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

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 ß-endorphin (ß-EP) secreting neurons in the brain of the tilapia Oreochromis mossambicus exposed to aquacultural stressors. Intensely stained ß-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 ß-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 ß-EP on reproductive axis prior to spawning. However, in fish exposed to stressors, several darkly stained ß-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 ß-EP secreting neurons in the NLT region might lead to the inhibition of LH secreting cells-ovary axis in fish.

Glucocorticoid synthesis inhibitor metyrapone blocks stress-induced suppression along luteinizing hormone secreting cells­ovary axis in the fish Oreochromis mossambicus.

We showed previously that exposure to mild acute stressors leads to inhibition of follicular development and luteinizing hormone (LH) secretion in tilapia. In this study, we examined whether the hypothalamo­pituitary­interrenal axis was involved in such inhibition. Administration (i.p.) of corticotropin-releasing hormone (CRH) to stripped Oreochromis mossambicus (eggs manually removed from mouth brooder) during the ovarian cycle for 22 days resulted in a significant increase in the serum levels of cortisol, and significantly lower gonadosomatic and hepatosomatic indices concomitant with complete absence of stage V (vitellogenic) follicles in the ovary compared to controls. Furthermore, the LH secreting cells at the proximal pars distalis (PPD) in the pituitary gland showed weak immunostaining in contrast to the intensely stained immunoreactive cells in controls during prespawning phase. On the other hand, while exposure of fish to aquacultural stressors produced effects similar to that of CRH treatment, treatment of glucocorticoid synthesis inhibitor metyrapone to stressed fish during the ovarian cycle did not show significant serum cortisol response. The LH secreting cells in these fish showed intense immunostaining at the PPD in the pituitary gland, and the ovary contained stage V follicles similar to that of controls prior to spawning phase. These results suggest that the inhibitory effects of CRH treatment on LH secretion and recruitment of follicles for vitellogenic growth are mediated through the stress hormone cortisol in O. mossambicus.

ß-endorphin-induced inhibition of vitellogenic follicular growth in the fish Oreochromis mossambicus: evidence for opioidergic mediation of ovarian stress response.

The opioid peptide ß-endorphin (ß-EP) is known to play modulatory role in vertebrate reproduction. The objective of the present study was to determine the influence of ß-EP and the opioid receptor antagonist naltrexone (NALT) on follicular development in normal, and stressed female tilapia Oreochromis mossambicus. Administration of 4 µg ß-EP, but not 0.5 or 1.5 µg ß-EP daily for 22 days caused a significant reduction in the mean GSI, HSI, and number of Stage IV (vitellogenic) follicles compared to experimental controls during the prespawning phase of the ovarian cycle. Furthermore, the Stage V (vitellogenic) follicles were completely absent during the prespawning phase concomitant with the significantly lower serum levels of estradiol and cortisol in 4 µg ß-EP treated fish compared to experimental controls. On the other hand, exposure of the fish to mild acute stressors for 22 days caused changes in the ovary similar to that of high dose of ß-EP, whereas administration of NALT attenuated these effects. The results reveal that ß-EP exerts inhibitory effect on the vitellogenic follicular growth through the suppression of ovarian steroidogenesis. The study provides evidence for the opioidergic mediation of the ovarian stress response for the first time in fish.

Naltrexone attenuates stress-induced suppression of LH secretion in the pituitary gland in the Cichlid fish Oreochromis mossambicus: evidence for the opioidergic mediation of reproductive stress response.

Opioid peptide ß-endorphin (ß-EP) plays a modulatory role in vertebrate reproduction. However, the role of opioid peptides in reproductive stress response is least understood in fishes. The aim of the present study was to determine the effect of different doses of ß-EP on luteinizing hormone (LH) secretion in normal and the opioid receptor antagonist naltrexone (NALT) in stressed female tilapia Oreochromis mossambicus. Administration of 4 µg ß-EP, but not 0.5 or 1.5 µg ß-EP, daily for 22 days caused suppression of LH-secreting cells at the proximal pars distalis of the pituitary gland, concomitant with a significant reduction in the mean GSI and HSI in 4 µg ß-EP-treated fish compared to controls. On the other hand, exposure of the fish to mild acute stressors for 22 days caused changes in the LH-secreting cells similar to that of high dose of ß-EP, whereas administration of NALT attenuated these effects. Taken together, the results indicate that increased concentration of ß-EP as may occur during stressful conditions can cause suppression of LH secretion, leading to the inhibition of spawning, and that treatment of NALT attenuates the stress-induced inhibition of LH secretion in fish.

Stress-induced inhibition of recruitment of ovarian follicles for vitellogenic growth and interruption of spawning cycle in the fish Oreochromis mossambicus.

The tilapia, Oreochromis mossambicus, shows a short ovarian cycle of 24-26 days in nonmouthbrooding condition. In this study, the stripped female O. mossambicus were exposed to repeated mild acute stressors such as handling, chasing, frequent netting and low water levels daily for a period of 26 days. The follicular dynamics did not show significant difference during previtellogenic phase (day 12), whereas the mean number of stage IV (vitellogenic) follicles remained significantly lower compared with controls at the end of vitellogenic phase (day 18). The stage V (vitellogenic, preovulatory) follicles were completely absent in contrast to their presence in controls prior to spawning (day 23). The control fish spawned spontaneously after 24 days and entered mouthbrooding phase, whereas those exposed to stressors did not spawn. Furthermore, the serum levels of estradiol (E(2)) remained significantly lower concomitant with a significant increase in the serum cortisol concentration during vitellogenic and prespawning phase compared with those of the controls. The LH cells in the PPD of the pituitary gland showed weak immunoreactivity through vitellogenic and prespawning phase in fish exposed to stressors indicating the diminished secretory activity in contrast to the intensely stained ir-material in controls. The study reveals the disruptive effects of aquacultural stressors on the spawning cycle through suppression of LH and E(2) secretion along the pituitary-ovary axis. The results suggest that the ovarian stress response depends on the phase of the cycle and that the interruption of the spawning cycle is due to inhibition of recruitment of preovulatory follicles in O. mossambicus.