Simultaneous demonstration of the activation of presynaptic dopamine autoreceptors and postsynaptic dopamine receptors in vitro by N,N-dipropyl-5,6-ADTN

A simple method for measuring dopamine release from rat brain slices

In many circumstances, rapid information is required about the effects of drugs on neurotransmitter release in brain, and a common method used is measurement of radiolabelled release from superfused brain slices or synaptosomes in vitro. However, the method requires expensive equipment and is not readily adapted to the measurement of endogenous release. The method described here uses readily available cheap chromatographic columns to measure both radiolabelled and endogenous dopamine (DA) release from striatal slices in repeated incubation samples. The results showed that the [3H]DA release is sensitive to temperature, K(+)-stimulation, and to both a DA agonist (pergolide) and an antagonist (eticlopride). Endogenous DA release was also stimulated by high K+ (20 mM) and sensitive to a DA agonist. Pergolide (100 microM) reduced both [3H]DA and endogenous DA release, while eticlopride (10 microM) increased [3H]DA, but not endogenous DA release. The results demonstrate an alternative cheap and quick way to study neurotransmitter release from brain in vitro.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Effect of SCH 23390 and quinpirole on novelty-induced grooming behaviour in spontaneously hypertensive rats and Wistar-Kyoto rats

Grooming behaviour induced by exposure to a novel environment was studied in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The dopamine D1 receptor antagonist, SCH 23390, and the dopamine D2 receptor agonist, quinpirole, were used to study brain dopamine systems in these rat strains, via their effects on grooming behaviour. The total grooming behaviour displayed in a 50-min observation period was significantly lower in SHR than in WKY. Except for the paw licking component no differences between the two strains were observed in the separate behavioural elements of grooming behaviour. SCH 23390 and quinpirole were found to suppress novelty-induced grooming behaviour of both strains. In SHR, grooming behaviour was less suppressed by SCH 23390, whereas the suppression by quinpirole was more pronounced than in WKY. These results indicate that there are alterations in central dopamine systems in SHR, probably involving changes both in dopamine D1 and D2 receptor mechanisms in the brain.

Effect of the dopamine D2 receptor agonist quinpirole on the in vivo release of dopamine in the caudate nucleus of hypertensive rats

Using an in vivo microdialysis method, we found that the extracellular concentrations of dopamine and its main metabolite dihydroxyphenylacetic acid (DOPAC) were lower in the caudate nucleus of 8-week-old spontaneously hypertensive rats (SHR) than in the same area of age-matched normotensive Wistar-Kyoto rats (WKY). No differences in the extracellular concentrations of dopamine and DOPAC were found between renal and deoxycorticosterone acetate (DOCA)-salt hypertensive rats when compared to their respective controls. After subcutaneous administration of the dopamine D2 receptor agonist quinpirole (10, 33 and 100 micrograms/kg), the amount of dopamine and DOPAC in the dialysates was diminished dose dependently. The quinpirole-mediated inhibition of dopamine release was more pronounced in SHR than in WKY, whereas inhibition of the extracellular DOPAC concentration was not different. Compared to WKY, the dose-response curve for the inhibition of dopamine release by quinpirole was shifted to the left in SHR and the maximal inhibition in response to the highest dose was significantly greater. Renal and DOCA-salt hypertensive rats showed no differences in the quinpirole-induced inhibition of the extracellular concentrations of striatal dopamine and DOPAC compared to their controls. The present findings on changes in dopaminergic neurotransmission and D2 autoreceptor-mediated modulation of dopamine release in genetically hypertensive rats but not in rats with experimentally induced hypertension provide further evidence for the hypothesis that alterations in the nigrostriatal dopamine system may be involved in the initiation of the development of spontaneous hypertension.

Lack of a dopamine autoreceptor selective profile of B-HT 920 in functional in vitro model systems of D2 receptors in rat striatum

Based on the results of in vivo studies, the thiazoloazepine derivative B-HT 920 has been proposed to be a selective agonist of dopamine autoreceptors. In the present study, we investigated the effects of B-HT 920 in two functional in vitro model systems of D2 receptors and compared these effects with the effects of the classical D2 agonist LY 171555. B-HT 920 and LY 171555 concentration dependently inhibited the electrically evoked release of radiolabeled dopamine and acetylcholine and the forskolin-induced stimulation of adenylate cyclase activity in rat striatal tissue slices with comparable efficacies. In striatal tissue slices prepared after 6-hydroxydopamine-induced destruction of dopaminergic terminals, both drugs were still able to inhibit forskolin-stimulated adenylate cyclase activity with a efficacy similar to that in tissue obtained from unlesioned rats. It is concluded that, in vitro, B-HT 920 is an agonist at both presynaptic and 'normosensitive' postsynaptic D2 receptors showing relatively high intrinsic activity.

4-Aminopyridine stimulates B-50 (GAP43) phosphorylation and [3H]noradrenaline release in rat hippocampal slices

In situ phosphorylation of the presynaptic protein kinase C substrate B-50 was investigated in rat hippocampal slices incubated with the convulsant drug 4-aminopyridine (4-AP). Phosphorylation of B-50 was significantly enhanced 1 min after the addition of 4-AP (100 microM). This increase by 4-AP was concentration dependent (estimated EC50 30-50 microM). Concomitant with the changes in B-50 phosphorylation, 4-AP also dose-dependently stimulated [3H]noradrenaline [( 3H]NA) release from the slices. 4-AP stimulated [3H]NA release within 5 min to seven times the control level. The B-50 phosphorylation induced by 4-AP remained elevated after removal of the convulsant, this is contrast to B-50 phosphorylation induced by depolarization with K+. A similar persistent increase was observed for [3H]NA release after a 5-min incubation period with 4-AP. These results give more insight into the molecular mechanisms underlying 4-AP-induced epileptogenesis and provide further evidence for the correlation between B-50 phosphorylation and neurotransmitter release in the hippocampal slice.

Electrically stimulated [3H]dopamine and [14C]acetylcholine release from nucleus caudatus slices: differences between spontaneously hypertensive rats and Wistar-Kyoto rats

Using an in vitro superfusion method it was found that nucleus caudatus slices of 8- and 12-week-old spontaneously hypertensive rats (SHR) release significantly less [3H]dopamine and [14C]acetylcholine upon electrical stimulation than do slices of normotensive Wistar-Kyoto rats (WKY) at all frequencies tested. At 4 weeks similar trends were seen, but the difference in [14C]acetylcholine release was not significant. That the difference in release of dopamine was already present prior to the onset of the development of hypertension, i.e. at the age of 4 weeks, indicates that it is probably not a consequence of, but rather associated with the development of hypertension. Addition of the dopamine uptake inhibitor nomifensine to the superfusion medium caused an increase in the net release of [3H]dopamine by inhibiting re-uptake, but did not influence the difference in release between SHR and WKY. The release of labelled dopamine and acetylcholine was inhibited in the presence of the dopamine D2 receptor agonist quinpirole. The concentration-response curve for the inhibition of the release of [3H]dopamine, but not that of [14C]acetylcholine, by quinpirole was shifted to the left and the maximum inhibition was higher for SHR than for WKY. These results suggest that the difference in stimulus-evoked release of labelled dopamine in the nucleus caudatus is not the consequence of changes in the uptake mechanism of dopamine, but is associated with differences between SHR and WKY in dopamine D2 autoreceptor regulation.

D-2 dopamine autoreceptor selective drugs: do they really exist?

The catecholamine dopamine plays an important role as a neurotransmitter or neurohormone in the brain and pituitary gland. Dopamine exerts its effects through activation of two types of receptors called D-1 and D-2. These receptors are distinguished by their different pharmacological characteristics and signal transduction mechanism(s). Release of dopamine inhibits the activity of dopaminergic neurons through activation of so-called dopamine autoreceptors which are of the D-2 type. In general, these receptors occur both in the soma-dendritic region of the dopaminergic neuron, where they are involved in the inhibition of the firing rate and on the dopaminergic terminals where they mediate the inhibition of dopamine synthesis and release. D-2 receptors occur also on the target cells of dopaminergic neurons both in the brain (postsynaptic D-2 receptors) and pituitary gland. On the basis of data gathered from in vivo (behavioral- as well as electrophysiological) studies it has been concluded that D-2 agonists are much more potent at dopamine autoreceptors as compared to postsynaptic D-2 receptors, indicating the possibility of a pharmacological distinction between these differentially located D-2 receptors. This concept led to the introduction of a whole group of drugs allegedly displaying a selective agonist profile at the dopamine autoreceptor. In contrast, biochemical (in vitro) studies with brain tissue as well as the pituitary gland, did not reveal any significant difference between the pharmacological profiles of autoreceptors and postsynaptic D-2 receptors. In the present minireview a balanced discussion is presented of these in vivo and in vitro findings and it is concluded that both autoreceptors as well as postsynaptic D-2 receptors are similar if not identical entities.

ACTH-(1-24) enhances the electrically stimulated release of [3H]dopamine from rat septal slices via a dopamine D2 receptor-independent mechanism

ACTH-(1-24) enhanced the basal as well as the electrically stimulated release of [3H]dopamine from rat septal slices in vitro. In the absence of Ca2+ from the superfusion medium the effect of ACTH-(1-24) on the electrically stimulated release of [3H]dopamine was abolished. The stimulus-evoked release of [3H]dopamine from septal slices appeared to be modulated through dopamine receptors of the D2 subtype: the dopamine D2 receptor agonists 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437) and quinpirole reduced, whereas the dopamine D2 receptor antagonist sulpiride enhanced the electrically stimulated release of [3H]dopamine. The magnitude of the effect of ACTH-(1-24) on [3H]dopamine release was the same in the presence or absence of N-0437, quinpirole and sulpiride. ACTH-(1-24) had no effect on either the basal or the electrically stimulated release of [3H]noradrenaline. Also when the electrically stimulated release of [3H]noradrenaline was reduced by the alpha 2-adrenoceptor agonist clonidine, the peptide was without effect. These results show that ACTH-(1-24) selectively enhances the release of [3H]dopamine from septal slices. The effect of the peptide is independent of the degree of activation of dopamine D2 receptors which modulate the stimulus-evoked release of [3H]dopamine. These results suggest that ACTH-(1-24) enhances the stimulus-evoked release of dopamine in the septum via a mechanism not associated with dopamine D2 autoreceptors.

Selective cortical infarction reduces [3H]sulpiride binding in rat caudate-putamen: autoradiographic evidence for presynaptic D2 receptors on corticostriate terminals

Although the existence of presynaptic D2 dopamine receptors on corticostriate terminals has been supported by numerous receptor-binding studies, recent autoradiographic data has failed to demonstrate loss of striatal D2 receptors following cortical lesions. In the present study, Long-Evans rats were subjected to unilateral middle cerebral artery (MCA) infarction in order to produce reproducible lesions of the neocortex without damaging subcortical structures. Animals were sacrificed 2 and 4 wk following lesion and brains were prepared for receptor autoradiography. D2 receptors were studied using the selective ligand [3H]sulpiride, while D1 dopamine receptors were examined using [3H]SCH 23390. Sodium-dependent, high-affinity choline uptake sites were labeled with [3H]hemicholinium-3, thereby providing a quantitative measure of cholinergic neuronal integrity. Unilateral cortical infarction resulted in approximately a 20% reduction in [3H]sulpiride binding in several discrete regions of the ipsilateral caudate-putamen (CPu), but not in the nucleus accumbens. D2 receptor binding was also reduced significantly in some areas of the contralateral CPu when compared with [3H]sulpiride binding in sham-operated, control animals. In contrast, D1 receptors (as identified by [3H]SCH 23390 and high-affinity choline uptake sites (labeled with [3H]-HC-3) were not affected by the cortical lesion. The results provide autoradiographic confirmation of the existence of presynaptic D2 receptors on corticostriate terminals.

Basal and electrically stimulated release of [3H]noradrenaline and [3H]dopamine from rat amygdala slices in vitro: effects of 4 beta-phorbol 12,13-dibutyrate, 4 alpha-phorbol 12,13-didecanoate and polymyxin B

The protein kinase C activator 4 beta-phorbol 12,13-dibutyrate (PDB) enhanced in a concentration-dependent manner the electrically stimulated release of [3H]noradrenaline ([3H]NA) and [3H]dopamine ([3H]DA) from rat amygdala slices in vitro. PDB enhanced the basal release of [3H]NA and [3H]DA as well. 4 alpha-Phorbol 12,13-didecanoate, which lacks the capacity to activate protein kinase C, was without effect on either basal or electrically stimulated release of [3H]NA and [3H]DA. Polymyxin B, which is a relatively selective protein kinase C inhibitor, decreased in a concentration-dependent manner the electrically stimulated release of both [3H]NA and [3H]DA from amygdala slices, whereas it enhanced the basal release of both neuromessengers. In the presence of 1.5 X 10(-7) M PDB, a concentration which when added to the superfusion medium alone doubled the electrically stimulated release of both [3H]NA and [3H]DA, polymyxin B again decreased in a concentration-dependent manner the release of both neuromessengers. At all polymyxin B concentrations used, the effect of the PKC inhibitor, expressed as percent inhibition, in the presence of PDB was approximately the same as that observed in the absence of PDB. This suggests that the antagonism between PDB and polymyxin B at the level of protein kinase C is not a competitive one. The effects of PDB and polymyxin B on basal release were additive. Taken together, these data suggest that in the amygdala presynaptically localized protein kinase C plays a role in signal transduction processes related to the exocytotic secretion of NA and DA from their nerve terminals.

Further pharmacological characterization of a D-2-like dopamine receptor on growth hormone producing cells in Lymnaea stagnalis

A preliminary study has revealed that a mammalian D-2-like dopamine (DA) receptor mediates hyperpolarization of the neuroendocrine growth hormone-producing cells (GHCs) in the snail Lymnaea stagnalis. An extensive pharmacological characterization of this receptor was performed in the present study. Several mammalian D-2 receptor agonists (e.g. aminotetralins) and antagonists (e.g.(-)-sulpiride) showed agonistic and antagonistic effects, respectively. However, some selective D-2 receptor agonists (e.g. N 0437) and antagonists (e.g. domperidone) failed to show agonistic or antagonistic effects, respectively. It is concluded that the dopamine receptor mediating hyperpolarization of the GHCs displays, besides some similarities, several differences from the mammalian D-2 receptor.

Inhibition of K+-stimulated [3H]dopamine and [14C]acetylcholine release by the putative dopamine autoreceptor agonist, B-HT 920

The inhibition of K+-stimulated [3H]dopamine and [14C]acetylcholine release from preloaded rat striatal slices was used to examine the presynaptic selectivity of the putative dopamine autoreceptor agonist, B-HT 920. In the micromolar range, B-HT 920 caused a concentration-dependent inhibition of the release of both labeled neurotransmitters as evoked by 20 mM K+. The effect of B-HT 920 on both [3H]dopamine and [14C]acetylcholine release was completely blocked by (+) butaclamol but not by (-) butaclamol. Sulpiride, a selective D2 antagonist, similarly blocked the inhibitory effect of B-HT 920 on the release of both labeled neurotransmitters indicating both responses were mediated by D2 receptors. (+) Butaclamol alone elevated stimulated [3H]dopamine release suggesting a significant amount of autoreceptor occupancy by endogenously released dopamine. Experiments with tolazoline and the alpha 2 agonist, B-HT 933, did not suggest any involvement of alpha-adrenoceptor activity in the inhibitory effects of B-HT 920 on the release of either transmitter. Inhibition of release was a selective effect of B-HT 920 as the drug was without effect on the K+-stimulated release of [3H]serotonin. The results indicate that in vitro B-HT 920 is active of both pre- and postsynaptic dopamine receptors in contrast to the pattern of effects observed after its in vivo administration.

Thalamic control of dopaminergic functions in the caudate-putamen of the rat--III. The effects of lesions in the parafascicular-intralaminar nuclei on D2 dopamine receptors and high affinity dopamine uptake

Dopamine receptor binding in the caudate-putamen was studied following bilateral lesions of the thalamostriatal pathway. Receptor binding was assayed using [3H]spiperone and defined with both (+)-butaclamol and S(-)-sulpiride. Radiofrequency lesions resulted in an increase in the Bmax of [3H]spiperone binding defined with both (+)-butaclamol and S(-)-sulpiride between 7 and 14 days following surgery. At longer survival times a fluctuating response was seen in which a decrease in receptor binding was observed at 28 days following lesion and a further rise again at 70 days. At no time point was significant change in Kd recorded. Further experiments were carried out to control for the possible effects of damage to fibres of passage and for inadvertent damage to habenula, as well as to define the receptor subtype involved. Ibotenic acid lesions resulted in similar effects to those reported with the radiofrequency method. Thus, 7 days following lesion, Bmax for (+)-butaclamol-defined [3H]spiperone binding increased by approximately 14-20% over that recorded in sham-lesioned animals. Using S(-)-sulpiride to define binding, Bmax was found to increase 13-17% in the same membrane preparations. Similar results were obtained in experiments at 14 days following ibotenic acid induced lesions. Again, no change in Kd was recorded. When radiofrequency lesions were made, which were largely restricted to habenula and associated fibres of passage, only small [(+)-butaclamol defined] or insignificant [S(-)-sulpiride defined] changes in Bmax were recorded. Combined radiofrequency lesions of habenula and ibotenic acid lesions of the thalamus resulted in a larger increase in Bmax for (+)-butaclamol defined binding than with S(-)-sulpiride defined binding. Our interpretation of these findings, in the light of the histology of the lesions, is that the predominant effect of removing thalamic input to the caudate-putamen is an increase in the number of D2 receptors, but without any change of affinity. A small component of the change in Bmax defined with (+)-butaclamol found with radiofrequency lesions may be due to a response at non-dopamine sites (possibly a 5-hydroxytryptamine receptor subtype) following damage to other caudate-putamen afferents which pass near the habenula or fasciculus retroflexus. Following unilateral ibotenic acid lesions of the thalamus, the number of high affinity uptake sites for dopamine was increased at long survival times.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of a long term dihydroergotoxine treatment on agonist and antagonist striatal dopamine binding sites are dose and age related

Chronic administration of dihydroergotoxine, at the two doses of 2.5 mg/kg and 5 mg/kg decreases the binding of dopamine 3H-agonists to striatal membranes. By contrast the binding of dopamine 3H-antagonists is decreased in the animals treated with the higher dose and increased in those treated with the lower one. In old rats, in which a partial loss of both 3H-antagonist and 3H-agonist binding sites is observed, the DHT treatment confirms to increase the binding of 3H-antagonists, without affecting that of 3H-agonists. Thus, aging and ergot alkaloids seem to discrimate between DA-agonist and DA-antagonist receptor sites suggesting that this receptors are separate entityes.

Muscarine receptor-mediated modulation of [3H]dopamine and [14C]acetylcholine release from rat neostriatal slices: selective antagonism by gallamine but not pirenzepine

The [3H]dopamine release induced from superfused rat neostriatal slices by 15 mM K+ was enhanced and that of [14C]acetylcholine was inhibited by oxotremorine in a concentration-dependent manner with a similar EC50 value (0.1 microM). Pirenzepine antagonized the modulatory effects of 1 microM oxotremorine on the release of both neurotransmitters with the same EC50 value (0.3 microM), whereas gallamine up to a concentration of 30 microM antagonized the inhibitory effect of oxotremorine on [14C]acetylcholine release only (EC50: 3 microM). These data strongly suggest that, although these functionally different populations of muscarine receptors cannot be regarded as being pharmacologically different (i.e. as M-1 and M-2 receptors, respectively), autoreceptors may still be selectively modified by drugs such as gallamine possibly acting on binding sites adjacent to the conventional muscarine receptor recognition site.

ACTH-(1-24) and alpha-MSH antagonize dopamine receptor-mediated inhibition of striatal dopamine and acetylcholine release

The effects of ACTH-(1-24), alpha-MSH and ACTH-(4-10) were studied on the electrically evoked release of 3H-dopamine and 14C-acetylcholine from striatal slices in the absence and presence of the dopamine receptor agonist TL-99. None of the peptides affected transmitter release when TL-99 was not present. ACTH-(1-24) and alpha-MSH concentration-dependently antagonized the inhibition of striatal transmitter release induced by dopamine receptor stimulation due to the presence of TL-99. ACTH-(1-24), 10(-7)M, reduced the TL-99-induced inhibition of the release of both dopamine and acetylcholine by approximately 50%, and 5 X 10(-6) M ACTH-(1-24) restored the release fully to control values. alpha-MSH was less effective by a factor 20-30 in counteracting the release-inhibiting effect of TL-99. ACTH-(4-10) had no effect at any of the concentrations tested. These results show that ACTH/MSH-like neuropeptides may act by modulating dopamine receptor functions in rat striatum.

Dopamine and the action of opiates: a reevaluation of the dopamine hypothesis of schizophrenia. With special consideration of the role of endogenous opioids in the pathogenesis of schizophrenia

It is suggested that the antipsychotic efficacy of opioids in patients suffering from schizophrenia may result from an interaction of opioids with the dopaminergic system. The modulatory effect of opioids on dopaminergic functions has already been demonstrated in basic experiments: Anatomical and biochemical data reveal an interaction between opioid receptors and dopamine (DA) actions on dopaminergic nerve terminals, cell bodies, and afferent nerve endings. Endogenous enkephalin levels correlate well with the endogenous dopamine content in various brain areas. Systemic or iontophoretic administration of morphine alters the spontaneous activity of ventral tegmental dopaminergic neurons. Morphine and enkephalin effectively enhance pituitary prolactin release, whereas dopamine inhibits it. Opioid agonists effectively alter DA release, DA reuptake, and DA metabolism in the striatum and substantia nigra. In reverse, chronic neuroleptic treatment enhances the synthesis and release of pituitary beta-endorphin. Opioids affect contralateral rotation elicited by dopamine agonists in animals with unilateral lesions of the nigrostriatal pathway. Phencyclidine, a psychotropic drug that shares certain pharmacological characteristics with the putative sigma-opioid receptor ligand SKF 10,047, indirectly mimics the effects of dopamine agonists on prolactin release, release of acetylcholine, etc. It is suggested that an imbalance of opiate-DA interaction might be involved in the pathogenesis of schizophrenia. Consequently, clinical studies on the effects of opioids on psychotic symptoms should also examine opioid influence on dopaminergic functions in these patients.

Kinetic and pharmacological profiles of the in vitro binding of the potent dopamine agonist [3H]N,N-dipropyl-5,6-dihydroxy-2-aminotetralin to rat striatal membranes

The in vitro binding of the new tritiated dopaminergic ligand [3H]N,N-dipropyl-5,6-dihydroxy-2-aminotetralin to rat striatal membranes is described. Binding assays were performed in 50 mM Tris-HCl pH 7.5 containing 1 mM EDTA and 0.01% ascorbic acid at 25 degrees C for 30 min. Specific binding at 0.5 nM [3H]DP-5,6-ADTN and 5 mg/ml membrane suspension was very high (87-93%) and was found to be linearly related with homogenate concentration over the range of 0.5-10 mg/ml (r = 0.9968). (+)Butaclamol 1 microM was used to define specific binding. Specific binding disappeared completely within 20 min when the tissue was incubated at 55 degrees C. Association and dissociation curves (obtained after addition of 1 microM (-)-DP-5,6-ADTN) were converted to linear logarithmic plots and the kinetic constants were computed (k1 = 0.054 min-1 and k-1 = 0.0188 min-1) as were the t1/2 for association (3.52 min) and dissociation (20.8 min). This resulted in an apparent KD of 0.34 nM. Increasing concentrations of [3H]DP-5,6-ADTN (0.05-3.0 nM) were found to saturate the receptor site. Linear transformation of the saturation curves and presentation of the curves as Eadie-Hofstee plots showed that only one set of receptors was labeled (nHill was 0.995 +/- 0.07) with a high affinity constant KD of 0.57 +/- 0.10 nM and a maximum number of binding sites Bmax of 18.6 +/- 2.4 pmol/g tissue. Various compounds were tested for their potency to displace the specific binding of [3H]DP-5,6-ADTN to striatal membranes (2.5 mg/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

Two dopamine receptors: biochemistry, physiology and pharmacology

In 1979, two categories of dopamine (DA) receptors (designated as D-1 and D-2) were identified on the basis of the ability of a limited number of agonists and antagonists to discriminate between these two entities. In the past 5 years agonists and antagonists selective for each category of receptor have been identified. Using these selective drugs it has been possible to attribute the effects of DA upon physiological and biochemical processes to the stimulation of either a D-1 or a D-2 receptor. Thus, DA-induced enhancement of both hormone release from bovine parathyroid gland and firing of neurosecretory cells in the CNS of Lymnaea stagnalis has been attributed to stimulation of a D-1 receptor. Likewise, the DA-induced inhibition of the release of prolactin and alpha-MSH from the pituitary gland, as well as of acetylcholine, DA and beta-endorphin from brain, the DA-induced inhibition of chemo-sensory discharge in rabbit carotid body and the DA-induced hyperpolarization of neurosecretory cells in the CNS of Lymnaea stagnalis have been attributed to stimulation of a D-2 receptor. Independently two categories of DA receptors (designated as DA-1 and DA-2) were identified in the cardiovascular system. Stimulation of a DA-1 receptor increases the vascular cyclic AMP content and causes a relaxation of vascular smooth muscle in renal blood vessels, whereas stimulation of a DA-2 receptor inhibits the release of norepinephrine from certain postganglionic sympathetic neurons. Recent studies with the newly developed drugs discriminating between D-1 and D-2 receptors suggest however that the independently developed schemata for classification of dopamine receptors in either the central nervous and endocrine systems or the cardiovascular system are similar although maybe not completely identical.

Pre-decapitation state of arousal of rats predetermines the effect of des-Tyr1-gamma-endorphin on dopamine release from nucleus accumbens slices in vitro

The non-opiate beta-endorphin fragment des-Tyr1-gamma-endorphin (DT gamma E) had a decreasing effect on K+-induced release of tritiated dopamine from nucleus accumbens slices in vitro, when tissue was used of rats which prior to decapitation were in a state of low arousal. When nucleus accumbens tissue was used of rats which were mildly stressed by exposure to a novel environment prior to decapitation, this effect was absent, while an enhancing effect of DT gamma E became evident on basal dopamine efflux. This latter effect resembled that of haloperidol, which dose-dependently enhanced basal dopamine efflux in vitro. Exposure of rats to ether vapor shortly before decapitation abolished both these in vitro effects of DT gamma E. The results are interpreted as indicating that the quality of the modulating effects of DT gamma E on dopamine release from dopaminergic neurons projecting to the nucleus accumbens is depending on the state of activity of these neurons, which, in its turn, is a reflection of the state of arousal of the rats.

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.

The purported dopamine agonist DPI inhibits [3H]noradrenaline release from rat cortical slices but not [3H]dopamine and [14C]acetylcholine release from rat striatal slices in-vitro

The effects of the purported dopamine (DA) receptor agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) upon the in-vitro K+-induced release of [3H]DA and [14C]acetylcholine from rat neostriatal slices, and of [3H]noradrenaline from rat neocortical slices have been investigated and compared with those of the DA receptor agonist TL-99 and the alpha-adrenoceptor agonist clonidine, respectively. The rapid decomposition of the catechol compounds DPI and TL-99 in the Krebs-Ringer bicarbonate superfusion medium was shown to be inhibited by both the chelating agent EDTA and the reducing agent ascorbic acid. The results suggest that in-vitro DPI is unable to stimulate striatal DA receptors, whereas it is effective in stimulating cortical alpha 2-adrenoceptors (EC50 = 61 nM). It is concluded that DPI should be considered as a mixed alpha 1/alpha 2-adrenoceptor agonist and that the designation of DPI as a DA receptor agonist should be abandoned.

Further functional in vitro comparison of pre- and postsynaptic dopamine receptors in the rabbit caudate nucleus

Slices of the rabbit caudate nucleus were preincubated with 3H-dopamine or 3H-choline and then superfused and stimulated electrically. DiPr-5,6-ADTN reduced the stimulation-evoked overflow of tritium over the same concentration range, independently of whether slices had been preincubated with 3H-dopamine or 3H-choline, and the same was true for apomorphine, NPA and pergolide. Three other putative dopamine receptor agonists, namely 3-PPP, DPI and SKF 38393, failed to decrease the evoked overflow of tritium. Each of six antagonists--(-)-sulpiride, (+)-sulpiride, CGP 11109 A, cis-flupentixol, domperidone and corynanthine--increased the evoked overflow over the same concentration range in experiments with 3H-dopamine and in those with 3H-choline. For each of these antagonists except cis-flupentixol, and also for chlorpromazine, haloperidol and rauwolscine, the pA2 values against apomorphine obtained in 3H-dopamine and in 3H-choline experiments were closely similar. The antagonist effect of cis-flupentixol against apomorphine was not purely competitive. (-)-Sulpiride was a more potent antagonist than (+)-sulpiride, and cis-flupentixol was more potent than trans-flupentixol. This study supplements a previous one in which (+/-)-sulpiride, metoclopramide and molindone were used as antagonists. It is a functional in vitro approach to receptor characterization, as opposed to radioligand binding studies or in vivo investigations. The results show that a large number of dopamine receptor agonists and antagonists are unable to distinguish between the presynaptic, release-inhibiting dopamine autoreceptors and those postsynaptic dopamine receptors which, when activated, depress the release of acetylcholine.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptors modulating [3H]acetylcholine release in slices of the cat caudate: effects of (-)-N-(2-chloroethyl)-norapomorphine

(-)-N-(2-Chloroethyl)-norapomorphine [-)-NCA) inhibited in a concentration-dependent manner the electrically evoked [3H]acetylcholine release in slices of cat caudate. The inhibition by (-)-NCA was reversible and antagonized by the benzamide neuroleptic S-sulpiride. Although (-)-NCA is an irreversible antagonist at some behaviorally relevant postsynaptic dopamine receptors, its effect as an agonist on dopamine receptors modulating [3H]acetylcholine release strongly resembles its action on presynaptic dopamine autoreceptors modulating [3H]dopamine release. Our results suggest that the dopamine receptor modulating [3H]acetylcholine release may not be an appropriate in vitro model for those behaviorally relevant postsynaptic dopamine receptors which are antagonized by (-)-NCA. It is more likely that it conforms to the characteristics of presynaptic release-modulating dopamine autoreceptors. The agonistic action of (-)-NCA at presynaptic dopamine receptors, in contrast to the irreversible antagonism of some postsynaptic dopamine receptors by (-)-NCA, should be interpreted with caution. Evidence is presented which suggests that (-)-NCA breaks down in solution into (-)-N-(2-hydroxylethyl)-norapomorphine [-)-NHA). Since (-)-NHA is an agonist at presynaptic dopamine receptors, this physicochemical breakdown product may be partly responsible for the apparent agonistic properties of (-)-NCA under our in vitro conditions.

Characterization of dopamine autoreceptor and [3H]spiperone binding sites in vitro with classical and novel dopamine receptor agonists

The specific D2 receptor agonist, LY 141865, but not the specific D1-receptor agonist, SK&F 38393, potently inhibited electrically evoked [3H]dopamine release from slices of the cat caudate. Similarly, LY 141865, but not SK&F 38393, inhibited [3H]spiperone binding to membranes of the cat caudate. The inhibition by dopamine receptor agonists of electrically evoked [3H]dopamine release was antagonized by the specific D2-receptor antagonist S-sulpiride. The inhibition of the electrically evoked release of [3H]dopamine by apomorphine was not, however, antagonized by the specific D1-receptor antagonist, bulbocapnine. Similarly, S-sulpiride but not bulbocapnine potently inhibited [3H]spiperone binding to membranes of the cat caudate. These results suggest that the dopamine autoreceptor modulating the depolarization-evoked release of [3H]dopamine, and the binding site of [3H]spiperone, are valid in vitro models for D2-dopamine receptors. Contrary to some previous reports, DPI was inactive in both in vitro dopamine receptor models. The IC50 values of a series of dopamine receptor agonists correlated very well in the two in vitro dopamine receptor models. One exception to this correlation was bromocriptine, which was more potent at [3H]spiperone binding sites than at the dopamine autoreceptor. With the exception of bromocriptine, all dopamine receptor agonists had one-hundred fold higher potency at the dopamine autoreceptor than at [3H]spiperone binding sites. [3H]Spiperone binding sites are localized primarily postsynaptic to dopamine terminals. Possible differences between the pharmacological properties of pre- and postsynaptic dopamine receptors should become apparent in the comparison of the two in vitro dopamine receptor models. However, the order of potency of dopamine receptor agonists with both in vitro models, dopamine autoreceptor and [3H]spiperone binding, was the same: N-n-propylnorapomorphine greater than TL-99 = 7-HAT greater than M-7 greater than Apomorphine greater than LY 141865.

Stereoisomers of apomorphine differ in affinity and intrinsic activity at presynaptic dopamine receptors modulating [3H]dopamine and [3H]acetylcholine release in slices of cat caudate

We investigated the effects of R(-)-apomorphine and S(+)-apomorphine on dopamine receptors modulating electrically evoked [3H]dopamine and [3H]acetylcholine release from slices of cat caudate nucleus. R(-)-Apomorphine inhibited the release of both [3H]dopamine and [3H]acetylcholine with an IC50 of 20 nM, while S(+)-apomorphine was without inhibitory action on the electrically evoked release of either neurotransmitter at concentrations up to 1 microM. At a concentration of 1 microM, however, S(+)-apomorphine antagonized the inhibition by R(-)-apomorphine, producing a parallel five-fold shift to the right in the concentration-response curve to R(-)-apomorphine. These results indicate that S(+)-apomorphine is devoid of intrinsic activity to stimulate presynaptic dopamine receptors modulating the electrically evoked release of dopamine and acetylcholine. In addition, S(+)-apomorphine has an approximately ten-fold lower affinity for presynaptic dopamine receptors compared to R(-)-apomorphine.

Stimulation of D2-dopamine receptors in rat neostriatum inhibits the release of acetylcholine and dopamine but does not affect the release of gamma-aminobutyric acid, glutamate or serotonin

D1-dopamine receptor stimulation did not affect the K+-induced release of [3H]GABA, [3H]glutamate, [3H]serotonin, [3H]dopamine and [14C]acetylcholine from slices of rat neostriatum. D2-dopamine receptor stimulation did not change the release of [3H]GABA, [3H]glutamate and [3H]serotonin, but inhibited the release of both [3H]dopamine and [14C]acetylcholine; this inhibition was antagonized by (-)-sulpiride. The release-inhibiting dopamine autoreceptors and the post synaptic dopamine receptors mediating the inhibition of acetylcholine release appear to be pharmacologically similar and can be classified as D2-receptors.

Is TL-99 a selective presynaptic dopamine receptor agonist?

The claim that the 2-aminotetralin analogue TL-99 is a selective presynaptic dopamine (DA) receptor agonist has been investigated both in vivo and in vitro in the rat. The pharmacological specificity of the hypomotility caused by TL-99 has been examined using various selective antagonists. In addition its effects on DA metabolism and noradrenaline (NA) and DA turnover (alpha-MT method) as well as its distribution in the brain have been studied. These in vivo studies provide evidence that although TL-99 is able to activate presynaptic DA receptors it is also a potent agonist of NA receptors as shown by the fact that the hypomotility could be partly reversed by the selective alpha 2-adrenoceptor antagonists yohimbine and piperoxan. Further supporting evidence for these findings was provided by in vitro studies on the inhibition of K+-induced [3H]dopamine, [14C]acetylcholine and [3H]noradrenaline release from striatal and cortical slices where it was shown that TL-99 is not only active at both pre- and postsynaptic DA receptors but also at alpha 2-NA receptors. For the latter receptor it had a potency comparable to that of the potent alpha 2-agonist clonidine and this may explain, to some extent, the hypomotility caused by TL-99. Thus, ascribing this hypomotility solely to an interaction with presynaptic DA receptors may be an oversimplification. It is therefore concluded that TL-99 should not be considered as a selective presynaptic DA receptor agonist.

Are presynaptic dopamine autoreceptors and postsynaptic dopamine receptors in the rabbit caudate nucleus pharmacologically different?

Slices of the rabbit caudate nucleus were preincubated with [3H]dopamine or [3H]choline and then superfused and stimulated electrically. Apomorphine reduced the stimulation-evoked overflow of tritium over the same concentration range, independently of whether slices had been pre-incubated with [3H]dopamine or with [3H]choline. Each of three antagonists--molindone, sulpiride and metoclopramide--increased the evoked overflow of tritium over the same concentration range in experiments with [3H]dopamine and those with [3H]choline. For each antagonist, the pA2 values against apomorphine obtained in [3H]dopamine experiments and in [3H]choline experiments were very similar. This study is a functional in vitro approach to receptor characterization, as opposed to radioligand binding studies or in vivo investigations. The results show that the dopamine receptor agonist apomorphine and three antagonists are unable to distinguish between the presynaptic, release-inhibiting dopamine autoreceptors and those postsynaptic dopamine receptors which, when activated, depress the release of acetylcholine. Although there are certainly more dopamine receptors in the caudate nucleus, these two physiologically important groups seem to be closely related.

Dopamine receptor mediated inhibition by pergolide of electrically-evoked 3H-dopamine release from striatal slices of cat and rat: slight effect of ascorbate

The dopamine receptor agonist pergolide inhibited the calcium-dependent, electrically evoked overflow of tritium from slices of the striatum of cat or rat prelabelled with 3H-dopamine. This inhibition of tritium overflow by nanomolar concentrations of pergolide was antagonized by the benzamide neuroleptic S-sulpiride (0.1 microM). In millimolar concentrations, L- ascorbate had slight or no effects on this dopamine receptor mediated inhibition, in striatal slices of either the cat or the rat. Since these same concentrations of ascorbate have been reported to completely block the specific binding of 3H-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADTN) and of 3H-apomorphine to presumed dopamine receptors, the present results suggest a dissociation between the characteristics of 3H-ADTN and 3H-apomorphine binding and the dopamine autoreceptor. Previous contradictory results concerning the existence of inhibitory dopamine receptors which modulate depolarization-evoked overflow of dopamine from the striatum of the rat are thus apparently not due to a species difference nor to the use of ascorbate, but rather to differences in experimental conditions.

Characterisation of the mechanisms by which purported dopamine agonists reduce spontaneous locomotor activity of mice

The spontaneous locomotor activity of mice, recorded in the first 20 min period after placement in individual cages fitted with photocells, was reduced dose-dependently by apomorphine, (-)-N-n-propylnorapomorphine, 2-dimethylamino-5,6-dihydroxytetralin (M-7), 2-di-n-propylamino-5,6-dihydroxytetralin, (3,4-dihydroxyphenylamino)-2-imidazoline (DPI), N-n-propyl-benzo(f) and (g) quinolines, bromocriptine, N-n-propyl-3-(3-hydroxyphenyl)-piperidine (3-PPP) and by the N,N-dimethyl-, -diethyl-, -dipropyl- and -dibutyl-dopamines. SK&F 38393, in a wide dose range, failed to modify spontaneous locomotion. The inhibition of spontaneous locomotion caused by 2-di-n-propylamino-5,6-dihydroxytetralin was reversed by spiroperidol, haloperidol, benperidol and (-)-sulpiride, used in doses which did not in themselves modify spontaneous locomotion, but not by similarly selected doses of trifluperidol, fluphenazine, oxiperomide, molindone, thioridazine, prazosin, picrotoxin, methysergide, yohimbine, propranolol, phentolamine, atropine or SK&F 38393. Spiroperidol was also shown to reduce the actions of N,N-diethyl-, N,N-dipropyl- and N,N-dibutyl-dopamine, M-7, bromocriptine, apomorphine and, less markedly, 3-PPP and the (f) and (g)benzoquinolines, but not the actions of N,N-dimethyldopamine or DPI. Yohimbine reversed the action of DPI but ont that of any other purported dopamine agonist, whilst prazosin was ineffective throughout the experiments. The neuroleptic agents did not in themselves cause significant increase in spontaneous locomotor activity. Data is discussed in terms of a dopamine-neuroleptic sensitive mechanism for motor inhibition.

Activation of presynaptic alpha-noradrenaline receptors in rat brain by the potent dopamine-mimetic N,N-dipropyl-5,6-ADTN

Rat cerebral cortex slices labeled with 3H-noradrenaline (NA) were superfused and 3H-NA release was induced with 20 mM K+. The release of 3H-NA was not affected by DA (1 micro M), but was inhibited by the 2-aminotetralins, 5,6-ADTN and N,N-dipropyl-5,6-ADTN (at a concentration of 1 micro M) by about 27% and 65%, respectively. The inhibitory effect of N,N-dipropyl-5,6-ADTN was concentration-dependent (3 x 10(-8) to 3 x 10(-7) M) and was antagonized by phentolamine but not by fluphenazine. The data indicate that, in addition to activating dopamine receptors, N,N-dipropyl-5,6-ADTN stimulates presynaptic alpha-NA receptors.