Calcitonin gene-related peptide rapidly downregulates nicotinic receptor function and slowly raises intracellular Ca2+ in rat chromaffin cells in vitro

Although calcitonin gene-related peptide (CGRP) modulates muscle-type nicot inic acetylcholine receptors (nAChRs) via intracellular second messenger-me diated phosphorylation, the action of this peptide on neuronal-type nAChRs remains unknown. Using neuronal nAChRs of rat chromaffin cells in vitro we studied the effect of CGRP, which is physiologically present in adrenal med ulla, on membrane currents and [Ca2+], transients elicited by nicotine. Our main novel observation was that CGRP (either bath-applied or focally appli ed for a few seconds or even co-applied with nicotine for a few millisecond s) selectively and rapidly blocked nAChRs (a phenomenon unlikely caused by intracellular messengers in view of its speed) without affecting GABA recep tors. The inhibitory effect of CGRP was independent of [Ca2+](i) or membran e potential and not accompanied by baseline current changes. Like the compe titive antagonist N,N,N-trimethyl-1-(4-trans-stilbenoxy)-2-propilammonium, CGRP induced a rightward, parallel shift the nicotine dose-response curve; during co-application of these blockers the nicotine dose-ratio value was t he sum of the values obtained with each antagonist alone. The block by CGRP was insensitive to the receptor antagonist hCGRP(8-37) but mimicked by CGR P(1-7). Persistent application of CGRP slowly increased [Ca2+](i), a phenom enon independent from external Ca2+, thus implying Ca2+ release from intern al stores, and suppressed by hCGRP(8-37), CGRP(1-7) had no significant effe ct on [Ca2+](i). We propose that the 1-7 amino acid sequence of CGRP was re sponsible for the direct, rapid block of nAChRs, whereas the full-length pe ptide molecule was necessary for the delayed rise in internal Ca2+ potentia lly able to trigger phosphorylation-dependent modulation of nicotinic recep tor function.