THE EFFECT OF NITRIC-OXIDE ON THE EFFICACY OF SYNAPTIC TRANSMISSION THROUGH THE CHICK CILIARY GANGLION

1 The effect of nitric oxide on the efficacy of synaptic transmission in the chick ciliary ganglion of post-hatched birds has been determine d by use of the size of the postganglionic compound action potential r esulting from chemical transmission through the ganglion as a measure of synaptic efficacy. 2 Sodium nitroprusside (100 muM) increased the s ynaptic efficacy by an average 26%. This is likely to be due to its ab ility to release nitric oxide, as potassium ferricyanide (100 muM) did not cause a potentiation. Sodium azide (100 muM), shown in sympatheti c ganglia to stimulate production of cyclic GMP, did not modulate syna ptic efficacy significantly. 3 8-Br-cyclic-GMP (100 muM) increased syn aptic efficacy by an average 61%. The addition of 8-Br-cyclic-AMP (100 muM) had less effect, increasing transmission by on average 46%. 4 Th e nitric oxide synthase blocker, N(G)-nitro-L-arginine methyl ester (L -NAME, 100 muM) was added prior to the tetanic stimulation of the preg anglionic nerves at 30 Hz for 20 s, a procedure known to produce both post-tetanic potentiation and long-term potentiation of synaptic trans mission through the ganglion. L-NAME reduced the long-term potentiatio n by an average of 47% but did not significantly change the post-tetan ic potentiation. 5 Following the brief application of 8-Br-cyclic AMP, 8-Br-cyclic GMP and sodium nitroprusside there was an enhancement of the efficacy of synaptic transmission that persisted after the withdra wal of the drugs. The maximum increase in synaptic efficacy following the brief addition of 8-Br-cyclic GMP was 116%, sodium nitroprusside w as 110% and 8-Br-cyclic AMP was 126%. 6 These results suggest that nit ric oxide modulates synaptic transmission through the ganglion by acti ng on an endogenous guanylate cyclase that produces cyclic GMP.