POTENTIATION BY TONIC A(2A)-ADENOSINE RECEPTOR ACTIVATION OF CGRP-FACILITATED [H-3] ACH RELEASE FROM RAT MOTOR-NERVE ENDINGS

1 The effect of calcitonin gene-related peptide (CGRP) on [H-3]-acetylcholine ([H-3]-ACh) release from motor nerve endings and its interaction with presynaptic facilitatory A(2a)-adenosine and nicotinic acetylcholine receptors was studied on rat phrenic nerve-hemidiaphragm preparations loaded with [H-3]-choline. 2 CGRP (100-400 nM) increased electrically evoked [H-3]-ACh release from phrenic nerve endings in a concentration-dependent manner. 3 The magnitude of CGRP excitation increased with the increase of the stimulation pulse duration from 40 mu s to 1 ms, keeping the frequency, the amplitude and the train length constants. With 1 ms pulses, the evoked [H-3]-ACh release was more intense than with 40 mu s pulse duration. 4 Both the nicotinic acetylcholine receptor agonist, 1,1-dimethyl-4-phenylpiperazinium, and the A(2a) adenosine receptor agonist, CGS 21680C, increased evoked [H-3]-ACh release, but only CGS 21680C potentiated the facilitatory effect of CGRP. This potentiation was prevented by the A(2a) adenosine receptor antagonist, PD 115,199. 5 Adenosine deaminase prevented the excitatory effect of CGRP (400 nM) on [H-3]-ACh release. This effect was reversed by the non-hydrolysable A(2a)-adenosine receptor agonist, CGS 21680C. 6 The nicotinic antagonist, tubocurarine, did not significantly change, whereas the A(2)-adenosine receptor antagonist, PD 115,199, blocked the CGRP facilitation. The A(1)-adenosine receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine, potentiated the CGRP excitatory effect. 7 The results suggest that the facilitatory effect of CGRP on evoked [H-3]-ACh release from rat phrenic motor nerve endings depends on the presence of endogenous adenosine which tonically activates A(2a)-adenosine receptors. Since both CGRP and A(2a)-adenosine receptors are positively coupled to the adenylate cyclase/cyclic AMP system, cooperation between these receptors might occur at the second messenger transduction system level.