Neural systems for early independent ingestion: regional metabolic changes during ingestive responding and dehydration.

The ingestive behavior of young rodents, studied independently of the mother and suckling, provides a system for the developmental analysis of the neurobiology of feeding and drinking. In these experiments regional neural metabolic activity, as assessed by semiquantitative deoxyglucose (DG) autoradiography, was related to dehydration and ingestive behavior in 6-day-old rat pups. During simple ingestive responding, changes in relative DG uptake, representative of changes in neural metabolic activity, occurred primarily in hindbrain sensory and motor nuclei. Producing cellular dehydration resulted in activity changes primarily in the basal forebrain. When pups were dehydrated and allowed to ingest during the DG-uptake period, activity changes were seen in both the hindbrain and forebrain areas that responded to ingestion or dehydration alone as well as in regions that were not affected by either manipulation alone. These latter changes, which result from an interaction of behavioral and physiological stimuli, call attention to areas that may subserve motivational aspects of behavior. Extracellular dehydration was found to produce fewer and different forebrain responses in neural metabolic activity. During ingestion, the only effects of extracellular dehydration that overlapped with those of cellular dehydration appeared in circumventricular hypothalamic regions and brain stem motor nuclei. Thus, there appeared to be only a limited final common pathway for these two types of dehydration-induced drinking. Taken together, these findings in infant rats depict distributed neural systems subserving ingestion and responding to state change. They provide a starting point early in ontogeny for the developmental analysis of neural substrates of ingestive systems.