Sexual dimorphism in the hypophysiotropic tyrosine hydroxylase-positive neurons in the preoptic area of the teleost, Clarias batrachus

Abstract

BackgroundDopamine (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.MethodsAdult, 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.ResultsNPP 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.ConclusionsNeurons 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.

Dopaminergic inhibition of gonadotropin-releasing hormone neurons in the cichlid fish,Astatotilapia burtoni

“…Previous studies demonstrated sexually dimorphic regulation of dopaminergic signaling in the hypothalamus and pituitary of both mammals and fish (Goebrecht et al, 2014;Saha et al, 2015;Scott et al, 2015). Although androgenic steroids regulate dopamine receptor expression in the fish POA (Pasqualini et al, 2009), there are, to our knowledge, no studies testing for sex-specific differences in dopamine receptor-mediated inhibition of POA neural activity.…”

“…Previously, dopaminergic cell bodies were observed near the POA in several teleost fish species, including A. burtoni (Goebrecht et al, 2014;O'Connell et al, 2010;Saha et al, 2015). We tested whether dopaminergic processes are closely apposed to GnRH1 neurons.…”

“…These results show that the inhibitory mechanisms postsynaptic to dopamine release onto POA GnRH1 neurons are not sexually dimorphic. However, presynaptic dopamine production or release could vary by sex; sexually dimorphic TH expression, as has been observed in fish and mammals (Saha et al, 2015;Scott et al, 2015), could result in sexually dimorphic regulation of dopaminergic control of reproductive behaviors (Goebrecht et al, 2014).…”

Dopamine system, cerebellum, and nucleus ruber in fish and mammals

“…This area may also contain A14 dopaminergic neurons in teleosts (Saha et al . ). However, establishing the boundary between different groups of dopaminergic neurons and determining neuronal homology between teleosts and humans remains difficult.…”

Role of Isotocin in the Regulation of the Hypophysiotropic Dopamine Neurones in the Preoptic Area of the Catfish, Clarias batrachus

“…Because NPPa contains hypophysiotropic DA neurones, we suggest a role for IST in the modulation of these neurones. The NPPa of teleost has been suggested as homologous to the mammalian anteroventral periventricular nucleus . Although the presence of oxytocin fibres has been demonstrated in the POA of mammals , its functional significance remains unexplored.…”