DIFFERENT PATHWAYS BY WHICH EXTRACELLULAR CA2+ PROMOTES CALCITONIN GENE-RELATED PEPTIDE RELEASE FROM CENTRAL TERMINALS OF CAPSAICIN-SENSITIVE AFFERENTS OF GUINEA-PIGS - EFFECT OF CAPSAICIN, HIGH K+ AND LOW PH MEDIA

Different modes by which Ca2+, entering the nerve terminal, promotes transmitter secretion as well as the ability of protons to release neuropeptides, have been shown in peripheral endings of capsaicin-sensitive afferents. We have studied these two aspects in the central endings of these neurons by measuring the release of calcitonin-gene related peptide-like immunoreactivity (CGRP-LI) from slices of the dorsal half of the guinea pig spinal cord. Although capsaicin (1-mu-M) released both CGRP-LI and substance P-like immunoreactivity (SP-LI), CGRP-LI was chosen as the sole suitable marker of peptides released from central terminals of capsaicin-sensitive afferents, since after in vitro desensitization to capsaicin (1-mu-M capsaicin for 20 min), high K+ (80 mM) failed to evoke CGRP-LI release, whereas SP-LI release was still observed. The capsaicin (1-mu-M)-evoked CGRP-LI release was entirely dependent on extracellular Ca2+. It was unaffected by 0.3-mu-M tetrodotoxin (TTX), slightly reduced by 0.1-mu-M omega-conotoxin (CTX) and blocked by 10-mu-M Ruthenium red (RR). The Ca2+-dependent K+ (80 mM)-evoked CGRP-LI release was unaffected by TTX, markedly reduced by CTX and only moderately inhibited by RR. Low pH (pH 5) produced a remarkable increase in CGRP-LI outflow that was abolished after exposure to capsaicin, reduced by about 50% in Ca2+-free medium and unaffected by TTX (0.3-mu-M). The Ca2+-dependent component of the proton-evoked CGRP-LI release was abolished in the presence of RR (10-mu-M) and slightly inhibited by CTX (0.1-mu-M). The mode by which capsaicin or high K+ promote Ca2+ entry into the central endings of capsaicin-sensitive afferents, and hence promote neuropeptide release may be distinguished on a pharmacological basis. Protons release CGRP in the spinal cord by a mechanism that shares a common pathway with that activated by capsaicin.