The effect of adenosine on spontaneous and evoked quantal secretion from different release sites of amphibian motor-nerve terminals

The effects of adenosine on the spontaneous quantal secretion from different release sites along terminal branches of toad (Bufo marinus) motor-nerve terminals was studied. Terminal branches were visualized using 3,3-diethyloxardicarbocyanine iodide (DiOC2(5))-fluorescence to assist in the placement of extracellular electrodes along different release sites of terminal branches. The maximum rate of spontaneous secretions (fe) observed with an extracellular electrode within any 10 microns length of terminal branch declined towards the distal end of the terminal branch as does the average number of evoked quantal secretions (mE). Adenosine (1-50 microM) depressed both fe and me. The ED50 of adenosine in depressing me was 5 microM. Adenosine (10 microM) produced on average a 43% decrease in fe and a 63% decrease in me, regardless of the initial values of fe and me. It is suggested that adenosine has qualitatively similar effects on both fe and me, regardless of their initial size, at different release sites within motor-nerve terminals.