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.

In the present study, we examined whether stress-induced suppression of reproduction is mediated through the catecholaminergic neurotransmitter dopamine (DA) in the female cichlid fish Oreochromis mossambicus. In the first experiment, application of antibody against tyrosine hydroxylase (TH; a marker for DA) in brain sections revealed the presence of intensely stained TH immunoreactive cells in the preoptic area (POA) and nucleus preopticus (NPO) during the previtellogenic phase. These cells showed weak immunoreactivity during the vitellogenic and prespawning phases concomitant with darkly stained luteinising hormone (LH) immunoreactive content in the proximal pars distalis (PPD) of the pituitary gland and fully ripened follicles (stage V) in the ovary of control fish. However, in fish exposed to aquacultural stressors, TH secreting cells remained intensely stained in POA and NPO regions during the prespawning phase, indicating increased synthetic and secretory activity, which was reflected by a significantly higher DA content compared to controls. Increased DA activity as a result of stress was associated with a decrease in the LH immunoreactive content in the PPD and an absence of stage V follicles in the ovary. In the second experiment, administration of DA caused effects similar to those in stressed fish, whereas DA receptor antagonist domperidone (DOM) treatment significantly increased the LH content in the PPD and the number of stage V follicles in unstressed fish. On the other hand, treatment of stressed fish with DOM resulted in dark accumulations of LH immunoreactive content in the PPD accompanied by the presence of stage V follicles in the ovary. Taken together, these results suggest an additional pathway for the inhibitory effects of stress through dopaminergic neurones along the reproductive axis.

The opioid peptide β-endorphin (β-EP) is known to play modulatory role in vertebrate reproduction. The objective of the present study was to determine the influence of β-EP and the opioid receptor antagonist naltrexone (NALT) on follicular development in normal, and stressed female tilapia Oreochromis mossambicus. Administration of 4 µg β-EP, but not 0.5 or 1.5 µg β-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 β-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 β-EP, whereas administration of NALT attenuated these effects. The results reveal that β-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.

Opioid peptide β-endorphin (β-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 β-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 β-EP, but not 0.5 or 1.5 μg β-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 β-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 β-EP, whereas administration of NALT attenuated these effects. Taken together, the results indicate that increased concentration of β-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.

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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 hypothalamo–pituitary–interrenal 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.

Summary Cortisol is the principal glucocorticoid released due to various forms of environmental as well as aquacultural stressors in fish. The aim of the present investigation was to determine cortisol‐induced alterations along the luteinizing hormone (LH)‐secreting cells–ovary axis in the tilapia Oreochromis mossambicus. Administration of cortisol to stripped O. mossambicus for a period of 22 days during the ovarian cycle caused significantly higher number of follicles with chromatin nucleoli (stage I) compared to those of initial controls and controls. Whereas the number of follicles at perinucleolar (stage II) and vitellogenic (stage IV) stages did not differ significantly between controls and cortisol‐treated fish, the number of follicles at cortical alveolar stage (stage III) was significantly lower in cortisol‐treated fish than in controls. While the stage V follicles (maturation stage) were absent in initial controls, their presence in controls was concomitant with intensely labelled LH‐secreting cells in the proximal pars distalis (PPD) region of the pituitary gland during prespawning phase. However, cortisol‐treatment resulted in complete absence of stage V follicles associated with weakly immunoreactive LH‐content in the PPD region of the pituitary gland during prespawning phase. These results suggest that chronic cortisol‐ treatment causes suppression of LH‐secreting cells activity and blocks progression of vitellogenic follicular development process in O. mossambicus.