342
Brain Research, 105 (1976) 342-346 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
The activity of identified secondary vestibular neurons during nystagmus
K. GRANT*, J. P. GUI~RITAUD, G. HORCHOLLE-BOSSAVIT AND S. TYC-DUMONT Laboratoire de Physiologie, C.H.U. St. Antoine, 75571 Paris (France)
(Accepted December 15th, 1975)
The mechanism responsible for the generation of vestibular nystagmus is still unknown although numerous studies have been devoted to the problem3,11. Rhythmic patterns correlated with the two phases of vestibular nystagmus have been observed in the vestibular nucleP, 7, in the brain stem reticular formation 4 and within the abducens nucleus 6. Secondary vestibular efferent axons recorded within the abducens nucleus have also been shown to fire in synchrony with one or other of the two phases of the nystagmic cycle 1°. These findings lead to two different hypotheses. The first postulates that the transformation of the tonic vestibular input into nystagmic oculomotor discharges takes place beyond the secondary vestibular neurons, via a polysynaptic circuit 8. In contrast, the second hypothesis attributes a key role to the secondary neurons. These first vestibular relay cells are thought to be fired rhythmically during nystagmus either by way of an agonist-antagonist inhibitory action between the bilateral vestibular nucleil2,14, or by a fast phase generator sensitive to bilateral vestibular information 1. This latter hypothesis is supported by the fact that secondary vestibular axons recorded within the abducens nucleus 1° and the trochlear nucleus 1 showed a rhythmic pattern related to the nystagmic cycle. However, no direct intracellular recordings from the soma of the secondary vestibular neurons sending their axons to the contralateral abducens nucleus have yet been provided during nystagmic events. In the experiments reported here, the intracellular activity of identified secondary vestibular neurons was recorded in the medial vestibular nucleus and their discharge pattern was examined during nystagmus. The experiments were performed on adult, alert, enc6phale isol6 cats. The spinal cord was sectioned at C1 under deep Fluothane anesthesia and artificial respiration. Stimulating electrodes were placed on the nerve originating from the ampulla of the horizontal semi-circular canal s. With the animal in the prone position, the abducens nerve was dissected free from the lateral rectus muscle and mounted on bipolar silver electrodes for stimulation and recording. Antidromic stimulation of the efferent secondary vestibular axons was performed within the abducens nucleus using a bipolar steel microelectrode introduced stereotaxically into the motor nucleus, the precise position of which was checked by recording the antidromic field potential following * French Exchange Scholar of the Medical Research Council.
343 stimulation of the abducens nerve. Glass microelectrodes, for intracellular recording or stimulation, were introduced into the medial vestibular nucleus according to stereotaxic coordinates at an angle of 30 °. These were filled with either 3 M KC1 or Procion yellow. After electrophysiological identification, the injection of dye allowed precise histological localization of the impaled cell. All the experiments were performed on an intact brain and with the skull resealed with wax in order to prevent damage to the structures located beneath the IVth ventricle. The location of the medial vestibular nucleus was ascertained by the characteristic field potential evoked by a single shock to the vestibular branch of the horizontal canaP 3. In 21 cats, 161 vestibular cells recorded intracellularly were tested for identification as second-order neurons sending their axons to the abducens motoneurons. The required criteria were the presence of both monosynaptic excitation following vestibular nerve stimulation and an antidromic invasion following stimulation of the abducens nucleus. In this sample of 161 impaled cells, 26 were identified as secondary vestibular neurons projecting to the contralateral abducens, as shown in Fig. 1. The intracellular recordings showed the antidromic activation (mean latency: 0.23 msec) of the neuron (Fig. 1A) with an infiexion on the rising phase indicating the sequential
-53mY
4
D
Fig. I. Identification of a secondary neuron in the medial vestibular nucleus, recorded intracellularly, showing direct activation via microelectrode (black horizontal bar). A: antidromic invasion following stimulation of the contralateral abducens nucleus (A). B: monosynaptic excitation by ipsilate'ral vestibular nerve stimulation at threshold (~). C: as B, with stimulation increased to twice threshold. Calibrations: upper trace 5 mV, lower trace 50 mV; time 1 msec. D: reconstruction, from light micrographs, of a secondary vestibular neuron injected with Procion yellow. Lower left: semischematic diagram of identified cell location showing soma (black dot) and area of dendritic field (shaded triangle). MLF, medial longitudinal fasciculus; M, medial vestibular nucleus; L, lateral vestibular nucleus; S, superior vestibular nucleus.
344
,~i ~ i
........
i~ ¸¸~¸!¸~¸¸~¸¸¸¸i
I
t _ ....... ~J J
E
't
Fig. 2. Discharge pattern of an identified secondary vestibular neuron recorded during vestibular nystagmus. A: intracellular stimulation (black bar) followed by antidromic invasion from stimulation of the contralateral abducens nucleus. B: fast sweep showing two stimulations during nystagmogenic stimulation (200 cycles/sec) of the ipsilateral vestibular nerve. Note that the first, monosynaptic, discharge is often followed by a second action potential of more variable latency and form. C: continuous record from the secondary vestibular neuron (upper trace), together with contralateral abducens nerve discharge (lower trace). Duration of nystagmogenic stimulation (200 cycles/sec) at threshold indicated by black arrow. D and E: as C, with stimulus strength raised to suprathreshold for nystagmus. The firing pattern show no modulation related to the nystagmic cycles. Calibrations: A and B, 5 mV and 50 mV; time: A and B, 2 msec, C-E, 500 msec.
345 invasion of the initial segment (IS spike) and the excitation of the soma (SD spike). In Fig. 1B, the same neuron was monosynaptically activated by stimulation at threshold of the ipsilateral vestibular nerve. The monosynaptic excitation following abovethreshold vestibular stimulation (Fig. 1C) evoked a full spike discharge with latencies that ranged from 0.8 to 1.5 msec. The l atencies of the monosynaptic EPSP ranged from 0.6 to 1.1 msec and were in accordance with those described by others 9,1a,16. In some instances the monosynaptic action potential was followed by another spike discharge, illustrated in Fig. 2B, with a variable latency and form which were related to the intensity of the stimulation. The remaining 135 identified neurons were not secondary neurons projecting to the contralateral abducens nucleus but were vestibular neurons, some of which were involved in nystagmic activity. The behavior of these cells will be described in a paper at present in preparation. After identification, the firing pattern of the 26 secondary vestibular neurons was observed during vestibular nystagmus produced by a repetitive electrical stimulation (200 cycles/sec) of the vestibular nerve. An example of the typical response of a secondary neuron is illustrated Fig. 2C-E. The neuronal discharge was recorded simultaneously with the activity of the contralateral abducens nerve showing nystagmic cycles. The neuron followed the 200 cycles/sec stimulation, being fired by each shock (Fig. 2B). In no case was a modulation of the rate of discharge observed which could be related to either phase of the nystagmic cycle recorded from the nerve. Following electrophysiological observations, some of the secondary neurons were injected with Procion yellow (Fig. 1D), allowing the reconstruction of the cell. These soma were located in the medial and rostral area of the medial vestibular nucleus and had an approximately triangular form with relatively long dendrites. In one case the supposed axon could be traced over 500 btm and in another over 1200 #m, running ventromedially in the medial vestibular nucleus and through the ipsilateral abducens nucleus to the medial longitudinal fasciculus. These findings indicate that the identified secondary vestibular neurons sending their axons to the contralateral abducens motoneurons exhibit no rhythmic pattern during vestibular nystagmus. They are excited by each single shock of the repetitive vestibular stimulation. This observation suggests that the soma of the first central relay cells located in the medial vestibular nucleus is not an important trigger zone for the genesis of nystagmus. Our results are in contrast to those reported by Maeda e t al. 1° who showed that the axons of these secondary neurons fired rhythmically in synchrony with the nystagmic discharges. However, this discrepancy might focus attention towards the capability of an axonal system to mediate integrative processes2,15. 1 BAKER, R., AND BERTHOZ, A., Organization of vestibular nystagmus in oblique oculomotor system, J. Neurophysiok, 37 (1974) 195-217. 2 BARRON,D. H., AND MATTHEWS,B. H. C., Intermittent conduction in the spinal cord, J. Physiol. (Lond.), 85 (1939) 73-103. 3 COHEN, B., The vestibulo-ocular reflex arc. In H. H. KORNHOBER(Ed.), Handbook of Sensory Physiology, Vol. 6, Pt. 1, Springer Verlag, Berlin, 1974, pp. 477-540.
346 4 DUENSING,F., UND SCHAEFER, K. P., Die locker gekoppelten Neurone der Formatio reticularis des Rhombencephalons beim vestibul~iren Nystagmus, Arch. Psychiat. Nervenkr., 196 (1957) 402-420. 5 DUENSING,F., UND SCHAEFER,K. P., Die Aktivit~it einzelner Neurone im Bereich der Vestibulariskerne bei Horizontalbeschleunigungen unter besonderer Berticksichtigung des vestibul/iren Nystagmus, Arch. Psychiat. Nervenkr., 198 (1958) 225-252. 6 GOGAN, P., GUERITAUD, J. P., HORCHOLLE-BOSSAVIT, G., AND TYc-DUMONT, S., Inhibitory nystagmic interneurons. Physiological and anatomical identification within the abducens nucleus, Brain Research, 59 (1973) 410~16. 7 HORCHOLLE, G., ET TYc-DuMONT, S., Activit6s unitaires des neurones vestibulaires et oculomoteurs au cours du nystagmus, Exp. Brain Res., 5 (1968) 16-31. 8 HORCHOLLE, G., ET TYc-DUMONT, S., Ph6nom6nes synaptiques du nystagmus, Exp. Brain Res., 8 (1969) 201-218. 9 KAWAI, N., ITO, M., AND NOZUE, M., Postsynaptic influences on the vestibular non-Deiters nuclei from primary vestibular nerve, Exp. Brain Res., 8 (1969) 190-200. l0 MAEDA, M., SHIMAZU, H., AND SHINODA, Y., Rhythmic activities of secondary vestibular efferent fibers recorded within the abducens nucleus during vestibular nystagmus, Brain Research, 34 (1971) 361-365. 11 PRECHT, W., The physiology of the vestibular nuclei. In H. H. KORNHUBER (Ed.), Handbook of Sensory Physiology, VoL 6, Pt. I, Springer Verlag, Berlin, 1974, pp. 353-416. 12 SHIMAZU, H., Vestibulo-oculomotor relations: dynamic responses. In A. BRODAL AND O. POMPEIANO (Eds.), Basic Aspects of Central Vestibular Mechanisms, Progr. Brain Res., VoL 37, Elsevier, Amsterdam, 1972, pp. 493-506. 13 SHIMAZU,H., AND PRECHT, W., Tonic and kinetic responses of cat's vestibular neurons to horizontal angular acceleration, J. Neurophysiol., 28 (1965) 991-1013. 14 SHIMAZU, H., AND PRECHT, W., Inhibition of central vestibular neurons from the contralateral labyrinth and its mediating pathway, J. Neurophysiol., 29 (1966) 467-492. 15 WAXMAN,S. G., Regional differentiation of the axon: a review with special reference to the concept of the multiplex neuron, Brain Research, 47 (1972) 269-288. 16 WILSON, V. J., WYLIE, R. M., AND MARCO, L. A., Synaptic inputs to cells in the medial vestibular nucleus, J. Neurophysiol., 31 (1968) 176--185.
Brain Research, 105 (1976) 342-346 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
The activity of identified secondary vestibular neurons during nystagmus
K. GRANT*, J. P. GUI~RITAUD, G. HORCHOLLE-BOSSAVIT AND S. TYC-DUMONT Laboratoire de Physiologie, C.H.U. St. Antoine, 75571 Paris (France)
(Accepted December 15th, 1975)
The mechanism responsible for the generation of vestibular nystagmus is still unknown although numerous studies have been devoted to the problem3,11. Rhythmic patterns correlated with the two phases of vestibular nystagmus have been observed in the vestibular nucleP, 7, in the brain stem reticular formation 4 and within the abducens nucleus 6. Secondary vestibular efferent axons recorded within the abducens nucleus have also been shown to fire in synchrony with one or other of the two phases of the nystagmic cycle 1°. These findings lead to two different hypotheses. The first postulates that the transformation of the tonic vestibular input into nystagmic oculomotor discharges takes place beyond the secondary vestibular neurons, via a polysynaptic circuit 8. In contrast, the second hypothesis attributes a key role to the secondary neurons. These first vestibular relay cells are thought to be fired rhythmically during nystagmus either by way of an agonist-antagonist inhibitory action between the bilateral vestibular nucleil2,14, or by a fast phase generator sensitive to bilateral vestibular information 1. This latter hypothesis is supported by the fact that secondary vestibular axons recorded within the abducens nucleus 1° and the trochlear nucleus 1 showed a rhythmic pattern related to the nystagmic cycle. However, no direct intracellular recordings from the soma of the secondary vestibular neurons sending their axons to the contralateral abducens nucleus have yet been provided during nystagmic events. In the experiments reported here, the intracellular activity of identified secondary vestibular neurons was recorded in the medial vestibular nucleus and their discharge pattern was examined during nystagmus. The experiments were performed on adult, alert, enc6phale isol6 cats. The spinal cord was sectioned at C1 under deep Fluothane anesthesia and artificial respiration. Stimulating electrodes were placed on the nerve originating from the ampulla of the horizontal semi-circular canal s. With the animal in the prone position, the abducens nerve was dissected free from the lateral rectus muscle and mounted on bipolar silver electrodes for stimulation and recording. Antidromic stimulation of the efferent secondary vestibular axons was performed within the abducens nucleus using a bipolar steel microelectrode introduced stereotaxically into the motor nucleus, the precise position of which was checked by recording the antidromic field potential following * French Exchange Scholar of the Medical Research Council.
343 stimulation of the abducens nerve. Glass microelectrodes, for intracellular recording or stimulation, were introduced into the medial vestibular nucleus according to stereotaxic coordinates at an angle of 30 °. These were filled with either 3 M KC1 or Procion yellow. After electrophysiological identification, the injection of dye allowed precise histological localization of the impaled cell. All the experiments were performed on an intact brain and with the skull resealed with wax in order to prevent damage to the structures located beneath the IVth ventricle. The location of the medial vestibular nucleus was ascertained by the characteristic field potential evoked by a single shock to the vestibular branch of the horizontal canaP 3. In 21 cats, 161 vestibular cells recorded intracellularly were tested for identification as second-order neurons sending their axons to the abducens motoneurons. The required criteria were the presence of both monosynaptic excitation following vestibular nerve stimulation and an antidromic invasion following stimulation of the abducens nucleus. In this sample of 161 impaled cells, 26 were identified as secondary vestibular neurons projecting to the contralateral abducens, as shown in Fig. 1. The intracellular recordings showed the antidromic activation (mean latency: 0.23 msec) of the neuron (Fig. 1A) with an infiexion on the rising phase indicating the sequential
-53mY
4
D
Fig. I. Identification of a secondary neuron in the medial vestibular nucleus, recorded intracellularly, showing direct activation via microelectrode (black horizontal bar). A: antidromic invasion following stimulation of the contralateral abducens nucleus (A). B: monosynaptic excitation by ipsilate'ral vestibular nerve stimulation at threshold (~). C: as B, with stimulation increased to twice threshold. Calibrations: upper trace 5 mV, lower trace 50 mV; time 1 msec. D: reconstruction, from light micrographs, of a secondary vestibular neuron injected with Procion yellow. Lower left: semischematic diagram of identified cell location showing soma (black dot) and area of dendritic field (shaded triangle). MLF, medial longitudinal fasciculus; M, medial vestibular nucleus; L, lateral vestibular nucleus; S, superior vestibular nucleus.
344
,~i ~ i
........
i~ ¸¸~¸!¸~¸¸~¸¸¸¸i
I
t _ ....... ~J J
E
't
Fig. 2. Discharge pattern of an identified secondary vestibular neuron recorded during vestibular nystagmus. A: intracellular stimulation (black bar) followed by antidromic invasion from stimulation of the contralateral abducens nucleus. B: fast sweep showing two stimulations during nystagmogenic stimulation (200 cycles/sec) of the ipsilateral vestibular nerve. Note that the first, monosynaptic, discharge is often followed by a second action potential of more variable latency and form. C: continuous record from the secondary vestibular neuron (upper trace), together with contralateral abducens nerve discharge (lower trace). Duration of nystagmogenic stimulation (200 cycles/sec) at threshold indicated by black arrow. D and E: as C, with stimulus strength raised to suprathreshold for nystagmus. The firing pattern show no modulation related to the nystagmic cycles. Calibrations: A and B, 5 mV and 50 mV; time: A and B, 2 msec, C-E, 500 msec.
345 invasion of the initial segment (IS spike) and the excitation of the soma (SD spike). In Fig. 1B, the same neuron was monosynaptically activated by stimulation at threshold of the ipsilateral vestibular nerve. The monosynaptic excitation following abovethreshold vestibular stimulation (Fig. 1C) evoked a full spike discharge with latencies that ranged from 0.8 to 1.5 msec. The l atencies of the monosynaptic EPSP ranged from 0.6 to 1.1 msec and were in accordance with those described by others 9,1a,16. In some instances the monosynaptic action potential was followed by another spike discharge, illustrated in Fig. 2B, with a variable latency and form which were related to the intensity of the stimulation. The remaining 135 identified neurons were not secondary neurons projecting to the contralateral abducens nucleus but were vestibular neurons, some of which were involved in nystagmic activity. The behavior of these cells will be described in a paper at present in preparation. After identification, the firing pattern of the 26 secondary vestibular neurons was observed during vestibular nystagmus produced by a repetitive electrical stimulation (200 cycles/sec) of the vestibular nerve. An example of the typical response of a secondary neuron is illustrated Fig. 2C-E. The neuronal discharge was recorded simultaneously with the activity of the contralateral abducens nerve showing nystagmic cycles. The neuron followed the 200 cycles/sec stimulation, being fired by each shock (Fig. 2B). In no case was a modulation of the rate of discharge observed which could be related to either phase of the nystagmic cycle recorded from the nerve. Following electrophysiological observations, some of the secondary neurons were injected with Procion yellow (Fig. 1D), allowing the reconstruction of the cell. These soma were located in the medial and rostral area of the medial vestibular nucleus and had an approximately triangular form with relatively long dendrites. In one case the supposed axon could be traced over 500 btm and in another over 1200 #m, running ventromedially in the medial vestibular nucleus and through the ipsilateral abducens nucleus to the medial longitudinal fasciculus. These findings indicate that the identified secondary vestibular neurons sending their axons to the contralateral abducens motoneurons exhibit no rhythmic pattern during vestibular nystagmus. They are excited by each single shock of the repetitive vestibular stimulation. This observation suggests that the soma of the first central relay cells located in the medial vestibular nucleus is not an important trigger zone for the genesis of nystagmus. Our results are in contrast to those reported by Maeda e t al. 1° who showed that the axons of these secondary neurons fired rhythmically in synchrony with the nystagmic discharges. However, this discrepancy might focus attention towards the capability of an axonal system to mediate integrative processes2,15. 1 BAKER, R., AND BERTHOZ, A., Organization of vestibular nystagmus in oblique oculomotor system, J. Neurophysiok, 37 (1974) 195-217. 2 BARRON,D. H., AND MATTHEWS,B. H. C., Intermittent conduction in the spinal cord, J. Physiol. (Lond.), 85 (1939) 73-103. 3 COHEN, B., The vestibulo-ocular reflex arc. In H. H. KORNHOBER(Ed.), Handbook of Sensory Physiology, Vol. 6, Pt. 1, Springer Verlag, Berlin, 1974, pp. 477-540.
346 4 DUENSING,F., UND SCHAEFER, K. P., Die locker gekoppelten Neurone der Formatio reticularis des Rhombencephalons beim vestibul~iren Nystagmus, Arch. Psychiat. Nervenkr., 196 (1957) 402-420. 5 DUENSING,F., UND SCHAEFER,K. P., Die Aktivit~it einzelner Neurone im Bereich der Vestibulariskerne bei Horizontalbeschleunigungen unter besonderer Berticksichtigung des vestibul/iren Nystagmus, Arch. Psychiat. Nervenkr., 198 (1958) 225-252. 6 GOGAN, P., GUERITAUD, J. P., HORCHOLLE-BOSSAVIT, G., AND TYc-DUMONT, S., Inhibitory nystagmic interneurons. Physiological and anatomical identification within the abducens nucleus, Brain Research, 59 (1973) 410~16. 7 HORCHOLLE, G., ET TYc-DuMONT, S., Activit6s unitaires des neurones vestibulaires et oculomoteurs au cours du nystagmus, Exp. Brain Res., 5 (1968) 16-31. 8 HORCHOLLE, G., ET TYc-DUMONT, S., Ph6nom6nes synaptiques du nystagmus, Exp. Brain Res., 8 (1969) 201-218. 9 KAWAI, N., ITO, M., AND NOZUE, M., Postsynaptic influences on the vestibular non-Deiters nuclei from primary vestibular nerve, Exp. Brain Res., 8 (1969) 190-200. l0 MAEDA, M., SHIMAZU, H., AND SHINODA, Y., Rhythmic activities of secondary vestibular efferent fibers recorded within the abducens nucleus during vestibular nystagmus, Brain Research, 34 (1971) 361-365. 11 PRECHT, W., The physiology of the vestibular nuclei. In H. H. KORNHUBER (Ed.), Handbook of Sensory Physiology, VoL 6, Pt. I, Springer Verlag, Berlin, 1974, pp. 353-416. 12 SHIMAZU, H., Vestibulo-oculomotor relations: dynamic responses. In A. BRODAL AND O. POMPEIANO (Eds.), Basic Aspects of Central Vestibular Mechanisms, Progr. Brain Res., VoL 37, Elsevier, Amsterdam, 1972, pp. 493-506. 13 SHIMAZU,H., AND PRECHT, W., Tonic and kinetic responses of cat's vestibular neurons to horizontal angular acceleration, J. Neurophysiol., 28 (1965) 991-1013. 14 SHIMAZU, H., AND PRECHT, W., Inhibition of central vestibular neurons from the contralateral labyrinth and its mediating pathway, J. Neurophysiol., 29 (1966) 467-492. 15 WAXMAN,S. G., Regional differentiation of the axon: a review with special reference to the concept of the multiplex neuron, Brain Research, 47 (1972) 269-288. 16 WILSON, V. J., WYLIE, R. M., AND MARCO, L. A., Synaptic inputs to cells in the medial vestibular nucleus, J. Neurophysiol., 31 (1968) 176--185.
