Seasonal Neuroendocrine Rhythms in the Male Siberian Hamster Persist After Monosodium Glutamate‐Induced Lesions of the Arcuate Nucleus in the Neonatal Period

Abstract

The aim of these experiments was to examine the role of the arcuate nucleus in the control of seasonal cycles of body weight, feed intake, moulting and reproduction in the Siberian hamster. The arcuate nucleus has previously been implicated as a central site where systemic feedback signals (e.g. leptin) might act to regulate feed intake and body weight, so it was predicted that hamsters with lesions of this structure would be unable to display the inhibitory effects of short days on these parameters. In the first series of studies, lesions that destroyed approximately 80% of the cells in the arcuate nucleus were produced by treating hamsters neonatally with monosodium glutamate (MSG; 4 mg/g body weight sc), and vehicle- and MSG-treated males were raised from birth in long days (LD) or short days (SD). In hamsters raised in LD, the initial gain in body weight and testicular growth were significantly reduced by MSG treatment, however, growth rate and testis weight were still significantly greater than in vehicle- or MSG-treated hamsters raised in SD. In the second study, hamsters treated neonatally with vehicle or MSG were raised in LD for 8 weeks and, subsequently, approximately half in each group were transferred to SD for 18 weeks. As expected, vehicle-treated hamsters showed a characteristic decline in body weight when exposed to SD, while those remaining in LD continued to increase body weight. Feed intake decreased in parallel with the decline in body weight in SD, a complete moult to the white winter pelage occurred by 16 weeks in SD, and testicular regression occurred. Responses to SD also occurred in the MSG-treated hamsters: body weight decreased in SD but increased in their lesioned litter mates remaining in LD, and feed intake paralleled body weight changes in these groups. The moult to winter pelage was significantly retarded in MSG-treated hamsters transferred to SD. The testes were completely regressed in sham- and MSG-treated hamsters exposed to SD, whereas testes weights in MSG-treated hamsters maintained in LD were intermediate between those in vehicle-treated hamsters in SD and LD. Thus, despite initial effects on growth, the MSG-treated hamsters bearing substantial lesions of the arcuate nucleus were able to show appropriate responses to photoperiod, although not always of the same magnitude as the unlesioned controls. We conclude that feedback mechanisms operating via the arcuate nucleus are not the major regulators of seasonal cycles of body weight, feed intake, pelage and reproduction.

Effects of monosodium-L-glutamate administration on serum levels of reproductive hormones and cholesterol, epididymal sperm reserves and testicular histomorphology of male albino rats

“…This finding is contrary to the reports of both testicular and ovarian hypogonadism in male and female mice following a subcutaneous administration of MSG to neonatal mice at the dosage between 2.2 and 4.2 g per kg body weight (Pizzi et al, 1977). Also, Ebling et al (1998), who made the same observation of hypogonadism of testes in hamsters, used a high dose of 4 g/kg body weight of MSG in their study in neonatal hamsters. Both contrary reports made use of higher doses of MSG and neonatal animals.…”

Monosodium glutamate-induced arcuate nucleus damage affects both natural torpor and 2DG-induced torpor-like hypothermia in Siberian hamsters

“…ARC MSG lesions have been successfully produced in Siberian hamsters, demonstrating that SP-induced changes in hamster body mass, food intake, fur color, and testis mass persist after ARC ablation (16). If MSG ablations in Siberian hamsters mimic the effect on circadian T b min in suckling rat pups (45), then ARC MSG ablations should eliminate photoperiod-dependent torpor in hamsters.…”

Decrease of food intake by MC4-R agonist MTII in Siberian hamsters in long and short photoperiods

“…This plasticity is particularly clear in mammals that have evolved under seasonal conditions and consequently display profound annual cycles of weight gain and loss (25). For example, under increasing or long photoperiods the Siberian hamster (Phodopus sungorus) defends its maximum body weight, whereas under decreasing or short photoperiods it enters a catabolic state and gradually loses body weight (10,22,31), mainly through a reduction in intraperitoneal fat stores (4). The reduction in body weight takes approximately 12-15 wk to reach the maximum weight loss of up to 40% of initial weight (24,29,31).…”