Plasticity of the electric organ discharge waveform of male Brachyhypopomus pinnicaudatus . II. Social effects

Abstract

Many electric fish produce sexually dimorphic electric organ discharges. Although electric organ discharges are comprised of action potentials, those of the Gymnotiform family Hypopomidae show significant plasticity in response to stress and time of day. We show here that male Brachyhypopomus pinnicaudatus (Hopkins 1991), adjusts the degree of sexual dimorphism in its electric organ discharge depending on immediate social conditions. Three to five days of isolation resulted in gradual decrease of two sexually dimorphic waveform characters: duration and amplitude. Introduction of a second fish to the experimental tank restored electric organ discharge duration and amplitude. Duration recovered quicker than amplitude, and both recovered faster in the presence of males than females. In studies of other electric fish species, treatment with steroid sex hormones have taken several days to increase sexual dimorphism in the electric organ discharge. The socially induced changes seen in this study are initiated too quickly to involve classic steroid action of genomic transcription and thus may depend on another mechanism. Socially induced regulation of the male's electric organ discharge waveform is consistent with the compromises in signaling strategy shown by other taxa with costly sexual advertisement signals.

Adrenocorticotropic Hormone Enhances the Masculinity of an Electric Communication Signal by Modulating the Waveform and Timing of Action Potentials within Individual Cells

“…These changes in the EODs of freeswimming fish are similar in time course and magnitude to EOD changes observed in response to 5-HT injections (Stoddard et al, 2003), social interactions (Franchina et al, 2001), and intraspecific aggression (Hagedorn and Zelick, 1989). We showed also that ACTH acts within minutes to modulate the EOD waveform of individual electrocytes in vitro, where changes in the waveform of the EOD parallel the EOD waveform changes elicited by in vivo ACTH injections and social interactions.…”

“…ACTH is released by the action of 5-HT in the hypothalamus and pituitary (Brown, 1994), and its levels in circulation show a pronounced circadian rhythm (Nichols and Tyler, 1967). Circulating ACTH levels also are elevated by stress (for review, see Sapolsky, 2002) and social encounters (Lyons et al, 1995;Hoglund et al, 2000), consistent with our findings that stressors and social encounters modulate the EOD within minutes (Franchina et al, 2001;Stoddard et al, 2003).…”

“…Occurring on an intermediate time scale of minutes to hours are EOD modulations, including circadian oscillations and EOD enhancements in response to social and environmental changes (Hagedorn and Zelick, 1989;Hagedorn, 1995;Franchina, 1997;Franchina and Stoddard, 1998;Franchina et al, 2001). The neuroendocrine pathways controlling these intermediate time-scale EOD modulations have not yet been well characterized.…”

Opposing actions of 5HT1A and 5HT2-like serotonin receptors on modulations of the electric signal waveform in the electric fish Brachyhypopomus pinnicaudatus

“…Social encounters and environmental stimuli modulate the EOD waveform by altering the membrane biophysics and discharge waveforms of individual electrocytes (Ferrari et al, 1995;McAnelly and Zakon, 2000;Mills and Zakon, 1991), resulting in either a larger or smaller waveform. Social encounters modulate the EOD within minutes and large waveforms and/or rapid enhancements are associated with dominance status (Franchina et al, 2001;Hagedorn and Zelick, 1989;Stoddard et al, 2003).We have shown that intramuscular injections of 5-HT cause male B. pinnicaudatus to increase their EOD amplitude and duration akin to the waveform changes observed during male-male social interactions (Stoddard et al, 2003). These effects of 5-HT do not result from its direct action on the electrocytes, whereas melanocortin peptides do modulate the discharge waveforms of single electrocytes directly (Markham and Stoddard, 2005).…”

“…Social encounters and environmental stimuli modulate the EOD waveform by altering the membrane biophysics and discharge waveforms of individual electrocytes (Ferrari et al, 1995;McAnelly and Zakon, 2000;Mills and Zakon, 1991), resulting in either a larger or smaller waveform. Social encounters modulate the EOD within minutes and large waveforms and/or rapid enhancements are associated with dominance status (Franchina et al, 2001;Hagedorn and Zelick, 1989;Stoddard et al, 2003).…”