Dogmas and controversies in the handling of nitrogenous wastes: 5-HT ₂-like receptors are involved in triggering pulsatile urea excretion in the gulf toadfish, Opsanus beta

When injected arterially, serotonin (5-hydroxytryptamine; 5-HT) has been shown to elicit naturally sized urea pulse events in the gulf toadfish, Opsanus beta. The goal of the present study was to determine which 5-HT receptor(s) was involved in mediating this serotonergic stimulation of the pulsatile excretion mechanism. Toadfish were surgically implanted with caudal arterial catheters and intraperitoneal catheters and injected with either 8-OH-DPAT (1 μmol kg ^-1), a selective 5-HT_1A receptor agonist, α-methyl-5-HT (1 μmol kg^-1), a 5-HT₂ receptor agonist, or ketanserin, a 5-HT₂ receptor antagonist (0.01, 0.1, 1 and 10 μmol kg^-1) plus α-methyl-5-HT. 8-OH-DPAT injection did not mediate an increase in urea excretion, ruling out the involvement of 5-HT_1A receptors in pulsatile excretion. However, within 5 min, α-methyl-5-HT injection caused an increase in the excretion of urea in >95% (N=2T) of the fish injected, with an average pulse size of 652±102 μmol N kg^-1 (N=26). With α-methyl-5-HT injection there was no corresponding increase in ammonia or [³H]PEG 4000 permeability. Urea pulses elicited by α-methyl-5-HT were inhibited in a dose-dependent fashion by the 5-HT₂ receptor antagonist ketanserin, which at low doses caused a significant inhibition of pulse size and at higher doses significantly inhibited the occurrence of pulsatile excretion altogether. However, neither 8-OH-DPAT nor α-methyl 5-HT injection had an effect on plasma cortisol or plasma urea concentrations. These findings suggest the involvement of a 5-HT₂-like receptor in the regulation of pulsatile urea excretion.