MEASUREMENT OF SYNAPTIC VESICLE EXOCYTOSIS IN AORTIC BARORECEPTOR NEURONS

The purpose of this study was to evaluate the use of the fluorescent m embrane label FM1-43 as a measure of synaptic terminal exocytosis duri ng stimulation of labeled aortic baroreceptor and unlabeled nodose gan glia neurons. Activation of the nerve terminals with electrical stimul ation or depolarization with 90 mM KCl in the presence of 2.0 mu M FM1 -43 resulted in bright, punctate staining of synaptic boutons. Additio nal depolarization in the absence of dye resulted in destaining with a time course that was consistent and repeatable in multiple boutons wi thin a given terminal. Destaining was dependent on calcium influx and was blocked by bath application of 100 mu M CdCl2. Whole cell patch-cl amp studies have reported that depolarization-induced calcium influx i n aortic baroreceptor cell bodies is predominantly caused by the activ ation of omega-conotoxin GVIA (omega-CgTx)-sensitive N-type calcium ch annels. In addition, these N-type channels have been shown to be inhib ited by activation of metabotropic glutamate receptors. In the present study, exocytosis in aortic baroreceptor terminals was not affected b y bath application of 5 mu M nifedipine and only partially inhibited b y bath application of 2.0 mu M omega-CgTx. However, depolarization-ind uced exocytosis was significantly inhibited by bath application of 200 mu M L-AP4, a type III metabotropic glutamate receptor agonist. Resul ts from this study suggest that 1) FM1-43 can be used to measure synap tic vesicle exocytosis in baroreceptor neurons; 2) the N-type calcium channel may not be involved in the initial phase of vesicle exocytosis ; and 3) activation of L-AP4-sensitive metabotropic glutamate receptor s inhibits 90 mM KCl-induced vesicle release.