Neuroanatomical evidence for the involvement of β-endorphin during reproductive stress response in the fish Oreochromis mossambicus

• β-EP shows cyclic changes during the ovarian cycle in fish.
• β-EP is involved in the regulation of ovarian cycle in fish.
• Stress-activates β-EP secreting neurons in the NLT region of the brain.
• Increased β-EP secretory activity during stress might suppress LH in pituitary.

Although neuroendocrine regulation of stress response involving hypothalamo-pituitary-adrenal axis is well established in mammals, the mechanism of functioning of endocrine-stress axis is not completely elucidated in fish. Our previous studies suggested a possible role for the opioidergic mediation of reproductive stress response in fish. In the present investigation, by immunocytochemical approach, we studied the distribution of β-endorphin (β-EP) secreting neurons in the brain of the tilapia Oreochromis mossambicus exposed to aquacultural stressors. Intensely stained β-EP immunoreactive neurons were encountered in the nucleus lateralis tuberis (NLT) region during previtellogenic and vitellogenic phases in both controls and in fish exposed to aquacultural stressors. Furthermore, at the end of the prespawning phase in controls, weak staining in β-EP neurons was accompanied by intensely stained luteinizing hormone (LH) immunoreactive cells in the proximal pars distalis (PPD) of the pituitary gland and a significantly higher gonadosomatic and hepatosomatic indices, suggesting the attenuation of inhibitory effect of β-EP on reproductive axis prior to spawning. However, in fish exposed to stressors, several darkly stained β-EP immunoreactive cells with dense fibre projections towards the hypothalamo-hypophysial tract were concomitant with weakly immunoreactive LH content in the PPD of the pituitary gland and a significantly lower gonadosomatic and hepatosomatic indices compared to those of controls. These results suggest that stress-induced activation of β-EP secreting neurons in the NLT region might lead to the inhibition of LH secreting cells-ovary axis in fish.