Paradoxical elevation of growth hormone by intraventricular somatostatin: possible ultrashort-loop feedback

Role of endogenous somatostatin in regulating GH output under basal conditions and in response to metabolic extremes

Somatostatin (SST) was first described over 30 years ago as a hypothalamic neuropeptide which inhibits GH release. Since that time a large body of literature has accumulated describing how endogenous SST mediates its effects on GH-axis function under normal conditions and in response to metabolic extremes. This review serves to summarize the key findings in this field with a focus on recent progress, much of which has been made possible by the availability of genetically engineered mouse models and SST receptor-specific agonists.

Gender-dependent role of endogenous somatostatin in regulating growth hormone-axis function in mice

It has been previously reported that male and female somatostatin (SST) knockout mice (Sst-/-) release more GH, compared with Sst+/+ mice, due to enhanced GH-secretory vesicle release. Endogenous SST may also regulate GH secretion by directly inhibiting GHRH-stimulated GH gene expression and/or by modulating hypothalamic GHRH input. To begin to explore these possibilities and to learn more about the gender-dependent role of SST in modulating GH-axis function, hypothalamic, pituitary, and liver components of the GH-axis were compared in male and female Sst+/+ and Sst-/- mice. Pituitary mRNA levels for GH and receptors for GHRH and ghrelin were increased in female Sst-/- mice, compared with Sst+/+ controls, and these changes were reflected by an increase in circulating GH and IGF-I. Elevated levels of IGF-I in female Sst-/- mice were associated with elevated hepatic mRNA levels for IGF-I, as well as for GH and prolactin receptors. Consistent with the role of GH/IGF-I in negative feedback regulation of hypothalamic function, GHRH mRNA levels were reduced in female Sst-/- mice, whereas cortistatin (CST) mRNA levels were unaltered. In contrast to the widespread impact of SST loss on GH-axis function in females, only circulating GH, hypothalamic CST, and hepatic prolactin receptor expression were up-regulated in Sst-/- male mice, compared with Sst+/+ controls. These results confirm and extend the sexually dimorphic role of SST on GH-axis regulation, and suggest that CST, a neuropeptide that acts through SST receptors to inhibit GH secretion, may serve a compensatory role in maintaining GH-axis function in Sst-/- male mice.

Expression of sstr1 and sstr2 in rat hypothalamus: correlation with receptor binding and distribution of growth hormone regulatory peptides

With the aim of elucidating the role of individual somatostatin receptors in the central control of growth hormone secretion, we have examined the distribution of sstr1 and sstr2 mRNAs in the hypothalamus of the adult rat by in situ hybridization using 35S-labelled antisense riboprobes. Both receptors were expressed strongly in the preoptic area, suprachiasmatic nucleus and arcuate nucleus. High sstr1, but low sstr2, expression was evident in the paraventricular and periventricular nuclei as well as in the ventral premammillary nucleus. Conversely, moderate to high sstr2, but low sstr1, mRNA levels were detected in the anterior hypothalamic nucleus, ventromedial and dorsomedial nuclei and medial tuberal nucleus. Within the arcuate nucleus, the distribution of cells expressing sstr1 and sstr2 was comparable to that of neurons which bind somatostatin-14 selectively, one third of which have been documented to contain growth hormone-releasing hormone. Within the periventricular nucleus, the distribution of cells expressing sstr1 and, to a lesser extent, sstr2 was reminiscent of that of both [125I]somatostatin-labelled and somatostatin-immunoreactive cells. Taken together, these results imply a role for both sstr1 and sstr2 receptors in the central regulation of growth hormone-releasing hormone and somatostatin secretion, and hence of growth hormone release, by somatostatin.

Model-projected mechanistic bases for sex differences in growth hormone regulation in humans

Models of physiological systems facilitate rational experimental design, inference, and prediction. A recent construct of regulated growth hormone (GH) secretion interlinks the actions of GH-releasing hormone (GHRH), somatostatin (SRIF), and GH secretagogues (GHS) with GH feedback in the rat (Farhy LS, Veldhuis JD. Am J Physiol Regul Integr Comp Physiol 288: R1649–R1663, 2005). In contrast, no comparable formalism exists to explicate GH dynamics in any other species. The present analyses explore whether a unifying model structure can represent species- and sex-defined distinctions in the human and rodent. The consensus principle that GHRH and GHS synergize in vivo but not in vitro was explicable by assuming that GHS 1) evokes GHRH release from the brain, 2) opposes inhibition by SRIF both in the hypothalamus and on the pituitary gland, and 3) stimulates pituitary GH release directly and additively with GHRH. The gender-selective principle that GH pulses are larger and more irregular in women than men was conferrable by way of 4) higher GHRH potency and 5) greater GHS efficacy. The overall construct predicts GHRH/GHS synergy in the human only in the presence of SRIF when the brain-pituitary nexus is intact, larger and more irregular GH pulses in women, and observed gender differences in feedback by GH and the single and paired actions of GHRH, GHS, and SRIF. The proposed model platform should enhance the framing and interpretation of novel clinical hypotheses and create a basis for interspecies generalization of GH-axis regulation.

Growth hormone-releasing hormone (GHRH) neurons in the arcuate nucleus (Arc) of the hypothalamus are decreased in transgenic rats whose expression of ghrelin receptor is attenuated: Evidence that ghrelin receptor is involved in the up-regulation of GHRH expression in the arc

GH secretagogue (GHS)/ghrelin stimulates GH secretion by binding mainly to its receptor (GHS-R) on GHRH neurons in the arcuate nucleus (Arc) of the hypothalamus. GHRH, somatostatin, and neuropeptide Y (NPY) in the hypothalamus are involved in the regulatory mechanism of GH secretion. We previously created transgenic (Tg) rats whose GHS-R expression is reduced in the Arc, showing lower body weight and shorter nose-tail length. GH secretion is decreased in female Tg rats. To clarify how GHS-R affects GHRH expression in the Arc, we compared the numbers of GHS-R-positive, GHRH, and NPY neurons between Tg and wild-type rats. Immunohistochemical analysis showed that the numbers of GHS-R-positive neurons, GHRH neurons, and GHS-R-positive GHRH neurons were reduced in Tg rats, whereas the numbers of NPY neurons and GHS-R-positive NPY neurons did not differ between the two groups. The numbers of Fos-positive neurons and Fos-positive GHRH neurons in response to KP-102 were decreased in Tg rats. Competitive RT-PCR analysis of GHRH mRNA expression in the cultured hypothalamic neurons showed that KP-102 increased NPY mRNA expression level and that NPY decreased GHRH mRNA expression level. KP-102 increased GHRH mRNA expression level in the presence of anti-NPY IgG. GH increased somatostatin mRNA expression. Furthermore, GH and somatostatin decreased GHRH mRNA expression, whereas KP-102 showed no significant effect on somatostatin mRNA expression. These results suggest that GHS-R is involved in the up-regulation of GHRH and NPY expression and that NPY, somatostatin, and GH suppress GHRH expression. It is also suggested that the reduction of GHRH neurons of Tg rats is induced by a decrease in GHS-R expression.

The somatostatin sst1 receptor: an autoreceptor for somatostatin in brain and retina?

The sst1 receptor was the first of the 5 somatostatin receptors to be cloned by homology with the glucagon receptor 13 years ago. It is a 7-transmembrane domain G-protein-coupled receptor that is negatively coupled to adenylyl cyclase, but can also trigger other transduction pathways. The distribution of sst1 mRNA, immunolabeling, and radioligand binding are not entirely overlapping, but the recent availability of knockout (KO) mice and a (still limited) number of selective agonists/antagonists has increased our knowledge about this receptor. These new tools have helped to reveal a role for the sst1 receptor in hippocampal, hypothalamic, basal ganglia, and retinal functions. In at least the latter 3 structures, the sst1 receptor appears to act as an inhibitory autoreceptor located on somatostatin neurons, whereas in the hippocampus such a role is still based on circumstantial evidence.

Expression analysis of hypothalamic and pituitary components of the growth hormone axis in fasted and streptozotocin-treated neuropeptide Y (NPY)-intact (NPY+/+) and NPY-knockout (NPY-/-) mice

In the fasted and the streptozotocin (STZ)-induced diabetic male rat, hypothalamic growth hormone (GH)-releasing hormone (GHRH) mRNA levels, and pulsatile GH release are decreased. These changes are believed to be due to a rise in hypothalamic neuropeptide Y (NPY) that inhibits GHRH expression. To directly test if NPY is required for metabolic regulation of hypothalamic neuropeptides important in GH secretion, NPY, GHRH and somatostatin (SRIH) mRNA levels were determined in fasted (48 h) and STZ-treated wild-type (NPY(+/+)) and NPY-knockout (NPY(-/-)) mice by ribonuclease protection assay. In addition, pituitary receptor mRNA levels for GHRH (GHRH-R), ghrelin (GHS-R) and SRIH (sst2) were assessed by RT-PCR. Under fed conditions the GH axis of NPY(+/+) and NPY(-/-) did not differ. In the NPY(+/+) mouse, fasting resulted in a 23% weight loss and >250% increase in NPY mRNA accompanied by a significant reduction in both GHRH and SRIH mRNA. These changes were associated with increases in pituitary expression of GHRH-R and GHS-R and a concomitant suppression of sst2. In the NPY(-/-) mouse, fasting also resulted in a 23% weight loss and comparable changes in GHRH-R and sst2, but failed to alter GHRH, SRIH and GHS-R mRNA levels. Fasting resulted in an overall increase in circulating GH, which reached significance in the fasted NPY(-/-) mouse. Induction of diabetes in NPY(+/+) mice, using a single, high-dose, STZ injection (150 mg/kg), resulted in modest weight loss (5%), and a 158% increase NPY expression which was associated with reciprocal changes in pituitary GHS-R and sst2 expression, similar to that observed in the fasted state, but no change in hypothalamic GHRH or SRIF expression was observed. Induction of diabetes in NPY(+/+) and NPY(-/-) mice, using a multiple, low-dose, STZ paradigm (5 consecutive daily injections of 40 mg/kg), did not alter body weight, hypothalamic neuropeptide expression or pituitary receptor expression, with the exception that sst2 mRNA levels were suppressed and GH levels did rise in the NPY(-/-) mouse. These observations demonstrate that NPY is not required for basal regulation of the GH axis, but is required for fasting-induced suppression of GHRH and SRIH expression, as well as fasting-induced augmentation of pituitary GHS-R mRNA. In contrast to the rat, fasting clearly did not suppress circulating GH levels in mice, but resulted in an overall rise in mean GH levels, similar to that observed in other mammalian species. The fact that many of the fasting-induced changes in the GH axis were observed in the high-dose STZ-treated mice, but were not observed in the multiple, low-dose paradigm, suggests STZ-mediated modulation of GH axis function is dependent on the severity of the catabolic state and not hyperglycemia.

Putative GH pulse renewal: periventricular somatostatinergic control of an arcuate-nuclear somatostatin and GH-releasing hormone oscillator

Growth hormone (GH) pulsatility requires periventricular-nuclear somatostatin(SRIF(PeV)), arcuate-nuclear (ArC) GH-releasing hormone (GHRH), and systemic GH autofeedback. However, no current formalism interlinks these regulatory loci in a manner that generates self-renewable GH dynamics. The latter must include in the adult rat 1) infrequent volleys of high-amplitude GH peaks in the male, 2) frequent discrete low-amplitude GH pulses in the female, 3) disruption of the male pattern by severing SRIF(PeV) outflow to ArC, 4) stimulation of GHRH and GH secretion by central nervous system delivery of SRIF, 5) inhibition of GH release by central exposure to GHRH, and 6) a reboundlike burst of GHRH secretion induced by stopping peripheral infusion of SRIF. The present study validates by computer-assisted simulations a simplified ensemble formulation that predicts each of the foregoing six outcomes, wherein 1) blood-borne GH stimulates SRIF(PeV) secretion after a long time latency, 2) SRIF(PeV) inhibits both pituitary GH and ArC GHRH release, 3) ArC GHRH and SRIF(ArC) oscillate reciprocally with brief time delay, and 4) SRIF(PeV) represses and disinhibits the putative GHRH-SRIF(ArC) oscillator. According to the present analytic construction, time-delayed feedforward and feedback signaling among SRIF(PeV), ArC GHRH, and SRIF(ArC) could endow the complex physiological patterns of GH secretion in the male and female.

Regulation by estradiol of hypothalamic somatostatin gene expression: possible involvement of somatostatin in the control of luteinizing hormone secretion in the ewe

In the ewe, the mediobasal hypothalamus (MBH) is the primary central site for estradiol to generate the preovulatory GnRH/LH surges and sexual behavior. This area contains numerous neurons expressing the estradiol receptor alpha, distributed in the ventromedial nucleus (VMN) and the infundibular nucleus (IN). A large proportion of these neurons express somatostatin, making this neuropeptide a potential candidate for transmission of the estradiol signal to the GnRH neurons located in the preoptic area. We tested this hypothesis using ovariectomized ewes that had been subjected to an artificial estrous cycle. In the first experiment, 22 h after progesterone removal, ewes received estradiol (treated ewes) or empty implants (control ewes) for 4 h and then were killed. Using in situ hybridization, we showed that this short estradiol treatment increased the somatostatin mRNA amount by about 50% in the VMN and 42% in the IN. In the second experiment, preovulatory estradiol signal was replaced by somatostatin intracerebroventricular (ICV) administration. This treatment abolished LH pulsatility and dramatically decreased the mean basal level of LH secretion while it did not affect the mean plasma GH concentration. We demonstrated that an increase in somatostatin mRNA occurs at the time of the negative feedback effect of estradiol on LH secretion during the early stage of the GnRH surge induction. As ICV somatostatin administration inhibits the pulsatile LH secretion by acting on the central nervous system, we suggest that somatostatin synthesized in the MBH could be involved in the estradiol negative feedback before the onset of the preovulatory surge.

Physiological neuroendocrinology of peptides, steroids and other hormones in cerebrospinal fluid

Cerebrospinal fluid acts as a conduit in neuroendocrine regulation. Valid assessment of normal cerebrospinal fluid levels of peptides, steroids and other hormones requires clarification of reference concentrations in control patients and normal volunteers. Awareness of factors which may alter neuronal activity and, in turn, the relative composition of cerebrospinal fluid constituents is essential to the accurate sampling and hormonal analysis of cerebrospinal fluid.

Localization of somatostatin receptors at the light and electron microscopial level by using antibodies raised against fusion proteins

Somatostatin mediates its multiple biological effects via specific plasma membrane receptors belonging to the family of G-protein coupled receptors with seven putative membrane-spanning domains. Five somatostatin receptor subtypes (sst1-sst5) have been cloned in human, mouse, and rat. We have raised specific antibodies against the five human somatostatin receptors by using the fusion protein technique. DNA sequences encoding C-terminal parts of the somatostatin receptors were inserted into a pGEX-2T plasmid vector. E. coli bacteria were transformed with the recombinant plasmid and fusion proteins were expressed and purified using the glutathione S-transferase Gene Fusion System. The fusion proteins were emulsified with Freund's complete adjuvant and polyclonal antibodies were raised in rabbits. The antisera were tested for specificity in Western blot analysis of membrane preparations from cell lines expressing the receptors and in membrane preparations of brain tissues. The receptors were visualized at the light microscopical level in paraformaldehyde fixed tissue sections by use of biotin labelled secondary antibodies as well as by amplification with biotinylated tyramide. The final step in the immunohistochemical visualization of the receptors was done by both peroxidase labelled streptavidin/biotin and different fluorophores. At the electron microscopical level, some of the receptors could be visualized in tissues fixed with a combination of paraformaldehyde and low concentrations of glutaraldehyde. In the hamster brain, sst2 receptors labelling was observed in both neuronal processes and perikarya. The staining was present in neo-, and allocortical areas of the forebrain, the hypothalamus, brain stem, and spinal cord. In the rat and human, sst1 receptor was shown to be an auto receptor on somatostatinergic neurons located in the hypothalamus. In the retina both sst1 and sst2 receptors were present. sst1 receptors were confined to amacrine cells, few ganglionic cells, and Müller cell-end feet. sst2 receptors were more widespread than the sst1 receptors. sst2-immunoreactivity was present in dopaminergic amacrine cells, the Müller cell-end feet, and in the inner segments of the cone photoreceptors. Thus, the availability of subtype specific antibodies against the five somatostatin receptors makes it possible to identify the receptors involved in the multiple somatostatinergic system in the body.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Immunohistochemical and cytochemical localization of the somatostatin receptor subtype sst1 in the somatostatinergic parvocellular neuronal system of the rat hypothalamus

Somatostatin is known to mediate its actions through five G-protein-coupled receptors (sst1-sst5). We have studied the expression of the sst1 receptor in the rat hypothalamus by using a subtype-specific antiserum. In Western blotting, the antiserum reacted specifically with a band with an apparent molecular weight of 80,000 in membranes prepared from hypothalamic tissue. The localization of the sst1 receptor was investigated by immunohistochemistry in hypothalamus sections. Additionally, an immunofluorescent double-labeling was performed for the sst1 receptor and somatostatin. Light microscopy revealed that the sst1 receptor is located in perikarya and nerve fibers in the rostral periventricular area surrounding the third ventricle as well as in nerve fibers projecting from the perikarya to the external layer of the median eminence. In these neuronal structures, sst1 immunoreactivity was found to be colocalized with somatostatin. Furthermore, the location of sst1 receptors was studied by immunoelectron microscopy in the median eminence. In the external layer, receptor immunoreactivity was confined to nerve terminals. Immunoreactive nerve terminals were seen to make synapse-like junctions with other both stained and unstained nerve terminals. Thus, the sst1 receptor is present in the classic somatostatinergic hypothalamic parvocellular system inhibiting hormone secretion from the anterior pituitary gland. These findings indicate that the sst1 receptor may act as an autoreceptor and inhibit the release of somatostatin from periventricular neurons projecting to the median eminence.

Chronic exposure to either somatostatin (SS) or octreotide, a long-lasting SS analogue, affects SS expression in the postnatal visual cortex of the rat

The peptide somatostatin (SS) is widely distributed in the mammalian brain where it modulates neuronal activity through interactions with specific membrane-bound receptor subtypes (ssts). Five different ssts were characterized so far (sst1-5) and their selective agonists were developed on the basis of their binding specificity. SS and ssts are transiently expressed in the developing brain, suggesting a functional role of somatostatinergic systems in neuronal maturation. In the present study, we investigated the effects of chronic exposure to either the SS synthetic analogue, SS-14 or octreotide (a long-acting sst2-preferring analogue) on the maturation of SS-immunoreactivity (-ir) in the primary visual cortex of the rat. SS-ir maturation was investigated both by an evaluation of the number of SS-immunoreactive cells and by radioimmunoassay (RIA) to measure the levels of SS in the postnatal visual cortex. In the visual cortex of normal rats, the number of SS-positive cells markedly increased during the second postnatal week and then significantly decreased until the adult value was reached at the third week. Early and repeated intracerebroventricular (i.c.v.) injections of either SS-14 or octreotide prevented the increase in the number of SS-positive cells, with adult values reached at the end of the first postnatal week. Similarly, administration of either SS-14 or octreotide significantly decreased the SS content of the visual cortex, measured at the end of the second postnatal week. These results show that high local concentrations of either SS-14 or octreotide interfere with SS expression in developing cortical neurons in a restricted postnatal period.

Somatostatin decreases somatostatin messenger ribonucleic acid levels in the rat periventricular nucleus

Growth hormone (GH) secretion from the pituitary is known to be under the dual control of GH-releasing factor (GRF) and somatostatin (SRIF). Hypothalamic SRIF, the major inhibitor of pituitary growth hormone secretion, inhibits its own release by a negative ultrashort-loop feedback mechanism. However, it is not known whether this negative regulation is mediated by inhibition of SRIF mRNA production. GRF may also inhibit its own release, thereby modifying pituitary GH secretion, possibly through an ultrashort-loop feedback mechanism. Thus, SRIF production and GRF release are both regulated by SRIF. Periventricular nucleus (PeN) and mediobasal hypothalamus (MBH) from adult male rats were incubated for 6 h in Waymouth's medium with either SRIF or the SRIF agonist analog RC 160 (10(-9) to 10(-6) M). Levels of SRIF mRNA were determined by an S1 nuclease protection assay using a 32[P]-labeled rat SRIF riboprobe. SRIF (10(-7) M) and RC 160 (10(-8), 10(-7) M) significantly (p< or =0.01) decreased SRIF mRNA levels in the PeN. The levels of SRIF mRNA in the MBH were not modified by either SRIF or RC 160. SRIF (10(-7) and 10(-6) M) significantly (p < or = 0.01 and p < or = 0.001, respectively) inhibited the release of GRF at 30 min in the MBH. Likewise, the release of GRF was slightly decreased by 10(-7) M RC 160, and significantly inhibited by 10(-6) M (p < or = 0.001) at 30 min. At 6 h, the levels of GRF were significantly reduced by 10(-7) M SRIF (p < or = 0.05) and by RC 160 (10(-7), 10(-6) M; p < or = 0.001 and p < or = 0.05, respectively). In contrast with these results, the SRIF analog was unable to alter SRIF release at 30 min. At 6 h incubation, RC 160 (10(-7) M) significantly (p < or = 0.001) reduced SRIF release from MBH fragments. These results demonstrate that SRIF and a SRIF analog decrease SRIF mRNA levels in the PeN and inhibit the release of SRIF from the nerve terminals of the MBH. Thus, SRIF appears to regulate its own gene expression by negative ultrashort-loop feedback. Therefore, when SRIF is secreted from these neurons in response to GRF, it down-regulates the preceding stimulatory input as well as its own secretion.

Passage of peptides across the blood-brain barrier: pathophysiological perspectives

Blood-borne peptides are capable of affecting the central nervous system (CNS) despite being separated from the CNS by the blood-brain barrier (BBB), a monolayer comprised of brain endothelial and ependymal cells. Blood-borne peptides can directly affect the CNS after they cross the BBB by nonsaturable and saturable transport mechanisms. The ability of peptides to cross the BBB to a meaningful degree suggests that the BBB may act as a modulatory pathway in the exchange of informational molecules between the brain and the peripheral circulation. The permeability of the BBB to peptides is a regulatory process affected by developmental, physiological, and pathological events. This regulation sets the stage for the relation between peptides and the BBB to be involved in pathophysiological events. For example, some of the classic actions of melanocortins on the CNS are explained by their abilities to cross the BBB, whereas aspects of feeding and alcohol-related behaviors are associated with the passage of other specific peptides across the BBB. The BBB should no longer be considered a static barrier but should be recognized as a regulatory interface controlling the exchange of informational molecules, such as peptides, between the blood and CNS.

Time course of hypothalamic growth hormone-releasing hormone and somatostatin content in streptozocin diabetic rats: evidence for early changes in hypothalamic regulation

Growth hormone secretion is markedly suppressed early in streptozocin induced diabetes mellitus of the rat. Our studies were designed to delineate early changes in hypothalamic regulation by growth hormone-releasing hormone (GHRH) and somatostatin (SS) with the aim of determining the best time period for hypothalamic secretion studies. Although hypothalamic GHRH content (ng/hypothalamus) and SS concentration (ng/mg wet weight) were unchanged at 17 to 20 days in previous studies, we anticipated changes earlier in the time course from transient imbalances in release and synthesis. We examined hypothalamic GHRH content and SS concentration in control, diabetic, and insulin treated diabetic rats (n = 5-13; streptozocin 100 mg/kg i.p.) at 0, 2, 4, 7, 10 and 21 days. In diabetic rats GHRH content was greater at day 2 (142 +/- 9% of control-same day, P < 0.05) and day 4 (139 +/- 17%, P < 0.05), but was less at day 10 (67 +/- 4%, P < 0.01). GHRH content of insulin treated diabetic rats was elevated at day 2 (158 +/- 10%, P < 0.05), but subsequently was unchanged from control. In diabetic rats SS concentration was decreased at day 4 (78 +/- 5%, P < 0.01) and at day 21 (91 +/- 3%, P < 0.05). Our results show earliest changes compared to control in GHRH content at 2 days and in SS concentration at 4 days. These findings support early changes in hypothalamic secretion, define a time period of 1 to 10 days for further studies of release and gene expression, and suggest complex relationships of gene expression, peptide synthesis, and peptide release.

Modulation by somatostatin of glutamate sensitivity during development of mouse hypothalamic neurons in vitro

Glutamate sensitivity development and interactions of somatostatin (SRIF) with AMPA/Kainate receptor-mediated glutamate responses were studied in dissociated hypothalamic neurons from 16-day-old mouse embryos grown in vitro. Only 18% of functionally innervated cells could be found at 6-9 DIV whereas the percentage of innervated neurons progressively increased thereafter to reach 100% at 19-22 DIV. The glutamate sensitivity, estimated from glutamate-induced peak inward current, was very low at 6-9 DIV, sharply increased at 11-14 DIV and developed at a low increase rate thereafter. SRIF either unaffected glutamate peak current (27% of the cells), or significantly decreased (50%) or increased it (23%). Pertussis Toxin pretreatment abolished the SRIF-induced decrease of the glutamate response without affecting the excitatory effect. The number of glutamate responsive neurons inhibited by SRIF increased with time in culture whereas that of neurons responding to SRIF by an increased glutamate response was not statistically modified by functional innervation. The present data suggest that increased glutamate sensitivity coincides with the onset of functional synaptogenesis in mouse hypothalamic neurons in culture. SRIF can modulate glutamate sensitivity of hypothalamic neurons with either synergistic or antagonistic effects. Since glutamate has been shown to stimulate SRIF synthesis and secretion from hypothalamic neurons, the reverse capacity of SRIF to modulate the glutamate response suggests that both transmitters exhibit complex reciprocal interactions.

Somatostatin in the prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: evidence for a nonsynaptic function

Neuropeptides are widely distributed throughout the nervous system and exert a large number of heterogeneous functions. While they are synthesized in the soma, release is thought to take place in axonal terminals of neurons. A good model system to investigate the role of peptides in the nervous system is provided by the central posterior/prepacemaker nucleus (CP/PPn) of pacemaker nucleus (Pn), a medullary cell group controlling the electric organ discharge (EOD). Previous immunocytochemical and in situ-hybridization studies employing topographical criteria indicated that PPn neurons may express the neuropeptide somatostatin (SS). In the present study, we unambiguously identified PPn neurons by in vitro tract tracing. By combining this technique with SS immunocytochemistry, we found that a large portion of retrogradely labelled PPn neurons exhibited SS-like immunoreactivity (72-89%, n = 708 cells in 10 fish examined). Surprisingly, however, neither the proximal PPn axons nor anterogradely labelled terminals innervating the Pn displayed significant amounts of SS-like immunolabelling (n = 10 fish examined in each experiment). These results and the lack of SS binding sites in the Pn [82] suggest that SS expressed by PPn cells is not synaptically released at the target site of their axons, the Pn, but acts via a nonsynaptic mechanism in the CP/PPn proper.

Patterns of expression of SSTR1 and SSTR2 somatostatin receptor subtypes in the hypothalamus of the adult rat: relationship to neuroendocrine function

The neuropeptide somatostatin is the major physiological inhibitor of growth hormone secretion. With the aim of identifying the receptor subtypes through which this neuropeptide may be exerting its neuroendocrine actions in the brain, we have examined by in situ hybridization the distribution of the messenger RNA for SSTR1 and SSTR2 isoforms in the hypothalamus of adult male and female rats. Both receptor subtypes were highly expressed in the medial preoptic area, suprachiasmatic nucleus and arcuate nucleus. High SSTR1, but low SSTR2, expression was evident in the para- and periventricular nuclei as well as in the ventral premammillary nucleus. Conversely, moderate to high SSTR2, but low SSTR1, messenger RNA levels were detected in the anterior hypothalamic nucleus, ventromedial and dorsomedial nuclei and medial tuberal nucleus. Taken together, these distributional patterns conform to those of somatostatin binding sites as visualized by in vitro autoradiography, suggesting that an important proportion of SSTR1 and SSTR2 receptors in the hypothalamus are associated with the perikarya and dendrites of intrinsic neurons. The distribution of SSTR1-expressing cells within the periventricular, paraventricular and suprachiasmatic nuclei was similar to that of neurons previously reported to contain and/or express somatostatin in the brain suggesting that some of the SSTR1 receptors may correspond to autoreceptors. Within the arcuate nucleus, the distribution of SSTR1 and SSTR2 messenger RNA-expressing cells was comparable to that of neurons previously found to selectively bind somatostatin-14 within this area. Given that over one third of these cells also contain and express growth hormone-releasing factor, the present findings suggest that both of these receptor subtypes are involved in the central regulation of growth hormone-releasing factor secretion by somatostatin. Taken together, the present results suggest that SSTR1 and SSTR2 somatostatin receptor messenger RNAs are heavily expressed in those neurons containing somatostatin and/or growth hormone-releasing factor and thereby imply a role for both SSTR1 and SSTR2 somatostatin receptor subtypes in neuroendocrine regulation of growth hormone secretion in both sexes of this species.

Distribution of somatostatin receptors 1, 2 and 3 mRNA in rat brain and pituitary

In this study sequence-specific antisense oligonucleotide probes have been used to investigate the distribution of the mRNAs coding for the somatostatin receptor subtypes termed somatostatin receptor 1, somatostatin receptor 2 and somatostatin receptor 3 in the rat brain and pituitary using in situ hybridization techniques. The three receptor subtype mRNAs were found to be widely distributed in the brain with different patterns of expression, but with some overlap. Somatostatin receptor 1 mRNA was particularly concentrated in the cerebral and piriform cortex, magnocellular preoptic nucleus, hypothalamus, amygdala, hippocampus, and several nuclei of the brainstem. Somatostatin receptor 3 mRNA was very abundant in the cerebellum and pituitary (in contrast to somatostatin receptor 1), but it was also found in hippocampus, amygdala, hypothalamus and in motor nuclei of the brainstem. Somatostatin receptor 2 mRNA levels were very low relative to the other two mRNAs evaluated. Receptor 2 mRNA was observed in the anterior pituitary, and in the brain it was found in the medial habenular nucleus, claustrum, endopiriform nucleus, hippocampus some amygdala nuclei, cerebral cortex and hypothalamus. None of the three somatostatin receptor mRNAs studied here was found in the caudate nucleus. Northern analysis revealed distinct sizes of mRNAs for each subtype, and displacement experiments showed that each probe sequence was subtype-specific.

Growth hormone-releasing factor increases somatostatin release and mRNA levels in the rat periventricular nucleus via nitric oxide by activation of guanylate cyclase

Previous work has shown that growth hormone-releasing factor (GRF) stimulates cGMP production and somatostatin [somatotropin (growth hormone)-release-inhibiting factor, SRIF] release without altering cAMP accumulation by fragments of median eminence incubated in vitro. Therefore, this study was undertaken to evaluate the effect of GRF and cGMP on SRIF mRNA and SRIF release in the periventricular nuclei of male rats in vitro. SRIF mRNA levels were determined in explants of periventricular nuclei incubated for 6 hr in Waymouth's medium in the presence of various substances. Steady-state levels of SRIF mRNA were measured by an S1 nuclease protection assay using a 32P-labeled rat SRIF RNA probe. SRIF release and cGMP formation were measured at 30 min and 6 hr by RIA. SRIF mRNA levels and SRIF release were significantly (P < 0.025) increased (approximately 2-fold) by 1 microM dibutyryl cGMP, whereas sodium butyrate had no effect. This augmentation was not influenced by cycloheximide, an inhibitor of protein synthesis. Sodium nitroprusside (10 microM), an activator of the guanylate cyclase pathway via its release of nitric oxide, augmented (P < 0.001) SRIF mRNA levels and significantly increased (P < 0.05) SRIF release. GRF (1 nM) increased SRIF mRNA (P < 0.001) and stimulated the release of SRIF at 30 min (P < 0.05) and 6 hr (P < 0.01). This stimulation was abolished by 10 microM NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of nitric oxide synthase, but not by NG-monomethyl-D-arginine (D-NMMA, the inactive isomer). GRF also increased cGMP formation. This effect was completely blocked by incubation with L-NMMA but not D-NMMA. These results indicate that GRF releases nitric oxide. The nitric oxide diffuses to the adjacent SRIF neurons, where it activates guanylate cyclase, leading to increased formation of cGMP. This cGMP increases SRIF mRNA and SRIF release in the periventricular nuclei of male rats.

Periventricular nucleus lesioning modulates specific somatostatin binding in various brain regions and anterior pituitary

The effects of periventricular nucleus (Pe) lesioning on the plasma growth hormone (GH) levels and the anterior pituitary (A.P.) and brain somatostatin (SRIF) receptors were studied. A transient significant increase in plasma GH level in lesioned rats was detected 1 day after the operation. This elevated level of plasma GH started to decrease 3 days after lesioning. These changes were paralleled by an increase in binding of 125I-Tyr11-SRIF-14 to the A.P. 1 day after lesioning. This result could further confirm that the SRIF inhibitory action on GH release takes place at the A.P. level. Also, a transient increase in binding of the radioligand was detected in some brain areas 1 and 4 days after the lesion. However, the mechanism by which this increase takes place remains to be elucidated.

Physiological consequences of the passage of peptides across the blood-brain barrier

Peptides given peripherally have been shown to affect the central nervous system (CNS). Peptides are also capable of crossing the blood-brain barrier (BBB). It is unclear, however, whether such crossing underlies the ability of peptides to affect the CNS. We review specific examples in which a peptide must cross the BBB to produce its effect. The effect elicited by passage often duplicates the effect elicited at peripheral sites of action. Other examples, however, are reviewed in which peptides have opposite effects after central and peripheral administration. Such paradoxical effects suggest that passage of peptides may be involved in feedback or counter-regulatory loops.

Expression of gonadotropin-releasing hormone receptors and autocrine regulation of neuropeptide release in immortalized hypothalamic neurons

The hypothalamic control of gonadotropin secretion is mediated by episodic basal secretion and midcycle ovulatory surges of gonadotropin-releasing hormone (GnRH), which interacts with specific plasma membrane receptors in pituitary gonadotrophs. Similar GnRH receptors and their mRNA transcripts were found to be expressed in immortalized hypothalamic neurons, which release GnRH in a pulsatile manner in vitro. Activation of these neuronal GnRH receptors elicited dose-related intracellular Ca2+ concentration responses that were dependent on calcium mobilization and entry and were inhibited by GnRH antagonists. Exposure of perifused neurons to a GnRH agonist analog caused a transient elevation of GnRH release and subsequent suppression of the basal pulsatile secretion. This was followed by dose-dependent induction of less frequent but larger GnRH pulses and ultimately by single massive episodes of GnRH release. The ability of GnRH to exert autocrine actions on its secretory neurons, and to promote episodic release and synchronized discharge of the neuropeptide, could reflect the operation of the endogenous pulse generator and the genesis of the preovulatory GnRH surge in vivo.

Intrahypothalamic neurohormonal interactions in the control of growth hormone secretion

The secretion of growth hormone (GH, somatotropin) is regulated by two neurohormones: one inhibitory, somatotropin release-inhibiting hormone (SRIH) or somatostatin, and one stimulatory, GH-releasing hormone (GHRH). There are several lines of evidence for reciprocal interactions between SRIH and GHRH neuronal networks. Anatomically, GHRH terminals contact SRIH-containing neurons in the periventricular nucleus and SRIH-containing fibres innervate GHRH-containing neurons in the arcuate nucleus. Physiologically, SRIH and GHRH are secreted into the portal blood in complementary oscillating patterns. Results from immunizations with anti-SRIH antisera suggest that endogenous SRIH blocks GHRH release from the median eminence. Intracerebroventricular injections of SRIH stimulate secretion of GHRH indirectly, probably via autoinhibition of SRIH neurons in the anterior periventricular region. High resolution autoradiography allowed us to visualize high affinity, specific [125]SRIH receptors on 30% of GHRH mRNA-containing cells in the ventrolateral portion of the arcuate nucleus. The functional importance of these receptors was demonstrated by blocking endogenous SRIH action with cysteamine, which resulted in an increase in GHRH mRNA levels and desaturation of SRIH receptors in the ventrolateral part of the arcuate nucleus. Alterations in GH production caused by hypophysectomy or acute and chronic GH hypersecretion have opposite effects on the synthesis of SRIH and GHRH, giving further evidence for reciprocal interactions between these two neurohormonal systems.

A novel hypothalamic-dispersed pituitary co-perifusion model for the study of growth hormone secretion

This study presents a novel, in vitro, hypothalamic-dispersed pituitary co-perifusion system (HPPS) developed to examine the influence of the hypothalamus on pituitary growth hormone (GH) secretion in a controlled environment. In this perifusion system, dispersed rat pituitary cells were loaded onto Biogel P-2 (P-2) beads in a 0.5-ml plexiglas chamber and were submerged in a 37 degrees C water bath. After stabilization, two hypothalami were placed into each chamber on a thin layer of P-2 beads and the chamber was re-equilibrated. To test the system, pituitary cells were stimulated either directly with growth hormone-releasing factor (GRF) or indirectly via the hypothalamus, with clonidine, an alpha 2-adrenergic (alpha 2) receptor agonist. Perifusion of HPPS or pituitary cells with GRF (40 ng/ml) induced a substantial endogenous GH surge. Clonidine (2 x 10(-8) M) treatment stimulated a GH surge in HPPS chambers, but not in chambers containing only pituitary cells. Thus, somatotrophs respond to hypothalamic factors released in response to clonidine and not directly to alpha 2 stimulation. To determine if the components involved in GH feedback are present in the perifusion system, HPPS chambers were sequentially perifused with hGH, clonidine, and GRF. hGH pretreatment suppressed the clonidine but not the GRF-induced GH surge(s) observed in chambers perifused with clonidine and GRF only. In chambers only containing pituitary cells, GH was only increased in response to GRF when sequentially perifused with all three substances. This study demonstrates the dynamic interaction between the hypothalamus and pituitary in the regulation of GH secretion in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroendocrine regulation of growth hormone secretion in sheep. IV. Central and peripheral cholecystokinin

The result of alterations in the levels of CCK, in the blood and in the cerebrospinal fluid, on the functioning of the growth hormone axis has been examined in sheep. Male Coopworth sheep of about 40 kg liveweight were given various doses of CCK either intracerebroventricularly (icv) or intravenously (iv). Other similar sheep were given various doses of a CCK antagonist (loxiglumide) by the same routes. Bolus iv administration of either 35 micrograms or 200 micrograms of CCK had no effect on plasma GH levels. When given icv, however, CCK resulted in a marked (P less than 0.01) prolonged depression in plasma GH levels. The decrease in GH secretion could be partially attenuated by concurrent administration of loxiglumide, but was completely unaffected by concurrent administration of antisomatostatin serum icv. Loxiglumide alone had no effect on plasma GH levels when given at up to 200 micrograms icv, but intravenous administration of 8 mg of the CCK antagonist resulted in an increase in plasma GH concentrations (P less than 0.05). Plasma levels of somatostatin, glucose and cortisol were unaffected by both icv and iv administration of CCK. These results show that CCK can have a strong GH-inhibiting effect in the brain. Furthermore, this effect seems to be independent of hypothalamic somatostatin, suggesting another GH-inhibiting system exists.

Interleukin-1 (IL-1) stimulates arginine vasopressin (AVP) release from superfused rat hypothalamo-neurohypophyseal complexes independently of cholinergic mechanism

We studied whether interleukin-1 (IL-1) affects the release of arginine vasopressin (AVP) from the superfused hypothalamo-neurohypophyseal complex (HNC) of rats. Involvement of the cholinergic system in the mediation of IL-1 on AVP release from HNC was also examined. Both human recombinant IL-1 alpha and -1 beta elicited a rapid increase of AVP from HNC in a dose-dependent manner at concentrations ranging from 0.1 to 10 nM. However, neither IL-1 alpha nor -1 beta at concentrations of 100 nM increased AVP, and even suppressed the stimulatory effect of 10 nM IL-1 alpha and -1 beta added later. Acetylcholine at concentrations of 1 to 100 nM caused a dose-dependent, rapid increase in AVP, whereas AVP release induced by 10 nM acetylcholine was completely suppressed by the combined presence of 10 microM hexamethonium, a nicotinic receptor antagonist, and 50 microM atropine, a muscarinic receptor antagonist. On the other hand, AVP release induced by 10 nM IL-1 alpha and -1 beta was not affected by the combination of the two antagonists. These results suggest that both IL-1 alpha and -1 beta may stimulate AVP release by acting directly on the hypothalamo-neurohypophyseal system, and that the stimulatory effect of IL-1 on AVP release may be independent of the cholinergic system.

Neuroendocrine regulation of growth hormone secretion in sheep. II. Effect of somatostatin on growth hormone and glucose levels

The effect of intravenous (iv) and intracerebroventricular (icv) administration of somatostatin on the plasma levels of growth hormone (GH) and glucose was studied in sheep. Intravenous somatostatin decreased (P less than 0.001) circulating GH when infused at the rate of 5 micrograms/min (150 ng/kg/min) over 1 hr, but when used at 1 microgram/min there was no effect on plasma GH levels during infusion. At both doses used there was an indication of an increase in GH following the cessation of somatostatin infusion. Somatostatin given at both these doses iv had no effect on plasma glucose levels. When given icv neither 1.8 micrograms, 18 micrograms nor 180 micrograms somatostatin had any significant effect of plasma GH levels, although there was a significant (P less than 0.05) elevation in GH levels 75 min after 180 micrograms somatostatin icv. Plasma glucose levels did not increase following injection of somatostatin icv at 1.8 or 18 micrograms, but there was a clear hyperglycaemic episode following 180 micrograms icv. Despite a lack of effect of somatostatin on GH release when given icv, there was a clear elevation (P less than 0.05) in plasma GH levels immediately following icv administration of a somatostatin antiserum. These data indicate that iv administration of somatostatin at pharmacological levels can depress unstimulated GH levels in sheep while administration icv does not. Central administration of somatostatin increases plasma glucose levels only at high doses and seems unlikely to be of physiological importance in glucose homeostasis.

Neuroendocrine regulation of growth hormone secretion in sheep. III. Serotoninergic systems

The role of serotoninergic pathways in the regulation of growth hormone secretion in the sheep has been investigated. Both peripheral and central routes of administration of serotonin agonists and antagonists have been used. Intravenous administration of the serotonin agonist, buspirone, at 1.2 mg/kg/h lowered plasma GH levels (P less than 0.001) but at 0.21 mg/kg/h there was no significant decrease. Intracerebroventricular (icv) administration of serotonin itself also depressed GH levels (P less than 0.01). The serotonin antagonist, cyproheptadine, failed to affect GH concentrations when given either intravenously (0.25 mg/kg/h) or intracerebroventricularly (4 mg). Neither serotonin nor cyproheptadine had any significant effect on plasma glucose or cortisol levels when administered icv. The possible role of somatostatin in mediating the serotonin associated decrease in GH was investigated by concurrent administration of serotonin and a specific, potent anti-somatostatin serum into a cerebral ventricle. This treatment also resulted in a marked, sustained depression in GH (P less than 0.001). These data suggest that serotonin can inhibit release of GH from the pituitary in sheep and that this is independent of hypothalamic somatostatin.

Rat growth hormone-releasing factor stimulates cyclic GMP formation and phosphatidylinositol metabolism in the median eminence

The effects of rat growth hormone releasing factor (rGRF) on somatostatin (SRIF) secretion, cyclic nucleotide production and phosphatidylinositol metabolism were investigated in the median eminence (ME), using an in vitro system. Medium was discarded and replaced by medium containing various concentrations of rGRF or rGRF plus epinephrine (E, 6 x 10(-7) M). rGRF had no effect on basal or E-stimulated release of cAMP. In the same experiments rGRF markedly stimulated SRIF release. These results suggested that cAMP is not involved in the stimulatory effect of GRF on SRIF release. However, GRF significantly stimulated release of both SRIF and cGMP in a dose-related manner. Maximal stimulation was observed at 10(-10) M GRF (p less than 0.005) which also produces maximal SRIF release. 2'0-monobutyrylguanosine 3'5' cyclic phosphate (mbcGMP, 10(-11) to 10(-10) M) stimulated SRIF release from ME fragments (p less than 0.001 at 10(-10) M) whereas the control, sodium butyrate (10(-6) M), had no effect. GRF caused significant elevation of 30.6% in the concentration of labelled inositol phosphates [( 3H]-IPs) in the ME. These data indicate that GRF stimulation of SRIF release is accompanied by increased cGMP production and phosphatidyl-inositol (PI) metabolism but does not alter cAMP production. Because mbcGMP can directly stimulate SRIF release, we suggest that GRF causes a receptor-mediated increase in the metabolism of phosphatidylinositol and cGMP formation. These actions therefore may be among the early metabolic events in the mechanism of GRF-stimulated SRIF release from the ME.

Central somatostatinergic regulation of growth hormone secretion in dwarf chickens

1. Basal circulating growth hormone (GH) concentrations in sex-linked-dwarf (SLD) chickens were unaffected by the intracerebroventricular (icv) injection of 10, 50 or 100 micrograms somatostatin (SRIF). 2. The GH response to systemic thyrotropin-releasing hormone (TRH; 10 micrograms/kg, iv) was, however, 'paradoxically' enhanced 20 min after icv SRIF administration. 3. A lower dose (1.0 micrograms) of SRIF had no effect on basal or TRH-induced GH release. 4. High-titre SRIF antisera (4 microliters) also had no acute effect on basal plasma GH concentrations, but augmented the GH response to TRH challenge. 5. SRIF would appear to act at central sites to modulate stimulated GH secretion in SLD chickens.

Intrahypothalamic injection of somatostatin, not GRF, stimulates prolactin secretion

The distribution of somatostatin and growth hormone releasing factor (GRF) fibres in the hypothalamus suggests that they may be involved in physiological functions in addition to growth hormone control. GRF or somatostatin were stereotaxically injected into anterior or basal hypothalamic regions of unanesthetized male rats and effects on plasma prolactin measured. Somatostatin caused a small, significant, dose-responsive stimulation of prolactin secretion when injected in both hypothalamic regions, while GRF was without effect. Somatostatin may therefore have a minor intrahypothalamic role in regulating prolactin.

Radioautographic analysis of somatostatin receptor sub-type in rat hypothalamus

Hypothalamic somatostatin (SRIF) receptors were examined in a qualitative and quantitative radioautographic study using [125I-Tyr0,D-Trp8]SRIF14 and the stable octapeptide analog [125I-Tyr3]SMS 201-995 as radioligands. The latter has been shown to bind selectively to the high-affinity SS1 receptor subtype. Both radioligands labeled specifically and with high resolution various hypothalamic nuclei. In addition, the labeling patterns obtained with the two probes were identical; in both cases specific binding density was highest in the preoptic area and lowest in the ventromedial hypothalamic nucleus. Inhibition of the specific binding of each radioligand by either SRIF14 or the SS1-selective (SMS 201-995) unlabeled competitor was assessed on serial sections throughout the hypothalamus. The proportions of both non-selective and SS1-selective binding, remaining in the presence of either SRIF14 or SMS 201-995 (micromolar concentrations) were identical. These results indicate the existence of a homogeneous class of SRIF binding sites of the SS1 type in the hypothalamus.

The effects of a growth hormone-releasing Peptide and growth hormone-releasing factor in conscious and anaesthetized rats

Abstract The growth hormone (GH) releasing ability of GH-releasing factor (GRF) and a GH-releasing hexapeptide, CHRP, have been studied in anaesthetized and conscious male and female rats. The GH responses to GHRP in anaesthetized rats were inconsistent, and this peptide was much less potent than GRF. Continuous iv infusions of GRF or GHRP both caused an initial GH release which was not maintained, and further GH release could be elicited by injection of GRF during an infusion of GHRP and vice versa. In contrast, conscious rats were much more sensitive to GHRP. Infusions of GHRP or GRF both caused an initial GH release. With GRF infusions, GH release continued in the normal episodic pattern whereas with GHRP infusion, GH secretion remained elevated over baseline and the normal pulsatile rhythm was disrupted. Plasma GH levels fell after stopping GHRP infusion, without an immediate resumption of normal GH pulsatility. Conscious male rats responded intermittently to injections of GRF given iv every 45 min, but when such serial injections of GRF were given during a continuous iv infusion of GHRP, the GH responses to GRF became regular and more uniform. These results suggest that GHRP prevents the normal cyclic refractoriness to GRF in male rats by disrupting cyclic somatostatin release. The greater potency of GHRP in conscious rats may also depend on the release of endogenous GRF since passive immunization with an anti-GRF serum reduced the plasma GH response to GHRP infusion. Thus in the conscious animal, GHRP may release GH by complex actions at both a hypothalamic and pituitary level.

Effect of naloxone and luteinizing hormone releasing hormone pulses on plasma luteinizing hormone concentration in ewe lambs

A study was conducted to determine the effect of pulses of naloxone on plasma LH concentration in prepubertal ewes and to see if this treatment affects the pituitary response to subsequent LHRH pulses. Prepubertal ewes (n = 5) received three intracarotidal pulses of naloxone (NAL, 1 mg/kg BW) and four pulses of Luteinizing Hormone Releasing Hormone (LHRH, 1 micrograms/pulse) at hourly intervals. Control ewes (n = 5) received saline instead of NAL followed by the LHRH pulses at the same frequency. Blood samples were collected from one hour before the first pulse of saline or NAL at 15 min intervals for four hours and at 30 min intervals for another four hours. Plasma LH concentration was measured by radioimmunoassay. The first pulse of NAL increased plasma LH levels compared to the basal levels and compared to control animals (P less than 0.01). The succeeding pulses were ineffective. LHRH provoked an increase in plasma LH concentration in control as well as in treated animals but the amplitude of the net peak height increased progressively up to the third pulse in NAL treated animals, while there was no increase in pituitary responsiveness to LHRH in control prepubertal ewes. Also, the area under the LHRH response curve was greater (P less than 0.05) in animals pretreated with NAL than in lambs pretreated with saline. The results suggest that there is an inhibitory opioid tone over LH secretion in female lambs. NAL increases the responsiveness to exogenous LHRH pulses, probably as a result of endogenous LHRH release.

The influence of human corticotropin-releasing hormone on somatostatin secretion in depressed patients and controls

Twenty subjects (10 patients with a major depressive episode and 10 individually matched healthy controls) received 100 micrograms synthetic human corticotropin-releasing hormone (hCRH) as an i.v. bolus dose. Healthy subjects and depressed patients exhibited a significant increase of plasma somatostatin (SRIH) concentrations with no difference between both comparison groups. Compared to controls, depressed patients showed a significant attenuation of corticotropin (ACTH) responses, while cortisol secretion in response to hCRH was normal. No correlations were found among basal plasma concentrations of SRIH, ACTH or cortisol and SRIH, ACTH or cortisol responses following hCRH. These findings are compatible with the hypothesis that hypothalamic-pituitary-adrenal (HPA) hyperactivity in the depressive state may primarily be due to central hypersecretion of CRH and support the view of a hCRH-induced SRIH secretion which is not related to HPA dysfunction associated with major depression.

Combined autoradiographic and immunohistochemical evidence for an association of somatostatin binding sites with growth hormone-releasing factor-containing nerve cell bodies in the rat arcuate nucleus

Abstract The regulation of growth hormone secretion depends upon the complex interplay between two hypothalamic hypophysiotropic factors: growth hormone-releasing factor and somatotropin release inhibiting factor or somatostatin. Interactions between these two neurohormones appear to be exerted both distally, at the level of pituitary somatotropes, and proximally, within the hypothalamus. In an attempt to detect a possible anatomical substrate for central interactions between the two neurohormones, we compared the autoradiographic distribution of specifically labeled somatostatin binding sites with the immunohistochemical distribution of growth hormone-releasing factor-containing neurons in the hypothalamus of adult rats. Somatostatin binding sites were labeled in vitro by incubating serial brain sections with [(125)l]TyrO-DTrp8-somatostatin. Growth hormone-releasing factor-immunoreactive neurons were visualized in a second set of animals, using an antiserum raised against synthetic rat growth hormone-releasing factor (1-29) NH(2). In light microscopic autoradiograms of sections incubated with [(125)l]somatostatin the label was found to be concentrated over small, round or oval neuronal perikarya clustered within the ventrolateral aspect of the arcuate nucleus. The topographic distribution of these [(125)l]somatostatin-labeled cells was similar to that of growth hormone-releasing factor-immunoreactive neurons detected within the same region. Moreover, the number of [(125)l]somatostatin-labeled cells was found to vary in parallel with that of growth hormone-releasing factor-immunoreactive neurons throughout the rostro-caudal extent of the arcuate nucleus (coefficient of correlation r = 0.80). These results suggest that somatostatin binding sites may be directly associated with the perikarya of arcuate growth hormone-releasing factor neurons. Such an association would provide an anatomical substrate for a direct regulation of growth hormone-releasing factor secretion by somatostatin at the hypothalamic level.

Methods for the study of somatostatin

It is clear from the above that there are a number of methods for study of SRIF release. From the standpoint of convenience, the in vitro static incubation of ME is the most practical technique at the present time. Using this preparation SRIF release has been found to be modified by a number of neurotransmitters and peptides, and studies on the mechanism of release of the peptide have been initiated. There is no doubt that such studies should be complemented by perifusion studies, by studies involving larger pieces of the hypothalamus which encompass the entire somatostatinergic neuron, and by in vivo studies to determine the correlation of in vivo and in vitro release. Among the in vivo techniques which have been utilized, the push-pull cannula technique employing cannulae implanted in hypothalamus or anterior pituitary gland offers the most promise. A summary of the effects of some neurotransmitters and neuropeptides on hypothalamic SRIF secretion is reported in Table. I.

Effects of elevated serum insulinlike growth factor-II on growth hormone and insulinlike growth factor-I mRNA and secretion

The insulinlike growth factors (IGF) appear to exert feedback control over their own production. In an effort to determine the physiologic mechanisms for this feedback modulation, we utilized a previously developed in vivo model in which rIGF-II secreting tumor cells are transplanted into immunodeficient rats to form IGF-II secreting tumors. The tumor-bearing rat have serum IGF-II concentrations sevenfold greater than those in controls (119 +/- 16 ng/mL [mean +/- SE] v 17 +/- 2 ng/mL, P less than .0001). Serum IGF-I concentrations were reduced among the tumor-bearing rats (438 +/- 42 ng/mL v 606 +/- 32 ng/mL, P = .002) and were negatively correlated with IGF-II concentrations (r = -.47, P = .025), suggesting that IGF-II suppressed the secretion of IGF-I. Increased serum IGF-II concentrations, however, did not affect basal growth hormone concentrations (tumor-bearing, 44 +/- 12 ng/mL; control 33 +/- 6 ng/mL, P = 0.96). The GH response to GH releasing factor was likewise similar in both groups. Moreover, pituitary GH mRNA level were not different in the two groups, suggesting that IGF-II does not have a significant effect on GH secretion in this in vivo model. There was no association between serum glucose and serum IGF-I or IGF-II concentrations. To examine the effect of IGF-II on IGF-I production from the liver, we measured IGF-I mRNA levels in a subset of animals. Despite these differences in serum IGF-I concentrations, the tumor-bearing rats did not have significantly lower liver IGF-I mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)