Regeneration of primary afferent neurons containing substance P-like immunoreactivity

Nerve stretch injury induced pain pattern and changes in sensory ganglia in a clinically relevant model of limb-lengthening in rabbits

We used a model of tibial lengthening in rabbits to study the postoperative pain pattern during limb-lengthening and morphological changes in the dorsal root ganglia (DRG), including alteration of substance P (SP) expression. Four groups of animals (naive; OG: osteotomized only group; SDG/FDG: slow/fast distraction groups, with 1 mm/3 mm lengthening a day, respectively) were used. Signs of increasing postoperative pain were detected until the 10(th) postoperative day in OG/SDG/FDG, then they decreased in OG but remained higher in SDG/FDG until the distraction finished, suggesting that the pain response is based mainly on surgical trauma until the 10(th) day, while the lengthening extended its duration and increased its intensity. The only morphological change observed in the DRGs was the presence of large vacuoles in some large neurons of OG/SDG/FDG. Cell size analysis of the S1 DRGs showed no cell loss in any of the three groups; a significant increase in the number of SP-positive large DRG cells in the OG; and a significant decrease in the number of SP-immunoreactive small DRG neurons in the SDG/FDG. Faster and larger distraction resulted in more severe signs of pain sensation, and further reduced the number of SP-positive small cells, compared to slow distraction.

Expression of TRPV1 channels after nerve injury provides an essential delivery tool for neuropathic pain attenuation

Increased expression of the transient receptor potential vanilloid 1 (TRPV1) channels, following nerve injury, may facilitate the entry of QX-314 into nociceptive neurons in order to achieve effective and selective pain relief. In this study we hypothesized that the level of QX-314/capsaicin (QX-CAP) - induced blockade of nocifensive behavior could be used as an indirect in-vivo measurement of functional expression of TRPV1 channels. We used the QX-CAP combination to monitor the functional expression of TRPV1 in regenerated neurons after inferior alveolar nerve (IAN) transection in rats. We evaluated the effect of this combination on pain threshold at different time points after IAN transection by analyzing the escape thresholds to mechanical stimulation of lateral mental skin. At 2 weeks after IAN transection, there was no QX-CAP mediated block of mechanical hyperalgesia, implying that there was no functional expression of TRPV1 channels. These results were confirmed immunohistochemically by staining of regenerated trigeminal ganglion (TG) neurons. This suggests that TRPV1 channel expression is an essential necessity for the QX-CAP mediated blockade. Furthermore, we show that 3 and 4 weeks after IAN transection, application of QX-CAP produced a gradual increase in escape threshold, which paralleled the increased levels of TRPV1 channels that were detected in regenerated TG neurons. Immunohistochemical analysis also revealed that non-myelinated neurons regenerated slowly compared to myelinated neurons following IAN transection. We also show that TRPV1 expression shifted towards myelinated neurons. Our findings suggest that nerve injury modulates the TRPV1 expression pattern in regenerated neurons and that the effectiveness of QX-CAP induced blockade depends on the availability of functional TRPV1 receptors in regenerated neurons. The results of this study also suggest that the QX-CAP based approach can be used as a new behavioral tool to detect dynamic changes in TRPV1 expression, in various pathological conditions.

Spinal or systemic TY005, a peptidic opioid agonist/neurokinin 1 antagonist, attenuates pain with reduced tolerance

BACKGROUND AND PURPOSE

The use of opioids in treating pain is limited due to significant side effects including somnolence, constipation, analgesic tolerance, addiction and respiratory depression. Pre-clinical studies have shown that neurokinin 1 (NK(1) ) receptor antagonists block opioid-induced antinociceptive tolerance and may inhibit opioid-induced rewarding behaviours. Here, we have characterized a bifunctional peptide with both opioid agonist and NK(1) antagonist pharmacophores in a rodent model of neuropathic pain.

EXPERIMENTAL APPROACH

Rats were evaluated for behavioural responses to both tactile and thermal stimuli in either an uninjured, sham- or nerve-injured state. TY005 (Tyr-DAla-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bn(CF(3) )(2) ) was delivered spinally or systemically to assess the antinociceptive effects after acute exposure. Motor skills were evaluated using the rotarod test to determine potential sedative effects. Spinal TY005 was given chronically to sham- or nerve-injured animals to determine the development of tolerance.

KEY RESULTS

Bolus injections of TY005 produced dose-dependent antinociception in non-injured animals and alleviated nerve injury-induced thermal and tactile hypersensitivities (i.e. antihyperalgesia) more effectively than morphine. Sedative effects were not evident from the rotarod test at doses that were antihyperalgesic, nor at doses threefold higher. Repeated administration of TY005 did not lead to the development of antihyperalgesic tolerance or alter sensory thresholds.

CONCLUSIONS AND IMPLICATIONS

Collectively, the data suggest that opioid agonist/NK(1) antagonist bifunctional peptides represent a promising novel approach to the management of chronic pain without the development of tolerance, reducing the need for escalation of doses and unwanted side effects associated with opiates alone.

Sensitization of cutaneous nociceptors after nerve transection and regeneration: possible role of target-derived neurotrophic factor signaling

Damage to peripheral nerves is known to contribute to chronic pain states, including mechanical and thermal hyperalgesia and allodynia. It is unknown whether the establishment of these states is attributable to peripheral changes, central modifications, or both. In this study, we used several different approaches to assess the changes in myelinated (A) and unmyelinated (C) cutaneous nociceptors after transection and regeneration of the saphenous nerve. An ex vivo recording preparation was used to examine response characteristics and neurochemical phenotype of different types of functionally defined neurons. We found that myelinated nociceptors had significantly lower mechanical and thermal thresholds after regeneration, whereas C-polymodal nociceptors (CPMs) had lower heat thresholds. There was a significant increase in the percentage of mechanically insensitive C-fibers that responded to heat (CHs) after regeneration. Immunocytochemical analysis of identified afferents revealed that most CPMs were isolectin B4 (IB4) positive and transient receptor potential vanilloid 1 (TRPV1) negative, whereas CHs were always TRPV1 positive and IB4 negative in naive animals (Lawson et al., 2008). However, after regeneration, some identified CPMs and CHs stained positively for both markers, which was apparently attributable to an increase in the total number of IB4-positive neurons. Real-time PCR analysis of L2/L3 DRGs and hairy hindpaw skin at various times after saphenous nerve axotomy suggested multiple changes in neurotrophic factor signaling that correlated with either denervation or reinnervation of the cutaneous target. These changes may underlie the functional alterations observed after nerve regeneration and may explain how nerve damage leads to chronic pain conditions.

Time course of substance P expression in dorsal root ganglia following complete spinal nerve transection

Recent evidence suggests that substance P (SP) is up-regulated in primary sensory neurons following axotomy and that this change occurs in larger neurons that do not usually produce SP. If this is so, then the up-regulation may allow normally neighboring, uninjured, and nonnociceptive dorsal root ganglion (DRG) neurons to become effective in activating pain pathways. By using immunohistochemistry, we performed a unilateral L5 spinal nerve transection on male Wistar rats and measured SP expression in ipsilateral L4 and L5 DRGs and contralateral L5 DRGs at 1-14 days postoperatively (dpo) and in control and sham-operated rats. In normal and sham-operated DRGs, SP was detectable almost exclusively in small neurons (< or =800 microm2). After surgery, the mean size of SP-positive neurons from the axotomized L5 ganglia was greater at 2, 4, 7, and 14 dpo. Among large neurons (>800 microm2) from the axotomized L5, the percentage of SP-positive neurons increased at 2, 4, 7, and 14 dpo. Among small neurons from the axotomized L5, the percentage of SP-positive neurons was increased at 1 and 3 dpo but was decreased at 7 and 14 dpo. Thus, SP expression is affected by axonal damage, and the time course of the expression is different between large and small DRG neurons. These data support a role for SP-producing, large DRG neurons in persistent sensory changes resulting from nerve injury.

Correlation between the appearance of neuropeptides in the rat trigeminal ganglion and reinnervation of the healing root socket after tooth extraction

The neuropeptide substance P (SP) modulates bone metabolism. This study examined the temporal appearance of the neuropeptides SP and brain-derived nerve growth factor (BDNF) and their receptors (neurokinin-1 receptor (NK₁-R) and Trk B, respectively) in the rat trigeminal ganglion to investigate the role of neuropeptides in healing after tooth extraction. Rats were anesthetized and their upper right first molars were extracted; the rats were sacrificed 3 hours and 1–21 days after extraction. Their trigeminal ganglion and maxilla were removed, and cryosections were prepared and immunostained using specific antibodies against SP, BDNF, NK₁-R, and Trk B. In the tooth sockets after extraction, new bone and a few SP-­immunoreactive nerve fibers were first seen at day 7, and bone completely filled the sockets at day 21. In the trigeminal ganglion, the proportions of NK₁-R-, BDNF-, and Trk B-immuno­reactive neurons changed similarly, i.e., they initially decreased, increased rapidly to ­maximum levels by day 3, and then decreased gradually to control levels until 21 days. These findings suggest that the appearance of neuropeptides in the trigeminal ganglion, the reinnervation of SP-immunoreactive nerve fibers, and bone repair in the tooth socket during healing after extraction were correlated.

The Role of Neurotransmitters in Neurite Outgrowth and Synapse Formation

Besides a well-established role in neuronal communication in the adult central nervous system, neurotransmitters have diverse tasks in the embryonic brain, ranging from early developmental functions in morphogenesis /13/, to later functions in target selection and synapse formation /87/. For example, growth cones of developing neurons are known to release transmitters /26,36,88,110,115/ and respond to transmitters released from other neurons /35,44,59, 61,70/. Moreover, depletion of transmitters during embryonic development results in developmental deficits of the brain /21,48,84,109/, suggesting that transmitters have crucial roles as morphogens and/or neurotrophic factors. Although recently the idea of neurotransmitters being important for neural development has been challenged /99/, there is a vast amount of literature that seems to support the hypothesis that neurotransmitter release in the developing central nervous system is crucial for proper brain development. In this review we focus on the roles that neurotransmitters play in neurite outgrowth, target selection and synapse formation, with particular emphasis on the effects of the transmitters serotonin and dopamine.

Changes in expression of two tetrodotoxin-resistant sodium channels and their currents in dorsal root ganglion neurons after sciatic nerve injury but not rhizotomy

Two TTX-resistant sodium channels, SNS and NaN, are preferentially expressed in c-type dorsal root ganglion (DRG) neurons and have been shown recently to have distinct electrophysiological signatures, SNS producing a slowly inactivating and NaN producing a persistent sodium current with a relatively hyperpolarized voltage-dependence. An attenuation of SNS and NaN transcripts has been demonstrated in small DRG neurons after transection of the sciatic nerve. However, it is not known whether changes in the currents associated with SNS and NaN or in the expression of SNS and NaN channel protein occur after axotomy of the peripheral projections of DRG neurons or whether similar changes occur after transection of the central (dorsal root) projections of DRG neurons. Peripheral and central projections of L4/5 DRG neurons in adult rats were axotomized by transection of the sciatic nerve and the L4 and L5 dorsal roots, respectively. DRG neurons were examined using immunocytochemical and patch-clamp methods 9-12 d after sciatic nerve or dorsal root lesion. Levels of SNS and NaN protein in the two types of injuries were paralleled by their respective TTX-resistant currents. There was a significant decrease in SNS and NaN signal intensity in small DRG neurons after peripheral, but not central, axotomy compared with control neurons. Likewise, there was a significant reduction in slowly inactivating and persistent TTX-resistant currents in these neurons after peripheral, but not central, axotomy compared with control neurons. These results indicate that peripheral, but not central, axotomy results in a reduction in expression of functional SNS and NaN channels in c-type DRG neurons and suggest a basis for the altered electrical properties that are observed after peripheral nerve injury.

Effects of substance P on excitability and ionic currents of normal and axotomized rat dorsal root ganglion neurons

Substance P (SP) may act within dorsal root ganglia (DRG) to modulate the transmission of nociceptive information. Because peripheral nerve injury (axotomy) alters the peptide content of sensory neurons, we used whole-cell recording to examine the effects of sciatic nerve section on the sensitivity of rat lumbar DRG neurons to SP (0.3--1 microM). At 1 microM, SP increased the excitability of 'small', putative nociceptive neurons but had little effect on the excitability of 'large' neurons. Two-four weeks after sciatic nerve section, however, the effect of SP on 'large' axotomized neurons was increased and its effect on 'small' neurons was decreased. SP did not affect Ca(2+) channel currents in control or axotomized neurons. The effects of SP on the current-voltage (I--V) relationship of 77% of neurons involved increased inward current at potentials below -30 mV and suppressed outward current at potentials above -20 mV. The effects of SP on the I--V relationship were similar in control and in axotomized neurons and the altered sensitivity of 'small' and 'large' cells could not be attributed to axotomy-induced changes in input resistance or membrane potential. The possible relevance of alterations in sensitivity, of 'large' DRG neurons to SP, to the generation of neuropathic pain is discussed.

Peripheral but not central axotomy promotes axonal outgrowth and induces alterations in neuropeptide synthesis in the nodose ganglion of the rat

We investigated the effects of central and peripheral axotomy of the sensory neurons in the nodose ganglion on neurite outgrowth and neuropeptide expression. Axonal outgrowth was studied in ganglia subjected to a conditioning lesion of the vagus nerve 6 days prior to in vitro explantation. In such cultures, a conditioning effect, i. e. a shorter initial delay and faster axonal outgrowth, was observed after peripheral axotomy, while central axotomy had no effect. Neuropeptide expression was measured by immunocytochemistry 3 days after axotomy. Peripheral axotomy induced an increase in the number of neurons expressing the C-terminal flanking peptide of neuropeptide Y (C-PON), galanin (GAL) and vasoactive intestinal peptide (VIP). In contrast, central axotomy did not affect neuropeptide expression. These results suggest that both axonal outgrowth and expression of neuropeptides in the sensory neurons of the nodose ganglion could be regulated by the contact of the cells with their peripheral, but not their central targets.

Recovery of C-fiber-induced extravasation following peripheral nerve injury in the rat

Peripheral nerve injury leads to substantial alterations in injured sensory neurons. These include cell death, phenotypic modifications, and regeneration. Primary sensory neurons have recently been shown not to die until a time beyond 4 months following a nerve crush or ligation and this loss is, moreover, limited to cells with unmyelinated axons, the C-fibers. The late loss of C-fibers may be due to a lack of target reinnervation during the regenerative phase. In order to investigate this, we have used a particular peripheral function, unique to C-fibers, as a measure of peripheral reinnervation: an increase in capillary permeability on antidromic activation of C-fibers, i.e., neurogenic extravasation. This was investigated in rats that had received a nerve crush injury 1 to 50 weeks earlier. Some recovery of the capacity of C-fibers to generate extravasation was detected at 8-10 weeks, which increased further at 12-14 weeks, and then plateaued at this level with no further recovery at 30 or 50 weeks. In intact and damaged sciatic nerves, A beta-fibers never induced extravasation. These findings are compatible with the hypothesis that those C-fibers which make it back to their peripheral targets do not subsequently die and those that do not, may die.

NK1, NMDA, 5HT1a, and 5HT2 receptor binding sites in the rat lumbar spinal cord: modulation following sciatic nerve crush

Quantitative receptor binding autoradiography was used to study the NK1, NMDA, 5HT1a, and 5HT2 receptor binding densities in the adult rat lumbar spinal cord from 3 days to 20 weeks following a unilateral crush lesion of the sciatic nerve. NK1 binding density increased unilaterally in the superficial dorsal horn on the side of the sciatic crush to reach levels 60% above controls by 4 weeks following the lesion and returned to control values by 12 weeks. NMDA binding density increased bilaterally and equally in both the dorsal and ventral horns to reach 300% of control values at 2 weeks following the crush and returned to near control values by 20 weeks following the lesion. Serotonergic receptor binding did not change. The changes in NK1 receptor binding density on postsynaptic dorsal horn cells are consistent with a response to the decrease and recovery in the synthesis and transport of tachykinins by the dorsal root ganglion cells following peripheral nerve injury. the bilateral changes in NMDA receptor binding are more likely mediated by polysynaptic pathways in the spinal cord that respond to the changes in metabolic events of the dorsal root ganglion cells evoked by axotomy and regeneration.

NT-3 modulates NPY expression in primary sensory neurons following peripheral nerve injury

Peripheral nerve transection induces significant changes in neuropeptide expression and content in injured primary sensory neurons, possibly due to loss of target derived neurotrophic support. This study shows that neurotrophin-3 (NT-3) delivery to the injured nerve influences neuropeptide Y (NPY) expression within dorsal root ganglia (DRG) neurons. NT-3 was delivered by grafting impregnated fibronectin (500 ng/ml; NT group) in the axotomised sciatic nerve. Animals grafted with plain fibronectin mats (FN) or nerve grafts (NG) were used as controls. L4 and L5 DRG from operated and contralateral sides were harvested between 5 and 240 d. Using immunohistochemistry and computerised image analysis the percentage, diameter and optical density of neurons expressing calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal peptide (VIP) and NPY were quantified. Sciatic nerve axotomy resulted in significant reduction in expression of CGRP and SP, and significant upregulation of VIP and NPY (P < 0.05 for ipsilateral vs contralateral DRG). By d 30, exogenous NT-3 and nerve graft attenuated the upregulation of NPY (P < 0.05 for NT and NG vs FN). However, NT-3 administration did not influence the expression of CGRP, SP or VIP. The mean cell diameter of NPY immunoreactive neurons was significantly smaller in the NT-3 group (P < 0.05 for NT vs FN and NG) suggesting a differential influence of NT-3 on larger neurons. The optical densities of NPY immunoreactive neurons of equal size were the same in each group at any time point, indicating that the neurons responding to NT-3 downregulate NPY expression to levels not detectable by immunohistochemistry. These results demonstrate that targeted administration of NT-3 regulates the phenotype of a NPY-immunoreactive neuronal subpopulation in the dorsal root ganglia, a further evidence of the trophic role of neurotrophins on primary sensory neurons.

Somatotopic redistribution of c-fos expressing neurons in the superficial dorsal horn after peripheral nerve injury

The functional somatotopic reorganization of the lumbar spinal cord dorsal horn after nerve injury was studied in the rat by mapping the stimulus-evoked distribution of neurons expressing proto-oncogene c-fos. In three different nerve injury paradigms, the saphenous nerve was electrically stimulated at C-fibre strength at survival times ranging from 40 h to more than six months: 1) Saphenous nerve stimulation from three weeks onwards after ipsilateral sciatic nerve transection resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous territory in laminae I-II, and an expansion of the saphenous territory into the denervated sciatic territory until 14 weeks postinjury. 2) Saphenous nerve stimulation from five days onwards after ipsilateral sciatic nerve section combined with saphenous nerve crush resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous nerve territory, and an expansion of the saphenous nerve territory into the denervated sciatic nerve territory. 3) Stimulation of the crushed nerve (without previous adjacent nerve section) at five days, but not at eight months resulted in a temporary increase in the number of Fos-immunoreactive neurons within the territory of the injured nerve, and no change in area at either survival time. The results indicate that nerve injury results in an increased capacity of afferents in an adjacent uninjured, or regenerating nerve, to excite neurons both in its own and in the territory of the permanently injured nerve in the dorsal horn. The onset and duration of the increased postsynaptic excitability and expansion depends on the types of nerve injuries involved. These findings indicate the complexity of the central changes that follows in nerve injuries that contain a mixture of uninjured, regenerating and permanently destroyed afferents.

Colchicine treatment of the sciatic nerve reduces neurogenic extravasation, but does not affect nociceptive thresholds or collateral sprouting in neuropathic or normal rats

The effect of topical colchicine treatment of the sciatic nerve on sciatic and saphenous nociceptive thresholds and neurogenic extra-vasation was investigated in normal and neuropathic rats. After a pilot investigation using several different concentrations of colchicine it was determined that treating the sciatic nerve with 5 mM colchicine did not usually affect the heat nociceptive threshold over the sciatic innervated plantar surface of the hindpaw. Mechanical nociception and motor function were also unchanged. Electrical stimulation of the sciatic nerve after intravenous injection of Evans blue dye causes extravasation of the dye in the cutaneous distribution of the nerve. The area and quantity of sciatic extravasation were measured 3 weeks after treating the sciatic nerve with colchicine. This treatment results in a marked loss of neurogenic extravasation, but there were no changes in the sciatic and saphenous mediated heat and mechanical nociceptive thresholds. The area of saphenous nociceptive innervation was mapped using pinch responses and saphenous neurogenic extravasation acutely after sciatic section. There was no change in the cutaneous distribution of saphenous nociceptive fibers when measured 3 weeks after the sciatic colchicine treatment. Some rats had their sciatic nerves transected immediately after colchicine treatment (5 and 50 mM) and the saphenous nociceptive thresholds and autotomy scores were followed postoperatively. Colchicine pretreatment of the sciatic nerve has no effect on the development of hyperalgesia or autotomy. Colchicine blocks axonal transport in peripheral nerve, including the orthograde transport of tachykinins, which probably explains its ability to induce prolonged reductions in sciatic neurogenic extravasation at concentrations that spare C-fiber nociceptor function. Sciatic nerve colchicine treatment does not trigger nociceptive fiber collateral sprouting from the adjacent saphenous nerve, nor does it influence the development of hyperalgesia and autotomy behavior after sciatic transection.

Functional reinnervation of the vasculature of the adult cat paw pad by axons originally innervating vessels in hairy skin

Sympathetic vasoconstrictor neurons that had previously innervated blood vessels in hairy skin were made to reinnervate the vasculature of the hairless skin of the paw pad by suturing the central stump of the cut sural nerve to the distal stump of the cut tibial nerve. After allowing sufficient time for the reinnervation, electrical stimulation of the vasoconstrictor pathway in the lumbar sympathetic trunk produced a reduction of the blood flow that was significantly greater than in control animals. There was also a clear sign of a "denervation supersensitivity" of the blood vessels as evidenced by a significantly increased vasoconstriction that followed the systemic application of the alpha 1-adrenoceptor specific agonist phenylephrine. Neurogenic vasodilation evoked by antidromic excitation of small diameter primary afferent neurones was significantly impaired although myelinated and unmyelinated primary afferents had re-grown into the target tissue. Electrical stimulation of the intact tibial nerve (containing sympathetic vasoconstrictor axons and nociceptive primary afferent fibres) in control animals, always produced vasodilatation indicating that the neurogenic vasodilatation can override the sympathetic vasoconstrictor response. By contrast, electrical stimulation of cross-unioned nerves consistently produced a robust vasoconstriction. We conclude that sympathetic vasoconstrictor neurons have a high capacity to functionally reinnervate autonomic effector organs in the adult cat. Despite this functional recovery, the blood vessels exhibited stronger than normal responses to an alpha 1-adrenoceptor agonist. The impaired neurogenic vasodilatation mediated by small diameter afferents may be due to their poor ability to re-establish their efferent vasodilatory function. Alternatively it may be masked by the strong vasoconstriction.

Ultrastructural studies on peptides in the dorsal horn of the rat spinal cord--III. Effects of peripheral axotomy with special reference to galanin

In this study co-localization of galanin- with calcitonin gene-related peptide (CGRP)-like immunoreactivity was examined in dorsal root ganglion neurons 14 days after sciatic nerve cut using a laser scanning confocal microscope. CGRP- and galanin-like immunoreactivities were also analysed in the dorsal horn of the spinal cord of these animals with immunofluorescence microscopy. The ultrastructural changes in galanin-immunoreactive, presumably primary afferent terminals in the superficial dorsal horn, were studied as well as the relationship between galanin-, substance P- and CGRP-like immunoreactivities in primary afferent terminals. Local galanin-positive neurons in lamina II were also analysed after peripheral axotomy. Under the confocal microscope, CGRP-like immunoreactivity was located in the perinuclear region, probably the Golgi complex, and in dot-like structures, probably representing large dense-core vesicles, in normal dorsal root ganglion neurons. However, after peripheral axotomy CGRP was mainly detected in dot-like structures. Only a slight decrease in percentage of CGRP neurons in dorsal root ganglion was seen after axotomy, and about 84% of the galanin-positive neurons contained CGRP. The field of galanin-positive nerve fibres in the superficial lumbar (L)4 and L5 dorsal horn expanded and the intensity of staining for CGRP was reduced in these regions 14 days after sciatic nerve cut. Using pre-embedding immunoelectron microscopy, several morphological changes were observed in galanin-positive terminals in laminae I and II ipsilateral to the lesion. Most importantly, the most frequently occurring type of galanin-positive terminals (type 1) showed distinct changes with a granular matrix, many immunoreactive, peripherally located large dense-core vesicles, empty large vesicles and synaptic vesicles which were displaced from the presynaptic zone. Other galanin-positive terminals underwent even more pronounced morphological changes, including extensive vesiculolysis, also of large dense-core vesicles, filamentous degeneration or formation of axonal labyrinths. An increased number of galanin-positive nerve terminals was observed in lamina III of the ipsilateral dorsal horn after axotomy. They did not form glomeruli and contained few large dense-core vesicles. Post-embedding immunocytochemistry combined with quantitative analysis revealed that significant changes occurred in a proportion of terminals also with regard to peptide content in large dense-core vesicles after axotomy. Thus, the percentage of galanin-positive large dense-core vesicles increased in several cases and that of substance P- and CGRP-immunoreactive ones decreased.(ABSTRACT TRUNCATED AT 400 WORDS)

Most dorsal root ganglion neurons of the adult rat survive nerve crush injury

Severe crush of the rat sciatic nerve does not result in any significant cell death among motor neurons (Swett et al., 1991a). The present study reports on the survival of the dorsal root ganglion (DRG) neurons in the same experiments. From 15 to 187 days after crush of the left sciatic nerve, the common peroneal or sural nerve was cut and labeled distal to the injury with a mixture of horseradish peroxidase (HRP) and its wheatgerm agglutinin conjugate (WGA:HRP). In other cases, the crush injury was made far enough distally on a peroneal or sural branch to permit labeling several millimeters proximal to the injury. The procedures for reconstructing the regenerated DRG neuron populations were identical to those used in an earlier study describing the normal sciatic DRG neuron populations in the rat (Swett et al., 1991b). The normal peroneal nerve contains 2699 +/- 557 DRG neurons. When the peroneal nerve was crushed near its point of origin from the sciatic and labeled 10 mm distal to the injury, 2186 +/- 163 DRG neurons were counted, suggesting a decrease of about 19% (p < 0.01). However, when the entire sciatic nerve was crushed, distal labeling of the peroneal nerve revealed a mean number of 2578 +/- 291 DRG neurons, an insignificant reduction (p > 0.2). When the peroneal nerve was labeled proximal to a peroneal crush site, a similar number of DRG neurons (2563 +/- 412) was counted. Results following sural nerve crush were similar. The sural nerve normally contains 1675 +/- 316 DRG neurons. When the nerve was labeled distal to the injury, 1558 +/- 64 DRG neurons were counted--a number almost identical to that found (1529 +/- 240) when this nerve was labeled proximal to the injury. The results demonstrate that within 6 months of severe crush injury of the rat sciatic nerve, the vast majority of DRG neurons survive and regenerate new axons distally beyond the injury site, presumably to reinnervate their original targets.

Invasion of the rat ventral root L5 by putative sympathetic C-fibers after neonatal sciatic nerve crush

The present study examines the occurrence of C-fibers in lumbar ventral roots after sciatic nerve crush in neonatal and adult rats. Electron microscopic analysis showed that the number of C-fibers in the ventral root L5 increased significantly on the lesion side after neonatal but not adult sciatic nerve crush and that the number of C-fibers was higher in the ventral root L5 on the unoperated side compared to this root in normal control rats. In order to determine whether the new C-fibers in the L5 root on the lesion side are sensory or sympathetic we made immunohistochemical studies on roots from neonatally crushed rats. We found that there was no obvious lesion side/contralateral side or operated rat/control rat difference with respect to the occurrence and general configuration of axons with substance P-, calcitonin gene-related peptide- or vasoactive intestinal polypeptide-like immunoreactivity. However, the occurrence of axons with tyrosine hydroxylase-like immunoreactivity appeared clearly higher in the ventral root L5 on the lesion side compared to the unoperated side in neonatally crushed rats. Moreover, these axons seemed to be more numerous also in the ventral root L5 on the unoperated side compared to normal control rats. No lesion side/contralateral side or operated rat/control rat differences were seen in the ventral root L4. We propose that the ventral root L5 is invaded by putative sympathetic C-fibers after sciatic nerve crush lesions in newborn rats.

Neurite outgrowth and GAP-43 mRNA expression in cultured adult rat dorsal root ganglion neurons: effects of NGF or prior peripheral axotomy

Adult dorsal root ganglion (DRG) cells are capable of neurite outgrowth in vivo and in vitro after axotomy. We have investigated, in cultured adult rat DRG cells, the relative influence of nerve growth factor (NGF) or a prior peripheral nerve lesion on the capacity of these neurons to produce neurites. Since there is evidence suggesting that the growth-associated protein GAP-43 may play a crucial role in axon elongation during development and regeneration, we have also compared the effect of these treatments on GAP-43 mRNA expression. NGF increased the early neurite outgrowth in a subpopulation of DRG cells. This effect was substantially less, however, than that resulting from preaxotomy, which initiated an early and profuse neurite outgrowth in almost all cells. No difference in the expression of GAP-43 mRNA was found between neurons grown in the presence or absence of NGF over 1 week of culture, in spite of the increased growth produced by NGF. In contrast, cultures of neurons that had been preaxotomized showed substantial increases in GAP-43 mRNA and NGF had, as expected, a significant effect on substance P mRNA levels. Two forms of growth may be present in adult DRG neurons: an NGF-independent, peripheral nerve injury-provoked growth associated with substantial GAP-43 upregulation, and an NGF-dependent growth that may underlie branching or sprouting of NGF-sensitive neurons, but which is not associated with increased levels of GAP-43 mRNA.

The resolution of neuropathic hyperalgesia following motor and sensory functional recovery in sciatic axonotmetic mononeuropathies

Nerve lesions producing extensive axonal loss can induce painful hyperalgesic states in man. The affect of axonal regeneration and end-organ reinnervation on hyperalgesia and pain is controversial. This study used two axonotmetic models, the sciatic crush injury (CI) and the sciatic chronic constrictive injury (CCI), to investigate the affects of nerve regeneration and reinnervation on hyperalgesia and presumed painful behavior in rats. The sciatic CI resulted in a transient loss of both sciatic motor function and the withdrawal response to pinch and heat in the sciatic distribution. Extensive recovery of motor function, pinch and heat response occurred over days 23-38 post-crush injury. This temporally corresponded with a plateau in the hindpaw autotomy score and a resolution of the saphenous-mediated pressure and heat hyperalgesia (adjacent neuropathic hyperalgesia; ANH) which developed over the medial dorsum of the hindpaw following the sciatic CI. In contrast, with sciatic transection and distal stump excision, no motor recovery occurs, large areas of the hindpaw remain unresponsive to heat and pinch, and the saphenous mediated ANH fails to resolve over a period of 3 months. When sciatic CI was compared to contralateral sciatic transection within the same rat, the bilateral saphenous-mediated pressure and heat thresholds were initially identical, but by 23-27 days post-crush, the crush side thresholds became hypoalgesic relative to the section side. This demonstrates an attenuation of the crush-induced ANH which temporally corresponds to the recovery of motor and sensory function. When the sciatic nerve was proximally crushed and distally transected (3 cm below the crush site), the saphenous-mediated pressure and heat threshold changes were identical (over 6 weeks of serial testing) to those produced by a contralateral sciatic transection within the same rat. This indicates that the microenvironments surrounding the regenerating axon tips did not differentially affect the development of ANH following sciatic CI or transection. The sciatic CCI resulted in a transient loss of hindpaw motor function without the loss of pinch or heat withdrawal responses in the sciatic distribution. Motor function recovery occurred primarily over days 23-59 post-ligature. During this prolonged period of motor function recovery there was a resolution of the sciatic-mediated plantar surface heat hyperalgesia and the saphenous-mediated heat ANH. The above data support the hypothesis that the successful regeneration of distal axons after axonotmetic lesions can initiate the resolution of neuropathic hyperalgesia.

Impaired induction of vasoactive intestinal polypeptide after sciatic nerve injury in the streptozotocin-diabetic rat

This study was designed to assess the effects of experimental diabetes and nerve crush injury upon vasoactive intestinal polypeptide (VIP) content and axonal transport in the dorsal root ganglia (DRG) and sciatic nerve. Sciatic nerve crush injury in control and 3-week streptozotocin-diabetic rats was followed 6.5 days later by placement of 2 constricting ligatures above the site of injury. After 12 h, the L4 and L5 DRG and sciatic nerve were removed for VIP radioimmunoassay. Similar samples were also taken from control and diabetic rats whose nerve had been ligated without a preceding crush. VIP was increased over 2-fold in ganglia and 4-fold in nerves of crush-injured controls compared to uninjured controls (both P < 0.01). Crush injury also increased ganglion and nerve VIP in diabetic rats (P < 0.05 and 0.01, respectively) but the increase was less than what occurred in crush-injured controls (both P < 0.05). The accumulation of VIP proximal to a sciatic ligature was similar in control and diabetic rats and was not altered in either group by crush injury, while retrograde transport of VIP was initiated by crush injury in both control and diabetic rats. These data show that short-term diabetes does not alter the amount and peripheral axonal transport of VIP in the sciatic nerve but impairs the ability of peripheral nerve to respond to injury.

The extraction and assay of substance P in the rat sciatic nerve

Published data for substance P (SP) content of sciatic nerves in control rats vary greatly. This study sought possible reasons for this variability by examining the influence of homogenisation procedures, freezing and selection of left/right nerve or proximal/distal segments. Substance P-like immunoreactivity (SP-LI) content (pg/mg nerve protein +/- SD) was significantly greater in sciatic nerves which had been homogenised using motor-powered equipment (615.4 +/- 146.3) as opposed to a hand-held pestle (445.4 +/- 111.8). Our second investigation revealed that freshly homogenised nerve tissue yielded greater SP-LI (508.8 +/- 88.7) than either tissue snap frozen in liquid nitrogen (307.6 +/- 77.9), snap-frozen and stored at -70 degrees C for 7 days (331.8 +/- 53.5), or tissue allowed to remain in the cadaver for 1 h and subsequently dissected and homogenised immediately (412.6 +/- 105.8). These data also show that storage at -70 degrees C imposes no further losses on those caused by freezing and extraction of frozen tissue. Two further studies indicated no variation between left and right sciatic nerves nor any proximal-distal gradients. Hence, this study illuminated the need for samples to be homogenised using motor-powered equipment immediately upon dissection, followed by SP extraction, for complete avoidance of losses of SP-LI.

Time course and characteristics of the capacity of sensory nerves to reinnervate skin territories outside their normal innervation zones

Factors involved in the outcome of regeneration of the saphenous nerve after a cut or crush lesion were studied in adult rats with electrophysiological recordings of low-threshold mechanoreceptor activity and plasma extravasation of Evans blue after electrical nerve stimulation that activated C fibers. In the first series of experiments, saphenous and sciatic nerve section was combined with anastomosis of the transected proximal end of the saphenous nerve to the distal end of the cut tibial nerve. Regeneration of saphenous nerve fibers involved in plasma extravasation and low-threshold mechanoreceptor activity in the glabrous skin was observed 13 weeks after nerve anastomosis. Substance P-, calcitonin gene-related peptide-, and protein gene product 9.5 (PGP-9.5)-immunoreactive (IR) thin epidermal and dermal nerve endings, as well as coarse dermal PGP-9.5-IR nerve fibers and Meissner corpuscles and Merkel cell-neurite-like complexes, were observed in the reinnervated glabrous skin at this time. In a second series of experiments, the time course of the regeneration of saphenous nerve axons to the permanently sciatic-nerve-denervated foot sole was examined. Saphenous-nerve-induced plasma extravasation and low-threshold mechanoreceptor activity in the saphenous nerve were found in the normal saphenous nerve territory 2, 3, 4, and 6 weeks after sciatic nerve cut combined with saphenous nerve crush in the left hindlimb. Saphenous-nerve-induced plasma extravasation was also present in the glabrous skin normally innervated by the sciatic nerve 3, 4, and 6 weeks after the sciatic cut/saphenous crush lesion. However, no low-threshold mechanoreceptor activity was detected in the saphenous nerve when the glabrous skin area was stimulated. In a third series of experiments, the fate of the expansion of the saphenous nerve territory after saphenous nerve crush was examined when the crushed sciatic nerve had been allowed to regenerate. Nerve fibers involved in plasma extravasation were observed in the glabrous skin of the hindpaw after saphenous nerve, as well as after tibial nerve, C-fiber stimulation 3, 12, and 43 weeks after the saphenous crush/sciatic crush lesion. Low-threshold mechanoreceptors from the regenerated saphenous nerve, which primarily innervates hairy skin, seem to be functional in the glabrous skin if the axons are guided by the transected tibial nerve by anastomosis. Furthermore, the results indicate that fibers from the regenerating saphenous nerve that have extended into denervated glabrous skin areas can exist even if sciatic nerve axons are allowed to grow back to their original territory.

Anterograde horseradish peroxidase tracing and immunohistochemistry of trigeminal ganglion tooth pulp neurons after dental nerve lesions in the rat

The peripheral reorganization of pulpal nerves after tooth injury was studied, in the rat, with anterograde horseradish peroxidase tracing techniques, and combined retrograde Fluorogold tracing and immunohistochemistry was employed to examine the effects of inferior alveolar nerve lesions or tooth injury on some cytochemical characteristics of pulpal trigeminal ganglion nerve cells, namely content of substance P, calcitonin gene-related peptide and the ganglioside GM1 (binding subunit of cholera toxin), as well as affinity to RT 97 (antibody to neurofilament protein) and the lectin Griffonia simplicifolia isolectin I-B4. Anterograde horseradish peroxidase tracing demonstrated that pulpal nerves either disappear or reinnervate novel targets after loss of pulpal tissue. There were no obvious signs of neuroma formation. Retrograde Fluorogold labelling with immunohistochemistry showed that after inferior alveolar nerve lesions with subsequent regeneration, a much higher proportion of Fluorogold cells (15%) were substance P-positive compared to normal (2%). In addition, 3% of the cells were Griffonia simplicifolia isolectin I-B4-positive. Such cells were very rare in controls. Proportions of calcitonin gene-related peptide-, GM1- and RT-97-positive cells were normal. After tooth lesions, the proportions of Fluorogold-positive substance P-, Griffonia simplicifolia isolectin I-B4-, GM1- and RT 97-labelled cells were similar to controls, while the proportion of calcitonin gene-related peptide-positive neurons was reduced. The results show that pulpal deafferentation may change the long-term cytochemical characteristics of affected trigeminal ganglion neurons.

End structure and neuropeptide immunoreactivity of axons in sciatic neuromas following nerve section in neonatal rats

The formation of neuromas after neonatal nerve injury was studied in rats. In neonatal pups, the sciatic nerve was cut and tightly ligated, and a portion of the distal stump was removed. After 6-10 weeks, a nerve-end neuroma had formed in about 70% of the animals. In the remaining animals the nerve had grown on the side of the ligature. The end structure of the neuroma axons was studied using anterogradely transported WGA-HRP injected into the L5 dorsal root ganglion. HRP labeling occurred in the entire proximal sciatic nerve. In the neuroma, labeled fibers branched profusely and either terminated with minor end swellings or turned in the retrograde direction. Immunohistochemistry showed that the fibers which projected into the neuroma presented a moderate immunoreactivity to substance P and neuropeptide Y and a strong reactivity to calcitonin gene-related peptide. The results show that many sensory and sympathetic sciatic nerve fibers survive chronic axotomy in the newborn and contribute to the formation of nerve-end neuromas. There are, however, important structural differences between adult and neonatally induced neuromas.

Rat dorsal root ganglion neurons in culture express vasoactive intestinal polypeptide (VIP) independently of nerve growth factor

Expression of vasoactive intestinal polypeptide (VIP) was studied in regenerating adult rat dorsal root ganglion (DRG) neurons in culture. VIP was not detected by radioimmunoassay in newly established cultures but increasing concentrations were observed between 2 and 4 days and VIP remained detectable for more than 2 weeks. Immunocytochemical staining revealed VIP-like immunoreactivity in virtually all neurons. Thus, VIP is expressed in regenerating DRG neurons in culture just as it is in vivo following axotomy. However, VIP concentrations in culture were the same whether nerve growth factor was present or not. Therefore, deprivation of retrogradely transported nerve growth factor is unlikely to explain VIP induction by axotomy in vivo.

Expression of beta-preprotachykinin mRNA and tachykinins in rat dorsal root ganglion cells following peripheral or central axotomy

The changes in gene expression and protein synthesis induced in neurons by axotomy usually lead to increased production of axon constituents and decreased production of molecules related to neurotransmission. Exceptions to this generalization occur, however, and it is unclear whether the injury itself changes the pattern of synthesis or whether individual mechanisms regulate the synthesis of the various axonal components. We used in situ hybridization histochemistry and immunocytochemistry to compare the changes in L4 and L5 rat dorsal root ganglion neuron levels of preprotachykinin mRNA and tachykinin peptides caused by sciatic nerve injury with those caused by dorsal root injury. Both lesions elicit regeneration, although only the axotomized peripheral processes re-establish functional contact with their targets. In the contralateral, intact dorsal root ganglia approximately 17% of neurons contained detectable levels of both mRNAs and peptides. Sciatic nerve section decreased by 70% the number of neurons labeled for preprotachykinin mRNA at three days post-operatively. Not all cells in the ganglion are axotomized by the sciatic nerve lesion; grain counts over the cells spared by the lesion showed an increased level of labeling, possibly a result of collateral sprouting by these spared cells. By two weeks, the number of cells labeled for preprotachykinin mRNA had decreased to 80% of control levels. The numbers of neurons labeled for tachykinin peptides decreased more slowly and reached approximately 50% of control numbers at two weeks. By six months post-operatively, when regeneration is largely complete, the number of neurons containing both mRNAs and peptides returned to normal. In contrast, dorsal root section did not elicit a decrease in the number of neurons labeled either for the mRNAs or the peptides at any of the post-operative intervals examined. These results indicate that axotomy is not the stimulus that elicits changes in the expression of genes coding for tachykinins. Evidence is considered indicating that interruption of the supply of peripherally derived nerve growth factor may be responsible for the changes in gene expression for tachykinins after axotomy.

Changes in lectin binding of lumbar dorsal root ganglia neurons and peripheral axons after sciatic and spinal nerve injury in the rat

The effects of chronic lesions of rat lumbar spinal or sciatic nerves on the binding of Glycine max (soybean) agglutinin to galacto-conjugates, in small- and medium-size primary sensory neurons of the L4 and L5 dorsal root ganglia, were examined over a 580-day period. Spinal nerve section resulted in a marked decrease in the population of stained neurons within 7 days. However, despite some retrograde morphological changes triggered by axonal injury, the proportion of stained nerve cells was normalized 180 days postoperatively. This temporary decrease in perikaryal lectin reactivity was initially associated with a marked accumulation of stained material in the nerve, proximal and distal to the site of section, with similar accumulations also being noticeable at each level of injury in sciatic nerves subjected to double ligature. This may reflect the presence of glycocompounds linked to the autolysis of nerve fibers during the phase of retrograde dying-back and Wallerian degeneration. At later stages, stained deposits could be seen scattered along central and peripheral axonal processes of the dorsal root ganglion neurons in the vicinity of the cell body. They may indicate a disturbance in the peripheral turnover of glycoproteins in chronically-transected nerves, with piling up of neuronal products. Sciatic nerve injury caused similar but less severe effects which, except for the L4 ganglion cells, were rapidly reversible.

Death of some dorsal root ganglion neurons and plasticity of others following sciatic nerve section in adult and neonatal rats

Newborn animals recover from neurological injury to a greater extent than adults in spite of the greater vulnerability of developing neurons to retrograde or transneuronal degeneration (Kennard, '42; Goldman, '74; Prendergast and Stelzner, '76; Bregman and Goldberger, '82, '83). The cellular mechanisms underlying this "infant lesion effect" are incompletely understood (Bregman and Goldberger, '82). The dorsal root ganglion (DRG) is an excellent model in which to compare the developing and adult nervous system with respect to the effects of axotomy on cell survival and cellular function. We studied the survival of L5 DRG neurons after section-ligation of the sciatic nerve of adult and neonatal rats and used qualitative and quantitative immunocytochemical methods to examine changes in intraspinal substance P immunoreactivity (SPIR). Retrograde transport of wheatgerm agglutinin-horseradish (WGA-HRP) peroxidase applied to the sciatic nerve of adult or neonatal rats demonstrated that 70% of the neurons in the normal L5 DRG project into the sciatic nerve at the site of transection. In adults 20% of all L5 DRG neurons died between 10 and 60 days postoperative; in newborns 50% of the neurons died between 5 and 10 days. These results indicate that 30% of axotomized neurons in adults and 75% in neonates die after sciatic nerve section and that neuron loss is both more rapid and more extensive in neonates. No cell death was observed in the L5 DRG of neonates after dorsal rhizotomy, thus suggesting that at this stage of development the survival of DRG neurons depends on the peripheral but not the central process. SPIR in laminae I and II of both adult and newborn operates decreased and then recovered, but the time course and extent of the recovery differ. In adults SPIR was depleted in the medial portion of the L5 segment ipsilateral to surgery by 10 days postoperative and remained depleted for at least 2 months. By 1 year partial recovery occurred, but remained incomplete even at the longest survival time studied (15 months). SPIR, which is present in the dorsal horn at birth, was diminished in ipsilateral laminae I and II by 4 days after nerve section on the day of birth. Between 30 days and 60 days, the density of SPIR in the dorsal horn ipsilateral to surgery became virtually indistinguishable from that on the contralateral, intact side, suggesting a more rapid and complete recovery than in adults.(ABSTRACT TRUNCATED AT 400 WORDS)

Substance P-, CGRP- and NPY-immunoreactive nerve fibers in rat sciatic nerve-end neuromas

Substance P (SP)-, calcitonin gene-related peptide (CGRP)- and neuropeptide Y (NPY)-immunoreactive nerve fibers were examined in experimental sciatic nerve-end neuromas in the rat with immunohistochemical techniques. At 1-3 days after nerve ligation and section of the sciatic nerve there was an accumulation of SP-like immunoreactivity (SP-LI). Six days after the lesion there was, however, a marked reduction and the neuromas remained virtually depleted from SP-LI at survival times between 8 days and 3 months. CGRP-LI was strong at 1-5 days post-operatively. By 8 days, CGRP-LI was reduced, but a large number of axons were still immunoreactive, and remained immunolabelled up to 3 months. CGRP-LI nerve fibers formed endbulbs, and appeared to grow in both anterograde and retrograde directions. Fine fibers sprouts were first observed at 8 days, but preterminal branching in neuromas aged less than a month was uncommon. At early stages (1-3 days) after ligation, there was a marked accumulation of NPY-LI proximal to the nerve constriction. NPY-LI was reduced from 5 days and on, but many fibers remained NPY-positive and their growth pattern through proximal and distal neuroma segments could be determined. The present results thus may indicate a differential effect of nerve injury on neuropeptide expression: immunohistochemically detectable SP-LI rapidly disappears from sciatic nerve fibers trapped in nerve-end neuromas, but CGRP-LI and NPY-LI remain and are useful as neuroanatomical markers for two subclasses of sprouting axons. Furthermore, the findings suggest that both CGRP and NPY, but not SP, could be involved in ectopic electrical activity in experimental neuromas.

Reciprocal regulation of tachykinin- and vasoactive intestinal peptide-gene expression in rat sensory neurones following cut and crush injury

The relative abundance of preprotachykinin- (PPT), actin- and vasoactive intestinal peptide- (VIP) mRNA's was measured in L5 dorsal root ganglia of rats after resecting or crushing the sciatic nerve. PPT-mRNA levels fell to 40% of control values 3, 6 and 9 days following nerve resection. Crushing produced a lesser fall at 3 and 6 days with a partial recovery at 9 days. Following resection actin-mRNA levels transiently rose to twice control values and had returned to normal by day 9. VIP-mRNA was not detectable in control ganglia but increasing amounts of VIP-mRNA were present 3, 6 and 9 days after nerve injury. The results are discussed in terms of the control mechanisms operating.

Dependence of GAP43 (B50, F1) transport on axonal regeneration in rat dorsal root ganglion neurons

The axonal transport of protein GAP43 (B50, F1) has been studied in rat sciatic nerve after application of L-[35S]methionine to the lumbar dorsal root ganglia. Within 24 h of a crush axotomy of the sciatic nerve a significant increase in labelling of transported GAP43 was detected. After 14 days, levels were approximately 60 times greater than in normal nerve, and thereafter declined, returning to the normal range by 114 days. In contrast, after a resection, which impeded regeneration, transport of labelled GAP43 remained elevated for at least 114 days. I conclude that GAP43 synthesis is regulated by a suppressive factor derived from the periphery which is depleted by axotomy and restored by regeneration. The time-course of down-regulation of GAP43 synthesis during regeneration is consistent with the target tissues being the source of the factor.

Differential effects of distal and proximal nerve lesions on carbonic anhydrase activity in rat primary sensory neurons, ventral and dorsal root axons

The effect of proximal and distal peripheral nerve injuries on the histochemistry of carbonic anhydrase (CA) in rat dorsal root ganglion (DRG) neurons, and myelinated (MyF) dorsal and ventral root fibers was studied. Sciatic neurectomy induced no change. Contrariwise, 7 days after lumbar spinal nerve section the numbers of CA-stained ventral root MyF and DRG cells at the L4 and L5 levels decreased to 73.2% and 51.9% of their original values respectively, although the numbers returned to normal by the 90th postoperative day. Dorsal root MyF followed a similar trend, albeit with some delay. Major morphological changes comprised atrophy of dorsal root sensory neurons and axons, particularly in long term experiments, as well as nuclear eccentricity in DRG neurons. These results suggest that, depending on the site of lesion, the rat peripheral nervous system (PNS) either maintains or quickly restores its capacity to synthesize CA. They stand in contrast to the long-lasting metabolic dysfunctions reported to occur when primary neurons are disconnected from the periphery. It is uncertain whether this difference is due to the critical role of CA in neuronal metabolism.

Substance P content of the skin, neurogenic inflammation and numbers of C-fibres following capsaicin application to a cutaneous nerve in the rabbit

The long-term effects of applying capsaicin briefly to a cutaneous nerve in the rabbit have been assessed 10 days after treatment. No changes in C-fibre numbers were seen in treated saphenous nerves and the average ratio of C-fibres to A-fibres was close to 7 in both control and treated nerves. However, the substance P content of the skin innervated by the saphenous nerve fell by 46% after capsaicin treatment compared with a fall of 65% after nerve section. Plasma extravasation in response to 7.5% mustard oil applied to the skin was also reduced following capsaicin treatment (by 43%) and following denervation (by 47%), although the response to 25% mustard oil was unaffected by previous capsaicin treatment. Thus, as in other species that have been examined (notably the rat), brief capsaicin treatment of rabbit skin nerves leads to a reduction in a neurogenic inflammatory response and in substance P content of the skin. However, unlike the rat, there is no degeneration of C-fibres. In the rabbit it is therefore possible to separate the neurotoxic, degenerative action of capsaicin from its ability to deplete substance P. The question of whether a similar dissociation between neurotoxic and other actions could be achieved in the rat by using lower capsaicin concentrations remains to be answered.

Effect of cutting or crushing the rat sciatic nerve on synthesis of substance P by isolated L5 dorsal root ganglia

Following cutting or crushing the rat sciatic nerve, synthesis of substance P by L5 dorsal root ganglia in vitro was reduced respectively to 20 and 40 per cent of control values. By day 64 the ability to synthesise substance P had been fully restored in crushed neurones but in cut neurones remained at a low level. We conclude that substance P synthesis is a more sensitive index of the effect of nerve injury than is substance P content and further that regenerating axons are able to support substance P synthesis before they reach their target tissue.

Effects of neonatal 6-hydroxydopamine administration on different substance P-containing sensory neurones

Rats were treated neonatally with 6-hydroxydopamine. This resulted in a greater than 93% depletion of noradrenaline content of all tissues examined in adult animals. There was a significant increase in the substance P content of semilunar ganglion, L5 dorsal root ganglion and of sciatic nerve. There was a corresponding increase in substance P synthesis as measured by axonal transport in the sciatic nerve. The noradrenaline content and ratio of noradrenaline: substance P in the iris of control animals were 2102 pg/mg and 24 respectively and, confirming the results of other workers, 6-hydroxydopamine treatment caused a doubling of this substance P content. Terminal fields in skin of face and paw had noradrenaline contents of 75 and 41 pg/mg respectively and noradrenaline: substance P ratios of 4.0 and 2.2 respectively and in these areas 6-hydroxydopamine treatment caused a much lesser increase in substance P content. We conclude that the response of sensory neurones to 6-hydroxydopamine is governed by the relative density of the sympathetic and sensory innervations in their terminal areas, possibly reflecting intensity of competition for nerve growth factor.