SEASONAL NEUROENDOCRINE RHYTHMS IN THE MALE SIBERIAN HAMSTER PERSIST AFTER MONOSODIUM GLUTAMATE-INDUCED LESIONS OF THE ARCUATE NUCLEUS IN THE NEONATAL-PERIOD

The aim of these experiments was to examine the role of the arcuate nu cleus in the control of seasonal cycles of body weight, feed intake, m oulting and reproduction in the Siberian hamster. The arcuate nucleus has previously been implicated as a central site where systemic feedba ck signals (e.g. leptin) might act to regulate reed intake and body we ight, so it was predicted that hamsters with lesions of this structure would be unable to display the inhibitory effects of short days on th ese parameters. In the first series of studies, lesions that destroyed approximately 80% of the cells in the arcuate nucleus were produced b y treating hamsters neonatally with monosodium glutamate (MSG; 4 mg/g body weight sc), and vehicle- and MSG-treated males were raised from b irth in long days (LD) or short days (SD). In hamsters raised in LD, t he initial gain in body weight and testicular growth were significantl y reduced by MSG treatment, however, growth rate and testis weight wer e still significantly greater than in vehicle- or MSG-treated hamsters raised in SD. In the second study, hamsters treated neonatally with v ehicle or MSG were raised in LD for 8 weeks and, subsequently, approxi mately half in each group were transferred to SD for 18 weeks. As expe cted, vehicle-treated hamsters showed a characteristic decline in body weight when exposed to SD, while those remaining in to continued to i ncrease body weight. Feed intake decreased in parallel with the declin e in body weight in SD, a complete moult to the white winter pelage oc curred by 16 weeks in SD, and testicular regression occurred. Response s to SD also occurred in the MSG-treated hamsters: body weight decreas ed in SD but increased in their lesioned litter mates remaining in LD, and feed intake paralleled body weight changes in these groups. The m oult to winter pelage was significantly retarded in MSG-treated hamste rs transferred to SD. The testes were completely regressed in sham- an d MSG-treated hamsters exposed to SD, whereas testes weights in MSG-tr eated hamsters maintained in LD were intermediate between those in veh icle-treated hamsters in SD and LD, Thus, despite initial effects on g rowth, the MSG-treated hamsters bearing substantial lesions of the arc uate nucleus were able to show appropriate responses to photoperiod, a lthough not always of the same magnitude as the unlesioned controls. W e conclude that feedback mechanisms operating via the arcuate nucleus are not the major regulators of seasonal cycles of body weight, feed i ntake, pelage and reproduction.