Impact of agonist-independent ghrelin receptor (GHSR1a) activity on calcium-mediated signals in primary hippocampal neuron cultures during synaptogenesis

RAINGO, J.; MARTINEZ DAMONTE, V.; LÓPEZ SOTO EJ

Congreso; Congreso Anual de la Society for Neuroscience; 2016

The ghrelin receptor or growth hormone secretagogue receptor type 1a GHSR1a is a Gproteincoupled receptor highly expressed in several regions of the central nervous system both preandpostsynaptically.How GHSR1a controls neuronal activity is still poorly understood butmultiple mechanisms seem to be implicated. GHRS1a displays two active modes, one dependenton ghrelin binding, its endogenous ligand, and another one termed constitutive active modeindependent from ghrelin binding and relying only on GHSR1a expression levels. Recent datafrom our laboratory suggest that GHSR1a modulates presynapticneurotransmitter release bycontrolling presynapticvoltagegatedcalcium channels (CaV2) by two independent mechanismssignaling through two different cellular pathways: an acute, Gqdependentand reversible one,mediated by ghrelin binding to the GHSR1a, and a chronic one, mediated by a Gi/odependentreduction of CaV2 membrane density due to GHSR1a constitutive activity. CaV2 are fundamentalstructures for neurotransmission, as they allow calcium influx to the presynapticterminal inresponse to action potentials and thus trigger synaptic vesicles, SVs, fusion to the plasmamembrane containing neurotransmitters. In this context, we hypothesize that this constitutivemodulation could be relevant in synapse formation when the establishment of CaV2 sublocalizationis critical. Thus, we evaluated if GHSR1a expression modifies synapse activity atearly stages in primary hippocampal cultured neurons from wild type versus GHSR1a knockoutmouse embryos by wholecellpatch clamp experiments. We first recorded CaV2 total currentsand found larger CaV2 currents in GHSR1a knockout mice cultures versus wildtypemicecultures. We then evaluated miniature spontaneous post synaptic currents, mSPSCs. We alsostudied evoked inhibitory and excitatory post synaptic currents, IPSCs, at different days in vitro.We found that GHSR1a constitutive activity CaV currents reduction correlates with smaller IPSCs.Based on our data, we propose that GHSR1a constitutive active mode differentially modulatesthe timing of spontaneous and action potentialevokedneurotransmitter release.