Stimulation by growth hormone of somatostatin release from the rat hypothalamus in vitro

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Somatostatin system: molecular mechanisms regulating anterior pituitary hormones

The somatostatin (SRIF) system, which includes the SRIF ligand and receptors, regulates anterior pituitary gland function, mainly inhibiting hormone secretion and to some extent pituitary tumor cell growth. SRIF-14 via its cognate G-protein-coupled receptors (subtypes 1-5) activates multiple cellular signaling pathways including adenylate cyclase/cAMP, MAPK, ion channel-dependent pathways, and others. In addition, recent data have suggested SRIF-independent constitutive SRIF receptor activity responsible for GH and ACTH inhibition in vitro. This review summarizes current knowledge on ligand-dependent and independent SRIF receptor molecular and functional effects on hormone-secreting cells in the anterior pituitary gland.

Influence of testicular hormones on the somatostatin-GH system during the growth promoted transition to puberty in sheep

The aim of the present study was to investigate whether the growth promoted transition to puberty in lambs involved changes in the effects of testicular hormones on somatostatin in hypothalamic neurons and GH secretion. The study was performed in infants (9-week-old) testis-intact (TEI) and orchidectomized (ORCHX) at the sixth week of age, and pubertal lambs (16-week-old) TEI and ORCHX at the 12th week of age (n = 20). In TEI lambs, the changes included a pubertal increase in immunoreactive somatostatin in the periventricular nucleus and median eminence with simultaneous neuropeptide depletion in the median eminence, and a decrease in the percentage of the hypophyseal area (PA) occupied by GH-immunoreactive cells (P < 0.05). The mean concentration of GH in the peripheral blood plasma was greater (P < 0.001) in early infancy (5 wk), because of the greater (P < 0.0001) pulse amplitude, and then uniformly low until puberty. The postnatal increase in the body weight (BW) was prominent (P < 0.01) in middle-late infancy (9-12 wk) because of the large daily live-weight gain. After orchidectomy somatostatin was abundant. This effect on nerve terminals in the median eminence was greater (P < 0.01) in infancy and lesser (P < 0.05) in puberty. Conversely, the PA occupied by GH cells was lower in the ORCHX pubertal lambs compared to TEI lambs (P < 0.05). The GH concentration and pulse characteristics were less (P < 0.05) in the infantile and pubertal ORCHX lambs compared to the TEI lambs. However, this effect was weak (P < 0.05) until middle infancy because of no influence on the GH basal concentration, and strong (P < 0.001) after late infancy. The BW did not differ (P > 0.05) between TEI and ORCHX lambs. Findings suggest activation of GH negative autofeedback loop in middle infancy. Testicular factors may play an inhibitory role in regulating somatostatin accumulation and a stimulatory role in GH secretion until puberty. The start of puberty is related to an attenuation in the stimulatory role of gonadal factors in regulating somatostatin depletion in nerve terminals associated with an intensification of the stimulatory role of gonadal factors in regulating GH secretion. From a somatic perspective of growth rate, these mechanisms do not seem to be important. Thus, testicular factors modulate mechanisms within the somatostatin-GH system to integrate somatotropic and gonadotropic functions at the time of growth-promoted sexual maturation in sheep.

Regulation of basal, pulsatile, and entropic (patterned) modes of GH secretion in a putatively low-somatostatin milieu in women

Somatostatin (SS) released by hypothalamic neurons inhibits GH exocytosis noncompetitively. Therefore, we postulated that attenuation of GH feedback-induced SS outflow would help to unmask covariates of endogenous secretagogue drive. To this end, 42 healthy pre- and postmenopausal women were randomly assigned to receive leuprolide plus estradiol (E(2)) or leuprolide plus placebo. A putatively low-SS milieu was imposed by L-arginine infusion. Deconvolution and regularity analyses were applied to 6-h GH concentration-time profiles. By two-way ANOVA, age negatively (P < 0.001) and E(2) positively (P = 0.001) determined pulsatile GH secretion in the presumptively SS-deficient milieu (P < 0.001). Comparable effects were exerted on the mass of GH secreted per burst per unit distribution volume (age P = 0.001, E(2) P < 0.001, overall P < 0.001). E(2) alone predicted basal (nonpulsatile) GH secretion (P = 0.004). Stepwise forward-selection multivariate regression demonstrated that age (P = 0.0017) and E(2) (P = 0.0002) together explained 46% of intersubject variability in pulsatile GH secretion (P < 0.001) and fully replaced the negative univariate effect of abdominal visceral fat (r(2) = 0.32, P < 0.001). Moreover, age and E(2) (but not AVF) interacted to supervise GH regularity (P = 0.007). We conclude that age and E(2) availability individually and together constitute primary predictors of basal, pulsatile, and patterned GH secretion in an inferentially feedback-silenced context in healthy women. Therefore, both factors must be considered in framing hypotheses of endogenous GH drive.

Cloning, tissue distribution and effects of food deprivation on pituitary adenylate cyclase activating polypeptide (PACAP)/PACAP-related peptide (PRP) and preprosomatostatin 1 (PPSS 1) in Atlantic cod (Gadus morhua)

Full-length complementary deoxyribonucleic acid sequences encoding pituitary adenylate cyclase activating polypeptide (PACAP)/PACAP-related peptide (PRP) and preprosomatostatin 1 (PPSS 1) were cloned from Atlantic cod (Gadus morhua) hypothalamus using reverse transcription and rapid amplification of complementary deoxyribonucleic acid ends. Semi-quantitative reverse transcriptase polymerase chain reaction shows that PRP/PACAP mRNA and PPSS 1 mRNA are widely distributed throughout cod brain. During development, PRP/PACAP and PPSS 1 were detected at the 30-somite stage and pre-hatching stage, respectively, and expression levels gradually increased up to the hatched larvae. PPSS 1, but not PRP/PACAP, appeared to be affected by food availability during early development. In juvenile cod, PPSS 1 expression levels increased and remained significantly higher than that of control fed fish throughout 30 days of starvation and during a subsequent 10 days refeeding period. In contrast, PRP/PACAP expression levels were not affected by 30 days of food deprivation, but a significant increase in expression levels was observed during the 10 days refeeding period in the experimental food-deprived group as compared to the control fed group. Our results suggest that PRP/PACAP and PPSS 1 may be involved in the complex regulation of growth, feeding and metabolism during food deprivation and refeeding in Atlantic cod.

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.

Aging and the growth hormone/insulin like growth factor-I axis

Growth hormone release and IGF-I synthesis decrease with increasing age. The regulation of the GH/IGF-I system is dependent on the integrity of the hypothalamus, pituitary and liver. During aging there are several changes which contribute to the decline in GH/IGF-I including changes in signal to the somatotrophs from growth hormone releasing hormone, somatostatin and other factors such as body composition, exercise, diet and sleep. All of these factors are discussed in detail within this review. The phenotypic similarities between aging and adult growth hormone deficiency syndrome combined with this decrease in GH/IGF-I with aging have prompted the question whether aging is a GH deficient state. The advent of recombinant growth hormone has led to a number of studies treating elderly patients with GH alone or in combination with sex steroids or exercise. The results of these studies would not back up the use of GH in elderly non-hypopituitary patients as they did not show efficacy, showed high rates of adverse events and there is also some evidence associating GH/IGF-I and risk of neoplasia. If GH therapy is to be used in this cohort of patients further long term efficacy and safety studies are required.

Gender modulates sequential suppression and recovery of pulsatile growth hormone secretion by physiological feedback signals in young adults

The basic mechanisms that drive the renewal of GH pulses in the human are not understood. Recent ensemble models predict that pulse regeneration requires quenching of an ongoing GH pulse by somatostatin outflow and evocation of a new burst by rebound GHRH release. We reasoned that related principles might explain why women consistently maintain higher-amplitude GH secretory bursts than men. Accordingly, the present study tests the hypothesis that gender modulates the successive dynamics of GH feedback and escape in the morning fasting, when GH pulses are larger in women. To this end, we infused single iv pulses of recombinant human (rh) GH (0, 1, and 3 microg/kg) in eight young men and six women on separate randomly ordered mornings fasting and quantitated serial inhibition and recovery of GH secretion by frequent sampling, immunochemiluminometry, a deconvolution procedure, and regularity analysis. Statistical contrasts revealed gender-comparable peak concentrations and kinetics of rhGH. However, women differed from men by way of: (1) 3.5- and 4.0-fold less feedback suppression of GH secretory-burst mass; (2) more irregular patterns of GH release during negative feedback; and (3) 12-and 14-fold greater postnadir rebound-like GH secretion after rhGH pulses. Mechanistic analyses based on a minimal feedback construct predicted that women generate higher endogenous secretagogue stimulation per unit somatostatin outflow than men. In summary, negative feedback induced by near-physiological GH pulses unmasks prominent gender-related contrasts in hypothalamo-pituitary autoregulation in young adults. A frugal but sufficient explanation of the ensemble outcomes is that women sustain greater hypothalamo-pituitary agonist input than men.

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.

Somatostatin and somatostatin receptor physiology

Since the discovery of somatostatin (SST) over three decades ago, its ubiquitous distribution and manifold functions are still being documented. SST is synthesized in the hypothalamus and transported to the anterior pituitary gland where it tonicaly inhibits GH and TSH secretion as well as being responsible for GH pulsatile release. Several internal feedback loops, sleep, exercise, and chemical agents control and influence SST release. SST also impacts the function of a wide variety of cells and organ systems throughout the body. Knowledge of the structures of the SSTs has resulted in recognition of the essential four core conserved residues responsible for their actions. The SSTs act through six separate SST cell surface receptors (SSTRs), members of the family of G protein-coupled receptors. Receptor ligand binding (SST/SSTR) results in cellular activities specific for each receptor, or receptor combinations, and their tissue/cell localization. Understanding the structure/function relationship of the SSTs and their receptors, including the internalization of SST/SSTR complexes, has facilitated the development of a variety of novel pharmacologic agents for the diagnosis and treatment of neuroendocrine tumors and unfolding new applications.

Effects of GH and IGF-I administration on GHRH and somatostatin mRNA levels: I. A study on ad libitum fed and starved adult male rats

The individual role played by GH and IGF-I in the regulation of hypothalamic GHRH and SRIF gene expression is still object of debate. We have investigated the effect of exogenously administered recombinant hGH (rhGH) and recombinant hIGF-I (rhIGF-I) in ad libitum fed control and starved rats, the latter an animal model which is characterized by low circulating levels of endogenous GH and IGF-I. Adult male rats were fed ad libitum (C) or food-deprived (S) for 72 hours; rats in either C or S groups were treated with systemic administration of rhGH and rhIGF-I for 3 days. GHRH, SRIF and GH mRNA levels were evaluated by Northern and slot blot hybridization. Administration of rhGH (250 micrograms/kg/twice daily, sc) induced a significant inhibition of GHRH and a significant stimulation of SRIF mRNA levels in C rats; GH treatment was, however, ineffective on both neuropeptide mRNA levels in the S group. Continuous infusion of rhIGF-I (300 micrograms/kg/day, sc) induced a significant increase of SRIF levels in both C and S rats but did not modify GHRH mRNA levels in either group. In the pituitary, GH mRNA levels followed a pattern very similar to that of GHRH. These results provide evidence for a direct role of GH in the inhibition of GHRH mRNA levels; IGF-I appears more involved in the direct stimulation of SRIF mRNA levels.

Differential effect of insulin-like growth factor-1 and growth hormone on hypothalamic regulation of growth hormone secretion in the rat

Pharmacological administration of either growth hormone (GH) or insulin-like growth factor 1 (IGF-1) were reported to inhibit endogenous GH release in humans and in the laboratory animal. We have evaluated the short-term differential mechanisms whereby the two hormones affect hypothalamic regulation of GH secretion. Wistar male rats (90 days old) were injected i.p. with either GH (recombinant GH NIAMDD, Baltimore, MD, USA), rIGF-1 (Fujisawa Pharmaceutical Co. Ltd., Osaka, Japan) or saline. Animals were sacrificed at 15, 30, 60 and 120 minutes following injection. Hypothalami were dissected and extracted immediately and the levels of growth hormone-releasing hormone (GHRH) and somatostatin were determined using specific antisera. Trunk blood was collected for GH and IGF-1 determination by RIA. Administration of IGF-1 or GH markedly decreased hypothalamic somatostatin stores by 77% and 54% respectively, within 15 minutes. Concomitantly, the wide range of GH levels found in the control group was reduced in the IGF-1 treated group suggesting that the pulsatile pattern of GH secretion was suppressed. Growth hormone administration induced an increase in hypothalamic GHRH stores (60% at 120 minutes). During this period serum IGF-1 levels were not altered. It is suggested that short term modulation of hypothalamic neurohormones by GH and IGF-1 is mediated by rapid stimulation of somatostatin release by both hormones, and inhibition of GHRH release is induced only by GH.

The regulation of growth hormone secretion

The regulation of GH secretion involves finely balanced systems with multiple components. As our knowledge of the physiology of GH regulation expands, so does our understanding of the bases for GH diseases. We now can identify several cellular loci that cause GH deficiency or GH excess. In addition, the recent increased understanding of GH physiology has resulted in an increase in potential therapies for growth disorders.

Pulsatile release and circadian rhythms of thyrotropin and prolactin in children with growth hormone deficiency

We have measured mean concentrations and have appraised the pulsatile nature of thyrotropin (TSH) and prolactin (PRL) release in children with classical GH deficiency (GHD; n = 4) and neurosecretory GH dysfunction (NSD; n = 4) and have compared the results with those obtained in children with constitutional delay (control; n = 4). Blood samples were obtained at 20-min intervals for 24 h. Pulse analysis of TSH and PRL was undertaken using the Cluster pulse detection algorithm. Circadian rhythmicity of TSH and PRL was assessed using cosinor analysis. The mean 24-h concentration of GH in the control subjects was significantly higher than that obtained in the GHD and NSD groups. With regard to TSH, the mean serum concentration in the GHD and NSD group were higher than that of the control subjects. This augmentation reflects TSH pulses of large amplitude and area, and a higher interpulse valley mean rather than a difference in peak number or peak duration. No differences in mean PRL concentration or characteristics of PRL pulses were found between the control and GHD and NSD subjects. When the 24 h data sets were divided into day (0800-2000 h) and night (2000-0800 h), the mean nighttime TSH concentration was higher than the daytime concentration in the control, GHD, and NSD groups. Although there were no day versus night differences in TSH pulse frequency in either group, peak amplitude, area, and interpulse valley means were increased during the night in the control group, and peak area, duration, and amplitude mean in the NSD group. The nighttime mean PRL concentrations in the control, GHD, and NSD subjects were higher than those found during the day. This increase was accounted for by increases in PRL peak amplitude, area in the control group, and peak area, amplitude, and interpulse valley mean in the GHD and NSD groups. Cosinor analysis of the 24-h TSH and PRL data revealed clear circadian rhythmicity in all groups of subjects. These data suggest that GHD and NSD are associated with an increase in pulsatile TSH secretion due to an increase in pulse amplitude and interpulse valley mean.

Somatogenic and lactogenic binding sites in rat brain and liver: quantitative autoradiographic localization

The distribution of somatogenic and lactogenic binding sites in female and male rat brain as well as in liver was studied by quantitative receptor autoradiography using 125I-human growth hormone (125I-hGH) as a ligand. Quantitative measurement of binding sites for 125I-hGH showed differences in the levels of these sites in the female and male brain and liver. Moreover, regional differences in the brain were also observed in each sex. In the female brain high levels of 125I-hGH binding sites were found in the choroid plexus. Intermediate levels were observed in the striatum, the hypothalamus and the hippocampus, whereas low levels of these sites were found in the central gray, the temporal, the piriform and the entorhinal cortices. In the male brain high levels of 125I-hGH binding sites were detected in the choroid plexus. Intermediate levels were observed in the parietal cortex, the hypothalamus and the hippocampus, whereas low levels were found in the tegmentum, the temporal cortex and the striatum. Quantification of 125I-hGH binding sites in the liver revealed higher levels in the female than in the male liver. In general, higher levels of binding sites (16%-77%) were observed in the female than in the male tissues. The quantification of rat growth hormone (rGH) by radioimmunoassay was also performed in this study. Varying amounts of rGH immunoreactivity were detected in the different brain regions, with the highest levels of rGH-like material being found in the midbrain and cortex of both sexes. Moreover, higher levels of rGH-like material were observed in the female than in the male brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone increases somatostatin release and messenger ribonucleic acid levels in the rat hypothalamus

Growth hormone (GH) suppresses its own secretion by stimulating somatostatin (SRIF) release. Thus, the possible regulation of GH-releasing factor (GRF) and SRIF release and SRIF messenger ribonucleic acid (mRNA) by GH was studied in the hypothalamus of male rats in vitro. The median eminences (ME's) were incubated in buffer containing 10(-7)-10(-11) M GH for 30 min. SRIF and GRF released into the medium were quantitated by RIA. The release of SRIF from ME fragments was significantly increased (P < 0.001) by 10(-9) M GH; however, 10(-9) M GH also inhibited (P < 0.01) GRF release from the ME. To determine the effect of GH on SRIF mRNA levels, periventricular nucleus (PeN) explants were cultured during 6 h in medium with 10(-7)-10(-11) M GH. Levels of SRIF mRNA (determined by an S1 nuclease protection assay) were significantly elevated in the presence of 10(-10)-10(-7) M GH. Likewise, 10(-9) M GH significantly stimulated SRIF release from PeN explants at 30 min and at 6 h. Surprisingly, 10(-9) M GH also significantly increased GRF release from the PeN explants at these times as well. This GRF was not responsible for the increased SRIF release or SRIF mRNA induced by GH since GRF antibody did not modify the GH-induced increases in SRIF release and mRNA levels. These results demonstrate a negative short-loop feedback of GH mediated at the ME by suppression of GRF and stimulation of SRIF release, whereas in the PeN GH increased both SRIF release and SRIF mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone therapy in normal short children induces a transitory decrease in plasma growth hormone releasing hormone levels and in human growth hormone responsiveness to exogenous growth hormone releasing hormone

A three-month study of the effect of growth hormone (hGH) therapy (0.1 U/kg/day sc) on plasma levels of GH releasing hormone (GHRH), somatostatin and insulin-like growth factor I (IGF-I) and on the hGH responsiveness to exogenous GHRH was carried out in 32 prepubertal short-stature children with normal GH secretion. Blood samples were collected prior to initiation of therapy, and at 5, 30 and 90 days of onset of therapy, as well as 2 and 90 days after termination of therapy. The nonconventional hGH therapy induced an increase in serum IGF-I levels which lasted as long as therapy was continued. Plasma GHRH levels showed an early transitory decrease after five days of therapy, whereas plasma somatostatin levels were unaltered. A slight suppression in hGH responsiveness to exogenous GHRH was found at 2 but not at 90 days after termination of hGH therapy. It is concluded that nonconventional hGH treatment does not cause permanent changes in physiological hGH secretion.

Acute changes in growth hormone-releasing hormone secretion after injection of BIM 23014, a long acting somatostatin analog, in rams

BIM 23014 is a somatostatin analog displaying an increased biological half life due to resistance to enzymatic degradation. This peptide inhibits GH release directly at the level of pituitary somatotrophs. In addition, an action of BIM 23014 at the level of the hypothalamus is possible since somatostatinergic fibers and receptors have been identified on GH-RH neurons. To evaluate the effect of BIM 23014 on GH-RH secretion, hypophysial portal blood (HPB) was continuously collected in conscious sheep. Twelve rams (40-45 kg, 9-month-old) with chronically implanted perihypophysial cannulae were i.v. injected with BIM 23014 (1 mg) or saline. HPB and jugular blood were collected for 3-5 hours before and after the injection for the determinations of GH-RH and GH concentrations respectively. The acute injection of BIM 23014 induced a rapid decrease of plasma GH within the first two hours. Simultaneously, GH-RH in HPB decreased significantly. After reaching a nadir, GH concentrations increased to values greater than baseline. A similar rebound in GH-RH levels in HPB was also observed. These data indicate that BIM 23014 acts at the level of GH-RH hypothalamic neurons, in addition to its well-know effect on the pituitary gland.

Galanin reinstates the growth hormone response to repeated growth hormone-releasing hormone administration in man

OBJECTIVE

To clarify the mechanism by which galanin, a 29-amino-acid peptide, increases GH secretion in man.

DESIGN

We studied the GH-releasing effect of this neurohormone (galanin, 15 micrograms/kg) infused over 60 minutes after 120 minutes of saline, following a previous GHRH bolus (GHRH 1 microgram/kg i.v. at 0 minutes, galanin infused from 120 to 180 minutes) and coadministered with the second of two consecutive GHRH boluses (GHRH every 120 minutes, galanin infused from 120 to 180 minutes).

PATIENTS

Fourteen healthy male subjects, aged 20-34 years, in two groups (group A, 20-31 years (n = 8); group B, 25-34 years (n = 6)) were studied.

MEASUREMENT

Blood samples were drawn every 15 minutes of 255 minutes. Serum GH was measured in duplicate by IRMA. Statistical analysis of the data was carried out by non-parametric ANOVA test.

RESULTS

The GH response to galanin infused 120 minutes after saline overlapped with that induced by the neuropeptide infused following previous GHRH bolus (AUC, mean +/- SEM: 317.3 +/- 73.2 vs 326.8 +/- 54.2 micrograms/l/h). The GH-releasing effect of the second GHRH bolus (126.9 +/- 32.3 micrograms/l/h) was lower than that of the first one (503.4 +/- 41.3 micrograms/l/h; P = 0.0002). Galanin markedly enhanced the GH responses to the second GHRH bolus (1118.0 +/- 212.7 micrograms/l/h; P = 0.0002 vs second GHRH bolus alone) so that it did not significantly differ from the first one (710.9 +/- 107.8 micrograms/l/h).

CONCLUSIONS

Our results show that the GH-releasing effect of galanin is not modified by GHRH pretreatment and that the neuropeptide reinstates the GH response to the repeated GHRH stimulation in man. They suggest that these effects are due to the inhibition of hypothalamic somatostatin release.

Growth factors and the anterior pituitary

Normal growth and secretion in the pituitary gland are dependent upon the co-ordinated action of a large number of extracellular growth factors, neuropeptides and peripheral hormones acting on their respective cellular receptors and via complex intracellular signalling pathways. The pituitary and hypothalamus are exposed to a large number of growth factors, several of which have well-documented effects on secretory function and may act as physiological modulators of pituitary hormone synthesis and release. IGF-I, for example, almost certainly acts as a feedback regulator of GH secretion. Despite well-documented mitogenic effects in other tissues, little is known about the role of these growth factors in normal pituitary cell turnover, compensatory hyperplasia or adenoma formation. There is now good evidence, however, that at least some of the hypothalamic releasing peptides are mitogenic for their respective pituitary cell subpopulations. The aetiology of pituitary tumours remains poorly understood but some appear to develop as a result of somatic mutation. Such mutations could enhance growth by causing altered expression of growth factors or their receptors, or constitutive activation of proteins involved in the intracellular mitogenic signal. Abnormalities have been documented at each of these levels in human pituitary tumours. The identification of an activating point mutation in the alpha subunit of Gs, the stimulatory regulatory peptide of adenylyl cyclase, in a proportion of somatotroph adenomas represents a major advance in our understanding of pituitary tumour pathogenesis. This and other findings may ultimately lead to new therapeutic approaches to the management of pituitary disease.

Critically ill patients have high basal growth hormone levels with attenuated oscillatory activity associated with low levels of insulin-like growth factor-I

OBJECTIVE

The aim was to study the relationship between growth hormone (GH) and insulin-like growth factor-I (IGF-I) in critically ill patients.

DESIGN

Case-control study of critically ill patients admitted to the intensive care unit was carried out.

PATIENTS

Six critically ill patients (51-78 years) who required ventilation and parenteral nutrition and six age, weight, height, and sex-matched healthy adults were studied.

MEASUREMENTS

The patients and controls were studied for two 24-hour periods; the patients before and after starting parenteral feeding, and the controls during a 36-hour fast and when taking meals equivalent in calories and protein to the patients' parenteral feed. Serum GH was measured at 20-minute intervals and analysed by a pulse detection algorithm (Pulsar) and Fourier transformation. IGF-I was measured at 0, 12, and 24 hours.

RESULTS

Patients had low serum IGF-I levels compared with controls, whether fasted or fed, despite having mean GH levels similar to fasted controls. For fasted patients vs fasted controls the mean (+/- 1 SD) GH levels were 4.5 +/- 2.0 vs 4.0 +/- 2.4 mU/l respectively, and IGF-I levels at the end of the fast were 0.17 +/- 0.11 vs 0.78 +/- 0.29 U/ml (P = 0.003). Patients showed elevated baseline GH levels compared with controls when fasted and during parenteral feeding (patients vs controls fasted 3.1 +/- 1.9 vs 0.8 +/- 0.5 mU/l, P = 0.01; patients vs controls fed 4.2 +/- 4.5 vs 0.5 +/- 0.04 mU/l, P = 0.028). Fourier transformation confirmed oscillatory GH levels in the controls, fasted or fed, but this activity was attenuated in the patients. Parenteral feeding had no effect on the GH profiles or IGF-I levels of patients, but controls showed greater mean GH levels during their fast than when fed.

CONCLUSIONS

We have demonstrated that critically ill patients have low IGF-I levels associated with augmented baseline GH levels which show reduced oscillatory activity. The results would be compatible with the hypothesis that there is an adaptive change in critically ill patients away from the indirect effects of GH (stimulation of IGF-I production and anabolism) and toward the direct effects (lipolysis and insulin antagonism) which increase the availability of energy substrates. The pattern of GH levels seen in our patients may be important in this adaptation.

In vitro release of growth hormone-releasing factor (GRF) from the hypothalamus: somatostatin inhibits GRF release

The manner of release of growth hormone-releasing factor (GRF) from the rat hypothalamus was studied in a perifusion system using a highly sensitive radioimmunoassay for rat GRF. The recovery of GRF in this system was 50-60%. The release of GRF from the rat hypothalamic blocks was almost stable for 20-240 min after the start of the perifusion and was stimulated by depolarization induced by high K+ concentration. The release of GRF was inhibited by somatostatin at concentrations of 10(-11) to 10(-8) M with maximum inhibition to 52.5% of the basal release at a concentration of 10(-9) M. These results suggest that this system is useful in studying the regulatory mechanism of GRF release and that, in addition to its action on the pituitary, somatostatin appears to act at the level of the hypothalamus in inhibiting GRF release in the regulation of GH secretion.

Beta-adrenoceptor activity change after prolonged treatment with growth hormone and somatostatin

1. The effect of 10 days treatment with growth hormone (GH) (1 mg/kg body wt/day) and somatostatin (SRIF) (0.25 mg/kg body wt/day) subcutaneously on the activity of beta-adrenoceptors in rat hypothalamic, pituitary and cerebral cortical membrane fractions was studied using [3H]dihydroalprenolol ([3H]DHA) as radioligand. 2. The administration of GH significantly increased the beta-adrenoceptor binding affinity and the administration of SRIF decreased the beta-adrenoceptor binding capacity in the hypothalamus. 3. In the pituitary the beta-adrenoceptor binding affinity was significantly decreased after both hormonal applications. 4. In the cerebral cortex the beta-adrenoceptor binding affinity was significantly decreased after the GH treatment and increased after the SRIF treatment. 5. The present study provides direct evidence for GH and SRIF effects on the activity of rat beta-adrenoceptors and supports the view about the involvement of beta-adrenergic mechanisms in the neurotransmitter regulation of GH secretion in the rat.

Regulation of somatostatin and growth hormone-releasing hormone gene expression in the rat brain

We have studied the regulation of somatostatin (SS) and growth hormone-releasing hormone (GHRH) gene expression in the brain of the laboratory rat. We report that hypophysectomy in the adult male reduces SS mRNA in cells of the periventricular nucleus (PeN), while GH reverses this effect. We demonstrate that cellular levels of SS mRNA in the PeN are higher in male compared to female animals. We report that castration reduces cellular levels of GHRH mRNA and SS mRNA in the arcuate nucleus and PeN, respectively, and that testosterone reverses this effect through an androgen receptor-dependent mechanism. Finally, we present a theoretical model to explain the generation of the ultradian rhythm in GH secretion, which implicates the reciprocal interaction between GH feedback and the transcriptional regulation of the SS and GHRH genes and the kinetics of these relationships.

Relationship between somatostatin and growth hormone messenger ribonucleic acid in human pituitary adenomas: an in-situ hybridization histochemistry study

We have used quantitative in-situ hybridization histochemistry to assess the somatostatin (SRIH) and GH messenger ribonucleic acid (mRNA) content of 81 pituitary adenomas. SRIH mRNA was found in the absence of GH mRNA in only one tumour and in the five cases in which GH and SRIH mRNA were both present, SRIH mRNA prevalence was approximately two orders of magnitude less than GH mRNA. However, no direct correlation between GH mRNA, SRIH mRNA and circulating GH levels was evident at the time of surgery. In contrast, there was no detectable SRIH mRNA probe binding to a thyrotroph adenoma, a gonadotroph adenoma, 29 endocrinologically inactive pituitary adenomas, nine lactotroph adenomas, two first trimester fetal and two normal adult pituitary glands obtained 24 and 36 h post-mortem. These findings corroborate previous reports of SRIH release from somatotroph adenomas in vitro and suggest that paracrine or autocrine mechanisms linking SRIH and GH mRNA operate within normal pituitary tissue. As somatotroph adenomas are thought to be predominantly clonal in origin, the data further suggest that GH and SRIH are synthesized in the same or closely related cells.

Aspects of growth hormone control in diabetes

Growth Hormone (GH) has been implicated in the development of retinal new vessels that characterise diabetic proliferative retinopathy. Formerly, pituitary ablation was successful in causing such new vessels to regress but this approach has been largely superseded by panretinal photocoagulation. A clearer understanding of the GH abnormalities in diabetes might not only shed light on the process of retinal new vessel formation but could also provide a means for pharmacological suppression of GH in those patients not fully responding to laser photocoagulation. In this review, GH control in diabetes is considered with particular reference to studies in patients with diabetic retinopathy.

Hypothalamic growth hormone-releasing factor (GRF) participates in the negative feedback regulation of growth hormone secretion

Effects of growth hormone (GH) excess on immunoreactive hypothalamic GH-releasing factor (GRF) and somatostatin (SRIF) were studied in rats. Hypothalamic GRF content significantly reduced after 7-day daily treatment with 160 micrograms of rat GH or after inoculation of GH-secreting rat pituitary tumors, MtT-F4 for 9 or 13 days and GH3 for 3 months. Basal and 59 mM K+-evoked release of GRF from incubated hypothalami diminished, more than the content, by 43-51% in MtT-F4 tumor- or by 67-83% in GH3 tumor-bearing rats. In contrast, there was a small but significant increase in content or release of SRIF in rats harboring the GH3 or MtT-F4 tumor, respectively. These results indicate the existence of a negative feedback loop via hypothalamic GRF as well as SRIF in control of GH secretion.

GHRF causes biphasic stimulation of SRIF secretion from rat hypothalamic cells

The complex interactions of the hypothalamic releasing peptides somatostatin (SRIF) and growth hormone (GH)-releasing hormone (GHRF), which regulate GH secretion, are still incompletely understood. To further scrutinize these interactions, we have studied the effects of GHRF on SRIF secretion from dispersed adult rat hypothalamic cells. Rat GHRF caused calcium- and dose-dependent stimulation of SRIF release in static 1-h incubations. SRIF release was stimulated by GHRF in a concentration range of 1-100 nM. However, the extended dose-response curve was biphasic in nature, with a significantly lower SRIF response in the presence of 1 microM GHRF vs. 100 nM GHRF. SRIF release, stimulated by another secretagogue (10 microM veratridine), was not affected by the presence or absence of 1 microM GHRF, suggesting the lack of toxic impairment of hypothalamic cell function by GHRF at this concentration. In conclusion, a biphasic stimulatory pattern of SRIF secretion in response to GHRF was observed in experiments employing dispersed rat hypothalamic cells. The biphasic SRIF response pattern to GHRF may be relevant in the physiological regulation of GH secretion.

Thyrotrophin-releasing hormone-induced growth hormone secretion in ducks: independence of peripheral plasma somatostatin, insulin, and glucagon

In young, but not old, ducks the iv infusion of thyrotrophin-releasing hormone (TRH) markedly increased peripheral plasma growth hormone (GH) concentrations, which remained elevated throughout the 30-min period of infusion. This GH response to TRH was suppressed by the simultaneous infusion of somatostatin, which increased the level of circulating somatostatin-like immunoreactivity (SLI) to supraphysiological levels. Basal concentrations of plasma SLI in both young and old birds were suppressed by TRH infusion. Concentrations of glucagon-like immunoreactivity (GLI) were increased by the infusion of TRH in young birds but not in adults, whereas plasma immunoreactive insulin (IRI) was decreased in young birds and increased in adults following TRH infusion. These results indicate that TRH-induced GH secretion in ducks is unrelated to changes in peripheral plasma SLI, GLI, or IRI induced by TRH infusion.

Identification of immunoreactive somatostatin in the rat harderian gland: regulation of its content by growth hormone, beta-adrenergic agonists and calcium channel blockers

Immunoreactive somatostatin (IRS) was identified in the male rat Harderian gland (HG) by radioimmunoassay. Tissue was extracted and a displacement curve performed; there were no significant differences between values obtained with serial dilutions of extracted tissue and those from purified somatostatin standard used in the radioimmunoassay. Basal values of HG-IRS were found to be in the nanomolar range (10.8 +/- 3.5 ng IRS/mg protein). Hypophysectomy did not change the HG-IRS but, in vivo growth hormone (GH) treatment led to a dramatic increase (6-7-fold) in the levels of IRS in the HG. Isoproterenol, a beta-adrenergic agonist, when administered in vivo significantly decreased the HG-IRS content. The effect of two different calcium channel blockers on the isoproterenol-induced decrease of HG-IRS was studied; no changes were observed with nifedipine but verapamil, injected one hour after isoproterenol administration, prevented the drop in HG-IRS levels. These data demonstrate the existence of IRS in a new location, the rat Harderian gland, and support a classical endocrine regulation for its tissue concentration.

Effect of sodium ion on atrial natriuretic factor release from rat hypothalamic fragments

The effects of Na ion and choline chloride on the release of atrial natriuretic factor (ANF) and growth hormone-releasing factor (GHRF) from rat hypothalamic fragments including the organum vasculosum of the lamina terminalis (OVLT) were examined in vitro. Although the release of ANF was stimulated by Na ion, choline chloride, and glucose in concentration-dependent manners, the release was more sensitive to a change in concentration of Na ion than to those of choline chloride and glucose. On the other hand, the change in Na ion concentration did not affect the release of GHRF. It can be therefore proposed that Na ion is the first candidate controlling ANF release from the brain tissue and that ANF in the hypothalamus and/or OVLT may play some role in the regulation of the Na ion and water balance in the central nervous system.

GH feedback occurs through modulation of hypothalamic somatostatin under cholinergic control: studies with pyridostigmine and GHRH

We have studied the effect of increased cholinergic tone on the GH response to growth hormone-releasing hormone (GHRH) and on GH feedback, using pyridostigmine, an acetylcholinesterase inhibitor. In six healthy male adult volunteers 120 mg oral pyridostigmine increased basal GH secretion compared to placebo and augmented the GH response to 100 micrograms i.v. GHRH (1-29) NH2; the effect was more than the additive effect of pyridostigmine and GHRH when each was given alone. Pretreatment with 2 IU methionyl-hGH given i.v. abolished the serum GH response to GHRH given 3 h later, demonstrating a negative feedback loop of GH on the response to GHRH; this inhibited response to GHRH was restored in subjects given pyridostigmine as well as methionyl-hGH. The data demonstrate that enhanced cholinergic tone releases GH, augments the serum GH response to GHRH and unblocks the negative feedback effect of methionyl-hGH pretreatment on the GH response to GHRH. These results suggest that GH negative feedback effects on its own secretion occur predominantly through increased hypothalamic somatostatin secretion; this somatostatin secretion is under inhibitory cholinergic control.

Somatostatin release from dispersed hypothalamic cells: effects of diabetes

We examined the release of growth hormone-release inhibiting factor (somatostatin) from dispersed hypothalamic cells obtained from mature diabetic rodents and normal age-matched controls, in an attempt to demonstrate a possible hypothalamic defect which might underlie some of the reported abnormalities in somatotrophic function in diabetes mellitus. Insulinopoenic diabetes was induced by either streptozotocin or alloxan. Somatostatin release from cells from diabetic rats was diminished both basally and after stimulation by membrane depolarisation. Stimulated release was calcium dependent in cells from both normal and diabetic animals. The defect was present in both streptozotocin and alloxan induced diabetes. We also compared hypothalamic somatostatin release from cells obtained from obese hyperinsulinaemic C57 BL/Ks db/db diabetic mice and non diabetic lean litter mates (db/-). Despite longstanding marked hyperglycaemia, no significant alteration in somatostatin release was apparent. Likewise, starvation of rats for 5 days did not result in significant diminution of somatostatin release. These observations document a defect in hypothalamic somatostatin release in experimentally induced insulinopoenic diabetes, which is not apparent in the db/db mouse, suggesting that glucose per se is not responsible. Rather than the anticipated increase in hypothalamic somatostatin release in insulinopoenic diabetes, a reduction in release was observed. These observations are compatible with the hypothesis that increased hypothalamic somatostatin release is not responsible for abnormal growth hormone secretion in this model.

Growth hormone and prolactin responses to bolus and sustained infusions of GRH-1-40-OH in man

To determine whether GRH stimulates PRL secretion we studied the effects of iv bolus injections and prolonged infusions of GRH 1-40-OH on PRL and GH serum levels in normal volunteers. Eight patients with acromegaly, two of whom had elevated basal levels of PRL, were also tested with single bolus injections. Six normal subjects given 3.3 micrograms/kg bolus injections of GRH showed a mean increment of GH of 22.0 +/- 1.7 ng/ml (mean +/- SE). A small rise in PRL was noted in 5 of the 6 subjects (mean peak level of 6.4 +/- 1.9 ng/ml vs basal level of 3.3 +/- 0.4 ng/ml, p less than 0.05). During the continuous intusion of GRH (10 ng/kg/min), GH levels rose gradually from a mean baseline of 1.1 +/- 0.1 ng/ml to a mean peak of 30.0 +/- 7.2 ng/ml at about 2 h and then slowly declined to a nadir of 4.2 +/- 0.4 ng/ml at 330 min. PRL levels did not rise significantly during the infusion. To determine whether the decline in GH levels in the face of continued infusion was due to loss of GH responsiveness, a 3.3 micrograms/kg bolus of GRH was given during the nadir at 330 min; this GH increment was significantly less than that obtained by the GRH bolus injection without the infusion (12.9 +/- 3.5 ng/ml vs 22.0 +/- 1.7 ng/ml, p less than 0.05). The PRL response to the GRH bolus was the same during the infusion of GRH as before. In each of 8 acromegalic patients (including two who had initially elevated basal PRL levels) GRH led to an increase in both GH and PRL levels. PRL and GH levels spontaneously fluctuated in parallel in 4 acromegalic cases studied with repeated samples over 6 h during placebo administration. These experiments show that GRH has significant, though weak, PRF effect in normals and that it is more potent PRF in acromegalic patients. Furthermore, the effects on GH and PRL of a sustained infusion of GRH for 5 1/2 h are both qualitatively and quantitatively different. These results suggest that the GRH effect is exerted either on different pituitary receptors for GH and PRL regulation, or that the releasable pools of the two hormones have different sizes and/or turnover times.

Effects of insulin-like growth factor I (IGF-I) on growth hormone-releasing factor (GRF) and thyrotropin-releasing hormone (TRH) stimulation of growth hormone (GH) secretion in the domestic fowl (Gallus domesticus)

Recent studies in mammalian species indicate that IGF-I may act as a negative feedback inhibitor of GH release through alteration of pituitary secretion or sensitivity to hypothalamic regulatory factors. Although avian GH secretion appears to be regulated by the differential release of hypothalamic inhibitory (somatostatin) and stimulatory (GRF and TRH) factors, feedback effects of IGF-I on in vivo GH release in birds have not been investigated. To study the effects of elevated IGF-I concentration on GRF- and TRH-stimulated GH secretion, 4-week-old chickens received an intravenous injection of recombinant human IGF-I either 15 min prior to (6 micrograms, study 1), or simultaneous with (10 micrograms, study 2). GRF (hGRF44NH2, 5 micrograms/kg) or TRH (0.5 microgram/kg) administration. Radioimmunoassay analysis of plasma collected prior to and following peptide treatment indicated that circulating IGF-I concentrations were elevated 83.9, 60.6, 77.9, and 88.8% at the time of TRH and GRF administration in studies 1 and 2, respectively. Peak GH concentrations (mean of +5- and +15-min samples) subsequent to TRH injection were significantly (P less than 0.01) depressed 45.1 and 48.2% in IGF-I-treated as compared with control chicks in the first and second studies, respectively. GRF-stimulated GH secretion was significantly (P less than 0.01) decreased by IGF-I administration in study 2 (41.3%) but not in study 1. An estimated half-life for IGF-I in the chicken is less than 15 min based on the disappearance rate of the elevation produced by exogenous IGF-I injections. Thus, IGF-I exerts a negative feedback effect on pituitary hormone secretion in avian as well as mammalian species.

Growth hormone pretreatment in man blocks the response to growth hormone-releasing hormone; evidence for a direct effect of growth hormone

The effect of pretreatment with biosynthetic methionyl human GH (hGH) on the GH response to GHRH has been studied in normal subjects. Eight volunteers were given either 4 IU hGH or placebo s.c. 12-hourly for 72 h before a GHRH test, or a single s.c. dose of 4 IU hGH 12 h before a GHRH test. Somatomedin-C (Sm-C) levels at the time of the GHRH tests were significantly elevated after treatment with hGH compared to placebo, and the GH response to GHRH was significantly attenuated. A further six subjects were given 2 IU hGH or placebo i.v., and i.v. GHRH 3 h later; there was no rise in Sm-C for the 5 h of the study after either treatment; nevertheless, the response to GHRH was completely abolished by pretreatment with hGH. These results demonstrate that GH can regulate its own secretion independently of changes in Sm-C levels, through a mechanism other than the inhibition of GHRH release. The attenuated response to GHRH in the presence of elevated Sm-C levels may be related to Sm-C, or be a more direct effect of the recently elevated GH levels.

Intracerebroventricular infusion of somatostatin selectively increases paradoxical sleep in rats

Chronic intracerebroventricular (i.c.v.) infusion of somatostatin (SRIF) elicited a specific increase of the daily duration of paradoxical sleep (PS) in rats. I.c.v. administration of cysteamine, a selective SRIF depletor, brought about a dose-dependent reduction of PS. Slow wave sleep (SWS) remained unaffected in both cases. These results suggest that SRIF may be involved in the regulation of PS.

Somatostatin in the central nervous system: physiology and pathological modifications

Since its discovery, at the beginning of 1973, somatostatin's multiple actions, in relation to its wide anatomical distribution have been widely documented. Its biochemical pathways have been elucidated with the discovery of other molecular forms as well as the mechanisms of its neuronal release. However, no definite proof is available concerning a neurotransmitter role for any peptide of the somatostatin family other than somatostatin-14. The precise determination of the roles of somatostatin in brain are still hampered by the poor pharmacology of the peptide. New tools are badly needed and in particular a true antagonist at the receptor site. The mechanisms of action of somatostatin are now well under way at least in the pituitary model. More information should come from this model and be applied to brain cells in vitro. The greatest challenge of somatostatin brain function lies in its role in the pathophysiology of neurological diseases such as Alzheimer's dementia and Huntington's disease. Nature has been using somatostatin-related molecules since inhibitory control was first needed in cell functions. Time will tell us if somatostatin is really an old peptide involved in senile dementia.

Exogenous growth hormone inhibits growth hormone-releasing factor-induced growth hormone secretion in normal men

Previous studies from this laboratory and by others in rats, monkeys, and humans support the concept that growth hormone (GH) can regulate its own secretion through an autofeedback mechanism. With the availability of human growth hormone-releasing factor (GRF), the possible existence of such a mechanism was reexplored by examining the effect of exogenous GH on the GH response induced by GRF-44-NH2 in six normal men (mean age, 32.4 yr). In all subjects the plasma GH response evoked by GRF-44-NH2 (1 microgram/kg i.v. bolus) was studied before and after 5 d of placebo (1 ml normal saline i.m. every 12 h), and then before and 12 h after 5 d of biosynthetic methionyl human GH (5 U i.m. every 12 h). The GH response to GRF (maximal increment over time 0 value) was significantly inhibited after GH treatment (0-1.3 vs. 2.3-11.2 ng/ml before treatment, P = 0.05), but was not significantly affected by placebo. This impaired pituitary response to GRF persisted for at least 24 h following exogenous GH treatment in two subjects who underwent further study. Serum somatomedin-C concentrations were significantly increased after 5 d of GH treatment (2.66-5.00 vs. 0.92-1.91 U/ml before treatment, P = less than 0.01). The impaired pituitary response to GRF may be mediated indirectly through somatomedin, somatostatin, by a direct effect of GH on the pituitary somatotropes, or by all of these mechanisms. These data suggest that after GH treatment, the blunted GH response to synthetic GRF is not solely a consequence of the inhibition of hypothalamic GRF secretion.

Deficiency of immunoreactive somatostatin in the median eminence of Snell dwarf mice

Snell dwarf mice (dw/dw) are characterized by a genetically determined, congenital lack of pituitary GH, TSH and prolactin. Given that hypothalamic somatostatin is involved in the regulation of pituitary GH and TSH release, it was decided to investigate the content of immunoreactive somatostatin (IRS) in the median eminence of dw/dw and phenotypically normal mice of the same strain. The content of IRS in the pyloric antrum and pineal gland of these animals was also examined. The effects of ovariectomy and of hyperprolactinemia (induced by a pituitary graft under the kidney capsule) on the median eminence content of IRS were also studied in both normal and dwarf mice. Median eminence IRS content was significantly lower in the dw/dw (23.6 +/- 1.8 ng) than in normal mice (57.4 +/- 7.1 ng); no difference was found in the pyloric IRS content of dw/dw (16.9 +/- 1.6 ng/mg of protein) and normal animals (13.8 +/- 1.9 ng/mg of protein), nor in the pineal content of IRS (639.4 +/- 64.4 pg/gland in the dw/dw; 732 +/- 265 pg/gland in normals). Neither ovariectomy nor hyperprolactinemia were found to affect the IRS content in the tissues studied in normal or dwarf mice. Treatment of an additional group of 9 dwarf mice with L-thyroxine (L-T4 2 micrograms/48 h. s.c. for 2 weeks) significantly increased the animals weight (10.2 +/- 0.4 g versus 7.4 +/- 0.3 g) and produced maturation of facial features; however, it did not change the IRS content in any of the tissues studied. It is concluded that the content of IRS in the median eminence of mice with a congenital lack of GH, TSH and prolactin is significantly reduced and that this is unlikely to be related to the deficiency of thyroid hormones in these animals.