Kunihiro Eguchi

Characterization of the neurons in the region of solitary tract nucleus during sleep.

Abstract Single-unit recording was made from neurons in the region of solitary tract nucleus (NTS) of cats. Neurons discharging in correlation with cardiac or respiratory cycle were identified outside the NTS. They showed no obvious change in discharge rate during sleep-wakefulness cycle, and were unresponsive to electrical stimulation of the mesencephalic reticular formation (MRF), suggesting that they are not involved in slow wave sleep (SS) mechanism. More than half of the neurons recorded in the NTS showed an increase in discharge rate during, but not prior to, SS. Most of non-NTS neurons had during SS a discharge rate similar to that during wakefulness. The NTS neurons may be more related to SS mechanism than non-NTS neurons. The effectiveness of electrical stimulation of the MRF in driving or inhibiting the neurons of the NTS region was measured to be expressed by an index. Generally speaking, responses with greater S/W index ratio were excitatory, while those with smaller were inhibitory. During paradoxical sleep the effectiveness was usually reduced.

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.

Relationship between positive sharp wave bursts and unitary discharges in the cat hippocampus during slow wave sleep.

In the cat hippocampus bursts of positive sharp waves (PSWs) appeared sporadically almost exclusively during slow wave sleep. The PSW burst was most often found in cell-rich areas in the CA1 and subiculum, and its occurrence was almost synchronized in different regions. An individual burst was usually composed of 3-5 PSWs of about 10 msec duration and showed a considerable fluctuation in amplitude. It was occasionally followed by a negative-going deflection of large amplitude and long duration (post-PSW negativity). The amplitude of PSWs and post-PSW negativity in the CA1 was high in the area giving a large sized-evoked response after stimulation of the contralateral CA3. The spike discharge rate during the burst was two or three times higher than that during the period just preceding the burst, but the discharge never occurred in the positive phase of the PSWs. During the initial part of the post-PSW negativity the high firing probability was maintained. Even when the PSW burst was not followed by a detectable post-PSW negativity, the firing probability during the period corresponding to the post-PSW negativity was still significantly higher than the pre-PSW period. It was suggested that the PSW bursts and post-PSW negativity were triggered off in cell-rich areas by diffuse excitatory inputs impinging possibly upon the hippocampal pyramidal cells and subicular principal cells. The rhythmic PSWs may be post-synaptic inhibitory potentials produced on the somata of those cells after activation of recurrent interneuronal circuits.

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.

Functional relationship between cat brainstem neurons during sleep and wakefulness.

Abstract The mode of interaction between so-called sleep-wakefulness centers in the cat lower brainstem was studied on 100 neurons. The magnitude of the spike response of a neuron in one center to electrical stimulation of another center was measured to calculate the index of responsiveness. During REM sleep the index was, in a great majority of cases, significantly smaller as compared with that during wakefulness and slow wave sleep. This reduction in the effectiveness of information transmission between different centers might be the basis of characteristic events occurring during REM sleep. From the behavior of the indexes during sleep-wakefulness cycle, it is suggested that different phases of sleep and wakefulness are realized by a complicated interplay of many sleep-wakefulness centers which would be communicating with one another not only through channels which are activated in a phase-specific manner, but also through a larger number of channels of which activity is modulated differentially during different phases.

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.

Stimulus-response relation of tooth pulp units in rat trigeminal nucleus

Electrical stimulation of the rat tooth pulp evokes a slow mass potential (SMP) in the trigeminal spinal nucleus. Single units in the SMP field were examined for their stimulus-response relation. The response profile of most single units differed from that of background SMPs in that the former tended to give a greater gradient or exponent when they were approximated by linear or power function, respectively. Most of single units with a short and stable latency could be described rather well by power function, while a majority of units with a longer and unstable latency had a relatively wide dynamic range to varied stimulus intensities and could be approximated more closely by linear function.

Attenuation during paradoxical sleep of signals from tooth pulp to thalamus.

Quantitative evaluation of the response of thalamic neurons to tooth pulp stimulation was made in chronically prepared cats. The latency, duration and intensity of the responses were measured from the post-stimulus time histograms to estimate, from various aspects, the alteration in the responsiveness during different phases of sleep and wakefulness. During slow wave sleep, tooth pulp-evoked impulses tended to be transmitted to the thalamus in a similar or slightly higher intensity compared to wakefulness. In contrast, during paradoxical sleep the signals were often attenuated in many aspects. The results seem to be in favor of the idea that the impairment of signal to noise ratio in a variety of neuronal networks is one of the characteristics of paradoxical sleep.

Cross-Correlation analysis of midbrain reticular neuron pairs during sleep-waking cycle of the cat

By simultaneously inserting three extracellular microelectrodes, separated by 1.0 mm from each other, into the cats midbrain reticular formation (MRF), temporal cross-correlation of firing of neuron pairs was measured to investigate the mode of interaction among the MRF neurons during different states of sleep and wakefulness. None of 97 neurons pairs studied gave clear-cut cross-correlograms suggesting cascade connection between two neurons. However, 29 neuron pairs showed weakly synchronized firing which occurred periodically with a mean interval of 1.23 s. This interval was different from the respiratory cycle. The rhythmic synchronization appeared most obviously during slow wave sleep. The synchronized firing was encountered more often in the pairs of adjacent neurons picked up with a single electrode and in the pairs picked up with separate electrodes positioned along the mediolateral axis, when compared with the neuron pairs located along the rostrocaudal axis. About 80% of the neurons (37 our of 46) which showed periodically synchronized discharge had no corresponding periodicity in their auto-correlograms. Simple synaptic linkage conceivable for the observed periodic synchronization of a neuron pair would be shared inhibition with the common inhibitory source activated periodically. Some neurons in the dorsal raphe nucleus and in the solitary tract nucleus were also examined for the cross-correlation with the MRF neurons. However, no positive relationship was found.

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.