Inhibitory synaptic currents in rat cerebellar Purkinje cells: modulation by postsynaptic depolarization.

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Abstract
1. Synaptic currents were recorded in voltage-clamped cerebellar Purkinje cells using the tight-seal whole-cell recording technique. Cells were dialysed with a CsCl solution and were held at -60 or -70 mV. Inhibitory interneurones (basket and stellate cells) were stimulated using an extracellular pipette positioned in the molecular layer. Blockers of excitatory glutamatergic synapses were included in the bath solution. 2. Evoked synaptic currents were observed after a latency of 3-4 ms. The time course of synaptic currents could in most cases be fitted to a biexponential curve, with a rise time constant, tau on, of 1-3 ms and a decay time constant, tau off, of 7-13 ms. These currents were blocked by bicuculline. 3. The mean amplitude of evoked synaptic currents increased in discrete steps when the voltage applied to the stimulating pipette was increased. At each level, very prominent fluctuations of the amplitude were observed among trials. 4. Complex synaptic currents corresponding to repetitive activity of the presynaptic interneurone were occasionally observed, particularly with high intensity presynaptic stimulation. This repetitive activity could lead to bursts of synaptic currents lasting for several seconds. 5. Following a depolarizing voltage train in the postsynaptic Purkinje cell, the amplitude of evoked synaptic currents was first inhibited, and then potentiated. The inhibition was accompanied by a small but consistent increase in tau off and by no alteration in tau on. When using small intensity presynaptic stimuli, it was found that the probability of failures was greatly enhanced. The inhibitory phase lasted for about 1 min before giving way to potentiation. The potentiation returned to the control with a time to half-decay of 12.9 +/- 0.9 min. 6. The present results give further evidence to a previously proposed hypothesis that the inhibition produced by Purkinje cell depolarization is mainly presynaptic. The longer lasting potentiation, on the other hand, has most probably a postsynaptic origin. Cerebellar Purkinje cells receive inhibitory GABAergic inputs from two classes of interneurones located in the molecular layer (reviewed in Palay & Chan-Palay, 1974; Ito, 1984). Basket cells are closest to the Purkinje cell layer and address their inhibitory signal primarily to the soma and to the main dendrites. Stellate cells are more externally located and have contacts with the more distal part of the dendritic arborization of Purkinje cells.(ABSTRACT TRUNCATED AT 400 WORDS)