EFFECTS OF ADENOSINE AND CADMIUM ON PRESYNAPTIC FIBER SPIKES IN THE CA1 REGION OF RAT HIPPOCAMPUS IN-VITRO

Adenosine was found to decrease the amplitude of presynaptic fiber spi kes recorded in stratum radiatum of the CA1 region of the hippocampus following stimulation of the Schaffer collateral and commissural affer ents. The presynaptic fiber spike reflects currents activated in the p resynaptic fibers and nerve terminals of this projection. However, the effect of adenosine was largely the result of: (a) temporal overlap b etween the presynaptic fiber spike and the field EPSP response and (b) an indirect effect most likely related to the hyperpolarization of CA 3 neurons giving rise to the Schaffer collateral fibers. When the CA3 region was separated by a knife cut from the CA1 region and the field EPSP blocked with 6,7-dinitroquinoxaline-2,3-dione, the effect of aden osine was markedly reduced, but was still significant (4.4 +/- 1.4% re duction in the amplitude of the presynaptic fiber spike). However, thi s action was not mimicked by the Ca2+ channel antagonist cadmium, sugg esting that it did not reflect an inhibition of Ca2+ currents by adeno sine. This effect was not observed in the presence of the K+ channel a ntagonists Ba2+ or diaminopyridine. These results suggest that adenosi ne may have a small hyperpolarizing effect on the presynaptic nerve te rminals, but this is unlikely to be the mechanism by which adenosine m odulates transmission in this brain region.