Increased serum growth hormone and somatic growth in adult hamsters with hippocampal transections

Feeding intervention potentiates the effect of mechanical loading to induce new bone formation in mice

The benefits of increased human lifespan depend upon duration of healthy, independent living; the healthspan. Bone-wasting disorders contribute significantly to loss of independence, frailty, and morbidity in older people. Therefore, there is an unmet need globally for lifestyle interventions to reduce the likelihood of bone fractures with age. Although many mechanisms are involved in disorders of bone loss, there is no single regulatory pathway and, therefore, there is no single treatment available to prevent their occurrence. Our aim in these studies was to determine whether fasting/feeding interventions alter the effect of mechanical loading on bone anabolic activities and increase bone mass. In young 17-week-old mice, 16-hour fasting period followed by reintroduction of food for 2 hours increased markedly the potency of mechanical loading, that mimics the effect of exercise, to induce new cortical bone formation. Consistent with this finding, fasting and re-feeding increased the response of bone to a loading stimulus that, alone, does not stimulate new bone formation in ad-lib fed mice. Older mice (20 months) experienced no potentiation of loading-induced bone formation with the same timing of feeding interventions. Interestingly, the pre-, prandial, and postprandial endocrine responses in older mice were different from those in young animals. The hormones that change in response to timing of feeding have osteogenic effects that interact with loading-mediated effects. Our findings indicate associations between timing of food ingestion and bone adaptation to loading. If translated to humans, such non-pharmacological lifestyle interventions may benefit skeletal health of humans throughout life-course and in older age.

Counterregulation of insulin by leptin as key component of autonomic regulation of body weight

A re-examination of the mechanism controlling eating, locomotion, and metabolism prompts formulation of a new explanatory model containing five features: a coordinating joint role of the (1) autonomic nervous system (ANS); (2) the suprachiasmatic (SCN) master clock in counterbalancing parasympathetic digestive and absorptive functions and feeding with sympathetic locomotor and thermogenic energy expenditure within a circadian framework; (3) interaction of the ANS/SCN command with brain substrates of reward encompassing dopaminergic projections to ventral striatum and limbic and cortical forebrain. These drive the nonhomeostatic feeding and locomotor motivated behaviors in interaction with circulating ghrelin and lateral hypothalamic neurons signaling through melanin concentrating hormone and orexin-hypocretin peptides; (4) counterregulation of insulin by leptin of both gastric and adipose tissue origin through: potentiation by leptin of cholecystokinin-mediated satiation, inhibition of insulin secretion, suppression of insulin lipogenesis by leptin lipolysis, and modulation of peripheral tissue and brain sensitivity to insulin action. Thus weight-loss induced hypoleptimia raises insulin sensitivity and promotes its parasympathetic anabolic actions while obesity-induced hyperleptinemia supresses insulin lipogenic action; and (5) inhibition by leptin of bone mineral accrual suggesting that leptin may contribute to the maintenance of stability of skeletal, lean-body, as well as adipose tissue masses.

Neuroendocrine correlates of separation stress in the Siberian dwarf hamster (Phodopus sungorus)

Siberian dwarf hamsters form monogamous male-female pair bonds. Disruption of the pair bond results in increases in body mass and behavioral alterations similar to profiles seen in human atypical depression. We examined behavioral and neuroendocrine correlates associated with separation of the male from his mate. Animals were paired (n = 28 pairs) for 3 weeks, then 15 pairs were separated and 13 pairs remained as controls. Behaviors of the males were observed in a novel environment following 3 weeks of pairing and 4 weeks of separation. The 5-min behavioral test monitored exploratory, territorial behavior, and locomotor activity. Separated males showed a significant increase in body mass (p < 0.01). paralleled by an increase in food consumption (p < 0.01). Separated males had decreased seminal vesicle mass (p < 0.05) and testicular mass (p < 0.05). Behavioral analysis revealed that separated males showed no significant differences in grooming, scent-marking, alert on 2 feet, or escape behavior when compared to paired males. Separated males did show significant increases in inactivity (p < 0.05). Plasma cortisol levels were significantly increased in separated animals (p < 0.05), but there were no significant effects on testosterone. Resting levels of plasma norepinephrine and epinephrine were less in separated males, but this was significant only for norepinephrine (p < 0.05). In conclusion, separation stress was accompanied by increased hypothalamic-pituitary-adrenal axis function and decreased peripheral sympathetic nervous system activity and decreased reproductive profiles.

Activity disc and cage for continuous measurement of running activity and core temperature in hamsters

We describe a design for the modular horizontal activity disc and tandem cages suitable for continuous monitoring of spontaneous running and of core temperatures in golden hamsters. An acrylic disc is equipped with a short brass axle. It is mounted inside a brass rotation sleeve at a 15 degrees angle off the horizontal plane. The disc module fits firmly inside either half of the tandem cage when activity measurements are needed. Easy removal allows for alternative use of cages. Minor modifications of disc dimensions and of disc base permit the use of activity modules with juvenile hamsters. The short distance between disc surface and cage floor permits continuous measurement of core temperature as well as running activity.

Photoperiodic responsiveness of hamsters with lesions of the lateral geniculate nucleus is related to hippocampal damage

We tested the hypothesis that the geniculohypothalamic tract is important for hamster photoperiodism. Adult male hamsters, maintained in a long photoperiod (LD 14:10), received either large bilateral neurotoxic lesions of the lateral geniculate nucleus (LGN) or sham lesions. One week later, half of the animals from each group were transferred to a short photoperiod (LD 8:16) where they were maintained for 15 weeks. Most lesions effectively destroyed the intergeniculate leaflet (IGL) and much of the lateral geniculate complex. They also caused substantial damage to the overlying hippocampus. The lesions had no effect on long-day animals, but significantly reduced the extent of testicular regression during short photoperiod exposure. This effect, however, appeared to be the result of hippocampal, rather than geniculate, damage. Four individuals with complete IGL lesions, but minimal hippocampal damage, underwent a pattern of regression that was indistinguishable from controls. Body weight was increased by lesions in short-day, but not long-day, animals. This effect was not related to the extent of hippocampal damage. We conclude that geniculate input to the suprachiasmatic nuclei is not essential for hamster photoperiodism and that hippocampal damage may modify the effect of short daylengths on testes size.

Lateral geniculate lesions alter circadian activity rhythms in the hamster

The suprachiasmatic nucleus (SCN) receives photic input via a direct retinohypothalamic tract (RHT) and an indirect geniculohypothalamic tract (GHT). The neurons giving rise to the GHT are in the intergeniculate leaflet (IGL) of the lateral geniculate nucleus (LGN) and contain neuropeptide-Y (NPY) immunoreactivity. The present study used the neurotoxin, N-methyl aspartate (NMA), to examine the effects of lesions of the LGN on circadian wheelrunning in the hamster. The results are compared to those from control lesioned animals and animals with parasigittal cuts through the hypothalamus. The effectiveness of the lesions was examined with NPY immunohistochemistry of the SCN and IGL. NMA injections destroyed the neurons of the IGL and the adjacent ventral and dorsal divisions of the LGN and greatly reduced NPY immunoreactivity in the SCN. The results of the rhythm studies were: 1) NMA injection into the LGN area produced phase advances if the injection occurred within the 12 hr preceding activity onset and delays or no effect if injected during the 12 hr after activity onset; 2) the NMA lesions reduced the rate of reentrainment to 6 hr shifts in the LD 14:10 photoperiod and advanced the entrained phase angles by about 10 min; 3) the knife cuts advanced the entrained phase angles by about 30 min; 4) neither NMA lesions nor knife cuts altered circadian period in constant dim light. Our results indicate that the GHT is not required for entrainment or normal expression of circadian rhythmicity, but that the GHT does exert an influence on entrainment.

Hippocampal serotonin mediates hypoactivity in dietarily obese hamsters: a possible manifestation of aging?

To determine whether endogenous opiates mediate hyperactivity in food restricted hamsters and serotonergic fibers innervating the hippocampus mediate hypoactivity in obese hamsters, food restriction and high-fat diet supplementation were used to produce significant body fat changes (8 vs. 21%). The levels and pattern of spontaneous running were examined after IP saline or naloxone HCl (20 mg/kg) and following the infusion of vehicle and 5,7-dihydroxytryptamine creatine sulfate (4 micrograms/2 microliters) into rostromedial septum of mature female hamsters. Septum-medial preoptic area (POA), hippocampus, hypothalamus, and cortex were dissected from the three groups as well as from two additional groups of hamsters receiving vehicle or neurotoxin. Concentrations of serotonin, norepinephrine, and dopamine were measured in these tissues by HPLC method. Fat-fed hamsters were hypoactive relative to food-restricted hamsters. Naloxone had no significant effect on running behavior. Serotonin neurotoxin increased the running activity of fat-fed hamsters to the level displayed by control hamsters by increasing the number of runs, the total activity level, the speed of running and by decreasing the duration of pauses. Neurotoxin led to selective deletion of serotonin in the hippocampus (77%) and parietal cortex (50%). Serotonergic fibers innervating the hippocampus thus appear to mediate the hypoactivity that is induced by dietary obesity in mature hamsters. Since serotonin mediates some other manifestations of aging, and slow weight increases characterize mid-portion of hamster life span, we hypothesize that serotonergic mediation of hypoactivity is another manifestation of aging.

Development and retention of phenotypically specialized cells in pituitary allografts in the hamster (Mesocricetus auratus)

We used immunohistochemistry to identify cells present in pituitary allografts in the hamster. Hypophyses removed from neonatal hamsters or adenohypophyses removed from adult females were placed beneath renal capsules of hypophysectomized adult females. Serum PRL, LH, and GH concentrations were measured at two, five and eight weeks after placement of allografts. Allografts were removed after eight weeks and stained for cells containing PRL, LH, FSH, GH, or ACTH. Allografts did not release LH or GH. Those of adult adenohypophyseal tissue released significantly more PRL. The morphology of allografts of neonatal hypophyseal tissue resembled that of the adult adenohypophysis in situ. Lactotrophs, corticotrophs, somatotrophs and LH-cells were observed; very few FSH-cells were present. Allografts of adult adenohypophyseal tissue contained pituitary cells, numerous cavities, often enclosing lymphoid cells, and fibrous tissue. Atypical lactotrophs were the numerically dominant cells in these allografts; all other cells were present. The LH-cells outnumbered FSH-cells. These observations suggest that: (a) development of normal adenohypophyseal morphology can occur in an ectopic position; (b) intracellular hormones are present in cells in an ectopic site; (c) development and retention of intracellular FSH is more dependent on occupation of the normal position of the adenohypophysis than is retention of intracellular LH; and (d) release of PRL occurs from atypical cells in allografts of adult adenohypophyseal tissue.

Rostromedial septal area controls pulsatile growth hormone release in the golden hamster

Limbic forebrain inhibits growth and growth hormone (GH) secretion in mature golden hamsters as shown by acceleration of growth and increases in serum GH concentrations following the electrolytic lesions of septum, transection of the hippocampus and surgical separation of these two regions. The growth-inhibitory function of this circuit is most probably mediated by somatostatinergic (SRIF) neurons. Such lesions induce hypoactivity possibly due to damage to endogenous opiatergic (EOP) neurons. EOP neurons facilitate spontaneous running in hamsters and mediate exercise-induced acceleration of growth and GH pulses. The coincidence of hypoactivity and growth acceleration after such lesions suggested the coexistence of SRIF and EOP fibers within the growth-inhibitory limbic forebrain circuit which control the rate of growth in mature hamsters by the variable inhibition of SRIF neurons by the EOP neurons. This hypothesis posits that accelerated growth is due to increased GH pulse frequency, and hypoactivity due to damage to EOP neurons, and was tested in this study by measuring pulsatile GH release (and as a measure of specificity, pulsatile prolactin release) in the presence and in the absence of opiate-receptor blocker naloxone in 21 female hamsters which sustained electrocoagulative lesions of rostromedial septum and 30 hamsters subjected to control surgery. Lesions doubled GH but not PRL pulse frequency, neither of which was affected by naloxone. Results support the hypothesis that opiatergic neurons facilitate pulsatile GH release by inhibiting the action of somatostatin neurons.

Hypothalamic control of GH secretion: pathophysiology and clinical implications

GH is secreted episodically. Its pattern is regulated by the interplay of a releasing and a release-inhibiting hormone of hypothalamic origin. Modulation occurs by metabolic factors (glucose, free fatty acids, ketone bodies, amino acids). Altered GH secretion has been observed in states of metabolic derangement such as diabetes mellitus, malnutrition and obesity. Further modulation occurs by extrahypothalamic CNS structures. In man--but not in animals, including subhuman primates--sleep has an important effect on GH secretion. A defective GH secretory pattern has been found to occur in several states of sleep disturbance, such as sleep deprivation, narcolepsy, severe psychosocial derangement, the apallic syndrome. Other CNS influences on GH secretion are related to stress, emotional changes and psychiatric disturbances. The exact mechanisms by which most of these influences are relayed to the GH secretory apparatus of the hypothalamus remain yet to be investigated.

A role for somatostatin in the control of hamster growth

Concentrations of somatostatin-like immunoreactivity (SRIF-LI) were measured in cerebral cortex, hippocampus, septum-POA, median eminence, gastric antrum, fundus and pancreas in adult female hamsters to determine whether changes in somatostatin could be related to increased growth hormone (GH) secretion and somatic growth that follow bilateral transections of hippocampus (n = 18; 17 controls). In addition, choline acetyltransferase (CAT) activity was measured in the four brain regions in hippocampectomized (n = 10) and control hamsters (n = 10) to gain insight into the relationship between these two neurotransmitters. Hippocampal transections induced: significant acceleration of somatic growth; increased serum GH concentrations; increased concentrations of SRIF-LI in septum-POA and gastric antrum; reduced concentrations of SRIF-LI in hippocampus and pancreas; and reduced CAT activity in the hippocampus. These results suggest that somatostatinergic and cholinergic projections to hippocampus via fornix suppress GH and somatic growth in adult hamsters and that reduced release of SRIF-LI in the gastric antrum may contribute to the acceleration of somatic growth through facilitated nutrient digestion and entry.

Basis for the hypoactivity that accompanies rapid weight gain in hamsters

The pattern of hypoactivity that accompanies rapid weight gain following septal lesions in hamsters was characterized. Lesioned hamsters displayed reduced levels of running, shorter and slower running bouts, and longer pauses. We examined whether this hypoactivity was due to reassignment of metabolic fuels from supporting physical activity to anabolism, or due to reduced capacity of running to induce psychomotor arousal and mobilize metabolic fuels. Septal lesions were associated with increased rate of ponderal growth and higher titers of circulating growth hormone and insulin. No difference in concentrations of muscle and liver glycogen, percentage of body fat, or the capacity of muscle homogenates to oxidize substrates were identified. Lesioned hamsters ran as fast and as long as control animals on electrical-shock reinforced treadmill, but were unable to generate as much heat in response to injection (0.8 mg/kg) of norepinephrine. We concluded that hypoactivity that accompanies rapid weight gain in hamsters results either from a reduced capacity of running to induce psychomotor arousal and provide incentives that normally motivate that behavior, or from a failure of running to mobilize metabolic fuels at a rate necessary to sustain normal running speed and duration, and not from reduced availability of metabolic fuels or reduced muscle capacity to oxidize metabolic substrates.

Role of limbic system in the control of hamster growth

Rostral septal lesions accelerate somatic growth in adult hamsters. This study tested the hypothesis that this effect results from damage to fibers of passage by observing the effects of transections of septohippocampal and septohypothalamic connections on growth. We attempted to identify these fibers further by (a) measuring spectrofluorometrically changes in the monoamine concentrations in hippocampus, cerebral cortex, corpus striatum, and diencephalon, (b) staining the degenerating axons after septal lesions and the two cuts, and (c) examining the correspondence between such damage and the acceleration of growth. Both knife cuts accelerated somatic growth and were associated (as well as septal lesions) with significant depletions of serotonin (-27 to -57%) and norepinephrine (-27 to -60%) in the hippocampus, with less consistent depletions of these monoamines in the cerebral cortex, and with no changes in regional dopamine content. All three procedures were associated with degeneration in the hippocampal formation and its fiber systems. Thus, fibers interconnecting hippocampus and brainstem, and passing through septum, exert tonic suppression over somatic growth in adult hamsters.