Hisao Nishijo

Taste Responses from the Entire Gustatory Apparatusa

The primary biological function of the gustatory system is to evaluate foods and fluids passing though the oral cavity. Within the brain, then, taste should interact with the neural systems that control feeding and drinking behavior. Taste is an anatomically distinct sense, and thus it should be possible to follow the gustatory system, synapse by synapse, into the CNS and use it to probe neural mechanisms involved in the maintenance of energy, water, and electrolyte balance. To paraphrase Freud, taste is the royal road to the neural mechanisms of feeding. This simple logic has guided much of the research in our laboratory and often is implicit in other experiments in the field. The road may be royal, but Dr. Freud neglected to add that it is neither straight nor paved. The logic may be sound, but some of the supporting facts and assumptions need work. First, the neural control of feeding behavior is true, but not pure. Control of ingestion is distributed throughout the brain and gut, and the same neural systems contribute to more than one function. The activity of hypothalamic neurons has been correlated with ingestion, gastric motility, blood pressure, and sudomotor response to name only a few. The function correlated with hypothalamic neural activity often has as much to do with the interests of the investigator as with the biology of the preparation. Similarly, the central gustatory system distributes widely within the brain, but in the process, somatic and visceral afferent neurons converge with the taste neurons to an extent that blurs the distinction of a dedicated chemosensory line. Even assuming that it plays a unique role in feeding, the import of taste as a physiological probe remains limited by our relatively poor understanding of its sensory characteristics. Taste buds are organized into subpopulations within the oral cavity based upon location and innervation (TABLE 1). Data on chemosensory responsiveness still

Parabrachial gustatory neural responses to monosodium glutamate ingested by awake rats

A sample of 41 gustatory neurons isolated in the parabrachial nuclei of awake, behaving rats was tested with sapid solutions of 0.1 M monosodium glutamate (MSG), 0.5 mM of guanosine 5-monophosphate (GMP), and a mixture of MSG and GMP as well as with 0.3 M sucrose, 0.1 M NaCl, 0.01 M citric acid, and 0.0001 M QHCl. Interneuronal correlation coefficients and factor analysis indicated that both the sodium cation and glutamic anion contributed to the activity elicited by MSG. Guanosine potentiated the responses to MSG, but only in neurons that also responded to sucrose. These results suggest that the gustatory contribution to the flavor denoted by the Japanese word umami may be mediated, in part, by neurons that also respond to chemical described by humans as sweet.

Contribution of amygdalar and lateral hypothalamic neurons to visual information processing of food and nonfood in monkey.

Visual information processing was investigated in the inferotemporal cortical (ITCx)-amygdalar (AM)-lateral hypothalamic (LHA) axis which contributes to food-nonfood discrimination. Neuronal activity was recorded from monkey AM and LHA during discrimination of sensory stimuli including sight of food or nonfood. The task had four phases: control, visual, bar press, and ingestion. Of 710 AM neurons tested, 220 (31.0%) responded during visual phase: 48 to only visual stimulation, 13 (1.9%) to visual plus oral sensory stimulation, 142 (20.0%) to multimodal stimulation and 17 (2.4%) to one affectively significant item. Of 669 LHA neurons tested, 106 (15.8%) responded in the visual phase. Of 80 visual-related neurons tested systematically, 33 (41.2%) responded selectively to the sight of any object predicting the availability of reward, and 47 (58.8%) responded nondifferentially to both food and nonfood. Many of AM neuron responses were graded according to the degree of affective significance of sensory stimuli (sensory-affective association), but responses of LHA food responsive neurons did not depend on the kind of reward indicated by the sensory stimuli (stimulus-reinforcement association). Some AM and LHA food responses were modulated by extinction or reversal. Dynamic information processing in ITCx-AM-LHA axis was investigated by reversible deficits of bilateral ITCx or AM by cooling. ITCx cooling suppressed discrimination by vision responding AM neurons (8/17). AM cooling suppressed LHA responses to food (9/22). We suggest deep AM-LHA involvement in food-nonfood discrimination based on AM sensory-affective association and LHA stimulus-reinforcement association.

Role of the hippocampal formation in sequence memory

It is suggested that episodic memory is defined as a context-dependent sequence memory, and that the hippocampal formation (HF) is essential in such memory. To investigate HF involvement in a context-dependent sequence memory, multiple single unit activities were recorded from the monkey HF during performance of real translocation and virtual navigation tasks. The results indicated that place-related neuronal activity in the HF was task-or context-dependent, and cross-correlation data suggest that the context-dependent information may be encoded by interaction among pyramidal neurons based on asymmetrical connections. Rat CA1 HF neurons were also recorded during a conditional sequence memory task. Consistent with the computational studies, 2 types of the HF neurons were found; some neurons responded to single item regardless of sequences in which the item was presented, while other neurons displayed sustained firing during serial presentation of several items. In humans, event-related potentials (ERPs) were recorded during a sound-sequence memory task. The results suggest that the ERPs around 300-700 msec latency were specifically involved in sound sequence information processing. Furthermore, equivalent dipoles for the ERPs were localized in the medial temporal lobe including the HF and parahippocampal gyrus. These results suggest that the HF is crucial in context-dependent sequence information processing, which may be the neural basis of episodic memory.