Endogenous inhibition of hippocampal LTD and depotentiation by vasoactive intestinal peptide VPAC1 receptors

Abstract

Vasoactive intestinal peptide (VIP), an important modulator of hippocampal synaptic transmission, influences exploration and hippocampal-dependent learning in rodents. Homosynaptic long-term depression (LTD) and depotentiation are two plasticity phenomena implicated in learning of behavior flexibility and spatial novelty detection. In this study, we investigated the influence of endogenous VIP on LTD and depotentiation induced by low-frequency stimulation (1 Hz, 900 pulses) of the hippocampal CA1 area in vitro in juvenile and young adult rats, respectively. LTD and depotentiation were enhanced by the VIP receptor antagonist Ac-Tyr(1) , D-Phe(2) GRF (1-29), and the selective VPAC1 receptor antagonist, PG 97-269, but not the selective VPAC2 receptor antagonist, PG 99-465. This action was mimicked by an anti-VIP antibody, suggesting that VIP, and not pituitary adenylate cyclase-activating polypeptide (PACAP), is the endogenous mediator of these effects. Selective inhibition of PAC1 receptors with PACAP (6-38) enhanced depotentiation, but not LTD. VPAC1 receptor blockade also revealed LTD in young adult rats, an effect abolished by the GABAA antagonist bicuculline, evidencing an involvement of GABAergic transmission. We conclude that inhibition of LTD and depotentiation by endogenous VIP occurs through VPAC1 receptor-mediated mechanisms and suggest that disinhibition of pyramidal cell dendrites is the most likely physiological mechanism underlying this effect. As such, VPAC1 receptor ligands may be considered promising pharmacological targets for treatment of cognitive dysfunction in diseases involving altered GABAergic circuits and pathological saturation of LTP/LTD like Down's syndrome and temporal lobe epilepsy.