Differential antagonism by chlorpromazine and diazepam of frog motoneurone depolarization induced by glutamate-related amino acids.

The effects of some central depressant drugs on amino acid-induced depolarization of motoneurons have been determined in the isolate hemisected frog spinal cord. Motoneuron depolarization was recorded from ventral roots and measurements were made in the presence or absence of procaine or tetrodotoxin to minimize indirect effects of both drugs and amino acids. Chlorpromazine (0.05-0.1 mM) and diazepam (0.5 mM) produced a similar differential pattern of depression of amino acid-induced depolarizations. Responses induced by L-homocysteate were markedly antagonized by these drugs, while responses to quisqualate were unaffected. L-Aspartate-induced responses were antagonized more than L-glutamate-induced responses. This pattern of antagonism resembles that previously described for Mg2+. In contrast, pentobarbital (0.1 or 0.3 mM), and the inhibitory amino acids GABA and beta-alanine (0.5-1.0 mM), depressed amino acid-induced responses in a more uniform manner. The differential effects observed with chlorpromazine and diazepam provide further support for the possibility that responses to excitant amino acids structurally related to L-glutamate may have different underlying mechanisms.