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Site of gustatory neural adaptation
SHORT COMMUNICATIONS
385
Site of gustatory neural adaptation Diamont and Zotterman 4 examined simultaneously the time courses of the electrical chorda tympani nerve responses and the psychophysical responses to prolonged gustatory stimulation of the human tongue, and found a close relation between the reduction of both responses after stimulation. They therefore suggested that the adaptation of gustatory sensation is mainly peripheral in nature. It is known that, in various kinds of animals as well as humans, the adaptation pattern of the primary taste afferent discharge is dependent on factors such as the kinds of taste stimuli and their concentrationsl,5,~, 11. However, little attention has been given to the mechanism by which the gustatory neural adaptation is produced. The present experiments were undertaken to examine this problem in the frog. Since it was recently found that in the frog a chemical synapse apparently exists between the taste cells and their axons 2,~, sensory adaptation may be concerned with the following factors: (1) receptor potential, (2) synaptic potential, and (3) spike-generating membrane activity. In the present experiments, mean time courses of the receptor potentials of the taste cells were compared with the time courses of the primary afferent neural discharge. The experimental results suggest that the adaptation of the gustatory neural discharge is mainly dependent on the synaptic or spike-generating mechanism rather than the receptor mechanism. Frogs of Rana catesbeiana and R. nigromaculata were used. When gustatory nerve activity was recorded the following procedure was used. After pithing, the whole glossopharyngeal nerve was cut centrally and dissected out, and the recording was done in situ. The amplified nerve activity was integrated with an electric integrator. Because the time course of the integrated nerve response was similar to that of the response recorded with a nerve impulse count summator 7 of 100-200 msec sampling time, a rising and falling time constant of 0.4 sec was used. In order to record the receptor potentials isolated tongues were used. Glass capillary microelectrodes (50-150 M ~ ) , filled with 2 M KC1, were inserted into single taste ceils of the fungiform papillae 12. Various concentrations of NaC1, quinine hydrochloride (Q-HC1), and acetic acid were used as taste stimuli. To eliminate the so-called water response the last two substances were dissolved in 0.01-0.1 M NaCl. The flow rate of the solutions applied over the tongue was 0.8 ml/sec for recording the whole nerve activity, but was much slower for recording the receptor potentials. Fig. 1A shows examples of the integrated neural responses to prolonged application of 0.5 M NaCI, 0.001 M Q-HCI and 0.002 M acetic acid. Fig. 1B, C and D shows the time course of the nerve responses to various concentrations of NaCI, Q-HC1 and acetic acid. The amplitude of responses was expressed as a percentage of initial peak response to each stimulus concentration. The responses to NaC1 were generally composed of two components, initial phasic and following tonic responses, although the 1 M NaCl stimulus produced a large second peak response at about 20 sec after its application (Fig. 1B). The initial phasic NaCI responses lasted for less than 5 sec and were followed by the tonic responses, whose amplitudes were relatively constant for 2 min or more except for the gradual decrease of the tonic responses to 1 M NaC1. As Brain Research, 34 (1971) 385-388
386
SHORT COMMUNICATIONS
B
A
100- ~ % o.5M NaCI
1
NaCI
2osec
lo'3M Q-HCI
3 5 ~ 0
•
D O ,'0
C
-~
1011% ~.Acetic acid
100I ~'HCI ••"10 -410 -210 -3M
4[~
•" 10-2 M A 5.10 -3
~\\
o 10-5 ~ =
~'
'
lb
'
Io' 8EC
'
i'o
'
SEC :~o
'
Fig. 1. Time course of the whole glossopharyngeal nerve responses to continuous flow of taste solutions over the tongue. A, Integrated responses to 3 kinds of stimuli, obtained from 3 different preparations. In each record the stimulus mark is shown as a thick line below the integrated response. The time calibration shown in record 1 is only for NaC1 response, and that in record 3 is for Q-HCI and acetic acid responses. B-D, Amplitudes of integrated responses (ordinates) plotted against time after the application of stimulus (abscissae). The amplitudes are expressed as percentage of initial maximal peaks to each concentration. The time of the peaks is shown as zero sec. Each symbol is the average value from 4 to 8 preparations. the NaCI c o n c e n t r a t i o n was increased, the tonic response amplitude expressed as the percentage of the initial phasic response clearly increased. O n the other hand, the initial peak response to Q-HCI or acetic acid decreased exponentially with time, a n d only small tonic responses were observed 15-20 sec after the peak (Fig. IC, D). The percentage of the steady response amplitudes tended to increase as the stimulus c o n c e n t r a t i o n was increased. But the difference in the amplitudes was small over a large range o f concentrations. The response amplitudes 20 sec after the initial peaks were reduced to 1 . 5 M . 5 ~ for 0.00014).01 M Q-HCI a n d to 5-13.5~o for 0.001-0.01 M acetic acid. These reduction rates were m u c h larger than the 21-86 ~ for 0.25-1 M NaCI stimuli for the same time course. The steady responses to Q-HC1 or acetic acid Brain Research, 34 (1971) 385-388
387
SHORT COMMUNICATIONS
o & •
0
I 0
Acetic acid NaCI Q-HCI
I
I
I I0
I
I
I
TIME AFTER PEAK
I 20
I
I
I
I
30 SEC
Fig. 2. Time course of the receptor potentials of single taste cells elicited by 3 kinds of taste stimuli. Amplitudes of the potentials are expressed as percentage of the peak height. The time of peak response is given as zero sec. Stimulus solutions: 0.5 M NaCl, 0.004 M Q-HCI and 0.016 M acetic acid. Each symbol is the average value from 5 to 6 taste cells.
were different from preparation to preparation, and sometimes no steady response appeared. This means that the tonic response for either Q-HC1 or acetic acid was unstable. On the contrary, the tonic response for NaC1 was very stable. So that the afferent neural activity could be compared with the receptor potential of the taste cells, microelectrodes were inserted into the cells. Fig. 2 shows the mean time course of intracellular receptor potentials to 0.5 M NaCl, 0.004 M Q-HC1 and 0.016 M acetic acid. Since it was difficult to maintain penetration of the cells for a long time, the responses to only these 3 kinds of solution were studied. The potentials to all solutions did not show the phasic component as seen in the afferent responses, and the amplitudes declined much more slowly than the afferent neural activity. The reduction rates of the receptor potentials 20 sec after the peak amplitudes were 72 ~ for NaC1, 46 ~ for Q-HC1 and 82 ~ for acetic acid. On the other hand, the reduction rates of the afferent responses to the same concentrations were 4 7 ~ for NaCl, 4 ~ for Q-HC1 and 13 ~ for acetic acid, as estimated from Fig. 1B, C and D. From a comparison of both groups of values, it is clear that the rate of decline of the gustatory afferent responses after peak response was faster than that of the receptor potentials. It is well known that the frequency of nerve impulses is proportional to the receptor or generator potentials s-10. Since, in the present experiments, such a proportional relationship was not found between the gustatory afferent discharges and the receptor potentials of the taste cells, chemical synapses located between the taste cells and axon endings may play an important role in the formation of the initial phasic and following tonic components of the afferent discharges. It is concluded that gustatory neural adaptation is mainly due to the adaptation properties of the synaptic potential and the impulse-generating membrane activity rather than the receptor potential. These properties of NaCl-sensitive fibers are probably different from those Brain Research, 34 (1971) 385-388
388
SHORT
COMMUNICATIONS
o f Q - H C 1 o r acetic acid-sensitive fibers, b e c a u s e the t i m e c o u r s e o f the d i s c h a r g e s o f t h e t w o g r o u p s o f fibers differed. I t h a n k P r o f . M . I c h i o k a f o r v a l u a b l e suggestions.
Department of Physiology, School of Dentistry, Tokyo Medical and Dental University, Tokyo (Japan)
TOSHIHIDE SATO*
1 BEIDLER, L. M., Properties of chemoreceptors of tongue of rat, J. Neurophysiol., 16 (1953) 595607. 2 DEHAN, R. S., AND GRAZIADEI, P. P. C., Functional anatomy of frog's taste organs, Experientia (Basel), 27 (1971) 823-825. 3 DEHAN, R. S., AND GRAZIADEI,P. P. C., Innervation of frog's taste organ, Histochemical studies, J. Neurobiol., in press. 4 DIAMONT, H., AND ZOTTERMAN, Y., A comparative study on the neural and psychophysical response to taste stimuli. In C. PFAFFMANN(Ed.), Olfaction and Taste, 111, Rockefeller Univ. Press, New York, 1969, pp. 428435. 5 FISHMAN,I. Y., Single fiber gustatory impulses in rat and hamster, J. cell. comp. Physiol., 49 (1957) 319-334. 6 HARPERN, B. P., Chemical coding - - temporal pattern. In Y. ZOTTERMAN(Ed.), Olfaction and Taste, I, Pergamon, Oxford, 1963, pp. 275-284. 7 ICHIOKA,M., KONDO, Y., AND SAKAMOTO,M., Nerve impulse count summator, Igaku no Ayumi, 54 (1965) 609-612. (In Japanese.) 8 KATZ, B., Depolarization of sensory terminals and initiation of impulses in the muscle spindle, J. Physiol. (Lond.), 111 (1950) 261-282. 9 MOR1TA, H., Electrical sign of taste receptor activity. In C. PFAFFMANN(Ed.), Olfaetion and Taste, IIl, Rockefeller Univ. Press, New York, 1969, pp. 370-381. 10 MOR1TA,H., AND YAMASHITA,S., Further studies on the receptor potential of chemoreceptors of the blowfly, Mere. Fac. Sci. Kyushu Univ. Set. E (Biol.), 4 (1966) 83-93. l l PFAEFMANN,C., AND POWERS, J. B., Partial adaptation of taste, Psychon. Sci., 1 (1964)41-42. 12 SATO, T., The response of frog taste cell (Rana nigromaculata and Rana catesbeiana), Experientia (Basel), 25 (1969) 709-710. (Accepted August 20th, 1971)
* Present address: Department of Biological Science, Unit 1, Florida State University, Tallahassee: Fla. 32306, U.S.A.
Brain Research, 34 (1971) 385-388
385
Site of gustatory neural adaptation Diamont and Zotterman 4 examined simultaneously the time courses of the electrical chorda tympani nerve responses and the psychophysical responses to prolonged gustatory stimulation of the human tongue, and found a close relation between the reduction of both responses after stimulation. They therefore suggested that the adaptation of gustatory sensation is mainly peripheral in nature. It is known that, in various kinds of animals as well as humans, the adaptation pattern of the primary taste afferent discharge is dependent on factors such as the kinds of taste stimuli and their concentrationsl,5,~, 11. However, little attention has been given to the mechanism by which the gustatory neural adaptation is produced. The present experiments were undertaken to examine this problem in the frog. Since it was recently found that in the frog a chemical synapse apparently exists between the taste cells and their axons 2,~, sensory adaptation may be concerned with the following factors: (1) receptor potential, (2) synaptic potential, and (3) spike-generating membrane activity. In the present experiments, mean time courses of the receptor potentials of the taste cells were compared with the time courses of the primary afferent neural discharge. The experimental results suggest that the adaptation of the gustatory neural discharge is mainly dependent on the synaptic or spike-generating mechanism rather than the receptor mechanism. Frogs of Rana catesbeiana and R. nigromaculata were used. When gustatory nerve activity was recorded the following procedure was used. After pithing, the whole glossopharyngeal nerve was cut centrally and dissected out, and the recording was done in situ. The amplified nerve activity was integrated with an electric integrator. Because the time course of the integrated nerve response was similar to that of the response recorded with a nerve impulse count summator 7 of 100-200 msec sampling time, a rising and falling time constant of 0.4 sec was used. In order to record the receptor potentials isolated tongues were used. Glass capillary microelectrodes (50-150 M ~ ) , filled with 2 M KC1, were inserted into single taste ceils of the fungiform papillae 12. Various concentrations of NaC1, quinine hydrochloride (Q-HC1), and acetic acid were used as taste stimuli. To eliminate the so-called water response the last two substances were dissolved in 0.01-0.1 M NaCl. The flow rate of the solutions applied over the tongue was 0.8 ml/sec for recording the whole nerve activity, but was much slower for recording the receptor potentials. Fig. 1A shows examples of the integrated neural responses to prolonged application of 0.5 M NaCI, 0.001 M Q-HCI and 0.002 M acetic acid. Fig. 1B, C and D shows the time course of the nerve responses to various concentrations of NaCI, Q-HC1 and acetic acid. The amplitude of responses was expressed as a percentage of initial peak response to each stimulus concentration. The responses to NaC1 were generally composed of two components, initial phasic and following tonic responses, although the 1 M NaCl stimulus produced a large second peak response at about 20 sec after its application (Fig. 1B). The initial phasic NaCI responses lasted for less than 5 sec and were followed by the tonic responses, whose amplitudes were relatively constant for 2 min or more except for the gradual decrease of the tonic responses to 1 M NaC1. As Brain Research, 34 (1971) 385-388
386
SHORT COMMUNICATIONS
B
A
100- ~ % o.5M NaCI
1
NaCI
2osec
lo'3M Q-HCI
3 5 ~ 0
•
D O ,'0
C
-~
1011% ~.Acetic acid
100I ~'HCI ••"10 -410 -210 -3M
4[~
•" 10-2 M A 5.10 -3
~\\
o 10-5 ~ =
~'
'
lb
'
Io' 8EC
'
i'o
'
SEC :~o
'
Fig. 1. Time course of the whole glossopharyngeal nerve responses to continuous flow of taste solutions over the tongue. A, Integrated responses to 3 kinds of stimuli, obtained from 3 different preparations. In each record the stimulus mark is shown as a thick line below the integrated response. The time calibration shown in record 1 is only for NaC1 response, and that in record 3 is for Q-HCI and acetic acid responses. B-D, Amplitudes of integrated responses (ordinates) plotted against time after the application of stimulus (abscissae). The amplitudes are expressed as percentage of initial maximal peaks to each concentration. The time of the peaks is shown as zero sec. Each symbol is the average value from 4 to 8 preparations. the NaCI c o n c e n t r a t i o n was increased, the tonic response amplitude expressed as the percentage of the initial phasic response clearly increased. O n the other hand, the initial peak response to Q-HCI or acetic acid decreased exponentially with time, a n d only small tonic responses were observed 15-20 sec after the peak (Fig. IC, D). The percentage of the steady response amplitudes tended to increase as the stimulus c o n c e n t r a t i o n was increased. But the difference in the amplitudes was small over a large range o f concentrations. The response amplitudes 20 sec after the initial peaks were reduced to 1 . 5 M . 5 ~ for 0.00014).01 M Q-HCI a n d to 5-13.5~o for 0.001-0.01 M acetic acid. These reduction rates were m u c h larger than the 21-86 ~ for 0.25-1 M NaCI stimuli for the same time course. The steady responses to Q-HC1 or acetic acid Brain Research, 34 (1971) 385-388
387
SHORT COMMUNICATIONS
o & •
0
I 0
Acetic acid NaCI Q-HCI
I
I
I I0
I
I
I
TIME AFTER PEAK
I 20
I
I
I
I
30 SEC
Fig. 2. Time course of the receptor potentials of single taste cells elicited by 3 kinds of taste stimuli. Amplitudes of the potentials are expressed as percentage of the peak height. The time of peak response is given as zero sec. Stimulus solutions: 0.5 M NaCl, 0.004 M Q-HCI and 0.016 M acetic acid. Each symbol is the average value from 5 to 6 taste cells.
were different from preparation to preparation, and sometimes no steady response appeared. This means that the tonic response for either Q-HC1 or acetic acid was unstable. On the contrary, the tonic response for NaC1 was very stable. So that the afferent neural activity could be compared with the receptor potential of the taste cells, microelectrodes were inserted into the cells. Fig. 2 shows the mean time course of intracellular receptor potentials to 0.5 M NaCl, 0.004 M Q-HC1 and 0.016 M acetic acid. Since it was difficult to maintain penetration of the cells for a long time, the responses to only these 3 kinds of solution were studied. The potentials to all solutions did not show the phasic component as seen in the afferent responses, and the amplitudes declined much more slowly than the afferent neural activity. The reduction rates of the receptor potentials 20 sec after the peak amplitudes were 72 ~ for NaC1, 46 ~ for Q-HC1 and 82 ~ for acetic acid. On the other hand, the reduction rates of the afferent responses to the same concentrations were 4 7 ~ for NaCl, 4 ~ for Q-HC1 and 13 ~ for acetic acid, as estimated from Fig. 1B, C and D. From a comparison of both groups of values, it is clear that the rate of decline of the gustatory afferent responses after peak response was faster than that of the receptor potentials. It is well known that the frequency of nerve impulses is proportional to the receptor or generator potentials s-10. Since, in the present experiments, such a proportional relationship was not found between the gustatory afferent discharges and the receptor potentials of the taste cells, chemical synapses located between the taste cells and axon endings may play an important role in the formation of the initial phasic and following tonic components of the afferent discharges. It is concluded that gustatory neural adaptation is mainly due to the adaptation properties of the synaptic potential and the impulse-generating membrane activity rather than the receptor potential. These properties of NaCl-sensitive fibers are probably different from those Brain Research, 34 (1971) 385-388
388
SHORT
COMMUNICATIONS
o f Q - H C 1 o r acetic acid-sensitive fibers, b e c a u s e the t i m e c o u r s e o f the d i s c h a r g e s o f t h e t w o g r o u p s o f fibers differed. I t h a n k P r o f . M . I c h i o k a f o r v a l u a b l e suggestions.
Department of Physiology, School of Dentistry, Tokyo Medical and Dental University, Tokyo (Japan)
TOSHIHIDE SATO*
1 BEIDLER, L. M., Properties of chemoreceptors of tongue of rat, J. Neurophysiol., 16 (1953) 595607. 2 DEHAN, R. S., AND GRAZIADEI, P. P. C., Functional anatomy of frog's taste organs, Experientia (Basel), 27 (1971) 823-825. 3 DEHAN, R. S., AND GRAZIADEI,P. P. C., Innervation of frog's taste organ, Histochemical studies, J. Neurobiol., in press. 4 DIAMONT, H., AND ZOTTERMAN, Y., A comparative study on the neural and psychophysical response to taste stimuli. In C. PFAFFMANN(Ed.), Olfaction and Taste, 111, Rockefeller Univ. Press, New York, 1969, pp. 428435. 5 FISHMAN,I. Y., Single fiber gustatory impulses in rat and hamster, J. cell. comp. Physiol., 49 (1957) 319-334. 6 HARPERN, B. P., Chemical coding - - temporal pattern. In Y. ZOTTERMAN(Ed.), Olfaction and Taste, I, Pergamon, Oxford, 1963, pp. 275-284. 7 ICHIOKA,M., KONDO, Y., AND SAKAMOTO,M., Nerve impulse count summator, Igaku no Ayumi, 54 (1965) 609-612. (In Japanese.) 8 KATZ, B., Depolarization of sensory terminals and initiation of impulses in the muscle spindle, J. Physiol. (Lond.), 111 (1950) 261-282. 9 MOR1TA, H., Electrical sign of taste receptor activity. In C. PFAFFMANN(Ed.), Olfaetion and Taste, IIl, Rockefeller Univ. Press, New York, 1969, pp. 370-381. 10 MOR1TA,H., AND YAMASHITA,S., Further studies on the receptor potential of chemoreceptors of the blowfly, Mere. Fac. Sci. Kyushu Univ. Set. E (Biol.), 4 (1966) 83-93. l l PFAEFMANN,C., AND POWERS, J. B., Partial adaptation of taste, Psychon. Sci., 1 (1964)41-42. 12 SATO, T., The response of frog taste cell (Rana nigromaculata and Rana catesbeiana), Experientia (Basel), 25 (1969) 709-710. (Accepted August 20th, 1971)
* Present address: Department of Biological Science, Unit 1, Florida State University, Tallahassee: Fla. 32306, U.S.A.
Brain Research, 34 (1971) 385-388