Hamster growth hormone. Species specificity and physiological changes in blood and pituitary concentrations as measured by a homologous radioimmunoassay

Enhancement and suppression of ultradian and circadian rhythms across the female hamster reproductive cycle

The impact of ovarian hormones on hamster ultradian rhythms (URs) is unknown. We concurrently monitored URs and circadian rhythms (CRs) of home cage locomotor activity during the estrous cycle, pregnancy, and lactation of Syrian hamsters. URs with a mean period of 4-5 h were evident during the dark phase in more than 90% of females on days 1 and 2 of the estrous cycle but were significantly less prevalent on cycle days 3 and 4. The period of the UR did not vary as a function of estrous cycle stage, but at all stages, the UR period was longer in the dark than the light phase. The UR acrophase occurred significantly earlier on cycle day 4 than on days 1 and 2, and UR robustness and amplitude were reduced on days 3 and 4. Robustness, mesor, and amplitude of CRs were greater during cycle days 3 and 4; timing of the CR acrophase was delayed on day 4 relative to all other cycle days. Effects of the estrous cycle on URs were evident only during the dark phase. The proportion of hamsters displaying dark phase URs increased significantly during early and late gestation and decreased during lactation. Pregnancy significantly increased UR complexity, robustness, and amplitude. The emergence of URs over gestation was paralleled by decrements in the robustness and amplitude of CRs, which also were absent in a significant proportion of dams during lactation but re-emerged at weaning of litters. The changing endocrine profile of the estrous cycle, hormonal dynamics of pregnancy and lactation, and nursing demands placed on dams are each associated with alterations in the expression of ultradian and circadian locomotor rhythms. Diminution of CRs and augmentation of URs may afford greater behavioral flexibility during life stages when interactions with mates and offspring are less predictable.

Effects of photoperiod and melatonin on the development of growth hormone cells and the pituitary-adrenal axis in the Djungarian hamster, Phodopus sungorus

The development of GH cells and the pituitary-adrenal axis was morphologically examined in male Djungarian hamsters (Phodopus sungorus) exposed to short days and those kept under long days and receiving daily afternoon injections of melatonin, from the time of weaning (20 days) until 100 days of age. The postnatal increase in area of ACTH cells under long days was inhibited in short-day-exposed or melatonin-treated animals. It was suggested that a short photoperiod may suppress, via melatonin, the development of ACTH cells. GH cells were not affected by age, photoperiod or exogenous melatonin. Under long days, the zona fasciculata decreased in volume with age, while the zona reticularis increased. Such changes in the volumes of these adrenocortical zones were depressed under short days. In addition, the volumes of the zona fasciculata and zona reticularis in long-day-housed animals became respectively larger and smaller subsequent to orchidectomy and melatonin administration. These results suggest that fasciculata cells in deeper levels become progressively differentiated into reticularis cells, that short photoperiod inhibits development of both zonae, and that such an inhibition is caused mainly by the decreased secretion of androgens.

Diurnal, photoperiodic, and age-related changes in plasma growth hormone levels in the golden hamster

The golden hamster has been used extensively as an animal model for the study of both circadian and seasonal rhythms, and their regulation by the light-dark (LD) cycle. More recently, this species has been used to examine how the generation and entrainment of circadian rhythms are altered in advanced age. Recent studies in both humans and rodents indicate that age-related changes in the diurnal rhythm of pituitary growth hormone (GH) release may mediate some of the adverse effects of aging on a variety of physiological systems. As a first step in determining whether or not age-related changes in circulating GH levels are associated with changes in the regulation and/or expression of circadian rhythms, the effects of age on both the ultradian and diurnal patterns of plasma GH levels were determined in 3- to 22-month-old male hamsters that were bled every 15 min for a 24-hr period while entrained to an LD 14:10 light cycle. An additional study involving a similar blood collection protocol examined whether or not the length of the day is involved in the regulation of plasma GH levels. Although the frequency of pulsatile GH release did not change with advanced age, both the mean levels of GH per sample and the mean amplitude per pulse of GH were significantly elevated in 3- to 4-month-old animals, compared to animals that were 12-13, 15-16, or 21-22 months of age. In hamsters aged 3-4 and 12-13 months, there was an increase in both mean levels and the mean amplitude per pulse of GH, but not pulse frequency, during the night as compared to daytime values. No such diurnal rhythm was detected in the two groups of older animals. A clear diurnal rhythm in GH levels was also detected in animals maintained in a short-day (LD 6:18) cycle, and the mean levels of GH per sample were greater in hamsters maintained on short compared to long (LD 14:10) days. These results indicate that there are pronounced age-related changes in pituitary GH release in the hamster, and that both the time of day and the length of the day influence the pattern of GH release.

Molecular cloning of cDNA encoding the precursor for hamster hypothalamic growth hormone-releasing factor

The structure of rat and mouse growth hormone-releasing factor (GRF) peptide and precursor shows considerable divergence from that of the human counterpart and also within rodents themselves. To study such structural divergence in another rodent, we cloned a cDNA encoding the GRF precursor from golden hamster. The hamster GRF (haGRF) cDNA clone had an open-reading frame that predicts a haGRF precursor protein with 107 amino acids. The haGRF precursor bore greater overall homology (82%) to the human than the same rodent homologue (58-64%) and contained two processing sites identical to the human sequence that would generate mature haGRF peptide. Furthermore, the haGRF peptide, like human but unlike rat or mouse GRF, consisted of 44 amino acids and also had greater homology to the human (89%) than the rodent sequence (64-75%), conserving a Tyr residue at the N-terminus and an amidated Leu residue at the C-terminus. Thus, both haGRF precursor and peptide are structurally more related to those of human than of other rodents, suggesting that rodent GRF precursor diverged from the human sequence at differential rates within the species.

Gender-associated differences in the development of 5-aminolevulinate synthase gene expression in the harderian gland of Syrian hamsters

The mRNA levels for aminolevulinate synthase (ALV-S), the rate-limiting enzyme in porphyrin synthesis, were studied in male and female Syrian hamsters during postnatal development. Sex-associated differences in the expression of ALV-S gene were evident at the end of the third week of postnatal development. Serum levels of luteinizing hormone (LH), testosterone, cortisol, thyroid hormones and insulin-like growth factor were also studied in order to correlate their concentrations with the mRNA levels for ALV-S. Among these hormones, serum LH levels showed a positive correlation with the ALV-S mRNA levels. However, the expected negative correlation with testosterone levels was not clearly observed. Thus, in order to test the effects of testosterone on ALV-S gene expression, 11-day-old male and female Syrian hamsters and adult female hamsters were injected with 50 micrograms of testosterone for 4 days. Testosterone administration decreased the levels of ALV-S mRNA in the adult females but did not influence those of young females. The possible explanation for the insensitivity to testosterone during these postnatal stages might involve the maturational state of androgen receptors in the Harderian glands.

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.

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.