Demonstration of an Autoreceptor Modulating the Release of [3H]5‐Hydroxytryptamine from a Synaptosomal‐Rich Spinal Cord Tissue Preparation

: A superfusion system employed to measure the K+ ‐stimulated release of [3H]5‐hydroxytryptamine ([3H]5‐HT,[3H]serotonin) from a synaptosomal‐rich spinal cord tissue preparation was carefully characterized, then used to examine the regulation of spinal 5‐HT release. Spinal 5‐HT release is apparently modulated by an autoreceptor. Exogenous 5‐HT depressed, in a concentration‐dependent manner, the K+‐stimulated release of [3H]5‐HT. Similarly, lysergic acid diethylamide (LSD) produced a concentration‐dependent decrease in [3H]5‐HT release. Methiothepin and quipazine blocked the inhibition of release induced by exogenous 5‐HT. The 5‐HT2 receptor antagonists spiperone and ketanserin failed to alter the action of 5‐HT at the spinal 5‐HT autoreceptor. Spiperone and ketanserin were shown, however, to alter the storage of [3H]5‐HT. When used in concentrations greater than 10 nM, the drugs evoked increases in basal [3H]5‐HT and [3H]5‐hydroxyindoleacetic acid ([3H]5‐HIAA) effluxes which were independent of the presence of calcium ions. A good agreement existed between the potencies of drugs for modifying autoreceptor function and their abilities to compete for high‐affinity [3H]5‐HT binding in the spinal cord (designated 5‐HT1). Furthermore quipazine, in concentrations that preferentially interact with the 5‐HT1B subtype, antagonized the actions of exogenous 5‐HT on K+ ‐stimulated release. Spiperone, in a concentration that approximated the affinity constant of 5‐HT1A sites for the drug, was ineffective in altering the ability of exogenous 5‐HT to modulate K+‐stimulated [3H]5‐HT release. These results suggest that 5‐HT1B sites are associated with serotonergic autoreceptor function in the spinal cord.