The responses of 111 cortical neurons to the four classical taste stimuli (sucrose, NaCl, HCl, and quinine HCl) applied to the anterior part of the tongue were recorded extracellularly in lightly anesthetized rats. Basic response properties of these cortical taste neurons were analyzed. The location of 88 of 111 neurons were histologically identified. They were distributed from anterodorsal to posteroventral direction in the insular cortex just dorsal to the rhinal sulcus and ventral to the somatic sensory area I. The receptive fields of 17 cortical neurons were examined. Most (94%) of the neurons had a narrow focus on the ipsilateral, contralateral, or bilateral sides of the tongue surface. Half of the foci were surrounded by a less-sensitive receptive field of relatively wide size. No apparent relationship was detected between the location of the cortical neurons and the site or extent of the receptive fields of those neurons, indicating a lack of topographical organization in the cortical gustatory area. The mean rate of the spontaneous discharges was 7.1 impulses/3 s, which is about 3 times larger than that in a first-order taste nerve (chorda tympani). The statistically significant difference of spontaneous discharges among response types of cortical neurons was observed only between the neurons responding in an excitatory manner to only one or two kinds of basic stimuli (6.2 impulses/3 s) and the neurons responding in an inhibitory manner to more than three kinds of taste stimuli (14.2 impulses/3 s). When the net responses (spontaneous rate subtracted) to each of the four tastes were compared with the spontaneous discharges in each neuron, the magnitude of spontaneous discharges was significantly negatively correlated with the net response to sucrose. This fact indicates that a neuron with a larger spontaneous discharge rate has a tendency to respond less to sucrose. Response characteristics of cortical taste neurons were quite distinct from those of the first-order taste neurons in the following respects: 1) a decrease in the average evoked discharge rate, which resulted in a small signal-to-noise ratio; 2) a tendency toward equalization of effectiveness of the four basic taste stimuli; 3) about 27% of the neurons decreased their firing rate during the first 3 s after the onset of taste stimulation; and 4) no clear initial phasic response, with a fluctuation in impulse discharges in some neurons.(ABSTRACT TRUNCATED AT 400 WORDS)
Corticotropin-releasing factor (CRF) expressed in the hypothalamus plays an important role in mediating behavioral responses to stressors. Restraining the body of an animal has been shown to activate and induce an enhanced expression of CRF in paraventricular neurons of the rat hypothalamus. Since aggressive biting behavior is known to suppress stress-induced noradrenaline secretion in the central nervous system and the formation of gastric ulcers, we investigated the effect of biting on restraint-induced CRF expression in the rat hypothalamus. The number of CRF-expressing neurons in the paraventricular nucleus increased significantly after short time restraint (30 or 60 min) followed by a 180-minute post-restraint period. Biting of a wooden stick during the restraint stress significantly suppressed the restraint-induced enhancement of CRF expression in the paraventricular nucleus. These observations suggest a possible anti-stress effect of biting and an important role of para-functional masticatory activity in coping with stressful events.
The present report was designed to investigate neural coding of taste information in the cerebral cortical taste area of rats. The magnitude and/or type (excitatory, inhibitory, or no-response) of responses of 111 cortical neurons evoked by single concentrations of the four basic taste stimuli (sucrose, NaCl, HCl, and quinine HCl) were subjected to four types of analyses in the context of the four proposed hypotheses of taste-quality coding: across-neuron response-pattern, labeled-line, matrix-pattern, and across-region response-pattern notions (88 histologically located neurons). An across-neuron response-pattern notion assumes that taste quality is coded by differential magnitudes of response across many neurons. This theory utilizes across-neuron correlation coefficients as a metric for the evaluation of taste quality coding. Across-neuron correlations between magnitudes of responses to any pairs of the four basic taste stimuli across 111 cortical neurons were very high and were similar. However, calculations made with net responses (spontaneous rate subtracted) resulted in less positive correlations but still similar values among the various pairs of taste stimuli. This finding suggests that across-neuron response patterns of cortical neurons become less discriminating among taste qualities compared with those of the lower-order neurons. A labeled-line notion assumes that there are identifiable groups of neurons and that taste quality is coded by activity in these particular sets of neurons. Some investigators have classified taste-responsive neurons into best-stimulus categories, depending on their best sensitivity to any one of the four basic stimuli, such as sucrose-best, NaCl-best, HCl-best, and quinine-best neurons; they have suggested that taste can be classified along four qualitative dimensions that correspond to these four neuron types (i.e., four labeled lines). The present study shows that responsiveness of each of the four best-stimulus neurons had similar profiles between peripheral and cortical levels. That is, when the stimuli were arranged along the abscissa in the order of sucrose, NaCl, HCl, and quinine, there is a peak response in one place, and the responses decreased gradually from the peak. However, such response characteristics do not favor the labeled-line theory, since they can be explained in the context of the across-neuron pattern theory. A matrix-pattern notion assumes that taste quality is coded by a spatially arranged matrix pattern of activated neurons.(ABSTRACT TRUNCATED AT 400 WORDS)
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