Pharmacological evidence for L‐aspartate as the neurotransmitter of cerebellar climbing fibres in the guinea‐pig.

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Abstract
Climbing fiber responses (c.f.r.) evoked by white matter stimulation and the depolarizations induced by iontophoretically applied L-glutamate and L-aspartate were recorded intracellularly from the proximal dendrites of Purkinje cells in in vitro slice preparations of the guinea pig cerebellum. Short pulses of L-glutamate and L-aspartate dose-dependently depolarized the Purkinje cell dendrite. Even small doses of these amino acids reduced the input resistance. The maximum decrease in input resistance induced by L-glutamate was 36% and that by L-aspartate was 38%. Intracellular injection of Cs+ allowed Purkinje cell dendrites to be depolarized to a range of -15 to +30 mV. The mean reversal potential for the c.f.r. (Ec) was found to be +10.2 mV (n = 4). The mean reversal potentials obtained for L-glutamate (Eg) and for L-aspartate (Ea) were +7.3 mV (n = 7) and +5.6 mV (n = 7), respectively. When external Na+ concentration was reduced, Ec, Ea and Eg were linearly and similarly shifted in the negative direction, indicating that all these reversal potentials are determined primarily by a Na+ conductance. The effects of the glutamate antagonists 2-amino-5-phosphonovaleric acid (APV), .gamma.-D-glutamylglycine (.gamma.-DGG), N-methyl-DL-aspartic acid (NMDLA) and glutamic acid diethylester (GDEE) were compared as to the responses to L-glutamate and L-aspartate and Ca2+-activated focal climbing fiber responses (c.f.c.f.r.) in order to investigate the receptor type at the synapses formed by the climbing fibers with Purkinje cell dendrites. APV was the most effective anatagonist of the c.f.c.f.r. Its action was reversible, selective for L-aspartate-induced depolarization and had no effect on the responses to L-glutamate. NMDLA, which has no activity as an agonist, was a greater suppressant of the responses to L-aspartate than those to L-glutamate. The receptor for the transmitter at the synapses formed by climbing fibers with Purkinje cell dendrites is of the L-aspartate-preferring type, and L-aspartate may be the endogenous transmitter at this synapse.