The depolarization of feline ventral horn group Ia spinal afferent terminations by GABA

The unmyelinated terminal regions of extensor muscle Ia afferent fibres were stimulated electrically near lumbar motoneurones in anaesthetised cats using 300 microseconds pulses of less than 1 microA passed through the central NaCl barrel of seven barrel micropipettes. Such terminations were identified by anodal blocking factors of less than four and the latency of the antidromic impulse recorded in the appropriate peripheral muscle nerve. Although the effects of microelectrophoretically administered GABA were occasionally complex, the most consistent finding was a reduction in termination threshold followed by an increase. Both this reduction in threshold by GABA, and that produced by tetanic stimulation of low threshold flexor afferents (PAD) were diminished by microelectrophoretic bicuculline methochloride. This GABA antagonist alone elevated the threshold of some terminations but did not reduce the depolarizing action of either potassium or L-glutamate. Furthermore, since reductions in threshold by GABA, but not by either potassium or L-glutamate, were associated with a decrease in PAD, GABA appears to increase terminal membrane conductance. Since neither GABA nor bicuculline methochloride influenced the threshold or afferent depolarization of non-terminal regions of Ia fibres, there results are consistent with the function of GABA as a depolarizing transmitter at gabergic axoaxonic synapses upon the terminals of Ia afferent fibres synapsing with motoneurones.