Two actions of vasopressin on neurons in the rat ventral hippocampus: a microiontophoretic study

Vasopressin (VP), applied by brief iontophoretic pulses on ventral hippocampus neurons in vivo, excited approximately 30% of the neurons tested. Glutamate (Glu) and acetylcholine (ACh) excited nearly all neurons recorded. A selective antagonist of vasopressin V1 receptors suppressed the VP-induced excitation and, in addition, suppressed the excitations induced by Glu but not those by ACh. The specificity of the action in the brain of this VP antagonist must therefore be doubted. Two excitatory amino acid antagonists, D(-)-2-amino-5-phosphonovaleric acid (2APV) and glutamic acid diethyl ester (GDEE), suppressed the responses to Glu and also those to VP. ACh excitations, tested in the same neurons, were little affected by 2APV and GDEE. The remaining 70% of VH neurons were not excitable with VP. However, the responses of these neurons to Glu but not to Ach, increased markedly both while the peptide was released and for tens of minutes thereafter. The increase in Glu responses induced by VP could not be prevented by the VP or excitatory amino acid receptor antagonists applied before the peptide. The possibility that the excitation and the potentiation of Glu responses caused by VP originated from two different actions of the peptide is discussed.