KOLODZIEJSKI, J.A.*; SMITH, G.T.; Indiana University, Bloomington; Indiana University, Bloomington: Sex and species differences in neuromodulatory control of communication: A comparative study of substance P and chirping in Apteronotus

Weakly electric fish often modulate their continuous electric organ discharge to produce social signals known as �chirps�. In the brown ghost knifefish, Apteronotus leptorhynchus, chirp rate is sexually dimorphic; males chirp more in response to social stimuli than females. In the black ghost knifefish, Apteronotus albifrons, chirp rates are lower than in A. leptorhynchus and are sexually monomorphic. Previous studies (Weld and Maler, 1992; Dulka et al 1995) indicated that sex differences in chirp rate in A. leptorhynchus were associated with sex differences in substance P-like immunoreactivity (SPlir) in the central posterior/prepacemaker nucleus (CP/PPn). In order to investigate the putative role of substance P in controlling chirp rate across species, we compared chirping behavior and SPlir within the CP/PPn of males and females of both A. leptorhynchus and A. albifrons. In agreement with previous research, chirp rate was sexually dimorphic in A. leptorhynchus, but not A. albifrons; and male A. leptorhynchus expressed significantly more SPlir in fine fibers of the CP/PPn than female A. leptorhynchus. Despite the fact that chirp rate in A. albifrons was sexually monomorphic, SPlir in the CP/PPn was sexually dimorphic. The amount of SPlir in the CP/PPn of male A. albifrons was significantly greater than that of females from either species and did not differ from that of male A. leptorhynchus. These results indicate that SPlir in the CP/PPn is not correlated with chirp rate across different Apteronotus species and suggest that substance P does not necessarily regulate chirp rate.