Gloria Shaffer Tannenbaum

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

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 [125l]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) NH2. In light microscopic autoradiograms of sections incubated with [125l]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 [125l]somatostatin‐labeled cells was similar to that of growth hormone‐releasing factor‐immunoreactive neurons detected within the same region. Moreover, the number of [125l]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.

Long-term alterations in growth hormone and insulin secretion after temporary dietary protein restriction in early life in the rat

ABSTRACT: Protein malnutrition early in life stunts subsequent physical growth in both humans and rats, but the mechanism(s) is unknown. To test the hypothesis that temporary early life dietary protein restriction produces long-term alterations in the growth hormone (GH) neuroendocrine axis, we examined the effects of 3 wk of exposure to dietary protein restriction in male rats postweaning (3–6 wk of age) on spontaneous and GH-releasing factor (GRF)-stimulated GH secretion at 12 wk of age. In comparison to rats weaned onto a normal diet (23% protein), rats weaned onto a low (4%) protein diet failed to catch up growth when transferred to the normal diet between 6 and 12 wk of age. Spontaneous 6-h GH secretory profiles of adult rats fed the low protein diet early in life showed a 41% reduction in mean GH peak amplitude and a significant suppression in overall mean 6-h plasma GH concentrations (37.5 ± 4.5 versus 56.9 ± 5.9 μ g/L; p < 0.02). The magnitude of the GH response to 1 μ g of rat GRF(1–29)NH2 i.v. challenge was augmented during the GH trough period in these rats (165.7 ± 30.4 versus 43.9 ± 17.6 μ g/L;p < 0.01). Although basal plasma IGF-I levels and glucose tolerance of protein-deprived rats were normal at 12 wk of age, the insulin response to ip glucose was severely blunted [insulin integrated area under the curve: 303.0 ± 32.7 versus 778.3 ± 105.0 pmol/L/75 min; p < 0.01]. These results demonstrate that temporary protein malnutrition early in life 1) blunts spontaneous pulsatile GH secretion, 2) augments GH responsiveness to GRF challenge, and 3) reduces the insulin secretory response to glucose in adulthood. Our findings suggest that dietary protein in early life is an important determinant for CNS control of GH secretion as well as for the development of pancreatic β-cell sensitivity to glucose. Such alterations in the GH neuroendocrine axis, together with the subnormal insulin secretion, likely contribute to the lack of catch-up growth in this model.

Genesis of the ultradian rhythm of GH secretion: a new model unifying experimental observations in rats

Growth hormone (GH) induces growth in animals and humans and also has important metabolic functions. The GH neuroendocrine axis consists of a signaling cascade from the hypothalamus to the pituitary, the liver, and peripheral tissues, including two major feedback mechanisms. GH is secreted from the pituitary into the circulating blood according to the effect on the somatotrophs of two hypothalamic peptides, GH-releasing hormone (GHRH) and its antagonist, somatostatin (SRIF). The typical GH profile in the male rat shows secretory episodes every 3.3 h, which are subdivided into two peaks. Focusing on the mechanisms for generation of this ultradian GH rhythm, we simulated the time course of GH secretion under a variety of conditions. The model that we propose is based on feedback of GH on its own release mediated both by GH receptors on SRIF neurons in the brain and by a delayed SRIF release into both the brain and portal blood. SRIF, with a resultant periodicity of 3.3 h, affects both the somatotroph cells in the pituitary and the GHRH neurons in the hypothalamus. The secretion of GHRH is postulated to occur in an approximately 1-h rhythm modulated by the level of SRIF in the hypothalamus. The model predicts a possible mechanism for the feminization of the male GH rhythm by sex steroids and vice versa, and suggests experiments that might reveal the proposed intrinsic 1-h GHRH rhythm.Growth hormone (GH) induces growth in animals and humans and also has important metabolic functions. The GH neuroendocrine axis consists of a signaling cascade from the hypothalamus to the pituitary, the liver, and peripheral tissues, including two major feedback mechanisms. GH is secreted from the pituitary into the circulating blood according to the effect on the somatotrophs of two hypothalamic peptides, GH-releasing hormone (GHRH) and its antagonist, somatostatin (SRIF). The typical GH profile in the male rat shows secretory episodes every 3.3 h, which are subdivided into two peaks. Focusing on the mechanisms for generation of this ultradian GH rhythm, we simulated the time course of GH secretion under a variety of conditions. The model that we propose is based on feedback of GH on its own release mediated both by GH receptors on SRIF neurons in the brain and by a delayed SRIF release into both the brain and portal blood. SRIF, with a resultant periodicity of 3.3 h, affects both the somatotroph cells in the pituitary and the GHRH neurons in the hypothalamus. The secretion of GHRH is postulated to occur in an ∼1-h rhythm modulated by the level of SRIF in the hypothalamus. The model predicts a possible mechanism for the feminization of the male GH rhythm by sex steroids and vice versa, and suggests experiments that might reveal the proposed intrinsic 1-h GHRH rhythm.

Somatostatin as a Physiological Regulator of Pulsatile Growth Hormone Secretion

Somatostatin plays an important role in the regulation of the episodic and ultradian rhythm of growth hormone (GH) secretion. Passive immunization of rats with specific antibodies to the 14 and 28 amino acid sequences caused a significant GH elevation. The fact that somatostatin antiserum was unable to block episodic GH surges indicates that this hormones release must be regulated by a dual mechanism. Indeed, GH-releasing factor (GRF) seems to be instrumental in the maintenance of pulsatile GH secretion. Moreover, exogenous GRF induced a further GH increase predominantly during the period of active secretion. Neutralization of endogenous somatostatin eliminated this time-dependent effect, indicating that this peptide blocks periodical spontaneous GH release. Food deprivation and changes in glucose homeostasis virtually obliterate the ultradian GH rhythm. In this context, peripheral somatostatin seems to play an important role. Also the central GRF/somatostatin interplay is responsible for a short-loop feedback control on pituitary somatotrops.

Physiological Role of Somatostatin in Regulation of Pulsatile Growth Hormone Secretion

Regulation of the secretion of anterior pituitary hormones, in general, and growth hormone (GH), in particular, is achieved by the remarkably complex interaction of neural and feedback regulatory components. It is now generally recognized that central nervous system (CNS) control of GH secretion occurs by way of the intricate interplay of at least two hypothalamic hypophysiotrophic hormones — one stimulatory, a GH-releasing factor, GRF, and one inhibitory, a GH-release inhibiting factor, somatostatin. The objective of this chapter is to review our current understanding of the regulatory mechanisms for the control of GH secretion, with particular emphasis on the physiological role of somatostatin in GH regulation.

Centrally Administered Insulin-Like Growth Factor II Fails to Alter Pulsatile Growth Hormone Secretion or Food Intake

Insulin-like growth factor II (IGF-II) peptide, mRNA, and receptors are widely distributed in the central nervous system, yet the physiological role of IGF-II in brain remains largely unknown. In the present study, we examined the in vivo effects of central administration of recombinant human IGF-II on pulsatile GH secretion and food intake. The IGF-II preparation used was shown to stimulate 3H-thymidine incorporation in MG-63 human osteosarcoma cells in vitro. Free-moving adult male rats bearing chronic intracerebroventricular (icv) and intracardiac venous cannulae were icv administered 10 microliters of either IGF-II (in doses of 300 ng and 1 microgram) or the vehicle solution, and blood samples were obtained every 15 min for 6 h. Vehicle-injected control animals exhibited the typical pulsatile pattern of GH secretion with most peak GH values greater than 100 ng/ml and trough levels less than 1.2 ng/ml. Central administration of IGF-II, at both doses, failed to alter the spontaneous 6-hour GH secretory profile; there were no significant differences in either GH peak amplitude, GH trough level, GH interpeak interval, or mean 6-hour plasma GH level, compared to vehicle-injected controls. There was also no effect of icv administered IGF-II on mean plasma glucose or insulin levels. Compared to vehicle-injected control rats, the icv injection of IGF-II (at doses of 300 ng and 1 microgram) did not significantly alter 24-h food intake or body weight gain in normal feeding rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous diabetes mellitus syndrome in the rat. III : pancreatic alterations in aglycosuric and untreated diabetic BB Wistar-derived rats

The pancreatic alterations in aglycosuric and untreated diabetic BB Wistar-derived rats are described. A common finding, often seen in young aglycosuric rats, is that of discrete foci of periductular and/or acinar aggregates of lymphocytes and macrophages. Sites of periductular mononuclear cell infiltrates usually lack endocrine cells. In contrast, foci of acinar infiltrates, although distinct from the predominant endocrine cell mass in the islets of Langerhans, often contain small numbers of alpha and/or beta cells. It is suggested that these clusters of endocrine cells may in some way be antigenically different from those resident in the principal islets and thus serve as an additional target for the immune system in rats bearing the BB genome. The development of overt diabetes requires a massive destruction of beta cells within the islets of Langerhans. Two forms of diabetes mellitus emerge in untreated animals. The more common, designated unstable diabetes, is severe and lethal unless treated with insulin. Less commonly, a stable type of diabetes mellitus ensues for which insulin therapy is not mandatory. In each, the concentration of pancreatic immunoreactive insulin is profoundly decreased, although relatively greater amounts are present in the stable form. Unstable diabetic rats demonstrate a reduction in the concentration of pancreatic immunoreactive glucagon and somatostatin, suggesting that alpha and delta cells also sustain injury in this model of insulin-dependent diabetes mellitus.

Elevated circulating acylated and total ghrelin concentrations along with reduced appetite scores in infants with failure to thrive.

Failure to thrive (FTT) is a term used to describe inadequate growth in infants. The immediate cause is undernutrition. Ghrelin is a potent orexigenic hormone that induces a positive energy balance and enhances appetite. There is no information regarding the possible role of ghrelin in infants with FTT. The aim of this study was 2-fold: 1) to examine circulating ghrelin levels in FTT infants, compared with those of normally growing infants; and 2) to evaluate appetitive behaviors in the two groups. Plasma acylated and total ghrelin concentrations were measured in nine FTT and five normally growing infants (age range, 9–18 mo). Appetite was assessed using three novel appetite measures. Both acylated and total ghrelin levels were significantly elevated in FTT infants compared with controls (p = 0.03 or less). Infants with FTT scored significantly lower than control infants on all appetite measures (p = 0.002 or less). Ghrelin levels were inversely related to appetite, weight velocity, weight/length z-scores, and weight z-score. These findings provide the first evidence that infants with FTT have higher circulating ghrelin concentrations but paradoxically lower appetite scores. Increased ghrelin secretion may reflect an adaptive mechanism attempting to increase appetite and preserve energy balance in response to poor nutritional state.

Immunohistochemical Distribution and Subcellular Localization of the Somatostatin Receptor Subtype 1 (sst1) in the Rat Hypothalamus

The aim of the present study was to examine the cellular and sub-cellular distribution of the somatostatin (SRIF) receptor subtype sst1 in the rat hypothalamus. Receptors were immunolabeled using an antibody directed against an antigenic sequence in the N-terminus of the receptor. Immunopositive neuronal cell bodies and dendrites were observed throughout the mediobasal hypothalamus, including the medial preoptic area, paraventricular, periventricular, and arcuate nuclei. Immunoreactive axons and axon terminals were also observed in the median eminence, suggesting that sst1 is also located pre-synaptically. Electron microscopic examination of the arcuate nucleus revealed a predominant association of immunoreactive sst1 with perikarya and dendrites. Most immunoreactive receptors were intracellular and localized to tubulovesicular compartments and organelles such as the Golgi apparatus, but 14% were associated with the plasma membrane. Of the latter, 47% were apposed to abbuting afferent axon terminals and 20% localized immediately adjacent to an active synaptic zone. These results demonstrate a widespread distribution of sst1 receptors in rat hypothalamus. They also show that somatodendritic sst1 receptors in the arcuate nucleus are ideally poised to mediate SRIF’s modulation of afferent synaptic inputs, including central SRIF effects on growth hormone-releasing hormone neurons documented in this area.

Effect of caffeine on thyroid and pituitary function in newborn rats.

Summary: The possibility that caffeine, a central nervous system stimulant used in neonatal apnea, may produce acute or chronic changes in growth hormone (GH), thyroxine (T4) and thyrotropin (TSH) was studied in the newborn rat. Five-day-old rats were separated into three groups: control (0) group receiving saline, Group I (low dose caffeine) receiving 5 mg/kg and Group II (high dose caffeine) receiving 50 mg/kg. Acute effects were studied at 2, 4, and 24 h after injection. Chronic effects were studied 24 h after the last of 10 daily injections. GH, T4, and TSH were measured by radioim-munoassay and caffeine by high pressure liquid chromatograph. GH was increased at all times and all doses after a single injection of caffeine. After chronic therapy, the increase in GH was small, suggesting depletion of pituitary reserve. A high dose of caffeine had a biphasic effect on T4 with an increase at 4 h and a decrease at 24 h. Thyrotropin-releasing hormone (TRH)-induced TSH release at 24 h was not influenced by caffeine administration. Chronic caffeine therapy stimulated both T4 and TSH; however, TRH-stimulated TSH release was decreased, suggesting that chronic therapy may blunt pituitary TSH response.