Caffeine and carbonyl cyanide m-chlorophenylhydrazone increased evoked andspontaneous release of luteinizing hormone-releasing hormone from intact presynaptic terminals

In bullfrog sympathetic ganglia, the ryanodine-sensitive Ca2+ store and mit ochondria modulate [Ca2+] within nerve terminals. We used caffeine (10 mM) and carbonyl cyanide m-chlorophenylhydrazone (10 mu M) to assess how these Ca2+ stores affect release of a neuropeptide, luteinizing hormone-releasing hormone, from these nerve terminals. Release of luteinizing hormone-releas ing hormone was evoked by electrical stimulation to presynaptic nerves and was monitored as a late slow excitatory postsynaptic potential in ganglioni c neurons. Caffeine increased release of luteinizing hormone-releasing horm one similarly whether the release was evoked by 4 or 20 Hz stimulations (by 2.7 +/- 1.1- and 3.2 +/- 0.9-fold, mean +/- S.E.M., n=27, respectively). C arbonyl cyanide m-chlorophenylhydrazone augmented release of luteinizing ho rmone-releasing hormone evoked by 4 Hz stimulation much more strongly (by 1 1.8 +/- 1.8-fold) than it increased the release evoked by 20 Hz stimulation (by 3.6 +/- 1.3-fold, n=25). We detected spontaneous release of luteinizin g hormone-releasing hormone as a slow hyperpolarization in response to a br ief application of an antagonist to the receptors for luteinizing hormone-r eleasing hormone in 65% (34 of 52) and 39% (11 of 28) of the ganglionic B a nd C neurons, respectively. Caffeine increased spontaneous release of lutei nizing hormone-releasing hormone by 2.3 +/- 0.7-fold (n = 6) whereas carbon yl cyanide m-chlorophenylhydrazone increased this release by 4.27- and 1.76 -fold (n = 2). Facilitation of Ca2+ release from the intracellular store by caffeine and i nhibition of mitochondrial Ca2+ removal by carbonyl cyanide m-chlorophenylh ydrazone increased spontaneous as well as evoked release of luteinizing hor mone-releasing hormone. Moreover, caffeine increments of evoked release did not depend on the firing frequency of the nerve whereas carbonyl cyanide m -chlorophenylhydrazone augmentations of evoked release strongly depended on the firing frequency. (C) 1999 IBRO. Published by Elsevier Science Ltd.