Effects of VPAC2 Receptor Activation on Membrane Excitability and GABAergic Transmission in Subparaventricular Zone Neurons Targeted by Suprachiasmatic Nucleus

Jul 3, 2009Journal of neurophysiology

How activating VPAC2 receptors changes electrical activity and inhibitory signals in neurons controlled by the brain’s internal clock

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Abstract

Approximately 50% of subparaventricular zone (SPZ) neurons responding to inputs from the suprachiasmatic nucleus (SCN) exhibited membrane depolarization when exposed to vasoactive intestinal polypeptide (VIP).

Bath-applied VIP (0.5-1 microM) caused depolarization in SPZ neurons, indicating the presence of postsynaptic receptors that activate a nonselective cationic conductance.
A subset of SPZ neurons showed increased amplitude of inhibitory postsynaptic currents (IPSCs) from SCN, suggesting presynaptic receptors facilitate GABA release.
The observed effects could be mimicked by the selective VPAC2 receptor agonist BAY 55-9837 (0.2-0.5 microM) and partially blocked by the VIP receptor antagonist VIP(6-28) (5 microM).
Findings suggest that VIP may modulate GABAergic signaling to different subpopulations of SPZ neurons through both post- and presynaptic VPAC2 receptors.

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The hypothalamic suprachiasmatic nucleus (SCN) harbors the master circadian pacemaker. SCN neurons produce the amino acid gamma-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 approximately 50% of the SPZ neurons receiving monosynaptic GABAA receptor-mediated inputs from SCN, bath-applied VIP (0.5-1 microM) resulted in a membrane depolarization caused by tetrodotoxin-resistant inward currents reversing at approximately -23 mV. These data suggest the existence of postsynaptic receptors that activate a nonselective cationic conductance. In addition, a subset of SPZ neurons showed an increase in the amplitude of SCN-evoked GABAergic inhibitory postsynaptic currents (IPSCs) and a decrease in their paired-pulse ratios. This, together with an increase in frequency of spontaneous and miniature IPSCs, implies the presence of presynaptic receptors that facilitate GABA release from SCN and possibly other synaptic terminals. The effects occurred in separate neurons and could be mimicked by the selective VPAC2 receptor agonist BAY 55-9837 (0.2-0.5 microM) and partially blocked by the VIP receptor antagonist VIP(6-28) (5 microM). The results indicate that VIP acts via both post- and presynaptic VPAC2 receptors to differentially modulate SCN GABAergic signaling to distinct subpopulations of SPZ neurons.

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Effects of VPAC2 Receptor Activation on Membrane Excitability and GABAergic Transmission in Subparaventricular Zone Neurons Targeted by Suprachiasmatic Nucleus

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