Impact of agonist-independent ghrelin receptor (GHSR) activity on calcium-mediated neurotransmitter release in hippocampal neurons

JESICA RAINGO

Tokio

Simposio; Annual Meeting of the Japan Neuroscience Society 2017; 2017

Japan Neuroscience Society

The ghrelin receptor or growth hormone secretagogue receptor type 1a (GHSR1a) is a G-protein coupled receptor highly expressed in several regions of the central nervous system both pre- and post-synaptically. How GHSR1a controls neuronal activity is still poorly understood but multiple mechanisms seem to be implicated. GHRS1a displays two active modes, one dependent on ghrelin binding, its endogenous ligand, and another one termed constitutive active mode independent from ghrelin binding and relying only on GHSR1a expression levels. Recent data from our laboratory suggest that GHSR1a modulates pre-synaptic neurotransmitter release by controlling pre-synaptic voltage-gated calcium channels (CaV2) by two independent mechanisms signaling through two different cellular pathways: a rapid, Gq-dependent and reversible one, mediated by ghrelin binding to the GHSR1a, and a chronic one, mediated by a Gi/o-dependent reduction of CaV2 membrane density due to GHSR1a constitutive activity. CaV2 are fundamental structures for neurotransmission, as they allow calcium influx to the pre-synaptic terminal in response to action potentials and thus trigger synaptic vesicles (SVs) fusion to the plasma membrane containing neurotransmitters. In this context, we hypothesize that this constitutive modulation could be relevant in pre-synaptic functions. Thus, we evaluated if GHSR1a expression modifies synapse activity in primary hippocampal cultured neurons from wild type versus GHSR1a knockout mouse embryos by whole-cell patch clamp experiments. We also rescued the KO cultures with GHSR1a overexpression with a lentiviral vector and reduced GHSR1a expression level with a RNAi based strategy. We first recorded CaV2 total currents and found larger CaV2 currents in GHSR1a knockout mice cultures versus wild-type mice cultures. We then evaluated miniature spontaneous post synaptic currents (mSPSCs) and AP evoked inhibitory and excitatory post synaptic currents (I/EPSCs). Based on our data, we propose that GHSR1a constitutive active mode only modulates action potential-evoked neurotransmitter release at GABAergic synapses. We are currently investigating the mechanisms underlying this remarkable specific effect.