Impulsive choice and impulsive action predict vulnerability to distinct stages of nicotine seeking in rats

BACKGROUND

Although heavy smoking has been associated with impulsivity in humans, it is not clear whether poor impulse control represents a risk factor in the etiology of nicotine dependence.

METHODS

To address this issue, rats were selected on the basis of individual differences in impulsivity in the delayed reward task (impulsive choice) and the 5-choice serial reaction time task (impulsive action). Subsequently, rats were subjected to a nicotine self-administration (SA) paradigm tailored to measure the motivational properties of nicotine and nicotine-associated stimuli. In separate groups, differences in electrically evoked dopamine release in slice preparations obtained from several mesolimbic brain regions were determined.

RESULTS

Impulsive action was associated with an enhanced motivation to initiate and maintain nicotine SA. In contrast, impulsive choice predicted a diminished ability to inhibit nicotine seeking during abstinence and an enhanced vulnerability to relapse upon re-exposure to nicotine cues. Impulsive action was associated with reduced dopamine release in the accumbens core and impulsive choice with reduced dopamine release in accumbens core, shell, and medial prefrontal cortex.

CONCLUSIONS

The strong association between sub-dimensions of impulsivity and nicotine SA implies that interventions aimed to improve impulse control might help to reduce susceptibility to nicotine dependence and/or lead to successful smoking cessation.

Interactions between CB1 cannabinoid and mu opioid receptors mediating inhibition of neurotransmitter release in rat nucleus accumbens core

We examined the occurrence of functional interactions between CB1 cannabinoid and mu opioid receptors in the core of rat nucleus accumbens (NAc core). To that end, receptor-mediated inhibition of depolarization (4-aminopyridine)-induced [3H]glutamate release and glutamate (NMDA) receptor-stimulated [14C]acetylcholine (ACh) and [3H]GABA release was studied in superfused NAc core slices. The inhibitory effects of the mu receptor agonist morphine and the CB1 receptor agonist HU210 on the release of these neurotransmitters were selectively antagonized by the mu receptor antagonist naloxone and the CB1 receptor antagonist SR141716A, respectively. Surprisingly, naloxone prevented the antagonistic action of SR141716A at CB1 receptors and SR141716A abolished that of naloxone at mu receptors mediating inhibition of [3H]glutamate and [3H]GABA release. Therefore, these antagonists seem to allosterically interact, indicating the involvement of physically associated mu opioid and CB1 cannabinoid receptors. Such an interaction between antagonists was not observed at the receptors mediating inhibition of [14C]ACh release. Moreover, dose-response curves of the agonists showed that mu and CB1 receptors mediating inhibition of [3H]glutamate release display a non-additive interaction, whereas these receptors synergistically interact regarding their inhibitory control of [3H]GABA release. Finally, the apparent allosteric interaction between antagonists was also observed regarding the effects of other receptor-selective agonists and antagonists at mu opioid and CB1 cannabinoid receptors (mediating inhibition of NMDA-induced [3H]GABA release) and must therefore be a unique property of the receptors involved. These data suggest the existence of physically associated mu opioid and CB1 cannabinoid receptors, whereby activation of these receptors results in either a non-additive (glutamate release) or a synergistic (GABA release) effect. It is proposed that these allosterically interacting mu and CB1 receptors in the NAc core may represent G-protein coupled heterodimeric receptor complexes.

Synergistically interacting dopamine D1 and NMDA receptors mediate nonvesicular transporter-dependent GABA release from rat striatal medium spiny neurons

Given the complex interactions between dopamine D1 and glutamate NMDA receptors in the striatum, we investigated the role of these receptors in transporter-mediated GABA release from cultured medium spiny neurons of rat striatum. Like NMDA receptor-mediated [(3)H]-GABA release, that induced by prolonged (20 min) dopamine D1 receptor activation was enhanced on omission of external calcium, was action potential-independent (tetrodotoxin-insensitive), and was diminished by the GABA transporter blocker nipecotic acid, indicating the involvement of transporter-mediated release. Interestingly, lowering the external sodium concentration only reduced the stimulatory effect of NMDA. Blockade of Na(+)/K(+)-ATPase by ouabain enhanced NMDA-induced but abolished dopamine-induced release. Moreover, dopamine appeared to potentiate the effect of NMDA on [(3)H]-GABA release. These effects of dopamine were mimicked by forskolin. mu-Opioid receptor-mediated inhibition of adenylyl cyclase by morphine reduced dopamine- and NMDA-induced release. These results confirm previous studies indicating that NMDA receptor activation causes a slow action potential-independent efflux of GABA by reversal of the sodium-dependent GABA transporter on sodium entry through the NMDA receptor channel. Moreover, our data indicate that activation of G-protein-coupled dopamine D1 receptors also induces a transporter-mediated increase in spontaneous GABA release, but through a different mechanism of action, i.e., through cAMP-dependent inhibition of Na(+)/K(+)-ATPase, inducing accumulation of intracellular sodium, reversal of the GABA carrier, and potentiation of NMDA-induced release. These receptor interactions may play a crucial role in the behavioral activating effects of psychostimulant drugs.

Unrestricted free-choice ethanol self-administration in rats causes long-term neuroadaptations in the nucleus accumbens and caudate putamen

In the present study, the reactivity of striatal dopamine and dopamine-sensitive neurons in super-fused striatal slices of ethanol-experienced rats was compared to that of ethanol-naive rats, 3 weeks after oral ethanol self-administration. During the acquisition phase (17 days), rats were offered increasing concentrations of ethanol (from 2 to 10%, 24 h per day) on an alternate-day schedule in a free choice with water. Following 2 weeks of unrestricted 10% ethanol consumption, the highest and lowest drinkers (representing about 25% of the upper and lower extremes of the total population) were selected. Preliminary experiments revealed that both groups of rats displayed a profound increase in ethanol consumption and preference 3 weeks after cessation of ethanol self-administration (deprivation effect). This deprivation effect was associated with an increase in electrically evoked release of [3H]dopamine from superfused nucleus accumbens slices, whereas the evoked [3H]dopamine release from caudate putamen slices remained unchanged. In slices of the caudate putamen, but not in nucleus accumbens slices, postsynaptic dopamine D1 receptor-stimulated cyclic AMP production was also enhanced. In addition, prior ethanol consumption enhanced the electrically evoked release of [14C]acetylcholine release in both striatal regions. Interestingly, the magnitude of these long-term neuroadaptations correlated with the amount of daily ethanol consumption, i.e. neuronal hyperresponsiveness in the striatum was more profound in the high than in the low ethanol drinkers. These data show for the first time that unrestricted free-choice ethanol consumption in rats is associated with a long-term increase in dopaminergic and cholinergic neurotransmission in the nucleus accumbens and caudate putamen. These (and other) neuroadaptations may underlie the enhanced motivation to self-administer ethanol and the maintenance of ethanol consumption long after deprivation.

Lack of alpha2-adrenoceptor autoregulation of noradrenaline release in rat nucleus accumbens slices

The role of release-inhibitory alpha2-adrenoceptors on or near noradrenergic nerve terminals was investigated in slices of rat nucleus accumbens and medial prefrontal cortex, representing major projection areas of mesocorticolimbic dopamine neurons. As expected, the electrically evoked release of [3H]noradrenaline from superfused medial prefrontal cortex slices was strongly inhibited (to about 25% of control values) by exogeneous noradrenaline and by the alpha2-adrenoceptor agonist oxymetazoline, while blockade of alpha2-adrenoceptors with phentolamine or rauwolscine caused a large increase (to almost 250% of control values) in neurotransmitter release. In striking contrast and much to our surprise, these drugs did not at all affect the electrically evoked release of [3H]noradrenaline from nucleus accumbens slices. These results indicate that alpha2-adrenoceptor autoregulation of noradrenaline release may not be a major feature of noradrenergic varicosities in rat nucleus accumbens. Thus, the functioning or presence of these receptors may vary with the brain region innervated by locus coeruleus neurons and may not represent a ubiquitous feature of noradrenergic neurons.

Kappa1- and kappa2-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 microM)-induced release of [3H]-dopamine and [14C]-acetylcholine (ACh) from superfused neostriatal slices were studied to investigate the possible occurrence of functional kappa-opioid receptor subtypes in rat brain. 2. The kappa receptor agonists (-)-ethylketocyclazocine ((-)-EKC), U69593 and the endogenous opioid peptide dynorphin A1-13 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 mu agonist Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO) and the delta agonist D-Pen2-D-Pen5-enkephalin (DPDPE) were ineffective in this respect. The inhibitory effect of submaximally effective concentrations of dynorphin A1-13, U69593 and (-)-EKC on NMDA-induced [3H]-dopamine release were not changed by the delta1/delta2-opioid receptor antagonist naltrindole (up to a concentration of 1 microM, but reversed by the kappa receptor antagonist nor-binaltorphimine (nor-BNI), with an IC50) as low as 0.02 nM, indicating the involvement of U69593-sensitive kappa1-opioid receptors. 3. NMDA-induced [14C]-ACh release was reduced in a naloxone-reversible manner by DPDPE (pD2 about 7.2), dynorphin A1-13 (pD2 6.7) and EKC (pD2 6.2), but not by U69593 and DAMGO. The inhibitory effect of a submaximally effective concentration of DPDPE, unlike those of dynorphin A1-13 and (-)-EKC, on NMDA-induced [14C]-ACh release was antagonized by naltrindole with an IC50 of 1 nM, indicating the involvement of delta-opioid receptors in the inhibitory effect of DPDPE. On the other hand, the inhibitory effects of dynorphin A1-13 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 U69593-insensitive kappa2-opioid receptors in the inhibitory effects of dynorphin A1-13 and (-)-EKC. 4. Although naloxone benzoylhydrazone (NalBzoH), displaying high affinity towards the putative kappa3-opioid receptor, antagonized the inhibitory effects of dynorphin A1-13 and (-)-EKC on [3H]-dopamine and [14C]-ACh release as well as that of U69593 on [3H]-dopamine release, it displayed a low apparent affinity (IC50 about 100 nM) in each case. 5. In conclusion, whereas activation of kappa1-opioid receptors causes presynaptic inhibition of NMDA-induced dopamine release, kappa2 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 kappa-opioid receptor subtypes in the brain.

Ethanol, like psychostimulants and morphine, causes long-lasting hyperreactivity of dopamine and acetylcholine neurons of rat nucleus accumbens: possible role in behavioural sensitization

Repeated treatment of rats with ethanol (1 g/kg, once daily for 15 days) enhanced the locomotor effect of morphine, 3 weeks post-treatment. This ethanol-induced long-term behavioural sensitization to morphine was associated with an increase in the electrically evoked release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) from nucleus accumbens slices. A similar enhanced responsiveness of accumbal dopaminergic and cholinergic neurons to depolarization was apparent 3 weeks after repeated morphine, amphetamine or cocaine administration. Prior ethanol exposure also caused a long-term enhancement of electrically evoked release of [3H]DA and [14C]ACh from slices of the caudate-putamen. Unlike the locomotor effect of morphine, that of amphetamine was not enhanced in ethanol-pretreated rats. These data indicate that ethanol administration may cause long-term behavioural sensitization associated with adaptive changes in dopaminergic and cholinergic neurons of rat nucleus accumbens and caudate-putamen. Furthermore, an enhanced reactivity of nucleus accumbens dopaminergic nerve terminals and dopamine-sensitive cholinergic neurons appears to be a common long-term neuroadaptive effect of distinct types of addictive drugs. However, since repeated ethanol exposure did not cause a long-term increase in the locomotor effect of amphetamine, these neuroadaptations may not always be sufficient to cause long-lasting behavioural (cross-)sensitization.

Dopamine displays an identical apparent affinity towards functional dopamine D1 and D2 receptors in rat striatal slices: possible implications for the regulatory role of D2 receptors

In this study we examined the selectivity of dopamine (DA) for rat striatal DA D1 and D2 receptors. In a Krebs-HEPES buffer, the Ki values of DA for D1 binding sites (labelled with [3H]SCH23390) and D2 binding sites (labelled with [3H]spiroperidol) in striatal membranes amounted to about 30 and 0.3 microM, respectively. However, the EC50s of DA (3 microM) and the DA releasing drug amphetamine (1 microM) were identical considering D1 receptor-stimulated and D2 receptor-inhibited adenylate cyclase activity in superfused striatal slices. Moreover, these EC50 values were also obtained studying DA- and amphetamine-induced D2 receptor activation, resulting in inhibition of the electrically evoked release of [14C]acetylcholine from the slices. Therefore, with regard to the apparent affinity of exogenous and endogenous DA for D1 and D2 receptors in rat striatal slices, the ligand-receptor binding data appeared to be misleading. Thus, our data show that in rat striatal slices DA has an identical apparent affinity towards functional D1 and D2 receptors, which is particularly intriguing in view of the very high receptor selectivity of synthetic D1 and D2 receptor agonists for these functional receptors in superfused brain slices as predicted on the basis of binding assays. This may have important implications for our understanding of central DA neurotransmission. For instance, since the inhibitory effect of opioid and muscarinic receptor activation on adenylate cyclase activity has been shown to be inversely related to the degree of DA D2 receptor activation, the degree of activation of D1 and D2 receptors by released DA is suggested to act as a functional gate allowing distinct neurotransmitters to play a role in striatal neurotransmission.

Repeated and chronic morphine administration causes differential long-lasting changes in dopaminergic neurotransmission in rat striatum without changing its delta- and kappa-opioid receptor regulation

Repeated, once daily morphine treatment (14 days) as well as chronic morphine administration (6 days) caused a rebound reduction in the electrically evoked release of [3H]dopamine from superfused rat striatal slices 1 day after the last subcutaneous injection. Interestingly, whereas [3H]dopamine release remained significantly reduced for at least 3 weeks following morphine withdrawal in chronically treated (tolerant/dependent) rats, neurotransmitter release from dopaminergic nerve terminals gradually increased above control values following cessation of repeated morphine administration. Postsynaptically, dopamine D1 receptor-stimulated adenylate cyclase appeared to be sensitized 1-3 days but was unchanged 3 weeks after chronic morphine treatment. In contrast, such an enhanced postsynaptic dopamine D1 receptor efficacy did not occur 1-3 days following repeated morphine administration, but appeared to develop slowly resulting in a profound increase of dopamine-sensitive adenylate cyclase 3 weeks after the last injection. The inhibitory effect of dynorphin A-(1-13) on [3H]dopamine release, as well as that of [Met5]enkephalin on dopamine D1 receptor-stimulated adenylate cyclase appeared to be unchanged subsequent to repeated or chronic morphine treatment. These data indicate that, long after cessation of drug treatment, chronic morphine treatment causes a reduction whereas repeated morphine administration gradually induces an enhancement of opioid receptor-regulated dopaminergic neurotransmission due to local adaptive changes within the rat striatum. Such distinct long-lasting alterations of dopaminergic neurotransmission induced by different temporal patterns of morphine administration in projection areas of mesencephalic dopaminergic neurons may be related to the enduring effects of drug abuse such as behavioural sensitization and drug craving.

Mu- and delta-opioid receptors inhibitorily linked to dopamine-sensitive adenylate cyclase in rat striatum display a selectivity profile toward endogenous opioid peptides different from that of presynaptic mu, delta and kappa receptors

The apparent affinities of endogenous opioid peptides for noncompetitively interacting mu and delta receptors, inhibitorily linked to dopamine (DA) D-1 receptor-stimulated adenylate cyclase, were investigated in superfused rat striatal slices exposed to 40 microM DA in the presence of 10 microM of the selective D-2 receptor antagonist (-)sulpiride. In the presence of peptidase inhibitors, a comparison was made with the apparent affinities of opioid peptides toward independent presynaptic opioid receptors in brain slices. beta-Endorphin had an about 100-fold higher apparent affinity (EC50: 1 nM) toward presynaptic mu-opioid receptors, mediating inhibition of the electrically evoked neocortical [3H]norepinephrine release, than for the striatal adenylate cyclase-coupled mu receptors. In contrast, the kappa-opioid receptor agonist dynorphin A1-13 displayed a similar apparent affinity (EC50: 0.1 microM) toward these functionally different mu receptors. Both Leu- and Met-enkephalin showed only a 3-fold higher apparent affinity (EC50: 30 nM) for presynaptic delta-opioid receptors, mediating inhibition of striatal [14C]acetylcholine release, than for presynaptic mu receptors. However, whereas Leu-enkephalin had a similar apparent affinity for presynaptic and adenylate cyclase-coupled delta receptors, Met-enkephalin displayed a 30-fold selectivity toward the latter receptors. Studying the inhibitory effect of Met-enkephalin on striatal adenylate cyclase stimulated by endogenously released (amphetamine-induced) DA, its very high affinity appeared to be inversely related to the activation of inhibitory DA D-2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-lasting reciprocal changes in striatal dopamine and acetylcholine release upon morphine withdrawal

In superfused rat striatal slices, the electrically evoked release of [3H]dopamine was reduced by about 30% and that of [14C]acetylcholine was enhanced by about 25% no less than 3 weeks after morphine withdrawal in rats chronically treated with the opiate for 6 days. The lasting nature of the reduced dopamine release at the level of dopaminergic nerve terminals and the concomittant enhanced excitability of neurons tonically inhibited by released dopamine, such as cholinergic interneurons, could play an important role in the maintenance of opiate dependence.

Opioid receptors and inhibition of dopamine-sensitive adenylate cyclase in slices of rat brain regions receiving a dense dopaminergic input

In slices of rat nucleus accumbens, olfactory tubercle, frontal cortex and mediobasal hypothalamus exposed to dopamine (DA), the activation of DA D1 receptors stimulated cyclic AMP (cAMP) formation whereas, in nucleus accumbens slices only, activation of D2 receptors appeared to inhibit D1 receptor-stimulated adenylate cyclase at the same time. Activation of mu-opioid receptors by [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO; 1 microM), but not of delta-opioid receptors by 1 microM [D-Pen2,D-Pen5]enkephalin (DPDPE), inhibited (by 35-40%) DA-stimulated cAMP production in slices of nucleus accumbens and olfactory tubercle. When adenylate cyclase was stimulated by selective D1 receptor activation, i.e. by DA in the presence of (-)-sulpiride, DPDPE reduced cAMP formation (by about 45%) in nucleus accumbens slices but not in slices of the other brain regions. The kappa-agonist, U 50,488, did not affect DA- or D1 receptor-stimulated adenylate cyclase activity in any of the brain regions. Preincubation of nucleus accumbens slices with the irreversible delta-ligand, fentanyl isothiocyanate (FIT; 1 microM), not only antagonized the inhibitory effect of DPDPE but also prevented the antagonism by naloxone of the inhibitory effect of DAMGO. Therefore, in nucleus accumbens opioids may inhibit DA-sensitive adenylate cyclase through activation of a mu/delta-opioid receptor complex, whereas in olfactory tubercle mu-receptors appear to mediate the inhibition of adenylate cyclase activity. Opioids do not seem to affect DA-stimulated cAMP formation in frontal cortex and mediobasal hypothalamus.

Opioid receptor antagonists discriminate between presynaptic mu and delta receptors and the adenylate cyclase-coupled opioid receptor complex in the brain

The present study addressed the question as to whether or not' interacting mu and delta opioid receptors, which may constitute an opioid receptor complex-inhibitory coupled to adenylate cyclase in rat neostriatum, display different antagonistic properties than the classical (noncomplexed) mu and delta receptors. In concentrations that antagonized the presynaptic inhibitory effect of [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO) on [3H]norepinephrine release from rat neocortical slices, the cyclic somatostatin-related mu opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 did not affect the inhibition of dopamine-sensitive adenylate cyclase caused by DAMGO in neostriatal slices. The delta opioid receptor antagonist naltrindole appeared to be about 200-fold more effective as an antagonist against inhibitory effect of [D-Ser2(O-tert-butyl),Leu5]enkephalyl-Thr6 on [14C]acetylcholine release from neostriatal slices than against the inhibitory effect of DAMGO on [3H]norepinephrine release from neocortical slices, in agreement with the involvement of presynaptic delta and mu receptors, respectively. However, regarding the inhibitory effect of DAMGO and [D-Ser2(O-tert-butyl),Leu5] enkephalyl-Thr6 on adenylate cyclase activity in neostriatal slices, naltrindole not only displayed a very low affinity but also only 10-fold delta-selectivity. In striking contrast to D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 and naltrindole, naloxone did not discriminate between the neurotransmitter release-and adenylate cyclase-inhibitory effects of DAMGO and [D-Ser2(O-tert-butyl), Leu5]enkephalyl-Thr6.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioid receptor-mediated inhibition of 3H-dopamine and 14C-acetylcholine release from rat nucleus accumbens slices. A study on the possible involvement of K+ channels and adenylate cyclase

The release of 14C-ACh from rat nucleus accumbens slices, induced by 15 mM [K+], was inhibited by the mu- and delta-opioid agonists DAMGO and DPDPE, respectively, whereas only the kappa agonist U50,488 reduced the release of 3H-DA. The opioid receptors involved appear to be localized on nerve terminals, since blockade of action potential propagation by 1 microM TTX did not diminish the inhibitory effects of DAMGO, DPDPE or U50,488. Enhancement of the potassium concentration in the superfusion medium to 56 mM with simultaneous reduction of the Ca2+ concentration from 1.2 mM to 0.12 mM induced a release similar to that caused by 15 mM K+ and 1.2 mM Ca+. Under this conditions, the inhibitory effects of both DAMGO and DPDPE on stimulated 14C-ACh release were reduced, whereas the inhibition of evoked 3H-DA release caused by U50,488 was not affected. Activation of mu- as well as delta-opioid receptors by DAMGO and DPDPE, respectively, inhibited forskolin-stimulated adenylate cyclase activity. However, increasing the intracellular cAMP levels with 0.3 mM 8-bromo-cAMP affected neither the depolarization-induced release of 14C-ACh or 3H-DA from accumbens slices nor the inhibitory effects of opioid receptor activation thereon. The results indicate that the mechanism by which functional mu and delta receptors presynaptically inhibit the depolarization-induced 14C-ACh release from nucleus accumbens slices is likely to involve an increase of potassium channel conductance. In contrast, activation of kappa-opioid receptors, which inhibits depolarization-evoked 3H-DA release, apparently does not result in a hyperpolarization of (dopaminergic) nerve terminals. In none of these inhibitory effects presynaptic adenylate cyclase appears to be involved.

Cyclic somatostatin analogues as potent antagonists at mu-, but not delta- and kappa-opioid receptors mediating presynaptic inhibition of neurotransmitter release in the brain

The opioid receptor antagonist properties of four conformationally constrained cyclic octapeptide analogues of somatostatin were investigated using in vitro functional paradigms of mu-, delta- and kappa-opioid receptors in the rat brain. The analogues examined were D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), D-Tic-CTOP (TCTOP) and D-Tic-CTAP (TCTAP). Activation of mu-receptors by the enkephalin analogue Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAGO) inhibited the (electrically evoked) release of [3H]noradrenaline (NA) from superfused cortical slices and this inhibitory effect was antagonized in a competitive fashion by all of the octapeptides tested (pA2 values: CTOP and CTAP 7.9-8.0, TCTOP and TCTAP 8.7-8.8). Selective activation of kappa-opioid receptors by the cyclohexylbenzeneaceamide U69593 (0.02 microM) inhibited (by 40-45%) the release of [3H]dopamine (DA) from striatal slices, whereas selective activation of delta-opioid receptors by [D-Ser2(O-t-butyl),Leu5]enkephalyl-Thr6 (DSTBULET; 0.1 microM) caused an inhibition (by 38-46%) of striatal [14C]acetylcholine (ACh) release. However, these inhibitory effects were not affected by any of the octapeptides in concentrations that caused full antagonism of the inhibitory effect (55-65%) of 0.1 microM DAGO on cortical [3H]NA release. Thus, the cyclic octapeptide somatostatin analogues CTOP, CTAP, TCTOP and TCTAP are potent and highly selective antagonists at the mu-opioid receptors mediating presynaptic inhibition of NA release in the brain. The mu-receptor affinity of the most potent of these antagonists, TCTOP and TCTAP, appears to be similar to that of naloxone but these antagonists have a much greater selectivity than the latter.

Effect of chronic prenatal morphine treatment of mu-opioid receptor-regulated adenylate cyclase activity and neurotransmitter release in rat brain slices

Timed-pregnant rats received a semisynthetic diet with or without morphine (0.5-1 mg/g) for 2 weeks. After 21 days of gestation the morphine-dependent dams were decapitated and the foetal brains were dissected. Chronic morphine administration caused a profound increase of adenylate cyclase activity stimulated by postsynaptic D1 dopamine receptors in striatal slices. The relative inhibitory effect of [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) on D1-stimulated cyclic AMP (cAMP) production was unaffected. In contrast, cAMP production induced via direct activation of the catalytic unit of adenylate cyclase with forskolin was not changed upon long-term morphine treatment, although DAGO strongly inhibited the effect of forskolin. The electrically evoked release of [3H]noradrenaline (NA) from superfused neocortical slices was strongly enhanced upon morphine treatment, whereas release induced by the calcium ionophore A23187, bypassing voltage-sensitive calcium channels, was unchanged. Again, the inhibitory effect of the mu receptor agonist DAGO was unaffected in neocortical slices from morphine-treated rats. It is suggested that tolerance to morphine may be caused by the fact that the opiate is acting against up-regulated signal transduction mechanisms, rather than by desensitization of central mu-opioid receptors. The pre- and postsynaptic changes may include an enhanced expression and/or biochemical modification of D1 receptors, Gs proteins and calcium channels in central neurons on which mu-opioid receptors are present. At the same time, these adaptive changes may underlie morphine withdrawal phenomena.

Opioid-receptor-mediated inhibition of [3H]dopamine but not [3H]noradrenaline release from rat mediobasal hypothalamus slices

The modulation of the electrically evoked release of [3H]dopamine (DA) and [3H]noradrenaline (NA) by opioid receptor activation was examined in superfused slices of rat mediobasal hypothalamus (MBH). [3H]DA release was inhibited (maximally by 30-35%) by both the selective kappa-agonist U 50,488 (1 nM to 1 microM) and the selective mu-agonist DAGO (0.01-1 microM) but not by the delta-selective agonist DPDPE (1 microM). Naloxone partly antagonized the inhibitory effect of U 50,488 and completely that of DAGO, whereas the selective kappa-antagonist norbinaltorphimine (nor-BNI) only antagonized the inhibition caused by U 50,488. The dopamine D2 receptor agonist quinpirole as well as the alpha 2-adrenoceptor agonist oxymetazoline both decreased (by 25-30%) the evoked overflow of [3H]DA. The evoked release of [3H]NA was not modulated by any of the opioid agonists nor by quinpirole. However, the alpha 2-adrenoceptor agonist oxymetazoline inhibited the release of [3H]NA by 30-40%. Activation of alpha 2-adrenoceptors by oxymetazoline prevented the inhibitory effect of U 50,488, but not DAGO, on evoked [3H]DA release, whereas the selective kappa-antagonist nor-BNI antagonized the inhibition by oxymetazoline of [3H]DA, but not [3H]NA, release. In conclusion, activation of both kappa- and mu-opioid receptors results in an inhibition of evoked DA release from MBH slices but does not modulate NA release. Therefore, several of the reported effects of opioids on hormone secretion may be an (indirect) consequence of a reduction of DA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-endorphin: a highly selective endogenous opioid agonist for presynaptic mu opioid receptors

In the presence of physiological cations (in Krebs-4-(2-hydroxyethyl)-1- piperazineethanesulfonic acid buffer) at 37 degrees C the Ki value's of beta-endorphin for mu- and delta-opioid receptor binding sites in rat neocortical membranes, labeled with [3H][D-Ala2,MePhe4,Gly- ol5]enkephalin (DAMGO) and [3H][D-Ala2-D-Leu5]enkephalin (in the presence of unlabeled DAMGO), respectively, amounted to about 9 and 22 nM. Surprisingly, a very different selectivity pattern for the endogenous opioid peptide was found when the affinity of beta-endorphin for functional presynaptic opioid receptors was examined. Thus, beta-endorphin strongly inhibited the electrically evoked release of [3H]NE from rat neocortical slices with an IC50 value of about 0.5 nM, whereas [14C] acetylcholine release from neostriatal slices was inhibited with an IC50 value of about 100 nM. On the other hand, the electrically evoked release of [3H]dopamine from striatal slices was not affected by beta-endorphin. The inhibitory effects of DAMGO and beta-endorphin on [3H]NE release from neocortical slices were equally well antagonized by naloxone. Moreover, 10 nM of the highly selective mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen- Thr-NH2 antagonized competitively the inhibitory effect of beta-endorphin on [3H]NE release.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological profile of various kappa-agonists at kappa-, mu- and delta-opioid receptors mediating presynaptic inhibition of neurotransmitter release in the rat brain

1. The potency, relative efficacy and selectivity of a series of kappa-opioid receptor agonists at the mu-, delta- and kappa-opioid receptors mediating inhibition of electrically-induced (radiolabelled) neurotransmitter release from superfused rat brain slices was determined. 2. With regard to their potencies at kappa-receptors mediating inhibition of striatal [3H]-dopamine release, the highest pD2 value (8.7) was found for bremazocine and the lowest (7.1) for U50488; the pD2 values for ethylketocyclazocine (EKC), tifluadom, U69593 and PD117302 were between 8.0 and 8.3. There were no marked differences between the relative efficacies of the kappa-agonists (maximum inhibition being 60-70%). In contrast to the other kappa-agonists, at a concentration of 1 microM, PD117302 caused a significant (25-40%) increase of the spontaneous efflux of tritium. 3. None of the kappa-agonists significantly affected striatal [14C]-acetylcholine (ACh) release, with the exception of a slight inhibitory effect of EKC. The delta-receptor-mediated inhibitory effect of [D-Ala2, D-Leu5]enkephalin (DADLE) on [14C]-ACh release was antagonized in a concentration-dependent manner by bremazocine (0.1 and 1.0 microM) and also partially by EKC (1 microM), but not by the other kappa-agonists. The pA2 value for bremazocine as an antagonist at the delta-receptors involved was 8.0, compared to 7.6 for naloxone. 4. None of the kappa-agonists significantly affected cortical [3H]-noradrenaline (NA) release, with the notable exception of tifluadom, which strongly inhibited release by activating mu-receptors. The mu-receptor-mediated inhibitory effect of Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO) on [3H]-NA release was antagonized in a concentration-dependent manner by bremazocine and EKC, but not by the other K-agonists. The pA2 value for bremazocine as an antagonist at the mu-receptors involved was 8.2, compared to 8.6 for naloxone. 5. Thus, whereas U69593 and PD1 17302 display high potency and selectivity towards K-opioid receptors, the potent benzomorphan K-agonists bremazocine and EKC also appear to be strong mu-opioid receptor antagonists.

Opioid receptor-mediated inhibition of dopamine and acetylcholine release from slices of rat nucleus accumbens, olfactory tubercle and frontal cortex

The modulation of the electrically evoked release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) by opioid receptor activation was examined in superfused slices from rat nucleus accumbens, olfactory tubercle, and frontal cortex. In all brain areas examined, [3H]DA release was inhibited by the kappa agonist, U 50,488 (1-100 nM), and this inhibition was fully antagonized by the selective kappa antagonist, norbinaltorphimine (nor-BNI). In the frontal cortex, the mu agonist, [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO, 0.01-1 microM), also inhibited the evoked release of tritium. However, further experiments (including the use of the D2-receptor agonist, LY 171555, and the alpha 2-adrenoceptor agonist, oxymetazoline) suggest strongly that in the frontal cortex DAGO only inhibits the release of [3H]catecholamine from noradrenergic nerve terminals, despite the use of desimipramine to prevent the uptake of [3H]DA into these terminals. [14C]ACh release from both the nucleus accumbens and olfactory tubercle, but not from the frontal cortex, was inhibited by DAGO (0.01-1 microM) and the delta agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE, 0.01-1 microM). These inhibitory effects were antagonized by 0.1 microM naloxone but not by 3 nM nor-BNI. The irreversible delta ligand, fentanyl isothiocyanate (FIT, 1 microM), only antagonized the inhibition caused by DPDPE. The results indicate that the inhibitory effects of opioids on the in vitro release of DA from dopaminergic nerve fibres arising from the substantia nigra and the ventral tegmental area are mediated by presynaptic kappa receptors only. In those regions where ACh release is modulated by opioids, the type of opioid receptor involved may depend on the type of neuron, i.e. interneuron or afferent neuron.

Ontogeny of mu-, delta- and kappa-opioid receptors mediating inhibition of neurotransmitter release and adenylate cyclase activity in rat brain

The ontogeny was examined of functional opioid receptors mediating presynaptic inhibition of neurotransmitter release and inhibition of dopamine (DA)-sensitive adenylate cyclase in the rat brain, using highly selective agonists for mu-, delta- and kappa-receptors. On gestational day 17 (E17) strong inhibitory effects of the selective mu-agonist DAGO on the electrically evoked release of [3H]noradrenaline from cortical slices and of the selective kappa-agonist U-50,488 on the electrically evoked release of [3H]DA from striatal slices were found. Electrically evoked release of [3H]acetylcholine from striatal slices was not detectable before postnatal day 7 (P7), but on that day it was already strongly inhibited by the selective delta-agonist DPDPE. Although mu- and delta-opioid receptors coupled to DA-sensitive adenylate cyclase in the striatum are likely to be physically associated in an opioid receptor complex in the adult, they were found to develop asynchronously. Whereas selective activation of mu-receptors with DAGO resulted in an inhibition of D1 dopamine receptor-stimulated adenylate cyclase activity on E17, activation of delta-receptors with DPDPE was not effective until P14. This study confirms the early appearance of mu- and kappa-opioid receptors and the relatively late development of delta-opioid receptors in the rat brain. Most importantly, it shows that in an early stage of development opioids are already able to mediate modulation of noradrenergic (via activation of mu-receptors) and dopaminergic (via activation of mu- and kappa-receptors) neurotransmission processes. Therefore, these opioid receptor types could play a role in brain development and/or developmental disturbances.

Selectivity of various opioid peptides towards delta-, kappa; and mu-opioid receptors mediating presynaptic inhibition of neurotransmitter release in the brain

The selectivity of a series of opioid peptides towards the mu-, delta- and kappa-opioid receptors mediating differential inhibition of electrically-induced neurotransmitter release from rat brain slices was studied, viz. cortical [3H]noradrenaline release (inhibited via mu-receptors), striatal [3H]dopamine release (inhibited via kappa-receptors) and striatal [14C] acetylcholine release (inhibited via delta-receptors). The highest affinity pD2 7.4) and selectivity towards mu-receptors was exhibited by Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAGO), whereas [D-Pen2, D-Pen5]enkephalin (DPDPE) was found to be the most selective delta-receptor agonist (pD2 7.3). Also the hexapeptides [D-Ser2]Leu-enkephalin-Thr (DSLET) and [D-Thr2]Leu-enkephalin-Thr (DTLET) showed a relatively high selectivity and, in addition, a high affinity (pD2 8.2-8.4) for delta-opioid receptors. Both dynorphin(1-13) and dynorphin(1-8) exhibited a high affinity for kappa-receptors (pD2 resp. 8.3 and 8.0), but the latter was far less selective. Both of the dynorphin A-related peptides showed affinity to mu-receptors (pD2 6.7-6.8), but dynorphin(1-8), in contrast to dynorphin(1-13), also displayed a high affinity to delta-receptors (pD2 7.6).

Mu- and delta-opioid receptor-mediated inhibition of adenylate cyclase activity stimulated by released endogenous dopamine in rat neostriatal slices; demonstration of potent delta-agonist activity of bremazocine

Rat neostriatal slices were superfused with medium containing 0.1 to 30 microM of the dopamine (DA)-releasing agent D-(+)-am-phetamine (AMPH) and the D-2 DA receptor antagonist (-)-sulpiride (10 microM) in the absence or presence of mu-, delta-, and kappa-selective opioids. AMPH dose-dependently enhanced the cyclic AMP production, as measured by its efflux from striatal slices, whereas simultaneous blockade of D-2 DA receptors by (-)-sulpiride strongly potentiated this effect. Both the mu-opioid receptor selective agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (0.01-3 microM) and the delta-opioid receptor selective agonist [D-Phe2-D-Pen5]enkephalin (DPDPE, 0.01-3 microM) inhibited the cyclic AMP efflux, stimulated by 10 microM AMPH in the presence of (-)-sulpiride, by 70 to 80%. The highly selective kappa-opioid receptor agonist U 50,488 (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrol-idinyl)- cyclohexyl]benzeneacetamide methanesulfonate hydrate) (0.01-1 microM) had no effect. In contrast, the purported kappa-opioid receptor agonist bremazocine (3-300 nM) inhibited the stimulated adenylate cyclase activity to a similar extent as did [D-Ala2-MePhe4,Gly-ol5]enkephalin and DPDPE. Moreover, the selective irreversible delta-antagonist fentanyl isothiocyanate reversed both the inhibition caused by DPDPE and that caused by bremazocine, whereas the kappa-selective antagonist norbinaltorphimine showed no differences in its potency to antagonize the inhibitory effects of the different opioid agonists. The results indicate that opioids, by activating mu- or delta-, but not kappa-opioid receptors may cause a profound inhibition of adenylate cyclase activity stimulated by activation of (postsynaptic) D-1 DA receptors upon the (presynaptic) release of DA.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium dependent 3H-noradrenaline release from rat neocortical slices in the absence of extracellular calcium: presynaptic modulation by mu-opioid receptor and adenylate cyclase activation

In Ca2+-free EGTA (1 mmol/l)-containing medium veratrine (3 mumol/l) and ouabain (100 mumol/l) strongly enhanced the efflux of 3H-noradrenaline from superfused rat brain neocortical slices prelabelled with the radioactive amine. In both cases 3H-noradrenaline release was prevented by tetrodotoxin (1 mumol/l). These effects of veratrine and ouabain were virtually additive and independent of whether the noradrenaline uptake carrier was blocked with 1 mumol/l desipramine or not. The adenylate cyclase activator forskolin (10 nmol/l - 10 mumol/l) strongly enhanced veratrine- and ouabain-induced 3H-noradrenaline release, without affecting spontaneous tritium efflux. The release induced by both stimuli was profoundly inhibited by the selective mu-opioid receptor agonist [D-Ala, MePhe4, Gly-ol5]enkaphalin (DAGO, 3 nmol/l - 1 mumol/l) in a concentration-dependent manner. The inhibitory effects of 1 mumol/l DAGO were abolished by 1 mumol/l naloxone. On the other hand, preincubation of the slices for 1 h with the delta-opioid receptor-selective irreversible ligand fentanyl isothiocyanate (1 mumol/l) did not change the inhibitory effects of DAGO. These data show that veratrine- and ouabain-induced 3H-noradrenaline release from central noradrenergic nerve terminals is facilitated by increasing intracellular cyclic AMP levels and reduced by activation of presynaptic mu-opioid receptors, indicating the involvement of exocytotic neurotransmitter release. The results provide further evidence for the hypothesis that under these conditions neurotransmitter release from central noradrenergic neurons is triggerred by a Na+-induced efflux of Ca2+ ions from intracellular stores.

Mu-, delta- and kappa-opioid receptor-mediated inhibition of neurotransmitter release and adenylate cyclase activity in rat brain slices: studies with fentanyl isothiocyanate

We investigated the effects of [D-Ala2,D-Leu5]enkephalin (DADLE). [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO), [D-Pen2,D-Pen5]enkephalin (DPDPE) (0.01-1 microM) and bremazocine (0.001-0.3 microM) on the electrically evoked release of radiolabelled neurotransmitters and on the dopamine (DA)-stimulated cyclic AMP efflux from superfused rat brain slices. The differential inhibitory effects of these agonists on the evoked neurotransmitter release indicate that the opioid receptors mediating presynaptic inhibition of [3H]noradrenaline (NA, cortex), [14C]acetylcholine (ACh, striatum) and [3H]DA (striatum) release represent mu, delta and kappa receptors, respectively. In agreement with this classification, preincubation (60 min) of the slices with the delta-opioid receptor-selective irreversible ligand, fentanyl isothiocyanate (FIT, 0.01-1 microM), antagonized the inhibitory effects of DADLE and DPDPE on striatal [14C]ACh release only. On the other hand, the D-1 DA receptor-stimulated cyclic AMP efflux from striatal slices appeared to be inhibited by activation of mu as well as of delta receptors. In this case, the reversible mu antagonist, naloxone (0.1 microM), fully antagonized the inhibitory effect of the mu agonist, DAGO, without changing the effect of the delta agonist DPDPE but was ineffective as an antagonist in slices pretreated with FIT (1 microM). The inhibitory effect of DAGO on the electrically evoked [3H]NA release was antagonized by naloxone whether the receptors were irreversibly blocked by FIT or not. These data not only further support the existence of independent presynaptic mu-, delta- and kappa-opioid receptors in rat brain but also evidence strongly that mu and delta receptors mediating the inhibition of DA-sensitive adenylate cyclase could share a common binding site (for naloxone and FIT) and, therefore, may represent constituents of a functional opioid receptor complex.

M-1 and M-2 muscarinic receptor-mediated inhibition of dopamine-sensitive adenylate cyclase in rat neostriatum: a permissive role for D-2 dopamine receptors

The interactions between dopamine and muscarinic receptor subtypes coupled to adenylate cyclase in superfused rat neostriatal slices were investigated using the efflux of cyclic AMP, in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, as a highly sensitive parameter of cyclic AMP production. Cyclic AMP efflux induced by simultaneous activation of (stimulatory) D-1 and (inhibitory) D-2 dopamine receptors by dopamine was reduced profoundly by the muscarinic receptor agonist oxotremorine and by inhibition of acetylcholinesterase with physostigmine, but not by the M-1 muscarinic receptor agonist McN-A-343. In contrast, upon blockade of D-2 receptors with (-)-sulpiride, dopamine-stimulated cyclic AMP efflux was inhibited by oxotremorine and physostigmine as well as by McN-A-343. Cyclic AMP efflux induced by isoprenaline, adenosine or vasoactive intestinal peptide was not affected by oxotremorine. The M-1 receptor-selective antagonist pirenzepine, unlike the nonselective antagonist atropine, was about 10 times less potent in antagonizing the inhibitory effects of (a near-maximally effective concentration of) oxotremorine upon simultaneous D-1 and D-2 receptor activation that upon selective D-1 receptor activation (i.e., upon blockade of D-2 receptors). In the latter case, pirenzepine was about 5 times more effective as an antagonist when muscarinic receptors were activated by McN-A-343 than upon exposure of the slices to oxotremorine or physostigmine, whereas the potency of atropine was independent of the agonist used.(ABSTRACT TRUNCATED AT 250 WORDS)

Fentanyl isothiocyanate reveals the existence of physically associated mu- and delta-opioid receptors mediating inhibition of adenylate cyclase in rat neostriatum

Dopamine D-1 receptor-stimulated cyclic AMP efflux from superfused rat neostriatal slices was strongly inhibited by the delta-opioid receptor agonist, [D-Pen2, D-Pen5]enkephalin (DPDPE, 1 microM), and by the mu-opioid receptor agonist, [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO, 1 microM). Naloxone (0.1 microM) fully antagonized the inhibitory effect of DAGO, leaving that of DPDPE virtually unchanged. Preincubation of the slices with the irreversible delta receptor ligand, fentanyl isothiocyanate (FIT, 1 microM) did not affect the inhibitory effect of DAGO, but prevented that of DPDPE. Naloxone no longer antagonized the inhibitory effect of DAGO when the delta receptors were selectively and irreversibly blocked by FIT. These data indicate that FIT and naloxone, acting on delta and mu receptors, respectively, may share a common binding site, suggesting the involvement of a functional mu, delta-opioid receptor-complex.

Alpha 2-adrenoceptor mediated inhibition of the release of radiolabelled 5-hydroxytryptamine and noradrenaline from slices of the dorsal region of the rat brain

Possible local interactions between noradrenergic and serotonergic systems in the dorsal raphe region of the rat were investigated by studying the effects of various drugs on depolarization (20 mmol/l K+)-induced release of [3H]5-hydroxytryptamine (5-HT) and [3H]noradrenaline (NA) in vitro using a superfusion method. Exogenous 5-HT did not influence the release of [3H]NA. However, NA (in the presence of 10 mumol/l desipramine) as well as the selective alpha 2-adrenoceptor agonists clonidine and oxymetazoline strongly inhibited [3H]5-HT release. The selective alpha 1-adrenoceptor agonists phenylephrine and methoxamine did not affect the release of either [3H]5-HT or [5H]NA. The inhibition by NA of both [3H]5-HT and [3H]NA release was not affected by the beta-adrenoceptor antagonist sotalol nor by the selective alpha 1-adrenoceptor antagonist prazosin. However, phentolamine and the selective alpha 2-adrenoceptor antagonists yohimbine and rauwolscine competitively antagonized the inhibitory effect of NA on [3H]NA release (respective pA2-values 7.5 and 8.3) and on [3H]5-HT release (respective pA2-values 7.7 and 8.2). Moreover, the release of [3H]NA and also, but to a lesser extent, that of [3H]5-HT were increased by the antagonists. It is concluded that the release of both 5-HT and NA in the dorsal raphe region may be subject to presynaptic inhibition by NA via activation of alpha 2-adrenoceptors.

Opioid and D-2 receptor mediated inhibition of forskolin-stimulated cyclic AMP efflux from rat neostriatal slices

Opioid and D-2 receptor agonists inhibit adenylate cyclase activity in neostriatal slices and homogenates. In the present study we used cyclic AMP efflux from rat neostriatal tissue as a parameter to estimate the effects of these drugs on cyclic AMP formation. Both the mu-opioid receptor agonist morphine and the D-2 dopamine receptor agonist LY 171555 were able to inhibit the forskolin-stimulated cyclic AMP efflux. The effects of morphine and LY 171555 could be reversed by naloxone and sulpiride, respectively. These data indicate that measurements of cyclic AMP efflux from brain slices is an accurate reflection of the effects of receptor stimulation on adenylate cyclase activity.

Morphine and enkephalins potently inhibit [3H]noradrenaline release from rat brain cortex synaptosomes: further evidence for a presynaptic localization of mu-opioid receptors

Synaptosomes prepared from rat cerebral cortex and labeled with [3H]noradrenaline (NA) were superfused with calcium-free Krebs-Ringer-bicarbonate medium and exposed to 10 mM K+ plus 0.1 mM Ca2+ so that [3H]NA release was induced. 6,7-Dihydroxy-N,N-dimethyl-2-aminotetralin (TL-99) strongly inhibited synaptosomal K+-induced [3H]NA release (EC50 = 5-10 nM) by activating alpha 2-adrenoceptors. Release was also inhibited (maximally by 40-50%) by morphine (EC50 = 5-10 nM), [Leu5]enkephalin (EC50 = approximately 300 nM), [D-Ala2,D-Leu5]enkephalin (DADLE), and Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAGO) (EC50 values = approximately 30 nM). In contrast to the mu-selective opioid receptor agonists morphine and DAGO, the highly delta-selective agonist [D-Pen2,D-Pen5]enkephalin (1 microM) did not affect [3H]-NA release. Furthermore, the inhibitory effect of DADLE, an agonist with affinity for both delta- and mu-opioid receptors, was antagonized by low concentrations of naloxone. The findings strongly support the view that, like alpha 2-adrenoceptors, mu-opioid receptors mediating inhibition of NA release in the rat cerebral cortex are localized on noradrenergic nerve terminals.

Inhibition of dopamine-sensitive adenylate cyclase by opioids: possible involvement of physically associated mu- and delta-opioid receptors

D-1 dopamine receptor-stimulated cyclic AMP efflux from rat neostriatal slices (induced by 30 microM dopamine + 10 microM (-)sulpiride) was concentration-dependently reduced by morphine, [D-Ala-D-Leu]-enkephalin (DADLE), [D-Pen-D-Pen]enkephalin (DPDPE) and bremazocine. Naloxone (0.1 microM) selectively antagonized the inhibitory effect of (a submaximally effective concentration of) morphine, whereas ICI 174864 (0.75 microM) completely blocked the inhibitory effects of DADLE, DPDPE and bremazocine without affecting that of morphine, indicating a role of mu- as well as delta-opioid receptors. Upon simultaneous activation of D-1 dopamine receptors and delta-opioid receptors the (mu-receptor-mediated) inhibitory effect of morphine was abolished, while it was not changed following simultaneous activation of D-1 and (inhibitory) D-2 dopamine receptors. Cyclic AMP efflux induced by isoprenaline or adenosine was not affected by the opioids and that induced by vasoactive intestinal peptide (VIP) was inhibited by morphine and DADLE only. In the latter case naloxone, but not ICI 174864, antagonized the inhibitory effects. These data show that D-1 dopamine receptor-stimulated adenylate cyclase activity in rat neostriatum, but not that stimulated through other receptors, is inhibited by two pharmacologically distinct opioid receptor subtypes. It is speculated that these mu- and delta-opioid receptors share a common inhibitory guanine nucleotide binding protein and may represent closely associated recognition sites of a functional opioid receptor complex.

Evidence for a presynaptic adenylate cyclase system facilitating [3H]norepinephrine release from rat brain neocortex slices and synaptosomes

The effects of drugs known to enhance intracellular cyclic AMP levels on depolarization-induced [3H]norepinephrine release from superfused rat neocortical slices and synaptosomes were investigated. The adenylate cyclase activator forskolin, the membrane-permeating cyclic AMP analogues 8-bromo-cyclic AMP and dibutyryl cyclic AMP, as well as the phosphodiesterase inhibitors isobutylmethylxanthine and 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrolidone (ZK 62771) enhanced the electrically evoked release of [3H]norepinephrine from superfused rat brain neocortex slices. 8-Bromo-cyclic GMP was without effect on the electrically evoked release. When [3H]norepinephrine release was enhanced by prolonging the electrical pulse duration from 2 msec to 10 msec, the relative inhibitory effect of the Ca2+ channel blocker Cd2+ and the relative facilitatory effect of the K+ channel blocker 4-aminopyridine remained unaffected. In striking contrast, the relative facilitatory effects of forskolin and 8-bromo-cyclic AMP were strongly reduced, whereas the effect of ZK 62771 was almost doubled. When veratrine-induced release of [3H]norepinephrine from cortex synaptosomes was examined, the facilitatory effects of forskolin, 8-bromo-cyclic AMP, and ZK 62771 were even more pronounced than in brain slices. The data strongly support the hypothesis that a presynaptic adenylate cyclase system plays a facilitatory role in the stimulus-secretion coupling process in central noradrenergic nerve terminals.

Kappa- and delta-opioid receptor agonists differentially inhibit striatal dopamine and acetylcholine release

At least three different families of endogenous opioid peptides, the enkephalins, endorphins and dynorphins, are present in the mammalian central nervous system (CNS). Immunocytochemical studies have demonstrated their localization in neurones, which supports the view that these peptides may have a role as neurotransmitter or neuromodulators. However, the target cells and cellular processes acted upon by the opioid peptides are still largely unknown. One possible function of neuropeptides, including the opioid peptides, may be presynaptic modulation of neurotransmission in certain neuronal pathways, for example, by inhibition or promotion of neurotransmitter release from the nerve terminals. Here we report that dynorphin and some benzomorphans potently and selectively inhibit the release of (radiolabelled) dopamine from slices of rat corpus striatum, by activating kappa-opioid receptors. In contrast, [Leu5]enkephalin and [D-Ala2, D-Leu5]enkephalin selectively inhibit acetylcholine release by activating delta-opioid receptors.

Presynaptic noradrenergic alpha-receptors and modulation of 3H-noradrenaline release from rat brain synaptosomes

The depolarization (15 mM K+)-induced release of 3H-NA from superfused rat brain synaptosomes and the effects of alpha-noradrenergic drugs thereon were studied. Noradrenaline (NA; in the presence of the uptake inhibitor desipramine) reduced synaptosomal 3H-NA release. Reduction of the concentration of calcium ions in the medium during K+ stimulation greatly enhanced the sensitivity of 3H-NA release to alpha-receptor-mediated inhibition. Under these conditions NA dose-dependently inhibited 3H-NA release from synaptosomes obtained from cortex or hypothalamus, but did not affect 3H-NA release from striatal (i.e dopaminergic) synaptosomes. Adrenaline, clonidine and oxymetazoline potently inhibited 3H-NA release from cortex synaptosomes at concentrations in the nanomolar range. Phentolamine by itself did not affect synaptosomal 3H-NA release, but antagonized the inhibitory effects of both noradrenaline and adrenaline. The data obtained further substantiate the hypothesis that the alpha-receptors mediating a local negative feedback control of NA release are localized on the varicosities of central noradrenergic neurons, Furthermore, noradrenergic nerve terminals in the hypothalamus appear to be less senstive to alpha-receptor-mediated presynaptic inhibition than those in the cortex.

Alpha-receptor-mediated modulation of 3H-noradrenaline release from rat brain cortex synaptosomes

The effects of oxymetazoline and noradrenaline (in the presence of desipramine) on the release of 3H-noradrenaline from rat brain cortex synaptosomes were studied using a superfusion technique. Both drugs (at 1 micrometer concentrations) were found to reduce the depolarization-induced (15 mM K+) release of 3H-noradrenaline. The release-modulating effect of noradrenaline was antagonized by phentolamine and yohimbine. The data provide direct evidence for the hypothesis that alpha-receptors modulating the release of noradrenaline are localized on varicosities of central noradrenergic neurones.