Neuropeptide Y in the Ventromedial and Suprachiasmatic Nuclei: Role in the Feeding Pattern of Monosodium Glutamate-Treated Rats

Neuropeptide Y (NPY) is an important mediator for the light/dark cycle entrainment of circadian rhythms including feeding rhythm. It is present in the main circadian oscillator, the suprachiasmatic nucleus which receives NPY efferences from the intergeniculate leaflet to form a pathway named the geniculo-hypothalamic tract (GHT). In the present experiment, we measured NPY in the arcuate nucleus (ARC) and in areas more or less related to feeding rhythm regulation in rats with brain lesion induced by monosodium glutamate (MSG). Newborn Long-Evans rats were subcutaneously injected three times during the first week of life either with MSG (4 mg/g BW; n = 15) or with saline (n = 10). One week after weaning, food intake and NPY concentrations in the GHT, ventromedial (VMN) and dorsomedial nuclei and lateral hypothalamus were measured. Daily food intake in the MSG-treated rats was slightly lower than in control (saline) rats (10.8 ± 0.3 vs 11.9 ± 0.4 g; p < 0.03). This diminution in food intake was only observed during the dark period (p < 0.02). NPY in the ARC rats was 50% lower in MSG-treated rats than in control rats (p < 0.001) confirming therefore the neurotoxicity of the neonatal injections of high doses of MSG. A smaller decrease was observed in the VMN of MSG-treated rats (-30%; p < 0.02) whereas no variations were observed in the GHT and in the other areas. We conclude that when the GHT is intact, gross regulation of feeding rhythm is maintained. The VMN might contribute to the slight change in the dark/light distribution of food intake through the NPY variation.