ROLE OF Q-TYPE CA2+ CHANNELS IN VASOPRESSIN SECRETION FROM NEUROHYPOPHYSEAL TERMINALS OF THE RAT

1. The nerve endings of rat neurohypophyses were acutely dissociated a nd a combination of pharmacological, biophysical and biochemical techn iques was used to determine which classes of Ca2+ channels on these ce ntral nervous system (CNS) terminals contribute functionally to argini ne vasopressin (AVP) and oxytocin (OT) secretion. 2. Purified neurohyp ophysial plasma membranes not only had a single high-affinity binding site for the N-channel-specific omega-conopeptide MVIIA, but also a di stinct high-affinity site for another omega-conopeptide (MVIIC), which affects both N- and P/Q-channels. 3. Neurohypophysial terminals exhib ited, besides L- and N-type currents, another component of the Ca2+ cu rrent that was only blocked by low concentrations of MVIIC or by high concentrations of omega-AgaIVA, a P/Q-channel-selective spider toxin. 4. This Ca2+ current component had pharmacological and biophysical pro perties similar to those described for the fast-inactivating form of t he P/Q-channel class, suggesting that in the neurohypophysial terminal s this current is mediated by a 'Q'-type channel. 5. Pharmacological a dditivity studies showed that this Q-component contributed to rises in intraterminal Ca2+ concentration ([Ca2+](i)) in only half of the term inals tested. 6. Furthermore, the non-L- and non-N-component of Ca2+-d ependent AVP release, but not OT release, was effectively abolished by the same blockers of a-type current. 7. Thus Q-channels are present o n a subset of the neurohypophysial terminals where, in combination wit h N- and L-channels, they control BVP but not OT peptide neurosecretio n.