Effects of l -DOPA on fusimotor control of triceps surae muscle spindles in the cat

Effects of l -DOPA on fusimotor control of triceps surae muscle spindles in the cat

ELSEVIER Neuroscienee Research 22 (1995) 307-314 NEUROSCIENCE RESEARCH Effects of L-DOPA on fusimotor control of triceps surae muscle spindles in t...

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ELSEVIER

Neuroscienee Research 22 (1995) 307-314

NEUROSCIENCE RESEARCH

Effects of L-DOPA on fusimotor control of triceps surae muscle spindles in the cat H ~ k a n J o h a n s s o n a, P e r S j 6 1 a n d e r *a, P e t e r S o j k a b, B r i t t - I n g e r W e n n g r e n c aDivision of Work Physiology, National Institute of Occupational Health, P.O. Box 7654, S-907 13 Ume~, Sweden bDepartment of Clinical Physiology. University of Ume~, S-901 85 Ume~, Sweden CDepartment of Otolaryngology, University of Ume~, S-901 85 Ume~, Sweden

Received 8 February 1995; accepted 28 March 1995

Abstract

The experiments were performed on lightly c~-chloraloseanaesthetised and spinalized cats. Alterations in fusimotor activity were assessed by recordings from single spindle afferents (90 primary and 12 secondary) from the triceps surae muscle, before and after i.v, administration of L-3-.3,4-dihydroxyphenylalanine(L-DOPA). The effects of L-DOPA on fusimotor reflexes from ipsi- and contralateral hind limb afferents were investigated by using extensions of the intact contralateral hind limb and tonic stretches of the ipsilateral posterior biceps and semitendinosus muscles as reflex stimuli. Prior to injection of L-DOPA, a low reflex responsiveness was found to both the ipsi- and the contralateral stimulation. After administration of L-DOPA, the reflex responsiveness as well as the resting activity of 1:hemuscle spindle afferents were increased as a result of enhanced activity in mainly dynamic fusimotor neurones. The results indicate that changes in fusimotor activity elicited after administration of L-DOPA are caused by release of transmission in interneuronal pathways mediating ipsi- and contralateral reflexes to mainly dynamic fusimotor neurones. The possible role of monoamineqlic descending control of fusimotor neurones in the regulation of muscle tone, tremor and rigidity is discussed. Keywords: Monoaminergic spinal control; Primary muscle spindle afferent; Secondary muscle spindle afferent; L-DOPA;

Fusimotor reflexes; Resting activity; Muscle tone; Rigidity

1. Introduction

There is a body of evidence that monoaminergic descending pathways originating in the brainstem influence the reflex effects of afferent inputs to the spinal cord. Administration of the monoamine precursors, L~-3,4-dihydroxyphenylalanine (L-DOPA) or 5-hydroxytryptamine (5-HT) in decerebrated and spinalized cats has been shown to irthibit pathways mediating shortlatency reflex effects fi:om flexor reflex afferents on o~motoneurones, and, at the same time, release transmission in reflex pathways mediating late long-lasting excitation (Andtn et al., 1966; Jankowska et al., 1967b; Lundberg, 1979). The discharge rate of interneurones in spinal pathways from group II muscle afferents is de* Correspondingauthor. Tel.: +46 90 167453;Fax: +46 90 165027; E-mail: [email protected]

pressed, whereas pathways from group I muscle at: ferents is unchanged after L-DOPA administration (Jankowska et al., 1967a; Schomburg and Steffens, 1988; Bras et al., 1989). It has, moreover, been demonstrated that monoaminergic descending pathways can alter the background activity, as well as the reflexly induced effects on lumbar 7-motoneurones (Grillner et al., 1967; Bergraans and Grillner, 1968, 1969; Grillner, 1969; Fromm and Noth, 1974; Sj6str6m and Zangger, 1976; Davey and Ellaway, 1988; Baker et al., 1991). In a decerebrated and spinalized preparation, Davey and Ellaway (1988) found that administration of L-DOPA and 5-HT increased the variability of firing of 7efferents and reduced the tendency for short-term sychronization between motoneurones. It was proposed that activity in monoaminergic descending spinal pathways, probably originating in the pontomedullary brainstem (Baker et al., 1991), suppress the synchrony of

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H. Johansson et aL / Neuroscience Research 22 (1995) 307-314

firing by inhibiting reflex pathways to 3,-motoneurones (Davey and Ellaway, 1988). Since synchronization of~,motoneurone activity could, via the muscle spindle loop, induce synchronization of ct-motoneurone activity (Windhorst, 1978), it seems possible that the descending monoaminergic control of-r-motoneurones might be involved in regulation of muscle tone in posture and movements and/or in mechanisms responsible for tremor and rigidity (Davey and Ellaway, 1988; Baker et al., 1991). In the investigations so far done regarding effects on 3,-motoneurones from monoaminergic descending pathways, the fusimotor activity has been recorded from 3,-efferents, or indirectly assessed from recordings of sensitivity changes of muscle spindle afferents at constant muscle length. With those techniques, it is not possible to determine whether fusimotor effects are directed onto static, dynamic or both static and dynamic ~,motoneurones. The purpose of the present study was to investigate the extent to which the two different types of 3,-motoneurones are influenced by L-DOPA-induced activation of segmental monoaminergic pathways/ neurones in spinalized cats. A method was chosen which permits quantitative evaluation of the contribution from static and dynamic fusimotor neurones to changes in the resting activity and the reflex responsiveness of muscle spindle afferents. 2. Methods

2.1. Animals and preparations The experiments were performed on 8 adult cats (weight: 2.8-4.0 kg) anaesthetized with ot-chloralose (60 mg/kg). If necessary, additional doses of 10 mg/kg were given during the experiment. A tracheostomy was made and end-expiratory CO2 was continuously monitored. Blood pressure was maintained above 80 mmHg, if necessary with infusion of dextran (Macrodex, 6% with sodium chloride, Pharmacia), Ringer solution, glucose (55 mg/ml) with sodium chloride or sodium chloride acetate. End-expiratory CO2 was kept below 5.2 kPa by suitably adjusted artificial respiration. The body temperature was held between 38 and 39°C with aid of heat lamps and a cushion (continuously perfused with hot water) placed under the belly of the animal. In the ipsilateral hind limb, conventional nervemuscle preparations were made for the lateral gastrocnemius, plantaris and soleus together (GS) and for the posterior biceps and semitendinosus together (PBSt). For the GS and PBSt muscles, the slack resting lengths and maximum physiological lengths were determined in situ and marked by appropriate reference labels in the surrounding tissue. The GS and the PBSt tendons were disconnected from their points of insertion and attached to separate electromagnetic pullers. Apart from the nerves to the GS and the PBSt muscles, the ip-

silateral hind limb was extensively denervated, including denervation of the ipsilateral hip regions and the tail. The contralateral hind limb was always left intact. A laminectomy, exposing the spinal cord between L 3 and Si level, was routinely performed, and the animals were spinalized either at the L 3 level or the CrC2 level. All exposed tissues were covered with liquid paraffin added to pools made of freed skin flaps. The temperature in the paraffin pools was maintained at approximately 38°C. The animals were mounted in a rigid metal frame in which the ipsilateral hind limb and the pelvis was carefully immobilised with pairs of horizontal pins firmly inserted into the bones of femur, tibia and pelvis.

2.2. Stretch stimulation of the receptor bearing muscles The ipsilateral GS tendon was connected to an electromagnetic puller (stiffness: 0.06 mm/N) with force and velocity feed-back. By aid of the puller, the muscle was stretched and released, using ramp-shaped stretches with a plateau duration of 15 s and a release phase of 10 s (velocity of stretch = 10 mm/s; plateau phase at 2 mm below maximum physiological length and release phase at 10 mm below maximum physiological length). Superimposed on each plateau phase was repetitive sinusoidal stretches at 1 Hz (peak-to-peak amplitude = 2 mm). This particular configuration of the sinusoidal stretch (i.e., frequency and amplitude) was picked in order to correspond to that used in previous studies describing stretch sensitivity changes of soleus muscle spindle afferents induced by controlled electrical stimulation of static and dynamic fusimotor neurones (Hulliger et al. 1977a,b). 2.3. Reflex stimulation To investigate if the size and character of fusimotor reflexes on GS muscle spindles were altered after LDOPA administration, two stimuli were chosen which in a similar preparation have been demonstrated to frequently induce both static and dynamic fusimotor reflexes on GS spindle afferents (cf. Appelberg et al. 1982, 1984; Johansson et al. 1991a). Thus, stretchsensitive receptor afferents in the ipsilateral PBSt muscles were activated by stretching the muscles from their resting lengths up to the maximum physiological lengths. The contralateral hind limb was stimulated by manually extending the hip, knee and ankle joints up to their maximum physiological joint angles. Precaution was always taken to avoid activation of nociceptors due to hyper-extension of the joints. The reflex stimuli were applied 10-15 s before and maintained during the data sampling. 2.4. Administration of L-DOPA All animals were given a single i.v. dose (60-100 mg/kg) of L-DOPA. The doses were low enough to avoid the appearance of a tonic stretch reflex in the

H. Johansson et al./ Neuroscience Research 22 (1995) 307-314

spinalized cat (cf. Grillner, 1969; Goodwin et al., 1973; Fromm and Noth, 1974). In two animals, small additional doses of L-DOPA were given during the course of the experiments. In four of the experiments, 30-90 min prior to the L-DOPA injection, a monoaminoxidase inhibitor (Nialamide: 12.5-50 mg/kg) was administrated i.v. in order to potentiate the effect of L-DOPA.

2.5. ]~ecording and analysis The activity of functionally single muscle spindle afferents from the ipsi]Lateral GS muscle was isolated from teased and cut filaments of the dorsal root L 7. Using silver-wire electrodes, the activity of single spindle afferents was recorded. The afferents were classified as muscle spindle afferents if they fell silent during maximal twitch contractions (elicited by electrical stimulation of the GS nerve), and were further classified as primary and secondary based on their conduction velocity (division line at 72 m/s). Alteration in the fusimotor activity to the ipsilateral GS muscle was assessed indirectly by monitoring the responses of muscle spindle afferents to sinusoidal stretches of the receptor bearing muscle. The data sampling was restricted to the plateau of the ramp-and-hold stretches. Control observations were made with the ipsilateral PBSt muscles at approximately resting lengths and with the intact contralateral hind limb in resting position (hip, knee and ankle joints at intermediate joint angles). Test observations were made during ongoing tonic stretch of the ipsilateral PBSt muscles or during full extension of the contralateral hip, knee anti ankle joints. For all muscle spindle afferents, control and test responses were assessedl before and after administration of L-DOPA and/or Nialamide. For each spindle afferent, the responses to ten successive sinusoidal stretching cycles were ave~raged on-line to generate cycle histograms, showing tlhe probability density of firing throughout the stretch cycle (cf. Fig. 1A-C). Simple sinusoids were fitted to the histograms using a leastsquare algorithm method which ignored periods of afferent silence (Hulliger et al., 1977a). Averaging and fitting of the sinusoid were performed on-line with a computer. The mean rate of discharge ('fitted mean') and the depth of modlulation of the fitted sines were taken as estimates of the responses (for details, see Appelberg et al., 1982). The numerical values of these parameters of each pair of control and test responses were then stored on files for off-line analysis. For all spindle afferents, changes in fusimotor effects were assessed by comparing the responses elicited during at least one control and one test measurement. By repeating the control and test recordings 5--12 times, most effects could be statistically analysed (paired t-test and two-tailed MannWhitney test; level for statistical significance: P < 0.05) and used for construction of scatter diagrams (cf. Fig.

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1D-F). The occurrence of predominantly dynamic, predominantly static or mixed static and dynamic fusimotor effects was inferred on the basis of comparison of the induced changes in fitted sine characteristics (test minus control values) to the sine changes known to be evoked by selective electrical stimulation of single static, single dynamic or of both static and dynamic fusimotor fibres (see Appelberg et al., 1982, where data from Hulliger et al., 1977b are recalculated and replotted). In the ipsilateral GS muscle, gross electromyographic (EMG) activity was recorded with surface electrodes placed on the muscle belly, or with needle electrodes inserted into the muscle. In order to permit detection of any possible relation between skeletomotor activity and fusimotor activity, the amplified EMG signal was continuously displayed on an oscilloscope in parallel with the muscle spindle afferent responses. 3. Results

3.1. Assessment of fusimotor reflex effects Fig. 1 shows the assessment of fusimotor reflex effect elicited on a primary muscle spindle afferent by full extension of the intact contralateral hind limb. The cycle histograms (A-C) display averaged responses of the primary muscle spindle afferent to 10 successive sinusoidal stretching cycles. A comparison of the control and test responses in Fig. 1A shows that activation of contralateral receptor afferents altered the sinusoidal response of the spindle afferent. The reflex stimulation elicited an increase in both depth of modulation and in fitted mean, and during both test and control recordings the afferent fell silent as a result of the release of the stretch. Taken together these observations indicate that the muscle spindle afferent sensitivity was increased due to reflex activation of dynamic GS fusimotor neurones (cf. Hulliger et al., 1977a; Appelberg et al., 1982, 1984). After administration of L-DOPA, an identical extension of the contralateral whole limb caused a considerably larger increase in the dynamic sensitivity of the primary spindle afferent (Fig. 1B). When the contralateral reflex stimulation was repeated after deepening of the general anaesthesia, only insignificant changes in the sensitivity of the spindle afferent were induced (Fig. 1C). All spindle afferents of the present study were examined with the single control-test procedure shown in Fig. 1A-C. Some of the muscle spindle afferents were, however, studied with a number of successive pairs of control and test measurements in order to determine the stability and the degree of consistency of the effects. The results obtained for the primary muscle spindle afferents of Fig. 1 during such series of control and test measurements are shown in the scatter diagrams (Fig. 1D-F). In these diagrams, the changes (test minus control) in responses evoked by full extension of the con-

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Fig. 1. Responses recorded from a primary muscle spindle afferent during full extension of the contralateral hind limb. Identical stimulations were performed during light anaesthesia before (A and D) and 30 min after a 75 mg/kg i.v. administration of L-DOPA (B and E), and during deep anaesthesia (administering of additional a-ehloralose, 8.5 mg/kg) after L-DOPA treatment (C and F). The upper cycle histograms (A-C) display single control and test responses. In all histograms, and for both control and test responses, the probability density of firing (ordinate) of averaged responses to 10 successive sinusoidai stretching cycles (at 1 Hz with a peak-to-peak amplitude of 2 ram; mean muscle length = 2 mm below the maximal physiological length) is plotted against the phase of the stretch cycle (abscissa), which began at 0°. A simple sinusoid is fitted to each histogram. Dotted lines: control responses recorded with the contralateral hind limb in resting position. Continuous lines: test responses recorded during full extension of the eontralateral hip, knee and ankle joints. In the scatter diagrams in D-F, the changes in sinusoidal responses for series of control and test pairs are shown (control and test values being the result of 10 averaged consecutive stretching cycles). For each of the control and test pairs, the change (i.e., test minus control value) in depth of modulation is plotted against the change in fitted mean. The single control and test pairs illustrated in A, B and C are marked by arrows in D, E and F respectively. The changes in fitted mean and in depth of modulation are statistically significant (P < 0.05, paired t-test) in D and E, but not in F. The slopes of the reference lines labelled 1'o (ratio 1.07), l's + "Yo (ratio 0.24) and 3¢s(ratio -0.47) correspond to the ratios of mean change in modulation/mean change in fitted mean found with selective electrical stimulation of dynamic fusimotor fibres alone, of a combination of both static and dynamic fusimotor fibres, and of static fusimotor fibres alone (see Appelberg et al. 1982 where data from Hulliger et al. 1977b was recalculated and replotted).

tralateral hind limb are displayed rather than the absolute size o f the responses ( A p p e l b e r g et al., 1984; Johansson et al., 1991 a). In Fig. 1D it can be clearly seen that c o n t r a l a t e r a l whole limb extension p r i o r to L - D O P A a d m i n i s t r a t i o n regularly induced increases b o t h in fitted m e a n a n d in depth o f m o d u l a t i o n . The d a t a points for change in m o d u l a t i o n are scattered a r o u n d the reference line for purely dynamic f u s i m o t o r effects (see legend). Consequently, the contralateral stimulation elicited changes in the response o f the spindle afferent, which are compatible with activation o f d y n a m i c f u s i m o t o r neurones. F u r thermore, from the d a t a shown in Fig. IE, it can be seen that the size o f the d y n a m i c fusimotor reflex effects, given by the m a g n i t u d e o f change in fitted mean a n d in depth o f m o d u l a t i o n , was significantly increased when identical reflex stimuli were p e r f o r m e d after administra-

tion o f L-DOPA. W h e n the anaesthesia was deepened, however, only insignificant changes in the sensitivity were induced (Fig. 1F). This strongly indicates that the changes in afferent response caused by extension o f the contralateral hind limb (Fig. 1D a n d E) could not have been attributed to mechanical interactions between the contralateral limb a n d the ipsilateral G S muscle, but rather to spinal fusimotor reflexes. M o r e o v e r , since no E M G activity was observed in the G S muscle either during control or d u r i n g test responses, reflex activation o f 1,- rather than /~-motoneurones seems to have been responsible for the sensitivity changes in the p r i m a r y muscle spindle afferent.

3.2. Effects on ipsi- and contralateral fusimotor reflexes Primary muscle spindle afferents. Altogether, 61 primary spindle afferents were studied with regard to

H. Johansson et al. / Neuroscience Research 22 (1995) 307-314

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Fig. 2. Histograms showing 1:he responsiveness (number of afferents with significant reflex effects/number of afferents tested) and the relative frequency of different types of reflex effects elicited on primary muscle spindle afferents before and after (0.5-3.0 h) administration of L-DOPA during full extension of the contralateral hind limb (A) and during tonic stretch of the ipsilaterai PBSt muscles (B). The relative frequencies are expressed as percentage of the total number of primary spindle afferents investigated with the contralateral stimulation (n = 61) and with the ipsilateral stimulation (n = 29). N, relative frequency of unresponsive afferents; D, relative frequency of afferents with excitatory dynamic fusimotor effects; M, relative frequency of afferents with excitatory dynamic and static fusimotor effects; I, relative frequency of afferents with iahibitory fusimotor effects.

fusimotor reflex effects elicited by full extension of the intact contralateral hind limb, and 29 primary spindle afferents were investiga:ted with tonic stretch of the ipsilateral PBSt muscles as reflex stimulus. Fig. 2 illustrates the responsiveness and the relative frequency of different types of reflex effects elicited by contralateral (in A) and ipsilateral stimuli (in B), before and after administration of L-DOPA. The analysis was based on the comparison between a number of consecutive control and test recordings, and statistically significant differences in fitted mean and/or in depth of modulation were regarded as true fusimotor reflex effects (see Methods). Reflex stimuli after [.-DOPA administration elicited statistically significant reflex effects in a larger number of spindle afferents as compared to prior to L-DOPA. The increased responsiveness was largely due to the appearance of dynamic', fusimotor reflex effects (i.e., significant changes both in fitted mean and in depth of modulation) in primary spindle afferents which prior to L-DOPA were unresponsive. For a few spindle afterents, however, the length sensitivity was either reduced,

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most likely as a result of inhibition of the fusimotor drive, or increased due to reflex actions onto both dynamic and static fusimotor neurones (i.e., significant changes in fitted mean only). Secondary muscle spindle afferents. Among the 10 secondary muscle spindle afferents investigated with contralateral whole limb extension, none showed any sign of fusimotor reflex effects before the L-DOPA injection. After the drug administration, four secondary afferents exhibited significant alteration in the stretch sensitivity. On these secondary spindle afferents, the contralateral reflex stimulus induced significant increases in fitted mean in combination with unchanged, or a slightly reduced, depth of modulation, thus indicating reflex effects mediated through static fusimotor neurones (cf. WadeU et al., 1991). Only two secondary spindle afferents were investigated for their responses to stretch of the ipsilateral PBSt muscles. None of these showed any fusimotor reflex effects, neither before nor after the LDOPA administration. For all afferents studied in the present investigation there are six pieces of evidence which, taken together, strongly suggest that the changes elicited on the GS muscle spindle afferents were caused by fusimotor reflex effects. First, the pelvis and the ipsilateral femur and tibia were firmly fixed to a rigid metal frame in order to avoid any potential mechanical interactions between the ipsilateral GS muscle and the ipsilateral PBSt muscles or the contralateral hind limb. Secondly, individual spindle afferents showed a considerable response variability in spite of identical mechanical stimulation. Thirdly, deterioration of the spinal circulation or deepening of the general anaesthesia always abolished or significantly reduced the effects. Fourthly, cutting of the nerve to the ipsilateral PBSt muscles, or cutting of the contralateral dorsal roots (L6-S0, invariably wiped out the effects caused by stretch of the ipsilateral PBSt muscles and/or by extension of the contralateral hind limb. Fifthly, some afferents which showed clear-cut fusimotor reflexes after L-DOPA administration, were found to be completely unresponsive, or to exhibit only very small fusimotor effects, to identical mechanical stimulation prior to the drug application. Sixthly, the induced changes in muscle spindle afferent activities were never correlated to extrafusal muscle activity, as deemed from comparisons of the EMG signal and the spindle afferent activity.

3.3. Effects on the resting activity in the ~/-muscle-spindle system The effects on the resting activity on GS primary muscle spindle afferents (i.e., sensitivity to sinusoidal GS stretch) were assessed by comparing averaged sinusoidal responses before and after administration of L-DOPA. The comparisons were based on control data (see Methods), i.e., from recordings with the contralateral

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H. Johansson et al./Neuroscience Research 22 (1995) 307-314

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Fig. 3. Scatter diagrams showing relative changes in resting activity of four primary muscle spindle afferents induced by administration of Nialamide (A) or Nialamide and L-DOPA (B-D). For each spindle afferent, the relative changes in depth of modulation and in fitted mean are plotted. Calculation of the relative changes for individual control trials after drug administration are based on mean values of 5-12 control trials recorded before drug administration. Prior to L-DOPA and/or Nialamide, the control activity of these four afferents ranged between 14.9-32.7 impulses/s in fitted mean, and between 20.7-48.8 impulses/s in depth of modulation. Recordings after drug administration started 20 rain after Nialamide/LDOPA injection and each afferent was tested with 2-3 min interval.

ankle, knee and hip joints in intermediate joint position and with the ipsilateral PBSt muscles in approximate resting lengths. Fig. 3 illustrates examples of L-DOPAinduced alteration in resting activity for four primary muscle spindle afferents. For each spindle afferent, the relative changes (in relation to mean values recorded prior to L-DOPA) in fitted mean and in depth of modulation are illustrated. The resting activity was investigated before and after L-DOPA administration on 29 primary muscle spindle afferents. These spindle afferents were selected since they were investigated with at least 5 control runs both before and after L-DOPA treatment. The resting activity was unchanged in 4 afferents (cf. Fig. 1) and increased in 25 afferents after L-DOPA administration. As assessed from statistically significant (P < 0.05: twotailed Mann-Whitney test) increases in fitted mean and in depth of modulation (Fig. 3A and D), the large majority of the afferents (20 out of 25) showed increased dynamic fusimotor drive. Five afferents displayed a combination of statistically significant (P < 0.05) increases in fitted mean and reductions (or unchanged values) in depth of modulation, suggesting enhanced activity in static or in both static and dynamic fusimotor neurones (Fig. 3B and C). It should be pointed out that for most spindle afferent the increases in resting were surprisingly large. Thus, an increased background activity in the order of 100%-200% was commonly found (Fig. 3A and B).

4. Discussion

The results accounted for in this paper show that administration of L-DOPA induces increased ieflex responsiveness and resting activity in muscle spindle afferents from the GS muscle in the spinalized cat. It was evidenced by a number of observations that the evoked changes in the sensitivity of muscle spindle afferent were due to segmental fusimotor actions. The findings suggest that segmental monoaminergic descending pathways (and/or interneurones) participate in the regulation of the GS 1,-muscle-spindle system by increasing the reflex responsiveness to afferent inputs from the ipsi- and contralateral hind limbs, and by enhancing the resting activity, on primarily dynamic fusimotor neurones. A number of previous studies have shown that both the resting activity and the reflex responsiveness of 7motoneurones are reduced after spinalization (for references, see Murthy, 1978; Hulliger, 1984). Spinal cat recordings from extensor muscle spindle afferents suggest that dynamic ~-motoneurones, in contrast to static, have a resting activity and can be reflexly activated by electrical nerve stimulation (Alnaes et al., 1965). The findings of the present study, that the resting activity and the reflex responsiveness were low and that only dynamic fusimotor reflex effects were observed prior to L-DOPA administration, are in accordance with these observations.

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By recording from gastroenemius 1'-efferents, Grillner (1969) found that the resting activity in fusimotor neurones was increased following L-DOPA administration in the spinalized cat. Furthermore, after L-DOPA injection a larger number of 1'-efferents were reflexly influenced by electrical or physiological activation of skin and muscle receptor afferents. Based on the observation that these effects were found in parallel with an elevated discharge rate in secondary muscle spindle afferents, it was suggested that the L-DOPA-mediated fusimotor effects on extensor muscles were confined to static 1"motoneurones, noweve:r, since unfortunately no recordings were made from primary muscle spindle afferents, any possible L-DOPA-induced alterations of the dynamic fusimotor activity would have passed undetected. The principal observations by Grillner (1969) were confirmed in this study, i.e., that L-DOPA increases the resting activity and the reflex responsiveness of fusimotor neurones in the spinal preparation. In addition, the present experiments demonstrate that the changes in fusimotor activity induced by L-DOPA are sufficiently clear-cut to be manifested as significant alterations in the discharge rates of both primary and seeondary extensor muscle spindle afferents (cf. Sjrstrrm and Zangger, 1976). By applying the generally accepted method for indirect classification of fusimotor effects (for references, see Hulliger, 1984; Johansson et al., I991b), by which changes in sinusoidal responses of muscle spindle afferents are compared with changes elicited by controlled electrical stimulation of static and dynamic fusimotor neurones, it was possible to conclude that L-DOPA alters the activity of both types of fusimotor neurones. However, in contrast to the dominance of static fusimotor effects tentatively suggested previously (Grillner, 1969), the results accounted for here demonstrate that L-DOPA-mediated activation of monoaminergic pathw~tys primarily increases the dynamic stretch sensitivity of GS primary muscle spindle afferents, most probably via activation of dynamic fusimotor neurones. The observation that L-DOPA administration had very little effect on the reflex responsiveness to ipsilateral PBSt stretch, whereas the responsiveness to contralateral whole limb extension was considerably increased, might appear somewhat puzzling. Yet, in previous studies on L-DOPA untreated cats with intact spinal cord, it was demonstrated that the responsiveness to extension of the inl~act contralateral hind limb was about twice as large as that to stretch of ipsilateral muscles (Appelberg et al. 1982, 1984; Johansson et al. 1991a). The most likely reason for this is that the contralateral stimulation activated a much larger number of afferents, including mechanosensitive receptor afferents from different muscles and joints, as well as from the skin. It is therefore conceivable that the L-DOPAmediated increase in reflex responsiveness on muscle spindle afferents requ!ires convergence from a reason-

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ably large number of active afferents, or, alternatively, from active joint and/or cutaneous afferents. In this context, it should be pointed out that the overall afferent input from the ipsilateral hindlimb was greatly impaired due to extensive denervations.

4.1. Possible functional implications In a number of studies it has been shown that activity in monoaminergic descending pathways exert modulatory actions on lumbar 1'-motoneurones. The results of the present investigation indicate that activation of monoaminergic descending or segmental pathways, via excitation of fusimotor neurones, primarily influence the dynamic sensitivity of the extensor muscle spindle afferents. Recent observations by Ellaway and coworkers suggest that pathways originating in the pontomedullatory brainstem might control the synchrony of 1'-motoneurone discharges (Davey and Ellaway, 1988; Baker et al., 1991), which through the muscle spindle loop may result in a higher degree of synchronization of the ct-motoneurone activity (Windhorst, 1978). Thus, by altering the degree of synchronization or the balance between static and dynamic fusimotor activity, it is conceivable that the monoaminergic control of the 1'muscle-spindle system may be involved in the normal regulation of muscle tone in posture and movements and/or in the neuromuscular mechanism responsible for tremor and rigidity. Several authors have proposed that pathological muscle rigidity and tremor might be attributed to a disturbed descending control of fusimotor neurones (Jansen, 1962; Hassler 1966; Andrews et al., 1972; Noth et al., 1988). An increased static fusimotor activity or a decreased dynamic fusimotor drive will reduce the sensitivity of muscle spindles to small muscle stretches, and thereby cause a depression of the short-latency phasic stretch reflex. From studies on rigid Parkinsonian patients, evidence have been reported for a significant reduction of the phasic stretch reflex which, however, is restored by L-DOPA treatment (Hassler, 1973; Scholz, et al., 1987; Noth et al., 1988). In the light of the present results it is tempting to speculate that the L-DOPAmediated re-establishment of the phasic stretch reflex may be due to an elevated activity in dynamic fusimotor neurones. The dynamic input to ot-motoneurones may be facilitated by a reduction of static influences via seeondary muscle spindle afferents, since it has been shown that L-DOPA depresses transmission in spinal pathways from group II muscle afferents, whereas the activity in pathways from group I muscle afferents is left unaffected (Schomburg and Steffens, 1988; Bras et al., 1989).

Acknowledgements This study was supported by grants from The Swedish Work Environment Fund and Centrum frr Idrottsforskning.

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