ATP RELEASE FROM DORSAL SPINAL-CORD SYNAPTOSOMES - CHARACTERIZATION AND NEURONAL ORIGIN

The present study determined the Ca2+-dependence of the release of ade nosine 5'-triphosphate (ATP) from dorsal spinal cord synaptosomes evok ed by depolarization with K+ and capsaicin, and the effect of intrathe cal capsaicin pretreatment, dorsal rhizotomy and intrathecal pretreatm ent with 6-hydroxydopamine on such release. Release of ATP evoked by K + was Ca2+-dependent, while that evoked by capsaicin was Ca2+-independ ent. Capsaicin pretreatment (60 mug, 7 days), which lesions small diam eter afferents, did not alter release of ATP evoked by either K+ or ca psaicin. Dorsal rhizotomy, which lesions all afferents, produced a sig nificant reduction in the amount of ATP released from the rhizotomized side compared to the intact side. Pretreatment with 6-hydroxydopamine (100 mug, 7 days) to destroy adrenergic nerve terminals, markedly red uced spinal cord noradrenaline levels, but did not alter the K+-evoked release of ATP. These results suggest that some K+-evoked release of ATP could originate from large but not small diameter afferent nerve t erminals in the spinal cord. ATP does not appear to originate from sma ll diameter afferents as, although ATP is released by in vitro exposur e to capsaicin, such release occurs only at high concentrations, relea se is Ca2+-independent and it is unaltered by pretreatment with capsai cin. The bulk of the ATP released from the spinal cord does not origin ate from descending noradrenergic nerve terminals.