Expression of vasoactive intestinal peptide receptor messenger RNA in the hypothalamus and pituitary throughout the turkey reproductive cycle

Vasoactive intestinal peptide (VIP) has been implicated in the regulation of avian reproductive activity and appears to act at the level of the hypothalamus and pituitary. This in situ hybridization histochemistry study describes the distribution of VIP receptor mRNA expression in the hypothalamus and the pituitary of reproductively active (laying) and quiescent (nonphotostimulated, incubating, and photorefractory) female turkeys and characterizes the differences observed in VIP receptor gene expression. VIP receptor mRNA, while expressed throughout the hypothalamus, was specifically expressed in areas known to contain GnRH-I neurons in the chicken, i.e., the lateral septum, medial preoptic area, anterior hypothalamus, and paraventricular nucleus. Significant differences in VIP receptor mRNA expression between different reproductive states was observed only within the infundibular nuclear complex. VIP receptor mRNA was markedly less in nonphotostimulated and photorefractory hens as compared with laying and incubating hens. The most dense VIP receptor mRNA was found in the anterior pituitary, where it was 2.4- and 3.0-fold greater in laying and incubating hens, respectively, as compared with that in nonphotostimulated ones. Hens that stopped incubating and became photorefractory displayed pituitary VIP receptor mRNA levels similar to those of nonphotostimulated birds. The changes in pituitary VIP receptor mRNA expression were positively correlated with known changes in pituitary prolactin (PRL) mRNA expression and PRL content and release. These findings indicate that the variations in PRL secretion observed across the turkey reproductive cycle are, in part, regulated by changes in VIP receptors at the pituitary level.     

Antisense Strategy Unravels a Dopamine Receptor Distinct from the D2 Subtype, Uncoupled with Adenylyl Cyclase, Inhibiting Prolactin Release from Rat Pituitary Cells

The antisense strategy was used to unravel the functional contribution of the mRNAs encoding dopamine (DA) receptors to the multiple transduction mechanisms operated by DA in rat pituitary cells. An antisense oligonucleotide was designed to recognize seven nucleotides upstream and 11 nucleotides downstream from the initiation translation codon of the mRNA that encodes the DA D2 receptor. Addition of the antisense oligonucleotide for 7 days to primary culture of rat pituitary cells resulted in a decreased expression of DA D2 receptor as shown by (a) the virtual disappearance of [³H]spiroperidol binding sites and (b) the marked reduction in the levels of both the long and the short splice variant of the D2 receptor mRNAs. After this treatment, the DA D2 receptor agonist bromocriptine lost its capability both to inhibit adenylyl cyclase activity and to reduce prolactin mRNA levels. On the contrary, the inhibition of prolactin release induced by bromocriptine was affected minimally by the antisense oligonucleotide treatment. These data indicate that (a) translation of the mRNA encoding DA D2 receptors results in receptors that are negatively coupled with adenylyl cyclase and functionally linked to inhibition of prolactin synthesis; and (b) the release of prolactin might be regulated, at least in part, by a DA receptor that is encoded by mRNA species distinct from those encoding the D2 receptor.     

Influence of VIP on prolactinemia in turkey anterior pituitary cells: role of cAMP second messenger in VIP-induced prolactin gene expression

Vasoactive intestinal peptide (VIP) is the avian prolactin (PRL)-releasing factor. In the turkey, hypothalamic VIP immunoreactivity and mRNA content, as well as VIP levels in hypophyseal portal blood, are closely related to the state of prolactinemia and the reproductive stage. The present study investigated the role of VIP on prolactinemia in turkey anterior pituitary (AP) cells through PRL gene expression and the role of a cAMP second messenger system on VIP-induced PRL expression. In primary AP cells harvested from hens in different prolactinemic states, steady state promoter activities were positively correlated with secreted PRL levels. VIP increased PRL promoter activities in AP cells from hens with intermediate PRL levels (laying), but not in AP cells from hypoprolactinemic hens (nonphotostimulated reproductively quiescent). However, in AP cells from hyperprolactinemic hens (incubating), PRL promoter activity was down-regulated by VIP. PRL mRNA steady state levels were significantly decreased by the cAMP analogue, 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), and PRL secretion was down-regulated by the phosphodiesterase blocker, 3-isobutyl-1-methylxanthine (IBMX) in a dose-dependent manner, suggesting that the cAMP second messenger system might be involved in the inhibitory action of dopamine upon VIP-stimulated PRL secretion and gene expression at the pituitary level. In a study of VIP immediate and long-term effects on c-fos expression in relation to PRL expression, VIP dramatically induced c-fos mRNA expression within 5 min, suggesting that VIP-induced c-fos expression might be involved in VIP-stimulated PRL secretion and gene expression. These results provide additional evidence of the functional significance of VIP in PRL gene expression and suggest that changes in PRL promoter activity by VIP may be one of the important inductive mechanisms leading to prolactinemia.     

Aspects of the neuroendocrine control of ovulation and broodiness in the domestic hen

The neuroendocrine control of ovulation and broodiness in the domestic hen involves complex interactions between hypothalamic neuropeptides, neurotransmitters, and ovarian steroids which regulate the secretion of luteinizing hormone (LH) and prolactin. Nuclear progesterone receptor is localized in many neurons throughout the hypothalamus but is absent from LHRH neurons. Hence, the positive feedback action of progesterone on LH release is not mediated by a genomic mechanism within the LHRH neuron. Precursors of 5-hydroxytryptamine (5HT) and dopamine (DA) inhibit the preovulatory release of LH, while the turnover rates of these neurotransmitters in the anterior hypothalamus decrease when preovulatory levels of LH are at their highest. Further, a population of receptors for 5HT which occurs in the anterior hypothalamus in laying birds is absent in nonlaying, incubating hens. Taken together, these observations suggest that the preovulatory surge of LH is mediated by a transitory decrease in the inhibitory action of 5HT and possibly DA, on the secretion of LHRH. Neurons containing 5HT may play a role in the regulation of prolactin release and, more specifically, in the control of broodiness. Drugs which enhance the function of 5HT neurons stimulate prolactin release while increased prolactin secretion in incubating hens is associated with an increase in the turnover of 5HT in the anterior hypothalamus. No receptors for 5HT were demonstrable in the anterior pituitary gland, showing that the prolactin-releasing activity of 5HT must be mediated by a prolactin-releasing factor (PRF). A candidate for a physiological PRF is vasoactive intestinal polypeptide (VIP).(ABSTRACT TRUNCATED AT 250 WORDS)     

Tachykinins and the control of prolactin secretion

Tachykinins are present in the pituitary gland and in brain areas involved in the control of the secretion of pituitary hormones. Tachykinins have been demonstrated to stimulate prolactin release acting directly on the anterior pituitary gland. These peptides have also been revealed to be able to act at the hypothalamic level, interacting with neurotransmitters and neuropeptides that have the potential to affect prolactin secretion. Tachykinins seem to act by stimulating or inhibiting the release of the factors that affect prolactin secretion. Among them, tachykinins have been demonstrated to stimulate oxytocin and vasopressin release, which in turn results in prolactin release. Tachykinins also potentiated the response to vasoactive intestinal peptide (VIP) and reinforced the action of glutamate, which in turn result in prolactin release. They have also been shown to interact with serotonin, a neurotransmitter involved in the control of prolactin secretion. In addition, tachykinins have been shown to inhibit GABA release, a neurotransmitter with prolactin-release inhibiting effect. This inhibition may result in an increased prolactin secretion by removal of the GABA inhibition. On the other hand, tachykinins have also been shown to stimulate dopamine release by the hypothalamus, an action that results in an inhibition of prolactin release. Dopamine is a well known inhibitor of prolactin secretion. In conclusion, although tachykinins have been shown to have a predominantly stimulatory effect on prolactin secretion, especially at the pituitary level, under some circumstances they may also exert an inhibitory influence on prolactin release, by stimulating dopamine release at the hypothalamic level.     

G protein preferences for dopamine D2 inhibition of prolactin secretion and DNA synthesis in GH4 pituitary cells

Dopamine is the primary inhibitory regulator of lactotroph proliferation and prolactin (PRL) secretion in vivo, acting via dopamine D2 receptors (short D2S and long D2L forms). In GH4C1 pituitary cells transfected with D2S or D2L receptor cDNA, dopamine inhibits PRL secretion and DNA synthesis. These actions were blocked by pertussis toxin, implicating G(i)/G(o) proteins. To address roles of specific G(i)/G(o)4 proteins in these actions a series of GH4C1 cell lines specifically depleted of individual Galpha subunits was examined. D2S-mediated inhibition of BayK8644-stimulated PRL secretion was primarily dependent on G(o) over G(i), as observed for BayK8644-induced calcium influx. By contrast, inhibitory coupling of the D2S receptor to TRH-induced PRL secretion was partially impaired by depletion of any single G protein, but especially G(i)3. Inhibitory coupling of D2L receptors to PRL secretion required G(o), but not G(i)2, muscarinic receptor coupling was resistant to depletion of any G(i)/G(o) protein, whereas the 5-HT1A and somatostatin receptors required G(i)2 or G(i)3 for coupling. The various receptors also demonstrated distinct G protein requirements for inhibition of DNA synthesis: depletion of any G(i)/G(o) subunit completely uncoupled the D2S receptor, the D2L receptor was uncoupled by depletion of G(i)2, and muscarinic and somatostatin receptors were resistant to depletion of G(i)2 only. These results demonstrate distinct receptor-G protein preferences for inhibition of TRH-induced PRL secretion and DNA synthesis.     

Involvement of D1 dopamine receptors in the nicotine-induced noradrenaline release from hypothalamic and preoptic noradrenaline nerve terminal systems

Nicotine (4 × 2 mg/kg, i.p.) was given every 30 min for 2 h to male rats. Some rats were pretreated with the D1 dopamine (DA) receptor antagonist SCH 23390 (1 mg/kg, i.p.) or with the D2 DA receptor antagonist raclopride (1 mg/kg, i.p.), 5 min before nicotine treatment. Hypothalamic and preoptic catecholamine levels were measured by quantitative histofluorimetry in discrete DA and noradrenaline nerve terminal systems.Nicotine treatment produced a depletion of catecholamine stores in noradrenaline and DA nerve terminals of the hypothalamus, the preoptic area and the median eminence, an action which was counteracted by SCH 23390 but not by raclopride.The results indicate that hypothalamic D1 DA receptors may regulate the sensitivity of the nicotinic cholinoceptors and increase their ability to release hypothalamic noradrenaline. A possible role of D1 DA receptor antagonists to reduce the ability of nicotine treatment to produce rapid increases in LH, prolactin and corticosterone secretion and tonic arousal is implicated.     

The effects of dopaminergic antagonism by sulpiride on TRH and VIP-induced prolactin release in nonsuckled lactating rats

Prolactin (PRL) release was studied in female rats during midlactation using pharmacologic manipulations designed to mimic the hypothalamic effects of suckling. In the first experiment pituitary dopamine (DA) receptors were blocked by sulpiride (10 micrograms/rat i.v.). One hour later, thyrotropin-releasing hormone (TRH, 1.0 micrograms/rat i.v.) was given to induce PRL release. TRH released significantly more PRL following DA antagonism than when no DA antagonism was produced, suggesting that DA receptor blockade increased the sensitivity of the AP to TRH. In a second experiment, VIP (25 micrograms/rat) increased plasma prolactin 3-4 fold but this effect was not enhanced significantly by prior dopamine antagonism with sulpiride. We conclude that dopamine antagonism enhances the PRL releasing effect of TRH but not VIP in lactating rats.     

Localization and possible function of peptidergic neurons and their interactions with central catecholamine neurons, and the central actions of gut hormones.

The localization of various neuropeptides is described in the gut and in the hypothalamus in the rat. Evidence is given for the presence of material resembling corticotropin-like intermediate peptide in arcuate and periarcuate neurons, projecting to various hypothalamic nuclei, limbic areas and the thalamus. beta-Endorphin and glucagon decrease dopamine turnover in the median eminence, while secretin increases dopamine turnover and vasoactive intestinal polypeptide (VIP) has no effect. beta-Endorphin, VIP, secretin, and glucagon all produce discrete changes in norepinephrine turnover in various hypothalamic nuclei. Mainly increases of norepinephrine turnover were observed. These catecholamine turnover changes appear to cause changes in the secretion of prolactin and growth hormone. The results therefore indicate that gut hormones and opioid peptides may act directly on the hypothalamus on specific types of receptors to participate in the control of hypothalamic functions such as control of hormone secretion from the anterior pituitary and of food intake. It seems possible that gastrointestinal peptides released from the gastrointestinal tract into the circulation under certain circumstances could reach the hypothalamus and modulate its activity via the above-mentioned mechanisms. It may therefore be speculated that disturbances in gastrointestinal functions could lead to pathological changes in food intake via modulation of hypothalamic activity.     

Hypothalamic monoamine turnover in ring doves (streptopelia risoria), courting,incubating and brooding young

1. The turnovers of hypothalamic 5-hydroxytryptamine (5HT), dopamine (DA) and noradrenaline (NE) were measured in male and female ring doves (Streptopelia risoria) at three stages of the breeding cycle: courtship, 3 days after pairing; early incubation, 1–2 days after egg laying; and brooding, 1–3 days after the squabs had hatched.2. In both sexes plasma LH decreased progressively from courtship through incubation to brooding young. Crop sacs were fully developed in doves brooding young but not at other stages of the reproductive cycle, indicating increased concentrations of plasma prolactin.3. The turnovers of 5HT and DA in both sexes were significantly higher (P<0.001) in doves brooding young than in birds incubating eggs or nest building. The turnover of DA was higher in females than in males at the onset of incubation. The turnover of NE was lower (P > 0.01) in females at the onset of incubation than during courtship or brooding.4. Increased turnover of hypothalamic DA may be more closely related to brooding behaviour than to changes in prolactin or LH secretion.5. Increased hypothalamic 5HT turnover in brooding doves appears to be more directly related to crop sac development, and by inference increased prolactin secretion, than to depressed plasma LH concentrations.     

NMDA Receptors in the Medial Zona Incerta Stimulate Luteinizing Hormone and Prolactin Release

1. The aim of the present work is to demonstrate the interaction between the glutamatergic/NMDA and dopaminergic systems in the medial zona incerta on the control of luteinizing hormone and prolactin secretion and the influence of reproductive hormones. 2. Proestrus and ovariectomized rats were primed with estrogen and progesterone to induce high or low levels of luteinizing hormone and prolactin. 2-Amino-7-phosphonoheptanoic acid, an NMDA receptor antagonist, and dopamine were injected in the medial zona incerta. Blood samples were withdrawn every hour between 1,600 and 2,000 hours or 2,200 hours via intracardiac catheter from conscious rats. Additional groups of animals injected with the NMDA receptor antagonist were killed 1 or 4 h after injection. Dopamine and its metabolite 3,4-dihydroxyphenylacetic acid were measured in different hypothalamic regions. 3. 2-Amino-7-phosphonoheptanoic acid blocked the ovulatory luteinizing hormone surge in proestrus rats. 2-Amino-7-phosphonoheptanoic acid also blocked the increase in luteinizing hormone induced by ovarian hormones in ovariectomized rats, an effect that was partially reversed by dopamine injection. Conversely, the increased release of luteinizing hormone and prolactin induced by dopamine was prevented by 2-amino-7-phosphonoheptanoic acid. We found that the NMDA antagonist injection decreased the dopaminergic activity--as evaluated by the 3,4-dihydroxyphenylacetic acid/dopamine ratio--in the medio basal hypothalamus and increased in the preoptic area. 4. Our results show an stimulatory role of NMDA receptors on the ovulatory luteinizing hormone release and on luteinizing hormone release induced by sexual hormones and demonstrate that the stimulatory effect of dopamine on luteinizing hormone and prolactin is mediated by the NMDA receptors. These results suggest a close interaction between the glutamatergic and dopaminergic incertohypothalamic systems on the control of luteinizing hormone and prolactin release.     

Localization and pharmacology of some dopamine receptor complexes in the striatum and the pituitary gland: synaptic and son-synaptic communication

The striatum receives massive dopaminergic projections from neurons in the ventral tegmental area, the substantia nigra and the retro-rubral cell group. Dopaminergic neurons in the arcuate nucleus and periventricular hypothalamic nuclei project to the median eminence and the neuro-intermediate lobe of the pituitary gland. The anterior lobe of the pituitary gland is not innervated by dopaminergic neurons, but receives dopamine via a vascular route from the median eminence. Two categories of dopamine receptors (D-1 and D-2) can be identified on the basis of the ability of various drugs to discriminate between these two entities. Dopamine stimulates both D-1 and D-2 receptors. The affinity of dopamine for the D-2 receptor is approximately 1000 times higher than for the D-1 receptor. Dopamine is involved in synaptic as well as non-synaptic communication. Examples of non-synaptic communication via D-2 receptors are the dopamine induced inhibition of prolactin release from the anterior pituitary gland and most likely the D-2 receptor mediated inhibition of the release of acetylcholine in the striatum. Examples of synaptic communication have been found in the striatum where (with ultrastructural techniques) synaptic contacts between dopaminergic nerve terminals and elements from cells containing GABA, substance P or enkephalin have been demonstrated. It is tempting to speculate that synaptic and non-synaptic communication occurs via D-1 and D-2 receptors respectively.     

Evidence for alternative splicing of a dopamine D2 receptor in a teleost

Previous attempts at identifying an alternatively spliced dopamine (DA) D2 receptor in teleosts have proven unsuccessful. We provide evidence of a splicing event of a goldfish D2 (gfD2b1) receptor in the neuroendocrine brain of adult goldfish that produces a spliced short isoform (gfD2b1S). We also identify an additional novel D2b paralog (gfD2b2) that does not appear to be alternatively spliced in adult fish during the reproductive cycle. Relatively high mRNA levels of gfD2b1S were observed in the neuroendocrine brain and pituitary of sexually immature fish compared with sexually regressing fish. Real-time RT-PCR revealed that intraperitoneal injection of either SCH 23390 or sulpiride-D1- or D2-specific antagonists, respectively-decreased mRNA levels of gfD2b1S by 3.9-fold without affecting the unspliced isoforms. We suggest that the expression of the spliced D2 receptor modulates the inhibitory tone of DA throughout the reproductive cycle. The deduced amino acid sequence of gfD2b1S lacks 29 amino acids in the same region as the short isoform of mammalian D2. We propose that the gfD2b1S splice variant is the teleost ortholog of mammalian D2S. The hypothesis that D2 receptor splicing is a relatively recent innovation in higher tetrapods is not supported by our results.     

Inhibition of nocturnal prolactin surges in the pregnant rat by incubation medium containing placental lactogen

Rats hysterectomized on Day 7 or 8 of pregnancy continued to have nocturnal prolactin surges 1 day later. Conditioned medium obtained from incubation of Day 11 placentas infused via the jugular vein completely blocked this nocturnal surge, indicating a negative feedback of placental secretions on prolactin. Infusion of an ultrafiltrate of the conditioned medium which only contained molecules with Mr above 10,000 also blocked the prolactin surge. Next, it was determined whether this feedback of placental secretions on prolactin may work by way of hypothalamic dopamine. Levels of dopamine in hypophysial stalk blood from pregnant rats on Day 12, a time when secretion of placental lactogen is high, were not different from those in rats in which placental lactogen was absent. It is concluded that termination of prolactin surges at midpregnancy may be due to feedback of placental secretions, possibly placental lactogen, on the hypothalamus and/or pituitary. However, these experiments do not support the hypothesis that this inhibition is mediated by alteration in hypothalamic dopamine secretion.     

Pituitary luteinizing hormone and prolactin messenger ribonucleic acid levels are inversely related in laying and incubating turkey hens.

The relationships of prolactin (PRL) and LH messenger (m) RNA to serum and pituitary content were determined for turkey hens at different phases of the reproductive cycle. In the nonphotostimulated, reproductively inactive hen, serum and pituitary PRL content and pituitary PRL mRNA levels were low. All three PRL values rose after photostimulation and peaked during the incubation phase. Relative to nonphotostimulated hens, hyperprolactinemic incubating hens showed 220-, 11-, and 57-fold increases in serum PRL, pituitary PRL content, and pituitary PRL mRNA levels, respectively. These peak levels declined 80-, 3-, and 6-fold, respectively, in photorefractory hens. In contrast to PRL levels, serum LH, pituitary LH, and pituitary LH beta-subunit mRNA levels did not change as dramatically. Serum LH showed no significant changes for the different reproductive phases. Pituitary LH peaked after photostimulation and declined to its lowest level in incubating hens. Pituitary LH-beta mRNA abundance was highest in photostimulated and laying hens and lowest in incubating and photorefractory hens. These results demonstrate that the abundance of LH-beta and PRL mRNA shows an inverse relationship in photostimulated/laying and incubating turkey hens.     

Expression and functional role of peroxisome proliferator-activated receptor-gamma in ovarian folliculogenesis in the sheep

Peroxisome proliferator-activated receptor (PPARgamma) is a nuclear receptor that is activated by fatty acids and derivatives and the antidiabetic glitazones, which plays a role in the control of lipid and glucose homeostasis. In the present work, we tested the hypothesis that PPARgamma plays a role in reproductive tissues by studying its expression and function in the hypothalamo-pituitary-ovary axis in the sheep. PPARgamma 1 and PPARgamma 2 proteins and mRNAs were detected in whole ovine pituitary and ovary but not in hypothalamic extracts. In situ hybridization on ovarian section localized PPARgamma mRNA in the granulosa layer of follicles. Interestingly, PPARgamma expression was higher in small antral (1-3 mm diameter) than in preovulatory follicles (>5 mm diameter) (P < 0.001) and was not correlated with healthy status. To assess the biological activity of ovarian PPARgamma, ovine granulosa cells were transfected with a reporter construct driven by PPARgamma-responsive elements. Addition of rosiglitazone, a PPARgamma ligand, stimulated reporter gene expression, showing that endogenous PPARgamma is functional in ovine granulosa cells in vitro. Moreover, rosiglitazone inhibited granulosa cell proliferation (P < 0.05) and increased the secretion of progesterone in vitro (P < 0.05). This stimulation effect was stronger in granulosa cells from small than from large follicles. In contrast, rosiglitazone had no effect on LH, FSH, prolactin and growth hormone secretion by ovine pituitary cells in vitro. Overall, these data suggest that PPARgamma ligands might stimulate follicular differentiation in vivo likely through a direct action on granulosa cells rather than by modulating pituitary hormone secretion.     

Localization of dopamine D1 and D2 receptor mRNAs in the rat systemic and pulmonary vasculatures.

The present study was designed to evaluate the expression of dopamine D1 and D2 receptor mRNAs in systemic and pulmonary vasculatures. Using specific antisense riboprobes for dopamine D1 and D2 receptor cDNAs, in situ hybridization histochemistry was performed in the aorta, common carotid artery, vertebral artery, pulmonary artery, and superior vena cava of the adult male Sprague Dawley rat. In the case of the aorta, common carotid artery, and vertebral artery, dopamine D1 receptor mRNAs localized mainly in the smooth muscle cells of the tunica media. However, the signals of dopamine D2 receptor mRNAs were found in the endothelium and subendothelial layer of tunica intima, and interstitial cells of tunica adventitia. In the case of the pulmonary artery, signals of dopamine D1 receptor mRNAs were detected within the tunica intima, media, and adventitia. Expression of D2 receptor mRNAs was detected in the walls of small blood vessels within the tunica adventitia of the pulmonary artery. There were no detectable signals of dopamine D1 and D2 receptor mRNAs in the vein. The uneven distribution of dopamine D1 and D2 receptor mRNAs in the rat systemic vasculatures and pulmonary artery suggests that dopamine differentially regulates the vasodilation of the systemic and pulmonary arteries through the differential stimulation of dopamine D1 and D2 receptor.     

The dopamine of the rat mammotroph in cell culture as a model for drug action

Dopamine can act directly on pituitary cells to inhibit prolactin release. This action can be blocked by dopamine receptor blocking drugs such as haloperidol, sulpiride and other neuroleptic agents. Comparison of the properties of the mammotroph dopamine receptor with the adenylate cyclase linked dopamine receptor of the limbic forebrain reveals some obvious differences. For example, dopamine receptor stimulants such as S-584 and lergotrile mesylate are inactive in stimulating the adenylate cyclase preparations but are potent in inhibiting pituitary prolactin secretion. Such inhibition of prolactin secretion can be reversed by haloperidol or sulpiride. In contrast to these observations, sulpiride does not block dopamine stimulation of cAMP formation. In addition, dopamine, apomorphine or lergotrile mesylate have no effect on a pituitary adenylate cyclase preparation and dopamine fails to elevate cAMP in the intact cells in culture. Despite the similarity between these two dopamine sensitive systems with respect to a number of agonists and antagonists, the exceptions described suggest that the pituitary system with further study may offer some greater reliability as a predictive test for clinically useful agents. These results also suggest that the receptors for dopamine, like that for norepinephrine, are of two types, only one of which is coupled to adenylate cyclase.