MODULATION BY ADENOSINE OF A NEURONAL INHIBITORY INTERACTION IN THE RAT HIPPOCAMPUS

Adenosine is acknowledged to have a primarily inhibitory function in t he central nervous system, but is believed to have little effect on in hibitory neurones themselves. It is, however, difficult to determine t he effect of adenosine on inhibitory synaptic potentials since adenosi ne directly depresses evoked potentials and, in the presence of bicucu lline to block GABA(A)-mediated inhibition, the bicuculline-resistant fraction of paired-pulse inhibition (ppi) is greater between pairs of small potentials than between pairs of larger potentials. Here, adenos ine increased bicuculline-resistant ppi when stimulus strength was con stant between adenosine and control but ppi of responses in adenosine was markedly less than ppi of control responses of the same size. Aden osine had less effect on the size of 'conditioned' potentials than on control potentials. It is concluded that adenosine can reduce the bicu culline-resistant fraction of paired-pulse inhibition in the hippocamp us. Further quantitative comparison of the effects of adenosine on ppi and on single evoked potentials excluded a difference in the potency of adenosine at excitatory and inhibitory terminals as an explanation for this activity. The results suggest that adenosine may diminish bic uculline-resistant paired-pulse inhibition by enhancing a simultaneous facilitatory component of the neuronal responses.