Neurochemical evidence that AMPA receptor-mediated tonic inhibition of hypothalamic dopaminergic neurons occurs via activation of inhibitory interneurons

Blockade of -amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors with 6-nitro-7-sulfamoyl-benzo-[f]quinoxaline-2,3(1H,4H)-dione (NBOX) activates tuberoinfundibular (TIDA) and periventricular-hypophysial dopaminergic (PHDA) neurons suggesting that these neurons are tonically inhibited by endogenous excitatory amino acid neurotransmitters acting at AMPA receptors. The purpose of the present study was to identify prospective inhibitory neurotransmitters mediating this effect by examining the ability of γ-aminobutyric acid (GABA)_A and κ-opioid receptor agonists to reverse the stimulatory effects of NBQX on the activity of these neurons (estimated from concentrations of 3,4-dihydroxyphenylacetic acid [DOPAC] in the median eminence and intermediate lobe of the pituitary). The GABA_A receptor agonist isoguvacine prevented the NBQX-induced increase in DOPAC concentrations in the median eminence but not in the intermediate lobe. The κ-opioid receptor agonist U-50,488 had no effect in the median eminence, but attenuated the NBQX-induced increase of DOPAC concentrations in the intermediate lobe. These results suggest that excitatory amino acid neurotransmitters activate AMPA receptors and increase release of GABA, which by acting at GABA, which by acting at GABA_A receptors tonically inhibits TIDA neurons. On the other hand, AMPA receptor-mediated tonic inhibition of PHDA neurons occurs, at least in part, by a mechanism involving endogenous κ-opioids.     

Dopamine and 7-OH-DPAT may act on D3 receptors to inhibit tuberoinfundibular dopaminergic neurons

Whether the tuberoinfundibular dopaminergic (TIDA) neurons resided in the dorsomedial arcuate nucleus (dmARN) can respond to dopamine and a dopamine D₃ receptor agonist, 7-hydroxydipropylaminotetralin (7-OH-DPAT), was the focus of this study. In studies using extracellular single-unit recording of dmARN neurons in brain slices obtained from ovariectomized rats, dopamine and 7-OH-DPAT inhibited 60.1% (n = 141) and 80.9% (n = 47) of recorded dmARN neurons, respectively. Other dopamine D₁ or D₂ receptor agonists were not as effective. Intracerebroventricular injection of 7-OH-DPAT (10⁻⁹ mol/3 μl) in ovariectomized, estrogen-primed rats significantly lowered the TIDA neuronal activity as determined by 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the median eminence. Co-administration of a putative D₃ receptor antagonist, U-99194A, could prevent the effect of 7-OH-DPAT. Unilateral microinjection of 7-OH-DPAT or dopamine itself (10⁻¹¹–10⁻⁹ mol/0.2 μl) into the right dmARN exhibited the same inhibitory effect on TIDA neurons. In all, dopamine may act on D₃ receptors to exhibit an inhibitory effect on its own release from the TIDA neurons.     

Comparison of the effects of the dopamine D2 agonist quinelorane on tuberoinfundibular dopaminergic neuronal activity in male and female rats

The purpose of the present study was to examine the effects of quinelorane (LY163502), a potent and selective D₂ dopaminergic (DA) receptor agonist, on the activity of tuberoinfundibular DA neurons in male and female rats as estimated by determining the concentration of the primary metabolite of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), in terminals of these neurons in the median eminence (ME). In males, quinelorane produced dose- and time-related increases in the concentration of DOPAC in the ME which was blocked by the D₂ receptor antagonist raclopride. The activity of tuberoinfundibular neurons in female rats is higher than it is in males because circulating levels of prolactin tonically stimulate these neurons in the female. In female rats, quinelorane markedly lowered plasma concentrations of prolactin but failed to alter DOPAC concentrations in the ME. Pretreatment of female rats with prolactin antiserum induced hypoprolactinemia and reduced DOPAC concentrations in the ME; in these animals quinelorane increased ME DOPAC concentrations. These results indicate that by acting on D₂ receptors quinelorane is able to stimulate tuberoinfundibular DA neurons in both male and female rats, but in female rats the ability of quinelorane to reduce circulating levels of prolactin indirectly reduces the activity of tuberoinfundibular DA neurons and thereby masks the stimulatory action of this drug on these neurons.     

Effect of electrical stimulation of the arcuate nucleus on neurochemical estimates of tuberoinfundibular and tuberohypophysial dopaminergic neuronal activities

The activity of nigrostriatal dopaminergic neurons has been estimated biochemically by measuring the rates of dopamine (DA) synthesis (accumulation of dihydroxyphenylalanine (DOPA) after NSD 1015) and turnover (decline of DA concentrations after alpha-methyltyrosine) in the striatum. It has been assumed that the activities of tuberoinfundibular dopaminergic (TIDA) and tuberohypophysial dopaminergic (THDA) neurons can also be estimated by making the same measurements in the terminals of these neurons in the median eminence and the posterior pituitary, respectively. In the present study, this assumption was tested directly by measuring the rates of DA synthesis and turnover in the median eminence and posterior pituitary following electrical stimulation of TIDS and THDA cell bodies in the arcuate nucleus. Electrical stimulation of the arcuate nucleus increased the rate of DOPA accumulation and the alpha-methyltyrosine-induced decline of DA concentrations in the median eminence and in the neural and intermediate lobes of the posterior pituitary. gamma-Butyrolactone (GBL), an anesthetic that selectively inhibits DA impulse flow, reduced the rates of DA synthesis and turnover in the median eminence. GBL also increased prolactin secretion which is tonically inhibited by DA released from TIDA neurons. Serum prolactin levels were significantly decreased by arcuate nucleus stimulation in GBL-anesthetized rats. These results indicate that the rates of DA synthesis and turnover within the median eminence and posterior pituitary reflect the activities of TIDA and THDA neurons, respectively.     

The metabolism of dopamine in the median eminence reflects the activity of tuberoinfundibular neurons

The purpose of the present study was to characterize the metabolism of dopamine (DA) in tuberoinfundibular (TI) neurons terminating in the median eminence and to examine the effects of procedures that alter the synthesis and turnover of DA in these neurons on the concentrations of dihydroxyphenylacetic acid (DOPAC) in the median eminence. The DA uptake inhibitor nomifensine (25 mg/kg, i.p.; 30 min) failed to alter median eminence DOPAC concentrations indicating that very little released DA is recaptured and metabolized by TIDA neurons. Within 5 min following the administration of the monoamine oxidase inhibitor pargyline (50 mg/kg, i.v.) median eminence DOPAC concentrations declined to 15% of control demonstrating that this metabolite has a high turnover rate and is rapidly removed from the median eminence. Median eminence DOPAC concentrations in diestrous female rats, whose TIDA neuronal activity is higher than in the male, were two-fold greater than in male rats. Prolactin (10 micrograms/rat, i.c.v.; 12 h), which increases TIDA neuronal activity, produced a corresponding increase in median eminence DOPAC concentrations in male rats. Restraint stress (30 min), which decreases TIDA neuronal activity, produced a corresponding decrease in median eminence DOPAC concentrations in diestrous female rats. The results from the present study suggest that DOPAC concentrations in the median eminence can be used as an index of TIDA neuronal activity.     

Effects of immunoneutralization of dynorphin1-17 and dynorphin1-8 on the activity of central dopaminergic neurons in the male rat.

The effects of administration of antibodies against dynorphin1-17 (DYN1-17-AB) and dynorphin1-8 (DYN1-8-AB) were examined on the activity of dopaminergic (DA) neurons comprising the nigrostriatal, mesolimbic, tuberoinfundibular and periventricular-hypophysial systems in the male rat brain. DA neuronal activity was estimated by measuring the concentration of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in brain (striatum, nucleus accumbens, median eminence) and pituitary regions (intermediate lobe) containing terminals of these neurons. The intracerebroventricular administration of either DYN1-17-AB or DYN1-8-AB produced a time-related increase in the activity of tuberoinfundibular and periventricular-hypophysial DA neurons, but failed to alter the activity of nigrostriatal or mesolimbic DA neurons. The ability of both DYN1-17-AB and DYN1-8-AB to enhance the activity of tuberoinfundibular and periventricular-hypophysial DA neurons was reversed by the kappa opioid agonist U-50,488. These results indicate that DYN1-17-AB and DYN1-8-AB, presumably by binding endogenous dynorphins, remove a tonic inhibitory action of these opioid peptides on tuberoinfundibular and periventricular-hypophysial DA neurons.     

Opioid receptor subtypes involved in the regulation of prolactin secretion during pregnancy and lactation

Afferent endogenous opioid neuronal systems facilitate prolactin secretion in a number of physiological conditions including pregnancy and lactation, by decreasing tuberoinfundibular dopamine (TIDA) inhibitory tone. The aim of this study was to investigate the opioid receptor subtypes involved in regulating TIDA neuronal activity and therefore facilitating prolactin secretion during early pregnancy, late pregnancy and lactation in rats. Selective opioid receptor antagonists nor-binaltorphimine (kappa-receptor antagonist, 15 micro g/5 micro l), beta funaltrexamine (mu-receptor antagonist, 5 microg/5 microl) and naltrindole (delta-receptor antagonist, 5 microg/5 microl) or saline were administered intracerebroventricularly (i.c.v.) on day 8 of pregnancy during a nocturnal prolactin surge, on day 21 of pregnancy during the ante partum prolactin surge or on day 7 of lactation before the onset of a suckling stimulus. Serial blood samples were collected at regular time intervals, via chronic indwelling jugular cannulae, before and after drug administration and plasma prolactin was determined by radioimmunoassay. TIDA neuronal activity was measured using the 3,4-dihydroxyphenylacetic acid (DOPAC) : dopamine ratio in the median eminence 2 h 30 min after i.c.v. drug injection. In each experimental condition, plasma prolactin was significantly inhibited by both kappa- and mu-receptor antagonists, whereas the delta-receptor antagonist had no effect compared to saline-injected controls. Similarly, nor-binaltorphimine and beta funaltrexamine significantly increased the median eminence DOPAC : dopamine ratio during early and late pregnancy, and lactation whereas naltrindole had no effect compared to saline-injected controls. These data suggest that TIDA neuronal activity, and subsequent prolactin secretion, is regulated by endogenous opioid peptides acting at both kappa- and mu-opioid receptors during prolactin surges of early pregnancy, late pregnancy and lactation.     

Activation of tuberoinfundibular and tuberohypophysial dopamine neurons following intracerebroventricular administration of bombesin.

The effect of bombesin on the activity of dopamine (DA) neurons comprising the nigrostriatal, mesolimbic, tuberoinfundibular and tuberohypophysial systems in the male rat was determined by measuring: (1) the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor, and (2) the concentration of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in brain (striatum, nucleus accumbens, median eminence) and pituitary regions (intermediate and neural lobes) containing terminals of these neurons. Intracerebroventricular (i.c.v.) injection of bombesin caused a dose- and time-related increase in the activity of DA neurons projecting to the median eminence and intermediate lobe of the pituitary, and a corresponding decrease in the concentrations of prolactin and alpha-melanocyte-stimulating hormone (alpha MSH) in the plasma. In contrast, doses of bombesin up to 10 ng i.c.v. failed to alter the activity of DA neurons terminating in the striatum, nucleus accumbens or neural lobe of the pituitary gland. Equimolar doses of bombesin and gastrin-releasing peptide (GRP), a bombesin-like peptide, increased the concentrations of DOPAC in the median eminence and intermediate lobe of the pituitary, suggesting that GRP-preferring receptors may be responsible for the stimulatory effects of bombesin on DA neuronal activity in these regions. The results of these studies suggest that bombesin increases the activity of tuberoinfundibular and tuberohypophysial DA neurons projecting to the median eminence and intermediate lobe of the pituitary, respectively, and thereby inhibits the secretion of prolactin and alpha MSH.     

Stimulatory role of prolactin on the development of tuberoinfundibular dopaminergic neurones in prepubertal female rats: studies with cysteamine and somatostatin

Cysteamine, a potent depletor of prolactin and somatostatin, was used to determine the role of prolactin and somatostatin in the control of central dopamine neurones in prepubertal rats. Cysteamine (100 mg/kg, i.p., twice daily) was injected for 7, 14 or 21 days in 28-day-old Sprague-Dawley female rats in one study and for 3 days in 35-day-old rats in another. In control rats, the 3, 4-dihydroxyphenylacetic acid (DOPAC) levels in the median eminence increased threefold from day 35 to day 49, and serum prolactin concentration increased about 50%. Cysteamine lowered serum prolactin concentrations to 20%, and median eminence DOPAC and dopamine levels to 32-50% of control levels in both studies. The DOPAC levels in the nucleus accumbens and striatum were also lowered, while both DOPAC and dopamine in the paraventricular nucleus and periventricular nucleus (A14) were increased by cysteamine. A single injection of rat prolactin (0.01, 0.1 or 1 mg/kg) significantly increased DOPAC or DOPA levels in the median eminence, nucleus accumbens and striatum, but not in the paraventricular nucleus or A14 at 14 h later in 28-day old female rats or in 40-day-old rats pretreated with cysteamine. In contrast, central injection of somatostatin dose (0.001-1 microg/rat) and time (30-90 min) dependently decreased the DOPAC levels in the median eminence, paraventricular nucleus and A14 and increased those in the nucleus accumbens and striatum of adult female rats. These results indicate that serum prolactin is important for the maturation and maintenance of dopamine systems in the median eminence, nucleus accumbens and striatum, while somatostatin exhibits inhibitory and stimulatory effects on hypothalamic and midbrain dopamine systems, respectively.     

Differential action of bromocriptine on nigrostriatal versus mesolimbic dopaminergic neurons

Summary The present study was undertaken to compare the abilities of the dopaminergic agonists apomorphine, bromocriptine, and lergotrile to inhibit the synthesis of dopamine (DA) in terminals of nigrostriatal and mesolimbic DA neurons. Thein vivo synthesis of DA was estimated by measuring the rate of accumulation of dihydroxyphenylalanine (DOPA) in terminals of nigrostriatal (striatum) and mesolimbic (nucleus accumbens, olfactory tubercle) neurons 30 min after the administration of NSD 1015, a decarboxylase inhibitor. The activation of DA autoreceptors in these regions was evaluated by measuring the abilities of the DA agonists to inhibit DA synthesis in brain regions of rats pretreated with gamma-butyrolactone (GBL). Apomorphine (0.03–1.0 mg/kg for 45 min) and bromocriptine (0.1–10 mg/kg for 90 min) produced dose-dependent decreases in the rate of DA synthesis in all three brain regions of both vehicle- and GBL-treated rats. A time course of the effects of the highest dose of bromocriptine (10 mg/kg), however, demonstrated dramatic regional differences in the ability of this drug to inhibit DA synthesis in saline-versus GBL-pretreated rats. Bromocriptine inhibited the GBL-induced increase in DA synthesis for 6 hours in all regions examined. In the striatum of saline-treated rats the decrease in DA synthesis was evident only at 1.5 hours after bromocriptine administration, while in the nucleus accumbens and olfactory tubercle DA synthesis remained inhibited for 6 hours. By contrast, lergotrile reduced DA synthesis to a similar extent in all three regions for at least 6 hours in both vehicle- and GBL-treated rats. These results suggest that there is no regional difference in the ability of bromocriptine to inhibit DA synthesis via DA autoreceptor mechanisms, but there appear to be differences in post-synaptic DA receptor-mediated mechanisms which regulate nigrostriatal versus mesolimbic DA neurons.     

Adenosine 3′,5′ cyclic monophosphate stimulates dopamine biosynthesis in the median eminence of rat hypothalamic slices

The regulation of dopamine biosynthesis in tuberoinfundibular dopaminergic (TIDA) neurons by adenosine 3',5'-cyclic monophosphate (cAMP) was investigated in the present study. Dopamine biosynthesis in TIDA neurons was estimated by the rate of in vitro dihydroxyphenylalanine (DOPA) accumulation in the median eminence of rat hypothalamic slices after incubation with a DOPA decarboxylase inhibitor. Addition of dibutyryl cAMP (db-cAMP) into medium caused an increase in the rate of DOPA accumulation in the median eminence in a dose- and time-dependent manner. 8-Bromo-cAMP also increased the rate of DOPA accumulation in the median eminence and cAMP was less effective than db-cAMP whereas neither adenosine nor sodium butyrate altered the rate of DOPA accumulation. An increase in the concentration of endogenous cAMP achieved by addition into medium of isobutylmethylxanthine, a phosphodiesterase inhibitor, or forskolin, an adenylate cyclase activator, was associated with an increase in the rate of DOPA accumulation in the medium eminence. db-cAMP, however, had an almost negligible effect on the secretion of dopamine from the median eminence. The stimulatory effect of db-cAMP on DOPA accumulation in the median eminence was not dependent upon the presence of extracellular Ca2+ and was not blocked by tetrodotoxin. Furthermore, the stimulation of DOPA accumulation in the median eminence induced by db-cAMP was additive with that induced by high potassium depolarization, which was Ca2+ -dependent. These results suggest that dopamine biosynthesis in TIDA neurons is regulated by two distinct mechanisms, one of which involves cAMP, and another of which involves Ca2+.     

Acute effects of morphine on neurochemical estimates of activity of incertohypothalamic dopaminergic neurons in the male rat

The effects of an acute injection of morphine on the activities of mesotelencephalic, tuberoinfundibular and incertohypothalamic dopamine (DA) neurons was estimated by measuring: the rate of turnover of DA (decline after alpha-methyltyrosine); and the concentration of the DA metabolite, dihydroxyphenylacetic acid (DOPAC), in brain regions containing cell bodies or terminals of these neurons (i.e. nucleus accumbens, striatum, median eminence and various hypothalamic nuclei). The rate of turnover of DA and the concentration of DOPAC were increased in nucleus accumbens and striatum and decreased in the median eminence 60 min after the administration of morphine (10 mg/kg, s.c.). Morphine increased the rate of turnover of DA and the concentration of DOPAC in brain regions containing both cell bodies (periventricular nucleus and medial zona incerta) and terminals (medial preoptic, preopticosuprachiasmatic and dorsomedial nuclei) of incertohypothalamic DA neurons. The effects of morphine in all brain regions were blocked by pretreatment with naltrexone. These results indicate that incertohypothalamic DA neurons are stimulated by the acute administration of morphine, and in this respect they resemble the extrahypothalamic mesotelencephalic DA neurons rather than hypothalamic tuberoinfundibular DA neurons.     

Characterization of opioid receptor-mediated regulation of incertohypothalamic dopamine neurons: lack of evidence for a role of 5-hydroxytryptaminergic neurons in mediating the stimulatory effects of morphine.

The purpose of the present study was to characterize opioid receptor-mediated regulation of incertohypothalamic dopaminergic (DA) neurons in the rat brain by examining the acute effects of selective mu or kappa opioid receptor agonists and antagonists on concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) in the medial zona incerta (MZI) and the dorsomedial hypothalamic nucleus (DMN) which contain cell bodies and terminals, respectively, of these neurons. Morphine caused a dose- and time-related increase in concentrations of DOPAC in MZI and DMN; this stimulatory effect was blocked by the mu opioid receptor antagonist naltrexone. In contrast, activation or blockade of kappa opioid receptors following administration of U-50,488 or nor-binaltorphimine, respectively, had no effect on DOPAC concentrations in either the MZI or DMN. The basal activity of incertohypothalamic DA neurons and their response to morphine was similar in male and female rats. Morphine also increased the concentrations of 5-hydroxyindoleacetic acid in MZI and DMN, indicating that morphine increases the activity of 5-hydroxytryptamine (5HT) neurons projecting to these regions. This might suggest that morphine-induced activation of incertohypothalamic DA neurons is mediated by 5HT neurons; but 5,7-dihydroxytryptamine-induced lesions of 5HT neurons did not alter the ability of morphine to increase DOPAC concentrations in MZI and DMN. These results indicate that the stimulatory effects of mu opioid receptor activation on incertohypothalamic DA neurons is not dependent upon the presence of 5HT neurons.     

Inhibition of Tuberoinfundibular Dopaminergic Neural Activity During Suckling: Involvement of μ and κ Opiate Receptor Subtypes

Previous studies have shown that mu (μ) and kappa ( κ ) opioid antagonists inhibit suckling-induced prolactin release. Prolactin responses elicited by pup suckling or opioid administration are mediated, at least in part, by suppression of dopamine (DA) release from tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus. We examined the effects of the μ opiate receptor antagonist, β -funaltrexamine ( β -FNA), and the κ opiate receptor antagonist, nor-binaltorphimine (nor-BNI) on the activity of TIDA neurons in lactating rats. TIDA neuronal activity was determined by measuring DOPA accumulation in the caudate putamen (CP) and median eminence (ME). The effects of opioid antagonist treatment were determined in pup-deprived (low circulating prolactin levels) or pup-suckled rats (high circulating prolactin levels). The accumulation of 5-hydroxytryptophan (5-HTP) in the medial preoptic area (MPOA), the anterior hypothalamus (AH) and the median eminence (ME) was quantified as an index of serotonergic activity in the same animals for comparative purposes.     

Interaction of amfonelic acid with antipsychotic drugs on dopaminergic neurons.

The effects of two inhibitors of dopamine (DA) reuptake, amfonelic acid and GBR 12909, on the clozapine- and haloperidol-induced increases in DA synthesis, release, and metabolism were investigated in the rat. In the striatum, as well as in the nucleus accumbens, the haloperidol-induced increase in tissue concentrations of dihydroxyphenylacetic acid (DOPAC) or the accumulation of dihydroxyphenylalanine (DOPA) was potentiated or unaltered, respectively, in rats treated with amfonelic acid. In contrast, amfonelic acid attenuated the stimulatory effects of clozapine on DOPAC concentrations and DOPA accumulation in both brain regions. GBR 12909 also differentially affected the haloperidol- and clozapine-induced increases in DOPAC concentrations. However, the clozapine-induced increase in DOPA accumulation in the median eminence was not significantly altered by treatment with amfonelic acid. The haloperidol-induced increase in the extracellular concentrations of DA and DOPAC in the striatum also was potentiated by amfonelic acid, whereas the increase elicited by clozapine was suppressed. The increase in extracellular DA produced by the administration of morphine or the coadministration of ritanserin, a 5-HT2 antagonist, and haloperidol also was potentiated by amfonelic acid. The ability of amfonelic acid to distinguish between the actions of clozapine and haloperidol on nigrostriatal and mesocorticolimbic DA neurons does not appear to be related to differences in the effects of the drugs on DA autoreceptors or 5-HT2 receptors. Moreover, the mechanism through which clozapine activates tuberoinfundibular DA neurons may differ from that which is involved in the activation of nigrostriatal or mesocorticolimbic DA neurons.     

β-endorphin alters dopamine uptake by the dopamine neurons of the hypothalamus and striatum

The study of hypothalamic dopamine (DA) neurons is complicated by the difficulty in distinguishing DA neurons from norepinephrine (NE) neurons and by the fact that they comprise only a small proportion of the catecholamine neuron population of the hypothalamus. We have studied DA uptake into nerve terminals of hypothalamic DA neurons using a synaptosomal preparation. Desmethylimipramine (DMI) was used to prevent uptake into synaptosomes from NE neurons, thus pharmacologically isolating dopaminergic from noradrenergic nerve terminals. This DMI-insensitive DA uptake in hypothalamus had all the properties of a high affinity uptake process; it was saturable, dependent on incubation time and incubation temperature, increased linearly with increasing amounts of tissue and was completely abolished by excess unlabeled DA. Also, it was completely abolished by benztropine, an inhibitor of amine uptake into DA neurons. We believe that DMI-insensitive DA uptake into hypothalamic synaptosomes represents uptake into DA nerve terminals.The DMI-insensitive DA accumulation in discrete areas of hypothalamus correlated well with the known prevalence of DA neurons relative to NE neurons in these areas: median eminence > median eminence-arcuate nucleus > mediobasal hypothalamus > whole hypothalamus. Comparison of the affinity constants for DA uptake into synaptosomes incubated without DMI revealed a 2-fold higher affinity constant for DA uptake in median eminence compared with striatum, but affinity constants in all the other hypothalamic regions examined (mediobasal hypothalamus, hypothalamus minus median eminence, whole hypothalamus) were identical to that of striatum. In contrast, comparison of the affinity constants for DA uptake in the presence of DMI revealed a 2–3-fold higher affinity constant for DA uptake in all these hypothalamic regions compared with striatum. It appears the tuberoinfundibular DA neurons and the other DA neurons of the hypothalamus have a high affinity uptake system for DA, although affinity for DA in all of these hypothalamic DA neurons appears to be 2–3-fold lower than that in striatal DA neurons. The data also suggest that the much larger mediobasal hypothalamus may serve as a model for studies of DA uptake into tuberoinfundibular DA neurons of the median eminence.     

Differential effects of haloperidol,clozapine, and fluperlapine on tuberoinfundibular dopamine neurons and prolactin secretion in the rat

Summary Two atypical neuroleptic agents, clozapine and fluperlapine, produced rapid elevations in plasma PRL concentrations that were similar in magnitude to those produced by haloperidol. However, the PRL response to clozapine or fluperlapine was of much shorter duration than that elicited by haloperidol. Clozapine, but neither fluperlapine nor haloperidol, produced a rapid increase in the activity of tuberoinfundibular dopamine (TIDA) neurons, as evidenced by an enhanced accumulation of dihydroxyphenylalanine (DOPA) in the median eminence after the inhibition of DOPA decarboxylase. The clozapine-induced increase in DOPA accumulation was evident within 30 minutes after its administration and persisted for at least 4 hours. The clozapine-induced increase in the activity of TIDA neurons may account, in part, for the abbreviated PRL response to this neuroleptic. In addition, ability to produce a short-lived increase in PRL secretion in the rat appears to be common to the atypicl neuroleptic drugs.     

Kisspeptin Regulates Tuberoinfundibular Dopaminergic Neurones and Prolactin Secretion in an Oestradiol‐Dependent Manner in Male and Female Rats

Prolactin (PRL) secretion is inhibited by hypothalamic dopamine. Kisspeptin controls luteinising hormone (LH) secretion and is also involved in PRL regulation. We further investigated the effect of kisspeptin‐10 (Kp‐10) on the activity of tuberoinfundibular dopaminergic (TIDA) neurones and the role of oestradiol (E₂) in this mechanism. Female and male rats were injected with i.c.v. Kp‐10 and evaluated for PRL release and the activity of dopamine terminals in the median eminence (ME) and neurointermediate lobe of the pituitary (NIL). Kp‐10 at the doses of 0.6 and 3 nmol increased plasma PRL and decreased 4‐dihydroxyphenylacetic acid (DOPAC) levels in the ME and NIL of ovariectomised (OVX), E₂‐treated rats but had no effect in OVX. In gonad‐intact males, 3 nmol Kp‐10 increased PRL secretion and decreased DOPAC levels in the ME but not in the NIL. Castrated males treated with either testosterone or E₂ also displayed increased PRL secretion and reduced ME DOPAC in response to Kp‐10, whereas castrated rats receiving oil or dihydrotestosterone were unresponsive. By contrast, the LH response to Kp‐10 was not E₂‐dependent in either females or males. Additionally, immunohistochemical double‐labelling demonstrated that TIDA neurones of male rats contain oestrogen receptor (ER)‐α, with a higher proportion of neurones expressing ERα than in dioestrous females. The dopaminergic neurones of periventricular hypothalamic nucleus displayed much lower ERα expression. Thus, TIDA neurones express ERα in male and female rats, and kisspeptin increases PRL secretion through inhibition of TIDA neurones in an E₂‐dependent manner in both sexes. These findings provide new evidence about the role of kisspeptin in the regulation of dopamine and PRL.     

Dopamine and 7-OH-DPAT may act on D(3) receptors to inhibit tuberoinfundibular dopaminergic neurons

Whether the tuberoinfundibular dopaminergic (TIDA) neurons resided in the dorsomedial arcuate nucleus (dmARN) can respond to dopamine and a dopamine D(3) receptor agonist, 7-hydroxydipropylaminotetralin (7-OH-DPAT), was the focus of this study. In studies using extracellular single-unit recording of dmARN neurons in brain slices obtained from ovariectomized rats, dopamine and 7-OH-DPAT inhibited 60.1% (n = 141) and 80.9% (n = 47) of recorded dmARN neurons, respectively. Other dopamine D(1) or D(2) receptor agonists were not as effective. Intracerebroventricular injection of 7-OH-DPAT (10(-9) mol/3 microl) in ovariectomized, estrogen-primed rats significantly lowered the TIDA neuronal activity as determined by 3, 4-dihydroxyphenylacetic acid (DOPAC) levels in the median eminence. Co-administration of a putative D(3) receptor antagonist, U-99194A, could prevent the effect of 7-OH-DPAT. Unilateral microinjection of 7-OH-DPAT or dopamine itself (10(-11)-10(-9) mol/0.2 microl) into the right dmARN exhibited the same inhibitory effect on TIDA neurons. In all, dopamine may act on D(3) receptors to exhibit an inhibitory effect on its own release from the TIDA neurons.     

Localization and Characterization of Dopamine D1 Receptors in Sheep Hypothalamus and Striatum

Dopamine receptors are pharmacologically grouped as D₁ and D₂ receptors. Previous research in the ewe has shown that central D₁ receptors may have a role in facilitating prolactin release. The aims of this study were therefore to localize and characterize D₁ binding sites in the hypothalamus of sheep. For comparison, a known D₁ receptor-rich tissue (striatum) was also studied. The bioactivities of several D₁ analogues were also assessed for their efficacy in sheep tissue. In vitro autoradiography with [¹²⁵I]-SCH23982 was used to localize D₁ binding sites. The ventromedial hypothalamic nucleus (VMH) displayed moderate levels of specific binding, localized to the medial portion of the nucleus. Low levels of specific binding were seen in the preoptic area, supraoptic nucleus and anterior hypothalamic area. The suprachiasmatic nucleus, median eminence and arcuate nucleus did not show specific binding. As expected the striatum displayed high levels of specific binding. The VMH, preoptic area, median eminence, striatum and anterior pituitary were examined with radioligand binding studies to quantify and characterize D₁ binding sites. Scatchard analysis gave K_D 1.04 nM and B_max 127.4 fmol/mg protein for VMH and K_D 1.99 nM and B_max 454.6 fmol/mg protein for striatum. While specific binding occurred in the preoptic area and median eminence this binding did not show saturation characteristics. Specific binding was not observed in the anterior pituitary. Affinities determined by competitive binding studies showed that the binding sites in both VMH and striatum have the characteristics of a D₁ receptor, that is, high affinity for the D₁ agonists and antagonists, low affinity for dopamine and the serotonergic antagonist ketanserin and extremely low affinity for the D₂ agonists and noradrenaline. Adenylate cyclase studies showed that in the striatum dopamine and the D₁ agonists, fenoldopam and SKF38393, were able to cause significant dose-dependent increases in adenylate cyclase activity. In contrast the D₁ agonist, SKF82958, was inactive in this system. The D₁ antagonists SCH23390 and SCH39166, but not SKF83566, abolished the adenylate cyclase response to 50 μM dopamine. In the VMH the D₁ agonist SKF38393, but not dopamine, stimulated adenylate cyclase activity. In conclusion, these results demonstrate that D₁ binding sites exist within the hypothalamus in the VMH and that these binding sites have the characteristics of D₁ receptors. These receptors are a potential site of action for dopamine in facilitating prolactin release. In addition, the results show that at least for some dopamine analogues, receptor binding affinity does not always correlate with biological activity.