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Acute synaptic modulation by nicotinic agonists in developing cerebellar Purkinje cells of the rat

Kazuyoshi Kawa

The synaptic properties of the immature mammalian cerebellum were studied with a focus on the nicotinic modulation of synaptic transmission. Synaptic currents in Purkinje neurones were recorded using whole-cell patch electrodes applied to cerebellar slices (200 µm thick) obtained from newborn rats at postnatal days 5-10 (P5-P10). When the membrane potential of a Purkinje cell was held at -40 mV, spontaneous synaptic currents occurring in the cell comprised both inward and outward components. The former was glutamatergic and the latter was GABAergic, as confirmed by measuring reversal potentials and by using the specific glutamate and GABA blockers, 6-cyano-7-nitroquinoziline-2,3-dione and bicuculline, respectively. Application of ACh (0.1-1000 µM) from a 'Y tube' enhanced the occurrence of both glutamatergic and GABAergic synaptic currents in Purkinje cells. These responses appeared within 1 s after the application of ACh, and they were mimicked by nicotinic agonists (10 µM nicotine, 10 µM cytisine, 10 µM 1,1-dimethyl-4-phenyl-piperazinium iodide, or 10 nM epibatidine), but were sensitive to a specific nicotinic antagonist (1 µM dihydro--erythroidine). When the generation of action potentials by cerebellar neurones in the slice preparation was blocked by the addition of TTX (1 µM) to the external saline, these ACh-induced responses almost disappeared. This indicates that the enhanced synaptic activities in Purkinje cells are induced via presynaptic nicotinic receptors on the excitatory and inhibitory interneurones, presumably on the proximal axons or somatodendritic domains of granule cells and basket cells in the cerebellar cortex. Interestingly, these nicotinic effects were remarkable in immature rats (P5-P10), but were barely detectable in older rats (more than 10 days of age), indicating that nicotinic ACh receptors are regulated developmentally and may play a novel role in the maturing cerebellum.

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