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The aim of this study was to determine the effect of stress on reproduction and the possible involvement of dopaminergic systems in the reproductive stress response in the mosquitofish Gambusia affinis. Exposure of fish to aquaculture stressors (four 10 min episodes of stress, each corresponding to a different stressor such as handling, chasing, frequent netting and low water levels), for a period of 30 days caused reduction in the mean numbers of stage I–IV follicles associated with lower number of pregnant females and embryos in most of the developmental stages compared with experimental controls. Besides, increase in the intensity of labelling and the per cent area of tyrosine hydroxylase (TH; a rate‐limiting enzyme in the biosynthetic pathway of catecholamines)‐ immunoreactive (ir) neurons was observed in the preoptic area (POA) and the nucleus preopticus (NPO) regions of the brain concomitant with reduction in the labelling of gonadotropin releasing hormone–immunoreactive (GnRH‐ir) fibres in the proximal pars distalis (PPD) of the pituitary gland in stressed fish compared with experimental controls. Treatment of domperidone (DOM) caused an increase in the number of stage II and V follicles and promoted pregnancy rate concomitant with an increase in the number of embryos at various developmental stages compared with those of experimental controls. Similar treatment to stressed fish caused an increase in the number of stages I–V follicles compared with those in stress alone group. The GnRH fibres showed increased immunolabelling in stress + DOM treated fish compared with stress alone fish. On the other hand, TH‐immunoreactivity in the POA and the NPO regions was reduced in stress + DOM treated fish compared with stress‐alone group. These results suggest that stress inhibits follicular development and subsequent hatching success through the suppression of GnRH and that the inhibition appears to be mediated through dopamine, for the first time in a viviparous fish.

Summary The present investigation was conducted to determine the influence of dopamine (DA) receptor agonist bromocriptine (BRO) on reproduction and onset of puberty in the viviparous fish Gambusia affinis. In the first experiment, the mean number of stage I and II follicles (previtellogenic) in 0.8 or 5 mg BRO treated fish did not show significant difference compared to those of experimental controls, whereas the mean number of stage III follicles were significantly lower in 5 mg BRO treated fish compared to experimental controls. However, treatment of 0.8 or 5 mg BRO resulted in significantly lower numbers of stage IV (early vitellogenic) and V (late vitellogenic) follicles compared to those of experimental controls. There was decrease in the percent occurrence of pregnancy and different stages of embryos in BRO treated fish compared with the experimental controls. Concomitant with this, sparsely distributed gonadotropin releasing hormone immunoreactive (GnRH‐ir) fibres were observed in the proximal pars distalis (PPD) region of the pituitary gland in BRO treated fish compared to those of dense accumulations of these fibres in the PPD region of the pituitary gland in experimental controls. In the second experiment, exposure of juveniles (25 DPH) to same doses of BRO for 45 days resulted in complete absence of vitellogenic follicles and presence of few GnRH‐ir fibres in 5 mg BRO treated juvenile in contrast to presence of vitellogenic follicles and dense aggregation of GnRH fibres in treatment controls. Overall, the results of the present investigation suggest that DA affects ovarian follicular and embryonic development and onset of puberty in viviparous species.