The peripheral terminal of the sensory C-fiber is a complex organelle, containing vesicularly packaged neuropeptides: calcitonin gene-related peptide (CGRP) and several tachykinins (TK). Depolarization of the peripheral terminal of the sensory C-fiber in the isolated trachea results in local Ca++ -dependent, tetrodotoxin (TTX)-insensitive release of CGRP/TK. Given the ability of pharmacological stimuli to excite or facilitate activity in the sensory C-fiber, it is not surprising that such stimuli also can evoke release of peptides from the terminals.
The specific aim of this work is to characterize the complex properties of the peripheral sensory C-fiber terminals that directly regulate the extracellular movement of CGRP/TK from the intra terminal pools. Four specific hypotheses will be addressed: 1. Release of peptides from the peripheral terminal of the C-fiber is regulated by specific classes of ion (Ca++, K+) channels and by increases in extracellular (H+). Peptide release evoked by various stimuli, e.g. K+, electrical field stimulation (EFS), capsaicin (CAP), bradykinin (BK), prostaglandins (PG), and the release potentiated by serotonin (5-HT) will be examined in the presence of: i) Ca++ channel agonists and antagonists of the L and N types; ii) K+ channel antagonists for Ca++ -dependent K+ channels; and iii) acid pH buffer. 2. Peripheral sensory C-fiber terminals that release CGRP/TK are subject to modulation by mediators secreted from local autonomic (sympathetic/ parasympathetic) nerve terminals. Effects of sympathetic and parasympathetic terminal activity on resting/stimuli evoked-release will be assessed following: i) chemical and surgical sympathectomy; ii) adrenergic receptor agonists and antagonists (alpha-1, alpha-2, beta- preferring); iii) NPY (neuropeptide Y) and its analogues; iv) acetylcholinesterase inhibitor; and v) muscarinic and nicotinic antagonists. 3. Arachidonic acid (AA) metabolites act as modulators of the peptide release from the peripheral terminals of primary sensory neurons.The role of AA products on resting/stimuli-evoked release will be examined following: i) cycloxygenase/ lipoxygenase inhibitors; ii) antagonists for prostanoid receptors: IP, DP, and TP; and iii) antagonists for leukotriene receptors. The release of eicosanoids (PGE2, 6-keto-PGF1 alpha), thromboxane (TXB2) and leukotriene (LTB4) into the tracheal perfusate by stimuli (see above) will also be examined. 4. Peripheral release of sensory neuropeptides will be augmented in states associated with an airway allergic reaction. Evoked release will be examined in the trachea of sensitized animals (chronic antigen challenge). Mechanisms of the release will be investigated using antagonists against AA products and autocoids. In summary, this study seeks to define the neurobiology of the peripheral terminal of the sensory C-fiber. Moreover, because of the potent effects of these peptides on the peri-terminal environment (vascular caliber, capillary, permeability, smooth muscle tone, and inflammatory cell activity), this study on the terminal properties governing release will provide direct insights into clinically significant events regulating airway reactivity.
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