Neurohypophysial hormones

MicroRNA profiling in the mouse hypothalamus reveals oxytocin-regulating microRNA

Oxytocin (Oxt), produced in the hypothalamic paraventricular and supraoptic nuclei for transport to and release from the posterior pituitary, was originally discovered through its role in lactation and parturition. Oxt also plays important roles in the central nervous system by influencing various behaviors. MicroRNAs (miRNAs), endogenous regulators of many genes, are a class of small non-coding RNAs that mediate post-transcriptional gene silencing. We performed miRNA expression profiling of the mouse hypothalamus by deep sequencing. Among the sequenced and cross-mapped small RNAs, expression of known miRNAs and unknown miRNAs candidates were analyzed. We investigated in detail one miRNA, miR-24, and found that it is a novel regulator of Oxt and controls both transcript and peptide levels of Oxt. These results provide insights into potential neurohypophysial hormone regulation mediated by miRNAs.

Cardiac effects of oxytocin: is there a role for this peptide in cardiovascular homeostasis?

Oxytocin is well known for its role in reproduction. However, evidence has emerged suggesting a role in cardiovascular and hydroelectrolytic homeostasis. Although its renal effects have been characterized, the cardiac ones have not been much studied. Therefore, we aimed to investigate the cardiac effects of oxytocin both in vivo and in vitro. In unanesthetized rats (n=6) intravenous oxytocin (1 mug) decreased dP/dt(max) by 15% (P<0.05) and heart rate by 20% (P<0.001), at the first minute after injection. dP/dt(max) was still lower in OT-treated rats than in controls (n=8) after 15 min (P<0.05), while heart rate returned to control values after 5 min. In isolated hearts, oxytocin was able to promote negative inotropic and chronotropic effects. Perfusion with 10(-5), 10(-6) and 10(-7)M oxytocin resulted in approximately 60% (P<0.01), 25% (P<0.01) and 10% (P<0.05) reduction of left ventricle developed pressure, without effect in lower concentrations (10(-10) to 10(-8) M). Also, dP/dt(max) was reduced by 45 and 20% (10(-5) e 10(-6) M; P<0.01), while diastolic pressure raised and heart rate fell only with 10(-5)M oxytocin (P<0.05). Intravenous oxytocin (1 mug; n=6) increased arterial pressure by 22% at the first minute (+23+/-3 mm Hg; P<0.001), returning to control value thereafter. Thus, oxytocin is able to promote directly negative inotropic and chronotropic effects, but its in vivo effect also involves a reflex mechanism, originated from its pressor effect.

Distinct rat neurohypophysial nerve terminal populations identified by size, electrophysiological properties and neuropeptide content

Voltage-gated ion channels are critical to excitation-secretion coupling in nerve terminals. We have identified two distinct populations of rat neurohypophysial (NHP) terminals distinguished by size, neuropeptide content and electrophysiological properties, including resting membrane potential, action potential (AP) properties, and K+ current and Na+ current characteristics. In large terminals (10-16 microm diameter), resting membrane potential was more negative than in small terminals (5-9.9 microm; -61+/-4 mV vs. -55+/-3 mV; p<0.01), action potential amplitude was larger (69+/-4 mV vs. 53+/-3 mV; p<0.01), peak IK was larger (1460+/-90 pA vs. 1140+/-70 pA; p<0.05) with a more negative V1/2 for activation (-3.1 mV vs. -0.6 mV; p<0.05), and Na+ current density was greater (approximately 470 pA/pF vs. approximately 300 pA/pF; p<0.01) with more negative V1/2 values for activation from -70 or -90 mV holding potentials (-44 mV vs. -24 mV; p<0.01). A positive linear correlation between INa amplitude and terminal size showed an inflection at a diameter of 9.2 microm. Neuropeptide content was generally segregated into a population of small terminals (<10 microm diameter) containing predominantly vasopressin and a population of large terminals (> or =10 microm diameter) containing predominantly oxytocin (OT); a small fraction of terminals in each group contained both peptides. These findings suggest that electrophysiological differences between small vasopressin-containing and large oxytocin-containing neurohypophysial terminals may contribute to their observed differential firing and peptide release patterns.