Testosterone increases the number of substance P-like immunoreactive neurons in a specific sub-division of the lateral hypothalamus of the weakly electric, brown ghost knifefish, Apteronotus leptorhynchus.

During spawning, male and female brown ghost knife fish modulate their electric organ discharge to produce discrete courtship signals known as chirps. However, non-spawning fish show clear sex differences in chirp responsiveness to electrosensory stimuli; males consistently chirp, whereas females do not. This behavioral dimorphism is paralleled by sex differences in substance P-like immunoreactivity (SPl-ir) in the prepacemaker nucleus (PPn) which regulates chirping. The PPn is densely innervated by SPl-ir fibers in males, but not in females. However, we have shown that both female chirping behavior and the expression of SPl-ir in the PPn are enhanced following adult testosterone treatment [J.G. Dulka, L. Maler, W. Ellis, Androgen-induced changes in electrocommunicatory behavior are correlated with changes in substance P-like immunoreactivity in the brain of the electric fish, Apteronotus leptorhynchus, J. Neurosci. 15 (1995) 1879-1890]. Thus, testosterone-induced changes in SPl-ir input to the PPn may modulate female chirping during spawning. Recent evidence suggests that SPl-ir projections to the PPn may originate from SPl-ir neurons in the lateral hypothalamus (Hl). If so, one might expect to see a greater number of SPl-ir neurons in the Hl of testosterone-implanted females compared to Blank-implanted controls. In making this comparison, we found that both groups of females had similar numbers of SPl-ir neurons in the anterior Hl, but that testosterone-implanted females had significantly (p<0.01) more SPl-ir neurons in a distinct part of the posterior Hl. This sub-population of testosterone-sensitive, SPl-ir neurons may contribute to the regulation of chirping, since an increase in their number is positively correlated with the appearance of SPl-ir fibers in the PPn [J.G. Dulka, L. Maler, W. Ellis, Androgen-induced changes in electrocommunicatory behavior are correlated with changes in substance P-like immunoreactivity in the brain of the electric fish, Apteronotus leptorhynchus, J. Neurosci. 15 (1995) 1879-1890].