Synthesis and Release of Acetylcholine by the Isolated Perifused Trout Caudal Neurosecretory System

Abstract

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 troutOncorhynchus mykisswas 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 × 105Bq/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 × 105Bq/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 (10−7M) nor urotensin II (10−6M) 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.