EFFECT OF THE PUTATIVE COGNITIVE ENHANCER, LINOPIRDINE (DUP 996), ON QUANTAL PARAMETERS OF ACETYLCHOLINE-RELEASE AT THE FROG NEUROMUSCULAR-JUNCTION

1 The subcellular mechanism and site of action of linopirdine or DuP 9 96 (3,3-bis(4-pyridinylmethyl-1-phenylindolin-2-one) was investigated at the frog neuromuscular junction, using miniature endplate potential (m.e.p.p.) counts and a new method for obtaining unbiased estimates o f n (number of functional release sites), p (probability of release), and var(s)p (spatial variance in p). 2 DuP 996 produced an increase in m (no. of quanta released), which was due -to an increase in n and p. The increase in m was concentration-dependent over a range of 0.1 - 1 00 muM and completely reversible with 15 min of wash. There was a satu ration in the increase in p, but not in the increase in m and n, for [ DuP 996] > 10 muM. By contrast, there was no major change in var(s)p. 3 Block of presynaptic Na+- and Ca2+-channels with 3 muM tetrodotoxin and 1.8 mM Co2+ prevented the m.e.p.p. frequency increase to DuP 996, and this effect was completely reversed by washing. 4 Application of t he neuronal Ca2+-channel blocker, omega-conotoxin GVIA (1 muM) brought about a rapid and profound decrease in the m.e.p.p. frequency increas e produced by DuP 996. The effect of the toxin was not reversed by pro longed washing. 5 Block of voltage-gated K+-channels with 100 muM 4-am inopyridine (4-AP) resulted in only a small (28 %) increase in m. The combination of 4-AP (100 muM) and DuP 996 (10 muM) produced an increas e in m (189%) which was much greater than the sum of the responses to each agent alone. This increase in m was due solely to an increase in n, as p and var(s)p were unchanged. 6 For [DuP 996] up to 100 muM, the re was no apparent change in the mean size, amplitude distribution, or time course of m.e.p.ps, signifying that it had no anticholinesterase activity. 7 It is concluded that DuP 996 increases the release of qua ntal transmitter but not the postsynaptic response to the quanta. This appears to involve an effect at the nerve terminal membrane, most lik ely an increase in Ca2+-conductance, and not an action to block K+-con ductance or to release Ca2+ from intraterminal organelles.