Novel regulator of vasopressin secretion: phoenixin

The newly described hypothalamic peptide, phoenixin, is produced in the hypothalamus and adenohypophysis, where it acts to control reproductive hormone secretion. Both phoenixin and its receptor GPR173 are expressed in the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei, suggesting additional, nonreproductive effects of the peptide to control vasopressin (AVP) or oxytocin (OT) secretion. Hypothalamo-neurohypophysial explants released AVP but not OT in response to phoenixin. Intracerebroventricular administration of phoenixin into conscious, unrestrained male and female rats significantly increased circulating AVP, but not OT, levels in plasma, and it increased immediate early gene expression in the supraoptic nuclei of male rats. Bath application of phoenixin in hypothalamic slice preparations resulted in depolarization of PVN neurons, indicating a direct, neural action of phoenixin in the hypothalamus. Our results suggest that the newly described, hypothalamic peptide phoenixin, in addition to its effects on hypothalamic and pituitary mechanisms controlling reproduction, may contribute to the physiological mechanisms regulating fluid and electrolyte homeostasis.

Hormone secretion in transgenic rats and electrophysiological activity in their gonadotropin releasing-hormone neurons

Expression of GFP in GnRH neurons has allowed for studies of individual GnRH neurons. We have demonstrated previously the preservation of physiological function in male GnRH-GFP mice. In the present study, we confirm using biocytin-filled GFP-positive neurons in the hypothalamic slice preparation that GFP-expressing somata, axons, and dendrites in hypothalamic slices from GnRH-GFP rats are GnRH1 peptide positive. Second, we used repetitive sampling to study hormone secretion from GnRH-GFP transgenic rats in the homozygous, heterozygous, and wild-type state and between transgenic and Wistar males after ∼4 yr of backcrossing. Parameters of hormone secretion were not different between the three genetic groups or between transgenic males and Wistar controls. Finally, we performed long-term recording in as many GFP-identified GnRH neurons as possible in hypothalamic slices to determine their patterns of discharge. In some cases, we obtained GnRH neuronal recordings from individual males in which blood samples had been collected the previous day. Activity in individual GnRH neurons was expressed as total quiescence, a continuous pattern of firing of either low or relatively high frequencies or an intermittent pattern of firing. In males with both intensive blood sampling (at 6-min intervals) and recordings from their GnRH neurons, we analyzed the activity of GnRH neurons with intermittent activity above 2 Hz using cluster analysis on both data sets. The average number of pulses was 3.9 ± 0.6/h. The average number of episodes of firing was 4.0 ± 0.6/h. Therefore, the GnRH pulse generator may be maintained in the sagittal hypothalamic slice preparation.

The anorexigenic and hypertensive effects of nesfatin-1 are reversed by pretreatment with an oxytocin receptor antagonist

Nesfatin-1 is an 82-amino acid protein encoded by the nucleobindin2 gene. When injected intracerebroventricularly, nesfatin-1, via a melanocortin ¾ receptor-dependent mechanism, potently decreased both food and water intakes and elevated mean arterial pressure in a dose-related manner. Because nesfatin-1 colocalized with oxytocin in hypothalamus and because nesfatin-1 had direct depolarizing effects on oxytocin-producing neurons in hypothalamic slice preparations, we hypothesized that the actions of nesfatin-1 required the presence of functional oxytocin receptors. We, therefore, pretreated conscious, unrestrained male rats with the oxytocin receptor antagonist, ornithine vasotocin (OVT), before treatment with nesfatin-1. We found that pretreatment with OVT reversed the effects of nesfatin-1 on both food and water intake and on mean arterial pressure, indicating that the central oxytocin system is a downstream mediator of these actions of nesfatin-1. Additionally, we found that OVT reversed the anorexigenic effect of α-melanocyte-stimulating hormone (α-MSH), suggesting that the central oxytocin system is downstream of the central melanocortin system. Taken together, these data suggest that nesfatin-1 acts through a serial neuronal circuit, in which nesfatin-1 activates the central melanocortin system, which, in turn, acts through the central oxytocin system, leading to an inhibition of food and water intake and an increase in mean arterial pressure.

Impact of Melatonin and Molecular Clockwork Components on the Expression of Thyrotropin β-Chain (Tshb) and the Tsh Receptor in the Mouse Pars Tuberalis

Photoperiodic regulation of reproduction in birds and mammals involves thyrotropin β-chain (TSHb), which is secreted from the pars tuberalis (PT) and controls the expression of deiodinase type 2 and 3 in the ependymal cell layer of the infundibular recess (EC) via TSH receptors (TSHRs). To analyze the impact of melatonin and the molecular clockwork on the expression of Tshb and Tshr, we investigated melatonin-proficient C3H wild-type (WT), melatonin receptor 1-deficient (MT1-/-) or clockprotein PERIOD1-deficient (mPER1-/-) mice. Expression of Tshb and TSHb immunoreactivity in PT were low during day and high during the night in WT, high during the day and low during the night in mPER1-deficient, and equally high during the day and night in MT1-deficient mice. Melatonin injections into WT acutely suppressed Tshb expression. Transcription assays showed that the 5′ upstream region of the Tshb gene could be controlled by clockproteins. Tshr levels in PT were low during the day and high during the night in WT and mPER1-deficient mice and equally low in MT1-deficient mice. Tshr expression in the EC did not show a day/night variation. Melatonin injections into WT acutely induced Tshr expression in PT but not in EC. TSH stimulation of hypothalamic slice cultures of WT induced phosphorylated cAMP response element-binding protein in PT and EC and deiodinase type 2 in the EC. Our data suggest that Tshb expression in PT is controlled by melatonin and the molecular clockwork and that melatonin activates Tshr expression in PT but not in EC. They also confirm the functional importance of TSHR in the PT and EC.