RESPONSES OF NEURONS IN THE PRIMATE TASTE CORTEX TO THE GLUTAMATE IONAND TO INOSINE 5'-MONOPHOSPHATE

To investigate the neural encoding of glutamate taste in the primate, recordings were made from taste responsive neurons in the cortical tas te areas in macaques. Most of the neurons were in the orbitofrontal co rtex taste area, with a small number in adjacent taste areas. First, i t was shown that single neurons that had their best responses to sodiu m glutamate also had good responses to glutamic acid. The correlation between the responses to these two tastants was higher than between an y other pair of tastants, which included glucose (sweet), sodium chlor ide (salty), HCl (sour), and quinine HCl (bitter). Accordingly, the re sponsiveness to glutamic acid clustered with the response to monosodiu m glutamate in a cluster analysis with this set of stimuli, and glutam ic acid was close to sodium glutamate in a space created by multidimen sional scaling. Second, it was shown that the responses of these neuro ns to the nucleotide umami tastant inosine 5'-monophosphate were more correlated with their responses to monosodium glutamate than to any pr ototypical tastant. Third, concentration response curves showed that c oncentrations of monosodium glutamate as low as 0.001 M were just abov e threshold for some of these neurons. Fourth, neurons have not yet be en found in this cortical region that showed synergism of monosodium g lutamate and the nucleotide inosine 5'-monophosphate: it was shown tha t mixtures of 0.0001 M inosine 5'-monophosphate with different concent rations (0.001, 0.01, and 0.1 M) of monosodium glutamate did not have a greater effect than the monosodium glutamate alone. Fifth, some neur ons in the orbitofrontal region, which responded to monosodium glutama te and other food tastes, decreased their responses after feeding with monosodium glutamate to behavioural satiety. In some cases this reduc tion was sensory-specific. These findings show that the taste neurons activated by monosodium glutamate can also be activated by other umami tastants, including glutamic acid and the nucleotide inosine 5'-monop hosphate. The responses to these umami tastants were more similar to e ach other than to any of the other prototypical tastants, providing ev idence that in this system umami is encoded differently from the other tastants. Moreover, the findings with these tastants provide addition al evidence that the responses to monosodium glutamate are not due jus t to activation of a sodium taste channel.