Somatic and prejunctional nicotinic receptors in cultured rat sympathetic neurones show different agonist profiles

1. The release of [H-3]-noradrenaline ([H-3]-NA) in response to nicotinic a cetylcholine receptor (nAChR) agonists was compared with agonist-induced cu rrents in cultured rat superior cervical ganglion (SCG) neurones. 2. [H-3]-NA release in response to high concentrations of nicotinic agonist s was reduced, but not fully inhibited, by the presence of either tetrodoto xin (TTX) or Cd2+ to block voltage-gated Na+ or Ca2+ channels, respectively . We used the component of transmitter release that remained in the presenc e of these substances (named TTX- or Cd2+-insensitive release) to pharmacol ogically characterize nAChRs in proximity to the sites of vesicular exocyto sis (prejunctional receptors). Prejunctional nAChRs were activated by nicot inic agonists with a rank order of potency of dimethylphenylpiperazinium io dide (DMPP) > nicotine > cytisine > ACh, and with EC50 values ranging from 22 mu M (DMPP) to 110 mu M(ACh). 3. [H-3]-NA release in response to low concentrations of nAChR agonists was fully inhibited by the presence of either TTX or Cd2+ (named TTX- or Cd2+- sensitive release). TTX-sensitive release was triggered by nicotinic agonis ts with a rank order of potency of DMPP > cytisine approximate to nicotine approximate to ACh, which due to its similarity to TTX-insensitive release indicates that it might also:be triggered by prejunctional-type nAChRs. The EC50 values for TTX (Cd2+)-sensitive release were less than 10 mu M for al l four agonists. 4. By contrast to transmitter release, somatic nAChRs as seen by patch clam p recordings were most potently activated by cytisine, with a rank order of potency of cytisine > nicotine approximate to DMPP > ACh. EC50 values for the induction of currents exceeded 20 mu M for all four agonists. 5. The nicotinic antagonist mecamylamine potently inhibited all transmitter release in response to nicotine. ct-Bungarotoxin (alpha-BuTX) was, on the other hand, without significant effect on nicotine-induced TTX-insensitive release. The competitive antagonist dihydro-P erythroidine (DH beta E) caus ed rightward shifts of the dose-response curves for both TTX-sensitive and TTX-insensitive transmitter release as well as for currents in response to nicotine, with pA(2) values ranging from 4.03 to 4.58. 6. Due to clear differences in the pharmacology of agonists we propose that nAChRs of distinct subunit composition are differentially targeted to soma tic or axonal domains.