Ionic channels involved in the LHRH and SRIF release from rat mediobasal hypothalamus.

A superfusion system was used in order to investigate the ionic requirements of luteinizing hormone releasing hormone (LHRH) and somatostatin (SRIF) release from mediobasal hypothalamic (MBH) slices of male adult rats. Slices were superfused with Hepes-buffered Locke medium at 37 °C in an atmosphere of Ch 95%–CO2 5% for 1 h. Bacitracin (2 × 10–5M) was added to the medium to prevent enzymatic degradation of neuropeptides. Depolarizing agents such as potassium (K+) or veratridine stimulated LHRH and SRIF release in a dose-dependent manner. Maximal effect was obtained with K+ 56 mM and veratridine 50 µM. The depolarizing effect of K+ 56 mM was specific and not due to the hypertonicity of the medium used. Neither Mg2+ nor chlorine was needed for the spontaneous or K+-evoked release of LHRH and SRIF. The amplitude of the secretory response to K+ 56 mM was related to Ca2+ concentration tested in the range of 0.2–8.8 mM; maximal responses were obtained between 0.8 and 1.8 mM. Removal of Ca2+ from the medium with or without replacement by Mg2+, as well as administration of voltage-sensitive Ca2+ channel blockers (D-600 10–4M, Mn2+ 3 mM) blocked both K+ and veratridine induced neuropeptide release. When sodium channels and the ‘early’ calcium channels were blocked by tetrodotoxin (5 × 10–7M) the stimulatory effect of veratridine was completely blocked whereas the stimulation of K+ was unaffected. These experiments indicate that: (1) both K+ and veratridine induced-LHRH and SRIF release is a Ca2+-dependent process; (2) Ca2+ concentration is critical for the amplitude of the secretory response; (3) the main Ca2+ channel involved in neuropeptide release corresponds to the voltage-sensitive Ca2+ channel, and (4) neither magnesium or chlorine is needed for either spontaneous or evoked release of LHRH and SRIF.