In vitro and in vivo evidence of neurotensin release from preganglionic axon terminals in the stellate ganglion of the cat

We have previously shown that the neurotensin (NT) store in preganglionic axon terminals of the cat stellate ganglion (SG) is reversibly depleted by prolonged preganglionic stimulation. The present study addresses the questions of whether the preganglionic axon terminals release NT in response to depolarizing stimuli in vitro and whether in vivo NT is released by the tonic firing of the sympathetic preganglionic neurons. Slices of the SG of the anaesthetized cat, maintained in oxygenated Ringer solution, released NT. The efflux increased when the K concentration was increased from 5 to 25 or 45 mM or when veratridine was added to the medium. In Ca-free medium, efflux was suppressed. The effect of veratridine was blocked by tetrodotoxin (TTX). In awake, freely moving cats, in which TTX was applied for 4 days to the preganglionic input of the right SG, the NT content of this ganglion doubled by comparison with the left SG. Since NT accumulates proximal to a ligature on the preganglionic input of the SG, the increased NT content is likely to result from suppression of action potential-dependent release while influx into the terminals persists. This result suggests that the steady state of the NT store in sympathetic preganglionic terminals is the result of a steady influx from the soma balanced by action potential-dependent loss, presumably release.