Feedback in the rabbit's central circadian system, revealed by the changes in its free-running food intake pattern induced by blinding, cervical sympathectomy, pinealectomy, and melatonin administration.

Long-term records of the number of food approaches per 30 min were obtained from 16 rabbits in constant light and after blinding. Optic nerve sectioning usually resulted in a shortening of the free-running period (tau; mean reduction = -0.26 hr) of the oscillator governing the rabbit's food intake pattern. Subsequent resection of both superior cervical ganglia always reduced this tau (mean = -0.41 hr), and it could be further shortened (mean = -0.24 hr) by chronic administration of melatonin. These observations led to experiments in which the food intake of 15 blinded rabbits was recorded before and after pinealectomy. This always resulted in a lengthening of tau (mean = +0.27 hr), which was never seen after subtotal or sham pinealectomy in 10 other blinded animals (mean = -0.05 hr). Sympathectomy also reduced tau (mean = -0.50 hr) in the pinealectomized rabbits, whereas it induced a smaller reduction (mean = -0.29 hr) after sham pinealectomy. These results, together with previous observations on the effect of sympathectomy on the retinal "dark" discharge and on the influence of the latter upon the food intake pattern, indicate that the cervical sympathetic nerves may form part of three feedback loops for the rabbit's central circadian system. These have separate postganglionic pathways; there are also differences in the circadian phasing of their activities and in their effects on tau.