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Studies on GABAergic synaptic transmission in neurons of the deep cerebellar nuclei

University of British Columbia

In the cerebellum, the corticonuclear projection subserves as the major efferent pathway for the cerebellar cortical networks. This pathway, consists of a direct axonal projection from the Purkinje cells to the neurons of the deep cerebellar nuclei (DCN). It has been demonstrated both in vivo and in vitro that stimulation of the Purkinje cell axons exerts a powerful inhibitory influence on DCN neurons mediated by the neurotransmitter, ƴ-aminobutyric acid (GABA). However, despite the wealth of anatomical and biochemical information, few electrophysiological studies have been done to characterize GABAergic synaptic transmission in DCN neurons. For example, it is not clear whether synaptic release of GABA activates pre- or postsynaptic GABAB receptors despite the finding that GABAB binding sites are present in the DCN. Furthermore, although GABAergic transmission in the DCN exhibits paired-pulse, frequency-dependent, as well as long-term depressions, the mechanisms underlying these plasticity's are yet to be resolved. In the present study, both perforated and whole-cell patch clamp recording techniques were utilized to determine whether preand postsynaptic GABAB receptors are present in the DCN and to test if endogenous release of GABA can activate either of the receptors. In addition, the contribution of GABAB receptors to paired-pulse and frequency-dependent depression of the deep nuclear inhibitory postsynaptic current (IPSC) was also assessed. Finally, experiments were conducted to investigate the properties of a tetanic stimulation-induced deep nuclear long-term depression (LTD) of the IPSC and to examine the role of Ca^2+ and protein phosphatases as potential mediators of the sustained depression. The results of the studies indicated that postsynaptic GABAB receptors are present on the membrane of DCN neurons. Activation of these receptors produces a G-protein-dependent response similar to that observed in other central neurons. In addition, presynaptic GABAB receptors are also present in the DCN. Activation of these receptors produces a suppression of deep nuclear IPSCs. However, deep nuclear preand postsynaptic GABAB receptors were found not to be activated by endogenous release of GABA. Furthermore, these receptors appear not to be involved in pairedpulse and frequency-dependent depressions of the IPSC. In voltage-clamped DCN neurons, LTD of the IPSC was induced reliably if the LTD-inducing train was delivered under current-clamp conditions where the membrane potential was allowed to fluctuate. Using this protocol in subsequent experiments, it was found that currents elicited by iontophoretic applications of THIP, a GABAA agonist, also exhibited LTD following a tetanic stimulation of the input. It was also demonstrated that LTD can be induced heterosynaptically. Furthermore, activation of the IPSC during the train was not required for LTD to occur. However, postsynaptic Ca^2+ accumulations via influx though A/-methyl-D-aspartate receptor-gated channels and/or voltage-gated Ca^2+ channels appear to play an important role in the generation of LTD. Moreover, protein phosphatase activity appears to be necessary for the induction of the depression. It is concluded that postsynaptic mechanisms contribute to LTD of GABAergic transmission in neurons of the DCN. Bhagavatula R. Sastry, Ph.D., Research Supervisor.

Thesis/Dissertation

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2009-02-18

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http://hdl.handle.net/2429/4783

Pharmaceutical Sciences

University of British Columbia

UBCV

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