GHSR constitutive activity impairs voltage-gated calcium channel (Ca V )-dependent inhibitory neurotransmission in hippocampal neurons

JESICA, RAINGO; SUSANA, RODRÍGUEZ SILVIA; VALENTINA, MARTÍNEZ DAMONTE

THE JOURNAL OF PHYSIOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2018

0022-3751

KEY POINTS:Presynaptic voltage-gated calcium channels (CaV 2) link action potentials arriving at the presynaptic terminal to neurotransmitter release. Hence, their regulation is essential to fine tuning brain circuitry. CaV 2 are highly sensitive to G protein-coupled receptor (GPCR) modulation. Our previous data indicated that ghrelin receptor GHSR constitutive activity impairs CaV 2 by decreasing their surface density. We present compelling support for the impact of CaV 2.2 inhibition by agonist-independent GHSR activity exclusively on GABA release in hippocampal cultures. We found that this selectivity arises from a high reliance of GABA release on CaV 2.2 rather than on CaV 2.1. Our data provides new information on the effects of the ghrelin/GHSR system on synaptic transmission, suggesting a putative physiological role of the constitutive signaling of a GPCR that is expressed at high levels in brain areas with restricted access to its natural agonist.ABSTRACT:GHSR (growth hormone secretagogue receptor) displays high constitutive activity, independent from its endogenous ligand, ghrelin. Unlike ghrelin-induced GHSR activity, the physiological role of GHSR constitutive activity and the mechanisms that underlie GHSR neuronal modulation remain elusive. We previously demonstrated that GHSR constitutive activity modulates presynaptic voltage-gated calcium channels (CaV 2). Here we postulate that GHSR constitutive activity-mediated modulation of CaV 2 could be relevant in the hippocampus since this brain area has high GHSR expression but restricted access to ghrelin. We performed whole-cell patch-clamp in hippocampal primary cultures from E16-18 day old C57BL6 wild type and GHSR deficient mice after manipulating GHSR expression with lentiviral transduction. We found that GHSR constitutive activity impairs CaV 2.1 and CaV 2.2 native calcium currents and that CaV 2.2 basal impairment leads to a decrease in GABA but not glutamate release. We postulated that this selective effect is related to a higher CaV 2.2 over CaV 2.1 contribution to GABA release (∼40% for CaV 2.2 in wild type vs. ∼20% in wild type GHSR overexpressing cultures). This effect of GHSR constitutive activity is conserved in hippocampal brain slices, where GHSR constitutive activity reduces local GABAergic transmission of the granule cell layer (intra-granule cell inhibitory postsynaptic currents (IPSCs) size ∼ -67 pA in wild type vs. ∼ -100 pA in GHSR deficient mice), whereas the glutamatergic output from the dentate gyrus to CA3 remains unchanged. In summary, we found that GHSR constitutive activity impairs IPSCs both in hippocampal primary cultures and in brain slices through a CaV 2-dependent mechanism without affecting glutamatergic transmission. This article is protected by copyright. All rights reserved.