Effect of interleukin-6 and tumor necrosis factor-alpha on GABA release from mediobasal hypothalamus and posterior pituitary

The release of cytokines during infection, inflammation and stress induces brain-mediated responses, including alterations of neuroendocrine functions. We examined the effect of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) on release of gamma-aminobutyric acid (GABA) from mediobasal hypothalamic (MBH) explants and posterior pituitaries (PP) of male rats. IL-6 (10 ng/ml) did not modify basal GABA release from MBH and PP, but significantly increased GABA release under depolarizing conditions (40 mM K(+)). This effect was abolished by incubation of the tissue with indomethacin, an inhibitor of cyclooxygenase activity, indicating that prostaglandins could mediate the stimulation of GABA release induced by IL-6. On the contrary, TNF-alpha (50 ng/ml) significantly decreased K(+)-evoked GABA release from both MBH and PP. This inhibitory effect was not modified by indomethacin. Neither IL-6 nor TNF-alpha affected nitric oxide synthesis, as measured by [(14)C]citrulline production. The current results indicate that IL-6 stimulates GABA release from both hypothalamus and posterior pituitary by a mechanism mediated by prostaglandins. On the contrary, TNF-alpha inhibits GABA release from both tissues. These results suggest the possibility that GABAergic activity in the hypothalamic-pituitary axis could be involved in neuroendocrine responses to cytokines.     

Changes in anterior pituitary hormone secretion and hypothalamic catecholamine metabolism during morphine withdrawal in the female rat

Studies were undertaken to evaluate the acute responses of hypothalamic noradrenergic and dopaminergic neurons and anterior pituitary hormones to naloxone (NAL)-precipitated morphine (MOR) withdrawal in the rat. Ovariectomized female rats were rendered MOR-dependent and injected with NAL (1 mg/kg b.w., s.c.). During precipitated MOR withdrawal, a decline in norepinephrine (NE) concentrations was preceded by an increase in the level of its metabolite normetanephrine (NME) in the medial basal hypothalamus (MBH) as well as the preoptic area-anterior hypothalamus (POA-AH). Both dopamine (DA) and its major acid metabolite, dihydroxyphenylacetic acid (DOPAC), showed increased concentrations in these two hypothalamic regions within 30 min of NAL administration. Elevated luteinizing hormone (LH) and beta-endorphin secretion was evident within 5 min of NAL injection to MOR-dependent rats, while serum prolactin (PRL) increased 15 min into MOR withdrawal. Both growth hormone (GH) and thyroid-stimulating hormone (TSH) were depressed over the course of MOR withdrawal. Although a cause and effect relationship cannot be established, during NAL-precipitated MOR withdrawal, a heightened hypothalamic monoaminergic neuronal activity is accompanied by a differential response of anterior pituitary hormones. The observed responses, which are similar to those seen during acute stress, indicate that MOR withdrawal may activate the same mechanisms which mediate the neuroendocrine response to stress.     

Reproductive experience increases striatal and hypothalamic dopamine levels in pregnant rats

The effects of parity on the dopaminergic function of rats were studied. Striatal and hypothalamic levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), norepinephrine (NE), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) as well as serum prolactin (PRL) levels of 7-days primigravid and multigravid rats were compared. Brains and trunk blood were collected from 1200–1400 h on day 7 of pregnancy and assayed for monoamines and their metabolites, and prolactin, respectively. Multigravid rats showed a significant increase in striatal and hypothalamic dopamine levels. A tendency to increase in striatal DOPAC levels was also observed in multigravid rats. Levels of other neurotransmitters and metabolites were not statistically different. Haloperidol (1 mg/kg) treatment induced a significant increase in multigravid 5-HT striatal levels. There was no statistical difference among primigravid and multigravid serum PRL levels after either saline or haloperidol treatment. These data suggest that prior parity produces a shift in dopaminergic activity in multigravid rats.     

Time course study on the effect of reserpine on hypothalamic immunoreactive CRF levels in rats

A time course study on the changes of rat hypothalamic corticotropin-releasing factor (CRF) levels and ACTH levels in plasma, pituitary and hypothalamus after an acute treatment with reserpine was examined using a rat CRF RIA. The massive and prolonged depletion of hypothalamic norepinephrine and dopamine levels provoked by a single injection of reserpine (2 and 8 mg/kg, i.p.) caused a transient decrease of hypothalamic CRF levels and ACTH levels in the anterior pituitary glands, and an increase in plasma ACTH levels. There was a strong correlation between the depletion of hypothalamic CRF and norepinephrine levels. These results suggest that: acute depletion of hypothalamic norepinephrine levels cause the initial release of CRF that stimulates pituitary ACTH secretion, and the depletion of CRF and ACTH stores at the early stage; and noradrenergic pathways may be involved in the inhibitory mechanism of CRF release.     

Hypothalamic dopamine may play a role in inducing prolactin in pituitary cells

The effects of hypothalamic dopamine on the functional differentiation of prolactin cells was investigated in the developing rat brain. The treatment of pregnant rats with alpha-methyl-p-tyrosine (tyrosine hydroxylase inhibitor) resulted in a noticeable decrease of dopamine concentration in the hypothalamus at birth and at 5 days of age of offspring. Moreover, treatment with this regimen caused a marked decrease in the population of prolactin cells and in the production of prolactin at birth and at 5 days of age of offspring. It seems, then, that hypothalamic dopaminergic neurons precede the functional differentiation of prolactin cells in the developing brains and that hypothalamic dopamine may play an important role in producing prolactin in the anterior pituitary during development.     

Decrease in hypothalamic epinephrine concentration and other neurochemical changes produced by quinpirole,a dopamine agonist,in rats

Summary Quinpirole, (4 aR-trans)-4, 4 a, 5, 6, 7, 8, 8a, 9-octahydro-5-propyl-1 H-pyrazolo[3, 4-g] quinoline, is a dopamine agonist selective for the D₂ subtype of dopamine receptors. In rats, quinpirole at doses of 0.3 mg/kg i.p. and higher decreased hypothalamic epinephrine concentrations. The doses required for this effect are only slightly higher than the minimum doses that decreased the concentration of dopamine metabolites in cerebral hemispheres. At higher doses of quinpirole (2–3 mg/kg i.p.), dopamine concentration was increased and norepinephrine concentration was decreased in hypothalamus, and MHPG sulfate (the norepinephrine metabolite) concentration was increased in brain stem and in hypothalamus. All of these neurochemical effects of quinpirole were blocked by pretreatment with spiperone, a dopamine antagonist. The effects were not produced by SKF 38393, a selective D₁ agonist, or by the dextrorotatory enantiomer of quinpirole, which lacks D₂ agonist activity. The data support the interpretation that quinpirole, by activating D₂ receptors, results in a decrease in dopamine metabolites, a decrease in hypothalamic epinephrine concentration, and an increased conversion of norepinephrine to MHPG sulfate in rat brain probably through enhanced norepinephrine release.Abbreviations DOPAC 3, 4-dihydroxyphenylacetic acid - HVA homovanillic acid - MHPG 3-methoxy-4-hydroxy-phenylethyleneglycol     

The effect of D-aspartate on luteinizing hormone-releasing hormone,alpha-melanocyte-stimulating hormone,GABA and dopamine release

Since D-aspartate stimulates prolactin and LH release, our objective was to determine whether D-aspartate modifies the release of hypothalamic and posterior pituitary factors involved in the control of their secretion and whether its effects on these tissues are exerted through NMDA receptors and mediated by nitric oxide. In the hypothalamus, D-aspartate stimulated luteinizing hormone-releasing hormone (LHRH), alpha-melanocyte-stimulating hormone (alpha-MSH) and GABA release and inhibited dopamine release through interaction with NMDA receptors. It increased nitric oxide synthase (NOS) activity, and its effects on LHRH and hypothalamic GABA release were blunted when NOS was inhibited. In the posterior pituitary gland, D-aspartate inhibited GABA release but had no effect on dopamine or alpha-MSH release. We report that D-aspartate differentially affects the release of hypothalamic and posterior pituitary factors involved in the regulation of pituitary hormone secretion.     

Ontogeny of catecholamine turnover rates in limbic and hypothalamic structures in relation to serum prolactin and gonadotropin levels

Norepinephrine (NE) and dopamine (DA) concentrations and turnover rates have been studied in the n. accumbens, medial preoptic area (MPO) and anterior and posterior parts of the mediobasal hypothalamus of developing rats. Turnover rates are determined by injection of -methyl-p-tyrosine 30 and 90 min prior to decapitation. NE concentrations and turnover in the n. accumbens were low in all age groups with slightly increased values between days 20 and 35 after birth whereas DA concentrations and turnover rates were low at day 15 and 20 and at high adult values by day 25 after birth. Medial preoptic and anterior mediobasal hypothalamic catecholamines exhibited a rather unique pattern. Concentrations and turnover rates were low in 15-day-old animals and increased between days 20 and 30 to very high values. Such high values were never observed in adult diestrous animals. The same pattern was also observed in the posterior mediobasal hypothalamus for NE concentrations and turnover rates whereas the respective values for DA showed relatively large fluctuations. On the basis of catecholamine measurements 30 and 90 min after blockade of tyrosine hydroxylase an attempt was also made to differentiate turnover rates of the functional and of the storage pool.

Serum LH levels in the 15-day-old animals showed large fluctuations. FSH levels were high and prolactin levels were low. At the time of increased preoptic and hypothalamic NE and DA turnover rates, serum prolactin levels were also high whereas serum LH levels were lowest between days 20 and 30 and then slightly increased. Serum FSH levels were uniformly low. The possibility is discussed that high NE turnover may stimulate pituitary LH and prolactin release by hypothalamic mechanisms. High serum prolactin levels may stimulate DA turnover which is inhibitory to pituitary LH release, thus counteracting the stimulatory effect of NE on LH-RH release. The dopaminergic inhibition of LH may be relieved at the time of puberty partially because the DA receptors become desensitized to the action of DA.     

Catecholaminergic mechanisms of the lateral hypothalamus: their role in the mediation of amphetamine anorexia.

The brain mechanisms mediating amphetamine's suppressive effect on feeding behavior were analyzed in rats with chronically implanted brain cannulas. Experiments in which drugs were injected directly into the anterolateral hypothalamus, the region found to be most responsive to amphetamine's action, yielded the following results. (1) Over a dose range of 6.25 nmoles (0.8 mug) to 400 nmoles (51.4 mug), hypothalamically injected D-amphetamine produced a reliable suppression of food consumption (20 percent at 6.25 nmoles, increasing to 88 percent at 200 nmoles) and was found to be approximately 3 times as potent as L-amphetamine in yielding this effect. (2) The anorexic effect of hypothalamically injected D-amphetamine was totally abolished by local administration of alpha-methyltyrosine, an inhibitor of dopamine, norepinephrine, and perhaps epinephrine synthesis, or by local administration of Fla-63, an inhibitor of only norepinephrine, and perhaps epinephrine, synthesis. (3) This effect of hypothalamic D-amphetamine was also antagonized by locally administered dopaminergic or beta-adrenergic receptor blockers but was unaffected by alpha-adrenergic, serotonergic, and cholinergic blockers. (4) Lateral hypothalamic administration of dopaminergic or beta-adrenergic receptor blockers, at quite low doses, was also effective in antagonizing the anorexia induced by peripherally administered D-amphetamine. These results strongly suggest that amphetamine, in suppressing feeding behavior, acts through the lateral hypothalamus, perhaps the anterior region, causing a release of dopamine and norepinephrine (or perhaps epinephrine) from lateral hypothalamic nerve endings and a subsequent stimulation of dopaminergic and beta-adrenergic receptors located in that region.     

Influence of the pineal gland on pituitary function in humans

In 14 men and 14 women we examined the relationship between melatonin and the secretion of pituitary hormones. Blood pressure, and the serum concentrations of melatonin, catecholamines, prolactin, thyrotropin, growth hormone, and cortisol were determined every 3-4 hr for 24 hr. In the normal, basal (unstimulated) condition there were no significant correlations (p greater than 0.05) between the systolic blood pressure and dopamine (r = 0.09), norepinephrine (r = 0.26), or epinephrine (r = 0.27), nor were there significant correlations between melatonin and dopamine (r = -0.01), norepinephrine (r = -0.26), or growth hormone (r = 0.17). The concentrations of melatonin correlated positively with those of prolactin (r = 0.56, p less than 0.05 for men and r = 0.58, p less than 0.001 for women) and thyrotropin (r = 0.62, p less than 0.001 for all subjects), but not with those of cortisol (r = 0.004, p greater than 0.05). We speculate that the increase in melatonin at night leads to a decrease in dopaminergic activity; the diminished release of dopamine may lead to a simultaneous increase in thyrotropin and prolactin.     

PACAP stimulates dopamine neuronal activity in the medial basal hypothalamus and inhibits prolactin

In sheep intracerebroventricular injection of PACAP (10 nmol) significantly (P<0.01) stimulated the levels of the dopamine metabolite DOPAC within the medial basal hypothalamus (as measured by in vivo microdialysis) and this effect was temporally correlated with a significant (P<0.05) suppression in peripheral prolactin concentrations. This result is in accord with the hypothesis that PACAP suppresses prolactin secretion from the anterior pituitary gland by stimulating dopamine release from tuberoinfundibular dopaminergic neurons.     

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.     

Norepinephrine regulation of growth hormone release from goldfish pituitary cells. I. Involvement of alpha2 adrenoreceptor and interactions with dopamine and salmon gonadotropin-releasing hormone

Adrenergic regulation of growth hormone (GH) release in the goldfish was examined in vitro using dispersed goldfish pituitary cells under column perifusion. Norepinephrine and epinephrine suppressed basal GH release from goldfish pituitary cells in a reversible and dose-dependent manner. At high doses, a transient rebound of GH release was observed after termination of norepinephrine and epinephrine treatment. In this study, the dose-dependence of adrenergic inhibition on basal GH release was mimicked by the alpha2 agonists clonidine and UK14304. Basal GH secretion, however, was not affected by the beta agonist isoproterenol and alpha1 agonist methoxamine. In addition, the inhibitory actions of norepinephrine and clonidine on basal GH release were blocked by the alpha2 antagonists yohimbine and RX821002. The beta antagonist propranolol and alpha1 antagonists prasozin and benoxathian were not effective in this respect. Salmon gonadotropin-releasing hormone (sGnRH) and dopamine, two known GH-releasing factors in fish, stimulated GH release from goldfish pituitary cells and their GH-releasing actions were inhibited by simultaneous treatment with norepinephrine. Furthermore, the GH rebound after norepinephrine treatment was significantly enhanced by prior exposure to sGnRH and this effect was not observed with dopamine treatment. These results, taken together, suggest that in the goldfish adrenergic input at the pituitary level inhibit basal GH release through activation of alpha2 adrenoreceptors. This alpha2 inhibitory influence may interact with dopaminergic and GnRH input to regulate GH secretion from goldfish pituitary cells. The 'post-inhibition' GH rebound after NE treatment and its sensitivity to sGnRH potentiation may also represent a novel mechanism for GH regulation in fish.     

The roles of dopamine and the neurointermediate lobe of the pituitary in the regulation of prolactin secretion during late pregnancy in rats

The major hypothalamic control over prolactin secretion from the anterior pituitary gland is inhibitory by means of dopamine released from tuberoinfundibular dopamine (TIDA) neurones. We have previously shown a dissociation between activity of TIDA neurones and prolactin secretion during late pregnancy, suggesting involvement of additional regulatory factors. The aim of the present study was to investigate the role of dopamine and the neurointermediate lobe (NIL) of the pituitary in the regulation of prolactin secretion during late pregnancy. To determine whether dopamine maintains inhibition of prolactin during late pregnancy, the D(2) receptor antagonist domperidone was administered at 12.00 h on days 18 and 21 of pregnancy. These times are characterized by high and low TIDA neuronal activity, respectively, and low prolactin secretion. Domperidone produced an immediate increase in plasma prolactin compared to vehicle-treated controls on both days 18 and 21. Thus, dopaminergic inhibition of prolactin secretion is maintained despite reduced TIDA neuronal activity at the end of pregnancy. The contribution of NIL-derived dopamine in regulating prolactin secretion was then examined by investigating the effect of surgical removal of the NIL. NIL removal produced significantly increased basal prolactin concentrations, indicating that dopamine from the NIL contributes to the suppression of prolactin before the antepartum prolactin surge. Furthermore, NIL removal also completely prevented the antepartum prolactin surge compared to sham-operated controls, which is consistent with the hypothesis that the NIL supplies a prolactin-releasing factor to the anterior pituitary to induce the antepartum prolactin surge.     

Release of luteinizing hormone-releasing hormone: interrelations between eicosanoids and catecholamines

The in vitro release of luteinizing hormone-releasing hormone (LHRH), prostaglandin (PG) E2 and leukotriene (LT) C4 from male rat median eminences (ME), was estimated by radioimmunoassay (RIA) in the presence of the catecholamines (CA), norepinephrine (NE) and dopamine (DA). NE increased the release of PGE2 in the presence and in the absence of the Ca2+ ionophore A23187 (5 x 10(-6) M), but it did not modify the A23187-induced release of LTC4 from endogenous precursors or radiolabelled arachidonic acid. DA also stimulated the A23187-induced release of PGE2 but inhibited that of LTC4. However, while NE increased both the basal and the A23187-induced release of LHRH, DA increased the basal release of LHRH and inhibited the A23187-induced LHRH release. Exogenous LTC4 cancelled the inhibitory effect of DA on LHRH release. Blockade of dopaminergic receptors with haloperidol suppressed the effects of DA on PGE2, LTC4 and LHRH release. Neither eicosanoid affected the K+-evoked [3H]DA release, whereas only PGE2 inhibited the K+-evoked [3H]NE release. We conclude that LTC4 does not interact with the noradrenergic pathway and that the stimulatory effect of both catecholamines on LHRH release involves PGE2, but the inhibitory effect of DA is associated with reduced LTC4 production.     

Decreased dopamine turnover in the median eminence in response to suckling in the lactating rat

The effects of suckling on the turnover of dopamine (DA) and norepinephrine (NE) were studied in terminal projection fields of the tuberoinfundibular (median eminence, ME), nigrostriatal (caudate nucleus, CN), incertohypothalamic (medial preoptic nucleus, MPN) and mesolimbic (nucleus accumbens, NA) dopaminergic neurons. Decreased dopamine turnover in the median eminence was found in suckled compared with nonsuckled rats at 10 days postpartum. This effect was specific as dopamine turnover in the CN, NA and MPN and norepinephrine turnover in the ME, NA and MPN were not affected by suckling. The suckling-induced prolactin response is markedly blunted in rats 20 days postpartum. In these rats, median eminence dopamine turnover did not decrease significantly in response to suckling. These results are consistent with the hypothesis that median eminence dopamine is a physiological prolactin inhibitory factor mediating suckling-induced prolactin release.     

Indomethacin prevents increased catecholamine turnover in rat brain following systemic endotoxin challenge

1. Key features of the acute phase response to infection are replicated by systemic administrations of lipopolysaccharide and may be mediated via the production of lymphokines and cytokines, including interleukin-1. Inhibition of prostaglandin synthesis may attenuate certain features of the acute phase response. 2. In the present study, the effects of systemic administration of the lipopolysaccharide (LPS, 250 micrograms/rat) and interleukin-1 (IL-1, 10 micrograms/rat) on catecholamine metabolism in different brain regions were compared and the effects of indomethacin, a cyclooxygenase inhibitor was determined. 3. The ratio of metabolite to parent amine was used as an index of turnover of catecholamines. 4. In hypothalamus, both epinephrine and norepinephrine concentrations were decreased and their major metabolite, 3-methoxy,4-hydroxyphenylglycol (MHPG), was elevated at 4, 8 and 24 hr following LPS. The major metabolite of dopamine (homovanillic acid, HVA) was increased at 8 hours in striatum, hypothalamus and medulla. LPS increased dopamine turnover at 8 and 24 hr and norepinephrine turnover at 4, 8 and 24 hr. 5. In all regions examined, IL-1 produced effects similar to LPS on amine and metabolite contents and norepinephrine and dopamine turnover. 6. Significantly, co-administration of a single dose of indomethacin (50 mg/kg) completely blocked LPS-induced changes in hypothalamic catecholamines and metabolites and the increase in turnover at 4 and 8 hr. Furthermore, the effects of IL-1 on hypothalamic MHPG content and norepinephrine turnover were also blocked by indomethacin, although the effects of IL-1 on regional catecholamines and HVA content and turnover were either not modified or partially antagonized by indomethacin. 7. The present results suggest that in the rat, activation of noradrenergic, dopaminergic and epinephrine-containing neurons in hypothalamus, as well as dopaminergic neurons in other regions is associated with the acute phase response to endotoxin and that synthesis of prostaglandins plays a pivotal role in catecholamine responses in all brain regions examined.     

Ablation of the hypothalamic arcuate-median eminence region reduces the concentration of vasoactive intestinal peptide in the anterior pituitary gland of male rats

This study was designed to determine the influence of the hypothalamus on the content of vasoactive intestinal peptide (VIP) in the anterior pituitary. Disruption of the hypothalamic-pituitary connection was performed by ablating the arcuate-median eminence (ARC-ME) region in adult male rats. Fifteen days later, there was a significant reduction in pituitary mass, adrenal and testicular weight and an increase in water consumption and serum prolactin levels indicating the elimination of hypothalamic influence on the pituitary gland in the ARC-ME group when compared to controls. Anterior pituitary VIP content was also significantly reduced in the lesion group. These data suggest that the hypothalamus is involved in the regulation of pituitary VIP.     

Nuclear translocation of STAT5 and increased expression of Fos related antigens (FRAs) in hypothalamic dopaminergic neurons after prolactin administration

Ample evidence indicates feedback relationships between pituitary prolactin and hypothalamic dopaminergic neurons. Since the presence of prolactin receptors was earlier demonstrated in hypothalamic dopaminergic neurons, our working hypothesis was that prolactin induced activation of prolactin receptor coupled signaling leads to increased neuronal activity in these neurons. The aim of this study was to correlate prolactin receptor mediated signaling and prolactin induced activation in hypothalamic dopaminergic neurons. We used nuclear translocation of STAT5 as a marker of prolactin receptor induced signaling and expression of Fos related antigens (FRAs) as an indicator of neuronal activation. We performed double label immunocytochemical studies to determine the time course of the presence of FRAs and STAT5 in the nuclei of hypothalamic dopaminergic neurons after ovine prolactin treatment. Exogenous ovine prolactin treatment of ovariectomized rats resulted in an increase in serum ovine prolactin levels and a decrease in endogenous serum prolactin levels, indicating that ovine prolactin activated mechanisms inhibited pituitary prolactin secretion. Indeed, ovine prolactin activated the prolactin receptors in most subpopulations of hypothalamic dopaminergic neurons, resulting in nuclear translocation of STAT5. Also, increased neuronal activity, indicated by expression of FRAs, was observed in the same neuron populations after ovine prolactin treatment. These results suggest that signal transduction mechanisms coupled to prolactin receptors in hypothalamic dopaminergic neurons resemble those observed in other tissues; and nuclear translocation of STAT5 can be used as a marker of prolactin receptor activation in hypothalamic dopaminergic neurons.