Satiety does not affect gustatory activity in the nucleus of the solitary tract of the alert monkey

Feeding to satiety decreases the acceptability of the taste of food. In order to determine whether the responsiveness of gustatory neurons in the nucleus tractus solitarius (NTS) is influenced by hunger, neural activity in the NTS was analyzed while monkeys were fed to satiety. Gustatory neural activity to glucose, fruit juice, NaCl, HCl and quinine HCl was measured before, while and after the monkey was fed to satiety with glucose, fruit juice or sucrose. While behavior turned from avid acceptance to active rejection upon repletion, the responsiveness of NTS neurons to the stimulus array, including the satiating solution, was unmodified. It is concluded that at the first central synapse of the taste system of the primate, neural responsiveness is not influenced by the normal transition from hunger to satiety. This is in contrast to the responses of a population of neurons recorded in the hypothalamus, which only occur to the taste of food when the monkey is hungry. Thus, NTS gustatory activity appears to occur independently of normal hunger and satiety, whereas hypothalamic neuronal activity is more closely related to the influence of motivational state on behavioral responsiveness to gustatory stimuli.