title: Sexual dimorphism in the hypophysiotropic tyrosine hydroxylase-positive neurons in the preoptic area of the teleost, Clarias batrachus.
abstract: BACKGROUND
  Dopamine (DA) neurons in the anteroventral periventricular nucleus (AVPV) in the preoptic area (POA) of mammals express estrogen receptors, regulate luteinizing hormone (LH) secretion, and show distinct sexual dimorphism. In teleosts, hypophysiotropic DA neurons of the nucleus preopticus periventricularis (NPP), located in the anteroventral POA, express estrogen receptors, innervate LH cells, and emerged as a neuroanatomical substrate for inhibiting LH cells. Interestingly, the NPP and AVPV seem to share several similarities. Whether DAergic neurons in the NPP show sexual dimorphism is, however, not known. Based on the proposed homology to AVPV and previous studies showing greater tyrosine hydroxylase (TH) mRNA and enzyme activity levels in the brain of female catfish, we hypothesize that females have greater number of DAergic neurons in the NPP and correspondingly more TH-immunoreactive fiber innervation of the pituitary.
  
  
  METHODS
  Adult, male and female Clarias batrachus collected during the prespawning phase of their reproductive cycle were used. Fish were anesthetized and perfused transcardially with phosphate-buffered saline (pH 7.4) and 4 % paraformaldehyde in phosphate buffer. Sections through the rostro-caudal extent of the POA and pituitary were processed for TH immunofluorescence. Using double immunofluorescence, the association between TH-immunoreactive fibers and LH cells in the pituitary was explored. Sections were analyzed using semiquantitative analysis.
  
  
  RESULTS
  NPP in POA of C. batrachus has two distinct subdivisions, viz, anterior (NPPa) and posterior (NPPp), and TH neurons were observed in both the subdivisions. Compared to that in the males, a significantly higher (P < 0.05) number of TH neurons was consistently observed in the NPPa of females. TH neurons in NPPp, however, showed no difference in the number or immunoreactivity. Since DA neurons in NPPa are hypophysiotropic, we compared TH-fiber innervation of the pituitary in both sexes. Compared to males, proximal pars distalis and LH cells in this region of the pituitary in females were densely innervated by TH fibers.
  
  
  CONCLUSIONS
  Neurons of NPPa and their innervation to the pituitary seem to be a distinct sexually dimorphic DAergic system in C. batrachus. The DAergic system may serve as a component of the neural mechanisms controlling the sexually dimorphic LH surge in teleosts. Given the similarities shared by NPPa and AVPV, homology between these two nuclei is suggested.
authors.affiliation: School of Biological Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar, 751 005 Odisha India ; Present address: Institut Pasteur, Affiliated to: Ecole des neurosciences Paris (ENP) Graduate program, 28, rue du docteur Roux, 75724 Paris, Cedex 15 France.
authors.forename: Soham
authors.initials: S
authors.lastname: Saha
authors.affiliation: School of Biological Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar, 751 005 Odisha India.
authors.forename: Saurabh
authors.initials: S
authors.lastname: Patil
authors.affiliation: School of Biological Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar, 751 005 Odisha India.
authors.forename: Uday
authors.initials: U
authors.lastname: Singh
authors.affiliation: School of Biological Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar, 751 005 Odisha India.
authors.forename: Omprakash
authors.initials: O
authors.lastname: Singh
authors.affiliation: School of Biological Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar, 751 005 Odisha India.
authors.forename: Praful S
authors.initials: PS
authors.lastname: Singru
keywords: Dopamine; Nucleus preopticus periventricularis (NPP); Pituitary; Preoptic area; Reproduction; Sexual dimorphism; Teleosts