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Chemosensitive neurons within the area postrema of the rat
Neuroscience Letters, 55 (1985) 137-140
137
Elsevier Scientific Publishers Ireland Ltd.
NSL 03217
CHEMOSENSITIVE NEURONS WITHIN THE AREA POSTREMA OF T H E R A T
A K I R A A D A C H I * and M O T O I K O B A S H I
Department of Physiology, Okayarna University Dental School, 2-5-1 Shikata-cho, Okayama 700 (Japan) (Received September 10th, 1984; Revised version received J a n u a r y 7th, 1985; Accepted January 10th, 1985)
Key words." area postrema - glucose-responsive neuron - sodium-responsive neuron
It is demonstrated that some neurons within the area postrema, if not all, are responsive to glucose or sodium ions applied topically by means of microelectro-osmotic techniques. Glucose-responsive neurons displayed a marked decrease in the discharge rate in response to the topical application of glucose. Two different types of sodium-responsive neurons were observed; one was characterized by increasing the frequency o f the discharges responding to microiontophoretic application of Na +, while the other showed the opposite response by decreasing the discharge rate in response to the same stimulation. They may serve enteroceptors in response to changes in the glucose or sodium concentrations of cerebrospinal fluid (CSF) or blood.
The function of the area postrema (AP) has been elucidated as a chemoreceptor trigger zone for vomiting, on the basis of an extensive series of physiological experiments [5]. Evidence suggesting other functions for the AP has grown in abundance recently. Among them, roles for this structure on the control o f food intake and on osmoreception [6, 7] are worthy of notice, since the A P has demonstrated neural and vascular connections [12] with the nucleus of the solitary tract (NTS) which receives hepatic glucose as well as sodium (osmo-) responsive afferents [I-4, 10]. However, these functions have been postulated, more or less speculatively for the AP, based on the results obtained by the recording of mass electrical activity or ablation experiments on the AP. The present study is an attempt to demonstrate chemosensitive neurons within the AP. Adult male Sprague-Dawley rats (Charles River), weighing 300-400 g were used. Under urethane-chloralose anesthesia [urethane (0.8 g/kg), chloralose (65 mg/kg) i.p.], the animals were mounted on a stereotaxic apparatus after jugular and carotid catheterizations. The occipital bone was removed, and the AP was exposed by partial decerebellation. A multibarrel glass pipette electrode (7 channels) was used for *Author for correspondence.
0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
138
Na* N a C I 60nA (ca)
A
,-Glucose-, Glucose 20 40 60nA (ca)
60 tar)
ul 3 Q.
E
0 3rain
B
Glucose(60 nA )
200 pV
5s Fig. I. A: pen-recording of the firing rate versus time of neuron within the AP which is responsive to the topical application of glucose by means of micro-electro-osmotic techniques. Small horizontal bars, application periods; numerals, intensities of constant electric current in hA. A marked decrease in the firing rate is recognized in response to the topical application of glucose at an intensity of 20, 40 or 60 nA. Note no significant change in the firing rate during the topical iontophoretic administration of Na + at an intensity of 60 nA. lntracarotid infusion of glucose (ca) induces a weak suppression in the firing rate after a long latency. The same infusion of hypertonic NaCI (1 M) elicits no response. B: oscillograph record of the discharges before, during (indicated by horizontal bar) and after the lopical administration of glucose at an intensity of 60 nA.
topical administration of glucose or Na + as well as for the recording of unit activities. Recording sites were marked by means of a dye injection (2% pontamine sky blue) determined histologically later. Further details on the micro-electroosmotic techniques for these topical applications have been reported by O o m u r a et al. [11]. The electrophysiological examination revealed that some neurons within the AP were responsive to the topical application of glucose by means of the micro-electroosmotic technique. As shown in Fig. 1, the response is characterized by a decrease in the discharge rate during topical application of glucose. Because the glucose which fills the micropipette is dissolved in isotonic saline, to eject the non-electrolyte by electro-osmotic force, the effects of Na + must always be examined in such a case. Fig. 1A clearly presents no effect of Na + on the activity of this neuron. Meanwhile, the application of glucose results in a marked suppression of the discharge rate. The longer latency and the aftereffect shown in Fig. 1B also provide evidence
139
"'30-
. "N~ , 50 60 50 6 0 n A
B 30-
LO
i.n
ul
i/I
n
Q.
E 0
.-.E 0 2rain
f-Na* .. 20 30 4 0 n A
2rain
Fig. 2. Pen recordings of the firing rate versus time of two different types of sodium-responsiveneurons within the AP. Note the marked increase in the firing rate in response to the topical application of Na + in A and the opposite response in B. Other descriptions as in Fig. 1. that the electric current applied is not effective but that the injected glucose exerts the responses [9]. Intracarotid infusion of glucose toward the brain (300 mM glucose, 16 ~d/s) also causes a lesser long-lasting suppression in the discharge rate. This weak response after a long latency period may be due to an indirect influence of this infusion on the AP, because the medulla oblongata has its main blood supply from the vertebral artery. No effect is produced by an identical infusion of hypertonic saline, so that the response of this neuron is specific for glucose. Records from two different types of sodium-responsive neurons within the A P are illustrated in Fig. 2A, B. Microiontophoretic application of Na + markedly increases the discharge rate of a neuron within the A P as shown in Fig. 2A. Another type of sodium-responsive neuron was also recognized within the AP. This type of sodiumresponsive neuron is characterized by a decrease in the discharge rate in response to the same application of Na + as shown in Fig. 2B. It is interesting that opposite neurons which show an antagonistic response to sodium have been widely recognized not only in the A P but also in the hepatic vagal afferent [1] and the taste nerve [8]. This might reflect c o m m o n biological factors. A m o n g a total of 27 neurons examined in this experiment, 7 responded to the topical administration of glucose in a fashion similar to that presented in Fig. 1. One neuron responded to Na + applied by topical iontophoretic injection in that its firing rate increased during this application (Fig. 2A). Three neurons behaved in the opposite way and their firing rates decreased in response to the same stimulation (Fig. 2B). Finally, it is assumed that these chemosensitive neurons may detect minor changes in the glucose or the sodium concentration of the CSF or the blood and in turn play a role in the regulatory reflexes which contribute to the maintenance of homeostatis. We are indebted to Prof. A. Niijima and Prof. Y. O o m u r a for their advice and
140
encouragement of the project. We also thank Miss Kahori Hirata for typing the manuscript. This work was supported by a research grant from the Ministry of Education of Japan. t Adachi, A., Niijima, A. and Jacobs, H.L., An hepatic osmoreceptor mechanism in the rat: electrophysiological and behavioral studies, Amer. J. Physiol., 231 (1976) 1043-1049. 2 Adachi, A., Electrophysiological study of hepatic vagal projecton to the medulla, Neurosci. Eett., 24 (1981) 19-23. 3 Adachi, A., Projection of the hepatic vagal nerve in the medulla oblongata, J. Auton. Nerv+ Syst., 10 (1984) 287-293. 4 Adachi, A., Shimizu, N., Oomura, Y. and Kobashi, M., Convergence of hepatoportal glucosesensitive afferent signals to glucose-sensitive units within the nucleus of the solitary tract, Neurosci. Lett., 46 (1984) 215-218. 5 Borison, H.L., Area postrema: chemoreceptor trigger zone for vomiting - is that all?, Kile Sci., 14 (1974) 1807-1817. 6 Carlisle, H.J. and Reynolds, R.W., Effect of amphetamine on food intake in rats with brain-stem lesions, Amer. J. Physiol., 201 (1961) 965-967. 7 Clemente, C.D., Sutin ,J. and Silverstone, J.T+, Changes in electrical activity of the medulla on the intravenous injection of hypertonic solutions, Amer. J. Physiol., 188 (1957) 193-198. 8 Cohen, M.J., Hagiwara, S. and Zotterman, Y., The response spectrum of taste fibers in the cat: a single fiber analysis, Acta Physiol. Scand., 33 (1955) 316-332. 9 Curtis, D.R. and Koizumi, K., Chemical transmitter substances in brain stem of cat, J. Neurophysiol., 24 (1961) 80-90. 10 Niijima, A., Glucose-sensitive afferent nerve fibers in the hepatic branch of the vagus nerve in the guinea-pig, J. Physiol. (Lond.), 332 (1982) 315-323. 11 Oomura, Y., Ono, T., Ooyama, H. and Wayner, M.J., Glucose and osmoreceptive neurons of the rat hypothalamus, Nature (Lond.), 222 (1969) 282-284. 12 Roth, G.I. and Yamamoto, W.S., The microcirculation of the area postrema in the rat, J. Comp. Neurol., 133 (1968) 329-340.
137
Elsevier Scientific Publishers Ireland Ltd.
NSL 03217
CHEMOSENSITIVE NEURONS WITHIN THE AREA POSTREMA OF T H E R A T
A K I R A A D A C H I * and M O T O I K O B A S H I
Department of Physiology, Okayarna University Dental School, 2-5-1 Shikata-cho, Okayama 700 (Japan) (Received September 10th, 1984; Revised version received J a n u a r y 7th, 1985; Accepted January 10th, 1985)
Key words." area postrema - glucose-responsive neuron - sodium-responsive neuron
It is demonstrated that some neurons within the area postrema, if not all, are responsive to glucose or sodium ions applied topically by means of microelectro-osmotic techniques. Glucose-responsive neurons displayed a marked decrease in the discharge rate in response to the topical application of glucose. Two different types of sodium-responsive neurons were observed; one was characterized by increasing the frequency o f the discharges responding to microiontophoretic application of Na +, while the other showed the opposite response by decreasing the discharge rate in response to the same stimulation. They may serve enteroceptors in response to changes in the glucose or sodium concentrations of cerebrospinal fluid (CSF) or blood.
The function of the area postrema (AP) has been elucidated as a chemoreceptor trigger zone for vomiting, on the basis of an extensive series of physiological experiments [5]. Evidence suggesting other functions for the AP has grown in abundance recently. Among them, roles for this structure on the control o f food intake and on osmoreception [6, 7] are worthy of notice, since the A P has demonstrated neural and vascular connections [12] with the nucleus of the solitary tract (NTS) which receives hepatic glucose as well as sodium (osmo-) responsive afferents [I-4, 10]. However, these functions have been postulated, more or less speculatively for the AP, based on the results obtained by the recording of mass electrical activity or ablation experiments on the AP. The present study is an attempt to demonstrate chemosensitive neurons within the AP. Adult male Sprague-Dawley rats (Charles River), weighing 300-400 g were used. Under urethane-chloralose anesthesia [urethane (0.8 g/kg), chloralose (65 mg/kg) i.p.], the animals were mounted on a stereotaxic apparatus after jugular and carotid catheterizations. The occipital bone was removed, and the AP was exposed by partial decerebellation. A multibarrel glass pipette electrode (7 channels) was used for *Author for correspondence.
0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
138
Na* N a C I 60nA (ca)
A
,-Glucose-, Glucose 20 40 60nA (ca)
60 tar)
ul 3 Q.
E
0 3rain
B
Glucose(60 nA )
200 pV
5s Fig. I. A: pen-recording of the firing rate versus time of neuron within the AP which is responsive to the topical application of glucose by means of micro-electro-osmotic techniques. Small horizontal bars, application periods; numerals, intensities of constant electric current in hA. A marked decrease in the firing rate is recognized in response to the topical application of glucose at an intensity of 20, 40 or 60 nA. Note no significant change in the firing rate during the topical iontophoretic administration of Na + at an intensity of 60 nA. lntracarotid infusion of glucose (ca) induces a weak suppression in the firing rate after a long latency. The same infusion of hypertonic NaCI (1 M) elicits no response. B: oscillograph record of the discharges before, during (indicated by horizontal bar) and after the lopical administration of glucose at an intensity of 60 nA.
topical administration of glucose or Na + as well as for the recording of unit activities. Recording sites were marked by means of a dye injection (2% pontamine sky blue) determined histologically later. Further details on the micro-electroosmotic techniques for these topical applications have been reported by O o m u r a et al. [11]. The electrophysiological examination revealed that some neurons within the AP were responsive to the topical application of glucose by means of the micro-electroosmotic technique. As shown in Fig. 1, the response is characterized by a decrease in the discharge rate during topical application of glucose. Because the glucose which fills the micropipette is dissolved in isotonic saline, to eject the non-electrolyte by electro-osmotic force, the effects of Na + must always be examined in such a case. Fig. 1A clearly presents no effect of Na + on the activity of this neuron. Meanwhile, the application of glucose results in a marked suppression of the discharge rate. The longer latency and the aftereffect shown in Fig. 1B also provide evidence
139
"'30-
. "N~ , 50 60 50 6 0 n A
B 30-
LO
i.n
ul
i/I
n
Q.
E 0
.-.E 0 2rain
f-Na* .. 20 30 4 0 n A
2rain
Fig. 2. Pen recordings of the firing rate versus time of two different types of sodium-responsiveneurons within the AP. Note the marked increase in the firing rate in response to the topical application of Na + in A and the opposite response in B. Other descriptions as in Fig. 1. that the electric current applied is not effective but that the injected glucose exerts the responses [9]. Intracarotid infusion of glucose toward the brain (300 mM glucose, 16 ~d/s) also causes a lesser long-lasting suppression in the discharge rate. This weak response after a long latency period may be due to an indirect influence of this infusion on the AP, because the medulla oblongata has its main blood supply from the vertebral artery. No effect is produced by an identical infusion of hypertonic saline, so that the response of this neuron is specific for glucose. Records from two different types of sodium-responsive neurons within the A P are illustrated in Fig. 2A, B. Microiontophoretic application of Na + markedly increases the discharge rate of a neuron within the A P as shown in Fig. 2A. Another type of sodium-responsive neuron was also recognized within the AP. This type of sodiumresponsive neuron is characterized by a decrease in the discharge rate in response to the same application of Na + as shown in Fig. 2B. It is interesting that opposite neurons which show an antagonistic response to sodium have been widely recognized not only in the A P but also in the hepatic vagal afferent [1] and the taste nerve [8]. This might reflect c o m m o n biological factors. A m o n g a total of 27 neurons examined in this experiment, 7 responded to the topical administration of glucose in a fashion similar to that presented in Fig. 1. One neuron responded to Na + applied by topical iontophoretic injection in that its firing rate increased during this application (Fig. 2A). Three neurons behaved in the opposite way and their firing rates decreased in response to the same stimulation (Fig. 2B). Finally, it is assumed that these chemosensitive neurons may detect minor changes in the glucose or the sodium concentration of the CSF or the blood and in turn play a role in the regulatory reflexes which contribute to the maintenance of homeostatis. We are indebted to Prof. A. Niijima and Prof. Y. O o m u r a for their advice and
140
encouragement of the project. We also thank Miss Kahori Hirata for typing the manuscript. This work was supported by a research grant from the Ministry of Education of Japan. t Adachi, A., Niijima, A. and Jacobs, H.L., An hepatic osmoreceptor mechanism in the rat: electrophysiological and behavioral studies, Amer. J. Physiol., 231 (1976) 1043-1049. 2 Adachi, A., Electrophysiological study of hepatic vagal projecton to the medulla, Neurosci. Eett., 24 (1981) 19-23. 3 Adachi, A., Projection of the hepatic vagal nerve in the medulla oblongata, J. Auton. Nerv+ Syst., 10 (1984) 287-293. 4 Adachi, A., Shimizu, N., Oomura, Y. and Kobashi, M., Convergence of hepatoportal glucosesensitive afferent signals to glucose-sensitive units within the nucleus of the solitary tract, Neurosci. Lett., 46 (1984) 215-218. 5 Borison, H.L., Area postrema: chemoreceptor trigger zone for vomiting - is that all?, Kile Sci., 14 (1974) 1807-1817. 6 Carlisle, H.J. and Reynolds, R.W., Effect of amphetamine on food intake in rats with brain-stem lesions, Amer. J. Physiol., 201 (1961) 965-967. 7 Clemente, C.D., Sutin ,J. and Silverstone, J.T+, Changes in electrical activity of the medulla on the intravenous injection of hypertonic solutions, Amer. J. Physiol., 188 (1957) 193-198. 8 Cohen, M.J., Hagiwara, S. and Zotterman, Y., The response spectrum of taste fibers in the cat: a single fiber analysis, Acta Physiol. Scand., 33 (1955) 316-332. 9 Curtis, D.R. and Koizumi, K., Chemical transmitter substances in brain stem of cat, J. Neurophysiol., 24 (1961) 80-90. 10 Niijima, A., Glucose-sensitive afferent nerve fibers in the hepatic branch of the vagus nerve in the guinea-pig, J. Physiol. (Lond.), 332 (1982) 315-323. 11 Oomura, Y., Ono, T., Ooyama, H. and Wayner, M.J., Glucose and osmoreceptive neurons of the rat hypothalamus, Nature (Lond.), 222 (1969) 282-284. 12 Roth, G.I. and Yamamoto, W.S., The microcirculation of the area postrema in the rat, J. Comp. Neurol., 133 (1968) 329-340.