Metabolic and feeding patterns: role of sympathetic and parasympathetic efferent pathways

The continuous transfer of fuel from the blood to tissues, and its adjustment to the rate of energy metabolism involve close relationships between the control of endogenous stores and that of the periodic intake of food. Neural and blood-borne signals to the brain and efferent sympathetic and parasympathetic pathways seem responsible for three different metabolic and feeding cycles. The matching of input and output of glucose to and from the blood, i.e. the regulation of the blood glucose level proper, is achieved by a short-term feedback mechanism which involves the action of insulin and glucagon on hepatic glucose production and peripheral glucose utilization. Oscillations in level of blood glucose, plasma insulin and glucagon in 8-11 min cycles, as observed in certain species, presumably reflect the play of this short-term regulation of the blood glucose level. The respective role of a pancreatic feedback and of a neural loop in this short-term regulation is discussed. Signals to brain targets and efferent pathways determine the periodic onset of meals after the energy ingested in the preceding meal has been exhausted in free-fed and briefly deprived rats. New evidence has been provided that the fall in blood glucose level exerts a decisive and direct action on the brain to determine meal onset and/or meal size. Superimposed on this prandial periodicity, a lipostatic mechanism modulates the feeding pattern in a dark-light periodicity by the alternation of fat synthesis and fat mobilization. The effects of VMH lesions and vagotomy, either combined or separately, on daily metabolic and feeding patterns suggest that the neural input to the pancreas and adipose tissues plays a primary role in this endogenous cycle and is also involved in the regulation of a constant fat body mass.