Synthesis and release of acetylcholine by the isolated perifused trout caudal neurosecretory system

The neurons of the caudal neurosecretory system of teleosts contain, in addition to urotensin I and urotensin II, a high concentration of acetylcholine (T. Ichikawa, 1978, Gen. Comp. Endocrinol. 35, 226-233). The isolated urophysis (and attached terminal spinal cord region) of the rainbow trout Oncorhynchus mykiss was incubated with [³H]choline (0.2 MBq/ml) for 45 min at 22° in the presence of the cholinesterase inhibitor, physostigmine. Unreacted choline was removed by perifusion with fish Ringer solution. Incorporation of radioactivity into newly synthesized [³H]acetylcholine was 4.9 ± 2.1 x 10⁵ Bq/g wet tissue wt. When incubations were carried out in the presence of hemicholinium-3, an inhibitor of high-affinity choline uptake, or when physostigmine was omitted from the incubation buffer and/or when [³H]-inulin was substituted for [³H]choline, the incorporation of radioactivity was greatly reduced (<0.5 x 10⁵ Bq/g). The release of [³H]acetylcholine from the preparation increased to 338 ± 59% of basal (P < 0.05) when the concentration of K⁺ in the perifusion buffer was raised to 41 mM, but neither urotensin I (10^-7 M) nor urotensin II (10^-6 M) had a significant effect on release. The data indicate that the trout caudal neurosecretory system possesses a high-affinity uptake system for choline and that newly synthesized acetylcholine is released in response to a depolarizing stimulus.