The role of endocannabinoid in long-term plasticity of vestibular GABAergic synapses and behavioral outcome in developing rats

Abstract

Endocannabinoid (eCB)-mediated long-term depression of inhibitory responses (iLTD) has emerged as a major theme of presynaptic plasticity in the mammalian brain. We hypothesize that eCB regulates the developmental plasticity of GABAergic synapses in the vestibular nucleus via the modulation of iLTD, thereby contributing to spatial recognition. Whole-cell patch-clamp experiments were performed on brainstem slice of postnatal rats. iLTD of GABAA receptor-mediated evoked-IPSCs was induced in 70% of the medial vestibular neurons tested in P5-7 rats but was reduced to 20% in P8-17 rats. In P5-7 rats, the proportion of neurons showing iLTD was reduced to 40% with bath application of AM251 (antagonist of cannabinoid CB1R). Similar results were obtained in P5-7 rats pre-implanted at P1 with AM251-loaded Elvax slice that allows controlled release of the drug into the underlying vestibular nucleus. In normal P8-10 rats, the proportion of neurons showing iLTD was increased to 60% with bath application of WIN55 (agonist of CB1R). Similar results were obtained in P8-10 rats pre-implanted at P1 with WIN55-loaded Elvax slice. However, the proportion was decreased to 20% in P11-17 rats pretreated with WIN55. To determine the receptor that triggers postsynaptic production and release of eCB, we applied to the bath antagonist/agonist of either Group I metabotropic glutamate receptor (mGluR) or serotonin receptor (5-HTR). When compared with the control, antagonist/agonist of Group I mGluR exerted no effect on iLTD induction. However, 5-HTR agonist increased the proportion of neurons expressing iLTD in P8-10 rats while 5-HTR antagonist decreased the proportion in P5-7 rats, suggesting that 5-HTR is involved in triggering eCB release from vestibular neurons. To attest the effects of eCB in the expression of vestibular behavior, rats pre-implanted at P1 with AM251- or WIN55-loaded Elvax were tested with negative geotaxis, a behavioral indicator of gravity-detection. When compared with normal rats which acquire the re-orienting behavior by P7, emergence of negative geotaxis was accelerated to P5 in AM251-treated rats but was delayed to P9 in WIN55-treated rats. Similar accelerated and delayed emergence of negative geotaxis was observed in rats pretreated with GABAA receptor agonist (muscimol) and antagonist (bicuculline) respectively. Our results therefore indicate that eCB plays an age-dependent role in modulating the efficacy of GABAergic synapses in the vestibular nucleus, thereby regulating activities of neural network responsible for the expression of gravity-detection behavior