Effects of VPAC2 receptor activation on membrane excitability and GABAergic transmission in subparaventricular zone neurons targeted by suprachiasmatic nucleus.

The hypothalamic suprachiasmatic nucleus (SCN) harbors the master circadian pacemaker. SCN neurons produce the amino acid γ-aminobutyric acid (GABA) and several peptide molecules for coordination and communication of their circadian rhythms. A subpopulation of SCN cells synthesizes vasoactive intestinal polypeptide (VIP) and provides a dense innervation of the subparaventricular zone (SPZ), an important CNS target of the circadian pacemaker. In this study, using patch-clamp recording techniques and rat brain slice preparations, the contribution of VIP to SCN efferent signaling to SPZ was evaluated by examining membrane responses of SPZ neurons to exogenous VIP receptor ligands. In ∼50% of the SPZ neurons receiving monosynaptic GABAA receptor–mediated inputs from SCN, bath-applied VIP (0.5–1 μM) resulted in a membrane depolarization caused by tetrodotoxin-resistant inward currents reversing at ∼−23 mV. These data suggest the existence of postsynaptic receptors that activate a nonselective cationic conducta...