Vasoactive intestinal polypeptide excitation of cerebral cortical neurons: lack of synergism with adenosine and noradrenaline

Vasoactive intestinal polypeptide (VIP), applied iontophoretically, excited 40% of the spontaneously firing rat cortical neurons tested. No neurons were depressed by VIP. When applied simultaneously with adenosine or noradrenaline, VIP depressed the firing of cortical neurons, but this depression could be reproduced by the passage of similar positive currents through a 50 mM NaCl-containing barrel of the multiple barrelled micropipette. VIP, therefore, excited rat cortical neurons and no depressant actions were apparent when VIP was applied together with adenosine or noradrenaline. Leakage of adenosine or noradrenaline during iontophoretic applications of the peptide may account for the reported inhibitory actions of VIP.