Blockade of Neuronal Facilitatory Nicotinic Receptors Containing 3 2 Subunits Contribute to Tetanic Fade in the Rat Isolated Diaphragm

Nicotinic receptor (nAChR) subtypes involved in preand postjunctional actions underlying tetanic fade were studied in rat phrenic-nerve hemidiaphragms. We investigated the ability of subtype-specific nAChR antagonists to depress nerve-evoked contractions and [H]-acetylcholine ([H]-ACh) release. Muscle tension was transiently increased during brief high frequency trains (50 Hz for 5 sec). The rank potency order of nAChR antagonists to reduce tetanic peak tension was -bungarotoxin d-tubocurarine mecamylamine hexamethonium. Reduction of maximal tetanic tension produced by dihydro-erythroidine (0.03–10 M), methyllycaconitine (0.003–3 M), and -conotoxin MII (0.001–0.3 M) did not exceed 30%. Besides reduction of peak tension d-tubocurarine (0.1–0.7 M), mecamylamine (0.1–300 M), and hexamethonium (30–3,000 M) also caused tetanic fading. With -conotoxin MII (0.001–0.3 M) and dihydro-erythroidine (0.03–10 M), tetanic fade was evident only after decreasing the safety factor of neuromuscular transmission (with high magnesium ions, 6–7 mM). The antagonist rank potency order to reduce evoked (50 Hz for 5 sec) [H]-ACh release from motor nerve terminals was -conotoxin MII (0.1 M) dihydro-erythroidine (1 M) d-tubocurarine (1 M) mecamylamine (100 M) hexamethonium (1,000 M). When applied in a concentration (0.3 M) above that producing tetanic paralysis, -bungarotoxin failed to affect [H]-ACh release. Data obtained suggest that postjunctional neuromuscular relaxants interact with -bungarotoxin-sensitive nicotinic receptors containing 1-subunits, whereas blockade of neuronal 3 2-containing receptors produce tetanic fade by breaking nicotinic autofacilitation of acetylcholine release. Synapse 49:77–88, 2003. © 2003 Wiley-Liss, Inc.