Decreased monoamine release in the median preoptic area following ventricular treatment with the angiotensin II antagonist saralasin in normotensive and spontaneously hypertensive rats

Previous findings have shown that some of the neurons in the median preoptic nucleus (MnPO) receive both catecholaminergic inputs from the brainstem and angiotensinergic inputs from the subfornical organ (SFO), and that alterations in the function of the brain renin-ANG system are implicated in hypertension, especially in spontaneous hypertensive rats (SHR). In an attempt to clarify the action of these inputs on MnPO neurons and to find the difference in the action between normotensive Wistar-Kyoto (WKY) rats and SHR, we used microdialysis to investigate the effects of injections of saralasin (Sar), an angiotensin II (ANG II) antagonist, into the third ventricle (3V) on monoamine release in the MnPO area of awake WKY and SHR. The content of noradrenaline (NA) in the MnPO area was significantly higher in SHR. No significant differences were observed between WKY and SHR in the concentrations of dopamine (DA) and of its two metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). In both WKY and SHR, Sar (Sar, 5 microg in 1 microl, three injections at 2-h intervals) injected into the 3V significantly decreased the extracellular concentrations of NA, DOPAC and HVA in the MnPO area. The decreases were much greater in SHR than in WKY rats. Similar injections of saline vehicle had no significant effect on the extracellular levels of NA, DA and the metabolites. These results suggest that central angiotensinergic circuits may serve to increase NA and DA release in the MnPO area, and support that a disorder in the ANG system may contribute, in part, to the elevated blood pressure of SHR.     

Effects of alterations in the activity of tuberohypophysial dopaminergic neurons on the secretion of alpha-melanocyte stimulating hormone

Administration of gamma-butyrolactone (GBL), an anesthetic which reduces dopaminergic neuronal activity, decreased the concentration of the dopamine (DA) metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the intermediate lobe of the pituitary gland, and increased alpha-melanocyte stimulating hormone (alpha MSH) concentrations in the serum of male rats. Bilateral electrical stimulation of the rostral arcuate nucleus, which contains perikarya of tuberohypophysial DA neurons, increased DOPAC concentrations in the intermediate lobe and decreased alpha MSH concentrations in the serum of GBL-anesthetized rats. Administration of the DA antagonist haloperidol prevented the decline in serum alpha MSH levels following arcuate nucleus stimulation, but had no effect on serum alpha MSH concentrations in sham-stimulated GBL-treated rats. These results indicate that GBL-induced decreases or stimulation-induced increases in the activity of tuberohypophysial DA neurons are accompanied by corresponding changes in the metabolism of DA in the intermediate lobe of the rat pituitary gland, and by reciprocal changes in the secretion of alpha MSH.     

Alpha-melanocyte-stimulating hormone mediates melanin-concentrating hormone-induced anorexigenic action in goldfish

In goldfish, intracerebroventricular (ICV) administration of melanin-concentrating hormone (MCH) inhibits feeding behavior, and fasting decreases hypothalamic MCH-like immunoreactivity. However, while MCH acts as an anorexigenic factor in goldfish, in rodents MCH has an orexigenic effect. Therefore, we examined the involvement of two anorexigenic neuropeptides, alpha-melanocyte-stimulating hormone (alpha-MSH) and corticotropin-releasing hormone (CRH), in the anorexigenic action of MCH in goldfish, using an alpha-MSH receptor antagonist, HS024, and a CRH receptor antagonist, alpha-helical CRH((9-41)). ICV injection of HS024, but not alpha-helical CRH((9-41)), suppressed MCH-induced anorexigenic action for a 60-min observation period. We then examined, using a real-time PCR method, whether MCH affects the levels of mRNAs encoding various orexigenic neuropeptides, including neuropeptide Y (NPY), orexin, ghrelin and Agouti-related peptide (AgRP), in the goldfish diencephalon. ICV administration of MCH at a dose sufficient to inhibit food consumption decreased the expression of mRNAs for NPY and ghrelin, but not for orexin and AgRP. These results indicate that the anorexigenic action of MCH in the goldfish brain is mediated by the alpha-MSH signaling pathway and is accompanied by inhibition of NPY and ghrelin synthesis.     

Neuronal interaction between melanin-concentrating hormone- and alpha-melanocyte-stimulating hormone-containing neurons in the goldfish hypothalamus

Intracerebroventricular (ICV) administration of melanin-concentrating hormone (MCH) inhibits food intake in goldfish, unlike in rodents, suggesting that its anorexigenic action is mediated by alpha-melanocyte-stimulating hormone (alpha-MSH) but not corticotropin-releasing hormone. This led us to investigate whether MCH-containing neurons in the goldfish brain have direct inputs to alpha-MSH-containing neurons, using a confocal laser scanning microscope, and to examine whether the anorexigenic action of MCH is also mediated by other anorexigenic neuropeptides, such as cholecystokinin (CCK) and pituitary adenylate cyclase-activating polypeptide (PACAP), using their receptor antagonists. MCH- and alpha-MSH-like immunoreactivities were distributed throughout the brain, especially in the diencephalon. MCH-containing nerve fibers or endings lay in close apposition to alpha-MSH-containing neurons in the hypothalamus in the posterior part of the nucleus lateralis tuberis (NLTp). The inhibitory effect of ICV-injected MCH on food intake was not affected by treatment with a CCK A/CCK B receptor antagonist, proglumide, or a PACAP receptor (PAC(1) receptor) antagonist, PACAP((6-38)). ICV administration of MCH at a dose sufficient to inhibit food consumption also did not influence expression of the mRNAs encoding CCK and PACAP. These results strongly suggest that MCH-containing neurons provide direct input to alpha-MSH-containing neurons in the NLTp of goldfish, and that MCH plays a crucial role in the regulation of feeding behavior as an anorexigenic neuropeptide via the alpha-MSH (melanocortin 4 receptor)-signaling pathway.     

Effects of orphanin FQ on central dopaminergic neuronal activities and prolactin secretion

Effects of orphanin FQ (OFQ) on central dopaminergic (DA) neurons and serum prolactin (PRL) were examined in ovariectomized, estrogen-primed Sprague-Dawley rats. The activities of central DA neurons, including the tuberoinfundibular (TI), nigrostriatal, mesolimbic, and incertohypothalamic ones, were determined by measuring the levels of 3,4-dihydroxyphenylacetic acid (DOPAC), the major metabolite of dopamine, in their projection regions in the brain by HPLC plus electrochemical detection. Intracerebroventricular administration of OFQ lowered DOPAC levels in the median eminence (ME), striatum, nucleus accumbens, and hypothalamic paraventricular nucleus in a dose (0.01-10 microg)- and time (30-90 min)-dependent manner. In contrast, OFQ increased DOPAC in the suprachiasmatic nucleus and had no effect in the periventricular nucleus. Serum PRL levels exhibited a typical inverse relationship with the activity of TIDA neurons, as determined by DOPAC levels in the ME. In the afternoon, we observed an endogenous decrease of ME DOPAC level accompanied by a PRL surge in estrogen-primed female rats. Although OFQ caused further decrease of ME DOPAC in the afternoon, it failed to augment the PRL surge level. Although pretreatment of an antisense oligodeoxynucleotide against the opioid receptor-like receptor gene had no effect on basal ME DOPAC levels in the morning or afternoon, it attenuated the afternoon PRL surge. Furthermore, it blocked the effects of exogenous OFQ on ME DOPAC and serum PRL levels, whereas the sense or missense oligodeoxynucleotide had no effect. These results indicate that OFQ and its receptors may be involved in the regulation of central DA neuronal activity and PRL secretion.     

alpha-MSH and gamma-MSH inhibit IL-1beta induced activation of the hypothalamic-pituitary-adrenal axis through central melanocortin receptors

Alpha-melanocyte-stimulating hormone (alpha-MSH) is a neuroimmunomodulatory peptide that is involved in the control of host responses trough modulation of production and action of proinflammatory cytokines in inflammatory cells in the periphery and within the central nervous system (CNS). However, little is known about the receptors that mediate the modulatory effects of alpha-MSH in the CNS. The objective of the present study was to establish the specific melanocortin receptors involved in the inhibition by MSH peptides of IL-1beta-induced activation of the HPA. i.c.v. injection of 12.5 ng of IL-1beta caused significant changes in plasma corticosterone, as compared to basal levels. The treatment with gamma-MSH (1 microg), an MC3 receptor agonist, resulted in significant reduction of the IL-1beta-induced plasma corticosterone levels. Administration of the MC3/MC4 receptor antagonist SHU9119 blocked this effect. Besides, treatment with a high dose of alpha-MSH (1 microg) increased plasma corticosterone. When alpha-MSH was given at a lower dose (0.1 microg), it did not modify corticosterone levels but caused an inhibitory effect on the corticosterone release induced by IL-1beta. The administration of SHU9119 or a more selective MC4 receptor antagonist like HS014 blocked the effects of alpha-MSH. In conclusion, our results suggest that both alpha-MSH and gamma-MSH are capable of inhibiting the effect of the IL-1beta on the activation of HPA axis acting at the CNS, and that this effect is mediated by specific central melanocortin receptors.     

Alpha2-adrenoceptor mediated co-release of dopamine and noradrenaline from noradrenergic neurons in the cerebral cortex

Previous results suggest that extracellular dopamine (DA) in the rat cerebral cortex originates from dopaminergic and noradrenergic terminals. To further clarify this issue, dialysate DA, dihydroxyphenylacetic acid (DOPAC) and noradrenaline (NA) were measured both in the medial prefrontal cortex (mPFC) and in the occipital cortex (OCC), with dense and scarce dopaminergic projections, respectively. Moreover, the effect of the alpha2-adrenoceptor antagonist RS 79948 and the D2-receptor antagonist haloperidol on extracellular DA, DOPAC and NA was investigated. Extracellular DA and DOPAC concentrations in the OCC were 43% and 9%, respectively, those in the mPFC. Haloperidol (0.1 mg/kg i.p.) increased DA and DOPAC (by 35% and 150%, respectively) in the mPFC, but was ineffective in the OCC. In contrast, RS 79948 (1.5 mg/kg i.p.) increased NA, DA and DOPAC, both in the mPFC (by approximately 50%, 60% and 130%, respectively) and the OCC (by approximately 50%, 80% and 200%, respectively). Locally perfused, the DA transporter blocker GBR 12909 (10 micro m) was ineffective in either cortex, whereas desipramine (DMI, 100 micro m) markedly increased extracellular NA and DA in both cortices. The weak haloperidol effect on DA efflux was not enhanced after DA- and NA-transporter blockade, whereas after DMI, RS 79948 markedly increased extracellular NA, and especially DA and DOPAC in both cortices. The results support the hypothesis that most extracellular DA in the cortex is co-released with NA from noradrenergic terminals, such co-release being primarily controlled by alpha2-adrenoceptors.     

Interaction of alpha-melanotropin (alpha-MSH) and noradrenaline in the median eminence in the control of female sexual behavior

In the present study, we examined the effects of the injection of alpha-melanotropin (alpha-MSH), noradrenaline (NA), and dopamine in the median eminence of ovariectomized-adrenalectomized rats on female sexual behavior. The animals were primed with l0 microg of estradiol benzoate, and 52-54 h later they were injected into the median eminence with either 1 microl of artificial cerebrospinal fluid, 1 microg/rat alpha-MSH, 200 ng/rat NA, 200 ng or 2 microg/rat dopamine, in 1 microl of artificial cerebrospinal fluid. Both alpha-MSH and NA significantly stimulated sexual behavior. This effect was antagonized by two beta-adrenergic antagonists: propranolol (500 ng/rat) and metoprolol (400 ng/rat) applied 15 min before the alpha-MSH or NA. The alpha-adrenergic antagonist prazosine (500 ng/rat) was ineffective in reducing the effect of alpha-MSH. The vehicle and dopamine at both doses had no effect on sexual activity. These results indicate that alpha-MSH and NA in the median eminence stimulate female sexual behavior and that NA mediates the action of alpha-MSH via beta-receptors.     

Cholinergic mediation in the ventral tegmental area of alpha-melanotropin induced excessive grooming: changes of the dopamine activity in the nucleus accumbens and caudate putamen

The effect of alpha-melanotropin (alpha-MSH) on the rat mesolimbic dopaminergic activity was estimated by measuring the changes in dihydroxyphenyl acetic acid (DOPAC) and dopamine (DA) endogenous levels in the nucleus accumbens (Ac) and caudate putamen (CP). A marked increase of DOPAC/DA ratios resulting from an increase in DOPAC and decrease in DA levels was found in the Ac 30 and 65 min after bilateral alpha-MSH-injections (1 microgram) into the ventral tegmental area (VTA). Similar changes were observed in the CP 65 min post-injections. These peptide-induced changes were completely inhibited by a previous VTA injection of atropine (1 microgram), at a dose that totally blocked the alpha-MSH-induced excessive grooming and motor activation. These results confirms that alpha-MSH affects a cholinergic afferent to the VTA which modifies the mesolimbic dopaminergic system involved in the alpha-MSH/ACTH-induced behaviors.     

The anorexic effect of alpha-melanocyte-stimulating hormone is mediated by corticotrophin-releasing factor in chicks

Alpha-melanocyte-stimulating hormone (alpha-MSH) is recognized as an anorexic peptide in the brain of vertebrates, but its mechanism of action has not been identified in birds. Therefore, we investigated whether the anorexic effect of alpha-MSH is mediated by corticotrophin-releasing factor (CRF) in the domestic chick. Firstly, we found that intracerebroventricular (i.c.v.) injection of alpha-MSH dose dependently increased plasma corticosterone (CORT) concentration. This effect was partly attenuated by co-injection of astressin, a CRF receptor antagonist, demonstrating that alpha-MSH stimulated CORT secretion by activating CRF neurons. The alpha-MSH-elicited CORT release was not attenuated by the injection of agouti-related protein, an endogenous melanocortin-4 (MC4) receptor antagonist, suggesting that alpha-MSH stimulated CRF neurons through MC4 receptor-independent pathways. Finally, we found that the anorexic effect of alpha-MSH was partly attenuated by astressin. The present results suggest that the anorexic effect of alpha-MSH in the chick brain is mediated in part by activation of CRF neurons.     

Effects of MCH/NEI in the striatum and interactions between them and with alpha-MSH on IP3 levels

In this study, we investigated: (a) the effect of melanocyte concentrating hormone (MCH) and neuropeptide glutamine (E)-isoleusine-(I) (NEI) on IP(3) production on an "in vitro" model using slices containing caudate putamen (CP) and accumbens nuclei (ACC); and (b) the interaction between these peptides and with alpha-melanocyte-stimulating-hormone (alpha-MSH) on the production of this second messenger. Only MCH at the highest dose studied (3.6 microM) increased the production of IP(3), whereas at the low concentration (0.6 microM) it did not affect IP(3) levels. NEI and alpha-MSH at both concentrations tested (0.6 and 3.6 microM), did not affect IP(3) production either. However, when NEI or alpha-MSH (at 3.6 microM) were added together with 3.6 microM MCH, the increase in the IP(3) content induced by this last peptide was blocked.     

Effects of prolactin-releasing peptide on tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in estrogen-treated female rats

Both systemic and central effects of a newly discovered prolactin (PRL)-releasing factor (PRF), prolactin-releasing peptide (PrRP), were determined in this study. Systemic injection of PrRP (1 and 10 microg/rat, i.v.) stimulated PRL secretion in ovariectomized, estrogen-treated rats similar to the effect of another PRF, thyrotropin-releasing hormone (TRH). Pretreatment with a dopamine D2 receptor antagonist, sulpiride (1 microg/rat, i.v.), potentiated the stimulatory effect of both PrRP and TRH on PRL secretion. Using the double-labeling immunohistochemical method, PrRP-immunoreactive terminals were found in close contact with tyrosine-hydroxylase-immunoreactive neurons in the hypothalamic arcuate nucleus. Central administration of PrRP (0.1-1,000 ng/rat, i.c.v.) stimulated tuberoinfundibular but not nigrostriatal dopaminergic neuronal activity in 15 min. Levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence and striatum were used as indices for tuberoinfundibular dopaminergic (TIDA) and nigrostriatal dopaminergic neuronal activities, respectively. The serum PRL level, however, was not significantly changed. Similar treatment with TRH (10 ng/rat, i.c.v.) stimulated and inhibited TIDA neuronal activity and serum PRL, respectively, at 30 min. In summary, PrRP may play a role in both the central and peripheral control of PRL secretion.     

Neurotensin-induced activation of hypothalamic dopaminergic neurons is accompanied by a decrease in pituitary secretion of prolactin and alpha-melanocyte-stimulating hormone.

The effects of neurotensin on the activity of hypothalamic tuberoinfundibular and periventricular-hypophysial dopaminergic (DA) neurons, and on the secretion of pituitary hormones that are tonically regulated by these neurons (i.e. prolactin and alpha-melanocyte-stimulating hormone [alpha MSH], respectively) were examined in estrogen-primed ovariectomized rats. The activity of tuberoinfundibular and periventricular-hypophysial DA neurons was estimated by measuring concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the terminals of these neurons in the median eminence and intermediate lobe of the posterior pituitary, respectively. Intracerebroventricular administration of neurotensin caused a dose- and time-related increase in DOPAC concentrations in both the median eminence and intermediate lobe, and a concurrent decrease in plasma levels of prolactin and alpha MSH. These results suggest that neurotensin-induced inhibition of secretion of prolactin and alpha MSH from the pituitary may be due to the stimulatory action of this neuropeptide on the release of dopamine from tuberoinfundibular and periventricular-hypophysial neurons.