Multiple receptors for dopamine

Uptake of radiocalcium by nerve endings isolated from rat brain: pharmacological studies

1 The uptake of radiocalcium by nerve-ending particles isolated from the striatum of rat brain was studied using lanthanum as a quenching agent. 2 High potassium-induced calcium uptake occurred in two phases: an initial rapid phase and a late slow phase. Following preincubation with CaCl2 2.2 mmol/l for 1 h, dopamine at 1 to 2 x 10(-4) mol/l reduced the high potassium-induced calcium uptake which occurred during the initial rapid phase by 66 and 25% at 2 and 4 s of incubation, respectively, but had no effect on the late slow uptake phase. 3 Haloperidol at 1 x 10(-6) mol/l abolished the inhibitory effect of dopamine on the initial rapid phase of the high potassium-induced calcium uptake. Haloperidol per se had no effect on the calcium uptake. 4 Dibutyryl cyclic adenosine monophosphate at 2.5 x 10(-3) mol/l or prostaglandin E1 (PGE1) at 1 x 10(-5) mol/l had no effect on the initial rapid phase of the high potassium-induced calcium uptake by striatal synaptosomes. Neither of these agents affect calcium uptake by whole brain synaptosomes. 5 It appears that in the striatum, dopamine regulates the depolarization-induced influx of calcium in presynaptic nerve endings. This mechanism could constitute a feed-back inhibition for transmitter release in the striatum.

Further evidence that 3H-cis(Z)flupenthixol binds to the adenylate cyclase-associated dopamine receptor (D-1) in rat corpus striatum

The proposal that 3H-cis(Z)flupenthixol (3H-FPT) preferentially binds to striatal dopamine receptors associated with adenylate cyclase activity (D-1), distinct from dopamine receptors to which 3H-haloperidol (3H-hal) binds (D-2), has been investigated further. The dopamine agonists bromocriptine and ergotamine were 60-times more potent as displacers of 3H-hal than 3H-FPT binding. Other dopamine agonists (ergometrine, dopamine, apomorphine, 2-amino-6,7-dihydroxy-tetralin (ADTN), and ergocornine) also shared this profile, although a smaller ratio was found for these compounds. Substituted benzamides (clebopride greater than sultopride greater than sulpiride greater than metoclorpramide greater than tiapride) displace 3H-hal but have only a very slight displacing effect towards 3H-FPT, and did not inhibit dopamine-stimulated adenylate cyclase activity. The same pattern is shared by oxiperomide and molindone. Together, these results support the idea that 3H-FPT labels another class of dopamine receptors than does 3H-hal, and that the former class most likely is associated with adenylate cyclase.

Dopamine inhibition of action potentials in a prolactin secreting cell line is modulated by oestrogen

Secretory activity of the anterior pituitary gland is regulated by the brain through stimulatory and inhibitory substances released from nerve endings in the median eminence of the hypothalamus and carried by the adenohypophysial portal blood system to their respective target cells. These hypothalamic influences are modulated by the feedback action of peripheral hormones. Prolactin (PRL) secreting cells are, at least partially, under the stimulatory influence of thyrotropin releasing hromone (TRH) and of oestrogens. However, they are mainly controlled by inhibitory substances among which dopamine (DA) is one of the most potent in vivo as well as in vitro. The inhibitory effect of DA is reversed by oestrogen in vitro. The mechanism by which these factors interact on their target cells is poorly understood. The recent discovery that anterior pituitary cells are excitable and that they are able to generate Ca2+-dependent action potentials has led to the suggestion that these effects are involved in a stimulus-secretion coupling at the membrane level. In this paper, we report that DA inhibits both the spontaneous and TRH-induced action potentials in clonal PRL pituitary cells. In addition, oestradiol-17 beta is able to reverse the inhibitory effect of DA.

Autoradiographic localization of neuroleptic and dopamine receptors in the caudate-putamen and substantia nigra: effects of lesions

The localization of neuroleptic receptors was studied in the caudate-putamen (CP) and the zona compacta of the substantia nigra using light microscopic autoradiography of 3H-spiperone binding sites. Lesion of the dopaminergic input to the caudate-putamen produced an increase in receptors in the CP, possibly reflecting denervation supersensitivity. Kainic acid lesions and decortication produced significant decreases of 61% and 18% in striatal receptors. This suggests that in the caudate-putamen most of the dopamine receptors are on intrastriatal neurons, but some are also localized to the afferents from the cortex. Lesion of the nigro-striatal dopaminergic pathway produced a large (48%) decrease in receptor sites in the substantia nigra zona compacta while kainic acid intrastrially and striato-nigral pathway lesions had no significant effect. These results suggest that the majority of dopamine receptors in the zona compacta which bind neuroleptics are located on cell bodies and processes of dopaminergic neurons and are anatomically distinct from dopamine-stimulated adenylate cyclase sites.

Dopaminergic agonists and their influence on the oxygenation and function activity of underperfused brain tissue

The effect of two dopaminergic agonists (apomorphine and bromocriptine) on electrical activity and oxygen supply of the brain was investigated in cats submitted to hypovolemic oligemia (mean arterial blood pressure: 45 mmHg). While both drugs stimulated the brain by prolonging the oligemia-induced seizures in the caudate nucleus and in the cerebral cortex, only apomorphine improved the pO2 distribution in the cortical tissue after 120 min oligemia. Bromocriptine, in contrast, had a beneficial effect of shorter duration. These data show that under conditions of incomplete ischemia the brain can still be activated. Furthermore, these results provide additional support for the biochemically founded hypothesis of different dopamine receptors in the brain.

Species variation in dopamine receptor binding

Binding of 3H-spiroperidol, 3H-apomorphine and 3H-ADTN (2-amino-6,7-dihydroxytetrahydronaphthalene) associated with dopamine receptors has been evaluated in corpus striatal membranes of calf, rat and human brains. Substantial species differences are apparent for numberous agonists and antagonists in competing for receptor binding. In general, dopamine receptor antagonists are more potent in rat and agonists more potent in calf. In competing for 3H-spiroperidol binding sulpiride, molindone and metaclopramide show the most pronounced species differences, being 3--10 times more potent in rat and human than in calf. In all three species agonists compete for 3H-spiroperidol binding with Hill coefficients less than one while antagonists inhibit 3H-spiroperidol binding with Hill coefficients of about 1.0. Conversely, 3H-apomorphine and 3H-ADTN binding in all three species is inhibited by antagonists with Hill coefficients less than 1.0 while agonists display Hill coefficients of about 1.0. In general agonists are more potent in competing for binding of 3H-apomorphine and 3H-ADTN than 3H-spiroperidol. However, a small component of dopamine, apomorphine and ADTN inhibition of 3H-spiroperidol binding displays very high affinity (IC50 about 1 nM). In human amygdala 3H-spiroperidol appears to label serotonin receptors predominantly.

Effects of estrogens on the characteristics of [3H]spiroperidol and [3H]RU24213 binding in rat anterior pituitary gland and brain

Since estrogens have been found to exert a marked inhibitory effect on dopaminergic action at the anterior pituitary and striatal levels in the rat, the effect of estrogen treatment has been studied on the binding characteristics of the dopamine (DA) antagonist [3H]spiroperidol and of the new DA agonist [3H]N-n-propyl-N-phenylethyl-beta-(3-hydroxyphenyl)ethylamine hydrochloride ([3H]RU24213) in rat striatum, nucleus accumbens + olfactory tubercle, frontal cortex and anterior pituitary gland. Specificity of binding was carefully examined in order to investigate possible changes of the agonist and antagonist states of the DA receptor. Estrogen treatment led to a small increase (approx. 20%) of [3H]spiroperidol and [3H]RU24213 binding in rat striatum, nucleus accumbens + olfactory tubercle and frontal cortex while no significant effect was found in the anterior pituitary gland. That the increased binding is due to a corresponding increased number of binding sites and not to higher affinity is indicated by the absence of effect of estrogen treatment on the IC50 values for displacement of the two labeled ligands by a variety of unlabeled compounds. Specificity of binding of DA agonists and antagonists remained unchanged after estrogen treatment. The present data suggest that the potent desensitizing effect of estrogen on DA action at the striatal and pituitary levels is exerted at a step subsequent to binding of DA to its receptor.

Is dopamine-sensitive adenylate cyclase involved in regulating the activity of striatal cholinergic neurons?

The dopamine (DA)-receptor mediated changes in striatal acetylcholine (ACh) levels have been studied to determine if this effect involves a D1-(adenylate cyclase dependent) or D2-(not linked to an adenylate cyclase) type of DA-receptor, Various DA-agonists (apomorphine, N-diphenethylamine derivatives) increased striatal ACh levels in both intact and 6-OHDA lesioned rats whereas only apomorphine stimulated the adenylate cyclase activity of striatal homogenates. The N-diphenethylamine compounds (RU 24213, RU 24926 and RU 26933) were without effect either on basal or DA-stimulated activities of this enzyme. In contrast, D-LSD (which acts as a partial agonist of the striatal DA-sensitive adenylate cyclase) did not modify the striatal ACh content. More interestingly, an intrastriatal injection of cholera toxin greatly stimulated striatal adenylate cyclase without altering ACh concentrations. Both haloperidol and methergoline antagonized the DA stimulation of adenylate cyclase, but only haloperidol decreased striatal ACh levels. These results indicate that the DA receptor involved in regulating the activity of striatal cholinergic neurons is of the D2-type.

Dopamine-receptor blockade and tardive dyskinesia

This study compares the sonographic and CT findings in a group of infants with macrocrania and correlates those findings with neurologic outcome to determine the diagnostic accuracy and prognostic value of sonography. Sonographic findings in 255 infants with macrocrania are described. Of the 195 term infants examined, 130 had normal sonograms, 11 (5.6%) had significant abnormalities, and 54 had increased intra- and/or extraaxial fluid spaces. Of the 60 former preterm infants, 33 had normal sonograms, four (6.7%) had significant abnormalities, and 23 had increased fluid spaces or small resolving germinal matrix hemorrhages. The patients with significant abnormalities usually had head circumferences greater than the 95th percentile and had neurologic abnormalities. There was good correlation between sonography and CT in 30 of the 36 patients evaluated by both. In six there was mild discrepancy in the volume of the extraaxial fluid. No significant abnormality was missed by sonography. CT did not contribute any additional information. Neurologic follow-up was available for 202 patients. Nineteen percent of the term infants and 24% of the former preterm infants were abnormal on neurologic follow-up. Most patients with normal sonograms were normal on follow-up. Twelve of the term and four of the preterm infants with normal sonograms were developmentally delayed on follow-up. Increased CSF in the ventricles and/or extraaxial spaces was a common abnormality, but it usually is associated with a normal neurologic outcome and represents “benign macrocrania.”

We conclude that an infant with an enlarged or enlarging head should have a neurologic examination and head circumference measurement. If the patient has a head circumference greater than the 95th percentile, particularly if there are abnormal neurologic findings, further evaluation is indicated. Sonography is the initial procedure recommended since it accurately evaluates ventricular size, extraaxial fluid, and congenital malformations. If sonography is normal or shows mildly increased fluid spaces, then follow-up head circumference measurement and clinical evaluation will probably suffice. CT is indicated if there is a significant abnormality on sonography that requires further clarification.

The molecular structure of adrenergic and dopaminergic substances

At the end of the last century it was established that the different nerve cells along a neuronal path do not come into direct physical contact with one another, but that there are narrow gaps between them, called synapses (Sherrington, 1897; Ramón y Cajal, 1906). Elliot (1905) made the basic experimental observation that the propagation of nerve impulses across a synapse might be mediated by specific chemical agents (see Fig. i). Such substances are now called neurotransmitters, and some 20 different compounds putatively responsible for synaptic transmission in different parts of the nervous system are known at present, e.g. a few recently isolated polypeptides. The most extensively studied transmitters are acetylcholine and the catecholamine group, consisting of dopamine (a), noradrenaline (b), and adrenaline (c).

Developments in understanding the physiology and pharmacology of parkinsonism

While the era of major advances in understanding and treating parkinsonism seems to be over, steady progress is being made in elucidating physiological and pharmacological aspects of extrapyramidal function. The dramatic impact of levodopa therapy has been followed by the recognition of serious limitations to its long-term use, which provides a continuing stimulus for efforts to analyze the physiological and pharmacological properties of the basal ganglia and substantia nigra. There are reasonable grounds for the hope that this research will lead to significant developments in therapy.

Sulpiride in tardive dyskinesia

Tardive dyskinesia can be suppressed by drugs that block dopaminergic receptors, but often at the cost of a concomitant increase in parkinsonism. Sulpiride (400 -- 2100 mg/day), a selective type-2 dopamine receptor antagonist, was evaluated in a blind, placebo-controlled trial in 11 patients with tardive dyskinesia. It significantly (P less than 0.01) reduced tardive dyskinesia without significantly affecting parkinsonism, although three patients had a increase in preexisting parkinsonian hypokinesia and tremor. During the placebo phase, the tardive dyskinesia and parkinsonian scores returned to the pretreatment values. There was no relationship between either tardive dyskinesia or parkinsonism and eye blinking rates. These results are consistent with the hypothesis that more than one population of dopamine receptors are involved in controlling extrapyramidal function. Sulpiride is an important tool for elucidating both the practical and heuristic aspects of subtypes of dopamine receptors and is a lead in the search for compounds that selectively affect dopaminergic mechanisms.