Modulatory effects of adenosine upon the transmitter release in the hippocampal slice preparation of rats

In the hippocampal slice preparation of rats, preloaded with 3H-glutamate a local perfusion technique was used in order to measure the labelled transmitter output. The electrical receptive stimulation of the Schaffer collateral-commissural projections resulted in a detectable 3H-glutamate release which was significantly increased in the presence of 4-aminopyridine (4-AP). Exogenous adenosine and its analogue L-PIA induced a slight unsignificant reduction of the release while in the presence of 4-AP the release was almost completely blocked. Adding adenosine deaminase (ADA) did not change the normal transmitter release while in the presence of 4-AP the elimination of the endogenous adenosine brought about a significantly higher 3H-glutamate output. It is concluded that the adenosine-induced presynaptic inhibition of the transmitter release could be a significant mechanism under conditions in which the synaptic activity is abnormally high. This effect seems to be mediated via the A1 adenosine receptor since the D-PIA isomer application has no significant inhibitory effects.