Voltage-operated Ca2+ channels involved in K+-evoked release of vasoactiveintestinal polypeptide from the rat hypothalamus

Rat brain hypothalami were exposed to various depolarizing stimuli and vaso active intestinal polypeptide-like immunoreactivity (VIP-LI) release was me asured by means of a radioimmunoassay (RIA) procedure. Under conditions of noradrenergic blockade, exposure to high K+ (40-100 mM) produced dose-depen dent increases in the VIP-LI release in a Ca2+-dependent manner. Exposure t o veratridine (3-100 muM) also induced concentration-dependent increases in VIP-LI release, an effect that was Ca2+-dependent and tetrodotoxin (TTX)-s ensitive. Specific ligands for the L, N, and P/Q-type voltage-operated Ca2 channels (VOCCs) were used to determine which channel subtypes were involv ed in the K+-evoked VIP-LI release. The L-type VOCC ligand, nifedipine (10 muM), had no effect on release. In contrast, the N-type VOCC blocker, omega -conotoxin GVIA (omega -CgTx GVIA) (0.1-100 nM), markedly reduced the K+-e voked response, with maximal inhibition of approximately 60 +/- 8%, omega - Agatoxin IVA (omega -Aga IVA) (1-50 nM). which binds P-type and, at high do ses, also Q-type VOCCs, produced dose-dependent inhibition of up to 25 +/- 3%, while the maximal inhibition observed with the non-selective VOCCs liga nd, omega -conotoxin MVIIC (omega -CmTx MVIIC) (1 nM-3 muM), amounted to 85 +/- 8%. These findings indicate that N and P-type Ca2+ channels play predo minant roles in the high K+-evoked release of VIP-LI from the rat hypothala mus. (C) 2001 Elsevier Science Ltd. All rights reserved.