Tatsuko Yokota

Correlated discharges of two neurons in rat gustatory cortex during gustatory stimulation

During application of tastants into the oral cavity correlated activities were observed in 27 of 64 neuron pairs recorded simultaneously in the gustatory cortex of anesthetized rats. The mean frequency of correlated discharges as assessed from the size of the peak appearing in the cross-correlogram was lower (0.2-0.33 spikes/s) and the mean width of the peaks was wider (8 ms) as compared to those reported in the gustatory relay stations in the brainstem. In a few cases troughs were formed or peaks appeared with a long delay.

Topographical representations of taste response characteristics in the rostral nucleus of the solitary tract in the rat

The rostral nucleus of the solitary tract (rNST) is the first-order taste relay in rats. This study constructed topographical distributions of taste response characteristics (best-stimulus, response magnitude, and taste-tuning) from spike discharges of single neurons in the rNST. The rNST is divided into four subregions along the rostrocaudal (RC) axis, which include r1-r4. We explored single-neuron activity in r1-r3, using multibarreled glass microelectrodes. NaCl (N)-best neurons were localized to the rostral half of r1-r3, while HCl (H)-best and sucrose (S)-best neurons showed a tendency toward more caudal locations. NaCl and HCl (NH)-best neurons were distributed across r2-r3. The mean RC values and Mahalanobis distance indicated a significant difference between the distributions of N-best and NH-best neurons in which N-best neurons were located more rostrally. The region of large responses to NaCl (net response >5 spikes/s) overlapped with the distribution of N-best neurons. The region of large responses to HCl extended widely over r1-r3. The region of large responses to sucrose was in the medial part of r2. The excitatory region (>1 spike/s) for quinine overlapped with that for HCl. Neurons with sharp to moderate tuning were located primarily in r1-r2, while those with broad tuning were located in r2-r3. The robust responses to NaCl in r1-r2 primarily contributed to sharp to moderate taste-tuning. Neurons in r3 tended to have broad tuning, apparently due to small responses to both NaCl and HCl. Therefore, the rNST is spatially organized by neurons with distinct taste response characteristics, suggesting that these neurons serve different functional roles.

Modulation during sleep of the cat trigeminal neurons responding to tooth pulp stimulation

Sleep-induced changes in the trigeminal neuron responses to electrical stimulation of the cat tooth pulp were studied. Two parameters were adopted: One was the evoked spike number at two times the threshold intensity (2 X T response magnitude), which would reveal the level shifting of the neuronal response by the sleep-regulatory system. Another was the rate of change in the response intensity when the stimulus was raised to a level of 0.7 time the arousal threshold during light slow wave sleep (sensitivity gradient), which would reflect the influences of the pain-modulatory system driven by strong noxious inputs. It was found that during sleep the two indexes tended to show a correlated change; the neurons which came to have a greater 2 X T response magnitude tended to have a smaller sensitivity gradient than during wakefulness, and vice versa. It was suggested that two contrasting populations of tooth pulp neurons might be differentiated, and that the sleep-regulatory system and the pain-modulatory system would have differential but correlated controls over these two kinds of neurons.

Differential taste coding of salt and acid by correlative activities between taste-sensitive neuron types in rat gustatory cortex

Using a multi-electrode recording technique, the present study aimed to elucidate the role of broadly-tuned taste-sensitive neurons in the rat gustatory cortex in discriminating between salt and acid. A majority of taste-sensitive neurons (94/119 neurons; 78%) were classified as NaCl (N)-, HCl (H)- or NaCl and HCl (NH)-best neurons. Of 63 neuron pairs (94 neurons), 31 showed significant peaks and/or troughs in their cross-correlograms (CCs) during taste stimulation periods. During NaCl stimulation, the incidence of significant correlation and the mean frequency of correlated discharges (FC) in the N/N and NH/NH pairs were higher than those in the other best-taste pairs. In contrast, during HCl stimulation both indices in the N/N or H/H pairs were very low, while those in the NH/NH pairs were high. These results suggest that (1) correlated activities between N-best neurons and those between NH-best neurons play a significant role in taste quality coding of salt, and that (2) correlated activities between NH-best neurons may be important for sour taste coding as well. Peak formation in CCs tended to be more frequent in the homo-types (N/N, H/H and NH/NH pairs) than in the hetero-types (N/NH, N/H and H/NH pairs). In contrast, troughs were observed mostly in the hetero-types. Inhibitory interaction in hetero-type pairs together with coactivation in homo-type pairs may enhance taste discrimination by taste-sensitive neuron populations.

Functional distribution of taste neurons in rat gustatory cortex —cross-correlation analysis

We previously showed that signal propagation in the piriform cortex (PC) elicited by stimulation of layer In was severely damaged when either layer Ia or Ib was cut. In the present study, double pulse stimulation was applied to layer Ia of slices where either layer Ia or Ib 0.4-0.7 mm caudal to the site of stimulation was cut, to investigate if temporal summation of excitation from the remaining fiber system to pyramidal cells could recover the damaged signal propagation. In 74% of samples with layer Ia cut and in 68% of those with layer Ib cut, double pulse stimulation, as compared with single pulse, obviously increased the excitability in the region caudal to the cut. The excitability induced by double pulse, however, reached to at most 50% of that in the same region induced by single pulse before the cut was made. Thus, it is suggested that convergence of inputs from afferent and association fibers onto pyramidal cells is indispensable for generating strong signal propagation in PC.

1913 Relations between correlated activities and response modalities in rat gustatory cortex

Tatsuko Yokota, Kunihiro Eguchi, Toyohiko Satoh Taste neurons in the gustatory cortex of anesthetized rats were recorded simultaneously with touch-sensitive neurons and/or apparently non-sensory neurons by the use of 4 glass micro-pipettes glued together. Four basic taste solutions were applied to the tongue and oral cavity. Cross-Correlation histograms (CChs) were constructed from the spike trains of a pair of neurons during taste stimulation, application of distilled water and no liquid delivering periods. 1) In about 50% of taste neuron pairs, peaks or troughs were detected in the CChs. However, in the taste/tactile pairs and the taste/non-sensory pairs, formation of peaks ortroughs occurred in less than 20%. The taste-neurons receive specific common inputs obviously more frequently than the neurons of other types. 2) The correlated activities in taste-pairs could be enhanced or attenuated during application of distilled water.

Potassium-ion activity in the trigeminal central nervous complex during tooth pulp-evoked slow mass potentials

Following electrical stimulation of the tooth pulp, extracellular K+ activity was enhanced by up to 0.1 mM only in the trigeminal motor nucleus which gave a positive slow mass potential (SMP) of low amplitude. No nuclear region giving a positive or negative SMP of high amplitude showed any detectable change. There are possible generation mechanisms of the negative and positive SMP.