Multiple receptors for dopamine

Effect of dopamine on pentagastrin-stimulated gastric acid secretion and mucosal blood flow in dogs with gastric fistula

The purpose of this study was to elucidate the effect of intravenously administered dopamine on dopamine receptors and adrenergic receptors in terms of its effect on gastric acid secretion, the kinetic mechanism, blood flow, and antral motility. Dopamine was used alone and in conjunction with selective blockade of alpha-, beta-, and dopaminergic receptors. A significant inhibition of gastric acid secretion was found with the highest dose of dopamine used (40 micrograms/kg/min). The kinetic study showed characteristics of a non-competitive type. The anti-secretory effect dopamine was significantly blocked by non-selective beta-blockade or by selective beta-blockade but not by alpha- or dopaminergic receptor blockade. This suggests that the inhibitory effect of dopamine on gastric secretion is mediated by beta-receptors. There was no significant effect on gastric mucosal blood flow, but the ratio between blood flow and acid secretion was significantly elevated during dopamine infusion, indicating that the acid inhibition was not secondary to changes in blood flow. It is concluded that the dopamine inhibition of acid secretion is mediated by beta 1-receptors, unlike the effect on antral gastric motility, which is mediated by dopamine receptors.

Chronic molindone treatment: relative inability to elicit dopamine receptor supersensitivity in rats

Chronic treatment of rats with the antipsychotic drug molindone (2.5 mg/kg) did not elicit behavioral supersensitivity to apomorphine (AP) (0.25 mg/kg) or increased striatal 3H-spiroperidol binding, whereas treatment with haloperidol (0.5-1.0 mg/kg) produced manifestations of dopaminergic supersensitivity in both paradigms. Chronic treatment with a high dose of molindone (20 mg/kg) elicited a small, but significant increase in behavioral sensitivity to AP (57%) which was, however, significantly less than that produced by 1 mg/kg haloperidol (126%, P less than 0.01). Apparent tolerance to elevation of striatal and frontal cortical 3,4-dihydroxyphenylacetic acid (DOPAC) levels was obtained with chronic molindone treatment (5 or 20 mg/kg). None of the molindone doses used (2.5-50 mg/kg) increased striatal dopamine receptor binding. Scatchard analyses revealed no change in either maximal binding capacity (Bmax) or dissociation constant (Kd). A significant (P less than 0.001) correlation of receptor binding activity and stereotypy score was obtained for haloperidol-, but not molindone-treated rats. These results with molindone in an animal model of tardive dyskinesia suggest that this drug may have a lower potential for eliciting this disorder in humans.

Slowing effects of dopamine and calcium-channel blockers on frequency of sodium spikes in rat pars intermedia cells

1. Spontaneous discharge of action potentials (Na spikes) in cells isolated from rat pars intermedia was slowed or arrested by Co(2+), Ni(2+) or Mn(2+), which block voltage-dependent Ca channels in these cells. The amplitude of persisting spikes was undiminished. The effects resembled those of dopamine.2. Action potential frequency decreased when the Ca(2+) concentration was lowered to 0.1 mM and increased when the Ca(2+) concentration was raised from this level to 1 mM or 2 mM or when Ba(2+) (2 mM) was introduced. These effects, together with those of Co(2+), Ni(2+) and Mn(2+), are consistent with the possibility that Ca(2+) participates in the regulation of spike discharge.3. Verapamil, methoxyverapamil (D600), and nifedipine reduced the amplitude of the individual Na spikes in concentrations that had little effect on voltage-dependent Ca channels. Action potential frequency was comparatively little affected by these drugs.4. K(+) (15 mM) stimulated action potential frequency and this effect too was suppressed by dopamine or Co(2+).5. The effect which dopamine had of slowing spontaneous discharge, like the inhibitory effect on secretion, was blocked by metoclopramide. But otherwise the mechanism is unclear: dopamine blocked voltage-dependent Ca channels in some cells but not in most others.6. The effects of K(+) and Ba(2+) of eliciting spikes, the suppression of Na-spike discharge by Co(2+) and related Ca-channel blocking cations, and the unspecific effects of the organic ;Ca channel blockers', all have implications for the use of these substances as tools to analyse stimulus-secretion coupling.

Roles of cyclic nucleotides in the synaptic transmission in sympathetic ganglia of rabbits

1. Two kinds of slow postsynaptic potentials can be elicitable in the rabbit superior cervical ganglion: a dopaminergic hyperpolarizing (slow IPSP) one and a muscarinic depolarizing (slow EPSP) one, respectively. 2. Cyclic AMP may not be involved in the generation of slow IPSP, whereas it certainly mediates the dopamine-induced enduring modulatory enhancement of the slow EPSP response. 3. Cyclic GMP appears to mediate at least one of the components in the mechanism underlying the slow EPSP by generating a depolarization with no change in membrane conductance. 4. Cyclic GMP does additionally regulate the dopamine for cyclic AMP)-induced modulatory enhancement of the slow EPSP by antagonizing it in a uniquely time-dependent fashion.

Identification of the D-1 dopamine receptor subunit in rat striatum after photoaffinity labeling

When rat striatal membranes, photolabeled with [3H]dopamine under assay conditions similar to those used for dopamine-sensitive adenylate cyclase, were subjected to sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis, several radioactively labeled bands appeared. Labeling of these bands was reduced in the presence of non-radioactive dopamine during photolysis, but was unaffected by the presence of sulpiride. Haloperidol preferentially reduced the labeling of the main band which had a molecular weight of about 57,000 rather than the other weakly labeled bands. Labeling of this 57,000 dalton protein was not apparent when rat cerebellar membranes were used and was markedly eliminated by kainic acid-induced lesions that destroyed the intrastriatal nerve cell bodies. These results indicate that this 57,000 dalton protein is the binding subunit of the D-1 dopamine receptor.

Stereoselectivity of presynaptic autoreceptors modulating dopamine release

The effects of the (R)- and (S)-enantiomers of sulpiride and butaclamol were studied on the spontaneous and field stimulation-evoked release of total radioactivity from slices of rabbit caudate nucleus prelabelled with [3H]dopamine. (S)-Sulpiride in concentrations ranging from 0.01--1 microM enhanced the electrically evoked release of [3H]dopamine while (R)-sulpiride was 10 times less potent than (S)-sulpiride. Exposure to (S)-butaclamol (0.01--1 microM) but not to (R)-butaclamol (0.1--10 microM) enhanced the field-stimulated release of [3H]dopamine. The facilitatory effects of (S)- and (R)-sulpiride and (S)-butaclamol on the stimulated release of the labelled neurotransmitter were observed under conditions in which these drugs did not modify the spontaneous outflow of radioactivity. Only the active enantiomers of sulpiride and butaclamol antagonized the inhibition by apomorphine (1 microM) of the stimulated release of [3H]dopamine. Our results indicate that the presynaptic inhibitory dopamine autoreceptors modulating the stimulation-evoked release of [3H]dopamine in the caudate nucleus are, like the classical postsynaptic dopamine receptors, chemically stereoselective.

Supersensitivity time course of dopamine antagonist binding after nigrostriatal denervation: evidence for early and drastic changes in the rat corpus striatum

The sensitivity of the [3H]haloperidol binding technique can be greatly increased by focusing tissue sampling on striatal regions where dopaminergic innervation is the richest. Such sampling is provided from pooled microdiscs punched out of 8 serial 500 micrometer thick sections of the rat brain. With these conditions, the density of receptor sites (Bmax) was found to be twice that of the whole striatum, without modification of the apparent dissociation constant (Kd) and of the Hill's slope. Such a procedure applied to rats with complete 6-hydroxydopamine-induced unilateral nigrostriatal lesions showed a moderate decrease in Bmax in the lesioned side up to 6 days after surgery, whereafter the value of Bmax increased progressively up to the thirtieth day, being then 160% over the control value. Conversely, the apparent Kd decreased significantly from the second to the sixth day postsurgery in the lesioned side, and then increased moderately up to the tenth day and drastically from the twenty-first to the thirtieth day. No change was observed in the corresponding intact side. The modifications appeared chronologically compatible with those corresponding to the behavioral denervation supersensitivity, evidencing drastic binding changes as compared with the whole striatum. The unexpected variations in Kd observed were well correlated with those in Bmax, suggesting that the new available binding sites might be of lower affinity. In the light of all these results, a hypothetical model is proposed.

Isolated horizontal cells from carp retina demonstrate dopamine-dependent accumulation of cyclic AMP

Horizontal cells of the carp retina were separated from other retinal cell types by using enzymatic dissociation and velocity sedimentation at unit gravity. Fractions containing horizontal cells were tested for their ability to accumulate cyclic AMP in the presence of various putative neurotransmitters. Micromolar concentrations of dopamine, when added in the presence of 3-isobutyl-1-methylxanthine, stimulated cyclic AMP accumulation in these isolated cells. The dopamine-dependent accumulation of cyclic AMP in intact isolated horizontal cells was blocked by nanomolar concentrations of dopamine antagonists such as haloperidol, (+)-butaclamol, and fluphenazine. The results indicate that there is a postsynaptic dopamine receptor on carp horizontal cells that is associated with adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1].

Mu and delta opiate receptors in rat brain are affected by GTP but not by MIF-1

The possible competition of MIF-1 (Pro-Leu-Gly-NH2) with 3H-naloxone or 3H-D-Ala-D-Leu-enkephalin (DADLE) for mu and delta opiate receptors in the rat striatum was examined under conditions of varying temperatures and concentrations of sodium and GTP. MIF-1, in concentrations ranging from 10(-14) to 10(-3) M, failed to compete with either 3H-DADLE or 3H-naloxone for delta or mu receptors, GTP (2 microM) facilitated binding (17.7 +/- 1.9%) at all concentrations of MIF-1 in the 1H-DADLE assay; unexpectedly, GTP facilitated binding in the 3H-naloxone assay (11.5 +/- 1.3%) under conditions used to potentiate antagonist binding (addition of NaCl and incubation at 4 degrees C). GTP showed optimal facilitation at a concentration of about 2 microM in both the 3H-DADLE and 3H-naloxone "antagonist" conditions. At higher doses of GTP, 3H-naloxone binding remained high, but 3H-DADLE binding decreased to near or below control values. MIF-1 did not affect these GTP responses, indicating further the lack of effect of this peptide on opiate binding.

Lithium dissociates haloperidol-induced behavioral supersensitivity from reduced dopac increase in rat striatum

Chronic lithium administration prevents both haloperidol-induced dopaminergic behavioral supersensitivity and increased tritiated neuroleptic binding to dopamine (DA) receptors in rat corpus striatum. Since chronic haloperidol treatment also induces tolerance to the activating effects to the drug on striatal DA synthesis, the ability of lithium to block neurochemical tolerance development was investigated. Whereas lithium treatment significantly (P less than 0.01) attenuated haloperidol-induced behavioral supersensitivity to apomorphine (0.33 and 0.66 mg/kg s.c.), it did not prevent tolerance to the elevation of striatal 3, 4-dihydroxyphenylacetic acid (DOPAC) levels 1 h after last treatment or in response to challenge with a low dose (0.1 mg/kg) of haloperidol during withdrawal. These results demonstrate a dissociation between the development of behavioral supersensitivity and the reduction in DOPAC increase. An assessment of lithium's demonstrated effects on supersensitivity development at various DA receptor sites suggests that tolerance may be mediated by presynaptic DA receptors on terminals of nigrostriatal neurons.

A comparison of the effects of neuroleptics on phencyclidine-induced behaviors in the rat

The dose-response effects of neuroleptic pretreatment on phencyclidine (PCP; 3 or 5 mg/kg)-induced locomotor activity, stereotyped behaviors and ataxia were quantified in groups of male rats using rating scales recently developed in this laboratory. Three butyrophenone neuroleptics consistently produced dose-dependent antagonism of the behavioral effects of PCP administration. Fluphenazine antagonized the behavioral effects produced by 3 mg/kg PCP but not those produced by 5 mg/kg PCP. Each of the other neuroleptics examined (chlorpromazine, thioridazine, mesoridazine, triflupromazine, cis-flupenthixol) had no consistent antagonistic effect or actually enhanced one or more of the behavioral effects of PCP. Some neuroleptics slightly reduced PCP locomotion or stereotypies at high doses, but these effects were probably a non-specific consequence of the synergistic ataxia-producing properties of these drugs. In a second set of experiments, atropine sulfate pretreatment increased PCP-induced locomotor activity and stereotyped behaviors but had no effect on ataxia; pretreatment with physostigmine produced opposite effects. Combined pretreatment with haloperidol and atropine sulfate significantly reduced only haloperidol antagonism of PCP-induced ataxia, thus suggesting that non-dopoaminergic effects of neuroleptics may interfere with their ability to antagonize PCP.

Possible mixed agonist--antagonist activity of D-sulpiride at dopamine receptor level in man

The effects of different doses of D-sulpiride (1, 6, 12 and 25 mg, i.v.) on arterial blood pressure (ABP), heart rate (HR) and prolactin (PRL), growth hormone (GH), insulin and gastrin secretions have been studied in 8 normal men. D-Sulpiride increased systolic ABP with a maximum effect rather 12 mg i.v., while it had only slight effects on diastolic ABP and HR. PRL secretion was increased by D-sulpiride in a dose-dependent way, while insulin secretion was lowered and GH secretion slightly enhanced only in a restricted range of doses (6 mg and 12 mg i.v., respectively). Gastrin secretion seemed to be unaffected by D-sulpiride at any of the tested doses. These results are discussed in view of a possible mixed agonist--antagonist activity of D-sulpiride at dopamine receptor level in contrast with the relatively pure antagonistic action of the levo isomer.

Comparison of the binding of [3H]spiperone and [3H]domperidone in homogenates of mammalian retina and caudate nucleus

The specific binding of [3H]spiperone and [3H]domperidone, as defined by 1 microM-(+)butaclamol, was compared in homogenates of bovine retina and caudate nucleus. Scatchard analyses of saturation data for [3H]spiperone binding yielded dissociation constants (Kd) of 0.35 nM in the retina and 0.64 nM in the caudate nucleus. Comparison of the maximum number of binding sites (Bmax) present in each tissue indicated that the density of sites in bovine caudate nucleus (270 fmol/mg protein) was approximately three times higher than in bovine retina (92 fmol/mg protein). This difference was even more marked in guinea pig tissues, with a ratio of 7:1 between corpus striatum and retina. The pharmacological analysis of [3H]spiperone binding in both the bovine retina and caudate nucleus indicated an interaction with dopaminergic rather than serotonergic sites. However, inhibition curves obtained to dopaminergic agonists in the bovine retina were significantly steeper than those observed in the bovine caudate nucleus, as reflected in the greater Hill coefficients obtained for these agents in the retina. Furthermore, only a small amount of specific [3H]domperidone binding was observed in either the bovine caudate nucleus or the guinea pig striatum, whilst no specific [3H]domperidone binding was detectable in homogenates of either bovine or guinea pig retina. These data suggest that the retina possesses only a small population of dopaminergic D2 sites and that these binding sites may differ from those present in the caudate nucleus.

Agonistic actions of DPI (2-(3,4-dihydroxyphenylimino)-imidazolidine) on alpha-adrenoceptors and dopamine receptors

Some of the pharmacological actions of 2-(3,4-dihydroxyphenylimino)-imidazolidine (DPI) were studied in vivo and vitro. DPI (1 nM-100 micro M) had a similar affinity but a lower intrinsic activity on alpha-adrenoceptors in rabbit aortic strips to noradrenaline (1 nM-10 micro M). DPI did not affect the uptake of [3H]noradrenaline in guinea-pig isolated atria. The effects of DPI on dopamine receptors were examined in the dog coronary and renal vasculature and in the rat isolated, perfused kidney. In the dog coronary vasculature DPI (0.005-10 micrograms, i.a.), like dopamine (5-50 micrograms, i.a.), caused an increase in coronary blood flow which was antagonized by the dopamine receptor antagonist, ergometrine. In the renal vasculature of both the rat and the dog, dopamine (5-50 micrograms, i.a.) caused vasodilatation but there was no evidence of an effect of DPI on dopamine receptors. It appears that DPI is a selective agonist for dopamine receptors in the coronary vasculature.

Dopaminergic modulation of adenylate cyclase stimulation by vasoactive intestinal peptide in anterior pituitary

The activation of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] by vasoactive intestinal peptide (VIP) was used as a model to investigate the molecular mechanisms triggered by the occupancy of dopamine recognition sites in rat anterior pituitary. Dopamine failed to change the basal enzyme activity, but it inhibited the stimulation of adenylate cyclase elicited by VIP. Apomorphine, 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene, and 2-bromo-alpha-ergocryptine mimicked the effect of dopamine, whereas (-)-sulpiride and and classical neuroleptics antagonized it. Dopamine failed to modulate the activation of pituitary adenylate cyclase by prostaglandin E1, which does not increase prolactin secretion. From these results we infer that stimulation of D-2 (dopamine) receptors may affect pituitary secretion by inhibiting the activation of anterior pituitary adenylate cyclase by VIP or other secretagogues.

Detection of dopamine receptors in the area postrema

[3H]Spiroperidol labeled dog area postrema membranes with high affinity (KD, 0.1 nM) and stereospecificity. The displacement of the radioligand by different dopamine agonists and antagonists indicated that the labeled sites were dopaminergic in nature. Guanine nucleotides decreased agonist affinity for the labeled sites without affecting the affinity for antagonists. Adenylate cyclase in the area postrema was stimulated by guanine nucleotides and sodium fluoride but not by dopamine. The results suggest that the dopamine receptor in the area postrema can be classified as D2-type receptors.

Rat dopaminergic function in the retina during aging

Parameters of dopaminergic transmission were studied in the retina of mature (3-4 months) and aged (23-24 months) rats. In the retina of senescent rats were found significantly higher dihydroxyphenylacetic acid (DOPAC) levels and a higher number of (3H-)spiroperidol binding sites. We detected also an increase of (3H)- methionine-enkephalin binding sites. The changes in the density of (3H)-spiroperidol and (3H)-Metenkephalin binding sites in the retina are opposite to those observed in the brain of aged rats.

Opiates inhibit the dopaminergic enhancement of the renal response to aldosterone in the rat

1. Both naloxone, an opiate antagonist, and levorphanol, an opiate agonist, inhibit the enhanced renal response to aldosterone produced by both l-dopa pretreatment and a high K+ diet. 2. This supports the evidence for a common mechanism of action for the enhancement of the renal response to aldosterone produced by l-dopa and a high K+ diet. Whether this mechanism is dopaminergic or opiate is uncertain. 3. The inhibition of the enhanced response, produced by l-dopa, by opiates is consistent with previous findings of displacement of 3H-dopamine from renal homogenates by opiates and supports the hypothesis that the binding sites relate to the renal response to aldosterone.

Effect of guanine nucleotides on dopaminergic agonist and antagonist affinity for [3H]sulpiride binding sites in rat striatal membrane preparations

[3H]Sulpiride bound to rat striatal membrane preparations with a saturable, high affinity component. This binding was displaced potently by dopamine antagonists (both classic neuroleptics and the benzamide, sulpiride) and less potently by dopamine agonists. GTP and its stable analogue Gpp(NH)p did not affect [3H]sulpiride binding to the membranes but altered the affinity for dopaminergic agonists. This effect was specific in that antagonist binding was not affected and only GTP, GDP, and Gpp(NH)p produced the effect. Similar alterations in ligand binding affinity caused by guanine nucleotides have been observed for binding sites linked to an adenylate cyclase. Such an interpretation for the case of [3H]sulpiride is contrary to suggestions that sulpiride labels only those dopamine receptors that are not cyclase linked.

Regional blockade by neuroleptic drugs of in vivo 3H-spiperone binding in the rat brain. Relation to blockade of apomorphine induced hyperactivity and stereotypies

The regional prevention by neuroleptic drugs of specific in vivo 3H-spiperone binding was studied in the rat brain. L-sulpiride, thioridazine and clozapine was found to reduce the 3H-spiperone binding selectively in the olfactory tubercle, septum, substantia nigra region and frontal cortex but not the striatum at dose levels which preferentially block apomorphine (APO) induced hyperactivity. The maximal prevention of specific 3H-spiperone binding by l-sulpiride and clozapine reached 60-80% in the former structures while the displacement of striatal 3H-spiperone binding did not exceed 40%. In contrast to l-sulpiride, thioridazine and clozapine both chlorpromazine and haloperidol reduced the 3H-spiperone binding to the same extent in all regions studied. Chlorpromazine and haloperidol were potent in prevention of striatal 3H-spiperone binding in vivo which reached 60-80% in this structure.

Interconverting mu and delta forms of the opiate receptor in rat striatal patches

The binding of a radiolabeled "mu receptor" prototype opiate, dihydromorphine (H2morphine), and the binding of a "delta receptor" prototype, [D-Ala2,D-Leu5]enkephalin (D-Enk), to slide-mounted rat caudate slices were simultaneously compared quantitatively and visualized by autoradiography. Generally, D-Enk bound to opiate receptors distributed evenly throughout the entire striatum (type 2 pattern), whereas H2morphine labeled discrete islands or patches of receptors (type 1 pattern). In the presence of Mn2+ (3 mM) or other divalent cations, however, Na+ and GTP at 25 degrees C caused an increase in D-Enk binding at the expense of H2morphine binding at striatal opiate receptor patches. Thus, these conditions shifted D-Enk binding from an even pattern to one that included both an even and patchy distribution. These incubation conditions not only promoted D-Enk binding to striatal patches but also enabled the opiate receptor to regulate adenylate cyclase with the same (P less than 0.01) ligand selectivity pattern as that obtained by the displacement of D-Enk binding. The relative affinity of opiate receptors in striatal patches for opiate peptides, naloxone, and morphine appears to be a function of incubation conditions and coupling to adenylate cyclase and is not indicative of distinctly different opiate receptors. We postulate a three-state allosteric model consisting of mu agonist-, mu antagonists-, and adenylate cyclase-coupled delta-agonist-preferring states, whose equilibrium may be regulated by a sulfhydryl group mechanism.

Modulation of slow postsynaptic potentials by dopamine, in rabbit sympathetic ganglion

(1) Temporary exposure of rabbit's superior cervical ganglion (SCG) to dopamine (DA), in the presence of an inhibitor of catechol-o-methyltransferase (COMT) is consistently followed by a potentiation of the slow (s)-EPSP and s-IPSP, lasting for some hours. The fast (f)-EPSP is not significantly increased, but it is better maintained than in control ganglia. (2) Exposure to the COMT-inhibitor U-0521 alone induces less but substantial potentiations of both s-PSPs. This effect is explained as due to protection of DA released intraganglionically at rest. (3) This evidence suggests that COMT may significantly limit the access of catecholamines to postsynaptic receptors, for at least certain types of neuron-to-neuron synaptic actions. (4) The potentiation of both s-PSPs, whether induced by DA in the presence of U-0521 or by U-0521 alone, is depressed by DA-1 antagonists that have been found to depress DA-stimulation of adenyl cyclase in rabbit SCG; these are spiroperidol, butaclamol and, to a lesser extent, bromocriptine. The specific 'DA-2' antagonists metoclopramide and sulpiride, and the alpha-adrenergic antagonist dihydroergotamine, did not depress potentiation. (5) Potentiation of s-EPSP is viewed as identical in nature to the previously discovered DA-modulatory enhancement of direct muscarinic depolarizing actions (by acetylcholine or its agonists). Potentiation of s-IPSP may be due to a similar DA-modulation of other muscarinic response(s) involved in mediating the s-IPSP. The consistency and comparative ease with which these DA-modulatory effects can be induced, under presently described experimental conditions, should facilitate future study of this mode of synaptic action.

Studies on the responses of plasma renin activity and aldosterone and cortisol levels to dopaminergic and opiate stimuli in man

Evidence for a role of dopamine and endogenous opioids in the control of the secretion of renin and adrenal steroids was sought in man. The effects of L-dopa, L-dopa plus carbidopa, dopamine, domperidone and naloxone were studied on the renin and aldosterone responses to head-up tilt. L-dopa diminished the rise in renin following tilt and this effect of L-dopa was abolished by carbidopa. Aldosterone was not significantly affected by any of the compounds. Cortisol secretion was stimulated by carbidopa plus L-dopa more than L-dopa alone, and was also increased by both dopamine and naloxone. The significance of these findings is discussed.