Changes in the somatotopic organization of the cat lumbar spinal cord following peripheral nerve transection and regeneration

Glass microelectrodes were used to record the activity of neurones in the left dorsal horn of the L6 segment of the spinal cord of normal cats and cats in which the left sciatic and saphenous nerves had been cut 1 or 9 months previously. In the normal animals the receptive fields of L6 dorsal horn neurones excited by tactile stimulation of the leg were somatotopically organized, with neurones in the medial and central dorsal horn having receptive fields on the distal parts of the leg, particularly the toes, and neurones in the lateral dorsal horn having receptive fields on the proximal parts of the leg, buttock and lower back. This somatotopy has been shown before. One month after nerve section no cells responded to tactile stimulation of the distal leg and cells in the medial and central parts of the dorsal horn now had receptive fields on the proximal leg, buttock and back. There did not appear to be any somatotopic organization of these new receptive fields. Lateral dorsal horn neurones had normal receptive fields. Nine months after nerve section neurones in the medial and central parts of the lumbar dorsal horn had receptive fields on the distal leg but they showed several abnormal features and there was no evidence of a return of the somatotopic organization seen in normal animals. Lateral dorsal horn cells still had normal receptive fields.     

Absence of mediolateral reorganization of dorsal horn somatotopy after peripheral deafferentation in the cat

The effect of chronic transection of the sciatic and saphenous nerves on the receptive fields of dorsal horn neurons in the L7 segment has been reinvestigated in six cats anesthetized with chloralose. Following nerve transection only a narrow lateral band of dorsal horn contained neurons with light touch receptive fields; these were situated on the proximal part of the hind limb. Dorsal horn neurons situated more than about 0.25 mm medial of the lateral edge (at the level of lamina IV) of the dorsal horn lost their light touch receptive fields, and did not acquire new light touch RFs on the proximal part of the hind limb for as long as 49 days after nerve transection. There was thus no sign of the extensive mediolateral reorganization of somatotopy described by some previous workers. Many affected neurons throughout laminae IV to VI became phasically responsive to mechanical stimulation of unidentified mechanoreceptors in deep tissue (e.g., muscle, tendon, joints, and fasciae) of the proximal part of the limb. Some of these neurons had quite low thresholds to mechanical distortion. A small proportion of neurons in medial lamina V and VI may acquire large, high-threshold cutaneous mechanoreceptive fields on the proximal part of the limb. The relation of these time-dependent changes to the known distribution of primary afferent fibers within the dorsal horn is discussed.     

Altered responses of nociceptive cat lamina I spinal dorsal horn neurons after chronic sciatic neuroma formation

The activity of lumbar spinal dorsal horn lamina I neurons with afferent drive from the sciatic nerve was studied in intact cats and in cats with acute sciactic nerve transection or chronic sciatic nerve transection with neuroma formation. The majority (51 of 75) of neurons recorded in lamina I ipsilateral to a neuroma had no receptive field and could only be identified by their responses to electrical stimulation of the sciatic nerve. The remainder could be activated by the sciatic nerve, but their responses to mechanical stimulation were irregular in comparison to the stable responses of cells recorded in control animals and to the responses of cells contralateral to chronic nerve lesions. Animals with acute nerve transections demonstrated as loss sciatic nerve-innvervated cells with receptive fields except for those cells located on the lateral edge of the dorsal horn, which had normal, proximal receptive fields and response characteristics. In addition, the characteristic somatotopy of lamina I cells was not observed in some cats with chronic neuromata. The mediolateral distribution of cell types indicated that some cells had altered receptive fields following chronic nerve transection. The data presented for lamina I neurons agrees with the observation of spinal cord plasticity first presented for cat dorsal horn cells. Since there is no evidence for a redistribution of intact afferent fibers following chronic nerve transection in adult mammals, the mechanism of altered somatotopy may involved alterations in synaptic efficacy at existing synapses.     

Alterations in the ipsi- and contralateral afferent inputs of dorsal horn cells produced by capsaicin treatment of one sciatic nerve in the rat

Dorsal horn cells in the lumbar spinal cord of decerebrate spinal rats were examined 7-21 days following local application of capsaicin to the sciatic nerve. Such local capsaicin treatment is known not to influence the size of the incoming A and C fibre afferent volley. The receptive field properties and primary afferent input of cells on both sides of the cord, that is ipsi and contralateral to the treated nerve, were examined. On the treated side, the percentage of cells excited by C fibres from the capsaicin treated nerve was 30% of normal and the number of cells responding to noxious heating of the cutaneous receptive field was reduced by 50%. A fibre input and low and high threshold mechanical input were normal. The receptive field size was larger in many cells innervated by the treated nerve. On the side opposite to the treated nerve, responses to noxious and non-noxious stimulation of the untreated limb were unaffected as was the input from the untreated sciatic nerve. Receptive fields were somewhat larger than normal. Effects were also observed from contralateral stimuli. Cells on both sides of the cord were found with excitatory contralateral receptive fields and excitatory responses to trains of high intensity stimulation to the contralateral sciatic nerve. In untreated animals the effect of such contralateral stimulation is inhibitory. The results show that peripheral nerve capsaicin treatment causes long lasting reduction of the C fibre input to dorsal horn cells on the treated side. However, it also results in changes in the inhibitory and excitatory receptive fields of cells on both sides of the cord.     

Plasticity in the nucleus gracilis of the rat

The nucleus gracilis of the rat contains a somatotopic map of the hindquarter similar to that in other mammals. Acute deafferentation of this dorsal column nucleus (DCN) either by dorsal root transection (roots L4, 5, and 6), by transection of the spinal cord at L3, or by peripheral nerve transection (the sciatic and saphenous nerves) creates as expected a clearly demarcated region in which cells have no receptive fields (RFs). Plasticity in the nucleus gracilis after four types of chronic deafferentiation was investigated: Experiment 1—15 to 20 days after transection of L4, 5, and 6 dorsal roots, cells in the acutely unresponsive region were found to be excited from nearby intact afferent fibers. This expansion of the effective input of intact afferent fibers is known to occur in the spinal cord. Experiment 2—Chronic transection of the sciatic and saphenous nerves was not followed by expansion of the intact input although this does occur in the spinal cord. Experiment 3—Neonatal treatment with capsaicin which destroys unmyelinated afferent fibers produced adult animals with somatotopically organized nucleus gracilis but with cells which exhibited very large RFs. This enlargement is also known to occur in the spinal cord. Experiment 4—Local application of capsaicin to a sciatic nerve of an adult enlarged the RFs of spinal cord cells after some days but had no effect on the functional organization of nucleus gracilis. Expansion of the RF of nucleus gracilis cells was, therefore, seen after dorsal root transection and after neonatal application of capsaicin but not after peripheral nerve transection or application of capsaicin to the adult single nerve. All four maneuvers produced RF expansion in the spinal cord. We conclude that the DCN has a more limited ability to respond to deafferentation than the spinal cord, and suggested that the former lacks a mechanism of connectivity control. The results are consistent with the hypothesis that primary afferent C fibers which innervate the spinal cord but not the DCN, play a role in the formation and stability of RFs formed by A fibers in adult animals.     

Effect of peripheral nerve injury on receptive fields of cells in the cat spinal cord

When the sciatic and saphenous nerves are cut and ligated in adult cats, the immediate effect is the production of a completely anesthetic foot and a region in medial lumbar dorsal horn where almost all cells have lost their natural receptive fields (RFs). Beginning at about 1 week and maturing by 4 weeks, some 40% of cells in the medial dorsal horn gain a novel RF on proximal skin, that is, upper and lower leg, thigh, lower back, or perineum. This new RF is supplied by intact proximal nerves and not by sciatic and saphenous nerve fibers that sprouted in the periphery. During the period of switching of RFs from distal to proximal skin there was no gross atrophy of dorsal horn grey matter and no Fink-Heimer stainable degeneration of central arbors and terminals of peripherally axotomized afferents. In intact animals medial dorsal horn cells showed no sing of response to mechanical stimulation of proximal skin. RFs of some of the cells had spontaneous variations in size and sensitivity, but these were not nearly sufficient to explain the large shifts observed after chronic nerve section. Tetanic electrical stimulation of skin or peripheral nerves often caused RFs to shrink, but never to expand. Although natural stimuli of proximal skin would not excite medial dorsal horn cells in intact or acutely deafferented animals, it was found that electrical stimulation of proximal nerves did excite many of these cells, often at short latencies. In the discussion we justify our working hypothesis that the appearance of novel RFs is due to the strengthening or unmasking of normally present but ineffective afferent terminals, rather than to long-distance sprouting of new afferent arbors within the spinal cord.     

Somatotopic organization of the dorsal horn in the lumbosacral enlargement of the spinal cord in the neonatal cat

The somatotopic organization of the light touch receptive fields of single unidentified dorsal horn neurons in the lumbosacral spinal cord has been studied in the neonatal cat anesthetized with chloralose. Satisfactory recordings were obtained from single dorsal horn neurons in kittens aged 3-6 days. Reconstruction of recording tracks from pontamine blue dye spots and comparisons of the depths of recording sites with Nissl-stained sections of cord showed that most single-unit recordings were obtained from laminae III and IV of Rexed. In animals of all ages neurons were found which responded briskly to light cutaneous mechanical stimulation. Their receptive fields varied widely in size, being smallest on the distal digits and largest on proximal skin. Receptive field areas were similar in proportion to the size of the hindlimb to those seen in the equivalent region in the adult cat. Because of the shape of the dorsal horn and the relatively narrow dorsal columns in neonatal kittens it proved difficult to locate units with receptive fields on proximal skin. Nevertheless the main features of the somatotopic organization of the dorsal horn were similar to those in the adult cat. Thus the somatotopic map of the kitten showed a medial representation of glabrous skin that was bounded laterally by the representation of the hairy skin of the toes. Proximal skin was represented in the lateral parts of the dorsal horn, a region which was not easily accessible for microelectrode recording. The individual toes were represented in a rostral to caudal sequence such that toe 2 was represented rostrally and toe 5 caudally. Around the toe representation the medial surface of the foot was represented rostrally, the ventrolateral surface caudally, and the dorsal surface laterally. The results indicate that the mature organization of light touch receptive fields of dorsal horn neurons in the lumbosacral cord of the cat is already largely present at birth.     

Somatotopic organization of cat brachial spinal cord

The representation of forelimb skin was mapped in the dorsal horn of spinal cord segments C4-T2 using single-unit recording techniques. The trajectory of segmental representation was found to course down the anterior surface of the forelimb, across the ventral surface of the forepaw, and up the posterior surface onto the anterior thorax, with shifting overlap, and little receptive field overlap between the preaxial and postaxial limb. Cells with receptive field centers on the proximal limb were generally located in the lateral dorsal horn and cells with distal receptive field centers tended to be located more medially. Cells with receptive fields on the ventral surface of the limb were located medially in the dorsal horn and those on the dorsal surface were in the lateral dorsal horn. Receptive field geometries varied with location on the limb. We conclude that forelimb segmental representation in the cervicothoracic cord obeys the same organizational rules as hind limb representation in the lumbosacral cord.     

PACAP mRNA is expressed in rat spinal cord neurons

This study examines the expression of pituitary adenylate cyclase activating polypeptide (PACAP) mRNA in the rat spinal cord during normal conditions and in response to sciatic nerve transection. Previously, PACAP immunoreactivity has been found in fibers in the spinal cord dorsal horn and around the central canal and in neurons in the intermediolateral column (IML). Furthermore, in the dorsal root ganglia, PACAP immunoreactivity and PACAP mRNA expression have been observed preferentially in nerve cell bodies of smaller diameter terminating in the superficial laminae of the dorsal horn. However, neuronal expression of PACAP mRNA in adult rat spinal cord appeared limited to neurons of the IML. By using a refined in situ hybridization protocol, we now detect PACAP mRNA expression in neurons primarily in laminae I and II, but also in deeper laminae of the spinal cord dorsal horn and around the central canal. In addition, PACAP mRNA expression is observed in a few neurons in the ventral horn. PACAP expression in the ventral horn is increased in a population of large neurons, most likely motor neurons, both after distal and proximal sciatic nerve transection. The proposed role of PACAP in nociception is strengthened by our findings of PACAP mRNA-expressing neurons in the superficial laminae of the dorsal horn. Furthermore, increased expression of PACAP in ventral horn neurons, in response to nerve transection, suggests a role for PACAP in repair/regeneration of motor neurons.     

Effect of nerve section on the spinal distribution of neighboring nerves

The spinal cord distribution of axonal terminals of peripheral nerves that innervate the skin of the upper medial thigh was examined in rats using transganglionic transport of horseradish peroxidase (HRP) and wheat-germ agglutinin-conjugated HRP (WGA-HRP). Chronic transection of the sciatic nerve or both the sciatic and saphenous nerves did not alter this distribution. Therefore, long-distance sprouting of intact 'thigh nerve' afferents in the dorsal horn is apparently not the mechanism whereby spinal dorsal horn neurons deafferented by sciatic and saphenous neurectomy, gain novel receptive fields in the cutaneous distribution of neighbouring intact nerves of the thigh.     

Crossed and uncrossed projections to cat sacrocaudal spinal cord: I. Axons from cutaneous receptors

Intra-axonal recording and horseradish peroxidase staining techniques were used to map terminal fields of primary afferent fibers from cutaneous receptors within the cat sacrocaudal spinal cord. It was hypothesized that projection patterns of cutaneous afferent fibers mirror the known somatotopic organization of sacrocaudal dorsal horn cells. Forty-three primary afferent fibers, innervating either slowly adapting type I receptors, hair follicles, or slowly adapting type II receptors, all on the tail, were recovered. All collaterals (N = 372) branched from parent axons in the dorsal columns. Most collaterals coursed rostromedially to the ipsilateral gray matter, penetrated the medial dorsal horn, and arborized within laminae III, IV, and to a lesser extent, V. Ipsilateral projections to dorsal horn were as follows: axons with dorsal or dorsolateral receptive fields (RFs; n = 20) to the lateral portion, axons with lateral RFs (n = 4) to the central portion, and axons with ventral or ventro-lateral RFs (n = 19) to the medial portion. Most axons (16 of 20) with dorsal or dorsolateral RFs also had contralateral projections to lateral dorsal horn and most axons (15 of 19) with ventral or ventrolateral RFs also had contralateral projections to medial dorsal horn. No axons with lateral RFs had crossed projections. These data represent the first complete mapping of the somatotopic organization of primary afferent fiber projection patterns to a spinal cord level. The findings demonstrate that ipsilateral projection patterns of sacrocaudal primary afferent fibers are in register with the somatotopic organization of the dorsal horn. Our earlier suggestion that crossed projections of primary afferent fibers give rise to crossed components of dorsal horn RFs spanning the midline is supported by these results.     

Increases in heart rate and blood pressure produced by injections of dermorphin into discrete hypothalamic sites

Single unit electrical activity has been recorded from dorsal horn neurones in the lumbar spinal cord of adult rats which had been treated at birth with either capsaicin (50 mg kg-1) or with the solvent-vehicle only. The responses of these neurones to electrical stimulation of A- and C-fibres in the sural nerve and to natural stimulation of their cutaneous receptive fields have been studied. In vehicle-injected rats, 54% of the units driven by electrical stimulation of the A-fibres in the sural nerve could also be driven by stimulation of the C-fibres in this nerve. In capsaicin-treated animals, only 30% of such units had a C-fibre input from the sural nerve. In vehicle-injected rats, 51.5% of the neurones with a C-fibre input showed a 'wind-up' effect on repetitive C-fibre stimulation of the sural nerve at 1 Hz. A similar proportion of neurones (55%) displayed this effect in capsaicin-treated rats. There were fewer neurones with very intense 'wind-up' in capsaicin-treated compared to vehicle-treated rats. In capsaicin-treated animals, greater proportions of neurones with 'wind-up' were superficially located in the dorsal horn, had small receptive fields and were driven only by cutaneous nociceptors. The proportions of neurones driven by innocuous mechanical stimulation of the skin, by noxious mechanical stimulation or by both forms of stimulation were similar in vehicle-injected and capsaicin-treated animals. In capsaicin-treated rats, more neurones had 'medium-sized' receptive fields than in vehicle-injected rats. In capsaicin-treated rats, more neurones had receptive fields in the foot and ankle than in vehicle-injected animals, where receptive fields in the toes were predominant. Some neurones showed expanded receptive fields after repetitive electrical stimulation of C-fibres at 1 Hz. This expansion occurred more often in neurones recorded from capsaicin-treated animals than in those of vehicle-injected rats. These results are discussed in relation to the role of afferent C-fibres in sensory mechanisms.     

The acute and chronic effects of capsaicin on slow excitatory transmission in rat dorsal horn

The acute and chronic effects of capsaicin on rat spinal dorsal horn neurons and the excitatory transmission in the dorsal horn were investigated by means of intracellular recording techniques in the spinal cord slice preparation. Bath application of capsaicin (1–2 × 10⁻⁵M) produced in a majority of cells a prolonged depolarization associated with an increase in synaptic activity and intense neuronal discharges. During and immediately following the capsaicin depolarization, repetitive stimulation of a dorsal root failed to elicit the slow depolarization.

After neonatal capsaicin treatment the proportion of dorsal horn neurons exhibiting the slow excitatory transmission was markedly reduced, however, the fast excitatory postsynaptic potentials were present in all examined cells. In addition, the proportion and sensitivity of the cells responding with a slow depolarization to substance P increased.     

Myelinated afferents sprout into lamina II of L3-5 dorsal horn following chronic constriction nerve injury in rats

In order to investigate the consequences of chronic constriction injury (CCI) to nerve, we explored the relationship between the development of mechanical allodynia and the reorganization of primary afferent terminals in the sensory lamina of the rat spinal cord dorsal horn. Following sciatic CCI neuropathy, mechanical allodynia developed in the corresponding footpad within two weeks and persisted throughout the experimental period which extended for an additional two weeks. The neuropathy of the sciatic injury includes extensive Wallerian-like degeneration of myelinated fibers but relative sparing of unmyelinated fibers. We observed that there was no significant change in the dorsal horn termination of unmyelinated C fibers in lamina II of the dorsal horn, using nerve injections of wheat germ agglutin-horseradish peroxidase for transganglionic axonal tracing of these fibers from the nerve injury site, and no evidence of sprouting into adjacent lamina. In contrast, myelinated afferent fibers were observed to be sprouting into lamina II of the dorsal horn, as indicated by cholera toxin beta-subunit-horseradish peroxidase retrograde axonal tracings. This region of the dorsal horn is associated with nociceptive-specific neurons that are not generally associated with myelinated fiber input from mechanical and proprioceptive receptors. As previously suggested in nerve transection and crush injuries, and now demonstrated in CCI neuropathy, these morphological changes may have significance in the pathogenesis of chronic mechanical allodynia.     

Myelinated afferents sprout into lamina II of L3–5 dorsal horn following chronic constriction nerve injury in rats

In order to investigate the consequences of chronic constriction injury (CCI) to nerve, we explored the relationship between the development of mechanical allodynia and the reorganization of primary afferent terminals in the sensory lamina of the rat spinal cord dorsal horn. Following sciatic CCI neuropathy, mechanical allodynia developed in the corresponding footpad within two weeks and persisted throughout the experimental period which extended for an additional two weeks. The neuropathy of the sciatic injury includes extensive Wallerian-like degeneration of myelinated fibers but relative sparing of unmyelinated fibers. We observed that there was no significant change in the dorsal horn termination of unmyelinated C fibers in lamina II of the dorsal horn, using nerve injections of wheat germ agglutin-horseradish peroxidase for transganglionic axonal tracing of these fibers from the nerve injury site, and no evidence of sprouting into adjacent lamina. In contrast, myelinated afferent fibers were observed to be sprouting into lamina II of the dorsal horn, as indicated by cholera toxin β-subunit-horseradish peroxidase retrograde axonal tracings. This region of the dorsal horn is associated with nociceptive-specific neurons that are not generally associated with myelinated fiber input from mechanical and proprioceptive receptors. As previously suggested in nerve transection and crush injuries, and now demonstrated in CCI neuropathy, these morphological changes may have significance in the pathogenesis of chronic mechanical allodynia.     

The effects of capsaicin applied to peripheral nerves on responses of a group of lamina I cells in adult rats

In adult rats, the sciatic and saphenous nerves on one side were treated topically with capsaicin. The capsaicin treatment had the effect of increasing the latency for withdrawal of the foot from hot water; 11–22 days later, the animals were decerebrated, and cells in the superficial dorsal horn of the lumbar cord with axons projecting in the contralateral dorsolateral funiculus (DLF) were examined electrophysiologically on the treated and untreated sides of the cord. HRP was applied to cut axons of the DLF at C4, in other rats, and retrograde labelling of cells in the lumbar cord indicated that most or all of the recordings in the capsaicin-treated animals were likely to originate from lamina 1. The dorsal horn cells, with receptive fields on the foot, showed decreased responses to electrically evoked afferent impulses in C fibres and grossly altered receptive fields. After capsaicin treatment, the proportion of cells responding to C afferents fell from 83% to 14%. The proportion responding only to C afferents and not to A afferents fell from 9% to 0%. The receptive fields (RFs) of these cells showed two gross abnormalities; 32% of the cells on the treated side had no apparent RF or an ill-defined, intermittent RF, whereas such cells were rare on the untreated side or in intact animals. By contrast 49% of the cells had grossly expanded RFs with an average area of 430 mm² against the normal average size of 130 mm². The antidromic conduction velocity of the axons of the cells was measured, and while there was no significant change in the number of cells with axons conducting above 3 m/second, an increased number of cells with lower velocities were recorded. It is suggested that the capsaicin treatment induced dendritic changes in cells with small axons allowing antidromic spikes to invade these cells.     

Spinal input pathways affecting the medullary gigantocellular reticular nucleus

Single neurons were recorded in nucleus reticularis gigantocellularis of anesthetized or decerebrate cats. Most of the neurons were isolated using contralateral dorsal funicular stimulation to initially identify the neuron, and all were tested for responsiveness to, or modulation by, input from the contralateral dorsal funiculus and the ipsilateral ventrolateral tracts in the presence of either complete spinal transection sparing the dorsal funiculi or transection of the dorsal half of the spinal cord. Two-thirds of the neurons were excited by way of both spinal pathways when the stimulus was applied rostal to the spinal lesion. The remaining one-third of the neurons were excited by one pathway (90% via dorsal funiculus and 10% via ventrolateral tracts) and inhibited by the other pathway. Most neurons responded within 2 to 8 ms after stimulation of either pathway, and 4.5% were driven antidromically from the ventrolateral tracts. In cats with only the dorsal funiculi intact, all neurons tested for cutaneous responsiveness had small excitatory receptive fields; 90% responded to touch and 10% to hair bending. The thresholds to peripheral nerve stimulation suggested that these neurons were excited by way of the large Aα fibers. Nearly three-fourths of the neurons tested responded to stimulation of the medial lemniscus just caudal to the thalamus; most responded within 5 ms. In cats with only the ventral half of the spinal cord intact, nearly all neurons tested responded to electrical stimulation of each of the four paws, but only 19% responded to cutaneous stimulation (tap or noxious pinch). Clear interactions were found between the two input pathways in nearly all neurons, the pattern of effects (inhibitory and facilitatory) varying with the nature of the effects from each pathway alone. In 4% of the neurons, spontaneous discharge was found to be increased (only ventral half of spinal cord intact) or decreased (only dorsal funiculi intact) during noxious pinching of the skin. Clearly, input by way of both spinal pathways can affect most, if not all, neurons in the gigantocellular nucleus of the medullary reticular formation.     

Crossed receptive field components and crossed dendrites in cat sacrocaudal dorsal horn

The hypothesis that sacrocaudal dorsal horn neurons with crossed receptive field components on the tail have dendrites which cross to the contralateral dorsal horn was tested in a combined electrophysiological and morphological study. Dorsal horn cells in the sacrocaudal spinal cord of anesthetized cats were penetrated with horseradish peroxidase-filled microelectrodes. After mapping their low threshold mechanoreceptive fields, cells were iontophoretically injected with horseradish peroxidase. A sample of 16 well-stained cells was obtained in laminae III and IV. Cells with receptive fields crossing the dorsal midline of the tail (n - 8) had somata in the lateral ipsilateral dorsal horn, and some of these cells (5/8) had dendrites which crossed to the lateral contralateral dorsal horn. Cells with receptive fields spanning the ventral midline (n - 2) were located near the center of the fused dorsal horn, and one of these had bilateral dendrites in this region. Cells with receptive fields on the lateral tail, crossing neither the dorsal nor the ventral midline (n - 6), had cell bodies in the middle of the ipsilateral dorsal horn; half had only ipsilateral dendrites, and half had crossed dendritic branches. Although the relationship between cell receptive field (RF) location (RF center, expressed as distance from tips of toes) and mediolateral location of the cell body was statistically significant, the correlation between crossed RF components and crossed dendritic branches was not significant. © 1993 Wiley-Liss, Inc.     

Slow excitatory transmission in rat dorsal horn: possible mediation by peptides

Repetitive stimulation of a dorsal root elicited a slow depolarization in about half of the dorsal horn neurons examined in the rat spinal cord slice preparation. The response was markedly depressed or abolished in the presence of substance P, substance P antagonists and capsaicin. In some dorsal horn neurons a slow hyperpolarization was also observed.     

A-fiber sprouting in spinal cord dorsal horn is attenuated by proximal nerve stump encapsulation

Peripheral nerve transection in the rat alters the spinal cord dorsal horn central projections from both small and large DRG neurons. Injured neurons with C-fibers exhibit transganglionic degeneration of their terminations within lamina II of the spinal cord dorsal horn, while peripheral nerve injury of medium to large neurons induces collateral sprouting of myelinated A-fibers from lamina I and III/IV into lamina II in rats, cats, and primates. To date, it is not known what sequelae are responsible for the collateral sprouting of A-fibers after peripheral nerve injury, although target-derived factors are thought to play an important role. To determine whether target-derived factors are necessary for changes in A-fiber laminar terminations in rat spinal cord dorsal horn, we unilaterally transected the sciatic nerve and ensheathed the proximal nerve stump in a silicone cap. Three days before sacrifice of rat, the injured sciatic nerve was injected with cholera toxin beta-subunit conjugated to horseradish peroxidase (betaHRP) that effectively labels both peripheral and central A-fiber axons. The effect of the ligature, axotomy, and silicone cap treatment was evaluated by analyzing the extent of betaHRP-, Substance P-(SP-), and isolectin B4- (IB4-) immunoreactive (ir) fibers in the somatotopically appropriate spinal cord dorsal horn regions. In all animals, 2-5 weeks after nerve transection (treated or otherwise), IB4- and SP-ir is absent from lamina II. Animals without nerve cap treatment exhibited robust fiber sprouting into lamina II at 2 weeks. In sharp contrast, animals treated with silicone caps did not exhibit betaHRP-ir fibers in lamina II at 2 weeks. This observation was extended up to 5 weeks postinjury. These results suggest that axotomy-induced expansion of betaHRP-ir primary afferent central terminations in the spinal cord dorsal horn is dependent on factors produced in the injury site milieu. While our understanding of local repair mechanisms of injured peripheral nerves is incomplete, it is clear that the time-dependent production of growth factors in the nerve injury microenvironment favor nerve fiber outgrowth, both peripherally and centrally.