Jorge Manzanares

3-methoxy-4-hydroxyphenylethyleneglycol concentrations in discrete hypothalamic nuclei reflect the activity of noradrenergic neurons

An analytical technique is described which permits the quantitation of picogram concentrations of 3-methoxy-4-hydroxyphenylethylene-glycol (MHPG) in acid hydrolyzed extracts of microdissected regions of the rat brain, and this procedure is used to determine if alterations in the activity of noradrenergic neurons are reflected by changes in the concentrations of MHPG in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the rat hypothalamus. MHPG was not detected in non-hydrolyzed samples of either the PVN or SON, but following acid hydrolysis (heating of samples at 94 degrees C for 5 min in 0.16 M perchloric acid) MHPG was detected in both of these regions. These results indicate that MHPG exists primarily as a conjugate in the PVN and SON. Neurotoxin-induced lesions of the ventral noradrenergic bundle decreased norepinephrine (NE) and MHPG concentrations in the PVN and SON, demonstrating that tissue levels of MHPG in these brain regions are dependent upon the presence of noradrenergic neurons. Electrical stimulation of the locus coeruleus increased MHPG concentrations in the PVN, but not in the SON, whereas electrical stimulation of the medial forebrain bundle increased MHPG concentrations in both of these regions. The alpha 2-adrenergic receptor antagonist idazoxan increased, while the alpha 2-adrenergic receptor agonist clonidine decreased MHPG concentrations in both the PVN and SON, but neither idazoxan nor clonidine altered NE concentrations in these regions. Immobilization of rats in the supine position increased MHPG concentrations in the PVN and SON, and this was accompanied by a decrease in NE concentrations in the SON.(ABSTRACT TRUNCATED AT 250 WORDS)

Sexual Differences in Kappa Opioid Receptor-Mediated Regulation of Tuberoinfundibular Dopaminergic Neurons

The purpose of the present study was to examine the acute effects of kappa opioid receptor blockade or activation on the activity of tuberoinfundibular dopaminergic (TIDA) neurons in gonadally-intact or castrated male and female rats. In the absence of drug treatment, the basal activity of TIDA neurons (accumulation of 3,4-dihydroxyphenylalanine, DOPA, in the median eminence after administration of a decarboxylase inhibitor) in male rats was approximately one third of that in diestrous females. In male rats, blockade of kappa opioid receptors following administration of the kappa antagonist norbinaltorphimine (NOR-BNI) increased the activity of TIDA neurons suggesting that these neurons are tonically inhibited by endogenous kappa opioids. By contrast, NOR-BNI had no effect on TIDA neuronal activity in gonadally-intact diestrous female rats, but increased the activity of these neurons in ovariectomized female rats. These results suggest that ovarian hormones block the inhibitory effects of endogenous kappa opioids on the activity of TIDA neurons. Activation of kappa opioid receptors following administration of the kappa agonist U-50,488 caused a dose-related decrease in TIDA neuronal activity in diestrous female rats. U-50,488 had no effect on TIDA neuronal activity in gonadally-intact male rats, but decreased the activity of these neurons in orchidectomized male rats. Taken together, these results reveal a sexual difference in the responsiveness of TIDA neurons to kappa opioid receptor agonists and antagonists, and suggest that gonadal steroid-induced gender differences in the basal activity of TIDA neurons may be due, in part, to differences in tonic inhibitory regulation of these neurons by endogenous kappa opioids.

Kappa-Opioid-Receptor-Mediated Regulation of α-Melanocyte-Stimulating Hormone Secretion and Tuberohypophysial Dopaminergic Neuronal Activity

The effects of the kappa-opioid receptor agonist trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene- acetamide methanesulfonate hydrate (U-50488) were examined on alpha-melanocyte-stimulating hormone (alpha-MSH) secretion and the activity of tuberohypophysial dopamine (DA) neurons in the male rat. Tuberohypophysial DA neuronal activity was estimated by measuring: (1) the rate of DA synthesis [accumulation of 3,4-dihydroxyphenylalanine (DOPA) following inhibition of aromatic L-amino acid decarboxylase], and (2) DA metabolism [concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC)] in the intermediate lobe of the pituitary. U-50488 produced a dose- and time-dependent increase in plasma concentrations of alpha-MSH which was accompanied by a decrease in the accumulation of DOPA and in the intermediate lobe. The effects of U-50488 were blocked by pretreatment with the DA agonist apomorphine but not by the beta-adrenergic antagonist propranolol. The effects of U-50488 on plasma alpha-MSH concentrations and intermediate-lobe DOPA accumulation were blocked by pretreatment with the selective kappa-opioid receptor antagonist nor-binaltorphimine. These results indicate that U-50488, by acting on kappa-opioid receptors, inhibits the activity of intermediate-lobe tuberohypophysial DA neurons, and through this action increases the secretion of alpha-MSH from melanotrophs.

Neurochemical evidence that estrogen-induced suppression of kappa-opioid-receptor-mediated regulation of tuberoinfundibular dopaminergic neurons is prolactin-independent.

The purpose of the present study was to examine the role of estrogen and prolactin in determining the responsiveness of tuberoinfundibular dopaminergic (TIDA) neurons to kappa-opioid receptor blockade in female rats. TIDA neuronal activity was estimated by measuring either dopamine synthesis [accumulation of 3,4-dihydroxyphenylalanine (DOPA) 30 min after the administration of the decarboxylase inhibitor NSD-1015] or metabolism [concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC)] in terminals of these neurons in the median eminence. Blockade of kappa-opioid receptors with the selective kappa-antagonist norbinaltorphimine (NOR-BNI) increased the concentrations of DOPAC in the median eminence of ovariectomized rats but had no effect in gonadally intact rats, suggesting that loss of endogenous ovarian hormones following ovariectomy results in an increase in kappa-opioid-receptor-mediated inhibition of TIDA neurons. Estrogen administration ot ovariectomized rats blocked NOR-BNI-induced increases in median eminence DOPAC concentrations, whereas treatment of gonadally intact or ovariectomized, estrogen-treated rats with prolactin antiserum had no effect on the insensitivity of these neurons to NOR-BNI. Administration of antiserum to dynorphin A1-8 increased DOPA accumulation in the median eminence of ovariectomized but not estrogen-treated ovariectomized rats. Taken together, these results reveal that estrogen, acting via a prolactin-independent mechanism, suppresses kappa-opioid-receptor-mediated inhibition of the activity of TIDA neurons, possibly by decreasing the release of endogenous dynorphin.

Effects of immunoneutralization of dynorphin1–17 and dynorphin1–18 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.

Activation of tuberohypophysial dopamine neurons following intracerebroventricular administration of the selective kappa opioid receptor antagonist nor-binaltorphimine

The effect of the kappa opioid receptor antagonist nor-binaltorphimine (NOR-BNI) was examined on the activity of dopamine (DA) neurons comprising the nigrostriatal, mesolimbic, and tuberohypophysial systems in the male rat. DA neuronal activity was estimated by measuring: (1) the concentration of the DA metabolite 3,4-dihydroxyphenylacetic acid and, (2) the accumulation of 3,4-dihydroxyphenylalanine after administration of a decarboxylase inhibitor in brain (striatum, nucleus accumbens) and pituitary regions (intermediate lobe, neural lobe) containing terminals of these neurons. The intracerebroventricular administration of NOR-BNI produced a dose- and time-related increase in the activity of tuberohypophysial DA neurons, but failed to alter the activity of nigrostriatal or mesolimbic DA neurons. The ability of NOR-BNI to enhance the activity of tuberohypophysial DA neurons was blocked by the kappa opioid agonist U-50,488. These results indicate that NOR-BNI, acting on kappa opioid receptors, activates tuberohypophysial DA neurons projecting to the neural and intermediate lobes of the pituitary.

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.

Sexual differences in the activity of periventricular-hypophysial dopaminergic neurons in rats.

The activities of periventricular-hypophysial dopaminergic (DA) neurons were compared in male and female rats by measuring dopamine synthesis (accumulation of 3,4-dihydroxyphenylalanine [DOPA] after inhibition of L-aromatic amino acid decarboxylase) and metabolism (concentrations of 3,4-dihydroxyphenylacetic acid [DOPAC]) in terminals of these neurons in the intermediate lobe of the pituitary. For comparison, the synthesis and metabolism of dopamine in the neural lobe of the pituitary and median eminence were also determined. The concentrations of DOPAC and accumulation of DOPA were higher in females than in males in both the intermediate lobe and median eminence, revealing a sexual difference in the basal activity of periventricular-hypophysial and tuberoinfundibular DA neurons. In contrast, there were no differences between male and female rats in activity of DA neurons terminating in the neural lobe. One week following gonadectomy, DOPA accumulation in the median eminence was decreased in females and increased in males, but remained unchanged in the intermediate lobe. These results indicate that sexual differences in the activity of periventricular-hypophysial DA neurons terminating in the intermediate lobe are not dependent upon the presence of circulating gonadal steroids, and in this respect, these neurons differ from tuberoinfundibular DA neurons.

Kappa opioid receptor-mediated regulation of prolactin and a-melanocyte-stimulating hormone secretion in male and female rats

Abstract The purpose of this study was to examine the effects of the kappa opioid receptor agonist U-50,488 and antagonist nor-binaltorphimine (NOR-BNI) on the secretion of prolactin and a -melanocyte-stimulating hormone ( a MSH) in male and female rats. Activation of kappa opioid receptors with U-50,488 increased plasma prolactin concentrations in both male and female rats, whereas blockade of kappa opioid receptors with NOR-BNI decreased plasma prolactin concentrations in male, but not in female rats. U-50,488 also increased and NOR-BNI decreased plasma concentrations of a MSH; similar effects were observed in both male and female rats. These results reveal a sexual difference in kappa opioid receptor-mediated regulation of prolactin, but not a MSH secretion.

Noradrenergic innervation to the VMN or MPN is not necessary for lordosis

The purpose of the present study was to determine the importance of noradrenergic neurons terminating in the ventromedial nucleus (VMN) and medial preoptic nucleus (MPN) of the hypothalamus for lordosis behavior in ovariectomized, estrogen/progesterone-treated female rats. Seven days following bilateral injections of the noradrenergic neurotoxin 5-amino-2,4-dihydroxy-alpha-methylphenylethylamine (5-ADMP) into the ventral noradrenergic bundle (VNAB), norepinephrine (NE) concentrations (ng/mg protein) were reduced to 30-35% of control in the VMN and MPN. 5-ADMP-induced lesions of the VNAB also reduced lordosis quotients in these animals, and this effect was reversed by intracerebral ventricular administration of the alpha 1-adrenergic receptor agonist phenylephrine. These results indicate that neurotoxin-induced disruption of noradrenergic neurons in the VNAB is associated with a deficit in sexual receptivity in female rats. To determine if the reduction in sexual receptivity following 5-ADMP-induced lesions of the VNAB resulted from loss of noradrenergic neuronal projections specifically to the VMN or MPN, lordosis quotients were determined in ovariectomized, estrogen/progesterone-treated rats in which noradrenergic terminals in these hypothalamic nuclei were selectively lesioned. Injection of 5-ADMP directly into either the VMN or MPN reduced NE concentrations to 17% of control in these hypothalamic nuclei, but failed to alter lordosis. Furthermore, injection of phenylephrine into either the VMN or MPN of VNAB-lesioned rats failed to reinstate lordosis to the levels comparable to sham-lesioned controls. Taken together, these results indicate that noradrenergic neurons terminating in either the VMN or MPN are not essential for gonadal steroid induction of sexual receptivity in ovariectomized female rats.