Evidence that D2 receptor-mediated activation of hypothalamic tuberoinfundibular dopaminergic neurons in the male rat occurs via inhibition of tonically active afferent dynorphinergic neurons

The purpose of the present study was to determine if D₂ receptor-mediated activation of hypothalamic tuberoinfundibular dopaminergic (TIDA) neurons occurs via afferent neuronal inhibition of tonically active inhibitory dynorphinergic neurons in the male rat. To this end, the effects of either surgical deafferentation of the mediobasal hypothalamus or administration of a κ opioid receptor agonist (U-50,488) or antagonist (nor-binaltorphimine (NOR-BNI)) on D₂ receptor-mediated activation of TIDA neurons were assessed. For comparison, the activity of mesolimbic DA neurons was also determined in these studies. TIDA and mesolimbic DA neuronal activities were estimated by measuring dopamine synthesis (accumulation of 3,4-dihydroxyphenylalanine (DOPA) following decarboxylase inhibition) and metabolism (concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC)) in terminals of these neurons in the median eminence and nucleus accumbens, respectively. Intraperitoneal administration of the D₂ receptor agonist quinelorane caused a dose-dependent increase in DOPAC in the median eminence and a decrease in DOPAC in the nucleus accumbens; surgical deafferentation of the mediobasal hypothalamus prevented the effect of quinelorane in the median eminence, but not the nucleus accumbens. Activation of κ opioid receptors with U-50,488 had no effect per se, but blocked quinelorane-induced increases in median eminence DOPA. In contrast, U-50,488 had no effect on DOPA in the nucleus accumbens of either vehicle- or quinelorane-treated rats. Blockade of κ opioid receptors with NOR-BNI increased median eminence DOPA, and prevented the stimulatory effects of quinelorane on dopamine synthesis. Administration of prolactin also increased median eminence DOPA, but did not alter the ability of quinelorane to stimulate dopamine synthesis. Neither NOR-BNI nor prolactin had any effect on DOPA in the nucleus accumbens of vehicle- or quinelorane-treated rats. These results suggest that D₂ receptor-mediated activation of TIDA neurons occurs via an afferent neuronal mechanism involving, at least in part, inhibition of tonically active inhibitory dynorphinergic neurons in the male rat.     

Prolactin regulation of tuberoinfundibular dopaminergic neurons: immunoneutralization studies

The purpose of this study was to determine the effects of acute hypoprolactinemia on tuberoinfundibular dopamine (DA) neurons using a rabbit anti-rat prolactin antiserum (PRL-AB) to immunoneutralize circulating prolactin under basal conditions and at various times after haloperidol-induced hyperprolactinemia. The specificity of PRL-AB for prolactin was determined by examining the ability of unlabelled hormone to displace binding of 125I-labelled prolactin to PRL-AB. Tuberoinfundibular DA neuronal activity was estimated by measuring the concentrations of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence which contains terminals of these neurons. Systemic (i.v.) administration of 200 microl of PRL-AB decreased plasma prolactin concentrations below detectable levels for at least 4 h, and this was accompanied by a pronounced decrease in DOPAC concentrations in the median eminence of females, but not males. Central (i.c.v.) administration of 2 microl PRL-AB diluted up to 1:100 mimicked the inhibitory effect of systemic administration of PRL-AB on median eminence DOPAC concentrations suggesting that the tonic stimulatory effect of prolactin on the basal activity of tuberoinfundibular DA neurons in females occurs via a central site of action. In male rats, blockade of anterior pituitary DA receptors with haloperidol (1 mg/kg; s.c.) caused an prompt (by 1 h) increase in plasma prolactin concentrations which was maintained for at least 12 h. Haloperidol-induced hyperprolactinemia also caused a delayed (at 6 and 12 h) increase in median eminence DOPAC concentrations in these animals which was blocked by PRL-AB. Exposure of rats to initial priming periods of endogenous hyperprolactinemia of up to 6 h duration (followed by 6 h or more of PRL-AB-induced hypoprolactinemia) failed to alter median eminence DOPAC concentrations unless prolactin exposure was reinstated by an i.c.v. injection of prolactin. These results confirm that prolactin mediates the stimulatory effects of haloperidol on tuberoinfundibular DA neurons, and reveal that delayed induced activation of these neurons by prolactin is dependent upon a priming period of sustained hyperprolactinemia longer than 3 h for initiation and maintenance of this response.     

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.     

Changes in the local metabolism of dopamine in the anterior and neural lobes but not in the intermediate lobe of the pituitary gland during nursing

Dopamine (DA), produced by tubero-infundibular dopaminergic (TIDA) neurons of the arcuate nucleus (ARN) is the established inhibitor of the secretion of prolactin (PRL). Changes in dopaminergic (DAergic) neuronal activity in the median eminence–long portal vessels (ME–LPV) and/or the concentration of DA in the anterior lobe (AL) are inversely related to the secretion of PRL. However, conflicting reports concerning DAergic neuronal activity during the suckling-induced release of PRL persist. In addition to TIDA neurons, PeVN-hypophysial DAergic (PHDA) and tubero-hypophysial DAergic (THDA) neurons which, respectively, innervate the intermediate lobe (IL) and the IL/neural lobe (NL) also have a significant role. We measured the concentrations of DA and its main metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), in the median eminence and the three pituitary lobes of lactating mothers. Concentrations of DA and DOPAC from tissues and the concentration of PRL in plasma were measured by HPLC-EC and RIA, respectively. There were no changes in the concentration of DA and DOPAC of the IL due to the suckling stimulus. In the NL, a decrease in the concentration of DOPAC was detected due to the suckling stimulus. In addition, there were no changes of DA or DOPAC in the outer zone of the AL (AL-OZ) due to suckling. However, a decrease in the concentrations of DA and DOPAC was detected in the inner zone of the AL (AL-IZ). These data suggest lactotrophs from the AL-IZ are responsible for the changes in the concentration of plasma PRL in response to the suckling stimulus. In addition TIDA and THDA neurons, but not PHDA neurons, regulate the control of the secretion of PRL in response to suckling.     

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.     

Fasting stimulates tuberoinfundibular dopaminergic neuronal activity and inhibits prolactin secretion in oestrogen-primed ovariectomized rats: involvement of orexin A and neuropeptide Y

Fasting up-regulates central orexigenic systems including orexin A and neuropeptide Y (NPY) and it also inhibits the secretion of prolactin. We hypothesized that fasting may act through orexin A and NPY to influence tuberoinfundibular dopaminergic (TIDA) neurones, the major regulator of prolactin secretion. The effects of orexin A and NPY on TIDA neuronal activity and prolactin secretion were determined in oestrogen-primed ovariectomized rats, and the effects of fasting and the involvement of orexin A and NPY were tested. Orexin A, NPY and its analogs were administered through preimplanted intracerebroventricular (i.c.v.) cannulae. TIDA neuronal activity was determined by measuring concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) or 3,4-dihydroxyphenylalanine in the median eminence. i.c.v. injection of NPY (10 microg) or orexin A (1 microg) concomitantly increased median eminence DOPAC and decreased serum prolactin concentrations. The effect of NPY was mimicked by a Y1 receptor agonist at lower doses (0.1 and 1 microg) and no additive effect was observed when orexin A and the Y1 agaonist were coadministered. Moreover, a Y1 receptor antagonist, BIBP, not only blocked the effect of Y1 agaonist, but also that of orexin A. Treatment with BIBP alone decreased median eminence DOPAC and increased serum prolactin concentrations, indicating that endogenous NPY may play a role. Moreover, fasting for 48 h significantly increased TIDA neuronal activity, both in the morning and afternoon, and the effect was reversed by treatment with either BIBP or an antibody against orexin A. The findings support our hypothesis that fasting stimulates TIDA neuronal activity and inhibits prolactin secretion through up-regulated central orexin A and NPY systems.     

Prolactin activates all three populations of hypothalamic neuroendocrine dopaminergic neurons in ovariectomized rats

Prior studies suggest that prolactin (PRL) stimulates release of dopamine (DA) from tuberoinfundibular dopaminergic (TIDA) neurons. In the present study, the time course over which PRL exerts its effects on all three populations of neuroendocrine dopaminergic (DAergic) neuron populations [TIDA, tuberohypophyseal (THDA) and periventricular-hypophyseal (PHDA)] was determined. Ten days following ovariectomy (OVX), groups of female rats were injected either with 15 microg of ovine PRL (oPRL) or saline at 0900 h. Rats were decapitated every 30 min from 0830 h-1100 h and hourly from 1200 h-1500 h. Trunk blood was assayed for rat PRL (rPRL) and oPRL using species-specific radioimmunoassays (RIAs). The concentration of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence (ME), as well as the anterior (AL), intermediate (IL) and neural (NL) lobes of the pituitary gland were determined by HPLC-EC. The concentration of rPRL in oPRL-treated animals, compared to saline-treated animals, was diminished by 1000 h and again between 1200 h-1500 h. DOPAC/DA ratio, an indicator of dopaminergic neuronal activity, increased spontaneously in the ME, IL, and NL during the afternoon in OVX rats. In animals injected with oPRL at 0900 h, the DOPAC/DA ratio increased in the ME, IL and NL within 1 h. Moreover, a secondary increase in the DOPAC/DA ratio in the IL and NL occurred during the afternoon in oPRL-treated rats. However, the second increase of DA turnover present in the ME of control animals never occurred in oPRL-treated animals. Furthermore, there were two increases in the concentration of DA in the AL: the first coincided with the increased turnover of DA in all three terminal areas and the second with increased DA turnover in the IL and NL. These data suggest that all three populations of hypothalamic neuroendocrine DAergic neurons are activated by PRL and that PHDA/THDA neurons have a second 'delayed' activation.     

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.     

Sexual differences in the stimulatory effects of bombesin on tuberoinfundibular dopaminergic neurons

The purpose of this study was to (1) examine the effects of central administration of bombesin on the activity of tuberoinfundibular dopaminergic (DA) neurons in male and female rats, and (2) determine if sexual differences in the responsiveness of these neurons to bombesin were due to the presence of prolactin or gonadal steroids. The activity of tuberoinfundibular DA neurons was estimated by measuring the concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in terminals of these neurons in the median eminence. The effects of bombesin on the secretion of prolactin and alpha-melanocyte stimulating hormone (alpha MSH), and the activities of nigrostriatal, mesolimbic, and periventricular-hypophysial DA neurons were also determined in gonadally intact male and female rats. Central administration of bombesin (10 ng/rat; i.c.v.) decreased prolactin secretion in gonadally intact male and female rats, but only in males was this associated with an increase in the activity of tuberoinfundibular DA neurons. In contrast, bombesin increased the activity of periventricular-hypophysial DA neurons terminating in the intermediate lobe of both male and female rats, and this was associated with a decrease in alpha MSH secretion in both sexes. Bombesin had no effect on the activities of nigrostriatal, mesolimbic, or periventricular-hypophysial DA neurons terminating in the neural lobe in either sex. The loss of endogenous gonadal hormones following ovariectomy rendered tuberoinfundibular DA neurons responsive to the stimulatory effects of bombesin, whereas immunoneutralization of endogenous prolactin following administration of prolactin antiserum had no effect on the inability of bombesin to alter the activity of these neurons in female rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Atrial natriuretic peptide-induced suppression of basal and dehydration-induced vasopressin secretion is not mediated by hypothalamic tuberohypophysial or tuberoinfundibular dopaminergic neurons

The purpose of this study was to examine the effects of atrial natriuretic peptide (ANP) on the secretion of vasopressin and the activities of hypothalamic tuberohypophysial and tuberoinfundibular dopaminergic neurons in normal and dehydrated male rats. Neuronal activity was estimated by measuring the concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) and dopamine (DA) in brain and posterior pituitary regions containing terminals of tuberohypophysial (neural lobe; intermediate lobe) and tuberoinfundibular (median eminence) DA neurons. Intracerebroventricular (icv) administration of 20 micrograms ANP decreased basal arginine vasopressin concentrations in the plasma, but had no effect on the concentrations of DOPAC or DA in any region examined. Water deprivation caused a time-dependent increase in plasma arginine vasopressin concentrations, with maximal levels measured 2 days after removal of water bottles. Water deprivation had no effect on DOPAC concentrations in the neural lobe, intermediate lobe or median eminence, but increased DA concentrations in the neural lobe. ANP (20 micrograms/rat; icv) decreased arginine vasopressin concentrations in the plasma of water-deprived rats without altering DOPAC or DA concentrations in the neural lobe, intermediate lobe or median eminence. These results indicate that ANP-induced suppression of basal and dehydration-induced vasopressin secretion is not mediated by tuberohypophysial or tuberoinfundibular DA neurons.     

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.     

Placental lactogen mimics prolactin in activating tuberoinfundibular dopaminergic neurons

The phasic patterns of prolactin release and tuberoinfundibular dopaminergic (TIDA) neuronal activity which occur during early pregnancy in the rat cease by midpregnancy. It has been suggested that this cessation is mediated by the increased secretion of placental lactogen which occurs at this time. The present experiments were undertaken to assess the effects of placental lactogen on TIDA neurons. The activity of these neurons was estimated from the in vivo rate of dihydroxyphenylalanine (DOPA) accumulation in the median eminence after the administration of an inhibitor of aromaticl-amino acid decar☐ylase. Female rats, ovariectomized 2 weeks previously, were injected intraventricularly (i.c.v.) 12 h prior to sacrifice with either rat prolactin (10 μg), human placental lactogen (100 μg) or 0.9% saline vehicle. The human placental lactogen increased the rate of DOPA accumulation in the median eminence, as did rat prolactin, suggesting that placental lactogen is capable of mimicking prolactin in activating TIDA neurons. Human placental lactogen (10–100 μg/10 μl, i.c.v.) stimulated TIDA neurons in a time-related manner reminiscent of that obtained with prolactin (i.e. the neurons were activated at 12 and 16 h but not at 4 or 8 h after i.c.v. infusion of prolactin). These actions of placental lactogen are not due to its somatotrophic properties, since low doses of rat growth hormone were without effect on these neurons. These data indicate that placental lactogen, acting in a prolactin-like fashion, can stimulate TIDA neurons.     

Effects of chronic bromocriptine treatment on tyrosine hydroxylase (TH) mRNA expression, TH activity and median eminence dopamine concentrations in ageing rats

The purpose of this study was to investigate the age-related changes in the responsiveness of tuberoinfundibular dopamine (TIDA) neurones to chronic hypoprolactinemia induced by treatment with bromocriptine, a dopamine receptor agonist. In one experiment, TIDA neuronal activity after acute hypoprolactinemia or exogenous prolactin was monitored by measuring tyrosine hydroxylase (TH) activity in the stalk median eminence of middle-aged cycling female rats (10-12 months), old constant oestrous rats (18-20 months) and old pseudopregnant rats (22-24 months). In another experiment, middle-aged cycling (10-12 months) rats were treated with bromocriptine for 6 or 12 months. TH activity was measured in the stalk median eminence, TH mRNA levels were measured in the arcuate nucleus and dopamine concentrations were measured in the arcuate nucleus and median eminence. Responsiveness of TIDA neurones to exogenous prolactin and to the withdrawal of bromocriptine in these rats was also tested. While the TIDA neurones in all three age groups responded to acute hypoprolactinemia by showing a reduction in TH activity, older rats failed to respond to exogenous prolactin administration. In contrast, chronic hypoprolactinemia for 12 months enabled the rats to retain TIDA neuronal responsiveness to exogenous prolactin. It also decreased TIDA neuronal function as measured by dopamine concentrations in the median eminence, TH activity in the stalk median eminence and TH mRNA in the arcuate nucleus of ageing rats. The restoration of the responsiveness of these neurones to prolactin stimulation in older rats demonstrates for the first time that hypoprolactinemia produced by chronic bromocriptine treatment indeed provides a neuroprotective effect on TIDA neurones. These results indicate that maintaining a low level of neuronal activity by lowering prolactin levels may be a contributing factor in retaining the plasticity of TIDA neurones.     

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.     

Morphine does not stimulate prolactin release during lactation

The ability of morphine to stimulate prolactin and growth hormone (GH) release was investigated in male rats and in female rats during diestrus, proestrus and lactation. In agreement with previous reports, acute morphine administration produced an increase in circulating levels of prolactin in male and in diestrous and proestrous female rats. In contrast to these results, morphine administration (10 or 15 mg/kg, s.c.; 5 mg/kg, i.v.; 5 or 10 micrograms, i.c.v.) did not produce an increase in prolactin levels in lactating dams. Morphine stimulates prolactin release in part by decreasing dopamine turnover in the tuberoinfundibular neurons in the median eminence. In order to assess the functional activity of these neurons during lactation, haloperidol (0.1 or 0.5 mg/kg, i.v.) was given to lactating dams. There was a significant increase in prolactin levels following haloperidol administration, suggesting that these dopaminergic neurons are participating in the modulation of prolactin release during lactation. In contrast to the insensitivity of the lactating rat to morphine stimulation of prolactin release, the intraventricular administration of two other opiate receptor agonists, beta-endorphin (10 or 20 micrograms) and [D-Ala-D-Leu]enkephalin (DADLE; 5 or 10 micrograms), produced significant increases in circulating levels of this hormone. The GH response to morphine, beta-endorphin and DADLE was also measured in these same rats. All these opiate receptor agonists stimulated GH release in male rats and in female rats during diestrus and proestrus as well as during lactation. These observations suggest that the suckling stimulus during lactation renders the rat refractory to morphine stimulation of prolactin release, possibly as a result of down-regulation of the mu-opiate receptor subtype.     

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.     

Distribution of 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylglycol (DOPEG) in microdissected brain structures and the pituitary gland: Metabolite changes in the median eminence in response to hyperprolactinemia and suckling

Summary Dopamine (DA), norepinephrine (NE), epinephrine (E), 3,4-dihydroxyphenylglycol (DOPEG) and dihydroxyphenylacetic acid (DOPAC) were determined simultaneously by a radioenzymatic, thin-layer chromatographic assay able to detect 1–10 pg of the parent compounds and 80–120 pg of their metabolites. A localization study of these compounds in 20 micro-dissected hypothalamic and limbic structures and the anterior and posterior pituitary glands of male rats was completed. DOPAC was detectable in 14 of 22 structures with the lowest DOPAC/DA ratio being found in the caudate nucleus (7.1%) and the highest in the medial aspect of the ventromedial nucleus of the hypothalamus (422.0%). There was a higher DOPAC/DA ratio in the lateral (21.5%) than in the medial (11.3%) portion of the median eminence suggesting that a greater portion of released DA in the medial median eminence enters the portal circulation. DOPEG was detectable in 6 of 22 structures with DOPEG/NE ratios ranging from 8% (interstitial nucleus of the stria terminalis, ventral aspect) to 32% (medial median eminence). A poor correlation exists between DOPAC and DA concentrations in the various brain regions while there was a stronger relationship between DOPEG and NE concentrations. Male rats were rendered hyperprolactinemic for 48 hours with injections of ovine prolactin (oPRL) every 8 hours (4 mg/kg body weight sc). In such rats there was a suppression of endogenous rat PRL (rPRL) secretion, the DOPAC/DA ratio increased 2.2-fold in the medial (MEm) and 1.9-fold in the lateral median eminence (ME1), and the DA concentration in the anterior pituitary also increased 2.6-fold. In 10 day postpartum lactating rats, suckling produced marked increases in serum rPRL but no change in DOPAC/DA ratios in the ME or in the DA concentration in the anterior pituitary. The data reveal a wide range of DOPAC/ DA ratios (7–422%) in brain regions containing cell bodies, axons and terminals of the different dopaminergic neuronal tracts in brain and pituitary.Considering the DOPAC/DA ratios in the MEm and ME1, it is suggested that a large perturbation of dopaminergic transmission produces a significant ratio change while a smaller perturbation is not detected by this index of neuronal metabolism.Supported by NIH grants NS-14611 (MS) and HD-15955 awarded to MS and Dr. Phyllis M. Wise.Recipient of NIH National Research Service Award AG-05351.Recipient of NIH National Research Service Award HD-06481.Recipient of NIH Research Career Development Award NS-00731.     

Ovarian steroid hormones modulate circadian rhythms of neuroendocrine dopaminergic neuronal activity

Dopamine (DA) is the primary inhibitor of prolactin (PRL) secretion. Three populations of neuroendocrine dopaminergic neurons (NDNs) designated tuberoinfundibular (TIDA), tuberohypophyseal (THDA) and periventricular hypophyseal DAergic (PHDA) neurons regulate PRL secretion. Given that ovarian steroids modulate both DA release and PRL secretion independently, we characterized the role of steroid hormones in coupling rhythmic NDN activity and PRL secretion. OVX rats under a standard 12:12 L:D cycle (L:D), constant dark (DD), or a 6-h phase-delayed L:D cycle (pdL:D) were treated with Estradiol-17beta (E) or E and Progesterone (E+P). NDN activity, defined by DA:DOPAC ratio in nerve terminals, was determined by HPLC-EC. E or E+P stimulated PRL surges in L:D that persisted under DD. In TIDA neurons, E or E+P treatment reduced the amount of DA released under L:D and DD and advanced the rhythm of DA turnover. E and E+P treatment reduced THDA and PHDA neuron activity under L:D, but did not affect these rhythms under DD. Circadian rhythms of PRL, corticosterone and DA turnover in NDN terminals from steroid treated rats entrained to a pdL:D cycle within 7 days. Therefore, ovarian steroids differentially adjust the timing and magnitude of NDN activity to facilitate coupling of DA release and PRL secretion.     

Studies on dopamine turnover in ovariectomized or hypo-physectomized female rats. effects of 17β-estradiol benzoate, ethynodioldiacetate and ovine prolactin

The effects of 17 beta-estradiol benzoate (EB), ethynodioldiacetate and ovine prolactin on dopamine (DA) turnover have been studied. As an index for a change in turnover, differences in DA depletion following tyrosine hydroxylase inhibition with alpha-methyl-tyrosine methylester were observed. DA was measured by means of mass fragmentographical analysis in the rat median eminence, the olfactory tubercle and the striatum. The actions of ethynodioldiacetate and ovine prolactin on DA turnover in various subregions of the rat median eminence were analysed by quantitative microfluorimetry. Repeated injections of EB to ovariectomized rats resulted in a significant increase of DA turnover in the median eminence. Administration of ethynodioldiacetate to ovariectomized rats almost significantly increased DA turnover in the olfactory tubercle. In the median eminence DA turnover was significantly increased only in the lateral palisade zone. In male hypophysectomized rats ovine prolactin increased DA turnover in both the lateral and the medial palisade zone of the median eminence. The results support the involvement of DA neurons in the control of prolactin and luteinizing hormone secretion. It is suggested that the tubero-infundibular DA neurons are involved in mediating the central inhibitory feedback actions of prolactin and gonadal steroids on prolactin and LH secretion, respectively.