Nociception induces a differential presynaptic modulation of the synaptic efficacy of nociceptive and proprioceptive joint afferents.

A previous study has indicated that during the state of central sensitization induced by the intradermic injection of capsaicin, there is a gradual facilitation of the dorsal horn neuronal responses produced by stimulation of the high-threshold articular afferents that is counteracted by a concurrent increase of descending inhibitory actions. Since these changes occurred without significantly affecting the responses produced by stimulation of the low-threshold articular afferents, it was suggested that the capsaicin-induced descending inhibition included a preferential presynaptic modulation of the synaptic efficacy of the slow conducting nociceptive joint afferents (Ramírez-Morales et al., Exp Brain Res 237:1629-1641, 2019). The present study was aimed to investigate more directly the contribution of presynaptic mechanisms in this descending control. We found that in the barbiturate anesthetized cat, stimulation of the high-threshold myelinated afferents in the posterior articular nerve (PAN) produces primary afferent hyperpolarization (PAH) in the slow conducting (25-35 m/s) and primary afferent depolarization (PAD) in the fast conducting (40-50 m/s) articular fibers. During the state of central sensitization induced by capsaicin, there is a supraspinally mediated shift of the autogenic PAH to PAD that takes place in the slow conducting fibers, basically without affecting the autogenic PAD generated in the fast conducting afferents. It is suggested that the change of presynaptic facilitation to presynaptic inhibition induced by capsaicin on the slow articular afferents is part of an homeostatic process aimed to keep the nociceptive-induced neuronal activity within manageable limits while preserving the proprioceptive information required for proper control of movement.

Descending inhibition selectively counteracts the capsaicin-induced facilitation of dorsal horn neurons activated by joint nociceptive afferents.

Previous studies from our laboratory showed that in the anesthetized cat, the intradermal injection of capsaicin in the hindpaw facilitated the intraspinal field potentials (IFPs) evoked by stimulation of the intermediate and high-threshold myelinated fibers in the posterior articular nerve (PAN). The capsaicin-induced facilitation was significantly reduced 3-4 h after the injection, despite the persistence of hindpaw inflammation. Although this effect was attributed to an incremented descending inhibition acting on the spinal pathways, it was not clear if it was set in operation once the capsaicin-induced effects exceeded a certain threshold, or if it was continuously operating to keep the increased neuronal activation within manageable limits. To evaluate the changes in descending inhibition, we now examined the effects of successive reversible spinal blocks on the amplitude of the PAN IFPs evoked at different times after the intradermal injection of capsaicin. We found that after capsaicin the PAN IFPs recorded in laminae III-V by activation of high-threshold nociceptive Aδ myelinated fibers increased gradually during successive reversible spinal blocks, while the IFPs evoked by intermediate and low threshold proprioceptive Aß afferents were only slightly affected. It is concluded that during the development of the central sensitization produced by capsaicin, there is a gradual increase of descending inhibition that tends to limit the nociceptive-induced facilitation, mainly by acting on the neuronal populations receiving inputs from the capsaicin-activated afferents without significantly affecting the information on joint angle transmitted by the low threshold afferents.

Supraspinal modulation of neuronal synchronization by nociceptive stimulation induces an enduring reorganization of dorsal horn neuronal connectivity.

KEY POINTS: The state of central sensitization induced by the intradermic injection of capsaicin leads to structured (non-random) changes in functional connectivity between dorsal horn neuronal populations distributed along the spinal lumbar segments in anaesthetized cats. The capsaicin-induced changes in neuronal connectivity and the concurrent increase in secondary hyperalgesia are transiently reversed by the systemic administration of small doses of lidocaine, a clinically effective procedure to treat neuropathic pain. The effects of both capsaicin and lidocaine are greatly attenuated in spinalized preparations, showing that supraspinal influences play a significant role in the shaping of nociceptive-induced changes in dorsal horn functional neuronal connectivity. We conclude that changes in functional connectivity between segmental populations of dorsal horn neurones induced by capsaicin and lidocaine result from a cooperative adaptive interaction between supraspinal and spinal neuronal networks, a process that may have a relevant role in the pathogenesis of chronic pain and analgesia. ABSTRACT: Despite a profusion of information on the molecular and cellular mechanisms involved in the central sensitization produced by intense nociceptive stimulation, the changes in the patterns of functional connectivity between spinal neurones associated with the development of secondary hyperalgesia and allodynia remain largely unknown. Here we show that the state of central sensitization produced by the intradermal injection of capsaicin is associated with structured transformations in neuronal synchronization that lead to an enduring reorganization of the functional connectivity within a segmentally distributed ensemble of dorsal horn neurones. These changes are transiently reversed by the systemic administration of small doses of lidocaine, a clinically effective procedure to treat neuropathic pain. Lidocaine also reduces the capsaicin-induced facilitation of the spinal responses evoked by weak mechanical stimulation of the skin in the region of secondary but not primary hyperalgesia. The effects of both intradermic capsaicin and systemic lidocaine on the segmental correlation and coherence between ongoing cord dorsum potentials and on the responses evoked by tactile stimulation in the region of secondary hyperalgesia are greatly attenuated in spinalized preparations, showing that supraspinal influences are involved in the reorganization of the nociceptive-induced structured patterns of dorsal horn neuronal connectivity. We conclude that the structured reorganization of the functional connectivity between the dorsal horn neurones induced by capsaicin nociceptive stimulation results from cooperative interactions between supraspinal and spinal networks, a process that may have a relevant role in the shaping of the spinal state in the pathogenesis of chronic pain and analgesia.

Dynamic synchronization of ongoing neuronal activity across spinal segments regulates sensory information flow.

Previous studies on the correlation between spontaneous cord dorsum potentials recorded in the lumbar spinal segments of anaesthetized cats suggested the operation of a population of dorsal horn neurones that modulates, in a differential manner, transmission along pathways mediating Ib non-reciprocal postsynaptic inhibition and pathways mediating primary afferent depolarization and presynaptic inhibition. In order to gain further insight into the possible neuronal mechanisms that underlie this process, we have measured changes in the correlation between the spontaneous activity of individual dorsal horn neurones and the cord dorsum potentials associated with intermittent activation of these inhibitory pathways. We found that high levels of neuronal synchronization within the dorsal horn are associated with states of incremented activity along the pathways mediating presynaptic inhibition relative to pathways mediating Ib postsynaptic inhibition. It is suggested that ongoing changes in the patterns of functional connectivity within a distributed ensemble of dorsal horn neurones play a relevant role in the state-dependent modulation of impulse transmission along inhibitory pathways, among them those involved in the central control of sensory information. This feature would allow the same neuronal network to be involved in different functional tasks.

Differential presynaptic control of the synaptic effectiveness of cutaneous afferents evidenced by effects produced by acute nerve section.

In the anaesthetized cat, the acute section of the saphenous (Saph) and/or the superficial peroneal (SP) nerves was found to produce a long-lasting increase of the field potentials generated in the dorsal horn by stimulation of the medial branch of the sural (mSU) nerve. This facilitation was associated with changes in the level of the tonic primary afferent depolarization (PAD) of the mSU intraspinal terminals. The mSU afferent fibres projecting into Rexed's laminae III-IV were subjected to a tonic PAD that was reduced by the acute section of the SP and/or the Saph nerves. The mSU afferents projecting deeper into the dorsal horn (Rexed's laminae V-VI) were instead subjected to a tonic PAD that was increased after Saph and SP acute nerve section. A differential control of the synaptic effectiveness of the low-threshold cutaneous afferents according to their sites of termination within the dorsal horn is envisaged as a mechanism that allows selective processing of sensory information in response to tactile and nociceptive stimulation or during the execution of different motor tasks.

A new feature extraction method for signal classification applied to cord dorsum potential detection.

In the spinal cord of the anesthetized cat, spontaneous cord dorsum potentials (CDPs) appear synchronously along the lumbo-sacral segments. These CDPs have different shapes and magnitudes. Previous work has indicated that some CDPs appear to be specially associated with the activation of spinal pathways that lead to primary afferent depolarization and presynaptic inhibition. Visual detection and classification of these CDPs provides relevant information on the functional organization of the neural networks involved in the control of sensory information and allows the characterization of the changes produced by acute nerve and spinal lesions. We now present a novel feature extraction approach for signal classification, applied to CDP detection. The method is based on an intuitive procedure. We first remove by convolution the noise from the CDPs recorded in each given spinal segment. Then, we assign a coefficient for each main local maximum of the signal using its amplitude and distance to the most important maximum of the signal. These coefficients will be the input for the subsequent classification algorithm. In particular, we employ gradient boosting classification trees. This combination of approaches allows a faster and more accurate discrimination of CDPs than is obtained by other methods.

Changes in correlation between spontaneous activity of dorsal horn neurones lead to differential recruitment of inhibitory pathways in the cat spinal cord.

Simultaneous recordings of cord dorsum potentials along the lumbo-sacral spinal cord of the anaesthetized cat revealed the occurrence of spontaneous synchronous negative (n) and negative-positive (np) cord dorsum potentials (CDPs). The npCDPs, unlike the nCDPs, appeared preferentially associated with spontaneous negative dorsal root potentials (DRPs) resulting from primary afferent depolarization. Spontaneous npCDPs recorded in preparations with intact neuroaxis or after spinalization often showed a higher correlation than the nCDPs recorded from the same pair of segments. The acute section of the sural and superficial peroneal nerves further increased the correlation between paired sets of npCDPs and reduced the correlation between the nCDPs recorded from the same pair of segments. It is concluded that the spontaneous nCDPs and npCDPs are produced by the activation of interconnected sets of dorsal horn neurones located in Rexed's laminae III­IV and bilaterally distributed along the lumbo-sacral spinal cord. Under conditions of low synchronization in the activity of this network of neurones there would be a preferential activation of the intermediate nucleus interneurones mediating Ib non-reciprocal postsynaptic inhibition. Increased synchronization in the spontaneous activity of this ensemble of dorsal horn neurones would recruit the interneurones mediating primary afferent depolarization and presynaptic inhibition and, at the same time, reduce the activation of pathways mediating Ib postsynaptic inhibition. Central control of the synchronization in the spontaneous activity of dorsal horn neurones and its modulation by cutaneous inputs is envisaged as an effective mechanism for the selection of alternative inhibitory pathways during the execution of specific motor or sensory tasks.

Changes in synaptic effectiveness of myelinated joint afferents during capsaicin-induced inflammation of the footpad in the anesthetized cat.

The present series of experiments was designed to examine, in the anesthetized cat, the extent to which the synaptic efficacy of knee joint afferents is modified during the state of central sensitization produced by the injection of capsaicin into the hindlimb plantar cushion. We found that the intradermic injection of capsaicin increased the N2 and N3 components of the focal potentials produced by stimulation of intermediate and high threshold myelinated fibers in the posterior articular nerve (PAN), respectively. This facilitation lasted several hours, had about the same time course as the paw inflammation and was more evident for the N2 and N3 potentials recorded within the intermediate zone in the L6 than in the L7 spinal segments. The capsaicin-induced facilitation of the N2 focal potentials, which are assumed to be generated by activation of fibers signaling joint position, suggests that nociception may affect the processing of proprioceptive and somato-sensory information and, probably also, movement. In addition, the increased effectiveness of these afferents could activate, besides neurons in the intermediate region, neurons located in the more superficial layers of the dorsal horn. As a consequence, normal joint movements could produce pain representing a secondary hyperalgesia. The capsaicin-induced increased efficacy of the PAN afferents producing the N3 focal potentials, together with the reduced post-activation depression that follows high frequency autogenetic stimulation of these afferents, could further contribute to the pain sensation from non-inflamed joints during skin inflammation in humans. The persistence, after capsaicin, of the inhibitory effects produced by stimulation of cutaneous nerves innervating non-inflamed skin regions may account for the reported reduction of the articular pain sensations produced by trans-cutaneous stimulation.

Patterns of primary afferent depolarization of segmental and ascending intraspinal collaterals of single joint afferents in the cat.

We have examined in the anesthetized cat the threshold changes produced by sensory and supraspinal stimuli on intraspinal collaterals of single afferents from the posterior articular nerve (PAN). Forty-eight fibers were tested in the L3 segment, in or close to Clarke's column, and 70 fibers in the L6-L7 segments within the intermediate zone. Of these, 15 pairs of L3 and L6-L7 collaterals were from the same afferent. Antidromically activated fibers had conduction velocities between 23 and 74 m/s and peripheral thresholds between 1.1 and 4.7 times the threshold of the most excitable fibers (xT), most of them below 3 xT. PAN afferents were strongly depolarized by stimulation of muscle afferents and by cutaneous afferents, as well as by stimulation of the bulbar reticular formation and the midline raphe nuclei. Stimulation of muscle nerves (posterior biceps and semitendinosus, quadriceps) produced a larger PAD (primary afferent depolarization) in the L6-L7 than in the L3 terminations. Group II were more effective than group I muscle afferents. As with group I muscle afferents, the PAD elicited in PAN afferents by stimulation of muscle nerves could be inhibited by conditioning stimulation of cutaneous afferents. Stimulation of the cutaneous sural and superficial peroneal nerves increased the threshold of few terminations (i.e., produced primary afferent hyperpolarization, PAH) and reduced the threshold of many others, particularly of those tested in the L6-L7 segments. Yet, there was a substantial number of terminals where these conditioning stimuli had minor or no effects. Autogenetic stimulation of the PAN with trains of pulses increased the intraspinal threshold in 46% and reduced the threshold in 26% of fibers tested in the L6-L7 segments (no tests were made with trains of pulses on fibers ending in L3). These observations indicate that PAN afferents have a rather small autogenetic PAD, particularly if this is compared with the effects of heterogenetic stimulation. Therefore, the depression of the PAN intraspinal fields produced by autogenetic stimulation described by Rudomin et al. (Exp Brain Res DOI 10.1007/s00221-006-0600-x, 2006) may be ascribed to other mechanisms besides a GABAa PAD. It is suggested that the small or no autogenetic PAD displayed by the examined joint afferents prevents presynaptic filtering of their synaptic actions and preserves the original information generated in the periphery. This could be important for proper adjustment of limb position.

Tonic and phasic differential GABAergic inhibition of synaptic actions of joint afferents in the cat.

The aim of this study was to examine the functional organization of the spinal neuronal networks activated by myelinated afferent fibers in the posterior articular nerve (PAN) of the anesthetized cat. Particular attention was given to the tonic and phasic GABAa inhibitory modulation of these networks. Changes in the synaptic effectiveness of the joint afferents were inferred from changes in the intraspinal focal potentials produced by electrical stimulation of the PAN. We found that conditioning stimulation of cutaneous nerves (sural, superficial peroneus and saphenous) and of the nucleus raphe magnus often inhibited, in a differential manner, the early and late components of the intraspinal focal potentials produced by stimulation of low and high threshold myelinated PAN afferents, respectively. The degree of the inhibition depended on the strength of both the conditioning and test stimuli and on the segmental level of recording. Conditioning stimulation of group I muscle afferents was less effective, but marked depression of the early and late focal potentials was produced by stimuli exceeding 5 xT. The i.v. injection of 1-2.5 mg/kg of picrotoxin, a GABAa blocker, had relatively minor effects on the early components of the PAN focal potentials, but was able to induce a significant increase of the late components. It also reduced the inhibitory effects of cutaneous and joint nerve conditioning on PAN focal responses. Conditioning autogenetic stimulation with high-frequency trains depressed the PAN focal potentials. The late components of the PAN responses remained depressed several minutes after discontinuing the conditioning train, even after picrotoxin administration. The present observations indicate that the neuronal networks activated by the low threshold PAN afferents show a relatively small post-activation depression and appear to be subjected to a minor tonic inhibitory GABAa control. In contrast, the pathways activated by stimulation of high threshold myelinated afferents have a strong post-activation depression and are subjected to a significant tonic GABAergic modulation. These contrasting features, together with the phasic differential GABAergic inhibition of the responses produced by stimulation of the different populations of joint afferents, may contribute to the preservation of the original information on joint position transmitted by large diameter joint afferents, in contrast with the tonic presynaptic inhibition exerted on the fine myelinated joint afferents, which may be involved in the adjustment of compensatory reactions to inflammation.

Persistence of PAD and presynaptic inhibition of muscle spindle afferents after peripheral nerve crush.

Two to twelve weeks after crushing a muscle nerve, still before the damaged afferents reinnervate the muscle receptors, conditioning stimulation of group I fibers from flexor muscles depolarizes the damaged afferents [M. Enriquez, I. Jimenez, P. Rudomin, Changes in PAD patterns of group I muscle afferents after a peripheral nerve crush. Exp. Brain Res., 107 (1996), 405-420]. It is not known, however, if this primary afferent depolarization (PAD) is indeed related to presynaptic inhibition. We now show in the cat that 2-12 weeks after crushing the medial gastrocnemius nerve (MG), conditioning stimulation of group I fibers from flexors increases the excitability of the intraspinal terminals of both the intact lateral gastrocnemius plus soleus (LGS) and of the previously damaged MG fibers ending in the motor pool, because of PAD. The PAD is associated with the depression of the pre- and postsynaptic components of the extracellular field potentials (EFPs) evoked in the motor pool by stimulation of either the intact LGS or of the previously damaged MG nerves. These observations indicate, in contrast to what has been reported for crushed cutaneous afferents [K.W. Horch, J.W. Lisney, Changes in primary afferent depolarization of sensory neurones during peripheral nerve regeneration in the cat, J. Physiol., 313 (1981), 287-299], that shortly after damaging their peripheral axons, the synaptic efficacy of group I spindle afferents remains under central control. Presynaptic inhibitory mechanisms could be utilized to adjust the central actions of muscle afferents not fully recovered from peripheral lesions.

Tonic differential supraspinal modulation of PAD and PAH of segmental and ascending intraspinal collaterals of single group I muscle afferents in the cat spinal cord.

We compared in the anesthetized cat the effects of reversible spinalization by cold block on primary afferent depolarization (PAD) and primary afferent hyperpolarization (PAH) elicited in pairs of intraspinal collaterals of single group I afferents from the gastrocnemius nerve, one of the pairs ending in the L3 segment, around the Clarke's column nuclei, and the other in the L6 segment within the intermediate zone. PAD in each collateral was estimated by independent computer-controlled measurement of the intraspinal current required to maintain a constant probability of antidromic firing. The results indicate that the segmental and ascending collaterals of individual afferents are subjected to a tonic PAD of descending origin affecting in a differential manner the excitatory and inhibitory actions of cutaneous and joint afferents on the pathways mediating the PAD of group I fibers. The PAD-mediating networks appear to function as distributed systems whose output will be determined by the balance of the segmental and supraspinal influences received at that moment. It is suggested that the descending differential modulation of PAD enables the intraspinal arborizations of the muscle afferents to function as dynamic systems, in which information transmitted to segmental reflex pathways and to Clarke's column neurons by common sources can be decoupled by sensory and descending inputs, and funneled to specific targets according to the motor tasks to be performed.

Effects of spinal and peripheral nerve lesions on the intersegmental synchronization of the spontaneous activity of dorsal horn neurons in the cat lumbosacral spinal cord.

In the anesthetized and paralyzed cat, spontaneous negative cord dorsum potentials (nCDPs) appeared synchronously in the L3 to S1 segments, both ipsi- and contralaterally. The acute section of both the intact sural and the superficial peroneal nerve increased the variability of the spontaneous nCDPs without affecting their intersegmental coupling. On the other hand, the synchronization between the spontaneous nCDPs recorded in segments L5-L6 was strongly reduced following an interposed lesion of the left (ipsilateral) dorsolateral spinal quadrant and it was almost completely abolished by an additional lesion of the contralateral dorsolateral quadrant at the same level. Our observations support the existence of a system of spontaneously active dorsal horn neurons that is bilaterally distributed along the lumbosacral segments and affects, in a synchronized and organized manner, impulse transmission along many reflex pathways, including those mediating presynaptic inhibition.

Differential modulation of primary afferent depolarization of segmental and ascending intraspinal collaterals of single muscle afferents in the cat spinal cord.

We examined primary afferent depolarization (PAD) in the anesthetized cat elicited in 109 pairs of intraspinal collaterals of single group I afferents from the gastrocnemius nerve, one of the pair ending in the L3 segment, around the Clarke's column nuclei, and the other in the L6 segment within the intermediate zone. Tests for refractoriness were made to assess whether the responses produced by intraspinal stimulation in the L3 and L6 segments were due to activation of collaterals of the same afferent fiber. PAD in each collateral was estimated by independent computer-controlled measurement of the intraspinal current required to maintain a constant probability of antidromic firing. In most fibers, stimulation of the ipsilateral posterior biceps and semitendinosus (PBSt) nerve with trains of pulses maximal for group I afferents had a qualitatively similar effect but produced a larger PAD in the L6 than in the L3 collaterals. Stimulation of cutaneous nerves (sural and superficial peroneus) with single pulses and of the posterior articular nerve, the ipsilateral reticular formation, nucleus raphe magnus and contralateral motor cortex with trains of pulses often had qualitatively different effects. They could produce PAD and/or facilitate the PBSt-induced PAD in one collateral, and produce PAH and/or inhibit the PAD in the other collateral. These patterns could be changed in a differential manner by sensory or supraspinal conditioning stimulation. In summary, the present investigation suggests that the segmental and ascending collaterals of individual afferents are not fixed routes for information transmission, but parts of dynamic systems in which information transmitted to segmental reflex pathways and to Clarke's column neurons by common sources can be decoupled by sensory and descending inputs and funneled to specific targets according to the motor tasks to be performed.

Intersegmental synchronization of spontaneous activity of dorsal horn neurons in the cat spinal cord.

Extracellular recordings of neuronal activity made in the lumbosacral spinal segments of the anesthetized cat have disclosed the existence of a set of neurons in Rexed's laminae III-VI that discharged in a highly synchronized manner during the occurrence of spontaneous negative cord dorsum potentials (nCDPs) and responded to stimulation of low-threshold cutaneous fibers (<1.5x T) with mono- and polysynaptic latencies. The cross-correlation between the spontaneous discharges of pairs of synchronic neurons was highest when they were close to each other, and decreased with increasing longitudinal separation. Simultaneous recordings of nCDPs from several segments in preparations with the peripheral nerves intact have disclosed the existence of synchronized spontaneous nCDPs in segments S1-L4. These potentials lasted between 25 and 70 ms and were usually larger in segments L7-L5, where they attained amplitudes between 50 and 150 micro V. The transection of the intact ipsilateral hindlimb cutaneous and muscle nerves, or the section of the dorsal columns between the L5 and L6, or between the L6 and L7 segments in preparations with already transected nerves, had very small effects on the intersegmental synchronization of the spontaneous nCDPs and on the power spectra of the cord dorsum potentials recorded in the lumbosacral enlargement. In contrast, sectioning the ipsilateral dorsal horn and the dorsolateral funiculus at these segmental levels strongly decoupled the spontaneous nCDPs generated rostrally from those generated caudally to the lesion and reduced the magnitude of the power spectra throughout the whole frequency range. These results indicate that the lumbosacral intersegmental synchronization between the spontaneous nCDPs does not require sensory inputs and is most likely mediated by intra- and intersegmental connections. It is suggested that the occurrence of spontaneous synchronized nCDPs is due to the activation of tightly coupled arrays of neurons, each comprising one or several spinal segments. This system of neurons could be involved in the modulation of the information transmitted by cutaneous and muscle afferents to functionally related, but rostrocaudally distributed spinal interneurons and motoneurons, as well as in the selection of sensory inputs during the execution of voluntary movements or during locomotion.

Modulation of synaptic transmission from segmental afferents by spontaneous activity of dorsal horn spinal neurones in the cat.

We examined, in the anaesthetised cat, the influence of the neuronal ensembles producing spontaneous negative cord dorsum potentials (nCDPs) on segmental pathways mediating primary afferent depolarisation (PAD) of cutaneous and group I muscle afferents and on Ia monosynaptic activation of spinal motoneurones. The intraspinal distribution of the field potentials associated with the spontaneous nCDPs indicated that the neuronal ensembles involved in the generation of these potentials were located in the dorsal horn of lumbar segments, in the same region of termination of low-threshold cutaneous afferents. During the occurrence of spontaneous nCDPs, transmission from low-threshold cutaneous afferents to second order neurones in laminae III-VI, as well as transmission along pathways mediating PAD of cutaneous and Ib afferents, was facilitated. PAD of Ia afferents was instead inhibited. Monosynaptic reflexes of flexors and extensors were facilitated during the spontaneous nCDPs. The magnitude of the facilitation was proportional to the amplitude of the 'conditioning' spontaneous nCDPs. This led to a high positive correlation between amplitude fluctuations of spontaneous nCDPs and fluctuations of monosynaptic reflexes. Stimulation of low-threshold cutaneous afferents transiently reduced the probability of occurrence of spontaneous nCDPs as well as the fluctuations of monosynaptic reflexes. It is concluded that the spontaneous nCDPs were produced by the activation of a population of dorsal horn neurones that shared the same functional pathways and involved the same set of neurones as those responding monosynaptically to stimulation of large cutaneous afferents. The spontaneous activity of these neurones was probably the main cause of the fluctuations of the monosynaptic reflexes observed under anaesthesia and could provide a dynamic linkage between segmental sensory and motor pathways.

Effects of PAD on conduction of action potentials within segmental and ascending branches of single muscle afferents in the cat spinal cord.

In anesthetized and paralyzed cats under artificial respiration, we examined the extent to which primary afferent depolarization (PAD) might affect invasion of action potentials in intraspinal axonal and/or terminal branches of single muscle afferents. To this end, one stimulating micropipette was placed at the L6 spinal level within the intermediate or motor nucleus, and another one at the L3 level, in or close to Clarke's column. Antidromically conducted responses produced in single muscle afferents by stimulation at these two spinal levels were recorded from fine lateral gastrocnemius nerve filaments. In all fibers examined, stimulation of one branch, with strengths producing action potentials, increased the intraspinal threshold of the other branch when applied at short conditioning testing stimulus intervals (<1.5-2.0 ms), because of the refractoriness produced by the action potentials invading the tested branch. Similar increases in the intraspinal threshold were found in branches showing tonic PAD and also during the PAD evoked by stimulation of group I afferent fibers in muscle nerves. It is concluded that during tonic or evoked PAD, axonal branches in the dorsal columns and myelinated terminals of muscle afferents ending deep in the L6 and L3 segmental levels continue to be invaded by action potentials. These findings strengthen the view that presynaptic inhibition of muscle afferents produced by activation of GABAergic mechanisms is more likely to result from changes in the synaptic effectiveness of the afferent terminals than from conduction failure because of PAD.

Presynaptic selection of afferent inflow in the spinal cord.

The synaptic effectiveness of sensory fibers ending in the spinal cord of vertebrates can be centrally controlled by means of specific sets of GABAergic interneurons that make axo-axonic synapses with the terminal arborizations of the afferent fibers. In the steady state, the intracellular concentration of chloride ions in these terminals is higher than that predicted from a passive distribution, because of an active transport mechanism. Following the release of GABA by spinal interneurons and activation of GABA(A) receptors in the afferent terminals, there is an outwardly directed efflux of chloride ions that produces primary afferent depolarization (PAD) and reduces transmitter release (presynaptic inhibition). Studies made by intrafiber recording of PAD, or by measuring changes in the intraspinal threshold of single afferent terminals (which is reduced during PAD), have further indicated that muscle and cutaneous afferents have distinctive, but modifiable PAD patterns in response to segmental and descending stimuli. This has suggested that PAD and presynaptic inhibition in the various types of afferents is mediated by separate sets of last-order GABAergic interneurons. Direct activation, by means of intraspinal microstimulation, of single or small groups of last-order PAD-mediating interneurons shows that the monosynaptic PAD elicited in Ia and Ib afferents can remain confined to some sets of the intraspinal collaterals and not spread to nearby collaterals. The local character of PAD allows cutaneous and descending inputs to selectively inhibit the PAD of segmental and ascending intraspinal collaterals of individual muscle spindle afferents. It thus seems that the intraspinal branches of the sensory fibers are not hard wired routes that diverge excitation to spinal neurons, but are instead dynamic pathways that can be centrally controlled to address information to selected neuronal targets. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.

Presynaptic inhibition in the vertebrate spinal cord revisited.

The present review examines the experimental evidence supporting the existence of central mechanisms able to modulate the synaptic effectiveness of sensory fibers ending in the spinal cord of vertebrates. The first section covers work on the mode of operation and the synaptic mechanisms of presynaptic inhibition, in particular of the presynaptic control involving axo-axonic synapses made by GABAergic interneurons with the terminal arborizations of the afferent fibers. This includes reviewing of the ionic mechanisms involved in the generation of primary afferent depolarization (PAD) by GABAergic synapses, the ultrastructural basis underlying the generation of PAD, the relationship between PAD and presynaptic inhibition, the conduction of action potentials in the terminal arborizations of the afferent fibers, and the modeling of the presynaptic inhibitory synapse. The second section of the review deals with the functional organization of presynaptic inhibition. This includes the segmental and descending presynaptic control of the synaptic effectiveness of group-I and group-II muscle afferents, the evidence dealing with the local character of PAD as well as the differential inhibition of PAD in selected collaterals of individual muscle-spindle afferents by cutaneous and descending inputs. This section also examines observations on the presynaptic modulation of large cutaneous afferents, including the modulation of the synaptic effectiveness of thin myelinated and unmyelinated cutaneous fibers and of visceral afferents, as well as the presynaptic control of the synaptic actions of interneurons and descending tract neurons. The third section deals with the changes in PAD occurring during sleep and fictive locomotion in higher vertebrates and with the changes of presynaptic inhibition in humans during the execution of a variety of voluntary movements. In the final section, we examine the non-synaptic presynaptic modulation of transmitter release, including the possibility that the intraspinal endings of primary afferents also release colocalized peptides in a similar way as in the periphery. The outcome of the studies presently reviewed is that intraspinal terminals of sensory fibers are not hard-wired conductors of the information generated in their peripheral sensory receptors, but dynamic systems that convey information that can be selectively addressed by central mechanisms to specific neuronal targets. This central control of information flow in peripheral afferents appears to play an important role in the generation of integrated movements and processing of sensory information, including nociceptive information.