Organization of enkephalinergic neuronal system in the central nervous system of the gecko Hemidactylus frenatus.

Enkephalins are endogenous opioid pentapeptides that play a role in neurotransmission and pain modulation in vertebrates. However, the distribution pattern of enkephalinergic neurons in the brains of reptiles has been understudied. This study reports the organization of the methionine-enkephalin (M-ENK) and leucine-enkephalin (L-ENK) neuronal systems in the central nervous system of the gecko Hemidactylus frenatus using an immunofluorescence labeling method. Although M-ENK and L-ENK-immunoreactive (ir) fibers extended throughout the pallial and subpallial subdivisions, including the olfactory bulbs, M-ENK and L-ENK-ir cells were found only in the dorsal septal nucleus. Enkephalinergic perikarya and fibers were highly concentrated in the periventricular and lateral preoptic areas, as well as in the anterior and lateral subdivisions of the hypothalamus, while enkephalinergic innervation was observed in the hypothalamic periventricular nucleus, infundibular recess nucleus and median eminence. The dense accumulation of enkephalinergic content was noticed in the pars distalis of the hypophysis. In the thalamus, the nucleus rotundus and the dorsolateral, medial, and medial posterior thalamic nuclei contained M-ENK and L-ENK-ir fibers, whereas clusters of M-ENK and L-ENK-ir neurons were observed in the pretectum, mesencephalon, and rhombencephalon. The enkephalinergic fibers were also seen in the area X around the central canal, as well as the dorsal and ventral horns. The widespread distribution of enkephalin-containing neurons within the central nervous system implies that enkephalins regulate a variety of functions in the gecko, including sensory, behavioral, hypophysiotropic, and neuroendocrine functions.

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.

Serotonin-immunoreactivity in the brain of the cichlid fish Oreochromis mossambicus.

Serotonin (5-HT) is an evolutionarily conserved monoaminergic neurotransmitter found in the central nervous system and peripheral nervous system across invertebrates and vertebrates. Although the distribution of 5-HT-immunoreactive (5-HT-ir) neurons is investigated in various fish species, the organization of these neurons in cichlid fishes is poorly understood. These fish are known for their adaptability to diverse environments, food habits, and complex mating and breeding behaviors, including parental care. In this paper, we describe the organization of 5-HT-ir neurons in the brain of the cichlid fish Oreochromis mossambicus. Aggregations of 5-HT-ir neurons were spotted in the granule cell layer of the olfactory bulb and near the ventricular border in the preoptic area and magnocellular subdivisions of the nucleus preopticus. Although the presence of 5-HT-ir cells and fibers in the hypothalamic and thalamic regions, cerebellum, and raphe nuclei was comparable to that of other teleosts, the current study reveals the occurrence of 5-HT-ir cells and fibers for the first time in some areas, such as the nucleus posterior tuberis, nucleus oculomotorius, and nucleus paracommissuralis in the tilapia. While the presence of 5-HT-ir cells and fibers in gustatory centers suggests a role for serotonin in the processing of gustatory signals, distinctive pattern of 5-HT immunoreactivity was seen in the telencephalon, pretectal areas, mesencephalic, and rhombencephalic regions, suggesting a cichlid fish specific organization of the serotonergic system. In conclusion, the 5-HT system in the tilapia brain may serve several neuroendocrine and neuromodulatory roles, including regulation of reproduction and sensorimotor processes.

GABA-immunoreactive neurons in the Central Nervous System of the viviparous teleost Poecilia sphenops.

Gamma-aminobutyric acid (GABA) functions as the primary inhibitory neurotransmitter within the central nervous system (CNS) of vertebrates. In this study, we examined the distribution pattern of GABA-immunoreactive (GABA-ir) cells and fibres in the CNS of the viviparous teleost Poecilia sphenops using immunofluorescence method. GABA immunoreactivity was seen in the glomerular, mitral, and granular layers of the olfactory bulbs, as well as in most parts of the dorsal and ventral telencephalon. The preoptic area consisted of a small cluster of GABA-ir cells, whereas extensively labelled GABA-ir neurons were observed in the hypothalamic areas, including the paraventricular organ, tuberal hypothalamus, nucleus recessus lateralis, nucleus recessus posterioris, and inferior lobes. In the thalamus, GABA-positive neurons were only found in the ventral thalamic and central posterior thalamic nuclei, whereas the dorsal part of the nucleus pretectalis periventricularis consisted of a few GABA-ir cells. GABA-immunoreactivity was extensively seen in the alar and basal subdivisions of the midbrain, whereas in the rhombencephalon, GABA-ir cells and fibres were found in the cerebellum, motor nucleus of glossopharyngeal and vagal nerves, nucleus commissuralis of Cajal, and reticular formation. In the spinal cord, GABA-ir cells and fibres were observed in the dorsal horn, ventral horn, and around the central canal. Overall, the extensive distribution of GABA-ir cells and fibres throughout the CNS suggests several roles for GABA, including the neuroendocrine, viscerosensory, and somatosensory functions, for the first time in a viviparous teleost.

The influence of the opioid pentapeptide methionine-enkephalin on seasonal and FSH-induced ovarian recrudescence in the gecko Hemidactylus frenatus.

Although methionine-enkephalin (M-ENK) is implicated in the regulation of reproductive functions in vertebrates, its function in reptiles is little understood. This study aims to elucidate the role of M-ENK on seasonal and follicle stimulating hormone (FSH)-induced ovarian recrudescence in the gecko Hemidactylus frenatus. In the first experiment, administration of 5 µg M-ENK did not affect germinal bed activity or follicular developmental stages I, II, and III (previtellogenic) and IV (vitellogenic), but there were no stage V (vitellogenic) follicles in the ovary. However, there was a significant decrease in the mean numbers of oogonia and primary oocytes in the germinal bed associated with the complete absence of stage IV and V follicles in 25 µg M-ENK-treated lizards in contrast to experimental controls. Furthermore, there was a significant decrease in gonadotropin-releasing hormone - immunoreactive (GnRH-ir) content in the median eminence (ME) and pars distalis (PD) of the pituitary gland and sparse labelling of hypothalamic GnRH-ir neurons in 25 µg M-ENK-treated lizards. In the second experiment, treatment with FSH during the regression phase of the ovarian cycle resulted in the appearance of stage IV and V follicles, in contrast to their absence in the initial controls and treatment controls. However, treatment with 25 µg M-ENK + FSH did not result in the appearance of these follicles, indicating the inhibitory effect of M-ENK on FSH-induced ovarian recrudescence. These findings suggest that M-ENK inhibits the germinal bed and vitellogenic follicular growth in a dose-dependent manner, possibly mediated through the suppression of GnRH release in the ME and PD. In addition, M-ENK may also act at the level of the ovary in the gecko.

The opioid peptide leucine-enkephalin disrupts seasonal and gonadotropin-induced ovarian recrudescence in the gecko Hemidactylus frenatus.

The enkephalins are known to regulate many physiological functions, including reproduction in vertebrates. However, the role of leucine-enkephalin (L-ENK) in the ovarian recrudescence activity of reptiles is not known. In the present study, we studied the influence of L-ENK on seasonal and FSH-induced ovarian recrudescence during the breeding and non-breeding phases of the cycle in the tropical and subtropical gecko Hemidactylus frenatus. In the first experiment, treatment with 5 and 25 µg L-ENK resulted in a dose-dependent inhibitory effect on the hypothalamic gonadotropin-releasing hormone (GnRH) neurons and ovary, as indicated by a significantly decreased percent area of GnRH-immunoreactive (GnRH-ir) fibres in the median eminence and pars distalis of the pituitary gland, concomitant with complete absence of stage V (late vitellogenic) follicles in the ovary compared to those of experimental controls. In the second experiment, administration of FSH to lizards in the regression phase stimulated the recruitment of stage IV and V (vitellogenic) follicles in contrast to their absence in initial controls or treatment controls. However, similar treatment of FSH in combination with 25 µg L-ENK did not result in the development of stage IV or V follicles. Together, these results suggest for the first time that treatment with 5 and 25 µg L-ENK exerts a dose-dependent inhibitory effect on the hypothalamic GnRH release into the median eminence and pituitary gland, leading to the blockade of ovarian recrudescence. These results also suggest a possible direct inhibitory effect of L-ENK at the level of the ovary in the gecko.

Organization of the galaninergic neuronal system in the brain of the gecko Hemidactylus frenatus.

Galanin (GAL) is a 29 amino acid peptide present in the central nervous system (CNS) as well as peripheral tissues in vertebrates. However, the brain distribution pattern of GAL is understudied in reptiles. The aim of this study was to determine the organization of galaninergic neuronal system in the brain of the gecko Hemidactylus frenatus, a tropical and sub-tropical lizard, using rabbit anti-galanin antibody. In the telencephalon, GAL-ir perikarya and fibres were found in the lateral septal nucleus, but only GAL-ir fibres were observed in the striatum, nucleus accumbens, anterior commissure, nucleus centralis amygdalae, dorsal and medial septal nuclei, nucleus of the diagonal band of Broca and in the optic chiasma. In the preoptic region, a cluster of GAL-ir cells and fibres was observed in the periventricular preoptic area and lateral preoptic area. GAL-ir perikarya and fibres were observed in hypothalamic areas such as the supraoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, periventricular nucleus of the hypothalamus, infundibular recess nucleus and in the median eminence, whereas GAL-ir fibres were present in the pars distalis of the pituitary gland. In the thalamus, GAL-ir fibres were observed in the dorsomedial, dorsolateral, and medial thalamic nuclei. GAL-ir fibres were also detected in mesencephalic areas such as the optic tectum, torus semicircularis, ventral tegmental area and substantia nigra, brain stem as well as the spinal cord. The organization of GAL-ir cells and fibres throughout the gecko brain suggests several neuroendocrine, neuromodulatory and behavioural functions for GAL in lizards. The study provides new insights into the evolutionarily conserved nature of GAL peptide in squamate reptiles and forms a valuable basis for future comparative studies.

Organization of the Melanin concentrating hormone secreting neuronal system in the brain of the cichlid fish Oreochromis mossambicus.

Melanin concentrating hormone (MCH) is a highly conserved cyclic peptide present in vertebrates. In this study, we describe the organization of MCH-immunoreactive (MCH-ir) cells and fibres in different regions of the brain in the cichlid fish Oreochromis mossambicus. Only MCH-ir fibres were observed in dorsal and ventral subdivisions of the telencephalon, the preoptic area and magnocellular and parvocellular divisions of the nucleus preopticus, and in hypothalamic areas such as the suprachiasmatic nucleus and tuberal area. Distinctly labelled MCH-ir perikarya were observed in the paraventricular organ, lateral and medial subdivisions of the nucleus lateralis tuberis, nucleus recessus lateralis and in the nucleus posterior tuberis. The pituitary gland showed MCH-ir fibres in the proximal pars distalis, neurohypophyseal ramifications and in pars intermedia where the dark accumulations of MCH-ir content corresponded to enlarged axon terminals. In the diencephalon, MCH-ir fibres were also labelled in the pretectal area, thalamic nuclei and preglomerular complex. In the midbrain tegmentum, a cluster of MCH-ir neurons was detected in the dorsal tegmental nucleus, whereas MCH-ir fibres were distributed in the torus semicircularis and optic tectum. In the rhombencephalon, MCH-ir fibres were located in the nucleus lateralis valvulae, cerebellum and secondary gustatory nucleus. Overall, the widespread distribution of MCH-ir cells and fibres in the brain suggests diverse roles for MCH such as regulation of sensorimotor and neuroendocrine functions in the tilapia.

Chronic exposure to aquacultural stressors affects pituitary-testis axis in the Mozambique tilapia Oreochromis mossambicus.

Reproduction in fish is modulated by several factors that include environmental and endocrine components. The aim of this study was to elucidate the effect of aquacultural stressors along the pituitary-testis axis in a continuously breeding cichlid fish Oreochromis mossambicus. The fish (35.05 ± 2.20 g) were divided into three groups (n = 10 in each group, n = 5 in each replicate), namely initial controls (euthanized on the day of initiation of experiment), time-matched controls (kept undisturbed), and stressed fish, which were subjected to different kinds of randomized aquacultural stressors such as handling, chasing, frequent netting, and low water levels, daily for a period of 21 days. Although the gonadosomatic index and the mean numbers of spermatogonia-A and spermatogonia-B did not differ significantly among different experimental groups, significant decrease was observed in the mean numbers of primary spermatocytes, secondary spermatocytes, early spermatids, and late spermatids in fish exposed to stressors compared to those of initial controls and time-matched controls. While the diameter of the seminiferous lobule was significantly lower, the size of the lumen and the serum levels of cortisol were significantly increased in stressed fish compared with initial controls and time-matched controls. Furthermore, weak androgen receptor immunoreactivity was observed in the Sertoli cells of the testis in contrast to the strongly immunoreactive androgen receptors in initial controls and time-matched controls. Concomitant with this, there was a significant decrease in the percent area and the intensity of luteinizing hormone (LH) immunoreactive content in the proximal pars distalis (PPD) region of the pituitary gland in stressed fish compared with initial controls and time-matched controls. Overall, these results suggest that exposure to chronic aquacultural stressors causes suppression of LH synthesis in the pituitary gland concomitant with decreased androgen receptor expression and blockade of recruitment of germline cells at the meiosis stage. This inhibition appears to be mediated through the hypothalamic-pituitary-interrenal axis in the tilapia O. mossambicus.

Melanin-concentrating hormone interferes with the hypothalamic-pituitary-gonad axis in the Mozambique tilapia.

This study was conducted to elucidate the influence of melanin-concentrating hormone (MCH) along the reproductive-axis in the female tilapia Oreochromis mossambicus. Administration of MCH (4 µg / 0.1 ml saline) for 22 days resulted in significantly lower gonadosomatic index compared to controls. Significant reduction in the mean numbers of follicles at different stages of development such as previtellogenic (stages I-III), vitellogenic (stage IV) and preovulatory (stage V) follicles was observed in MCH-treated fish compared with controls. On the other hand, the rate of atresia was significantly higher in follicles at stages II, III and IV in MCH-treated fish. In addition, in the pituitary gland, sparsely labelled gonadotropin releasing hormone (GnRH)-immunoreactive fibres were observed in MCH-treated fish in contrast to their intense labelling in controls. The serum level of luteinizing hormone (LH) showed significant decrease, but the serum cortisol level rose significantly following MCH treatment compared to those of controls. Collectively, these results indicate for the first time, that MCH treatment blocks follicular development during the ovarian cycle, possibly through the suppression of GnRH-LH axis in fish. The results also indicate that MCH may activate the stress-axis pathway in fish.

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.

The effect of α-MSH treatment on the hypothalamic-pituitary-gonad axis in the cichlid fish Oreochromis mossambicus.

In this investigation, we examined the influence of alpha-melanocyte stimulating hormone (α-MSH), a proopiomelanocortin-derived peptide, along the hypothalamic-pituitary-gonad axis in a cichlid fish Oreochromis mossambicus. Administration of α-MSH (40 µg/0.1 ml saline) for 22 days did not affect the number of stage I (previtellogenic) follicles but caused significant reduction in the mean numbers of previtellogenic (stages II and III), vitellogenic (stage IV) and preovulatory (stage V) follicles compared to those of controls. While the gonadosomatic index was significantly lower, the rate of follicular atresia in stages II, III and IV remained significantly higher in α-MSH-treated fish compared to the controls. Furthermore, the mean percent area of gonadotropin-releasing hormone-immunoreactive (GnRH-ir) fibres and luteinizing hormone-immunoreactive (LH-ir) cells were significantly reduced in the proximal pars distalis  of the pituitary gland in α-MSH-treated fish compared with the controls. Together, our findings suggest for the first time that the treatment of α-MSH blocks the follicular developmental process during the ovarian cycle, possibly through the inhibition of GnRH-LH pathway in teleosts.

Neuroanatomical organization of methionine-enkephalinergic system in the brain of the Mozambique tilapia Oreochromis mossambicus.

Enkephalins are a class of opioid peptides implicated in several physiological and neuroendocrine responses in vertebrates. In this study, using immunocytochemical or immunofluorescence technique, we examined the neuroanatomical distribution of methionine enkephalin (M-ENK) immunoreactivity in the central nervous system (CNS) of the cichlid fish Oreochromis mossambicus. In the telencephalon, no M-ENK-like-immunoreactive (M-ENK-L-ir) perikarya, but sparsely distributed fibres were detected in the glomerular layer and the granule cell layer of the olfactory bulb. Although intensely labeled M-ENK-L-ir cells and fibres were found in the pallium, no M-ENK immunoreactivity was observed in the subpallium. The preoptic area showed a few M-ENK-L-ir somata and dense innervations of fibres. In the hypothalamic area, M-ENK-L-ir cells and fibres were located in magnocellular and parvocellular subdivisions of the nucleus preopticus, and medial and lateral subdivisions of the nucleus lateralis tuberis. Surrounding the recessus lateralis of the third ventricle, several intensely stained and packed M-ENK-L-ir cells and fibres were seen in dorsal, lateral and ventral subdivisions of the nucleus recessus lateralis. In the diencephalon, M-ENK immunoreactivity was restricted to the habenula, the thalamus, the pretectal area and the nucleus posterior tuberis. Dense aggregations of M-ENK-L-ir fibres were seen in the mesencephalic subdivisions, the optic tectum and the torus semicircularis, whereas a few fusiform M-ENK-L-ir cells and fibres were scattered in the midbrain tegmentum. In the rhombencephalon, different populations of ovoid or spindle shaped M-ENK-L-ir cells were observed in the secondary gustatory nucleus, the sensory trigeminal nerve nucleus, the nucleus reticularis medialis and the vagal motor nucleus, whereas bands of fibres were seen in the rostral spinal cord. Collectively, the widespread distribution of M-ENK immunoreactivity in the CNS suggests a role for this opioid peptide in regulation of neuroendocrine control of reproduction and modulation of sensorimotor functions in fish.

Alpha-melanocyte stimulating hormone immunoreactivity in the brain of the cichlid fish Oreochromis mossambicus.

This study reports the distribution of a pro-opiomelanocortin-derived neuropeptide α-MSH in the brain of the cichlid fish Oreochromis mossambicus. α-MSH-ir fibres were found in the granule cell layer of the olfactory bulb, the medial olfactory tract, the pallium and the subpallium, whereas in the preoptic area of the telencephalon, few large α-MSH-ir perikarya along with extensively labeled fibres were observed close to the ventricular border. Dense network of α-MSH-ir fibres were seen in the hypothalamic areas such as the nucleus preopticus pars magnocellularis, the nucleus preopticus pars parvocellularis, the suprachiasmatic nucleus, the nucleus anterior tuberis, the paraventricular organ, the subdivisions of the nucleus recessus lateralis and the nucleus recessus posterioris. In the nucleus lateralis pars medialis, some α-MSH-ir perikarya and fibres were found along the ventricular margin. In the diencephalon, numerous α-MSH-ir fibres were detected in the nucleus posterior tuberis, the nucleus of the fasciculus longitudinalis medialis and the nucleus preglomerulosus medialis, whereas in the mesencephalon, α-MSH-ir fibres were located in the optic tectum, the torus semicircularis and the tegmentum. In the rhombencephalon, α-MSH-ir fibres were confined to the medial octavolateralis nucleus and the descending octaval nucleus. In the pituitary gland, densely packed α-MSH-ir cells were observed in the pars intermedia region. The widespread distribution of α-MSH-immunoreactivity throughout the brain and the pituitary gland suggests a role for α-MSH peptide in regulation of several neuroendocrine and sensorimotor functions as well as darkening of pigmentation in the tilapia.

The stress - Reproductive axis in fish: The involvement of functional neuroanatomical systems in the brain.

The neuroendocrine-stress axis of nonmammalian species is evolutionarily conserved, which makes them useful to serve as important model systems for elucidating the function of the vertebrate stress response. The involvement of hypothalamo-pituitary-adrenal (HPA) axis hormones in regulation of stress and reproduction is well described in different vertebrates. However, the stress response is a complex process, which appears to be controlled by a number of neurochemicals in association with hypothalamo-pituitary-interrenal (HPI) axis or independent of HPI axis in fish. In recent years, the participation of neurohormones other than HPI axis in regulation of stress and reproduction is gaining more attention. This review mainly focuses on the involvement of functional neuroanatomical systems such as the catecholaminergic neurotransmitter dopamine (DA) and opioid peptides in regulation of the stress-reproductive axis in fish. Occurrences of DA and opioid peptides like ß-endorphin, enkephalins, dynorphin, and endomorphins have been demonstrated in fish brain, and diverse roles such as pain modulation, social behaviour and reproduction are implicated for these hormones. Neuroanatomical studies using retrograde tracing, immunohistochemical staining and lesion methods have demonstrated that the neurons originating in the preoptic region and the nucleus lateralis tuberis directly innervate the pituitary gland and, therefore, the hypophysiotrophic role of these hormones. In addition, heightened synthetic and secretory activity of the opioidergic and the dopaminergic neurons in hypothalamic areas of the brain during stress exposure suggest potentially intricate relationship with the stress-reproductive axis in fish. Current evidence in early vertebrates like fish provides a novel insight into the underlying neuroendocrine mechanisms as additional pathways along the stress-reproductive axis that seem to be conserved during the course of evolution.

The opioid peptide dynorphin suppresses pituitary-ovary axis in the tilapia Oreochromis mossambicus.

The opioid peptides are involved in the regulation of neuroendocrine functions in vertebrates. Nonetheless, the influence of an opioid peptide, dynorphin A (DYN), on reproduction in fish is understudied. The aim of this work was to study the influence of DYN on the pituitary-ovary axis in Oreochromis mossambicus. Daily injections (ip) of 250 µg DYN kg-1 body weight for 22 days during the ovarian cycle caused a reduction in the intensity and the per cent area of luteinizing hormone (LH) immunoreactive content in the proximal pars distalis region of the pituitary gland compared with an intense immunostaining in time-matched controls. In the ovary, DYN treatment caused a decrease in the number of stage I (previtellogenic) follicles compared with time-matched controls. No difference was observed in the number of stage IV (vitellogenic) follicles among different experimental groups, whereas the numbers of stage II and stage III follicles (previtellogenic) were higher in DYN-treated fish than in time-matched controls. Nonetheless, there was a reduction in the number of stage V (preovulatory) follicles in DYN-treated fish compared with time-matched controls. Taken together, these results indicate that DYN exerts an inhibitory effect on follicular recruitment at the late vitellogenic stage, through the suppression of LH secretion in fish.

Tyrosine hydroxylase-immunoreactive neurons in the brain of tadpole of the narrow mouthed frog Microhyla ornata.

Catecholamines serve as a neuromodulators of many behavioral and endocrine responses in different vertebrates including amphibians. However, the neuroanatomical studies on catecholamines, especially in the tadpole brain are limited. In this study, we report the distribution of catecholaminergic neurons in different areas of the brain in the tadpole of Microhyla ornata at metamorphic climax stage. Application of antisera against tyrosine hydroxylase (TH) revealed the presence of catecholaminergic cells and fibres in the olfactory bulb, the telencephalon, the diencephalon, the mesencephalon, the spinal cord and the pituitary gland. Whereas densest aggregations of TH-immunoreactive (TH-ir) fibres were noticed in the nucleus accumbens and the amygdala pars medialis regions of the telencephalon, highest population of TH-ir cells with dorsolaterally and rostrocaudally oriented fibres was observed in the preoptic area. Larger and distinct TH-ir cell bodies along with few dorsolaterally oriented TH-ir fibres were scattered throughout the suprachiasmatic nucleus. While moderate to intensely stained clusters of TH-ir cells were observed in dorsal and ventral hypothalamic regions, conspicuous TH-ir cells and fibres were seen in the pars distalis of the pituitary gland. In the nucleus tuberculi posterioris, numerous moderate sized TH-ir cells were found along the margin of the third ventricle and the fibres from this region were oriented dorsolaterally towards the torus semicircularis and tectal regions, whereas well organized largest TH-ir cells and fibres were seen in the tegmentum. In the spinal cord, medium sized TH-ir cells along with numerous laterally running fibres were encountered. Overall, widespread distribution of the TH-ir cells and fibres in the brain and the pituitary gland of the tadpole suggest diverse roles for the catecholamines in regulation of locomotion, olfaction, skin pigmentation and endocrine responses during final stages of metamorphosis in M. ornata.

Influence of endomorphins along the pituitary-ovary axis in the Mozambique Tilapia Oreochromis mossambicus.

Endomorphins (EM-1 and EM-2) are the tetrapeptides involved in pain and neuroendocrine responses with a high affinity for µ-opioid receptors in vertebrates. However, their role in fish reproduction is not clear. The aim of this study was to investigate the influence of EM-1 and EM-2 on the pituitary-ovary axis in the Mozambique tilapia Oreochromis mossambicus. The experimental set-up consisted of four groups, namely, initial controls, controls, EM-1- and EM-2-treated groups (n = 10 in each group consisting of two replicates). Although the number of stage IV (vitellogenic) follicles was significantly lower (P < 0.05) in controls compared to initial controls, the stage V (preovulatory) follicles were present in controls in contrast to their absence in initial controls. Treatment of 40 µg EM-1/0.1 ml saline/fish/day for 22 days resulted in significant increase (P < 0.05) in the number of stage I follicles compared to controls. While similar treatment of EM-2 did not significantly alter the number of stage I follicles compared to controls, the number of stage II follicles was significantly lower (P < 0.05) in this group compared to those of controls and EM-1 treated fish. The number of stage III and IV follicles did not significantly differ among controls, EM-1- and EM-2-treated groups. However, a significant reduction (P < 0.05) in the mean number of stage V follicles was observed in EM-1- and EM-2-treated fish compared to controls. These changes were concomitant with significant reduction (P < 0.05) in the intensity and the percent area of immunoreactivity of luteinizing hormone (LH) secreting cells in the proximal pars distalis (PPD) of the pituitary gland and significantly higher (P < 0.05) percent occurrence of follicular atresia in EM-1- and EM-2-treated fish compared to those of controls. Taken together, these results suggest an inhibitory effect for endomorphins along the pituitary-ovary axis, for the first time in fish.

Leucine-enkephalin-immunoreactive neurons in the brain of the cichlid fish Oreochromis mossambicus.

Enkephalins are the pentapeptides involved in pain relief and neuroendocrine responses with high affinity for delta opioid receptors in vertebrates. In the present investigation, we studied the distribution of leucine-enkephalin-immunoreactive (L-ENK-ir) neurons in the brain of the cichlid fish Oreochromis mossambicus. Application of the antisera against L-ENK revealed the presence of numerous L-ENK-ir perikarya and fibres in subdivisions of the dorsal and the ventral telencephalon, the medial olfactory tract and the nucleus entopeduncularis, whereas intensely labelled L-ENK-ir fibres were noticed in the olfactory bulb. Furthermore, the presence of L-ENK-ir cells and dense accumulations of fibres in the preoptic area and its subdivisions, the nucleus preopticus pars magnocellularis and the nucleus preopticus pars parvocellularis suggested a role for this peptide in regulation of reproduction. While intensely labelled cells and fibres were found in the nucleus lateralis tuberis pars lateralis as well as the nucleus lateralis tuberis pars medialis, some L-ENK-ir fibres were seen at the hypothalamo-hypophyseal tract indicating the possible hypophysiotrophic role for this peptide. Numerous L-ENK-ir cells and dense network of fibres were observed in the subdivisions of the nucleus recess lateralis and the pretectal area, whereas intensely labelled thick network of L-ENK- fibres were found in the ventromedial thalamic nucleus, the sub-layers of the optic tectum and the rostral spinal cord. The widespread distribution of L-ENK-immunoreactivity in the olfactory bulb, the telencephalon, the diencephalon and the mesencephalon regions of the brain as well as the spinal cord suggests the possible involvement of this peptide in the regulation of diverse functions such as neuroendocrine, antinociceptive, visual and olfactory responses in O. mossambicus.