SYNCHRONIZED DISCHARGE OF TASTE NEURONS RECORDED SIMULTANEOUSLY IN RAT PARABRACHIAL NUCLEUS

Cross-correlation analysis was made in the taste-sensitive neuron pairs recorded simultaneously from the parabrachial nucleus (PBN) of rats. Three indexes were adopted to evaluate the activities of the taste neurons; namely, 1) spike response density (RD value), which is the net spike density to stimulation with the four basic tastes; 2) the frequency of correlated discharges (FC value in spikes per second), which was determined by measuring the area of the peak appearing in the cross-correlogram (CC) during application of the test fluids; and 3) the weight of correlated discharge (WC = FC/RD), which shows the relative importance of correlated discharges in the taste signals delivered by a component neuron of a given pair. In 11 of 23 pairs, the CCs exhibited peaks during stimulation with tastants. These 11 pairs, which were recorded in the pontine taste area, were composed of 18 NaCl-best (most sensitive to NaCl) and 4 HCl-best neurons. In eight pairs, the best-stimulus of both of the component neurons was NaCl, and it was HCl in one pair (homo-type pairs). The remaining two pairs were composed of an NaCl-best and an HCl-best neuron (hetero-type pairs). In eight homo-type pairs (7 NaCl-best pairs and 1 HCl-best pair), each pair exhibited the maximal FC value during stimulation with the best-stimulus of the component neurons (2.3 ≤ maximal FC ≤ 26.6 Hz). In the remaining three pairs, the maximal FC values were low (0.8-1.9 Hz). Two of them were hetero-type, and, in one homo-pair, the maximal FC value was obtained during stimulation with the tastant different from the best-stimulus of the component neurons. The peaks in the CCs were usually straddling the origin, and their highest point was located at the origin or very close to it. The width of the peaks appearing in the CCs was broad (3-8 ms) in seven pairs and narrow (0.5-2 ms) in four pairs. In all of the broad-peak pairs, the best-stimulus of both the component neurons was identical (homo-type), NaCl or HCl. In six broad-peak pairs, small peaks appeared also during application of distilled water. In the narrow-peak pairs, two were of homo-type (NaCl-best), and their maximal FC value was obtained during NaCl stimulation. Their maximal FC value was higher than that of the other two narrow-peak pairs, which were of hetero-type and exhibited no peaks during water application. In the neuron pairs showing no peaks in the CC during stimulation with tastants, the water-induced peaks were never observed, suggesting that sensory inputs from the oral cavity, which are possibly tactile, assist the formation of across-neuron activity pattern in the PBN. Most of the neurons giving a high WC value belonged to the homo-type pair, and their maximal FC values were also high and obtained during stimulation with the best-stimulus of the component neurons (NaCl or HCl). The broad-peaks are supposed to have been produced by the feedback control from the higher, gustatory relay stations; some NaCl- and HCl-best neurons in these relay stations sending through internuncial neurons return projection back to the PBN neurons, which are NaCl- and HCl-best, respectively. Some of the narrow-peaks have been produced possibly by the commonly innervating neurons in the nucleus tractus solitarii (NTS).