SUBSTANCE-P-LIKE IMMUNOREACTIVITY IN THE BRAIN OF THE GYMNOTIFORM FISH APTERONOTUS-LEPTORHYNCHUS - PRESENCE OF SEX-DIFFERENCES

The distribution of substance P-like immunoreactivity (SPli) was charted in the brain of gymnotiform fish Apteronotus leptorhynchus, and correlated with the circuitry underlying intraspecific electro-communication. Cell bodies were found predominantly in the lateral hypothalamus and in certain paraventricular organs: nucleus preopticus preiventricularis, anterior subdivision; anterior hypothalamus; nucleus posterioris periventricularis; nucleus recessus lateralis, medial subdivision 2; nucleus recessus posterioris and nucleus recessus lateralis, lateral subdivision. Cell bodies were also found in the rostral olfactory nucleus, ventral telencephalon (ventral and central subdivisions), the habenula, the vagal sensory and motor nuclei and in the subtrigeminal nucleus. The distribution of SPli fibers was similar in some respects to that reported for other vertebrates. SPli was found in the rhombencephalon associated with vagal afferent fibers and in the funicular nucleus (possibly related to nociception). In the diencephalon and midbrain SPli fibers were found in the habenular-interpeduncular tract, in the hypothalamus and pituitary. SPli fibers were also found in preoptic and forebrain areas. The most striking result was the sexually dimorphic SPli innervation of certain hypothalamic and septal nuclei, and of the prepacemaker nucleus (PPn), a diencephalic cell group which controls communication ('chirping') in gymnotiforms. The PPn and septal/hypothalamic nuclei were densely innevated by SPli in males but devoid of SPli in females.