Long-term modulation of the axonal refractory period.

The main question addressed in this study was whether the refractoriness of nerve fibres can be modulated by their depolarisation and, if so, whether depolarisation of nerve fibres evokes a long-term decrease in the duration of the refractory period as well as the previously demonstrated increase in their excitability. This was investigated on nerve fibres within the dorsal columns, dorsal roots and peripheral nerves in deeply anaesthetised rats in vivo. The results revealed major differences depending on the sites of fibre stimulation and polarisation. Firstly, the relative refractory period was found to be shorter in epidurally stimulated dorsal column fibres than in fibres stimulated at other sites. Secondly, the minimal effective interstimulus intervals reflecting the absolute refractory period were likewise shorter for nerve fibres within the dorsal columns even though action potentials evoked by the second of a pair of stimuli were similarly delayed with respect to the preceding action potentials at all the stimulation sites. Thirdly, the minimal interstimulus intervals were reduced by epidurally applied cathodal direct current polarisation but not at other stimulation sites. Consequently, higher proportions of dorsal column fibres could be excited at higher frequencies, especially following their depolarisation, at interstimulus intervals as short as 0.5-0.7 ms. The results demonstrate that epidural depolarisation results in long-lasting effects not only on the excitability but also on the refractoriness of dorsal column fibres. They also provide further evidence for specific features of afferent fibres traversing the dorsal columns previously linked to properties of their branching regions.

The plasticity of nerve fibers: the prolonged effects of polarization of afferent fibers.

The review surveys various aspects of the plasticity of nerve fibers, in particular the prolonged increase in their excitability evoked by polarization, focusing on a long-lasting increase in the excitability of myelinated afferent fibers traversing the dorsal columns of the spinal cord. We review the evidence that increased axonal excitability 1) follows epidurally applied direct current (DC) as well as relatively short (5 or 10 ms) current pulses and synaptically evoked intrinsic field potentials; 2) critically depends on the polarization of branching regions of afferent fibers at the sites where they bifurcate and give off axon collaterals entering the spinal gray matter in conjunction with actions of extrasynaptic GABAA membrane receptors; and 3) shares the feature of being activity-independent with the short-lasting effects of polarization of peripheral nerve fibers. A comparison between the polarization evoked sustained increase in the excitability of dorsal column fibers and spinal motoneurons (plateau potentials) indicates the possibility that they are mediated by partly similar membrane channels (including noninactivating type L Cav++ 1.3 but not Na+ channels) and partly different mechanisms. We finally consider under which conditions transspinally applied DC (tsDCS) might reproduce the effects of epidural polarization on dorsal column fibers and the possible advantages of increased excitability of afferent fibers for the rehabilitation of motor and sensory functions after spinal cord injuries.NEW & NOTEWORTHY This review supplements previous reviews of properties of nerve fibers by surveying recent experimental evidence for their long-term plasticity. It also extends recent descriptions of spinal effects of DC by reviewing effects of polarization of afferent nerve fibers within the dorsal columns, the mechanisms most likely underlying the long-lasting increase in their excitability and possible clinical implications.

Branching points of primary afferent fibers are vital for the modulation of fiber excitability by epidural DC polarization and by GABA in the rat spinal cord.

The aim of the study was to examine whether the sustained increases in the excitability of afferent fibers traversing the dorsal columns evoked by their polarization depend on the branching points of these fibers. To this end, the effects of epidural polarization were compared in four spinal regions in deeply anesthetized rats; two with the densest collateralization of muscle afferent fibers (above motor nuclei and Clarke's column) and two where the collateralization is more sparse (rostral and caudal to motor nuclei, respectively. The degree of collateralization in different segments was reconstructed in retrogradely labeled afferent fibers in the rat. Nerve volleys evoked in peripheral nerves by electrical stimulation of the dorsal columns within these regions were used as a measure of the excitability of the stimulated fibers. Potent increases in the excitability were evoked by polarization above motor nuclei and Clarke's column, both during constant direct current (DC) polarization (1 µA for 1 min) and for at least 30 min following DC polarization. Smaller excitability increases occurred during the polarization within other regions and were thereafter either absent or rapidly declined after its termination. The postpolarization increases in excitability were counteracted by the GABAA receptor antagonist bicuculline and the α5GABAA extrasynaptic receptor antagonist L655708 and enhanced by the GABAA receptor agonist muscimol and by ionophoretically applied GABA. As extrasynaptic α5GABAA receptors have been found close to Na channels within branching points, these results are consistent with the involvement of branching points in the induction of the sustained postpolarization increases in fiber excitability.NEW & NOTEWORTHY Polarization of sensory fibers traversing dorsal columns of the spinal cord may considerably increase the excitability of these fibers. We show that this involves the effects of current at branching points of afferent fibers and depends on extrasynaptic effects of GABA. These results contribute to our understanding of the mechanism underlying plasticity of activation of nerve fibers and may be used to increase the effectiveness of epidural stimulation in humans and recovery of spinal functions.

Interactions Between Baclofen and DC-induced Plasticity of Afferent Fibers within the Spinal Cord.

The aims of the study were to compare effects of baclofen, a GABAB receptor agonist commonly used as an antispastic drug, on direct current (DC) evoked long-lasting changes in the excitability of afferent fibers traversing the dorsal columns and their terminal branches in the spinal cord, and to examine whether baclofen interferes with the development and expression of these changes. The experiments were performed on deeply anesthetized rats by analyzing the effects of DC before, during and following baclofen administration. Muscle and skin afferent fibers within the dorsal columns were stimulated epidurally and changes in their excitability were investigated following epidural polarization by 1.0-1.1 µA subsequent to i.v. administration of baclofen. Epidural polarization increased the excitability of these fibers during post-polarization periods of at least 1 h. The facilitation was as potent as in preparations that were not pretreated with baclofen, indicating that the advantages of combining epidural polarization with epidural stimulation would not be endangered by pharmacological antispastic treatment with baclofen. In contrast, baclofen-reduced effects of intraspinal stimulation combined with intraspinal polarization (0.3 µA) of terminal axonal branches of the afferents within the dorsal horn or in motor nuclei, whether administered ionophoretically or intravenously. Effects of DC on monosynaptically evoked synaptic actions of these fibers (extracellular field potentials) were likewise reduced by baclofen. The study thus provides further evidence for differential effects of DC on afferent fibers in the dorsal columns and the preterminal branches of these fibers and their involvement in spinal plasticity.

Long-lasting increase in axonal excitability after epidurally applied DC.

Effects of direct current (DC) on nerve fibers have primarily been investigated during or just after DC application. However, locally applied cathodal DC was recently demonstrated to increase the excitability of intraspinal preterminal axonal branches for >1 h. The aim of this study was therefore to investigate whether DC evokes a similarly long-lasting increase in the excitability of myelinated axons within the dorsal columns. The excitability of dorsal column fibers stimulated epidurally was monitored by recording compound action potentials in peripheral nerves in acute experiments in deeply anesthetized rats. The results show that 1) cathodal polarization (0.8-1.0 µA) results in a severalfold increase in the number of epidurally activated fibers and 2) the increase in the excitability appears within seconds, 3) lasts for >1 h, and 4) is activity independent, as it does not require fiber stimulation during the polarization. These features demonstrate an unexplored form of plasticity of myelinated fibers and indicate the conditions under which it develops. They also suggest that therapeutic effects of epidural stimulation may be significantly enhanced if it is combined with DC polarization. In particular, by using DC to increase the number of fibers activated by low-intensity epidural stimuli, the low clinical tolerance to higher stimulus intensities might be overcome. The activity independence of long-lasting DC effects would also allow the use of only brief periods of DC polarization preceding epidural stimulation to increase the effect.NEW & NOTEWORTHY The study indicates a new form of plasticity of myelinated fibers. The differences in time course of DC-evoked increases in the excitability of myelinated nerve fibers in the dorsal columns and in preterminal axonal branches suggest that distinct mechanisms are involved in them. The results show that combining epidural stimulation and transspinal DC polarization may dramatically improve their outcome and result in more effective pain control and the return of impaired motor functions.

Evidence that some long-lasting effects of direct current in the rat spinal cord are activity-independent.

The effects of trans-spinal direct current (DC) stimulation (tsDCS) on specific neuronal populations are difficult to elucidate, as it affects a variety of neuronal networks. However, facilitatory and depressive effects on neurons processing information from the skin and from muscles can be evaluated separately when weak (0.2-0.3 µA) DC is applied within restricted areas of the rat spinal cord. The effects of such local DC application were recently demonstrated to persist for at least 1 h, and to include changes in the excitability of afferent fibres and their synaptic actions. However, whether these effects require activation of afferent fibres in spinal neuronal pathways during DC application, i.e. whether they are activity-dependent or activity-independent, remained an open question. The aim of the present study was to address this question by analysing the effects of local DC application on monosynaptic actions of muscle and skin afferents (extracellular field potentials) and afferent fibre excitability. The results revealed that long-lasting post-polarization changes evoked without concomitant activation of afferent fibres replicate changes evoked by stimuli applied during, before and after polarization. The study leads to the conclusion that the reported effects are activity-independent. As this conclusion applies to the local effects of DC application in at least two spinal pathways and to the effects of both cathodal and anodal polarization, it indicates that some of the more widespread effects of trans-spinal and trans-cranial stimulation (both tsDCS and transcranial DC stimulation) may be activity-independent. The results may therefore contribute to the design of more specific DC applications in clinical practice.

Mechanical compression and nucleus pulposus application on dorsal root Ganglia differentially modify evoked neuronal activity in the thalamus.

A combination of mechanical compression caused by a protruding disc and leakage of nucleus pulposus (NP) from the disc core is presumed to contribute to intervertebral disc hernia-related pain. Experimental models of disc hernia including both components have resulted in changes in neuronal activity at the level of the dorsal root ganglion (DRG) and spinal cord, but changes within the brain have been less well studied. However, acute application of NP to a DRG without mechanical compression rapidly increases neuronal activity in the thalamus, a major brain relay nucleus processing information from sensory pathways including ascending nociceptive tracts. The combination of mechanical compression and NP might therefore result in further increases in central neuronal activity. Using an experimental disc herniation rat model including both mechanical compression and NP the present study aimed to investigate changes in neuronal activity in the contralateral thalamic ventral posterior lateral nucleus in vivo. Measurements were obtained while electrically stimulating the ipsilateral sciatic nerve at Aδ fiber intensities. The L4 DRG was subjected to light mechanical compression and NP exposure, and acute changes in evoked thalamic responses were recorded for up to 40 min. In order to compare effects in naïve animals with effects following a longer period of NP exposure, animals that were either disc-punctured or sham-operated 24 h previously were also included. In all animals, light mechanical compression of the DRG depressed the number of evoked neuronal responses. Prior NP exposure resulted in less potent changes following mechanical compression (80% of baseline) than that observed in naïve animals (50%). During the subsequent NP application, the number of evoked responses compared to baseline increased in pre-exposed animals (to 87%) as well as in naïve animals (72%) in which the removal of the mechanical compression resulted in a further increase (106%). The contribution of acute DRG compression and disc material leakage to changes in transmission in central neuronal networks is likely to be complex and to involve both short-term and long-term effects. Since a light mechanical compression may reduce transmission in nociceptive pathways, it is possible that the presence or absence of NP is crucial for pain development in the acute phase of disc herniation.

Interactions between spinal interneurons and ventral spinocerebellar tract neurons.

Recent evidence indicates that ventral spinocerebellar tract (VSCT) neurons do not merely receive information provided by spinal interneurons but may also modulate the activity of these interneurons. Hence, interactions between them may be mutual. However, while it is well established that spinal interneurons may provide both excitatory and inhibitory input to ascending tract neurons, the functional consequences of the almost exclusively inhibitory input from premotor interneurons to subpopulations of VSCT neurons were only recently addressed. These are discussed in the first part of this review. The second part of the review summarizes evidence that some VSCT neurons may operate both as projection neurons and as spinal interneurons and play a role in spinal circuitry. It outlines the evidence that initial axon collaterals of VSCT neurons target premotor inhibitory interneurons in disynaptic reflex pathways from tendon organs and muscle spindles (group Ia, Ib and/or II muscle afferents) to motoneurons. By activating these interneurons VSCT neurons may evoke disynaptic IPSPs in motoneurons and thus facilitate inhibitory actions of contralateral muscle afferents on motoneurons. In this way they may contribute to the coordination between neuronal networks on both sides of the spinal cord in advance of modulatory actions evoked via the cerebellar control systems.

Excitatory inputs to four types of spinocerebellar tract neurons in the cat and the rat thoraco-lumbar spinal cord.

The cerebellum receives information from the hindlimbs through several populations of spinocerebellar tract neurons. Although the role of these neurons has been established in electrophysiological experiments, the relative contribution of afferent fibres and central neurons to their excitatory input has only been estimated approximately so far. Taking advantage of differences in the immunohistochemistry of glutamatergic terminals of peripheral afferents and of central neurons (with vesicular glutamate transporters VGLUT1 or VGLUT2, respectively), we compared sources of excitatory input to four populations of spinocerebellar neurons in the thoraco-lumbar spinal cord: dorsal spinocerebellar tract neurons located in Clarke's column (ccDSCT) and in the dorsal horn (dhDSCT) and ventral spinocerebellar tract (VSCT) neurons including spinal border (SB) neurons. This was done on 22 electrophysiologically identified intracellularly labelled neurons in cats and on 80 neurons labelled by retrograde transport of cholera toxin b subunit injected into the cerebellum of rats. In both species distribution of antibodies against VGLUT1 and VGLUT2 on SB neurons (which have dominating inhibitory input from limb muscles), revealed very few VGLUT1 contacts and remarkably high numbers of VGLUT2 contacts. In VSCT neurons with excitatory afferent input, the number of VGLUT1 contacts was relatively high although VGLUT2 contacts likewise dominated, while the proportions of VGLUT1 and VGLUT2 immunoreactive terminals were the reverse on the two populations of DSCT neurons. These findings provide morphological evidence that SB neurons principally receive excitatory inputs from central neurons and provide the cerebellum with information regarding central neuronal activity.

Bilateral postsynaptic actions of pyramidal tract and reticulospinal neurons on feline erector spinae motoneurons.

Trunk muscles are important for postural adjustments associated with voluntary movements but little has been done to analyze mechanisms of supraspinal control of these muscles at a cellular level. The present study therefore aimed to investigate the input from pyramidal tract (PT) neurons to motoneurons of the musculus longissimus lumborum of the erector spinae and to analyze to what extent it is relayed by reticulospinal (RS) neurons. Intracellular records from motoneurons were used to evaluate effects of electrical stimulation of medullary pyramids and of axons of RS neurons descending in the medial longitudinal fasciculus (MLF). The results revealed that similar synaptic actions were evoked from the ipsilateral and contralateral PTs, including disynaptic and trisynaptic EPSPs and trisynaptic IPSPs. Stimulation of the MLF-evoked monosynaptic and disynaptic EPSPs and disynaptic or trisynaptic IPSPs in the same motoneurons. All short-latency PSPs of PT origin were abolished by transection of the MLF, while they remained after transection of PT fibers at a spinal level. Hence, RS neurons might serve as the main relay neurons of the most direct PT actions on musculus (m.) longissimus. However, longer-latency IPSPs remaining after MLF or PT spinal lesions and after ipsilateral or contralateral hemisection of spinal cord indicate that PT actions are also mediated by ipsilaterally and/or contralaterally located spinal interneurons. The bilateral effects of PT stimulation thereby provide an explanation why trunk movements after unilateral injuries of PT neurons (e.g., stroke) are impaired to a lesser degree than movements of the extremities.

Differential modulation by monoamine membrane receptor agonists of reticulospinal input to lamina VIII feline spinal commissural interneurons.

Noradrenaline and serotonin have previously been demonstrated to facilitate the transmission between descending reticulospinal tracts fibres and commissural interneurons coordinating left-right hindlimb muscle activity. The aim of the present study was to investigate the contribution of subclasses of monoaminergic membrane receptors to this facilitation. The neurons were located in Rexed lamina VIII in midlumbar segments and identified by their projections to the contralateral gastrocnemius-soleus motor nuclei and by lack of projections rostral to the lumbosacral enlargement. The effects of ionophoretically applied membrane receptor agonists [phenylephrine (noradrenergic alpha(1)), clonidine (noradrenergic alpha(2)), 8-OH-DPAT (5-HT(1A), 5-HT(7)), 2-me-5-HT (5-HT(3)), 5-me-5-HT (5-HT(2)) and alpha-me-5-HT (5-HT(2))] were examined on extracellularly recorded spikes evoked monosynaptically by electric stimulation of descending reticulospinal fibres in the medial longitudinal fascicle. Application of alpha(1) and 5-HT(2) agonists resulted in a facilitation of responses in all investigated neurons while application of alpha(2), 5-HT(1A/7) and 5-HT(3) agonists resulted in a depression. These opposite modulatory effects of different agonists suggest that the facilitatory actions of noradrenaline and serotonin on responses of commissural interneurons reported previously following ionophoretic application are the net outcome of the activation of different subclasses of monoaminergic membrane receptors. As these receptors may be distributed predominantly, or even selectively, at either pre- or postsynaptic sites their differential modulatory actions could be compatible with a presynaptically induced depression and a postsynaptically evoked enhancement of synaptic transmission between reticulospinal neurons and commissural interneurons.

Thalamic activation in a disc herniation model.

STUDY DESIGN: A novel approach combining a rodent disc herniation model with electrophysiologic recordings of thalamic evoked responses. OBJECTIVE: To assess short-term effects of nucleus pulposus (NP) application on dorsal root ganglions (DRG) on high threshold afferent fiber evoked activation in the thalamus. SUMMARY OF BACKGROUND DATA: Epidural application of NP in combination with mechanical compression induces pain related behavior in rats associated with enhanced activity of pain-processing neurons in the dorsal horn of the spinal cord. However, possible effects on neuronal activity in the pain processing ventral posterior lateral (VPL) thalamic nucleus following NP application on DRG have not been investigated. METHODS: Responses in the contralateral VPL evoked by electrical stimulation of the sciatic nerve and of the fourth lumbar (L4) DRG were recorded in adult Sprague-Dawley rats. Records were obtained before and during application (5, 10, and 20 minutes) of NP or of adipose tissue (AT) to the L4 DRG. AT was used as control for mechanical effects of NP application. RESULTS: Application of NP resulted in an increase of evoked thalamic responses to 138% +/- 10% of control after 20 minutes (P < 0.01), whereas AT application for 20 minutes resulted in a reduction of evoked responses to 77% +/- 4% (P < 0.05). Recordings in control animals (i.e., with no application) demonstrated stable evoked neuronal thalamic activity for up to 40 minutes. CONCLUSION: The study demonstrates that NP application onto DRG increases afferent fiber evoked responses in the thalamus and in view of the opposite effects of AT application suggests that these effects may be specific for NP. The results show that NP affects sensory transmitting pathways within a few minutes, possibly due to rapid and reversible alterations in the neuronal excitability. The study thus introduces a rodent model for studying sensory afferent evoked thalamic activity related to DRG injury which may be used to evaluate analgesics and anti-inflammatory drugs used for pain relief in disc herniation and neuropathic pain patients.

Differential projections of excitatory and inhibitory dorsal horn interneurons relaying information from group II muscle afferents in the cat spinal cord.

Dorsal horn interneurons with input from group II muscle spindle afferents are components of networks involved in motor control. Thirteen dorsal horn interneurons with monosynaptic group II input were characterized electrophysiologically and labeled intracellularly with Neurobiotin. Their axonal projections were traced, and neurotransmitter content was established by using immunocytochemistry. Two subpopulations were identified: five interneurons had axons that contained vesicular glutamate transporter 2 and hence were glutamatergic and excitatory. Terminals of the remaining eight interneurons were immunoreactive for the glycine transporter 2 or were apposed to gephyrin but did not contain the GABA-synthesizing enzyme glutamic acid decarboxylase and were therefore glycinergic and inhibitory. Excitatory cells were located mainly in the central region of lamina IV and had relatively small somata and restricted dendritic trees. In contrast, inhibitory interneurons were located more ventrally, in lamina V and had relatively larger somata and more extensive dendritic trees. Axonal projections of the two subpopulations differed considerably. Excitatory interneurons predominantly projected ipsilaterally, whereas most inhibitory interneurons projected both ipsilaterally and contralaterally. Three inhibitory axons formed contacts with large cholinergic cells in motor nuclei, thus revealing a novel direct coupling between inhibitory dorsal horn interneurons and motoneurons. The organization of the excitatory interneurons is consistent with current knowledge of reflex pathways to motoneurons, but the existence and connections of the inhibitory subpopulation could not be predicted from previous data. Our results indicate that these latter interneurons exercise widespread inhibitory control over a variety of cell types located on both sides of the spinal cord.

The actions of monoamines and distribution of noradrenergic and serotoninergic contacts on different subpopulations of commissural interneurons in the cat spinal cord.

Modulatory actions of monoamines were investigated on spinal commissural interneurons which coordinate left-right hindlimb muscle activity through direct projections to the contralateral motor nuclei. Commissural interneurons located in Rexed lamina VIII, with identified projections to the contralateral gastrocnemius-soleus motor nuclei, were investigated in deeply anaesthetized cats. Most interneurons had dominant input from either the reticular formation or from group II muscle afferents; a small proportion of neurons had input from both. Actions of ionophoretically applied serotonin and noradrenaline were examined on extracellularly recorded spikes evoked monosynaptically by group II muscle afferents or reticulospinal tract fibres. Activation by reticulospinal fibres was facilitated by both serotonin and noradrenaline. Activation by group II afferents was also facilitated by serotonin but was strongly depressed by noradrenaline. To investigate the possible morphological substrates of this differential modulation, seven representative commissural interneurons were labelled intracellularly with tetramethylrhodamine-dextran and neurobiotin. Contacts from noradrenergic and serotoninergic fibres were revealed by immunohistochemistry and analysed with confocal microscopy. There were no major differences in the numbers and distributions of contacts among the interneurons studied. The findings suggest that differences in modulatory actions of monoamines, and subsequent changes in the recruitment of subpopulations of commissural interneurons in various behavioural situations, depend on intrinsic interneuron properties rather than on the patterns of innervation by monoaminergic fibres. The different actions of noradrenaline on different populations of interneurons might permit reconfiguration of the actions of the commissural neurons according to behavioural context.

Networks of inhibitory and excitatory commissural interneurons mediating crossed reticulospinal actions.

Axonal projections and neurotransmitters used by commissural interneurons mediating crossed actions of reticulospinal neurons were investigated in adult cats. Eighteen interneurons, located in or close to lamina VIII in midlumbar segments, that were monosynaptically excited by reticulospinal tract fibres and projected to contralateral motor nuclei were labelled by intracellular injection of tetramethylrhodamine-dextran and Neurobiotin. The nine most completely labelled interneurons were analysed with combined confocal and light microscopy. None of the stem axons gave off ipsilateral axon collaterals. Seven cells had axon collaterals that arborized in the contralateral grey matter in the ventral horn of the same segments. Transmitters were identified by using antibodies raised against vesicular glutamate transporters 1 and 2, glutamic acid decarboxylase and the glycine transporter 2. The axons of two cells were immunoreactive for the glycine transporter 2 and hence were glycinergic. Three cells were immunoreactive for the vesicular glutamate transporter 2 and hence were glutamatergic. None of the axons displayed immunoreactivity for glutamic acid decarboxylase. Electron microscopy of two cells revealed direct synaptic connections with motoneurons and other neurons. Axonal swellings of one neuron formed synapses with profiles in motor nuclei whereas those of the other formed synapses with other structures, including cell bodies in lamina VII. The results show that this population of commissural interneurons includes both excitatory and inhibitory cells that may excite or inhibit contralateral motoneurons directly. They may also influence the activity of motoneurons indirectly by acting through interneurons located outside motor nuclei in the contralateral grey matter but are unlikely to have direct actions on interneurons in the ipsilateral grey matter.

Neuronal basis of crossed actions from the reticular formation on feline hindlimb motoneurons.

Pathways through which reticulospinal neurons can influence contralateral limb movements were investigated by recording from motoneurons innervating hindlimb muscles. Reticulospinal tract fibers were stimulated within the brainstem or in the lateral funiculus of the thoracic spinal cord contralateral to the motoneurons. Effects evoked by ipsilaterally descending reticulospinal tract fibers were eliminated by a spinal hemisection at an upper lumbar level. Stimuli applied in the brainstem evoked EPSPs, IPSPs, or both at latencies of 1.42 +/- 0.03 and 1.53 +/- 0.04 msec, respectively, from the first components of the descending volleys and with properties indicating a disynaptic linkage, in most contralateral motoneurons: EPSPs in 76% and IPSPs in 26%. EPSPs with characteristics of monosynaptically evoked responses, attributable to direct actions of crossed axon collaterals of reticulospinal fibers, were found in a small proportion of the motoneurons, whether evoked from the brainstem (9%) or from the thoracic cord (12.5%). Commissural neurons, which might mediate the crossed disynaptic actions (i.e., were antidromically activated from contralateral motor nuclei and monosynaptically excited from the ipsilateral reticular formation), were found in Rexed's lamina VIII in the midlumbar segments (L3-L5). The results reveal that although direct actions of reticulospinal fibers are much more potent on ipsilateral motoneurons, interneuronally mediated actions are as potent contralaterally as ipsilaterally, and midlumbar commissural neurons are likely to contribute to them. They indicate a close coupling between the spinal interneuronal systems used by the reticulospinal neurons to coordinate muscle contractions ipsilaterally and contralaterally.

Modulatory effects of alpha1-,alpha2-, and beta -receptor agonists on feline spinal interneurons with monosynaptic input from group I muscle afferents.

Previous studies have shown that monoamines may modulate operation of spinal neuronal networks by depressing or facilitating responses of the involved neurons. Recently, activation of interneurons mediating reciprocal inhibition from muscle spindle (Ia) afferents and nonreciprocal inhibition from muscle spindle and tendon organ (Ia/Ib) afferents in the cat was found to be facilitated by noradrenaline (NA). However, which subclass membrane receptors are involved in mediating this facilitation was not established; the aim of the present experiments was to investigate this. Individual Ia- and Ia/Ib-inhibitory interneurons were identified in the cat lumbar spinal cord, and NA agonists were applied close to these neurons by ionophoresis. The agonists included the alpha1-receptor agonist phenylephrine, the alpha2-receptor agonists clonidine and tizanidine, and the beta-receptor agonist isoproterenol. Effects were measured by comparing changes in the number of extracellularly recorded spike potentials evoked by electrical stimulation of muscle nerves and changes in the latency of these potentials before, during, and after application of the tested compounds. Results show that the facilitatory effect of phenylephrine is as strong as that of NA, whereas the facilitatory effect of isoproterenol is weaker. Clonidine depressed activity of both Ia- and Ia/Ib-inhibitory interneurons, whereas tizanidine had no effect. These findings lead to the conclusion that beneficial antispastic effects of clonidine and tizanidine in humans are unlikely to be associated with an enhancement of the actions of Ia- and Ia/Ib-inhibitory interneurons, and the findings also support previous proposals that these compounds exert their antispastic actions via effects on other neuronal populations.

Modulation of responses of feline ventral spinocerebellar tract neurons by monoamines.

Ventral spinocerebellar tract neurons located in laminae V-VII of cat lumbar spinal cord were tested for the effects of ionophoretically applied monoamines and receptor selective agonists. Extracellularly recorded responses, monosynaptically evoked by group I afferents in a muscle nerve, were compared before, during, and after ionophoresis. They were analyzed with respect to changes in the number of evoked spikes and in the latency. Both serotonin (5-HT) and noradrenaline (NA) were found to facilitate responses of all neurons tested. Ionophoresis of three serotonin subtype receptor agonists (5-carboxamidotryptamine maleate, 5 methoxytryptamine HCl, and alpha-methyl 5-hydroxytryptamine) and of two NA receptor agonists (phenylephrine and isoproterenol) likewise had a facilitatory effect. However, three other 5-HT receptor agonists (8-hydroxy-dipropylaminotetraline hydrobromide), 2-methyl 5-hydroxytryptamine, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl and two NA receptor agonists (tizanidine and clonidine) had the opposite effect because they depressed responses of the tested neurons. These results show that information forwarded by means of the ventral spinocerebellar tract may be modulated by monoamines and that several receptor subtypes, located pre- or postsynaptically, may be involved. The results also demonstrate that transmission by means of group I muscle afferents may not only be facilitated by monoamines but also depressed by selective receptor subtype activation.

Serotoninergic and noradrenergic axons make contacts with neurons of the ventral spinocerebellar tract in the cat.

Contacts between monoaminergic fibers and electrophysiologically identified neurons of the ventral spinocerebellar tract were investigated in the cat. Five neurons were labeled intracellularly with rhodamine dextran, and monoaminergic fibers were revealed with antibodies against serotonin and dopamine beta-hydroxylase. The distribution of appositions between monoaminergic varicosities and the soma and the whole length of dendrites of these neurons was examined by using a three-channel confocal microscope. The analysis showed that close appositions between monoaminergic fibers and labeled processes occurred over the whole surface of the neurons. The highest percentage of such appositions was found on proximal dendrites, for both serotonin (37%) and noradrenaline (57%). The total number of serotoninergic contacts (66-134 per neuron) by far exceeded that of noradrenergic contacts (3-36 per neuron). Contacts between serotoninergic fibers and two neurons were analyzed by using electron microscopy. These neurons were labeled intracellularly with horseradish peroxidase, and serotoninergic varicosities were identified by immunocytochemistry. Six of 10 serially analyzed boutons in apposition to proximal dendrites were found to form morphologic synapses. The identification of the remaining four was inconclusive. These results indicate that many of the appositions seen in confocal microscopy may represent direct synaptic contacts. They also indicate that monoaminergic neurons may modulate activity of neurons of the ventral spinocerebellar tract by direct postsynaptic actions in addition to any effects evoked by means of volume transmission.