Effects of growth hormone-releasing factor in the brain

Neuroendocrine and physiological regulation of intake with particular reference to domesticated ruminant animals

The central nervous system undertakes the homeostatic role of sensing nutrient intake and body reserves, integrating the information, and regulating energy intake and/or energy expenditure. Few tasks regulated by the brain hold greater survival value, particularly important in farmed ruminant species, where the demands of pregnancy, lactation and/or growth are not easily met by often bulky plant-based and sometimes nutrient-sparse diets. Information regarding metabolic state can be transmitted to the appetite control centres of the brain by a diverse array of signals, such as stimulation of the vagus nerve, or metabolic 'feedback' factors derived from the pituitary gland, adipose tissue, stomach/abomasum, intestine, pancreas and/or muscle. These signals act directly on the neurons located in the arcuate nucleus of the medio-basal hypothalamus, a key integration, and hunger (orexigenic) and satiety (anorexigenic) control centre of the brain. Interest in human obesity and associated disorders has fuelled considerable research effort in this area, resulting in increased understanding of chronic and acute factors influencing feed intake. In recent years, research has demonstrated that these results have relevance to animal production, with genetic selection for production found to affect orexigenic hormones, feeding found to reduce the concentration of acute controllers of orexigenic signals, and exogenous administration of orexigenic hormones (i.e. growth hormone or ghrelin) reportedly increasing DM intake in ruminant animals as well as single-stomached species. The current state of knowledge on factors influencing the hypothalamic orexigenic and anorexigenic control centres is reviewed, particularly as it relates to domesticated ruminant animals, and potential avenues for future research are identified.

Effects of chronic ethanol exposure on sleep in rats

Sleep disturbance is a common complaint in alcoholics. When polysomnographic studies are performed in alcoholics, reductions in slow wave sleep are a common finding; however, few studies have evaluated the effects of chronic alcohol exposure on sleep in animal models. In the present study, the sleep EEG was evaluated in 40 Wistar rats who were exposed to chronic alcohol or control conditions in vapor chambers. Rats were exposed to ethanol vapors or control chambers for 6 weeks and then withdrawn. Sleep EEG was recorded before exposure (baseline), immediately following exposure, and 5 weeks after withdrawal from the ethanol/control chambers. In the ethanol-exposed animals, blood ethanol levels averaged 192 mg/dL over 6 weeks of exposure. Chronic ethanol exposure and withdrawal was not found to affect either slow wave sleep latency or slow wave sleep duration; however, overall spectral power as well as power in the delta, theta, and beta frequencies were significantly reduced following chronic exposure (2-4 Hz, [F(1, 17) = 18.11, p = 0.001], 4-6 Hz, [F(1, 17) = 15.98, p = 0.001], 6-8 Hz [F(1, 17) = 15.52, p = 0.001], 8-16 Hz band [F(1, 17) = 18.73, p < 0.0001], 16-32 Hz [F(1, 17) = 10.13, p = 0.005], and 1-50 Hz [F(1, 17) = 17.03, p = 0.001]. After 5 weeks of withdrawal, significant decreases still persisted in the delta and theta frequencies (2-4 Hz [F(1, 16) = 6.21, 0.024], 4-6 Hz [F(1, 16) = 6.26, 0.024], and 6-8 Hz [F(1, 16) = 4.84, p = 0.043]). These findings suggest that spectral analysis of the EEG is a highly sensitive measure of the effects of ethanol on sleep. These findings additionally demonstrate that chronic ethanol exposure can produce persistent diminution in the systems that generate cortical slow waves in the rat and thus may provide a model for understanding the mechanisms underlying sleep disturbances associated with alcoholism.

Intrapreoptic microinjection of GHRH or its antagonist alters sleep in rats

Previous reports indicate that growth hormone-releasing hormone (GHRH) is involved in sleep regulation. The site of action mediating the nonrapid eye movement sleep (NREMS)-promoting effects of GHRH is not known, but it is independent from the pituitary. GHRH (0.001, 0. 01, and 0.1 nmol/kg) or a competitive antagonist of GHRH (0.003, 0.3, and 14 nmol/kg) was microinjected into the preoptic area, and the sleep-wake activity was recorded for 23 hr after injection in rats. GHRH elicited dose-dependent increases in the duration and in the intensity of NREMS compared with that in control records after intrapreoptic injection of physiological saline. The antagonist decreased the duration and intensity of NREMS and prolonged sleep latency. Consistent alterations in rapid eye movement sleep (REMS) and in brain temperature were not found. The GHRH antagonist also attenuated the enhancements in NREMS elicited by 3 hr of sleep deprivation. Histological verification of the injection sites showed that the majority of the effective injections were in the preoptic area and the diagonal band of Broca. The results indicate that the preoptic area mediates the sleep-promoting activity of GHRH.

Growth hormone-regulatory peptides (GHRH and somatostatin) and feeding: a model for the integration of central and peripheral function

The present paper provides an overview of findings that implicate growth hormone-releasing hormone (GHRH) and somatostatin (SS), the two peptides that regulate growth hormone secretion, in the central regulation of feeding. Evidence is presented that GHRH and SS increase food intake, in the rat, via a common centrally mediated mechanism involving the suprachiasmatic nucleus. Food intake is increased by increasing motivation to eat as evidenced by facilitation of operant behavior. Macronutrient-choice studies indicate that GHRH (and possibly SS) selectively facilitate protein consumption. Time of day is also important, with evidence that endogenous GHRH and SS-induced feeding is most strong in the early nocturnal period. GHRH and SS, together with other nutrient-specific signals, such as neuropeptide Y, noradrenaline and galanin, may determine the circadian expression of food intake in animals. Other behavioral and physiological effects of these peptides, both central and peripheral, are reviewed in the context of a possible mechanism by which these peptides integrate diverse, but complimentary, central and peripheral functions related to nutrition, metabolism and growth.

GRF-induced feeding: evidence for protein selectivity and opiate involvement

Growth hormone-releasing factor (GRF) is a hypothalamic peptide named for its ability to induce release of growth hormone from the anterior pituitary. GRF also acts as a neurotransmitter in the suprachiasmatic nucleus/medial preoptic area (SCN/MPOA) to stimulate food intake. The purpose of this series of experiments was to explore the nature of GRF-induced feeding, with a particular emphasis on macronutrient selectivity, and to examine the role of opiate activity in the paraventricular nucleus of the hypothalamus (PVN). Chow intake stimulated by GRF microinjection (1 pmol/0.5 microliters) into the SCN/MPOA was blocked by injection of methyl-naltrexone (3 micrograms/0.5 microliters) into the PVN. In animals habituated to macronutrient diets (Teklad, WI), GRF preferentially stimulated intake of protein at 2 and 4 h postinjection, whereas it had no effect on carbohydrate intake. Further, this effect was blocked by injection of naloxone (40 nmol/0.5 microliters) into the PVN. Microinjection of morphine (0, 1, 10, and 17 micrograms/0.5 microliter) into the PVN also specifically stimulated protein intake at 2 and 4 h postinjection. These results suggest that feeding derived from GRF actions in the SCN/MPOA is macronutrient selective, and is dependent on PVN opiate activity for expression.