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The properties of lingually induced IPSPs in the masseteric motoneurons
648
Brain Research, 168 (1979) 648-651 © Elsevier/North-Holland Biomedical Press
The properties of lingually induced IPSPs in the masseteric motoneurons
MITSURU TAKATA and SATOSHI FUJITA
Department of Physiology, School of Dentistry, Tokushima University, 3 Kuramoto-cho, Tokushima (Japan) (Accepted February 1st, 1979)
In intracellular recordings from the masseteric motoneurons (Mass.Mns) it has been reported that inhibitory postsynaptic potentials (IPSPs) evoked by lingual nerve stimulation were the composite of an early and a late component 1,8,4. With regard to an early IPSP it was found that the early IPSP was blocked by intravenous injection of strychnine and was reversed to a depolarizing potential by displacement of the membrane potential to hyperpolarization 3,6. However, the properties of the late IPSP are still unknown. In order to examine the properties of the late IPSP, the effect of membrane polarization on the late IPSP and changes of membrane conductance during the late 1PSP were explored and are described in the present paper. In addition, the synaptic linkage of bilateral lingual nerve afferents to the Mass.Mns was also explored. Ten adults cats weighing 2.5-3.5 kg were used. Experiments were performed on decerebrated and decerebeUated cats anesthetized with pentobarbital sodium (Nembutal, Abott) 30 mg/kg. They were immobilized by intravenous injection of Flaxedil and respiration was maintained artificially. Sleeve electrodes were used to stimulate the cut central ends of the ipsilaterai masseteric nerve and the lingual nerves of either side. Glass micropipettes filled with 2 M potassium citrate were used both for recording and for current injection. Input conductance was measured by current injection 2. The IPSPs produced in a Mass.Mn by stimulation of either the ipsilateral or the contralateral lingual nerve (ipsi-L or contra-L) were composed of an early and a late component as illustrated in Fig. 1A. The two components were more obvious when evoked by stimulation of the contra-L (Fig. lAb-l) than with ipsilateral stimulation (a-l). The threshold-stimulus for the lingual nerve was determined by recording the incoming nerve volley at the semilunar ganglion 5. In the experiment of Fig. IA the ipsi-L and the contra-L was stimulated with an intensity 4.0 times the nerve threshold. After the administration of strychnine (0.08 mg/kg) stimulation ot the contra-L evoked only the late IPSP (strychnine insensitive IPSP) as shown in b-2, since the early IPSP was blocked. In this case, the ipsi-L stimulus produced a spike, followed by the late IPSP (a-2). The records a-2 and b-2 are also shown on a slower sweep speed in a-3
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Fig. 1. The properties of lingually induced IPSPs. A: effect of strychnine on IPSPs. a-1 and b-l, control IPSPs set up by ipsi-L and contra-L stimulation, a-2 and b-2, 3 rain after injection of strychnine (0.08 mg/kg). In a-3 and b-3 are shown the recordings of a-2 and b-2 at slower sweep speed and low amplification.B-a and B-b, tracings ofPSPs procuded by ipsi-L and the contra-L stimulation. Tracing A before and tracing B after the injection. By subtracting a tracing B from a tracing A, tracing C was obtained (B-b). The broken line plots the increase of amplitude of the late IPSP. C: inhibition of induced spikes. C-I, spikes induced by the injection of 5 nA depolarizing current into a cell. A single stimulus to the ipsi-L (C-2) and the contra-L (C-3) depressed the induced spikes. and b-3, respectively. The duration of the early IPSP (strychnine sensitive IPSP) set up by lingual nerve stimulation was 60 msec as illustrated in the tracings of Fig. 1B-b. A tracing labelled A shows both components of the IPSPs produced by contra-L stimulation and the tracing B the contra-IPSP after injection of strychnine. By subtracting tracing B from A, curve C was obtained, which indicates the time course of the early IPSP. The broken line of a tracing C shows the increase of the amplitude of the late IPSP (by about 1 mV) after the injection of the drug. The tracings shown in B-a are the ipsi-IPSPs. Tracings A and B were made before and after strychnine was given. As in the case of the contra-IPSPs, the late IPSP set up by ipsi-L stimulation was also increased in amplitude after the administration of the drug. The increased amplitude of the late IPSP indicates that strychnine releases from inhibition of the inhibitory interneurons generating the late IPSP. In a preliminary report, Takata and Akita reported that the late IPSP set up by ipsi-L stimulation is always larger in amplitude than that produced by contra-L stimulation 6. However, in strychninized cat the late IPSP produced by stimulation of the ipsi-L and contra-L
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Fig. 2. The properties of the late IPSP. A: measurement of membrane conductance. A-a, antidromic spike. A-b, responses of a neuron to ipsi-L stimulation in strychninized cat. c-1,2,3 and 4, membrane resistance during the rising phase of late IPSP. d-2,3,4 and 5, membrane resistance during the decay phase of late IPSP. c-5, d-1 and d-6, resting membrane resistance. B: effects of membrane polarization on the late IPSP. In series B-a and B-b are shown the effect of membrane polarization on the ipsi- and the contra- late IPSP. C: amplitude of the ipsi- (white circles) and the contra- late IPSP (black circles) as a function of injected current. HP and DP denote the injection of hyperpolarizing and depolarizing current into a cell.
nerves had the same amplitude. This indicates that the inhibitory interneurons generating the late IPSP are more powerfully inhibited in the contralateral pathway than in the ipsilateral pathway. Inhibition of firings of a Mass.Mn by the late IPSP was examined in a strychninized cat (Fig. IC). Steady depolarizing current was passed across the membrane, while the spike discharge was recorded. In C-l, injection of 5 nA depolarizing current caused this motoneuron to fire at a rate of 100 impulses per sec. When a single shock was applied to the ipsi-L (C-2) or the contra-L (C-3), the indiced spikes were suppressed during the late IPSP, indicating that the late component is also postsynaptic in nature. In order to examine the ionic mechanism for the late IPSP, the membrane conductance was measured during the late IPSP conventional pulse techniques. In Fig. 2Aa is shown an antidromic spike produced in a Mass.Mn by stimulation of the masseteric nerve. After administration of strychnine, ipsi-L stimulus evoked an excitatory postsynaptic potential followed by the late IPSP (A-b). By injection of a brief hyperpolarizing pulse into this neuron the membrane resistance was measured during
651 the rising phase (c-l, 2, 3 and 4) and the decay phase of the late IPSP (d-2, 3, 4 and 5) as illustrated in Fig. 2A-c and d. Records c-5, d-1 and d-6 show the resting membrane resistance. As clearly seen, no conductance increase could be detected during the late IPSP. In the following study the relation between the amplitude of the late IPSP and the membrane potential displacements was examined in a strychninized cat (Fig. 2B, C). In some Mass.Mns stimulation of the ipsi-L and the contra-L evoked only the late IPSP after the injection of strychnine. In series B-a and B-b are shown the effect of membrane polarization on the ipsi- and the contra- late IPSP. Even when a hyperpolarizing current was passed across the membrane as indicated by downward deflexion of current monitor (4 and 5 in both series), no reversal of the late IPSP was obtained. When steady depolarizing current was passed across the membrane as indicated by upward deflexion of current monitor, neither the ipsi-(2 and 1 in series B-a) nor the contra-late IPSP (2 and 1 in series B-b) showed any remarkable change in amplitude. Record 3 of both series shows the late IPSP at resting membrane potential. In a graph of Fig. 2C is represented the effect of membrane polarization on the ipsilateral (white circles) and the contralateral late IPSP (black circles) as a function of amounts of injected current. In a preliminary report, it has been reported that the reversal of the early IPSP corresponded to the membrane potential displacements caused by 5 nA hyperpolarizing current 6. However, no reversal of the late IPSP was obtained when the cell membrane was hyperpolarized. With regard to the ionic mechanism for IPSPs produced in the Mass.Mns by lingual nerve stimulation, it has earlier been reported that the early IPSP is reversed to a depolarizing potential by an increase of intracellular concentration of C l - ions, but the late IPSP is insensitive to C1- ions 6. The present experiment revealed that the late, strychnine-insensitive IPSP produced in the Mass.Mns by lingual nerve stimulation is not associated with measurable conductance changes. The authors wish to thank Professor Y. Kawamura, Department of Oral Physiology, Osaka University for his encouragement and Professor G. Somjen, Department of Physiology, Duke University for his helpful criticism in preparing this manuscript.
1 Goldberg, L. J. and Nakamura, Y., Lingually induced inhibition of masseteric motoneurons, Experientia (Basel), 24 (1968) 371-373. 2 Gustafsson, B., Lindstr6m, S. and Takata, M., After-hyperpolarization mechanism in the dorsal spinocerebellar tract cells of the cat, J. Physiol. (Lond.), 275 (1978) 283-301. 3 Kidokoro, Y., Kubota, K., Shuto, S. and Sumino, R., Reflex organization of cat masticatory muscles, J. Neurophysiol., 31 (1968) 695-708. 4 Takata, M. and Kawamura, Y., Studies on the postsynaptic potential of masseter motoneurons. In R. Dubner and Y. Kawamura (Eds.), Oral-Facial Sensory and Motor Mechanisms, Appleton Century Crofts, New York, 1971, pp. 205-215. 5 Takata, M., Ito, K. and Kawamura, Y., Inhibition of hypoglossal motoneurons by stimulation of the jaw-opening muscle afferents, Jap. J. Physiol., 25 (1975) 453-465. 6 Takata, M. and Akita, R., The two components of IPSPs in the masseteric motoneurons, J. physiol. Soc. Japan, 38 (1976) 89.
Brain Research, 168 (1979) 648-651 © Elsevier/North-Holland Biomedical Press
The properties of lingually induced IPSPs in the masseteric motoneurons
MITSURU TAKATA and SATOSHI FUJITA
Department of Physiology, School of Dentistry, Tokushima University, 3 Kuramoto-cho, Tokushima (Japan) (Accepted February 1st, 1979)
In intracellular recordings from the masseteric motoneurons (Mass.Mns) it has been reported that inhibitory postsynaptic potentials (IPSPs) evoked by lingual nerve stimulation were the composite of an early and a late component 1,8,4. With regard to an early IPSP it was found that the early IPSP was blocked by intravenous injection of strychnine and was reversed to a depolarizing potential by displacement of the membrane potential to hyperpolarization 3,6. However, the properties of the late IPSP are still unknown. In order to examine the properties of the late IPSP, the effect of membrane polarization on the late IPSP and changes of membrane conductance during the late 1PSP were explored and are described in the present paper. In addition, the synaptic linkage of bilateral lingual nerve afferents to the Mass.Mns was also explored. Ten adults cats weighing 2.5-3.5 kg were used. Experiments were performed on decerebrated and decerebeUated cats anesthetized with pentobarbital sodium (Nembutal, Abott) 30 mg/kg. They were immobilized by intravenous injection of Flaxedil and respiration was maintained artificially. Sleeve electrodes were used to stimulate the cut central ends of the ipsilaterai masseteric nerve and the lingual nerves of either side. Glass micropipettes filled with 2 M potassium citrate were used both for recording and for current injection. Input conductance was measured by current injection 2. The IPSPs produced in a Mass.Mn by stimulation of either the ipsilateral or the contralateral lingual nerve (ipsi-L or contra-L) were composed of an early and a late component as illustrated in Fig. 1A. The two components were more obvious when evoked by stimulation of the contra-L (Fig. lAb-l) than with ipsilateral stimulation (a-l). The threshold-stimulus for the lingual nerve was determined by recording the incoming nerve volley at the semilunar ganglion 5. In the experiment of Fig. IA the ipsi-L and the contra-L was stimulated with an intensity 4.0 times the nerve threshold. After the administration of strychnine (0.08 mg/kg) stimulation ot the contra-L evoked only the late IPSP (strychnine insensitive IPSP) as shown in b-2, since the early IPSP was blocked. In this case, the ipsi-L stimulus produced a spike, followed by the late IPSP (a-2). The records a-2 and b-2 are also shown on a slower sweep speed in a-3
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Fig. 1. The properties of lingually induced IPSPs. A: effect of strychnine on IPSPs. a-1 and b-l, control IPSPs set up by ipsi-L and contra-L stimulation, a-2 and b-2, 3 rain after injection of strychnine (0.08 mg/kg). In a-3 and b-3 are shown the recordings of a-2 and b-2 at slower sweep speed and low amplification.B-a and B-b, tracings ofPSPs procuded by ipsi-L and the contra-L stimulation. Tracing A before and tracing B after the injection. By subtracting a tracing B from a tracing A, tracing C was obtained (B-b). The broken line plots the increase of amplitude of the late IPSP. C: inhibition of induced spikes. C-I, spikes induced by the injection of 5 nA depolarizing current into a cell. A single stimulus to the ipsi-L (C-2) and the contra-L (C-3) depressed the induced spikes. and b-3, respectively. The duration of the early IPSP (strychnine sensitive IPSP) set up by lingual nerve stimulation was 60 msec as illustrated in the tracings of Fig. 1B-b. A tracing labelled A shows both components of the IPSPs produced by contra-L stimulation and the tracing B the contra-IPSP after injection of strychnine. By subtracting tracing B from A, curve C was obtained, which indicates the time course of the early IPSP. The broken line of a tracing C shows the increase of the amplitude of the late IPSP (by about 1 mV) after the injection of the drug. The tracings shown in B-a are the ipsi-IPSPs. Tracings A and B were made before and after strychnine was given. As in the case of the contra-IPSPs, the late IPSP set up by ipsi-L stimulation was also increased in amplitude after the administration of the drug. The increased amplitude of the late IPSP indicates that strychnine releases from inhibition of the inhibitory interneurons generating the late IPSP. In a preliminary report, Takata and Akita reported that the late IPSP set up by ipsi-L stimulation is always larger in amplitude than that produced by contra-L stimulation 6. However, in strychninized cat the late IPSP produced by stimulation of the ipsi-L and contra-L
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Fig. 2. The properties of the late IPSP. A: measurement of membrane conductance. A-a, antidromic spike. A-b, responses of a neuron to ipsi-L stimulation in strychninized cat. c-1,2,3 and 4, membrane resistance during the rising phase of late IPSP. d-2,3,4 and 5, membrane resistance during the decay phase of late IPSP. c-5, d-1 and d-6, resting membrane resistance. B: effects of membrane polarization on the late IPSP. In series B-a and B-b are shown the effect of membrane polarization on the ipsi- and the contra- late IPSP. C: amplitude of the ipsi- (white circles) and the contra- late IPSP (black circles) as a function of injected current. HP and DP denote the injection of hyperpolarizing and depolarizing current into a cell.
nerves had the same amplitude. This indicates that the inhibitory interneurons generating the late IPSP are more powerfully inhibited in the contralateral pathway than in the ipsilateral pathway. Inhibition of firings of a Mass.Mn by the late IPSP was examined in a strychninized cat (Fig. IC). Steady depolarizing current was passed across the membrane, while the spike discharge was recorded. In C-l, injection of 5 nA depolarizing current caused this motoneuron to fire at a rate of 100 impulses per sec. When a single shock was applied to the ipsi-L (C-2) or the contra-L (C-3), the indiced spikes were suppressed during the late IPSP, indicating that the late component is also postsynaptic in nature. In order to examine the ionic mechanism for the late IPSP, the membrane conductance was measured during the late IPSP conventional pulse techniques. In Fig. 2Aa is shown an antidromic spike produced in a Mass.Mn by stimulation of the masseteric nerve. After administration of strychnine, ipsi-L stimulus evoked an excitatory postsynaptic potential followed by the late IPSP (A-b). By injection of a brief hyperpolarizing pulse into this neuron the membrane resistance was measured during
651 the rising phase (c-l, 2, 3 and 4) and the decay phase of the late IPSP (d-2, 3, 4 and 5) as illustrated in Fig. 2A-c and d. Records c-5, d-1 and d-6 show the resting membrane resistance. As clearly seen, no conductance increase could be detected during the late IPSP. In the following study the relation between the amplitude of the late IPSP and the membrane potential displacements was examined in a strychninized cat (Fig. 2B, C). In some Mass.Mns stimulation of the ipsi-L and the contra-L evoked only the late IPSP after the injection of strychnine. In series B-a and B-b are shown the effect of membrane polarization on the ipsi- and the contra- late IPSP. Even when a hyperpolarizing current was passed across the membrane as indicated by downward deflexion of current monitor (4 and 5 in both series), no reversal of the late IPSP was obtained. When steady depolarizing current was passed across the membrane as indicated by upward deflexion of current monitor, neither the ipsi-(2 and 1 in series B-a) nor the contra-late IPSP (2 and 1 in series B-b) showed any remarkable change in amplitude. Record 3 of both series shows the late IPSP at resting membrane potential. In a graph of Fig. 2C is represented the effect of membrane polarization on the ipsilateral (white circles) and the contralateral late IPSP (black circles) as a function of amounts of injected current. In a preliminary report, it has been reported that the reversal of the early IPSP corresponded to the membrane potential displacements caused by 5 nA hyperpolarizing current 6. However, no reversal of the late IPSP was obtained when the cell membrane was hyperpolarized. With regard to the ionic mechanism for IPSPs produced in the Mass.Mns by lingual nerve stimulation, it has earlier been reported that the early IPSP is reversed to a depolarizing potential by an increase of intracellular concentration of C l - ions, but the late IPSP is insensitive to C1- ions 6. The present experiment revealed that the late, strychnine-insensitive IPSP produced in the Mass.Mns by lingual nerve stimulation is not associated with measurable conductance changes. The authors wish to thank Professor Y. Kawamura, Department of Oral Physiology, Osaka University for his encouragement and Professor G. Somjen, Department of Physiology, Duke University for his helpful criticism in preparing this manuscript.
1 Goldberg, L. J. and Nakamura, Y., Lingually induced inhibition of masseteric motoneurons, Experientia (Basel), 24 (1968) 371-373. 2 Gustafsson, B., Lindstr6m, S. and Takata, M., After-hyperpolarization mechanism in the dorsal spinocerebellar tract cells of the cat, J. Physiol. (Lond.), 275 (1978) 283-301. 3 Kidokoro, Y., Kubota, K., Shuto, S. and Sumino, R., Reflex organization of cat masticatory muscles, J. Neurophysiol., 31 (1968) 695-708. 4 Takata, M. and Kawamura, Y., Studies on the postsynaptic potential of masseter motoneurons. In R. Dubner and Y. Kawamura (Eds.), Oral-Facial Sensory and Motor Mechanisms, Appleton Century Crofts, New York, 1971, pp. 205-215. 5 Takata, M., Ito, K. and Kawamura, Y., Inhibition of hypoglossal motoneurons by stimulation of the jaw-opening muscle afferents, Jap. J. Physiol., 25 (1975) 453-465. 6 Takata, M. and Akita, R., The two components of IPSPs in the masseteric motoneurons, J. physiol. Soc. Japan, 38 (1976) 89.