Adenosine depresses spontaneous transmitter release from frog motor nerve terminals by acting at an A 1-like receptor

Abstract

Adenosine (1μM to 1mM) depressed spontaneous transmitter release from frog motor nerve terminals without producing any observable postsynaptic effects. Since this action of adenosine was blocked by 20μM theophylline and 1μM 8-phenyltheophylline, adenosine probably acts at a specific receptor on motor nerve terminals to reduce spontaneous transmitter output. The effects of the adenosine analogs, L-N 6-phenylisopropyladenosine (L-PIA, 100pM to 1μM), D-PIA (100nM to 100μM), and 5'-N-ethylcarboxamidoadenosine (NECA, 10nM to 100μM), were tested on spontaneous transmitter release at the frog neuromuscular junction. L-PIA The rank-order potency of these analogs indicates that adenosine acts at an A 1-like receptor to depress spontaneous transmitter release. Inhibitory actions of maximally effective concentrations of adenosine and L-PIA were also blocked by the A 1-specific antagonist, 1-3-dipropyl-8-cyclopentylxanthine (DPCPX) at a concentration of 100nM. Micromolar concentrations of NECA, an agonist with approximately equal affinity for the A 1 and A 2 receptors, produced biphasic effects on mepp frequency. Thus, a second adenosine receptor, perhaps of the A 2 subtype, may be present on motor nerve terminals and may mediate an icnreaaspontaneous transmitter release. spontaneous transmitter release.