The opioid peptide leucine enkephalin modulates hypothalamic-hypophysial axis in the cichlid fish Oreochromis mossambicus.

In vertebrates, opioid peptides are thought to be involved in the regulation of reproduction; however, the significance of enkephalins in testicular function remains unclear. We examined the influence of δ-opioid receptor agonist leucine enkephalin (L-ENK) on the hypophysial-testicular axis of the cichlid fish Oreochromis mossambicus. Treatment with a low dose of L-ENK (60 µg) caused a significant increase in the numbers of primary and secondary spermatocytes and early and late spermatids, concomitant with intense immunolabelling of testicular androgen receptors, but did not significantly alter serum luteinizing hormone (LH) and 11-ketotestosterone (11-KT) levels compared to those of controls. Nevertheless, treatment with a high dose of L-ENK (200 µg) caused a significant reduction in the numbers of secondary spermatocytes as well as late spermatids associated with marginal immunolabelling of androgen receptors and significantly lower concentrations of serum 11-KT and LH compared to controls. In addition, the serum cortisol level was not affected in low-dose L-ENK-treated fish, but its level was significantly increased in the high-dose L-ENK-treated group. Together, these findings indicate that a low dose of L-ENK stimulates the germ cells at the meiosis stage and promotes further stages of spermatogenesis, whereas a high concentration of L-ENK inhibits spermatogenesis at the advanced stages. This effect appears to be mediated through the suppression of testicular steroidogenesis and the reduction of LH release in the pituitary gland of tilapia. The findings also suggest that elevated L-ENK levels in teleosts may exert their inhibitory influence on the hypophysial-testicular axis via glucocorticoids.

The opioid peptide ß-endorphin interferes with the pituitary-testis axis in the Mozambique tilapia Oreochromis mossambicus.

Although the involvement of ß-endorphin (ß-ERP) in vertebrate reproduction has been suggested, its role in testicular activity is not clear in fish. We describe the influence of ß-ERP on spermatogenesis in a cichlid fish in the present paper. In comparison to the control group, the administration of ß-ERP (3 µg) caused a significant increase in the number of spermatogonia-A and spermatids. Following treatment with ß-ERP (6 µg), a significant increase in the number of spermatogonia-A was observed, whereas the numbers of all the other germ cells, excluding spermatogonia-B, significantly decreased in comparison to those in the control group. In addition, treatment of fish with 6 µg ß-ERP resulted in a significant reduction in the dimensions of the lumen and seminiferous lobules, the level of immunopositive androgen receptor (AR) expression in Sertoli cells, and the percentage of luteinizing hormone (LH) immunolabeled in the pituitary compared to those in the control group or the group treated with 3 µg ß-ERP. In contrast, the intensity of AR immunoreactivity and the percentage of LH immunolabeling were substantially increased in fish treated with 3 µg ß-ERP compared to those in the control group. These findings reveal for the first time that a low dose of ß-ERP stimulates the recruitment of spermatogonia as well as spermateleosis, whereas a high concentration affects the recruitment of germ cells prior to meiotic division in tilapia. These results suggest that ß-ERP exerts modulatory effects at the testicular and hypophysial levels through alterations in AR expression and LH secretory activity, respectively, in teleosts.

Organization of serotonergic system in Sphaerotheca breviceps (Dicroglossidae) tadpole brain.

The monoaminergic neurotransmitter 5-hydroxytryptamine (5-HT) is known to be involved in several physiological, behavioural and neuroendocrine functions in vertebrates. In this study, we investigated the distribution of 5-HT neuronal system in the central nervous system (CNS) of Sphaerotheca breviceps tadpoles at metamorphic climax stage. In the telencephalon, there was no 5-HT-immunoreactive (5-HT-ir) perikarya, but conspicuous fibres were observed in the olfactory bulb, pallium, subpallium and amygdala complexes. The preoptic area showed dense 5-HT-ir somata and cerebrospinal fluid contacting fibres, whereas a few varicose 5-HT-ir fibres were noticed in the suprachiasmatic nucleus. 5-HT-ir cells and fibres were found in the ventral, lateral dorsal subdivisions of the hypothalamus and in the nucleus tuberculi posterioris, but only 5-HT-ir fibres were localised in the periventricular area and pituitary gland. Numerous 5-HT-ir cells and/or fibres were detected in the thalamus, entopeduncular area and mesencephalic subdivisions. In the rhombencephalon, although 5-HT-ir cells and fibres were noticed in the subdivisions of the raphe nucleus and reticular formation, a moderate plexus of fibres was observed in the cerebellum, parabrachial nucleus and solitary tract. Distinct 5-HT-ir fibres, but no perikarya, were observed in the rostral spinal cord. Overall, extensively labelled 5-HT-ir cells and fibres in the CNS of the metamorphic tadpole suggest possible roles for the involvement of 5-HT in various somatosensory, behavioural and neuroendocrine functions during final stages of development.

Serotonin (5-hydroxytryptamine)-immunoreactive neurons in the brain of the viviparous fish Gambusia affinis.

The monoaminergic neurotransmitter serotonin (5-HT) acts as a neuromodulator and is associated with a wide range of functions in fish. In this investigation, 5-HT immunoreactivity was studied in the central nervous system (CNS) of the viviparous mosquitofish Gambusia affinis. 5-HT-immunoreactive (5-HT-ir) cells/fibres were observed throughout the subdivisions of ventral and dorsal telencephalon including the olfactory bulb. Several intensely stained 5-HT-ir cells and/or fibres were detected in different areas of the hypothalamus as well as the proximal pars distalis of the pituitary gland. 5-HT-ir cells were restricted to the dorsal and ventral part of the pretectal diencephalic cluster, but only fibres were detected in the anterior, ventromedial and posterior subdivisions of the thalamic nucleus and in the preglomerular complex. In the mesencephalon, 5-HT-ir perikarya, and fibres were seen in the optic tectum, midbrain tegmentum and torus semicircularis. A cluster of prominently labelled 5-HT-ir neurons was observed in the superior raphe nucleus, whereas numerous 5-HT-ir fibres were distributed throughout the rhombencephalic divisions. In addition, a bundle of rostrocaudally running 5-HT-ir fibres was noticed in the spinal cord. This is the first detailed neuroanatomical study in a viviparous teleost, reporting a widespread distribution of 5-HT-ir somata and fibres in the CNS. The results of this study provide new insights into the evolutionarily well conserved nature of the monoaminergic system in the CNS of vertebrates and suggest a role for 5-HT in regulation of several physiological, behavioural and neuroendocrine functions in viviparous teleosts.

Domperidone treatment attenuates stress-induced suppression of reproduction in viviparous mosquitofish Gambusia affinis.

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.

Distribution of tyrosine hydroxylase-immunoreactive neurons in the brain of the viviparous fish Gambusia affinis.

Tyrosine hydroxylase (TH) is the common precursor enzyme involved in the biosynthetic pathway of the catecholaminergic neurotransmitters, dopamine and norepinephrine. In this investigation, the neuroanatomical distribution of TH-immunoreactivity was studied in the brain of the female mosquitofish Gambusia affinis. Numerous intensely stained TH-immunoreactive (ir) neurons were scattered in the olfactory bulb with their fibres extending towards the medial olfactory tract, whereas few telencephalic TH-ir cells with distinct fibres were observed in the dorsal nucleus of area ventralis telencephali and the posterior nucleus of area ventralis telencephali regions. Large TH-ir cell populations were seen in the suprachiasmatic nucleus and the nucleus dorsomedialis thalami regions of the diencephalon. Distinct TH-ir cells with long fibres were found at the preoptic area and the nucleus preopticus pars magnocellularis as well as the nucleus preopticus pars parvocellularis regions. Numerous intensely stained TH-ir cells were observed in the paraventricular organ and the nucleus posterior tuberis regions, whereas moderately stained cells were present in the nucleus of recessus lateralis medialis. Several TH-ir neurons were detected in medial and lateral subdivisions of the nucleus lateralis tuberis. Furthermore, the projections of the TH-ir fibres were seen in the proximal pars distalis region of the pituitary gland, where gonadotropin-secreting cells are located, suggesting the communication between TH cells and gonadotrope cells. In the rostral spinal cord, dense aggregations of the TH-ir fibres were noticed. Overall, the widespread distribution of the TH-ir neurons throughout the brain and their fibres in the spinal cord and the pituitary gland suggests diverse roles for the catecholaminergic neurons in various physiological functions including reproduction in the mosquitofish.