Characterization of serotonin receptors in fish brain: Polyamines inhibit the binding process

Serotonin (5‐HT) binding sites were characterized in the brain synaptosomal preparations from telencephalon and diencephalon of teleost fish, rainbow trout (Oncorhynchus mykiss). Serotonin specific binding curve exhibited a sigmoidal shape with Hill interaction coefficient (n) = 3 in both the parts of the brain, indicating the presence of co‐operative type of receptor sites. The telencephalic and diencephalic regions had multiple types of serotonin binding sites. In the diencephalic region, addition of spiperone produced a shallow curve for the inhibition of 3H‐5‐HT binding, indicating the presence of both low and high affinity binding sites. In both parts of the brain, the total specific binding of 3H‐5‐HT to the receptor sites was inhibited by spiperone and an agonist of 5‐HT1A receptor subtype (8‐OH‐DPAT), indicating the presence of 5‐HT1A serotonergic receptors in the fish brain. Interestingly, the polyamines, putrescine (Put), spermidine (Spd) and spermine (Spm) inhibited the binding of 3H‐5‐HT to the receptor sites in both parts of the brain i.e. telencephalon and diencephalon. The IC50 values for Put, Spd and Spm, were, respectively, 50 μM, 35 μM, 50 μM for telecephalon and 100 μM, 70 μM and 60 μM for diencephalon. The action of polyamines was not reversed by ifenprodil (a polyamine binding site antagonist), suggesting that the polyamine effects are not mediated via the site present on NMDA receptors. On the contrary, ifenprodil inhibited the serotonin binding to the brain synaptosomes. Addition of Mn2+ increased the binding of serotonin to the receptor sites, whereas Mg2+ failed to influence the same significantly.