Localization of P450 Aromatase in The Brain of Adult Catfish Heteropneustes fossilis and Regional Distribution of Estradiol-17β and Testosterone: Gender and Seasonal Differences

P450 aromatase is the key enzyme that converts aromatizable androgens to estrogens. It is coded by the gene that exists in two forms: ovarian type and brain type. In the present study, we localized aromatase protein and gene (brain type) by immunocytochemical and in situ hybridization techniques, respectively, and the gene expression by RT-PCR. Aromatase immunoreactivity and transcript signals were observed in basophils of the proximal pars distalis of the pituitary, purported to be the gonadotrophs. The transcript signals were detected in the periventricular grey zone of optic tectum (PGZ), ventral telencephalic region, and radial glial cells lining the telencephalon, hypothalamus and olfactory bulb. Transcript levels of cyp19a1b showed sex, seasonal and regional differences. We measured testosterone (T) and estradiol-17β (E2) levels by specific ELISA kits. In the brain, T level is low in comparison to E2 level. Testosterone level is higher in postspawning phase and lower in spawning phase. The dimorphic expression varied with seasons, being higher in males in the preparatory and prespawning phases. E2 level is higher in female brain than in male brain especially in the preparatory and prespawning phases. E2 level is the highest in prespawning phase and lowest in postspawning phase. The steroids showed brain regional distribution with significant seasonal and dimorphic variations. Olfactory bulb has the highest T: E2 ratio in the resting, preparatory and prespawning phases in comparisons to other brain regions. All brain regions elicited a high level of E2, which showed dimorphic and seasonal variations, indicating high rate of aromatization. Differential levels of aromatase transcript, E2 and T in brain regions suggest a high level of neuroestrogen formation and E2 involvement in various neuroendocrine functions such as regulating spawning behaviour, plasticity, neurogenesis, etc.