Representation of taste stimuli in the brain.

Ingestive behavior is controlled by various neural systems in the central nervous system such as oromotor and taste neural systems. Since the taste system is the final arbiter by which an animal determines whether a chemical or food will be acceptable (Travers and Norgren, 1986), the taste system is one of the main neural systems for ingestive behavior. The brainstem contains a basic neural system for ingestive behavior, that includes brainstem taste areas such as the nucleus of the solitary tract (NTS) and the pontine parabrachial nucleus (PBN), by which the animal can manifest different oromotor responses to four basic tastes (Grill and Norgren, 1978). It has been suggested that the forebrain, including the amygdala (AM) and hypothalamus, which receives taste information from the brainstem taste areas, is a higher center which modulates activity of this basic neural system by its descending projections (Norgren, 1995). The AM and hypothalamus, which receives massive afferent fibers from the AM, have been reported to be important in motivation, feeding behavior and evaluation of the biological significance of sensory stimuli (Rolls, 1976; Nishijo et al., 1988). Lesions of the AM and hypothalamus altered food preference in monkeys and rats (Murray et al., 1996; Isaacson, 1982) and attenuated behavioral responses to both preferred and aversive taste stimuli in rats (Kemble and Schwartzbaum, 1969). Furthermore, decerebration and electrical stimulation or inactivation of the forebrain modulated activity of taste neurons in the NTS and PBN (Matsuo et al., 1984; Mark et al., 1988; Di Lorenzo, 1990). In the present study, to investigate the nature of taste information processing in the AM, neuronal activity was recorded from the AM during discrimination of sensory stimuli associated with various taste solutions and ingestion of taste solutions.