Endocannabinoid-mediated presynaptic plasticity regulates postnatal acquisition of spatial navigation

Abstract

Long-term depression (LTD) of inhibitory responses, as mediated by endocannabinoid (eCB), plays an important role on presynaptic plasticity in the mammalian brain. However, the involvement of eCB on developmental plasticity of the vestibular system remains unclear. We hypothesize that eCB regulates the efficacy of GABAergic synapses in the vestibular nucleus (VN) via the modulation of LTD, thereby contributing to spatial recognition. With the use of whole-cell patch-clamp experiments, GABAA receptor-mediated evoked-IPSCs were recorded on brainstem slice of postnatal rats. In normal P5-8 rats, LTD was induced in 70% of the neurons in the medial VN but was reduced to 20% in P9-17 rats. The proportion of neurons showing LTD at P5-8 decreased to 30% with bath application of AM251 (antagonist of CB1 receptor). Similar results were observed in P5-8 rats preimplanted at P1 with AM251-loaded Elvax slice that allowed slow release of the drug into the underlying VN. In normal P9-11 rats, the proportion of neurons showing LTD increased to 70% with bath application of WIN55 (agonist of CB1R). Similar results were observed in P9-11 rats preimplanted at P1 with WIN55-loaded Elvax slice. Conversely, the proportion decreased to 20% in P12-17 rats pretreated with WIN55 at P1. That a custom-made blocking G-protein receptor kinase (GRK) blocking peptide could increase the occurrence of LTD to 90% in P12-17 rats pretreated with WIN55 at P1 further suggests that postnal desensitization of CB1R operates through the GRK pathway. In vitro experiments were also conduted to determine the receptor that triggers postsynaptic production and release of eCB. Agonist/antagonist of Group I metabotropic glutamic receptor exerts no effect on LTD induction. On the other hand, agonist of 5-HT receptor increased the proportion of LTD-expressing VN neurons in P9-11 rats while antagonists of 5-HT receptor decreased the proportion in P5-8 rats, suggesting that 5-HT receptor is involved in triggering eCB release. Furhtermore, using double-labeled immunofluorescence, we found that the expression of CB1R decreased during postnatal development. Pretreatment with WIN55 or AM251 could increase or decrease the expression of CB1R, respectively. To investigate the effects of eCB in the expression of vestibular behavior, rats preimplanted at P1 with AM251-or WIN55-loaded Elvax were tested with negative geotaxis, an indicator for gravity-detection. Normal rats acquire the reorienting behavior by P7. The emergence of this behavior was accelerated to P5 in AM251-treated rats, but delayed to P9 in WIN55-treated rats. In the dead-reckoning test, we found that rats pretreated with WIN55 at P1 showed difficulty in finding their way home (i.e. increase in heading angle and return time). That drug perturbation done at P8 and P12 showed no effect on spatial navigation, suggest a postnatal critical period for the regulation of spatial navigation. Altogether, our findings indicate that eCB modulates the efficacy of GABAergic synapses within the VN in an age-dependent manner, thereby regulating activities of neural circuitry responsible for the expression of vestibular behavior.