Somatostatin-like immunoreactivity in the pituitary and brain of three teleost fish species: somatostatin as a potential regulator of prolactin cell function

Endocrine regulation of prolactin cell function and modulation of osmoreception in the Mozambique tilapia

Prolactin (PRL) cells of the Mozambique tilapia, Oreochromis mossambicus, are osmoreceptors by virtue of their intrinsic osmosensitivity coupled with their ability to directly regulate hydromineral homeostasis through the actions of PRL. Layered upon this fundamental osmotic reflex is an array of endocrine control of PRL synthesis and secretion. Consistent with its role in fresh water (FW) osmoregulation, PRL release in tilapia increases as extracellular osmolality decreases. The hyposmotically-induced release of PRL can be enhanced or attenuated by a variety of hormones. Prolactin release has been shown to be stimulated by gonadotropin-releasing hormone (GnRH), 17-β-estradiol (E2), testosterone (T), thyrotropin-releasing hormone (TRH), atrial natriuretic peptide (ANP), brain-natriuretic peptide (BNP), C-type natriuretic peptide (CNP), ventricular natriuretic peptide (VNP), PRL-releasing peptide (PrRP), angiotensin II (ANG II), leptin, insulin-like growth factors (IGFs), ghrelin, and inhibited by somatostatin (SS), urotensin-II (U-II), dopamine, cortisol, ouabain and vasoactive intestinal peptide (VIP). This review is aimed at providing an overview of the hypothalamic and extra-hypothalamic hormones that regulate PRL release in euryhaline Mozambique tilapia, particularly in the context on how they may modulate osmoreception, and mediate the multifunctional actions of PRL. Also considered are the signal transduction pathways through which these secretagogues regulate PRL cell function.

Distribution of somatostatin immunoreactive neurons and fibres in the central nervous system of a chondrostean, the Siberian sturgeon (Acipenser baeri)

Somatostatin (SOM) is a neuropeptide that is widely distributed in the central nervous system of vertebrates. Two isoforms of somatostatin (SS1 and SS2) have been characterized in sturgeon and in situ hybridisation studies in the sturgeon brain have demonstrated that mRNAs of the two somatostatin precursors (PSS1 and PSS2) are differentially expressed in neurons [Trabucchi, M., Tostivint, H., Lihrmann, I., Sollars, C., Vallarino, M., Dores, R.M., Vaudry, H., 2002. Polygenic expression of somatostatin in the sturgeon Acipenser transmontanus: molecular cloning and distribution of the mRNAs encoding two somatostatin precursors. J. Comp. Neurol. 443, 332-345.]. However, neither the morphology of somatostatinergic neurons nor the patterns of innervation have yet been characterized. To gain further insight into the evolution of this system in primitive bony fishes, we studied the distribution of somatostatin-immunoreactive (SOM-ir) cells and fibres in the brain of the Siberian sturgeon (Acipenser baeri). Most SOM-ir cells were found in the preoptic area and hypothalamus and abundant SOM-ir fibres coursed along the hypothalamic floor towards the median eminence, suggesting a hypophysiotrophic role for SOM in sturgeon. In addition, SOM-ir cells and fibres were observed in extrahypothalamic regions such as the telencephalon thalamus, rhombencephalon and spinal cord, which also suggests neuromodulatory and/or neurotransmitter functions for this peptide. Overall there was a good correlation between the distribution of SOM-ir neurons throughout the brain of A. baeri and that of PSS1 mRNA in Acipenser transmontanus. Comparative analysis of the results with those obtained in other groups of fishes and tetrapods indicates that widespread distribution of this peptide in the brain is shared by early vertebrate lines and that the general organization of the somatostatinergic systems has been well-conserved during evolution.

Distribution of somatostatin-like immunoreactivity in the brain of the caecilian Dermophis mexicanus (Amphibia: Gymnophiona): comparative aspects in amphibians

The organization of the somatostatin-like-immunoreactive (SOM-ir) structures in the brain of anuran and urodele amphibians has been well documented, and significant differences were noted between the two amphibian orders. However, comparable data are not available for the third order of amphibians, the gymnophionans (caecilians). In the present study, we analyzed the anatomical distribution of SOM-ir cells and fibers in the brain of the gymnophionan Dermophis mexicanus. In addition, because of its known relationship with catecholamines in other vertebrates, double immunostaining for SOM and tyrosine hydroxylase was used to investigate this situation in the gymnophionan. Abundant SOM-ir cell bodies and fibers were widely distributed throughout the brain. In the telencephalon, pallial and subpallial cells were labeled, being most numerous in the medial pallium and amygdaloid region. Most of the SOM-ir neurons were found in the preoptic area and hypothalamus and showed a clear projection to the median eminence. Less conspicuously, SOM-ir structures were found in the thalamus, tectum, tegmentum, and reticular formation. Both SOM-ir cells and fibers were demonstrated in the spinal cord. The double-immunohistofluorescence technique revealed that catecholaminergic neurons and SOM-ir cells are largely intermingled in many brain regions but form totally separated populations. Many differences were found between the distribution of SOM-ir structures in Dermophis and that in anurans or urodeles. Some features were shared only with anurans, such as the abundant pallial SOM-ir cells, whereas others were common only to urodeles, such as the organization of the hypothalamohypophysial SOM-ir system. In addition, some characteristics were found only in Dermophis, such as the localization of the SOM-ir spinal cells and the lack of colocalization of catecholamines and SOM throughout the brain. Therefore, any conclusions concerning the SOM system in amphibians are incomplete without considering evidence for gymnophionans.

Brain mapping of three somatostatin encoding genes in the goldfish

In the present study the brain distribution of three somatostatin (SRIF)-encoding genes, PSS-I, PSS-II, and PSS-III, was analyzed by in situ hybridization (ISH) in the goldfish. The PSS-I mRNA showed the widest distribution throughout the brain, whereas PSS-II transcripts were restricted to some hypothalamic nuclei. On the other hand, PSS-III presents an intermediate distribution pattern. All SRIF encoding genes are expressed in hypophysiotropic nuclei supporting the idea that, in addition to SRIF-14, [Pro(2)] SRIF-14, and gSRIF-28 have pituitary-controlling functions. Moreover, each of the genes is expressed in nuclei directly associated with feeding behavior, suggesting a role for SRIF peptides in the central control of food intake and energy balance. Alternatively, they might have a role in processing sensory information related with feeding behavior, since PSS genes are expressed in the main gustatory, olfactory, and visual centers, which project to the hypothalamic feeding center in teleost fish.

Somatostatin in the brain of the cave salamander,Hydromantes genei (Amphibia, Plethodontidae): Immunohistochemical localization and biochemical characterization

The distribution of somatostatin-like immunoreactivity in the brain of the cave salamander Hydromantes genei (Amphibia, Plethodontidae) was investigated by using two distinct antisera raised against somatostatin-14. Most somatostatin-positive cells were detected in the ependymal cell layer surrounding the ventricles. These cells possessed the typical morphological characteristics of tanycytes or radial glial cells. Double-labeling with an antiserum against somatostatin and a monoclonal antibody against glial fibrillary acidic protein showed that somatostatin-immunoreactive cells lining the ventricles also exhibited GFAP-like immunoreactivity. Injection of the neurotracer biocytin into the lateral ventricle revealed that neurons lining the ventricles did not contain somatostatin-like immunoreactivity. In the telencephalon, somatostatin-like immunoreactivity was confined to radial glial cells. In the diencephalon, in addition to somatostatin-immunoreactive cells in the ependyma, positive cell bodies were also found in the periventricular preoptic nucleus, the infundibular nucleus, the epiphysis, and the subcommissural organ. In the metencephalon, positive cell bodies were found in the auricula cerebelli, whereas in the rhombencephalon numerous somatostatin-immunoreactive cells were seen lining the ventricular cavity. Immunoreactive nerve fibers were observed in the hypothalamus-median eminence complex. In the pituitary, a discrete group of somatostatin-positive cells was found in the pars distalis. High-performance liquid chromatography analysis of brain extracts revealed that the immunoreactive material coeluted with somatostatin-14. The present results show that the somatostatin peptidergic system in the brain of the cave salamander has a more simple organization than those described in the brain of frog and other vertebrates. This feature is probably related to the expression of high pedomorphic characters in plethodontids. The distribution of somatostatin-like immunoreactivity suggests that, in the cave salamander, somatostatin may act as a neurotransmitter and/or neuromodulator, a central regulator of fluid homeostasis, and a hypophysiotropic neurohormone.

Somatostatin-like immunoreactivity in the brain of the urodele amphibian Pleurodeles waltl. Colocalization with catecholamines and nitric oxide

The neuronal structures with somatostatin-like immunoreactivity have been studied in the brain of the urodele amphibian Pleurodeles waltl. Intense immunoreactivity was observed in neurons and fibers distributed throughout the brain. Within the telencephalon, the subpallial regions were densely labeled containing both cells and fibers, primarily in the striatum and amygdala. The majority of the somatostatin immunoreactive neurons were located in the preoptic area and hypothalamus, although less numerous cells were also found in the thalamus. A conspicuous innervation of the median eminence was revealed, which arises from the hypothalamic cell populations. In the brainstem, intense fiber labeling was present in the tectum and tegmentum, whereas cell bodies were located only in the tegmentum of the mesencephalon and in the interpeduncular, raphe and reticular nuclei of the rhombencephalon. Longitudinal fiber tracts throughout the brainstem were observed and they continued into the spinal cord in the laterodorsal funiculus. The localization of somatostatin in catecholaminergic and nitrergic neurons was studied by double labeling techniques with antisera against tyrosine hydroxylase and nitric oxide synthase. Catecholamines and somatostatin only colocalized in a cell population in the ventral preoptic area. In turn, the striatum and amygdala contained neurons with somatostatin and nitric oxide synthase. Our results demonstrated that the somatostatin neuronal system in the brain of Pleurodeles waltl is consistent with that observed in anuran amphibians and shares many characteristics with those of amniotes. Colocalization of somatostatin with catecholamines and nitric oxide is very restricted in the urodele brain, but in places that can be easily compared to those reported for mammals, suggesting that interactions between these neurotransmitter systems are a primitive feature shared by tetrapod vertebrates.

Isolation and characterization of a homologue of mammalian prolactin-releasing peptide from the tilapia brain and its effect on prolactin release from the tilapia pituitary

In the tilapia (Oreochromis mossambicus), as in many teleosts, prolactin (PRL) plays a major role in osmoregulation in freshwater. Recently, PRL-releasing peptides (PrRPs) have been characterized in mammals. Independently, a novel C-terminal RF (arginine-phenylalanine) amide peptide (Carrasius RF amide; C-RFa), which is structurally related to mammalian PrRPs, has been isolated from the brain of the Japanese crucian carp. The putative PrRP was purified from an acid extract of tilapia brain by affinity chromatography with antibody against synthetic C-RFa and HPLC on a reverse-phase ODS-120 column. The tilapia PrRP cDNA was subsequently cloned by polymerase chain reaction. The cDNA consists of 619 bp encoding a preprohormone of 117 amino acids. Sequence comparison of the isolated peptide and the preprohormone revealed that tilapia PrRP contains 20 amino acids and is identical to C-RFa. Incubation of the tilapia pituitary with synthetic C-RFa (100 nM) significantly stimulated the release of two forms of tilapia PRL (PRL188 and PRL177). However, the effect of C-RFa was less pronounced than the marked increase in PRL release in response to hyposmotic medium. The ability of C-RFa to stimulate PRL release appears to be specific, since C-RFa failed to stimulate growth hormone release from the pituitary in organ culture. In contrast, rat and human PrRPs had no effect on PRL release. C-RFa was equipotent with chicken GnRH in stimulating PRL release in the pituitary preincubated with estradiol 17beta. Circulating levels of PRL were significantly increased 1 h after intraperitoneal injection of 0.1 microg/g of C-RFa in female tilapia in freshwater but not in males. These results suggest that C-RFa is physiologically involved in the control of PRL secretion in tilapia.

Distribution of somatostatin in the brain and of somatostatin and thyrotropin-releasing hormone in peripheral tissues of the chicken

Our research group recently presented the distribution of thyrotropin-releasing hormone (TRH) in the chicken brain. In this study we measured somatostatin (SRIH) concentrations in different brain parts and nuclei. The distribution of SRIH and TRH in peripheral tissues was also studied. Although the highest SRIH content was found in endocrine areas like diencephalon and median eminence (ME), high levels were also recorded in brain stem and several hypothalamic nuclei which do not project to the ME. SRIH immunoreactivity was also found within the pituitary. In peripheral tissues, SRIH was mainly present in gonads, thyroid and intestine. Low amounts were found in duodenum, kidney, heart and lung. SRIH concentrations were barely detectable (liver, blood cells) or undetectable (muscle, skin, spleen) in other peripheral tissues investigated. Although TRH was found in all tissues collected, it was also most abundant in brain, pituitary, thyroid and gonads. Our results suggest that also in the chicken SRIH and TRH are implicated in the control of several physiological processes like growth, reproduction and digestion.

Social status controls somatostatin neuron size and growth

Many animal species show flexible behavioral responses to environmental and social changes. Such responses typically require changes in the neural substrate responsible for particular behavioral states. We have shown previously in the African cichlid fish, Haplochromis burtoni, that changes in social status, including events such as losing or winning a territorial encounter, result in changes in somatic growth rate. Here we demonstrate for the first time that changes in social status cause changes in the size of neurons involved in the control of growth. Specifically, somatostatin-containing neurons in the hypothalamus of H. burtoni increase up to threefold in volume in dominant and socially descending animals compared with cell sizes in subordinate and socially ascending fish. Because somatostatin is known to be an inhibitor of growth hormone release, the differences in cell size suggest a possible mechanism to account for the more rapid growth rates of subordinate and socially ascending animals compared with those of dominant or socially descending fish. These results reveal possible mechanisms responsible for socially induced physiological plasticity that allow animals to shift resources from reproduction to growth or vice versa depending on the social context.

Effects of thyrotropin-releasing hormone and its metabolites, Cyclo(His-Pro) and TRH-OH, on growth hormone and prolactin synthesis in primary cultured pituitary cells of the common carp, Cyprinus carpio

The effects of thyrotropin-releasing hormone (TRH) and its metabolites, cyclo(His-Pro) and TRH-OH, on growth hormone (GH) and prolactin (PRL) synthesis were investigated using primary cultured pituitary cells of the common carp, Cyprinus carpio. The effects of these pep tides on GH and PRL were compared to those of human GH-releasing hormone (hGHRH) and somatostatin (somatotropin-releasing inhibiting factor; SRIF). GH and PRL synthesis were determined by measuring the incorporation of [3H]leucine into GH and PRL. TRH stimulated the release of newly synthesized GH and PRL, but not thyroid-stimulating hormone. In addition, TRH stimulated a dose-related increase in the release of newly synthesized GH and PRL at 10(-9) to 10(-7) M. Cyclo(His-Pro) stimulated the release of newly synthesized GH dose- dependently. TRH, cyclo(His-Pro), and hGHRH stimulated GH synthesis, while SRIF inhibited this at 10(-7) M. The release of newly synthesized PRL into culture medium was also stimulated by TRH and hGHRH, but inhibited by SRIF. PRL synthesis was not affected by TRH-OH and cyclo(His-Pro). Intracellular contents of GH and PRL in the pituitary did not change significantly. The present study demonstrates that TRH plays an important role in both GH and PRL synthesis and release. This is the first report in which the effects of cyclo(His-Pro) on GH synthesis in teleosts are demonstrated.

Distribution of somatostatin-like immunoreactivity in the brain of the frog, Rana esculenta, during development

The anatomical distribution of somatostatin-like immunoreactivity in the central nervous system of the frog, Rana esculenta, during development and in juvenile specimens was investigated by indirect immunofluorescence. Soon after hatching, at stages II-III, somatostatin-like immunoreactive structures were found in the preoptic-median eminence complex. In stage VI tadpoles, new groups of immunopositive perikarya and nerve fibers appeared in the diencephalon, within the ventral infundibular nucleus and in the ventral area of the thalamus, as well as in the medial pallium. In stages XII-XIV of development, immunopositive perikarya were also present in the dorsal infundibular nucleus of the hypothalamus and ventrolateral area of the thalamus. A small group of somatostatin-like immunoreactive neurons appeared in the posteroventral nucleus of the rhombencephalon. However, these neurons were not seen in later stages of development. Tadpoles in stages XVIII, XXI-XXII and in juveniles were characterized by a wider distribution of immunoreactive cell bodies and fibers in the pallium. New groups of immunoreactive neurons were found in the dorsal and lateral pallium. The presence of positive perikarya in the lateral pallium is a transient expression found only in these stages. The organization of the somatostatinergic system was most complex during the metamorphic climax, with the appearance of positive cell bodies in the posterocentralis area of the thalamus, and in juvenile animals with the presence of perikarya in the ventral part of the medial pallium and within the central grey rhombencephali. In contrast to the adult frog, somatostatin neurons were not observed in the mesencephalon of tadpoles and juveniles.

Immunocytochemical localization of somatostatin and autoradiographic distribution of somatostatin binding sites in the brain of the African lungfish, Protopterus annectens

The anatomical distribution of somatostatin-immunoreactive structures and the autoradiographic localization of somatostatin binding sites were investigated in the brain of the African lungfish, Protopterus annectens. In general, there was a good correlation between the distribution of somatostatin-immunoreactive elements and the location of somatostatin binding sites in several areas of the brain, particularly in the anterior olfactory nucleus, the rostral part of the dorsal pallium, the medial subpallium, the anterior preoptic area, the tectum, and the tegmentum of the mesencephalon. However, mismatching was found in the mid-caudal dorsal pallium, the reticular formation, and the cerebellum, which contained moderate to high concentrations of binding sites and very low densities of immunoreactive fibers. In contrast, the caudal hypothalamus and the neural lobe of the pituitary exhibited low concentrations of binding sites and a high to moderate density of somatostatin-immunoreactive fibers. The present results provide the first localization of somatostatin in the brain of a dipnoan and the first anatomical distribution of somatostatin binding sites in the brain of a fish. The location of somatostatin-immunoreactive elements in the brain of P. annectens is consistent with that reported in anuran amphibians, suggesting that the general organization of the somatostatin peptidergic systems occurred in a common ancestor of dipnoans and tetrapods. The anatomical distribution of somatostatin-immunoreactive elements and somatostatin binding sites suggests that somatostatin acts as a hypophysiotropic neurohormone as well as a neurotransmitter and/or neuromodulator in the lungfish brain.

Estradiol inhibits plasma somatostatin 14 (SRIF-14) levels and inhibits the response of somatotrophic cells to SRIF-14 challenge in vitro in rainbow trout, Oncorhynchus mykiss

In the present study, the effects of 17 beta-estradiol (E2) treatment on plasma growth hormone (GH) and somatostatin 14 (SRIF-14) concentrations were investigated, as well as the effect of in vivo E2 treatment on the in vitro GH response to SRIF-14 challenge in sexually immature rainbow trout (Oncorhynchus mykiss). Two weeks after receiving a steroid hormone implant, plasma E2 and GH levels were significantly (P < 0.05) elevated, and plasma SRIF levels were significantly (P < 0.05) lowered relative to the control. Pituitary glands were taken from E2-primed and control fish and challenged with a single pulse of SRIF-14 (10(-8) M) in a perifusion unit to evaluate the effect of E2 on the response of somatotrophs to the effect of SRIF-14. Whereas SRIF-14 challenge significantly (P < 0.01) inhibited GH release from pituitary fragments taken from control fish, there was no such response in E2-primed fish. Furthermore, GH release following SRIF-14 administration (at the point of maximal inhibition) was significantly depressed in control fish with respect to the E2 treatment group. These data suggest that E2 treatment may increase plasma GH concentrations by altered somatotroph responsiveness to SRIF-14 inhibition. Furthermore, E2 may increase plasma GH by suppressing plasma SRIF-14 levels, although the role of circulating SRIF-14 on the regulation of GH release has not been fully determined in teleosts.

Somatostatin-like immunoreactivity in the brain of an electric fish (Apteronotus leptorhynchus) identified with monoclonal antibodies

The immunohistochemical localization of somatostatin-like immunoreactive (SSir) cells and fibers in the brain of the gymnotiform teleost (Apteronotus leptorhynchus) was investigated using well-characterized monoclonal antibodies directed against somatostatin-14 and -28. Large populations of SSir neurons occur in the basal forebrain, diencephalon and rhombencephalon and a dense distribution of fibers and terminal fields is found in the ventral, dorsomedial and dorsolateral telencephalon, hypothalamus, centralis posterior thalamus, subtrigeminal nucleus, the motor nucleus of vagus and in the ventrolateral medulla. Immunoreactive neurons in the forebrain are concentrated mainly in the ventral telencephalic areas, the region of the anterior commissure and entopeduncular nucleus. In a fashion similar to the large basal telencephalic cells of other species, the cells of the rostral nucleus entopeduncularis have a significant projection to the dorsal telencephalon. The preoptic region and the peri- and paraventricular hypothalamic nuclei are richly endowed with SSir cells; some of these cells contribute fibres to the pituitary stalk and gland. In the thalamus, only the n. centralis posterior stands out for the density of SSir cells and terminals; these cells appear to project to the prepacemaker nucleus, thus suggesting an SS influence on electrocommunication. In the mesencephalon most SSir cells occur in the optic tectum, torus semicircularis and interpeduncular nucleus. The rhombencephalic SSir cells have a wider distribution (central gray, raphe, sensory nuclei, reticular formation, electrosensory lateral line lobe and surrounding the central canal). The results of this study show the presence of SS in various sensory systems, electromotor system and specific hypothalamic nuclei, suggesting a modulatory role in the processing of sensory information, electrocommunication, endocrine and motor activities.

Galanin-like immunoreactivity in the brain of teleosts: distribution and relation to substance P, vasotocin, and isotocin in the Atlantic salmon (Salmo salar)

The presence of galanin-like substances and their relation to substance P-, vasotocin-, and isotocin-immunoreactive neurons and fibers in the brain of teleosts was investigated with immunohistochemical methods. Two specific antisera against synthetic porcine galanin (GAL) revealed cell bodies and fibers in the brain of four different teleost species (Salmo salar, Carassius carassius, Gasterosteus aculeatus, and Anguilla anguilla). In all four species the main location of galanin immunoreactivity was in the hypothalamo-pituitary region. A detailed study of the distribution of galanin immunoreactivity in S. salar showed that galanin immunoreactive (GALir) perikarya were present in the nucleus preopticus periventricularis, an area that may be compared to the supraoptic nucleus in mammals, and in the nucleus lateralis tuberis, a nucleus involved in pituitary control in fishes that may be compared with the arcuate nucleus in mammals. GALir perikarya were found also in the nucleus recessus lateralis and in the nucleus recessus posterior. Numerous GALir fibers were present in the telencephalon and diencephalon, whereas only small numbers of fibers were found in the brainstem. In contrast to the situation in mammals, no GALir perikarya were observed in the brainstem areas corresponding to the noradrenergic locus coeruleus and serotonergic raphe nuclei in S. salar. We did not find any coexistence of GALir substances with arginine vasotocin or isotocin in neurosecretory neurons, as has been shown for galanin with the mammalian counterparts vasopressin and oxytocin. Also, the galanin-like substance(s) and their structurally closest related peptide family, the tachykinins, belong to separate neuronal systems in teleosts. The presence of GALir neurons in brain areas known to be involved in pituitary control, and a massive GALir innervation of the pituitary, strongly indicate a role for galanin-like substances in pituitary control also in teleosts. Furthermore, the presence of extrahypothalamic GALir fibers suggests involvement of galanin-like substances in other brain functions in teleosts. In conclusion, there are general similarities between teleosts and mammals concerning the distribution of galanin-like substances. However, there seem to be substantial differences in their distribution relative to functionally related peptides within the hypothalamo-pituitary system. Whereas galanin appears to be colocalized and released together with vasopressin and oxytocin in mammals, in teleosts the homologous substances are contained within different sets of neurons that innervate the same target, the pituitary.

Distribution of serotonin- and dopamine-immunoreactivity in the brain of the teleost Clarias gariepinus

The distribution of serotonergic and dopaminergic cell bodies and varicose fibres in the brain of the teleost Clarias gariepinus was studied immunohistochemically using antisera against formaldehyde-conjugated serotonin and dopamine. Many serotonergic and dopaminergic fibres innervated the areas dorsalis telencephali pars medialis and pars lateralis dorsalis, as well as the area ventralis telencephali pars ventralis. In the diencephalon, a large number of serotonergic and some dopaminergic fibres were found in the preoptic nucleus, innervating the cells of this nucleus. In addition, serotonergic and dopaminergic fibres were observed in the pituitary stalk and in all regions of the pituitary gland. Moreover, the diencephalon contained the highest number of serotonin- or dopamine-immunoreactive cell bodies. These cells were confined to the same periventricular nuclei as the nucleus ventromedialis thalami, the nucleus posterior periventricularis, the nucleus lateralis tuberis, the nuclei recessus lateralis and recessus posterioris. Most cells of these nuclei were in contact with the cerebrospinal fluid of the third ventricle. The brainstem contained serotonergic cell bodies in the raphe nuclei and a few serotonergic and dopaminergic fibres. The torus semicircularis was densely innervated by serotonergic fibres and, to a lesser extent, dopaminergic fibres. In the midbrain of Clarias gariepinus, no dopaminergic homologue of the substantia nigra was observed. The results are discussed both in a comparative and a physiological context. In this regard, special attention has been paid to the contribution of hypothalamic monoamines in the regulation of gonadotropin secretion as an essential step in the neuro-endocrine control of reproduction.

In vitro responses of rainbow trout (Oncorhynchus mykiss) somatotrophs to carp growth hormone-releasing factor (GRF) and somatostatin

To study the hypothalamic control of growth hormone (GH) release in lower vertebrates, we employed an in vitro technique using a monolayer cell culture system of rainbow trout pituitary glands. Two newly purified carp brain growth hormone-releasing factors, carp GRF(1-45) and carp GRF(1-29), and synthetic somatostatin-14 (SST-14) were applied to the cultured pituitary cells. The results indicate that: (1) The carp GRFs had a dose-related potency in stimulating growth hormone release. The dose of half maximum effect (ED50) for carp GRF(1-45) was 0.107 nM, and an equal potency for carp GRF(1-29) was 0.388 nM. (2) SST-14 inhibited GH release having a dose-dependent potency with an ED50 of 0.186 nM. (3) Osmotic pressure did not influence SST-14 inhibited GH secretion but did affect spontaneous GH release. (4) The response of cultured cells was not affected by length of incubation period with SST-14 or carp GRF but was affected by cell density.

Distribution of two forms of somatostatin in the brain, anterior intestine, and pancreas of adult lampreys (Petromyzon marinus)

The distribution of two major immunoreactive forms of somatostatin, somatostatin-14 and somatostatin-34, within the brain, pancreas and intestine of adult lampreys, Petromyzon marinus, was identified using antisera raised against these peptides. Immunostaining of the brain is similar in juveniles and upstream migrants, and somatostatin-14 is the major somatostatin form demonstrated. A few somatostatin-34-containing cells are localized within the olfactory bulbs, thalamus and hypothalamus, but cells immunoreactive to anti-somatostatin-34 in the hypothalamus and thalamus do not co-localize somatostatin-14. Immunostaining of pinealocytes within the pineal pellucida with anti-somatostatin-14 may infer a novel function for this structure. Somatostatin-14 and somatostatin-34 are co-localized within D-cells of the cranial pancreas and caudal pancreas of juveniles and upstream migrants. Numerous somatostatin-34-immunoreactive cells are distributed within the epithelial mucosa of the anterior intestine but not all of these cells cross-react with anti-somatostatin-14. It appears that somatostatin-34 is the major somatostatin in the pancreo-gastrointestinal system of adult lampreys.

Chronological appearance of the different hypophysial hormones in the pituitary of sea bass larvae (Dicentrarchus labrax) during their early development: an immunocytochemical demonstration

Antisera raised against chum salmon prolactin (PRL), rainbow trout growth hormone (GH), mammalian adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), and luteinizing hormone (LH) were used to study the chronological appearance of immunoreactivity for PRL, GH, ACTH, TSH, LH, and melanocyte-stimulating hormone (MSH) in the pituitary of sea bass larvae (Dicentrarchus labrax) during the first 26 days after hatching. The anti-ACTH gives positive immunostaining in the ACTH cells as well as in the MSH cells; however, the two cell types can easily be distinguished by their different localization in the pituitary: ACTH in the rostral pars distalis, MSH in the pars intermedia. The first day after hatching cells immunoreactive for TSH, GH and ACTH could already be noticed, ACTH reacted strong in the pars intermedia but very weak in the rostral pars distalis. Cells immunopositive for PRL became visible between Days 9 and 15. With anti-LH, no positive reaction could be obtained during the first 26 days after hatching.

Effects of GnRH-associated peptide and its component peptides on prolactin secretion from the tilapia pituitary in vitro

The rostral pars distalis (RPD), containing mainly prolactin (PRL)-secreting cells, of the pituitary from immature and mature tilapia was incubated for 16 hr at 27 degrees in hypoosmotic medium (300 mOsm/kg) in the presence (10(-8) and 10(-11) M) or absence of the human GnRH-associated peptide (GAP) molecule, a potent PRL-inhibiting factor in mammals (Nikolics et al., Nature (London) 316, 511, 1985), and of a series of its component peptides. The release of the two forms of PRL in tilapia into the medium was measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by densitometry. The variability inherent in this method was normalized by calculating PRL release as the percentage of the total hormone present in both tissue and medium. Newly synthesized PRL was detected by incorporation of [35S]methionine, introduced into the culture medium, by the PRL molecules. In immature tilapia, GAP inhibited the release of total PRL while stimulating the release of newly synthesized large PRL. Among the GAP fragments tested, 28-36 was the fragment that most significantly affected PRL secretion. Both concentrations of fragment 28-36 stimulated the release of newly synthesized PRL from immature rostral pars distalis (RPDs). This stimulation appears to be dependent on the osmotic pressure of the medium since this fragment did not affect PRL secretion in hyperosmotic medium (340 mOsm/kg). Fragment 38-49 inhibited total PRL release from mature RPDs. Fragment 51-66 stimulated the release of total PRL from mature RPDs. Examination of tissue and medium values in densitometric units after incubation with fragments 28-36 and 51-66 indicated that while the tissue content of PRL was decreased, the medium content of PRL was not affected. This suggests that fragments 28-36 and 51-66, in opposition to the situation found when the data are expressed as percentage release of PRL, may not stimulate PRL release but may instead decrease the tissue content of PRL. These results suggest that the entire human GAP molecule, as well as some of its fragments, may have direct effects on the PRL cells in the tilapia pituitary.

Localization of growth hormone-releasing factor-like immunoreactivity in the hypothalamo-hypophysial system of some teleost species

An antiserum to growth hormone-releasing factor (GRF) 1-44 was applied on brain and pituitary sections of nine teleost species. Immunoreactive (ir) perikarya were demonstrated in parvo- and magnocellular portions of the preoptic nucleus (PON) and occasionally in the nucleus lateralis tuberis. The two tracts originating in the PON ran ventro-laterally toward the optic chiasm and then caudally in the basal hypothalamus. In the pars distalis (PD) of the eel, carp, goldfish and salmonids, GRF-ir fibers did not enter the rostral PD and few fibers passed close to somatotropes. In Myoxocephalus and Mugil, a variable number of ir-fibers passed close to cells of the rostral and proximal PD. In the neurointermediate lobe, GRF-ir fibers were located exclusively in the neural tissue of the eel and trout. In goldfish, carp and Myoxocephalus, GRF-ir fibers entered the intermediate lobe. This antiserum also labeled corticotrops and, to a lesser extent, melanotrops in the pituitary of cyprinids. A variable number of perikarya contained both GRF and vasotocin in the PON of the eel. In all teleost species studied so far, the distribution patterns of GRF are different, and the function of the various adenohypophysial cell types appears to be differently modulated, according to the variable distribution of GRF in the pituitary.

The tilapia prolactin cell: A model for stimulus-secretion coupling

The tilapia prolactin (PRL) cell responds rapidly (10-20 min) to small physiological changes in medium osmotic pressure (OP), releasing increasing quantities of hormone as medium OP is reduced. This release is rapidly (≤ 10 min) inhibited by somatostatin (SRIF). There is now extensive evidence that tilapia PRL cell function is regulated through the second messengers Ca(++) and cAMP. Our studies have shown that PRL release is augmented by treatments that lead to increased levels of intracellular Ca(++) or cAMP. On the other hand, PRL release is blocked when tissues are incubated in Ca(++)-depleted medium or upon the addition of Co(++), an inhibitor of Ca(++)-mediated processes. The use of(45)Ca(++) to characterize the movement of Ca(++) into PRL cells has provided evidence that an increase in the influx of extracellular Ca(++) may participate in PRL release upon exposure to hyposmotic medium. Our studies have also shown that SRIF suppresses the increase in(45)Ca(++) accumulation that is brought about when OP is reduced. We have also examined the effects of OP and SRIF on cAMP levels. The reduction of medium OP did not alter cAMP metabolism during 20 min of incubation. By contrast, cAMP accumulation in the presence of IBMX was enhanced at 1 hr of incubation in reduced OP. Thus, an increase in cAMP turnover may play a role in maintaining PRL release under sustained stimulation. SRIF reduced the accumulation of cAMP during 10 min of incubation with IBMX and also reduced the forskolin-stimulated increase in cAMP. Thus, SRIF may suppress adenylate cyclase activity. Finally, our studies have revealed that the forskolin-stimulated increase in cAMP levels is not dependent upon medium Ca(++). The presence of Ca(++) in the medium is required, however, for PRL release even when the cAMP messenger system has been activated. Moreover, cAMP accumulation was augmented when intracellular Ca(++) was increased. This raises the possibility that reduced OP may stimulate an increase in cAMP turnover indirectly through its action(s) on cytosolic Ca(++).

Relationship between serum growth hormone levels and the brain and pituitary content of immunoreactive somatostatin in the goldfish, Carassius auratus L

In this study, the relationships between endogenous brain and pituitary immunoreactive somatostatin (irSRIF) and circulating growth hormone (GH) levels in the goldfish were examined using two approaches. First, the amount of irSRIF in extracts of the pituitary gland and various brain regions was measured by radioimmunoassay several times throughout the year and was compared to serum GH levels at each time. The amounts of irSRIF in extracts of the pituitary gland, hypothalamus, and telencephalon were found to be inversely related to seasonal changes in serum GH levels, such that irSRIF was highest in these regions when serum GH levels were lowest (November and February). Conversely, irSRIF in these regions was lower in May, June, and July when serum GH levels were highest. These results suggest that endogenous irSRIF in the pituitary and forebrain may participate in the regulation of seasonal changes in serum GH levels in the goldfish. In extracts from other brain regions (thalamus + midbrain and cerebellum + medulla), some changes in the amount of irSRIF were observed among the various sample times, but these variations were not related to changes in serum GH levels. In a second set of experiments, the origin of irSRIF fibers innervating the goldfish pituitary gland was examined by using brain lesioning techniques to destroy regions of the forebrain known to contain irSRIF perikarya and fibers, and subsequently measuring the amount of irSRIF in the pituitary gland. Lesions in the preoptic area of the forebrain resulted in increased serum GH levels concomitant with a decrease in pituitary irSRIF content. This provides direct evidence that the preoptic area is the origin of a somatostatinergic projection inhibiting GH secretion from the goldfish pituitary. Lesions centered in the nucleus lateral tuberis (NLT) pars anterioris did not influence serum GH levels or the pituitary content of irSRIF. In contrast, more posterior lesions centered in the NLT pars posterioris (NLTp) resulted in a dramatic reduction in the amount of irSRIF in the pituitary. This suggests that the majority of irSRIF projections to the goldfish pituitary pass through the area destroyed by the lesion centered in the NLTp; it is also possible that perikarya within this area may be the origin of at least some of the irSRIF-containing fibers in the goldfish pituitary. Together, results from the present study provide evidence of a functional relationship between circulating levels of GH and endogenous brain and pituitary irSRIF in the goldfish.

Immunocytochemical demonstration of close relationships between neuropeptidergic nerve fibers and hormone-producing cell types in the adenohypophysis of the sea bass (Dicentrarchus labrax)

Light microscopic double immunocytochemical stainings, performed on sea bass hypothalamo-hypophysial sections, revealed the projection of different neuropeptide-immunoreactive neurons innervating the hormone-producing cell populations in the pituitary gland. In the rostral pars distalis (PD) the ACTH cells were found in close proximity to fibers immunoreactive for somatostatin (SRIF), growth hormone-releasing hormone (GRF), corticotropin-releasing hormone (CRF), vasotocin (VT), isotocin (IT), substance P (SP), neurotensin, and galanin (GAL), while the PRL cell zone seemed only innervated by nerve fibers immunopositive for GAL. In the proximal PD, fibers immunoreactive for SRIF, GRF, VT, IT, cholecystokinin, SP, neuropeptide Y, and GAL formed a close relationship with the growth hormone cells. The gonadotrophs were observed near nerve fibers immunostained for gonadotropin-releasing hormone, IT, and less obviously GRF and VT, while fibers positive for GRF, CRF, VT, IT, SP, and GAL penetrated between and formed a close association with the thyrotrophs. In the pars intermedia the MSH cells and the PAS-positive (PAS+) cells seemed both innervated by separate nerve fibers immunoreactive for GRF, CRF, melanin concentrating hormone, VT, IT, and SP. All these results suggest a functional role of the neuropeptides in the adenohypophysis of the sea bass, possibly in the synthesis and/or release of hypophysial hormones from the different cell types.

Immunocytochemical localization of LH-RH in the brain and pituitary of the catfish, Clarias batrachus (Linn.)

An elaborate organization of luteinizing hormone-releasing hormone (LH-RH) immunoreactive (ir) cells and fibers was encountered in the olfactory system of Clarias batrachus. In addition to the ir structures in the olfactory nerve, peripheral area of the olfactory bulb, and the medial olfactory tract (MOT), ir cells and fibers were prominently seen in the lamellae of the olfactory organ. Perikarya showing varying degrees of intensity of immunoreaction were observed along the base of the forebrain in the nucleus preopticus basalis lateralis, nucleus preopticus periventricularis, nucleus preopticus, nucleus lateralis tuberis pars posterior, and the pituitary. Some cells were also noticed in the midbrain tegmentum. A well-defined system of ir fibers from the MOT penetrated the telencephalon and curved dorsocaudally into the pars supracommissuralis above the anterior commissure (AC); while some fibers decussate in the AC, others extended posteriorly into the diencephalon. A fairly dense network of beaded ir fibers was seen in the basal forebrain, conspicuous around the organum vasculosum laminae terminalis and caudally traceable as far as the neurohypophysis; some immunostained fibers appear to be directly contacting with the cells of the proximal pars distalis. Fibers were also witnessed in the optic chiasma and in the inner plexiform layer of the retina. Solitary fibers were noticed in certain circumscribed telencephalic areas, caudal hypothalamus, posterior commissure, midbrain tegmentum, cerebellum, and ventral medulla oblongata. The highly organized LH-RH containing system in C. batrachus is indicative of its elaborate role in synchronization of the reproductive processes and the environmental cues.

Peptidergic innervation of the adrenocorticotropic hormone (ACTH)- and growth hormone (GH)-producing cells in the pars distalis of the sea bass (Dicentrarchus labrax)

Due to its unique organization, the teleost pituitary is an ideal model in which to investigate the relationship of the nervous system with the pituitary endocrine cells. A light microscope immunocytochemical study of the sea bass pituitary revealed six different neuropeptides in nerve fibers which projected into the pituitary neurohypophysis and bordered the adenohypophysial cells. Double staining showed separate nerve fibers immunoreactive for corticotropin-releasing factor (CRF), vasotocin (VT), somatostatin (SRIF), growth hormone-releasing factor (GRF), and neurotensin (NT) in the vicinity of the adrenocorticotropic hormone-releasing cells (ACTH-cells) in the rostral pars distalis (PD). In the proximal PD cholecystokinin (CCK)-, SRIF-, GRF-, and VT-immunoreactive fibers penetrated between the growth hormone-releasing cells (GH-cells). These results suggest a possible role for CCK, GRF, SRIF, and VT in the modulation of GH-cell activity, while the synthesis and/or secretion of the ACTH-cells might be affected by the release of VT, CRF, SRIF, GRF, and NT.

Novel effect of vasoactive intestinal polypeptide and peptide histidine isoleucine: inhibition of in vitro secretion of prolactin in the tilapia, Oreochromis mossambicus

The effects of vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) on the in vitro secretion of two prolactins (PRL) from the rostral pars distalis (RPD) and of growth hormone (GH) from the proximal pars distalis (PPD) of the pituitary of the tilapia (Oreochromis mossambicus) were studied. RPDs were incubated for 20 hr in hypoosmotic (280-300 mOsm) or hyperosmotic (340-350 mOsm) Krebs-Ringer bicarbonate medium with added peptide concentrations of 0 (control), 0.3, 3.0, 30, and 300 nM; similarly, PPDs were incubated with the same peptide concentrations in isoosmotic (325 mOsm) medium supplemented with cortisol. PRL and GH in the tissue and secreted into the medium were measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by soft laser densitometry of the protein band(s). Neither VIP nor PHI has a detectable effect on the secretion of GH. Secretion of the two PRLs is significantly inhibited by VIP and PHI in both hyperosmotic and hypoosmotic medium. In hyperosmotic medium, 300 nM VIP inhibits secretion of both PRLs by 47%, whereas in hypoosmotic medium, 300 nM VIP inhibits their secretion by 27%. PHI inhibits their secretion by ca. 65% in hyperosmotic medium and by 40% in hypoosmotic medium. There is preliminary immunocytochemical evidence for some VIP-like immunoreactivity (IR), but no conclusive indication of PHI-like IR, in the hypothalamo-hypophysial area. The inhibitory actions of VIP and PHI on PRL secretion in tilapia are in contrast to the known stimulatory actions of VIP and PHI on PRL secretion in tetrapods.

Different cellular distributions of two somatostatins in brain and pancreas of salmonids, and their associations with insulin- and glucagon-secreting cells

Invariant somatostatin-14 (SST-14) and somatostatin-25 (SST-25), isolated from coho salmon pancreas (Plisetskaya et al., 1986a) are likely coded by two distinct somatostatin genes. The present study was undertaken to investigate whether these genes are expressed in the same or in different cell types in the pancreatic islets and in the brain of two salmonids: rainbow trout and coho salmon. Antibodies generated against SST-14, mammalian (m) SST-28(1-14), salmon (s) SST-25, salmon insulin, and salmon glucagon were used as immunocytochemical probes. Two distinct cell types containing SSTs were revealed in the pancreas of both salmonid species: one cell type immunoreactive to both SST-14 and mSST-28(1-14) and the other cell type immunoreactive only to sSST-25. The SST-14/mSST-28(1-14)-positive cells were limited to the more central parts of the islets, in apposition to the insulin-positive cells: sSST-25-positive cells were located more peripherally and were associated topographically with the glucagon-positive cells. In contrast to the pancreas, neurons in the neurohypophysis and hypothalamus of the rainbow trout and coho salmon contained only SST-14-like and mSST-28(1-14)-like immunoreactivities, while immunoreactivity to sSST-25 was completely absent. These results suggest that differentiation in the pancreas and brain of salmonid fishes results in cell types in which SST genes are separately expressed. The close topographical association of sSST-25 with glucagon cells, and of SST-14 with insulin cells, in the pancreatic islets implies yet unknown functional regulatory relationships that require detailed study.

Sexually dimorphic age-related differences in the immunocytochemical distribution of somatostatin in the platyfish

Somatostatin (SRIF) was localized by immunocytochemistry in the brains and pituitary glands of male and female platyfish (Xiphophorus maculatus) between the ages of 8 and 30 months (average life span is 30 months). In the brain, immunoreactive (ir-) SRIF is found in the perikarya of the ventrobasal hypothalamus (nucleus lateralis tuberis, nucleus anterior tuberis), dorsomedial hypothalamus, dorsal thalamus, ventral tegmentum and rhombencephalic basal plate; in the pituitary it is localized surrounding the ir-growth hormone containing cells of the caudal pars distalis, and in older fish it is also found within certain cells of the pars intermedia. We have demonstrated that there are consistent, sexually dimorphic, age-related changes in the intensity and/or distribution of ir-SRIF. The sexual dimorphism of the changes in SRIF immunoreactivity during the aging of platyfish may be related to the different patterns of growth observed in males and females.

Studies on the regulation of growth hormone release from the proximal pars distalis of male tilapia, Oreochromis mossambicus, in vitro

The in vitro effects of several factors, including cortisol, somatostatin (SRIF), and medium osmotic pressure, on growth hormone (GH) release from the tilapia pituitary were examined in relation to fish size. Spontaneous GH release from the proximal pars distalis (PPD) of approximately 60-g fish was significantly less than that from tissue of fish weighing either approximately 120 or approximately 280 g when incubated in 340 m phi smolal medium. While GH content of the PPD cultures (tissue + medium measured by densitometry) increased consistently with fish size, GH concentration (per microgram of tissue protein) was variable, being highest in 120-g fish and lowest in 280-g fish. Moreover, GH concentration was not related to GH release. Fish size also appeared to be important in the responsiveness of GH cells to stimulation by cortisol (Nishioka et al., 1985) and by increased osmotic pressure. In cultures of PPD from approximately 60-g fish, in which spontaneous release was relatively low, cortisol and increased medium osmotic pressure significantly enhanced release. Cortisol and hyperosmotic medium were without significant effect, however, on GH release from PPD of approximately 120-g fish, which showed high spontaneous release. In contrast, SRIF, a potent inhibitor of GH secretion, was effective in lowering GH release regardless of fish size. Nevertheless, SRIF was apparently more effective in inhibiting GH release from tissue of 60-g fish than from tissue of 120-g fish. Our data suggest that GH secretion may be augmented when smaller tilapia (approximately 60 g) are transferred to seawater, a situation in which blood cortisol and osmotic pressure would presumably be elevated.

Somatostatin and altered medium osmotic pressure elicit rapid changes in prolactin release from the rostral pars distalis of the tilapia, Oreochromis mossambicus, in vitro

Prolactin (PRL) cells in the rostral pars distalis of the tilapia Oreochromis mossambicus respond to somatostatin (SRIF) and reduced medium osmotic pressure within 10-20 min of exposure during perifusion incubation. Pieces of rostral pars distalis tissue were removed from freshwater-adapted tilapia and were preincubated in [3H]leucine in static culture (355 m phi smolal) for 48 hr. Following preincubation, they were placed in the perifusion apparatus and baseline release was established for 3 hr in hyperosmotic medium (355 m phi smolal). Exposure to hyposmotic medium (280 m phi smolal) resulted in a rapid and steep rise in the release of [3H]PRL, which remained elevated for more than 2 hr. When SRIF was added simultaneously with hyposmotic medium, the rise in PRL release normally initiated by reduced osmotic pressure was prevented. Somatostatin also quickly reduced release that had been previously elevated by exposure to hyposmotic medium. The time course of these changes suggests that SRIF and altered osmotic pressure act on PRL secretion in at least partial independence of effects which they may have on PRL synthesis in the tilapia pituitary.

In vitro effects of somatostatin and urotensin II on prolactin and growth hormone secretion in tilapia, Oreochromis mossambicus

The control of release of two recently characterized forms of prolactin (PRL) of molecular mass 24 and 20 kDa was investigated. The rostral pars distalis of male tilapia was incubated singly in a hypotonic modified Krebs-Ringer bicarbonate medium in order to stimulate PRL release; for comparison, the proximal pars distalis containing growth hormone (GH) cells was incubated in isotonic medium with or without 1 microgram/ml cortisol to stimulate GH release. The release of both PRLs and GH into the medium was measured by sodium dodecyl sulfate (SDS)--polyacrylamide gel electrophoresis followed by densitometry. Both somatostatin and synthetic (Gillichthys) urotensin II, a partial somatostatin homolog and analog from the teleost caudal neurosecretory system, significantly inhibited the release of both PRLs. Somatostatin significantly inhibited GH release, but urotensin II had no significant effect.

Estradiol-17 beta and thyrotropin-releasing hormone stimulate prolactin release from the pituitary gland of a teleost fish in vitro

The effects of estradiol-17 beta (E2) and thyrotropin-releasing hormone (TRH) on prolactin (PRL) release were investigated using the organ-cultured rostral pars distalis (RPD) of the tilapia, Oreochromis mossambicus. Spontaneous PRL release into hyperosmotic medium increased in a dose-related manner following E2 pretreatment in vitro. In addition, TRH stimulated a dose-related increase in PRL release from E2-preincubated RPD's, but had no effect on tissues not previously exposed to E2. The maximal PRL response, nearly three times control levels, occurred at 50 nM TRH. Higher doses of TRH were less effective in stimulating PRL release. These findings indicate that TRH may be an important hypothalamic prolactin-releasing factor in the tilapia. Furthermore, the marked potentiation of the action of TRH on PRL release following exposure to E2 suggest that there may be a shift in the control of PRL secretion with changes in the reproductive state of the tilapia.