Effects of gamma-aminobutyric acid and glycine on synaptic excitability of neurones in the solitary tract nucleus.

In the solitary tract nucleus, neuronal responsiveness to synaptic input from peripheral afferent fibres has been found to decrease as the frequency of that input is increased. The present study investigated the possibility that glycine and gamma-aminobutyric acid (GABA) are (1) involved in this phenomenon of "frequency-dependent inhibition" or (2) capable of otherwise modifying neuronal responsiveness to synaptic input. In 32 of 57 neurones, application of glycine reduced responsiveness to input from visceral afferents. gamma-Aminobutyric acid reduced the responsiveness in 22 of 56 neurones but induced an increase in another 6. Selectivity of agonist effects was verified using glycinergic antagonist strychnine and the GABAergic antagonists picrotoxin and bicuculline. Of 67 neurones examined, 32 exhibited decreased numbers of action potentials with increasing stimulus frequency. Strychnine disrupted the frequency-dependent inhibition in 3 of 11 neurones, while picrotoxin and bicuculline prevented it in 2 of 7 and 4 of 9 neurones, respectively. These results indicate that both glycinergic and GABAergic systems may modulate the responsiveness of neurones in the solitary tract nucleus but that neither fully accounts for the expression of frequency-dependent inhibition.