Comparison of effects of monoamines on transmission in spinal pathways from group I and II muscle afferents in the cat

Serotoninergic and noradrenergic axonal contacts associated with premotor interneurons in spinal pathways from group II muscle afferents

We investigated the possibility that monoaminergic axons make contacts with spinal interneurons which project to motor nuclei and are monosynaptically activated by group II muscle afferents. Interneurons in midlumbar spinal segments of adult cats were characterized electrophysiologically and intracellularly labelled with tetramethylrhodamine dextran. Serotoninergic and noradrenergic axons were identified with immunofluorescence in sections containing labelled cells. Contacts between monoaminergic axons and interneurons were investigated with three-colour confocal laser scanning microscopy and analysed with a computer reconstruction program. Cell bodies and dendritic trees of five cells were reconstructed and putative contacts were plotted. The average number of contacts formed by serotoninergic axons was 140 and the average number of noradrenergic contacts was 38. The majority (95%) of contacts were formed with dendrites; these were distributed over the entire dendritic tree, even on the most distal branches. These findings provide a morphological basis for the modulatory actions of monoamines on premotor spinal interneurons in pathways from group II muscle afferents.

Major role for sensory feedback in soleus EMG activity in the stance phase of walking in man

1. Sensory feedback plays a major role in the regulation of the spinal neural locomotor circuitry in cats. The present study investigated whether sensory feedback also plays an important role during walking in 20 healthy human subjects, by arresting or unloading the ankle extensors 6 deg for 210 ms in the stance phase of gait. 2. During the stance phase of walking, unloading of the ankle extensors significantly (P < 0.05) reduced the soleus activity by 50 % in early and mid-stance at an average onset latency of 64 ms. 3. The onset and amplitude of the decrease in soleus activity produced by the unloading were unchanged when the common peroneal nerve, which innervates the ankle dorsiflexors, was reversibly blocked by local injection of lidocaine (n = 3). This demonstrated that the effect could not be caused by a peripherally mediated reciprocal inhibition from afferents in the antagonist nerves. 4. The onset and amplitude of the decrease in soleus activity produced by the unloading were also unchanged when ischaemia was induced in the leg by inflating a cuff placed around the thigh. At the same time, the group Ia-mediated short latency stretch reflex was completely abolished. This demonstrated that group Ia afferents were probably not responsible for the decrease of soleus activity produced by the unloading. 5. The findings demonstrate that afferent feedback from ankle extensors is of significant importance for the activation of these muscles in the stance phase of human walking. Group II and/or group Ib afferents are suggested to constitute an important part of this sensory feedback.

Effects of monoamines on interneurons in four spinal reflex pathways from group I and/or group II muscle afferents

Effects of locally applied serotonin (5-HT) and noradrenaline (NA) were tested on extracellularly recorded responses of single spinal interneurons in deeply anaesthetized cats. These effects were tested on: (i) interneurons mediating reciprocal inhibition from group Ia afferents; (ii) interneurons mediating non-reciprocal inhibition from group Ia and Ib afferents; (iii) intermediate zone interneurons co-excited by group I and II afferents; and (iv) dorsal horn interneurons excited by group II afferents. Effects of monoamines were tested on responses evoked at latencies compatible with monosynaptic coupling. Responses evoked by group Ia and/or Ib muscle afferents were facilitated in all of the tested interneurons both by NA and 5-HT. Responses evoked by group II muscle afferents were depressed in the majority of the interneurons but were facilitated in some of them. 5-HT depressed these responses in all dorsal horn interneurons and in one subpopulation of intermediate zone interneurons, while it facilitated them in another subpopulation of intermediate zone interneurons. NA depressed them in all intermediate zone interneurons and in one subpopulation of dorsal horn interneurons, while it facilitated them in another subpopulation of dorsal horn interneurons. The results of this study lead to the conclusions that: (i) modulation of synaptic actions of muscle spindle and tendon organ afferents on spinal interneurons by 5-HT and NA is related to both the type of the afferent and the functional type of the interneuron; and that (ii) 5-HT and NA counteract each others' actions on some interneuronal types but mutually enhance them on the others.

Recovery of locomotion after ventral and ventrolateral spinal lesions in the cat. II. Effects of noradrenergic and serotoninergic drugs

The effects of serotoninergic and noradrenergic drugs (applied intrathecally) on treadmill locomotion were evaluated in two adult cats subjected to a ventral and ventrolateral spinal lesion (T13). Despite the extensive spinal lesion, severely damaging important descending pathways such as the reticulo- and vestibulospinal tracts, both cats recovered quadrupedal voluntary locomotion. As detailed in a previous paper, the locomotor recovery occurred in three stages defined as early period, when the animal could not walk with its hindlimbs, recovery period, when progressive improvement occurred, and plateau period, when a more stable locomotor performance was observed. At this latter stage, the cats suffered from postural and locomotor deficits, such as poor lateral stability, irregular stepping of the hindlimbs, and inconsistent homolateral fore- and hindlimb coupling. The present study aimed at evaluating the potential of serotoninergic and/or noradrenergic drugs to improve the locomotor abilities in the early and late stages. Both cats were implanted chronically with an intrathecal cannula and electromyographic (EMG) electrodes, which allowed determination, under similar recording conditions, of the locomotor performance pre- and postlesion and comparisons of the effects of different drugs. EMG and kinematic analyses showed that norepinephrine (NE) injected in early and plateau periods improved the regularity of the hindlimb stepping and stabilized the interlimb coupling, permitting to maintain constant locomotion for longer periods of time. Methoxamine, the alpha1-agonist (tested only at the plateau period), had similar effects. In contrast, the alpha2-agonist, clonidine, deteriorated walking. Serotoninergic drugs, such as the neurotransmitter itself, serotonin (5HT), the precursor 5-hydroxytryptophan (5HTP), and the agonist quipazine improved the locomotion by increasing regularity of the hindlimb stepping and by increasing the step cycle duration. In contrast, the 5HT1A agonist 8-hydroxy-dipropylaminotetralin (DPAT) caused foot drag in one of the cats, resulting in frequent stumbling. Injection of combination of methoxamine and quipazine resulted in maintained, regular stepping with smooth movements and good lateral stability. Our results show that the effects of drugs can be integrated to the residual voluntary locomotion and improve some of its postural aspects. However, this work shows clearly that the effects of drugs (such as clonidine) may depend on whether or not the spinal lesion is complete. In a clinical context, this may suggest that different classes of drugs could be used in patients with different types of spinal cord injuries. Possible mechanisms underlying the effect of noradrenergic and serotoninergic drugs on the locomotion after partial spinal lesions are discussed.

Active motor neurons potentiate their own sensory inputs via glutamate-induced long-term potentiation

Adaptive motor control is based mainly on the processing and integration of proprioceptive feedback information. In crayfish walking leg, many of these operations are performed directly by the motor neurons (MNs), which are connected monosynaptically by sensory afferents (CBTs) originating from a chordotonal organ that encodes vertical limb movements. An in vitro preparation of the crayfish CNS was used to investigate a new control mechanism exerted directly by motor neurons on the sensory inputs themselves. Paired intracellular recordings demonstrated that, in the absence of any presynaptic sensory firing, the spiking activity of a leg MN is able long-lastingly to enhance the efficacy of the CBT-MN synapses. Moreover, this effect is specific to the activated MN because no changes were induced at the afferent synapses of a neighboring silent MN. We report evidence that long-term potentiation (LTP) of the monosynaptic EPSP involves a retrograde system of glutamate transmission from the postsynaptic MN, which induces the activation of a metabotropic glutamate receptor located presynaptically on the CBTs. We demonstrate that LTP at crayfish sensory-motor synapses results exclusively from the long-lasting enhancement of release of acetylcholine from presynaptic sensory afferent terminals, without inducing any modifications in postsynaptic MN properties. Our data indicate that this positive feedback control represents a functional mechanism that may play a key role in the auto-organization of sensory-motor networks.

Modulation of responses of feline gamma-motoneurones by noradrenaline, tizanidine and clonidine

1. Effects of noradrenaline (NA) and the alpha2 agonists tizanidine and clonidine were tested on extracellularly recorded responses of gamma-motoneurones in deeply anaesthetized cats. Two types of responses were used; firstly, short latency phasic responses evoked by electrical stimulation of group II afferents in a muscle nerve and, secondly, tonic background discharges. 2. Responses evoked by group II muscle afferents were depressed when NA and tizanidine were applied ionophoretically close to a gamma-motoneurone and when clonidine was applied systemically. The number of spike potentials evoked by stimulation of these afferents decreased and their latencies increased. Responses evoked by flexor or extensor afferents in gamma-motoneurones innervating flexors or extensors were similarly depressed. 3. Tonic discharges were inconsistently and/or insignificantly affected by locally applied NA and tizanidine but were depressed by systemically applied clonidine. 4. Control tests indicate specific effects of NA and tizanidine application since similarly ionophoresed H+ ions did not change responses of gamma-motoneurones to stimulation of group II afferents, or only weakly enhanced their background discharges. Furthermore, serotonin ejected from a solution with a similar pH facilitated rather than depressed responses of gamma-motoneurones. 5. The results indicate that some antispastic effects of clonidine and tizanidine may be due to the depression of group II-evoked responses of gamma-motoneurones, resulting in weaker responses of muscle spindles to muscle stretches.

Medium-latency response to muscle stretch in human lower limb: estimation of conduction velocity of group II fibres and central delay

In standing subjects, ankle dorsiflexion evoked short-latency responses (SLRs) at 41 and 57 ms, on the average, in soleus (Sol) and flexor digitorum brevis (FDB), respectively. Medium-latency responses (MLRs) occurred at 70 and 95 ms. The time between the MLRs was 25 ms and between the SLRs was 16 ms. The difference between these two values represents the extra-time to conduct the FDB volley for MLR from distal to proximal muscle, in excess to that for SLR. The velocity of the afferents mediating the FDB MLR (21.4 m/s on average) was estimated by dividing the distance between the two muscles by the sum of the above extra-time and the conduction time of Ia fibres along the same distance. The central delay of FDB MLR (6.7 ms on average) was obtained by dividing the distance between FDB and spinal cord by the sum of afferent and efferent MLR conduction times. The central delay of FDB SLR (1.4 ms) was analogously obtained. These findings give an estimation of the conduction velocity of the group II afferent fibres in humans and support the hypothesis that the FDB MLR is relayed through a spinal oligosynaptic pathway.

Modulation of responses of four types of feline ascending tract neurons by serotonin and noradrenaline

Modulation of responses of four types of ascending tract cells by noradrenaline and serotonin was compared in order to investigate how information forwarded by these cells may be gated by monoaminergic tract neurons. Spinocervical tract, postsynaptic dorsal column and dorsal spinocerebellar tract neurons located in Clarke's column and in the dorsal horn were identified by their axonal projections. Noradrenaline and serotonin were applied ionophoretically close to a selected neuron, and their effects were tested on extracellularly recorded responses of this neuron to electrical stimulation of low-threshold skin afferents and group II muscle spindle afferents. The modulatory actions of noradrenaline and serotonin were estimated from changes in the number of responses evoked by 30 successive stimuli, the minimal latencies of these responses, and their firing frequency. All four populations of ascending tract neurons investigated were modulated by serotonin and noradrenaline, but not in the same way. The responses were most often depressed by noradrenaline and facilitated by serotonin, but in some types of neuron they were affected in the same direction. Transmission from low-threshold skin and group II muscle afferents changed in the same direction in some types of neuron but in the opposite direction in other types. The results indicate that transfer of information from skin and group II muscle afferents to supraspinal centres may be gated by descending monoaminergic pathways in a highly differentiated manner, and is adjusted to the requirements of various behavioural situations.

SEROTONERGIC pontomedullary neurons are not activated by antinociceptive stimulation in the periaqueductal gray

The antinociceptive and cardiovascular effects of midbrain periaqueductal gray (PAG) stimulation are mediated through a relay in the pontomedullary raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC). To test whether the neurons important in mediating PAG-evoked effects are SEROTONERGIC, the responses of pontomedullary SEROTONERGIC-LIKE cells to PAG stimulation were tested. SEROTONERGIC-LIKE neurons (n = 21) were recorded extracellularly in halothane-anesthetized Sprague Dawley rats. Serotonergic-like neurons were distinguished by their slow and steady background discharge. Two neurons that were physiologically characterized as SEROTONERGIC-LIKE were intracellularly labeled and processed for serotonin immunoreactivity; both cells tested contained immunoreactive serotonin. Train stimulation of sites within the midbrain PAG, at intensities of </=50 microA, suppressed the tail withdrawal from noxious heat and evoked changes in blood pressure and heart rate. No SEROTONERGIC-LIKE cells were activated by single-pulse or short-train (two to five pulses) stimulation of the PAG at antinociceptive intensities. In most cases, SEROTONERGIC-LIKE cells were unaffected by long-train stimulation (5-6 sec) of the PAG, which produced antinociception and cardiovascular changes. In contrast, >50% of the cells in two nonserotonergic-like cell classes were activated at short latency by such PAG stimulation. In conclusion, monosynaptic excitation of SEROTONERGIC cells in RM/NRMC is unlikely to be necessary for the nociceptive and autonomic modulatory effects of PAG stimulation.

A confocal microscopic survey of serotoninergic axons in the lumbar spinal cord of the rat: co-localization with glutamate decarboxylase and neuropeptides

Patterns of co-localization of serotonin with glutamate decarboxylase (the synthetic enzyme for GABA) or each one of eight neuropeptides (calcitonin gene-related peptide, dynorphin, enkephalin, galanin, neuropeptide Y, neurotensin, substance P and somatostatin) were investigated with dual-colour confocal laser scanning microscopy in the lumbar spinal cords of three adult rats. Four regions of the gray matter were studied (laminae I-II, V, IX and X). The extent of co-localization was estimated by direct assessment of merged pairs of optical sections and by automated image analysis. Co-localization of serotonin and glutamate decarboxylase was found only in a few axons of laminae I-II but was not detected in other laminae. Peptides were not co-localized with serotonin in the superficial dorsal horn but considerable co-localization was found in motor nuclei and sparse co-localization was found in laminae V and X. Galanin and substance P frequently co-existed with serotonin in lamina IX but some co-localization with dynorphin, somatostatin, [Met]enkephalin and neuropeptide Y was also detected. Galanin, substance P and dynorphin were also co-localized with serotonin in a few axons of the deep dorsal horn and in the gray matter around the central canal. Neurotensin and calcitonin gene-related compound did not co-exist with serotonin in any of the laminae investigated. This evidence suggests that different populations of serotoninergic axons project to different regions of the spinal gray matter. Those containing glutamate decarboxylase terminate in the superficial dorsal horn and are likely to be involved in antinociception, whereas those containing peptides terminate principally in motor nuclei and are likely to modulate motor activity.

Antispastic effects of L-dopa

Antispastic effects of the noradrenaline and dopamine precursor l-3,4-dihydroxyphelanine (L-dopa) were investigated in 11 subjects in which exaggerated stretch reflexes developed after spinal cord injuries. The effects were evaluated from changes in the electromyographic (EMG) response of the quadriceps muscle during tendon jerks evoked by standardized taps over the patellar tendon, in clonus and in resistance to passive movements of the limb. After administration of L-dopa, EMG responses occurring 30-150 ms after the tendon tap decreased to about 50% of control, and clinical tests revealed a marked decrease in the resistance to muscle stretches and in the degree of clonus. The effects were maximal within about 1 h. The depressive actions of L-dopa are interpreted as being exerted primarily at the spinal level, since they were evoked in paraplegics and tetraplegics. The results support the previous hypothesis that group II muscle afferents contribute to.the exaggerated stretch reflex in spastic patients because L-dopa depresses transmission from group II but not from group I muscle afferents. They also indicate the possibility of using L-dopa in the treatment of spastic patients.

Synaptic relationships between serotonin-immunoreactive axons and dorsal horn spinocerebellar tract cells in the cat spinal cord

Dorsal horn spinocerebellar tract cells were identified according to electrophysiological criteria in adult cats and labelled intracellularly with horseradish peroxidase. Sections containing labelled neurons were processed to reveal serotonin immunoreactivity and examined with light and electron microscopy. Numerous contacts were observed on cell bodies, and on proximal and intermediate parts of dendrites. Electron microscopic examination of contacts revealed that synaptic junctions were usually present at the region of apposition. It is concluded that serotonin has a postsynaptic action on dorsal horn spinocerebellar tract cells and that this action is mediated through conventional synapses.

Time course of 'set'-related changes in muscle responses to stance perturbation in humans

1. In standing subjects, toe-down rotation of a supporting platform elicits a medium-latency response (MLR) in tibialis anterior (TA) muscle and a long-latency response (LLR) in soleus (Sol). Toe-up rotation induces a short-latency response (SLR) in Sol and a LLR in TA. When subjects steadily hold onto a stable frame, all responses are decreased, except Sol SLR. The aim of this investigation was to assess whether the response modulation is dependent on information from the hand touching the frame, or whether it anticipates the holding task. 2. The time course of the changes in response amplitude was studied in a time interval centred around the act of holding, performed in a reaction-time mode. Subjects kept their extended arm close to the frame in front of them and brought the hand in contact with the frame in response to a visual go-signal. The platform was moved at different intervals prior to or after the go-signal. Surface EMGs of Sol, TA and deltoid (Delt) were recorded. 3. TA MLR began to decrease when the platform was displaced at an interval of 140 ms after the go-signal, about 200 ms before subjects touched the frame and 120 ms before termination of Delt EMG. Four hundred milliseconds after the go-signal the response reached and maintained maximal inhibition, similar to that occurring under the stationary holding condition. The time course of inhibition of Sol LLR and TA LLR was similar to that of TA MLR, except that LLRs began to decrease at an earlier interval. Due to the different response latency from the onset of the perturbations, the beginning of inhibition of both MLRs and LLRs occurred almost simultaneously. 4. The changes in amplitude of leg muscle responses are not triggered by the go-signal, contact with the frame, or arm motion, suggesting that the modulation is related to the transition to a new, stabilized postural 'set'. The similar extent and parallel time course of MLR and LLR suppression, possibly transmitted through different pathways, points to the spinal cord as the site of action. The lack of depression of the monosynaptic SLR suggests an effect at premotoneuronal level. On the basis of selectivity, latency and time course of the effect, we favour the hypothesis that a monoaminergic pathway from the brainstem is involved.

Dopaminergic control of transmission from group II muscle afferents to spinal neurones in the cat and guinea-pig

The effects of dopamine and its agonists on transmission from muscle afferents to spinal neurones were investigated in the cat and guinea-pig spinal cord, by measuring the drug effects on the amplitude of monosynaptic field potentials evoked by electrical stimulation of group I and group II muscle afferents. Local iontophoretic application of dopamine, the dopamine D1/D5 agonist SKF-38393 and the D2/D3/D4 agonist quinpirole all depressed the group II field potentials evoked at the base of the dorsal horn. Group II field potentials in the intermediate zone were depressed by dopamine to a similar degree as the dorsal horn field potentials, whereas the dopamine agonists were without effect upon them. The intermediate zone field potentials evoked by group I muscle afferents were not depressed by any of the drugs. The dopamine-evoked depression of the group II-evoked field potentials in the dorsal horn in the guinea-pig spinal cord was reduced by the simultaneous application of haloperidol. The results demonstrate that dopamine receptors mediate the depression of transmission from group II muscle afferents to interneurones in the dorsal horn, but not to neurones in the intermediate zone of the spinal cord.

Effects of serotonin on dorsal horn dorsal spinocerebellar tract neurons

Effects of ionophoretic application of serotonin and of one of its agonists were tested on responses of dorsal horn dorsal spinocerebellar tract neurons evoked by electrical stimulation of peripheral nerves. Both drugs depressed monosynaptically evoked actions of group II muscle afferents; they decreased the number and/or increased the latency of spike potentials evoked by these afferents. In contrast, synaptic actions of low-threshold cutaneous afferents (mono- or oligosynaptic) were facilitated in the majority of the neurons, as judged by decrease in the latency of spike potentials evoked by stimulation of a cutaneous nerve and/or an increase in the number of these potentials. It is proposed that facilitatory actions assist in maintaining tonic discharges of dorsal spinocerebellar tract neurons in some movements and that the selective control of group II input is used to correlate activity of spinal and supraspinal neurons. Both actions may be subserved by tight contacts between serotoninergic nerve fibres and dorsal spinocerebellar tract neurons, which have been revealed in a parallel study.

Contacts between serotoninergic fibres and dorsal horn spinocerebellar tract neurons in the cat and rat: a confocal microscopic study

Contacts between serotoninergic nerve fibres and dorsal horn dorsal spinocerebellar tract neurons were analysed in order to investigate the morphological basis of actions of serotonin upon dorsal spinocerebellar tract neurons. In a series of experiments dorsal spinocerebellar tract neurons were labelled with intracellularly injected rhodamine-dextran in the cat. The neurons were monosynaptically excited by group II muscle afferents and cutaneous afferents and were identified by antidromic activation following stimuli applied in the cerebellum. In the second series of experiments dorsal spinocerebellar tract neurons were labelled by retrograde transport of Fluorogold injected into the cerebellum in the rat. In both series, serotoninergic fibres were labelled by using a specific anti-serotonin antiserum and were revealed by immunofluorescence. Appositions between the serotoninergic fibres and the cells were inspected with a dual channel confocal microscope. The merged images obtained with the two channels of the microscope were viewed in single optical planes 2 microns apart and in rotated three-dimensional reconstructions. Serotoninergic nerve fibres were found in apposition to cell bodies of all feline dorsal spinocerebellar tract neurons (n = 7) and of 75% of rat dorsal spinocerebellar tract neurons (n = 90). The numbers of putative contacts on cell bodies varied between less than 100 and nearly 300 (mean 160) in the cat and between about five and 30 in the rat. Contacts with dendrites of feline neurons were seen on 96% of 72 dendrites within 300 microns from soma and on 91% of 23 dendrites at distances of 300-500 microns. The number of such contacts varied from less than five to 150 on a single dendrite within these ranges of distances. Their total number within 100 microns from the soma was comparable or exceeded the number of contacts on the soma.

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

The experiments were performed on lightly alpha-chloralose anaesthetised 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-beta-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 the muscle 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 monoaminergic descending control of fusimotor neurones in the regulation of muscle tone, tremor and rigidity is discussed.

Selective depression of medium-latency leg and foot muscle responses to stretch by an alpha 2-agonist in humans

1. In standing humans, toe-up rotation of a platform induces a short-latency (SLR) and a medium-latency response (MLR) in both soleus (Sol) and flexor digitorum brevis (FDB) muscles. Toe-down rotation evokes a MLR in the tibialis anterior (TA). The SLR is the counterpart of the monosynaptic stretch reflex, but the origin of the MLR is still debated. By means of tizanidine (an alpha 2-adrenergic receptor agonist) we tested the hypothesis that the MLR is relayed by group II afferent fibres, since animal data indicate that tizanidine or stimulation of monoaminergic brainstem centres decrease the excitability of spinal interneurones supplied by those fibres. In addition, we compared the effect of the drug on these responses with that induced by stabilization of posture. 2. Eight subjects received tizanidine (150 micrograms kg-1 orally) or placebo, in a single-blind design. Platform rotations were delivered prior to administration and for 3 h afterwards. Both TA- and FDB-MLRs decreased in size, starting from about 1 h after tizanidine administration. Sol-SLR was unaffected. Response latencies were unchanged. Placebo induced no changes in any response. In each subject, the extent of TA-MLR depression induced by holding onto a frame and by tizanidine was superimposable. 3. The selective effect of tizanidine on MLR supports the notion that it is relayed through group II afferent fibres. The similar effects of holding and tizanidine on the response suggests that it is modulated by monoaminergic centres.

Effects of monoamines on transmission from group II muscle afferents in sacral segments in the cat

The effects of one 5-HT1A serotonin agonist (8-OH-DPAT) and of two alpha 2 noradrenaline agonists (tizanidine and B-HT 933) were tested on the transmission between group II muscle afferents and spinal neurons in the sacral segments of the spinal cord in the cat. These agonists have previously been found to depress transmission from group II muscle afferents either in the dorsal horn or in the intermediate zone of midlumbar segments, and this study addressed the question of whether their actions in the sacral segments are similarly selective. The drugs were applied ionophoretically and their effects were tested on field potentials evoked from group II muscle afferents. As judged by changes in the amplitude of the early components of these field potentials, the transmission is effectively depressed by the serotonin agonist (to 56 +/- 26% after 2 min of ionophoresis of 8-OH-DPAT) but not by the noradrenaline agonists (to 97 +/- 12% after 6 min of ionophoresis of B-HT 933 and to 95 +/- 17% after 6 min of ionophoresis of tizanidine). These data suggest that transmission from group II muscle spindle afferents in the sacral segments is under control of serotonin releasing neurons, as in the dorsal horn of midlumbar segments, but leave open the question of the similarities or differences in the mechanisms (pre- and/or postsynaptic) of this control.

Peripheral proprioceptive modulation in crayfish walking leg by serotonin

In vitro serotoninergic modulation of intracellularly recorded sensory responses was examined in primary afferent terminals of a crayfish leg proprioceptor, the coxo-basal chordotonal organ (CB CO). The effects of different concentrations of serotonin (5-HT) on static and dynamic sensory responses were analysed following bath or pressure applications of the monoamine directly on the strand of the mechanoreceptor. Consequently, the reported effects result from the direct peripheral action of 5-HT on the sensory organ itself. Serotonin modulates the sensory activity by modifying the sensory discharge frequency. The firing discharge of the primary afferents is increased in a dose-dependent manner. The maximal effect is obtained with a concentration of 10(-6) M. Higher concentrations are less effective, and for 20% of the recorded cells, 10(-4) M 5-HT induces a decrease of the sensory discharge, i.e. has an inhibitory effect. Alteration in the pattern of sensory firing, resulting in bursting discharge, was observed in some units. All the recorded sensory units were responsive to the neuromodulator whatever their functional properties. The effects of 5-HT lasted as long as the amine was applied and were reversible after wash. The results suggest that 5-HT could exert a modulatory action on the proprioceptive feedback, by peripheral action on the sensory organ. The natural modalities of 5-HT action are discussed on the basis of immunohistochemistry data suggesting: (i) connections between CB CO and central serotoninergic cells, (ii) 5-HT content in sensory cells of the CB CO. Since the CB CO is involved in the control of leg movement and position, the modulation of its primary afferents might influence the organization of the locomotor pattern. The functional significance of this peripheral sensory neuromodulation was approached by the analysis of the motor reflex activity.

Presynaptic modulation of spinal reflexes

Recent evidence suggests that independent sets of interneurons mediate presynaptic inhibition of primary and secondary muscle spindles and of tendon organ afferents. There is also evidence that the information which flows through different intraspinal collaterals of a single muscle spindle or tendon organ afferent fiber is selectively affected by electrical stimulation of the motor cortex. These studies suggest that presynaptic inhibition plays an important role in the selection of the sensory signals required for the execution of a specific motor task.

Gating of transmission to motoneurones by stimuli applied in the locus coeruleus and raphe nuclei of the cat

1. Neuronal systems activated by stimulation in the region of the locus coeruleus/subcoeruleus (LC/SC) and raphe nuclei have previously been shown to depress transmission from group II muscle afferents in regions of the midlumbar spinal segments in which premotor interneurones are located. The aim of the present investigation was to determine the extent to which such depression is paralleled by depression of the reflex actions of group II afferents on motoneurones. 2. The effects of short trains of conditioning electrical stimuli applied within the LC/SC and raphe nuclei were examined on postsynaptic potentials (PSPs) evoked by group I and group II muscle afferents in hindlimb motoneurones. The effects were examined over a wide range of conditioning-test intervals but particular emphasis was placed on the effects produced at long intervals (> 100 ms) since such effects are more likely to be mediated by the descending noradrenergic and serotonergic neurones of the LC/SC and raphe nuclei which are of slow conduction velocity. In addition, conditioning stimuli alone evoked PSPs in motoneurones (with latencies of 7-15 ms and a duration of 50-80 ms) and effects evoked at short conditioning-test intervals might therefore have been secondary to changes in motoneurone membrane properties. 3. At conditioning-test intervals between 100 and 350 ms synaptic actions of group II origin were strongly and consistently depressed. Both EPSPs and IPSPs were affected, two-thirds of those tested being reduced in amplitude by 50% or more. A similar depression was exerted on PSPs evoked from the quadriceps and deep peroneal nerves mediated predominantly by interneurones located in the midlumbar segments and on PSPs evoked from the hamstring and triceps surae nerves mediated by interneurones located in more caudal segments. It is thus concluded that neuronal systems activated by stimuli applied in the LC/SC and raphe nuclei are capable of gating transmission in all those interneuronal pathways which mediate the reflex actions of group II afferents on motoneurones in anaesthetized animals.

Interactions between pathways controlling posture and gait at the level of spinal interneurones in the cat

The properties of three interneuronal populations controlling posture and locomotion are briefly reviewed. These are interneurones mediating reciprocal inhibition of antagonistic muscles and interneurones in pathways from secondary muscle spindle afferents to ipsilateral and contralateral motoneurones, respectively. It will be shown that these interneurones subserve a variety of movements, with functionally specialized subpopulations being selected under different conditions. Mechanisms for gating the activity of these neurones appear to be specific for each of them but to act in concert. Interneurones which are active during locomotion and postural reactions are distributed over many segments of the spinal cord and over several of Rexed's laminae, both in the intermediate zone and in the ventral horn (Berkinblit et al., 1978; Bayev et al., 1979; Schor et al., 1986; Yates et al., 1989). The location of neurones discharging during neck and labyrinthine reflexes is illustrated in Fig. 1A and B but indications that neurones with an even wider distribution contribute to locomotion, scratching and the related postural reactions have been provided by neuronal markers which preferentially label active neurones (WGA-HRP; see Noga et al., 1987) or neurones with active genetic transcription (c-fos; I. Barajon, personal communication; Dai et al., 1991). Such a wide distribution indicates a high degree of non-homogeneity, since neurones of different functional types are usually located in different laminae. It has been demonstrated that some of these neurones may be particularly important for setting up the rhythm of muscle contractions specific for different gaits or scratching, as part of their "pattern generators" (see, e.g., Grillner, 1981). Other neurones may be primarily involved in initiation of these movements or in postural adjustments combined with them. A considerable proportion of neurones mediating these movements are nevertheless likely to be used not in one particular type of movement but in a variety of movements, and contribute to postural reactions and locomotion as well as to various segmental reflexes and centrally initiated movements; they are likely to operate as last order (premotor) interneurones of several spinal pathways to motoneurones. One of the indications that this is the case is the overlap between the areas of location of interneurones active during postural reactions, locomotion, or scratching and the areas of location of premotor interneurones (Fig. 1C,D). The latter were labelled by loading motoneurones with WGA-HRP and by its subsequent retrograde transneuronal transport (see Harrison et al., 1986).(ABSTRACT TRUNCATED AT 400 WORDS)

Do noradrenergic descending tract fibres contribute to the depression of transmission from group II muscle afferents following brainstem stimulation in the cat?

Two alpha 2 noradrenaline antagonists, idazoxan and yohimbine, were injected in midlumbar segments of the spinal cord to test whether they counteract depression of field potentials evoked by group II muscle afferents by conditioning stimuli applied in the brainstem. The tested field potentials were those evoked monosynaptically in the intermediate zone of midlumbar segments. Their depression reflected thus the depression of transmission between group II fibres and their first relay neurones. The conditioning stimuli were applied either within the ipsilateral locus coeruleus/subcoeruleus or outside these nuclei (in the raphe magnus, raphe obscurus, or cuneiform nuclei). The brainstem evoked depression of the tested field potentials (n = 12) was reduced following injection of idazoxan or yohimbine to about two thirds of that which was evoked originally but in three cases to about one half. The study leads thus to the conclusion that noradrenergic descending tract neurones contribute to the depression of transmission from group II afferents to spinal interneurones and that such noradrenergic neurones are activated by stimuli applied within as well as outside their nuclei. However, the relative contribution of monoaminergic and non-monoaminergic descending tract neurones to the control of transmission from group II afferents to these neurones remains to be established.

Limitedly selective action of a delta-agonistic leu-enkephalin on the transmission in spinal motor reflex pathways in cats

1. The influence of the delta-opioid receptor agonist (D-Ser2)-leu-enkephalin (Thr6) (DSLET) on different spinal reflex pathways was investigated in anaemically decapitated, high spinal cats. Monosynaptic reflexes were tested to analyse excitatory and inhibitory flexor reflex afferent (FRA) pathways from nociceptive (from the skin of the central pad) and non-nociceptive (from skin, joint or group II muscle) afferents, as well as an excitatory nociceptive non-FRA pathway from the central pad to plantaris and intrinsic foot extensors and the inhibitory pathway from Ib muscle afferents. 2. DSLET suffused over the spinal cord (concentration 10(-3)-10(-6) M) caused a concentration-dependent depression of transmission in nociceptive and non-nociceptive FRA pathways. The excitatory FRA pathways including those from group II muscle afferents were more sensitive than the inhibitory ones. The nociceptive non-FRA pathway from the central pad to plantaris and intrinsic foot extensors was less affected than the FRA pathways. The inhibitory pathway from Ib muscle afferents remained almost unaffected. 3. Intravenous injection of DSLET (0.5-3.6 mg/kg) induced dose-dependent effects similar to those from local spinal application. The main difference was that I.V. injection more readily caused depression of the inhibitory FRA pathways to extensors. 4. The effects of local spinal application and of I.V. injection of DSLET were antagonized by I.V. injection of naloxone (0.1-1 mg/kg). 5. The effects of DSLET on spinal reflex pathways in many respects resemble that of monoamines. Possibly there is an interaction and a co-operation of enkephalins and monoamines in motor control.

Contralaterally projecting lamina VIII interneurones in middle lumbar segments in the cat

Peripheral input to lamina VIII interneurones was investigated by using extracellular and/or intracellular records from them. The interneurones were located in the L4-L5 spinal segments and projected to contralateral motor nuclei in the L7 segment. They constituted a non-homogeneous population but their input from muscle afferents (mainly group II afferents of quadriceps, flexor digitorum longus and pretibial flexors and group I afferents of triceps surae and hamstring nerves) and from cutaneous and joint afferents resembled the input to ipsilaterally projecting laminae V-VII interneurones of the same segments rather than the input to more caudally located lamina VIII interneurones. Since the ipsilaterally projecting laminae V-VII interneurones with such an input might be involved in locomotion, it is proposed that this is also the case for the contralaterally projecting lamina VIII midlumbar interneurones, especially those excited by stimuli applied in the cuneiform nucleus (mesencephalic locomotor region).

Concomitant depression of locus coeruleus neurons and of flexor reflexes by an alpha 2-adrenergic agonist in rats: a possible mechanism for an alpha 2-mediated muscle relaxation

The alpha 2-agonist tizanidine, clinically used as an antispastic drug, also strongly reduces polysynaptic flexor reflexes. The hypothesis was tested that the noradrenergic coerulespinal system exerts a tonic facilitation on spinal reflexes and that the depressant effects of tizanidine may be explained by an alpha 2-mediated autoinhibition of the tonic activity of locus coeruleus neurons, resulting in a disfacilitation of the spinal reflexes. The following results support this working hypothesis: (1) systemic injections of tizanidine markedly decreased the spontaneous activity of locus coeruleus neurons, but not of non-locus coeruleus neurons. The alpha 2-antagonist yohimbine reversed this effect. (2) The time course of diminished locus coeruleus activity paralleled that of depressed flexor reflexes. (3) Flexor reflexes were also markedly depressed by the alpha 1-adrenergic antagonist prazosin, administered alone, which is in line with the proposition that the noradrenergic system exerts a tonic facilitation on spinal neurons by way of alpha 1-adrenergic receptor activation. (4) Flexor reflexes were facilitated by conditioning microstimulation of locus coeruleus neurons, and this effect was reversed by prazosin. (5) Flexor reflexes significantly diminished in size following placement of an irreversible lesion in the ipsilateral locus coeruleus. Although these results strongly support the above hypothesis regarding a descending modulatory function of the descending locus coeruleus system on spinal reflexes, possible additional mechanisms, perhaps also involving the ascending projection of the locus coeruleus to supraspinal motor structures, remain to be elucidated.