Brain Research, 85 (1975) 99-102 © Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands
99
Reciprocal la inhibition during the late reflexes evoked from the flexor reflex afferents after DOPA
T.-C. FU*, E. JANKOWSKA AND A. LUNDBERG Department of Physiology, University of G6teborg, Gdteborg (Sweden)
(Accepted November 15th, 1974)
After an intravenous injection of L-DOPA (spinal cats), volleys in flexor reflex afferents (FRA) evoke late reflex discharges in ipsilateral flexor and in contralateral extensor motoneurones. A mutual reciprocal inhibition then occurs between the interneuronal pathways mediating late excitation to flexor and extensor motoneurones respectively9,10. Such a half-centre organization can give rise to alternating activation of flexors and extensors and it has been suggested that the interneuronal network mediating the late reflexes after DOPA constitutes a spinal locomotor centre 9. Evidence is now forthcoming that this interneuronal network is used in stepping in spinal cats 1,2 and also in the mesencephalic cat 4. In the mesencephalic preparation it has been shown that stepping depends on a-y-linkage 12 and the network released after DOPA does indeed give parallel reflex activation of a- and 7-motoneuronesa, 9. It has also been shown that many neuronal pathways, both descending and segmental, with effects on ct- and 7-motoneurones exert similar effects on the interneurones of the reciprocal la inhibitory pathway 6, presumably giving 'a-7-1inkage in reciprocal inhibition '5. If this hypothesis also applies to the 'spinal locomotor centre' then it would be expected that the late reflex activation after DOPA should be accompanied by a facilitation of the reciprocal la inhibitory pathway. The present investigation has indeed shown that such an action does occur. All experiments (14) were made on anaemically decorticated low spinal cats. Conventional techniques were used for intracellular recording from motoneurones and extracellular recording from interneurones. All observations reported were made after intravenous injection of L-DOPA (100 mg/kg) during the period when volleys in the FRA evoked late excitation in motoneurones 9. The evidence for excitatory action on la inhibitory interneurones is three-fold. (1) During late activation of flexor and extensor motoneurones (from the ipsiand contralateral FRA respectively) after DOPA there is a concomitant facilitation of transmission in the Ia inhibitory pathway to antagonist motoneurones. This was * Present address: Department of Physiology, College of Medicine, National Taiwan University, Taipei, Taiwan.
100 A
Q 1.2 ,,
B
C
Q 1.4
,i .
-
O 2.0
D
rest
H
F
DP
I
G
i
coil
Tf
L6VR+Q 114
CO Sur
t00 ms (E-I)
Fig. 1. ExUacellular records from an interneurone (E-I and upper traces in A - D , Lower traces being the cord surface potentials) identified as a Ia inhibitory interneurone by records A - D (see text). Abbreviations: Q, ipsilateral quadriceps nerve; DP, ipsilateral deep peroneat nerve including branches to anterior tibial and extensor digitorum longus; j, ipsilateral posterior knee joint nerve; coil, contralateral hamstring nerve; coSur, contralateral sural nerve; VR, ventral root. Strength of nerve stimulation is expressed in multiples of threshold. Nerves in records F - I were stimulated with the strength about 20 x threshold.
evidenced by the finding that volleys in the F R A enhanced the IPSP evoked in them by a submaximal la volley. (2) The inhibitory interneurones interposed between the group la muscle spindle afferents and motoneurones 7,11 become strongly excited from the FRA after injection of DOPA. lnterneurones monosynaptically la activated from flexors, which inhibit extensors, are excited from the ipsilateral FRA. while those which inhibit flexors are excited from the contralateral FRA. Thus they do assist the ipsilateral flexion and the crossed extension reflexes respectively. Records from an interneurone subserving the latter reflex are shown in Fig. 1. Records A - D identify it as a la inhibitory interneurone 7,1t because it was excited by group la afferents (A), followed high frequency stimulation (B) and was inhibited via m o t o r axon collaterals and Renshaw cells (C, D). After injection o f D O P A it was firing irregularly at rest with a relatively high frequency (E) and was excited from the contralateral (H,I) nerves and inhibited from most o f the ipsilateral (F, G). At any time interval ventral root stimulation invariably gave complete inhibition for about 50 rnsec of the discharge evoked from the FRA. All interneurones with convergence of monosynaptic 1a excitation and late, longlasting excitation were inhibited from ventral roots. However, a large part ofinterneurones in which late, longlasting excitation was evoked from the F R A after D O P A received neither la excitation nor recurrent inhibition from motor axon collaterals. Some of these interneurones are likely to be part of the excitatory pathway to motoneurones 9J°. (3) Intracellular records from extensor motoneurones after injection of DOPA revealed late, longlasting hyperpolarizations from the ipsilateral FRA, while in flexor motoneurones similar responses were evoked from the contralateral FRA. These late
101
A
I.
coil 20
.
.
.
.
.
.
C
coH+VR
.
10+50 ms (A,C) B
L6 VR
D
5 mvl
~"~r 1+5 ms (B,D) Fig. 2. lntracellular records from a posterior biceps semitendinosus motoneurone (upper traces) with corresponding records from the surface of the spinal cord (lower records). Record in D shows the expanded central part of record C. Abbreviations as in Fig. 1.
hyperpolarizing responses were identified as |PSPs since they displayed appropriate changes in magnitude during passage of hyper- or depolarizing currents through the recording electrode. These IPSPs were almost totally removed during a period of about 50 msec following a single ventral root stimulus. Fig. 2B shows that such stimulation may have little effect on the motoneurone membrane when applied separately but repolarizes it to the resting level when applied during a late IPSP after DGPA (Fig. 2C, D). Tests during hyper- and depolarization showed that there was a close correlation between the size of the repolarization and the 1PSP, which shows that the repolarization is a disinhibition 8,13. Thus the late IPSP evoked from the FRA after DOPA is mediated by interneurones which are under inhibitory control from Renshaw ceils. These interneurones must be identical with the interneurones mediating reciprocal la inhibition of motoneurones because recurrent inhibition from motor axon collaterals reaches these interneurones selectively 6. From discharges recorded in Ia inhibitory interneurones (Fig. l) it was to be expected that volleys in the FRA should evoke IPSPs in motoneurones after DOPA. The finding of a virtually complete removal of the late FRA IPSPs by stimulation of ventral roots has given the additional information that these late IPSPs are evoked largely, if not exclusively, via the reciprocal la inhibitory pathway. By contrast the short-latency IPSP evoked from the FRA in motoneurones of acute spinal cats not injected with DOPA is only in part mediated by the Ia inhibitory interneurones; the major part is via a 'private' interneuronal pathway from the FRA not shared with la afferentsL The transmission in the latter pathway to motoneurones is inhibited after DOPA 9 and the 'private' inhibition from the F R A instead is exerted at an interneuronal level, giving the mutual inhibitory action between the excitatory lines to extensors and flexors, which is the essential element in the half-centre organization TM. The present findings suggest that 'a-y-linkage in reciprocal inhibition' also operates during the late reflexes released after DOPA and, given the hypothesis that
102 the interneuronal network mediating these reflexes functions as a spinal locomotor centre, also during locomotion. This work was supported by the Swedish Medical Research Council (project No. 94).
t BUDAKOVA,N. N., Stepping movements evoked by a rhythmic stimulation of a dorsal root in mesencephalic eat, Fiziol. Zh. (Leningrad), 57 (1971) 1632-1640 (in Russian). 2 FORSSBERG,H., ANDGR1LLNER,S., The locomotion of the acute spinal cat injected with clonidine i.v., Brain Research, 50 (1973) 184-186. 3 GRILLNER,S., The influence of DOPA on the static and the dynamic fusimotor activity to the triceps surae of the spinal cat, Acta physiol, scand., 77 (1969) 490-509. 4 GRILLNER,S., AND SnlK, M. L., On the descending control of the lumbosacral spinal cord from the 'mesencephalic locomotor region', Acta physiol, scand, 87 (I973) 320-333. 5 HONGO,T,, JANKOWSKA,E., AND LUNDBERG,A., The rubrospinal tract. II. Facilitation of interneuronal transmission in reflex paths to motoneurones, Exp. Brain Res., 7 0969) 365-391. 6 HULTBORN,H., Convergence on interneurones in the reciprocal Ia inhibitory pathway to motoneurones, Acta physiol, scan&, 85, Suppl. 375 (1972) 1-42. 7 HULTBORN,H., JANKOWSKA,E., ANDLINDSTRt)M,S., Recurrent inhibition of interneurones monosynaptically activated from group Ia afferents, J. Physiol. (Lond.), 215 (1971) 613-636. 8 HULTBORN,H., JANKOWSKA,E., L~NDSTR6M, S., AND ROBERTS, W., Neuronal pathway of the recurrent facilitation of motoneurones, J. Physiol. (Lond.), 218 (1971) 495-514. 9 JANKOWSKA,E., JUKES, M. G. M., LUND, S., AND LUNDBERG,A., The effect of DOPA on the spinal cord. 5. Reciprocal organization of pathways transmitting excitatory action to alpha motoneurones of flexors and extensors, Acta physiol, scand., 70 (1967) 369-388. 10 JANKOWSKA,E., JUKES,M. G. M., LUND, S., ANDLUNDBERG,A., The effect of DOPA on the spinal cord. 6. Half-centre organization of interneurones transmitting effects from the flexor reflex afferents, Acta physiol, scand., 70 (1967) 389-402. 11 JANKOWSKA,E., AND ROBERTS,W. J., Synaptic actions of single interneurones mediating reciprocal la inhibition of motoneurones, J. Physiol. (Lond.), 222 (1972) 623-642. 12 SEVEmN,F. V., ORLOVSKY,G. N., ANDSINK, M. L., Work of the muscle receptors during controlled locomotion, Biophysics, 12 (1967) 575-586. 13 WILSON,V. J., AND BURGESS,P., Disinhibition in the cat spinal cord, J. NeurophysioL~ 25 (1962) 392-404.
99
Reciprocal la inhibition during the late reflexes evoked from the flexor reflex afferents after DOPA
T.-C. FU*, E. JANKOWSKA AND A. LUNDBERG Department of Physiology, University of G6teborg, Gdteborg (Sweden)
(Accepted November 15th, 1974)
After an intravenous injection of L-DOPA (spinal cats), volleys in flexor reflex afferents (FRA) evoke late reflex discharges in ipsilateral flexor and in contralateral extensor motoneurones. A mutual reciprocal inhibition then occurs between the interneuronal pathways mediating late excitation to flexor and extensor motoneurones respectively9,10. Such a half-centre organization can give rise to alternating activation of flexors and extensors and it has been suggested that the interneuronal network mediating the late reflexes after DOPA constitutes a spinal locomotor centre 9. Evidence is now forthcoming that this interneuronal network is used in stepping in spinal cats 1,2 and also in the mesencephalic cat 4. In the mesencephalic preparation it has been shown that stepping depends on a-y-linkage 12 and the network released after DOPA does indeed give parallel reflex activation of a- and 7-motoneuronesa, 9. It has also been shown that many neuronal pathways, both descending and segmental, with effects on ct- and 7-motoneurones exert similar effects on the interneurones of the reciprocal la inhibitory pathway 6, presumably giving 'a-7-1inkage in reciprocal inhibition '5. If this hypothesis also applies to the 'spinal locomotor centre' then it would be expected that the late reflex activation after DOPA should be accompanied by a facilitation of the reciprocal la inhibitory pathway. The present investigation has indeed shown that such an action does occur. All experiments (14) were made on anaemically decorticated low spinal cats. Conventional techniques were used for intracellular recording from motoneurones and extracellular recording from interneurones. All observations reported were made after intravenous injection of L-DOPA (100 mg/kg) during the period when volleys in the FRA evoked late excitation in motoneurones 9. The evidence for excitatory action on la inhibitory interneurones is three-fold. (1) During late activation of flexor and extensor motoneurones (from the ipsiand contralateral FRA respectively) after DOPA there is a concomitant facilitation of transmission in the Ia inhibitory pathway to antagonist motoneurones. This was * Present address: Department of Physiology, College of Medicine, National Taiwan University, Taipei, Taiwan.
100 A
Q 1.2 ,,
B
C
Q 1.4
,i .
-
O 2.0
D
rest
H
F
DP
I
G
i
coil
Tf
L6VR+Q 114
CO Sur
t00 ms (E-I)
Fig. 1. ExUacellular records from an interneurone (E-I and upper traces in A - D , Lower traces being the cord surface potentials) identified as a Ia inhibitory interneurone by records A - D (see text). Abbreviations: Q, ipsilateral quadriceps nerve; DP, ipsilateral deep peroneat nerve including branches to anterior tibial and extensor digitorum longus; j, ipsilateral posterior knee joint nerve; coil, contralateral hamstring nerve; coSur, contralateral sural nerve; VR, ventral root. Strength of nerve stimulation is expressed in multiples of threshold. Nerves in records F - I were stimulated with the strength about 20 x threshold.
evidenced by the finding that volleys in the F R A enhanced the IPSP evoked in them by a submaximal la volley. (2) The inhibitory interneurones interposed between the group la muscle spindle afferents and motoneurones 7,11 become strongly excited from the FRA after injection of DOPA. lnterneurones monosynaptically la activated from flexors, which inhibit extensors, are excited from the ipsilateral FRA. while those which inhibit flexors are excited from the contralateral FRA. Thus they do assist the ipsilateral flexion and the crossed extension reflexes respectively. Records from an interneurone subserving the latter reflex are shown in Fig. 1. Records A - D identify it as a la inhibitory interneurone 7,1t because it was excited by group la afferents (A), followed high frequency stimulation (B) and was inhibited via m o t o r axon collaterals and Renshaw cells (C, D). After injection o f D O P A it was firing irregularly at rest with a relatively high frequency (E) and was excited from the contralateral (H,I) nerves and inhibited from most o f the ipsilateral (F, G). At any time interval ventral root stimulation invariably gave complete inhibition for about 50 rnsec of the discharge evoked from the FRA. All interneurones with convergence of monosynaptic 1a excitation and late, longlasting excitation were inhibited from ventral roots. However, a large part ofinterneurones in which late, longlasting excitation was evoked from the F R A after D O P A received neither la excitation nor recurrent inhibition from motor axon collaterals. Some of these interneurones are likely to be part of the excitatory pathway to motoneurones 9J°. (3) Intracellular records from extensor motoneurones after injection of DOPA revealed late, longlasting hyperpolarizations from the ipsilateral FRA, while in flexor motoneurones similar responses were evoked from the contralateral FRA. These late
101
A
I.
coil 20
.
.
.
.
.
.
C
coH+VR
.
10+50 ms (A,C) B
L6 VR
D
5 mvl
~"~r 1+5 ms (B,D) Fig. 2. lntracellular records from a posterior biceps semitendinosus motoneurone (upper traces) with corresponding records from the surface of the spinal cord (lower records). Record in D shows the expanded central part of record C. Abbreviations as in Fig. 1.
hyperpolarizing responses were identified as |PSPs since they displayed appropriate changes in magnitude during passage of hyper- or depolarizing currents through the recording electrode. These IPSPs were almost totally removed during a period of about 50 msec following a single ventral root stimulus. Fig. 2B shows that such stimulation may have little effect on the motoneurone membrane when applied separately but repolarizes it to the resting level when applied during a late IPSP after DGPA (Fig. 2C, D). Tests during hyper- and depolarization showed that there was a close correlation between the size of the repolarization and the 1PSP, which shows that the repolarization is a disinhibition 8,13. Thus the late IPSP evoked from the FRA after DOPA is mediated by interneurones which are under inhibitory control from Renshaw ceils. These interneurones must be identical with the interneurones mediating reciprocal la inhibition of motoneurones because recurrent inhibition from motor axon collaterals reaches these interneurones selectively 6. From discharges recorded in Ia inhibitory interneurones (Fig. l) it was to be expected that volleys in the FRA should evoke IPSPs in motoneurones after DOPA. The finding of a virtually complete removal of the late FRA IPSPs by stimulation of ventral roots has given the additional information that these late IPSPs are evoked largely, if not exclusively, via the reciprocal la inhibitory pathway. By contrast the short-latency IPSP evoked from the FRA in motoneurones of acute spinal cats not injected with DOPA is only in part mediated by the Ia inhibitory interneurones; the major part is via a 'private' interneuronal pathway from the FRA not shared with la afferentsL The transmission in the latter pathway to motoneurones is inhibited after DOPA 9 and the 'private' inhibition from the F R A instead is exerted at an interneuronal level, giving the mutual inhibitory action between the excitatory lines to extensors and flexors, which is the essential element in the half-centre organization TM. The present findings suggest that 'a-y-linkage in reciprocal inhibition' also operates during the late reflexes released after DOPA and, given the hypothesis that
102 the interneuronal network mediating these reflexes functions as a spinal locomotor centre, also during locomotion. This work was supported by the Swedish Medical Research Council (project No. 94).
t BUDAKOVA,N. N., Stepping movements evoked by a rhythmic stimulation of a dorsal root in mesencephalic eat, Fiziol. Zh. (Leningrad), 57 (1971) 1632-1640 (in Russian). 2 FORSSBERG,H., ANDGR1LLNER,S., The locomotion of the acute spinal cat injected with clonidine i.v., Brain Research, 50 (1973) 184-186. 3 GRILLNER,S., The influence of DOPA on the static and the dynamic fusimotor activity to the triceps surae of the spinal cat, Acta physiol, scand., 77 (1969) 490-509. 4 GRILLNER,S., AND SnlK, M. L., On the descending control of the lumbosacral spinal cord from the 'mesencephalic locomotor region', Acta physiol, scand, 87 (I973) 320-333. 5 HONGO,T,, JANKOWSKA,E., AND LUNDBERG,A., The rubrospinal tract. II. Facilitation of interneuronal transmission in reflex paths to motoneurones, Exp. Brain Res., 7 0969) 365-391. 6 HULTBORN,H., Convergence on interneurones in the reciprocal Ia inhibitory pathway to motoneurones, Acta physiol, scan&, 85, Suppl. 375 (1972) 1-42. 7 HULTBORN,H., JANKOWSKA,E., ANDLINDSTRt)M,S., Recurrent inhibition of interneurones monosynaptically activated from group Ia afferents, J. Physiol. (Lond.), 215 (1971) 613-636. 8 HULTBORN,H., JANKOWSKA,E., L~NDSTR6M, S., AND ROBERTS, W., Neuronal pathway of the recurrent facilitation of motoneurones, J. Physiol. (Lond.), 218 (1971) 495-514. 9 JANKOWSKA,E., JUKES, M. G. M., LUND, S., AND LUNDBERG,A., The effect of DOPA on the spinal cord. 5. Reciprocal organization of pathways transmitting excitatory action to alpha motoneurones of flexors and extensors, Acta physiol, scand., 70 (1967) 369-388. 10 JANKOWSKA,E., JUKES,M. G. M., LUND, S., ANDLUNDBERG,A., The effect of DOPA on the spinal cord. 6. Half-centre organization of interneurones transmitting effects from the flexor reflex afferents, Acta physiol, scand., 70 (1967) 389-402. 11 JANKOWSKA,E., AND ROBERTS,W. J., Synaptic actions of single interneurones mediating reciprocal la inhibition of motoneurones, J. Physiol. (Lond.), 222 (1972) 623-642. 12 SEVEmN,F. V., ORLOVSKY,G. N., ANDSINK, M. L., Work of the muscle receptors during controlled locomotion, Biophysics, 12 (1967) 575-586. 13 WILSON,V. J., AND BURGESS,P., Disinhibition in the cat spinal cord, J. NeurophysioL~ 25 (1962) 392-404.
