

Summary In superfused rat hypothalamic slices prelabelled with [³H]-noradrenaline, the ₂-adrenoceptor agonist UK 14304 inhibited in a concentration-dependent manner the electrically-evoked release of tritium. This inhibition was antagonized by the ₂-adrenoceptor blocking agent idazoxan, which by itself increased the electrically-evoked tritium overflow. Exposure to forskolin, an adenylate cyclase activator, increased the electrically-evoked release of [³H]-noradrenaline. In the presence of forskolin (1 mol/l), both the inhibitory effect of UK 14304 and the increasing effect of idazoxan on the electrically-evoked release of [³H]-noradrenaline were less pronounced than in the absence of the adenylate cyclase activator. Exposure to forskolin and to the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine shifted to the right the concentration-effect curve for UK 14304 in a similar manner as that observed in the presence of forskolin alone. Exposure to phorbol-12,13-dibutyrate (0.01–10 mol/l), a drug which activates protein kinase C, increased the electrically-evoked release of [³H]-noradrenaline. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 mol/l), the concentration effect curve for UK 14304 on tritium overflow was significantly shifted to the right. The increasing effect of idazoxan on tritium overflow was significantly less pronounced in the presence of 1 mol/l phorbol-12,13-dibutyrate.In superfused rat hypothalamic slices prelabelled with [³H]-5-hydroxytryptamine, the ₂-adrenoceptor agonist UK 14304 significantly inhibited the electrically-evoked release of tritium. Exposure to forskolin increased in a concentration-dependent manner [³H]-5-hydroxytryptamine overflow, but did not modify the UK 14304-mediated inhibition. Exposure to 3-isobutyl-1-methylxanthine enhanced the electrically-evoked release of [³H]-5-hydroxytryptamine. In the presence of both forskolin (1 mol/l) and 3-isobutyl-l-methylxanthine (1 mmol/l), the concentration-response curve for UK 14304 was significantly shifted to the right. Exposure to phorbol-12,13-dibutyrate (0.01–10 mol/l) enhanced in a concentration-dependent manner the electrically-evoked overflow of [³H]-5-hydroxytryptamine. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 mol/l), UK 14304 was significantly less potent to inhibit tritium release than in the absence of the protein kinase C activator.It is concluded that both cyclic AMP and phosphoinositide turnover are involved in the modulation of noradrenaline and 5-hydroxytryptamine release by presynaptic ₂-adrenoceptors in rat hypothalamic slices. However, these interactions do not represent definitive proof for a cause-effect relationship for the second messengers mediating the ₂-adrenoceptor induced inhibition of transmitter release either as autoreceptor or as heteroreceptor.Send offprint requests to S. Z. Langer at the above address     

Effects of acute ethanol on β-endorphin release in the nucleus accumbens of selectively bred lines of alcohol-preferring AA and alcohol-avoiding ANA rats

Rationale

The selectively bred lines of alcohol-preferring alko alcohol (AA) and alcohol-avoiding alko nonalcohol (ANA) rats have been used to demonstrate differences in relevant neurotransmitters which could account for their difference in alcohol consumption. Studies have demonstrated differences in distinct components of the endogenous opioid system in various brain regions associated with the process of reinforcement between the AA and ANA lines of rats.

Objectives

The goal of this current study was to investigate the hypotheses that the AA and ANA rats will show differences in the release of β-endorphin at the level of nucleus accumbens (NAC) and in locomotor activity in response to acute systemic administration of ethanol.

Materials and methods

AA and ANA rats were unilaterally implanted with a guide cannula to aim microdialysis probes at the level of NAC. Intraperitoneal injections of 0.0, 1.5, 2.0, and 2.5 g ethanol/kg body weight were administered. Dialysate samples were collected at 30-min intervals prior to and following the injection. Radioimmunoassay specific for β-endorphin was used to determine the dialysate β-endorphin content.

Results

The 2.5-g/kg ethanol dose induced a transient increase in extracellular β-endorphin at the level of NAC of AA but not of ANA rats. The 2.5-g/kg ethanol dose also attenuated locomotor activity in the AA but not in the ANA rats.

Conclusions

The lack of an increase in the β-endorphin concentration in the NAC of ANA rats in response to ethanol may partially account for their lower alcohol consumption and lower alcohol-induced attenuation of locomotor activity compared to AA rats.     

Opposing actions of D-1 and D-2 dopamine receptor-mediated alterations of adenosine-3′,5′-cyclic monophosphate (cyclic AMP) formation during the amphetamine-induced release of endogenous dopamine in vitro

Summary Changes in the formation of cyclic AMP following d-amphetamine (0.1 to 20 pmol/1) were examined in vitro in striatal slices of the rat. d-Amphetamine caused a doserelated increase in cyclic AMP content. This action of d-amphetamine was abolished by tissue pretreatment with reserpine (2.5 mg/kg, i.p.) and 3-iodotyrosine (1 mmol/1). By contrast, both clorgyline (0.1 pmol/l) and nomifensine (30 mol/l) enhanced the d-amphetamine-induced increase in cyclic AMP formation. In superfusion experiments, a strong correlation between endogenous dopamine and cyclic AMP release was observed before, during and after d-amphetamine exposure. Finally, Sch 23390 (10 mol/1) abolished while (–)sulpiride (10 mol/1) enhanced the amphetamine-induced increase in cyclic AMP content. These results suggest that d-amphetamine enhances the formation of cyclic AMP through the release of endogenous dopamine into the synapse where it can interact with both D-1 and D-2 dopamine receptors. These results provide direct evidence that the antagonistic properties of D-1 and D-2 receptors on cyclic AMP formation are apparent at striatal synapses during release of endogenous neuronal dopamine.Abbreviations DA dopamine - 5-HT serotonin - CAMP cyclic AMP adenosine-3,5-cyclic monophosphate Supported in part by the West Virginia University Medical Corporation and a grant from the Fraternal Order of Eagles. Some of the findings were presented at the Annual meeting of the Society for Neurosciences, Washington, DC (Azzaro and Liccione 1986) Send offprint requests to A. J. Azzaro at the above address     

Comparison of the behavioural effects induced by administration in rat nucleus accumbens or nucleus caudatus of selective μ and δ opioid peptides or kelatorphan an inhibitor of enkephalin-degrading-enzymes

The effects of selective agonists for opioid receptors: [D-Thr², Leu⁵]-enkephalyl-Thr⁶ (DTLET) and receptors: [D-Ala², MePhe⁴, Gly-ol⁵]-enkephalin (DAGO) and of (R)-3-(N-hydroxyl-carboxamido-2-benzylpropanoyl)-L-alanine (kelatorphan), a complete inhibitor of enkephalin degrading enzymes, on the motor activity of rats was examined after their local administration into the nucleus accumbens (NA) or nucleus caudatus (NC). In both structures DTLET dose dependently enhanced locomotor activity as measured in the open-field test. This strong effect was reversed by the selective antagonist: ICI 174,864. Contrastingly, DAGO induced hypoactivity followed by hyperactivity 150 min later. This biphasic effect was blocked by systemic injection of naloxone, but not by ICI 174,864. The physiological relevance of these effects was ascertained by the naloxone-reversible stimulatory responses induced by kelatorphan, supporting a role for endogenous enkephalins in the control of behavior through receptor stimulation.     

Blockade of dopamine receptors reverses the behavioral effects of endogenous enkephalins in the Nucleus caudatus but not in the Nucleus accumbens: differential involvement of δ and μ opioid receptors

We have previously (Daugé et al. 1988) demonstrated that injection of the agonist [d-Ala², MePhe⁴, Gly-ol⁵]-enkephalin (DAGO) or the agonist [d-Thr², Leu⁵]-enkephalyl-Thr⁶ (DTLET) into the rat Nucleus accumbens (N.Acc.), or Nuceeus caudatus (N.Caud.) induced a hypoactivity followed by hyperactivity 150 min later in the case of the agonist and a hyperactivity in the case of the agonist. Moreover, naloxone reversible delta-type responses were obtained by local infusion of kelatorphan, ([(R)-3(N-hydroxylcarboxamido-2-benzylpropanoyl)-l-alanine]), a complete inhibitor of enkephalin catabolism, suggesting a tonic control of the behavioral activity of rat by the endogenous opioid peptides. In this work, the putative involvement of the dopaminergic system in these behavioral responses was investigated by using the DA antagonist thioproperazine. In the N.Acc., the behavioral effects of kelatorphan, or of or agonists were not altered by thioproperazine-induced blockade of dopamine receptors. In contrast, the hyperactivity produced by DTLET or by kelatorphan in the N.Caud. was reversed by thioproperazine while the time-dependent biphasic effect resulting from DAGO injection remained unaffected by the DA antagonist. This blocking effect of thioproperazine is in agreement with the previously described -selective enhancement of the release of newly synthesized DA in the striatum but not in the N.Acc.     

Drug discrimination and neurochemical studies in α7 null mutant mice: tests for the role of nicotinic α7 receptors in dopamine release

Rationale  The nicotine discriminative stimulus has been linked to β2-containing (β2*) nicotinic receptors, with little evidence of a role for α7 nicotinic receptors, because nicotine discrimination was very weak in β2 null mutant mice but normal in α7 mutants. Objectives  As both α7 and β2* nicotinic receptors have been implicated in nicotine-stimulated dopamine overflow, this study focused on the dopamine-mediated element in the nicotine stimulus by examining cross-generalisation between amphetamine and nicotine. Materials and methods  Male α7 nicotinic receptor null mutant mice and wild-type controls were bred in-house and trained to discriminate nicotine (0.8 mg/kg) or (+)-amphetamine (0.6 mg/kg) from saline in a two-lever procedure with a tandem VI-30 FR-10 schedule of food reinforcement. Dopamine release from striatal slices was determined in parallel experiments. Results  An α7 nicotinic receptor-mediated component of dopamine release was demonstrated in tissue from wild-type mice using choline as a selective agonist. This response was absent in tissue from null mutant animals. The mutation did not influence acquisition of drug discriminations but subtly affected the results of cross-generalisation tests. In mice trained to discriminate nicotine or amphetamine, there was partial cross-generalisation in wild-type mice and, at certain doses, these effects were attenuated in mutants. Further support for an α7 nicotinic receptor-mediated component was provided by the ability of the α7 nicotinic receptor antagonist methyllycaconitine to attenuate responses to nicotine and amphetamine in wild-type mice. Conclusions  These findings support the concept of an α7 nicotinic receptor-mediated dopaminergic element in nicotine discrimination, warranting further tests with selective dopamine agonists.     

Iptkalim inhibits cocaine challenge—induced enhancement of dopamine levels in nucleus accumbens and striatum of rats by up—regulating Kir6.1 and Kir6.2 mRNA expression

目的:研究钾通道开放剂埃他卡林对急慢性可卡因应用引起的伏隔核、纹状体和额叶皮层的多巴胺和谷氨酸水平变化的影响及其机制。方法:采用高效液相色谱与电化学检测、荧光检测联用的方法测定各脑区谷氨酸和多巴胺的含量;采用半定量逆转录聚合酶链反应(RT-PCR)研究ATP敏感性钾通道亚单位Kir6.1、Kir6.2、SUR1和SUR2 mRNA表达的变化。结果:埃他卡林不影响急性可卡因应用引起纹状体和伏隔核中多巴胺和谷氨酸水平的增高(P>0.05),能够逆转激发剂量可卡因诱导的慢性成瘾大鼠纹状体和伏隔核的多巴胺含量增高(P<0.05),对激发后皮层和伏隔核谷氨酸水平增高有降低趋势但差异无显著性(P>0.05)。激发剂量可卡因能提高可卡因预处理组和埃他卡林预处理组纹状体和伏隔核的Kir6.1和Kir6.2 mRNA表达以及皮层的Kir6.2 mRNA表达,而且IPT预处理组的升高幅度显著高显著高于可卡因慢性处 理组。结论:埃他卡林通过上调Kir6.1和Kir6.2 mRNA表达抑制可卡因激发引起的纹状体和伏隔核的多巴胺水平的增高。     

The α₁-, but not α₂-, adrenoceptor in the nucleus accumbens plays an inhibitory role upon the accumbal noradrenaline and dopamine efflux of freely moving rats

In vivo microdialysis was used to analyse the role of the α(1)- and α(2)-adrenoceptor subtypes in the regulation of noradrenaline and dopamine efflux in the nucleus accumbens of freely moving rats. Intra-accumbal infusion of α(1)-adrenoceptor agonist methoxamine (24pmol) failed to alter the noradrenaline efflux, but decreased the dopamine efflux. The intra-accumbal infusion of α(1)-adrenoceptor antagonist prazosin (6, 600 and 6000pmol) produced a dose-related increase and decrease of the noradrenaline and dopamine efflux, respectively. An ineffective dose of prazosin (6pmol) counteracted the methoxamine (24pmol)-induced decrease of dopamine efflux. The prazosin (6000pmol)-induced increase of noradrenaline efflux, but not the decrease of dopamine efflux, was suppressed by the co-administration of an ineffective dose of methoxamine (0.024pmol). Neither the α(2)-adrenoceptor agonist clonidine (300pmol) and UK 14,304 (300pmol) nor the α(2)-adrenoceptor antagonist RX 821002 (0.6, 3, 600 and 6000pmol) significantly affected the accumbal noradrenaline and dopamine efflux. The doses mentioned are the total amount of drug over the 60-min infusion period. The present results show that (1) accumbal α(1)-adrenoceptors which are presynaptically located on noradrenergic nerve terminals inhibit the accumbal noradrenaline efflux, increasing thereby the accumbal dopamine efflux, (2) accumbal α(1)-adrenoceptors which are postsynaptically located on dopaminergic nerve terminals inhibit the accumbal dopamine efflux, and (3) accumbal α(2)-adrenoceptors play no major role in the regulation of accumbal efflux of noradrenaline and dopamine.     

Comparison of the effects of haloperidol,remoxipride and raclopride on “pre”- and postsynaptic dopamine receptors in the rat brain

Summary The ability of the dopamine receptor antagonists haloperidol, raclopride and remoxipride to prevent the B-HT 920-induced decrease in striatal and limbic L-DOPA accumulation in gamma-butyrolactone (GBL)- and NSD 1015-treated rats (termed GBL-reversal) was used to define the effects of these compounds on presynaptic dopamine receptors. The doses of the dopamine antagonists producing antagonism of GBL-reversal were in each case roughly similar to the doses required to increase dopamine turnover in striatal and limbic areas. The potencies of haloperidol, raclopride and remoxipride in the GBL model were compared with their potencies in behavioural models for postsynaptic dopamine receptors. Haloperidol produced antagonism of GBL-reversal over a similar dose range to that required for antagonism of apomorphine-induced hyperactivity and stereotypy syndromes. Raclopride was effective in the order of potency: antagonism of apomorphine-induced hyperactivity > antagonism of GBL-reversal > antagonism of apomorphine-induced stereotypy. For remoxipride, the dose-response curve for antagonism of GBL-reversal was superimposable over that for antagonism of apomorphine-induced stereotypies, with an ED₅₀ value about 12 times higher than that for antagonism of apomorphine-induced hyperactivity. Thus, the relative potencies of dopamine receptor antagonists at pre- and postsynaptic dopamine receptors vary considerably from compound to compound. Send offprint requests to O. Magnusson