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Physicochemical studies of taste reception. III. Interpretation of the water response in taste reception.

The model membrane composed of a Millipore filter paper and the total lipids from bovine tongue epithelium or phosphatidylcholine from egg yolk simulated well the water response of a living taste cell, The water response observed with the model membrane adapted to various salt solutions was interpreted in terms of changes in electric potential at the membrane-solution interface, i.e. the water response was attributed to the e.m.f. change produced by diffusion of the electrolytes dissolved in (or adsorbed on) the membrane surface into the bulk solution. The water response of the frog tongue was also investigated by measuring the neural response of the glossopharyngeal nerve. The results obtained were consistent with the mechanism proposed in the present paper. The response of the frog to Ca2+ was examined under the condition where the water response was suppressed, and it was concluded that the water response of the frog is different from the response to Ca2+.

Animals↗

Pysicochemical studies of taste reception. V. Suppressive effect of salts on sugar response of the frog.

The tast responses of frog to various kinds of sugars were measured quantitatively by use of the glossopharyngeal nerve activity under an appropriate condition where the water response was completely suppressed. The concentration dependences of response of frog tongue to D-fructose, D-glucose, and sucrose were almost the same, D-galactose, however, elicited a much larger response in comparison with the other sugars in the whole range of concentrations examined. The sugar response was suppressed extensively by the presence of small amount of salts in the stimulating sugar solution. The suppressive effects of NaCl, KCl, MgCl2, MgSO4, and K4Fe(CN)6 were examined with a fixed concentration of sugar. The results obtained with these salts, added in various concentrations, fell on a single curve when the data were plotted against the ionic strength in the stimulating solution. The present results were consistent with the notion that the taste receptor potential for salts or acids is attributable to a change in the phase boundary potential at the membrane-solution interface as proposed in the previous papers of this series.

Animals↗

Depolarization induced by injection of cyclic nucleotides into frog taste cell.

In order to identify the intracellular transmitter involved in the taste transduction process, cyclic nucleotides were iontophoretically injected into the frog taste cells while membrane potentials were recorded intracellularly. Injection of either cyclic GMP or cyclic AMP induced a depolarization response of about 5 mV in the taste cells, but injection of Cl- had no effect. The rate of a repolarization after the depolarization elicited by cyclic GMP was larger than that after cyclic AMP. The possible role of cyclic nucleotide in the taste transduction was discussed.

Animals↗

Corticofugal effects on the activity of thalamic taste cells.

To examine corticofugal influences on afferent thalamic cell responses in the gustatory system, effects of cortical conditioning stimuli on the responses of thalamic taste cells to peripheral stimulation were examined in the rat. Two types of excitability change of thalamic cells were observed; one is inhibitory (for about 60 msec) (20%) and the other is inhibitory (for about 10 msec)--facilitatory (for about 60 msec) (40%) to conditioning stimulus applied to the ventral gustatory area, and half of the cells belonging to the latter type also showed a similar pattern with more marked and prolonged excitability changes to conditioning stimulus applied to the dorsal gustatory area. It was suggested that some of the response characteristics of thalamic and cortical taste cells were attributed to the corticofugal feedback loop.

Afferent Pathways↗

The effect of nerve stump length upon mammalian taste responses.

A decline in the magnitude of the summated responses evoked by taste solutions began about 1 h following IXth nerve transection. In 41 IXth nerves transected at different lengths we found a linear relationship between the length of the IXth nerve stump remaining attached to the tongue and the time for the taste response amplitude to fall to 50% of its control magnitude. The existence of a nerve stump length dependency suggests that IXth nerve taste responses in the gerbil are maintained by the axonal transport of trophic substances to the tongue.

Animals↗

Taste responses in the superior secondary gustatory nucleus of the carp, Cyprinus carpio L.

Single unit discharges in the superior secondary gustatory nucleus of the carp, Cyprinus carpio L., were studied electrophysiologically in response to chemical stimulation of the external chemoreceptors of the facial skin surface. Of 36 gustatory neurons recorded, 80.6% were facilitated by taste stimuli and 19.4% were inhibited. The gustatory neurons were classified according to their responsiveness to the 4 basic taste substances and, except the inhibitory type, did not differ remarkably from the primary and secondary gustatory neurons. More inhibitory type (19.4%) neurons occurred at higher levels of the gustatory system. As in the primary and secondary levels, sodium chloride and acetic acid solutions were more effective stimuli than quinine HCl and sucrose. The ascending secondary gustatory fibers project bilaterally to the superior gustatory nucleus of the carp. About 20% of the gustatory neurons respond to stimulation of only the contralateral facial skin while 27.8% respond to stimulation of either side of the face. The latter neuron type showed very complicated responses, and were classified into 'Uniform', 'Summation', 'Contra. greater than Ipsi.' and 'Quality field' types. The remaining 50% of the neurons respond only to stimulation on the ipsilateral side.

Animals↗

Electrophysiological evidence for the topographical arrangement of taste and tactile neurons in the facial lobe of the channel catfish.

Neural responses in the facial lobe of th channel catfish to chemical and mechanical stimulation of the external skin surface were studied electrophysiologically. Taste and tactile neurons in the lobe were organized in a somatotopic manner, which confirms the anatomical reports of the facial lobe in the bullhead catfish, but is markedly different from that of the Cyprinidae. The taste neurons were arranged generally in the more dorsal regions of the tactile sensitive areas and responded with highest frequency to L-alanine or L-arginine HCI among several amino acids tested. The mechanically responsive neurons in the deeper layer of the antero-medial portion of the lobe, possibly corresponding to the intermediate nucleus of the facial lobe, had large receptive fields ranging from 100 mm to the whole body surface; in addition, some of these neurons showed lateral inhibition. The present study revealed that the facial lobe of the channel catfish is a center not only for gustatory input, but also for tactile information.

Animals↗

Surface and intramedullary potentials evoked by stimulation of the glossopharyngeal nerve in frogs.

The bulbar potentials evoked by afferent volleys in the bullfrog glossopharyngeal nerve and in its 2 distal branches were studied. Following supramaximal electric stimulation of the peripheral nerve, the potential consisting of 2 triphasic deflections (S1 and S2) of presynaptic origin and 4 postsynaptic negative waves, N1, N2, N3 and N4, having the peak latency of 5, 8, 30 and 80 ms, respectively, was obtained along the nucleus fasciculus solitarius. By lowering the stimulus intensity to the threshold for exciting mechanosensitive fibers, only S1 followed by N1, N3 and N4 was produced, whereas, at higher intensities, S2 which accompanied by N2 became apparent. N1 and N2 waves were distributed over the bulbar dorsal surface with the maximal amplitude at 1-2 mm rostral to the obex and 0.5-1 mm lateral from the midline, the negativity being found maximal at the depth 0.5-1 mm from the surface. The surface-recorded N2 potential evoked by stimulation of the medial branch distributed caudal to that produced by stimulation of the lateral branch. Of intramedullary-recorded 4 negative waves, only N1 caused by volleys in the lateral branch distributed deeper layer than the one evoked by those in the other branch.

Afferent Pathways↗