Opposing actions of 5HT sub(1) sub(A) and 5HT sub(2)-like serotonin receptors on modulations of the electric signal waveform in the electric fish Brachyhypopomus pinnicaudatus

Serotonin (5-HT) is an indirect modulator of the electric organ discharge (EOD) in the weakly electric gymnotiform fish, Brachyhypopomus pinnicaudatus. Injections of 5-HT enhance EOD waveform ''masculinity'', increasing both waveform amplitude and the duration of the second phase. This study investigated the pharmacological identity of 5-HT receptors that regulate the electric waveform and their effects on EOD amplitude and duration. We present evidence that two sets of serotonin receptors modulate the EOD in opposite directions. We found that the 5HT sub(1) sub(A)R agonist 8-OH-DPAT diminishes EOD duration and amplitude while the 5HT sub(1) sub(A)R antagonist WAY100635 increases these parameters. In contrast, the 5HT sub(2)R agonist alpha -Me-5-HT increases EOD amplitude but not duration, yet 5-HT-induced increases in EOD duration can be inhibited by blocking 5HT sub(2) sub(A) sub(/) sub(2) sub(C)-like receptors with ketanserin. These results show that 5-HT exerts bi-directional control of EOD modulations in B. pinnicaudatus via action at receptors similar to mammalian 5HT sub(1) sub(A) and 5HT sub(2) receptors. The discordant amplitude and duration response suggests separate mechanisms for modulating these waveform parameters.