Dopamine D2 receptors are restricted to the prolactin cells in the rabbit pituitary gland: a combined autoradiographic and immunocytochemical study

Distribution of dopamine D2 receptor mRNAs in the brain and the pituitary of female rainbow trout: an in situ hybridization study

The distribution of D(2)R (dopamine D(2) receptor) mRNAs was studied in the forebrain of maturing female rainbow trout by means of in situ hybridization using a (35)S-labeled riboprobe (810 bp) spanning the third intracytoplasmic loop. A hybridization signal was consistently obtained in the olfactory epithelium, the internal cell layer of the olfactory bulbs, the ventral and dorsal subdivisions of the ventral telencephalon, and most preoptic subdivisions, with the notable exception of the magnocellular preoptic nucleus, and the periventricular regions of the mediobasal hypothalamus, including the posterior tuberculum. In the pituitary, the signal was higher in the pars intermedia than in the proximal and the rostral pars distalis, but no obvious correspondence with a given cell type could be assigned. Labeled cells were also located in the thalamic region, some pretectal nuclei, the optic tectum, and the torus semicircularis. These results provide a morphologic basis for a better understanding on the functions and evolution of the dopaminergic systems in lower vertebrates.

Tyrosine hydroxylase- and/or aromatic L-amino acid decarboxylase-expressing neurons in the rat arcuate nucleus: ontogenesis and functional significance

This study has evaluated in vivo, ex vivo and in vitro the ontogenesis and functional significance of the neurons of the arcuate nucleus (AN) expressing either individual enzymes of dopamine (DA) synthesis, tyrosine hydroxylase (TH) or aromatic L-amino acid decarboxylase (AADC) as well as both of them in rats from the 17th embryonic day (E) till adulthood. Immunocytochemistry, image analysis, confocal microscopy, high performance liquid chromatography with electrochemical detection and radioimmunoassay were used to solve this problem. Monoenzymatic TH-containing neurons were initially observed on E18 located in the ventrolateral AN whereas the neurons expressing only AADC or both AADC and TH first appeared on E20 in the dorsomedial AN. On E21, the monoenzymatic TH- or AADC-expressing neurons comprised more than 99% of the whole neuron population expressing the DA-synthesizing enzymes. In spite of an extremely small number (<1%) of the neurons expressing both enzymes (DArgic neurons), the dissected AN (ex vivo) and its primary cell culture (in vitro) contained a surprisingly high amount of DA and L-dihydroxyphenylalanine (L-DOPA) which were released in response to membrane depolarization. Furthermore, DA production in the AN of fetuses occurred to be sufficient to provide an inhibitory control of prolactin secretion, as in adults. The above data suggest that DA could be synthesized, at least in the AN of fetuses, by monoenzymatic neurons containing either TH or AADC, in co-operation. This hypothesis may be extended to adult animals as their AN contained the same populations of the neurons expressing DA-synthesizing enzymes as in fetuses though the proportion of true DArgic neurons increased up to 38%. During ontogenesis, the monoenzymatic TH- and AADC-containing neurons established axosomatic and axo-axonal junctions that might facilitate the L-DOPA transport from the former to the latter. Moreover, the monoenzymatic AADC-expressing neurons project their axons to the median eminence, thereby, providing the pathway for the DA transport toward the hypophysial portal circulation. Thus, DA appears to be synthesized in the AN not only by DArgic neurons but also by monoenzymatic TH- and AADC-expressing neurons in co-operation.

Non-dopaminergic neurons expressing dopamine synthesis enzymes: differentiation and functional significance

The development and functional significance of neurons in the arcuate nucleus expressing tyrosine hydroxylase and/or aromatic L-amino acid decarboxylase were studied in rat fetuses, neonates, and adults using immunocytochemical (single and double immunolabeling of tyrosine hydroxylase and aromatic L-amino acid decarboxylase) methods with a confocal microscope and computerized image analysis, HPLC with electrochemical detection, and radioimmunological analysis. Single-enzyme neurons containing tyrosine hydroxylase were first seen on day 18 of embryonic development in the ventrolateral part of the arcuate nucleus. Neurons expressing only aromatic L-amino acid decarboxylase or both enzymes of the dopamine synthesis pathway were first seen on day 20 of embryonic development, in the dorsomedial part of the nucleus. On days 20-21 of embryonic development, dopaminergic (containing both enzymes) neurons amounted to less than 1% of all neurons expressing tyrosine hydroxylase and/or aromatic L-amino acid decarboxylase. Nonetheless, in the ex vivo arcuate nucleus and in primary neuron cultures from this structure, there were relatively high leveLs of dopamine and L-dihydroxyphenylalanine (L-DOPA), and these substances were secreted spontaneously and in response to stimulation. In addition. dopamine levels in the arcuate nucleus in fetuses were sufficient to support the inhibitory regulation of prolactin secretion by the hypophysis, which is typical of adult animals. During development, the proportion of dopaminergic neurons increased, reaching 38% in adult rats. Specialized contacts between single-enzyme tyrosine hydroxylase-containing and aromatic L-amino acid decarboxylase-containing neurons were present by day 21 of embryonic development; these were probably involved in transporting L-DOPA from the former neurons to the latter. It was also demonstrated that the axons of single-enzyme decarboxylase-containing neurons projected into the median eminence, supporting the secretion of dopamine into the hypophyseal portal circulation. Thus, dopamine is probably synthesized in the arcuate nucleus not only by dopaminergic neurons, but also by neurons expressing only tyrosine hydroxylase or aromatic L-amino acid decarboxylase.

Dopamine function in obsessive-compulsive disorder: growth hormone response to apomorphine stimulation

Indirect observations suggest that the dopaminergic system may be involved in the pathophysiology of obsessive-compulsive disorder (OCD). The dopaminergic function of 15 patients with OCD and 15 age/sex-matched controls was evaluated by measuring the growth hormone (GH) responses to stimulation with the dopaminergic agonist apomorphine (APO), which increases growth hormone-releasing hormone (GHRH), GH, and somatomedine C (SMD-C) secretions. Therefore, we measured basal plasma GH and SMD-C concentrations and GH responses to GHRH stimulation to exclude that a downstream pathology of the somatotropic axis could obscure the significance of the results of the APO test. The response of prolactin (PRL) to APO inhibition were also measured. Basal plasma levels of GH, SMD-C, and PRL, GH responses to GHRH stimulation, and PRL responses to APO inhibition did not differ in the two groups of subjects. GH responses to APO stimulation were blunted in obsessive-compulsive (OC) patients. The emetic response to the same stimulation was stronger in patients than in controls. These responses suggest that in our OC patients there is a dysregulation of the dopaminergic system, which is possibly expressed in different ways in the various areas of the central nervous system.

Stereospecificity of the dopamine receptor mediating the growth hormone response to apomorphine in man. Short communication

The stereospecificity of the D-2 receptor mediating the growth hormone (GH) response to apomorphine (Apo) and the D-2 receptor regulating prolactin (PRL) secretion were investigated in 10 normal men by examining the effects of cis-flupenthixol (cis-Fx) and trans-flupenthixol (trans-Fx). cis-Fx (1 mg six hourly times four doses) antagonized the GH response to Apo HCl (0.5 mg sc) and increased basal serum PRL concentrations whereas the trans-isomer showed no effect. These findings (a) provide further evidence that the GH response to Apo is mediated by stimulating dopamine (DA) receptors, and, (b) demonstrate stereospecificity of the DA receptor mediating the GH response to Apo and the DA receptor regulating PRL secretion.

Multiple fluorescent ligands for dopamine receptors. II. Visualization in neural tissues

Selective dopamine receptor ligands, (R,S)-5-(4'-aminophenyl)-8-chloro-2,3,4, 5-tetrahydro-3-methyl-[1H]-3-benzazepin-7-ol, the 4'-amino derivative of the high affinity D1 receptor antagonist SCH 23390, the high affinity D2 receptor antagonist N-(p-aminophenethyl)-spiperone or NAPS, and the D2 selective agonist, 2-(N-phenethyl-N-propyl)-amino-5-hydroxytetralin or PPHT were chemically coupled to the fluorescent compounds, Bodipy, Cascade blue, coumarin, fluorescein, rhodamine, or Texas red. The utility of the 6 fluorescent moieties linked to the 3 dopamine receptor binding ligands for anatomical study of regional and cellular distribution patterns of the two dopaminergic receptor subtypes has been assessed in frozen sections of the rat striatum and compared to our previous report using the rhodamine-labeled antagonists. The regional staining for the two dopaminergic receptor binding sites supports previous work using in vitro receptor autoradiographic analyses; the D1 receptor binding was more robust than that of D2 receptors in the caudate nucleus. The cellular element which most frequently expressed striatal D1 binding sites had a medium-diameter cell body. Medium-sized cells also exhibited fluorescence for the D2 binding site, as did a much larger diameter element; potentially the cholinergic interneuron of the caudate nucleus. The pharmacological specificity for each of the different D1 fluorescent antagonist ligands in the tissues was determined by competition with 100-fold excess of unlabeled SCH 23390 (non-specific binding), spiroperidol (binding selectivity), the stereoactive paired isomers of butaclamol, and the serotonin 5-HT2 receptor antagonist ketanserin. The same criteria were used to assess the different D2 fluorescent agonist and antagonist ligand derivatives. The anatomical efficacy of these novel ligands was determined using selective dichroic filters to stimulate the fluorescent moieties in the optimal excitation wavelength, and the amount of fluorescent dopamine receptor binding was photographically measured and contrasted for each of the newly synthesized fluoroprobes. Using the most pharmacologically specific and anatomically efficient of these novel fluoroprobes, we determined the localization pattern of the D1 and D2 dopamine receptor binding sites in tissues reported to exhibit both subtypes of the receptor. The cellular distribution of the dopamine receptor binding sites was determined concurrently using fluoroprobes in the forebrain, mesencephalon, pituitary, retina, and superior cervical ganglion of the rodent, and bovine adrenal medullary chromaffin cells were examined using the rhodamine-labeled antagonists.

Effect of some peptides on dopaminergic function in man

Thyrotropin-releasing hormone (TRH) (200 micrograms iv) and 1-desamino-8-D-arginine vasopressin (DDAVP) (4 micrograms iv) antagonized the growth hormone (GH) response to apomorphine HCl (Apo) (0.5 mg sc) in 10 normal men. Apo had no effect on basal prolactin (PRL) levels but antagonized the PRL response to TRH. DDAVP plus Apo decreased PRL compared to placebo or DDAVP alone. These observations are compatible with (a) an inhibitory effect of TRH on hypothalamic and pituitary lactotrophe dopamine (DA) function (b) a facilitory effect of DDAVP on lactotrophe DA function and an inhibitory effect on hypothalamic DA function. Whether these are direct or indirect effects on DA mechanisms is unclear.

Apomorphine in the evaluation of dopaminergic function in man

1. Apomorphine (Apo), a short acting dopamine (DA) receptor agonist, stimulates growth hormone (GH) secretion, decreases prolactin secretion, induces yawning, penile erections and other physiological effects in man. An effect on behavior, movement disorders and alcoholism has also been described. 2. Apo-mediated responses are used to evaluate DA function in psychiatric and neurological disorders. Many of the studies in schizophrenia using the GH response to Apo as an index of central DA function are difficult to interpret because of failure to control for key variables. 3. The GH response to Apo is a useful system to evaluate the effects of various drugs including peptides which may not cross the blood brain barrier on DA function in man. 4. Apo is a potent sedative. Specific antimanic, antischizophrenic, and anticraving effects in alcoholics have not been convincingly demonstrated. Side effects of Apo and failure to use active placebo make double-blind studies difficult. 5. Apo improves parkinsonian symptoms and certain forms of reflex epilepsy but beneficial effects in other involuntary movement disorders requires further documentation. 6. Apo may be a useful agent to evaluate DA function in impotent patients and predict a therapeutic response to long-acting dopaminergic agents. 7. Impairment of DA function may play a role in diabetic impotence. 8. The development of a simple polygraphic method to monitor the yawning response to Apo may facilitate clinical studies on the basic physiology of yawning in man and the use of the yawning response as a measure of central DA function in schizophrenia and other clinical disorders. 9. The use of Apo with 18F-fluorodeoxyglucose positron emission tomography to examine regional DA function in man opens up a promising area of research. 10. Though long-acting orally active aporphine DA agonists and antagonists have been developed the problem of tolerance may limit their therapeutic potential.