Effect of glutamate, aspartate and related derivatives on cerebellar Purkinje cell dendrites in the rat: an in vitro study

Abstract

The responses of Purkinje cells to short duration (pulse) ionophoretic applications of L-Asp, L-Glu, N-methyl DL-aspartate (NMDLA) and quisqualic acid in their dendritic fields were studied in vitro on sagittal slices of lobules IX and X of the adult rat cerebellum. Pulse application of L-Asp or L-Glu evoked transient and dose-dependent increases in the firing rate of the simple spikes recorded extracellularly as single units. When the ionophoretic electrode was positioned in the dendritic field of the Purkinje cells, the lowest thresholds for L-Glu and L-Asp mediated excitations of the cells were as low as 25 and 35 pC, respectively, with a latency for maximal responses as brief as 7 ms. In intracellular recordings these excitatory responses consisted of depolarizations of up to 18 mV in amplitude and with depolarizing slopes up to 0.52 mV/ms. They were generally unaccompanied by changes in cell input resistance in contrast to the marked decrease which occurred in response to steady applications of large doses of L-Asp and L-Glu. The spatial distribution of the excitatory sites confirmed that the dendritic sensitivity to L-Glu was greater than that of the soma and showed that the same was true for L-Asp. In 34% of cells the sensitivity for L-Asp declined markedly in the upper region of the molecular layer, while it remained high for L-Glu; no such differential sensitivity was detected in the remaining 66% of the cells. Inhibitory responses, antagonized by 10-5 M-bicuculline in the bath, were also induced in Purkinje cells by L-Glu and L-Asp when the ionophoretic electrode was withdrawn from the excitatory sites by as little as 8 .mu.m and up to 40 .mu.m upward or downward along the track of parallel fibers or positioned as far as 250 .mu.m laterally. Whenever it was applied in the molecular layer, the pulse application of NMDLA elicited no excitatory response in Purkinje cells recorded extra- or intracellularly. Slow depolarizations accompanied by a slight increase in cell input resistance were obtained with steady applications of 20-50 nA of the drug for 20-30 s. In contrast, pulse application of quisqualic acid appeared to have the same type of fast excitatory effect on Purkinje cells as L-Asp and L-Glu, but its potency was greater and its action more prolonged. Its steady application led to an abrupt and marked decrease in cell membrane resistance. The excitatory effects of L-Asp, L-Glu and quisqualic acid were antagonized by L-glutamic acid diethyl ester more consistently than by D-.alpha.-aminoadipate, suggesting that L-Asp and L-Glu act on Purkinje cells via quisqualic acid rather than via NMDLA receptors.