NPY STIMULATION OF FOOD-INTAKE IN SIBERIAN HAMSTERS IS NOT PHOTOPERIOD DEPENDENT

Siberian hamsters (Phodopus sungorus sungorus) show naturally occurrin g seasonal cycles of food intake that are triggered by changes in the photoperiod. In long ''summer-like'' days (LD) food intake is at its p eak, whereas in short ''winter-like'' days (SD) food intake reaches a nadir. Although the mechanisms underlying these changes in food intake are unknown, results from previous studies suggest that the ability t o stimulate or inhibit food intake in Siberian hamsters complements th e naturally occurring food intake cycle. Thus, inhibitors of food inta ke are more effective in SDs, whereas stimulators of food intake are m ore effective in LDs. A stimulator of food intake in a wide variety of species is neuropeptide Y (NPY). Therefore, we explored the ability o f NPY to stimulate food intake in Siberian hamsters. In addition, we t ested whether the efficacy of NPY to stimulate food intake was photope riod dependent. In Experiment 1, LD-housed adult male hamsters were gi ven a series of NPY doses (0.078-10.0 mu g) intracerebroventricularly (ICV) into the third ventricle and food intake was measured 30 min, 1, 2, and 4 h postinjection. NPY was a potent stimulator of food intake with the 7.5 mu g dose of NPY producing the greatest increase at 30 mi n. In Experiment 2, adult male hamsters were housed in LDs or SDs and were given various doses of NPY ranging from 0.039-7.5 mu g. NPY given ICV stimulated food intake to the same degree in LDs as in SDs with t he greatest increases in food intake occurring in the hamsters receivi ng the 2.5 and 5.0 mu g dose of NPY. In addition, Siberian hamsters we re very sensitive to NPY with the lowest effective dose (0.0585 mu g) that stimulated food intake being six times smaller than in other rode nts tested. Collectively, these results showed that Siberian hamsters were more sensitive to the stimulatory effect of NPY on food intake th an any other species, but that the ability of NPY to stimulate feeding was not photoperiod dependent.