Ingrowth of sensory axons into an end-to-side coapted nerve stump after donor nerve crush in the rat.

BACKGROUND: Target innervation through an end-to-side (ETS) nerve coaptation depends on axonal sprouting from the donor nerve. Terminal axonal sprouting in a partially denervated target tissue is more extensive from a crushed donor nerve than from an intact donor nerve. We hypothesized that axonal sprouting into an ETS coapted recipient nerve could be stimulated by crushing the donor nerve. METHOD: Twenty-seven rats were randomised into 3 groups. In all, the distal stump of the transected peroneal nerve was sutured to the side of the sural nerve in place of the epineural window. The control group received no additional treatment. In the experimental groups, the sural donor nerve was crushed either 8 mm proximal (proximal crush group) or 8 mm distal to the coaptation site (distal crush group). Sixteen weeks after the surgery, histomorphometric analysis of the recipient peroneal nerve stump 4 mm distal to the coaptation site was performed. FINDINGS: The number of myelinated axons in the recipient peroneal nerve stump was 758 +/- 247 in the control group, 503 +/- 246 in the distal crush group and 211 +/- 96 in the proximal crush group. The differences between the groups were statistically significant (p < 0.05). The majority of myelinated axons were thin myelinated axons and the frequency distribution of their cross-sectional areas was similar in all groups. CONCLUSION: Contrary to our expectations, a significantly lower number of myelinated axons were present in recipient nerves in the proximal and distal crush groups than in the control group. This suggests that sensory axon ingrowth into an ETS coapted nerve cannot be enhanced by crushing the donor nerve.

Which myelinated sensory axons sprout into an end-to-side coapted peripheral nerve in the rat?

BACKGROUND: The high-threshold sensory afferents, which express trkA, are predominantly involved in terminal collateral sprouting in the skin of adult mammals. We explored which sensory axons are capable of sprouting into the end-to-side coapted nerve in the rat. METHOD: The distal stump of the transected peroneal nerve was sutured to the side of the uninjured sural nerve. After 36 weeks, sprouting of sensory axons into the end-to-side coapted nerve was assessed by the electrophysiologic measurements of compound action potential and by counting the myelinated axons. The neurons in the dorsal root ganglia (DRG) L5 whose axons sprouted into the end-to-side coapted nerve were retrogradely labelled by the fluorescent dye Fluorogold. The expression of trkA in sprouting DRG neurons was investigated by immunohistochemistry. FINDINGS: Predominantly thin myelinated axons were found in the end-to-side coapted peroneal nerve. Their mean conduction velocity (CV) was between the average CVs of the Adelta and Abeta fibres in the normal sural nerves. About 90% of the sprouting DRG neurons were small and medium sized, and about 10% were large. About 85% of sprouting DRG neurons was immunoreactive to trkA, but the rest were not. CONCLUSIONS: Mostly the high-threshold sensory afferents sprouted into the end-to-side coapted nerve, which resembles the collateral sprouting of sensory axons in the skin. However, our results suggest that also some low-threshold mechanoreceptors can sprout after the end-to-side nerve repair.

Articaine is more effective than lidocaine or mepivacaine in rat sensory nerve conduction block in vitro.

The reasons for the relatively high failure rate after inferior alveolar nerve block in dentistry are not fully understood. Therefore, the effectiveness of different anesthetic solutions (2% and 4% lidocaine, 3% mepivacine, 2% and 4% articaine) in depressing the compound action potential amplitude of the sensory fibers in the rat sural nerve was examined under strictly controlled conditions in vitro. After application of an anesthetic solution and stimulation of the nerve with a supramaximal electrical stimulus, a complete disappearance of the compound action potential of the C fibers, but not of the A fibers, was observed in all the experimental groups. Both 2% and 4% articaine more effectively depressed the compound action potential of the A fibers than did other anesthetic solutions. These results are discussed in the light of recent clinical reports finding no differences in the effectiveness between 4% articaine and 2% lidocaine regarding the inferior alveolar nerve block.

Sex-related difference in collateral sprouting of nociceptive axons after peripheral nerve injury in the rat.

Possible sex-related differences in the extent of collateral sprouting of noninjured nociceptive axons after peripheral nerve injury were examined. In the first experiment, peroneal, tibial, and saphenous nerves were transected and ligated in female and male rats. Eight weeks after nerve injury, skin pinch tests revealed that the nociceptive area of the noninjured sural nerve in the instep skin expanded faster in females; the final result was a 30% larger increase in females than in males. In the second experiment, the end-to-side nerve anastomosis was used as a model for axon sprouting. In addition to the previous procedure, the end of an excised peroneal nerve segment was sutured to the side of the intact sural nerve. Eight weeks later, collateral sprouting of nociceptive axons into the anastomosed peroneal nerve segment was assessed by the nerve pinch test and axon counting. There was no significant difference with respect to the percentages of male and female rats with a positive nerve pinch test. The number of myelinated axons in the anastomosed nerve segment was significantly larger in female (456 +/- 217) than in male (202 +/- 150) rats, but the numbers of unmyelinated axons were not significantly different. In normal sural nerves, the numbers of either all myelinated axons or thin myelinated axons did not significantly differ between the two sexes. Therefore, the more extensive collateral axon sprouting observed in female than in male rats is probably due to the higher sprouting capacity of thin myelinated sensory axons in females.

Effect of thyroid hormones on acetylcholinesterase mRNA levels in the slow soleus and fast extensor digitorum longus muscles of the rat.

In the rat, the level of acetylcholinesterase messenger RNA in the typical slow soleus muscles is only about 20-30% of that in the fast extensor digitorum longus muscles. The expression of contractile proteins in muscles is influenced by thyroid hormones and hyperthyroidism makes the slow soleus muscle faster. The influence of thyroid hormones on the levels of acetylcholinesterase messenger RNA level in the slow soleus and fast extensor digitorum longus muscle of the rat was studied in order to examine the effect of thyroid hormones on muscle acetylcholinesterase expression. Hyperthyroidism was induced in rats by daily thyroid hormone injection or thyroid hormone releasing tablet implantation. Hind-limb suspension was applied to produce muscle unloading. Muscle denervation or reinnervation was achieved by sciatic nerve transection or crush. Acetylcholinesterase messenger RNA levels were analyzed by Northern blots and evaluated densitometrically. Hyperthyroidism increased the levels of acetylcholinesterase messenger RNA in the slow soleus muscles close to the levels in the fast extensor digitorum longus. The effect was the same in the unloaded soleus muscles. Acetylcholinesterase expression increased also in the absence of innervation (denervation), in the presence of changed nerve activation pattern (reinnervation), and under enhanced tonic neural activation of the soleus muscle (electrical stimulation). However, the changes were substantially smaller than those observed in the control soleus muscles. Enhancement of acetylcholinesterase expression in the soleus muscles by the thyroid hormones is, therefore, at last in part due to hormonal effect on the muscle itself. On the contrary, increased level of the thyroid hormones had no influence on acetylcholinesterase expression in the normal fast extensor digitorum longus muscles. However, some enhancing influence was apparent whenever the total number of nerve-induced muscle activations per day in the extensor digitorum longus muscle was increased. Thyroid hormones seem to be an independent extrinsic factor of acetylcholinesterase regulation in the slow soleus muscle.

Interneuronal signalling is involved in induction of collateral sprouting of nociceptive axons.

Collateral sprouting of cutaneous nociceptive axons into the adjacent denervated skin critically depends on the nerve growth factor, presumably originating from the degenerated neural pathways and denervated skin. We hypothesised that the degenerated neural pathways are necessary, but not sufficient, to induce collateral sprouting of nociceptive axons, and, in addition, that the interaction between the injured and non-injured neurones within a dorsal root ganglion can trigger sprouting of nociceptive axons also in the absence of the denervated skin. End-to-side nerve anastomosis, made in female Wistar rats by suturing the end of an excised peroneal nerve segment to the side of the intact sural nerve, was used as a model for sprouting which allowed us to study the putative induction mechanisms separately. If the nerves adjacent to the sural nerve were transected concomitantly with the coaptation of the end-to-side anastomosis, robust nociceptive axon sprouting into the anastomosed nerve segment was observed by the nerve pinch test and counting of myelinated axons. Collateral sprouting did not occur, however, either if the cells in the anastomosed nerve segment were killed by freezing and thawing, or if the adjacent nerves had not been injured. However, if the ipsilateral dorsal cutaneous nerves, having their neurones in the same dorsal root ganglia as the sural nerve, were transected, but no other nerves were injured, then the sural nerve axons sprouted in abundance through the anastomosis even in the absence of denervated skin around the sural nerve terminals. From these results we suggest that cells (probably proliferating Schwann cells) in the degenerated neural pathways are necessary but not sufficient to induce collateral sprouting of nociceptive axons, and that interactions between the injured and non-injured neurones within the dorsal root ganglion (i.e. direct or indirect interneuronal signalling) are important in this regard.

Long-term changes in myosin heavy chain composition after botulinum toxin a injection into rat medial rectus muscle.

PURPOSE: To study long-term changes of extraocular muscles after botulinum toxin (Botx) A-induced paralysis, with special emphasis on myosin heavy chain (MyHC) isoform pattern in muscle fibers. METHODS: Botx A (5 IU) was injected into the ocular medial rectus (MR) muscles of adult rats. After 1, 5, and 8 months muscle cross sections were examined immunohistochemically, histochemically, and morphometrically. MyHC content was analyzed by gel electrophoresis. RESULTS: Paralyzed MR muscles displayed mildly atrophic and hypertrophic muscle fibers and decreased oxidative metabolism, due to decreased succinate dehydrogenase activity. However, muscle morphology was not grossly disturbed. MyHC profile was shifted toward slower isoforms. Electrophoretic analysis showed that the share of MyHCI, and especially of MyHCIIa and MyHCIIx/d, increased several fold, whereas the share of MyHCIIb decreased heavily during the first 5 months. Immunohistochemical analysis generally mirrored the results obtained by electrophoresis. Moreover, specific extraocular MyHC isoform MyHCeom disappeared and could not be detected during the whole experimental period. The portion of MyHCIIb relatively increased 8 months after Botx A injection, although the MyHC profile was still far from normal. CONCLUSIONS: These long-lasting changes in Botx A-paralyzed ocular MR muscles most probably reflect their inability to regain their unique functional characteristics after new motor end plate formation and recovery of muscle contraction.

Muscle activity-resistant acetylcholine receptor accumulation is induced in places of former motor endplates in ectopically innervated regenerating rat muscles.

Expression of acetylcholine receptors (AChRs) in the extrajunctional muscle regions, but not in the neuromuscular junctions, is repressed by propagated electric activity in muscle fibers. During regeneration, subsynaptic-like specializations accumulating AChRs are induced in new myotubes by agrin attached to the synaptic basal lamina at the places of former motor endplates even in the absence of innervation. We examined whether AChRs still accumulated at these places when the regenerating muscles were ectopically innervated and the former synaptic places became extrajunctional. Rat soleus muscles were injured by bupivacaine and ischemia to produce complete myofiber degeneration. The soleus muscle nerve was permanently severed and the muscle was ectopically innervated by the peroneal nerve a few millimeters away from the former junctional region. After 4 weeks of regeneration, the muscles contracted upon nerve stimulation, showed little atrophy and the cross-section areas of their fibers were completely above the range in non-innervated regenerating muscles, indicating successful innervation. Subsynaptic-like specializations in the former junctional region still accumulated AChRs (and acetylcholinesterase) although no motor nerve endings were observed in their vicinity and the cross-section area of their fibers clearly demonstrated that they were ectopically innervated. We conclude that the expression of AChRs at the places of the former neuromuscular junctions in the ectopically innervated regenerated soleus muscles is activity-independent.

Schwann cell dependence of regenerating rat sensory neurons is inversely related to the quality of axon growth substratum.

It is still controversial to what extent elongation of regenerating sensory axons depends on proliferating Schwann cells (SCs) in an injured peripheral nerve. We hypothesized that such regeneration was independent of SC support early after nerve injury, but later became SC-dependent. The sural nerve in rats was crushed, and freezing destroyed cells but not their basal laminae (BL) in the distal nerve segment. Sensory axon elongation was assessed by the nerve pinch test and their abundance was examined immunohistochemically. Sensory axons regenerated fairly rapidly during the first week even if SC migration was prevented. Thereafter, they ceased to elongate and withdrew until their terminals contacted the SCs migrating from the proximal nerve segment. Intrinsic neuronal capacity for growth without cell support, however, had not been lost. Rather, progressive degradation of the former SC BL and loss of laminin in the acellular segment arrested axon growth. Further elongation occurred only when SC migration was possible, corroborating our hypothesis. Sensory neurons continued to elongate and maintain their axons in spite of deteriorating growth substratum if, prior to injury the axons had been allowed to sprout into the denervated skin. Previous sprouting exposed the sensory neurons to high levels of NGF.

Effect of mechanical load on acetylcholinesterase mRNA levels in the slow soleus muscle of the rat.

The influence of mechanical load on the levels of acetylcholinesterase (AChE) mRNA was studied in order to examine to which extent different loading conditions may be responsible for differences in AChE regulation between the soleus, which is an antigravity muscle, and the fast EDL muscle. Forty-eight female rats were randomly assigned to three groups: a group with hindlimb suspension producing soleus muscle unloading, a group with ablation of synergistic gastrocnemius muscle causing overload of the soleus muscle, and the control group. The soleus muscles were isolated after 8 days of treatment AChE mRNA levels were analyzed by Northern blots and evaluated densitometrically. The values were normalized with beta-actin mRNA level, and then a value of 100% was assigned to the mRNA level in the control EDL muscle. Muscle unloading did not produce a significant increase of the AChE transcript levels, but the levels were rather variable. However, a statistically significant increase of AChE mRNA levels was observed in overloaded soleus muscles. These results corroborate the hypothesis that the slow and fast patterns of activity appear more important then muscle loading for the differences in regulation of AChE mRNA levels in fast and slow muscles.

Role of axon-deprived Schwann cells in perineurial regeneration in the rat sciatic nerve.

The role of Schwann cells (SC) in perineurial regeneration after nerve injury has not yet been resolved. It was hypothesized that SC alone are able to induce at least partial morphological restoration of the destroyed orthotopic perineureum (PN). To test the hypothesis, a permanently denervated segment of the rat sciatic nerve was made acellular by freeze-thawing, except in its most proximal part where non-neuronal cells were left intact. Restoration of the frozen segment by these cells was examined by electron microscopy and immunohistochemistry of the SC marker, S-100 protein, 4 and 8 weeks after injury. The PN regenerated from undifferentiated fibroblast-like cells. In the presence of migrant SC without axons, regenerated cells in the place of the former PN were stacked in several layers and, in accordance with the hypothesis, partially expressed typical features of the perineurial cells (PC): pinocytotic vesicles, short fragments of basal lamina and tight junctions. Migrant SC induced formation of pseudo-minifascicles even in the epineurium. In these, SC organized the adjacent fibroblasts into a multilayered circular sheath, and induced their partial differentiation towards perineurial cells. Further experiments demonstrated that regenerating axons are required for complete morphological differentiation of the regenerated perineurial cells either in the orthotopic PN or in minifascicles.

Modification of skeletal muscle AChE expression by a novel method of stimulus application to the peripheral nerve.

We have developed a new method for chronic application of electrical stimuli to the rat peripheral nerve in vivo. This method has the following advantages: (1) the amplitude, duration and pattern of stimulation can be adjusted before and during the course of experiment, (2) the set-up allows the animal to move freely during the experiment, and (3) the set-up is constructed from inexpensive, of-the-shelf components that can be reused several times. The new method was used to study the influence of the pattern of muscle activation on acetylcholinesterase (AChE) regulation in extensor digitorum longus (EDL) and tibialis anterior (TA), fast skeletal muscles. Northern blot analysis of the chronically stimulated, fast EDL and TA revealed a rapid decrease of AChE mRNA level to a level typical for a slow, skeletal muscle.

Fibre types and myosin heavy chain expression in the ocular medial rectus muscle of the adult rat.

Myosin heavy chain (MHC) expression was determined immunohistochemically in individual muscle fibre types characterised by activities of ATPase and the key oxidative and glycolytic enzymes in rat ocular medial rectus (MR) muscles. In the global layer (GL), glycolytic activity of muscle fibres was higher and oxidative activity lower, than in the orbital layer (OL). Muscle fibres in the former displayed rosette-like organisation with a slow fibre surrounded by several fast fibres, which expressed either MHCIIa or MHCIIb, but many co-expressed both isoforms. In the OL some slow fibres co-expressed MHCIIa. Extraocular MHC isoform (MHCeom) could not be determined immunohistochemically and no pure MHCIIx/d containing fibres were found, suggesting that these isoforms, demonstrated electrophoretically, are co-expressed with others. Slow muscle fibres in both layers co-expressed MHCbeta slow, MHCalpha cardiac and MHC-slow tonic. Neonatal isoform (MHCneo) was co-expressed in several fast and slow muscle fibres in the orbital, but not global layer. Slow fibres in the GL displayed very low oxidative activity. Electrophoretic analysis of ocular MR muscle homogenates revealed that about 50% of total MHC was MHCIIb, MHCeom was quite prominent (25%), and MHCIIa, MHCIIx/d and MHCI contributed each about 8%. MHCneo, MHCslow tonic and MHCalpha cardiac could not be identified as separate bands.

Differences in expression of acetylcholinesterase and collagen Q control the distribution and oligomerization of the collagen-tailed forms in fast and slow muscles.

The collagen-tailed forms of acetylcholinesterase (AChE) are accumulated at mammalian neuromuscular junctions. The A(4), A(8), and A(12) forms are expressed differently in the rat fast and slow muscles; the sternomastoid muscle contains essentially the A(12) form at end plates, whereas the soleus muscle also contains extrajunctional A(4) and A(8) forms. We show that collagen Q (ColQ) transcripts become exclusively junctional in the adult sternomastoid but remain uniformly expressed in the soleus. By coinjecting Xenopus oocytes with AChE(T) and ColQ mRNAs, we reproduced the muscle patterns of collagen-tailed forms. The soleus contains transcripts ColQ1 and ColQ1a, whereas the sternomastoid only contains ColQ1a. Collagen-tailed AChE represents the first evidence that synaptic components involved in cholinergic transmission may be differently regulated in fast and slow muscles.

Recovery of cutaneous pain sensitivity after end-to-side nerve repair in the rat.

OBJECT: The hypothesis that collaterally sprouting axons from an uninjured donor nerve may provide recovery of pain sensitivity in the skin after end-to-side nerve repair was investigated in rats. In addition, the effect of this technique on the donor nerve was examined. METHODS: The distal stump of the transected peroneal nerve was sutured end to side to the intact sural nerve. No epineurial window or perineurial slit was made in the sural nerve at the site of coaptation. Other nerves in the leg were transected and ligated. Eighteen weeks later, the sural nerve was transected at a site distal from the coaptation site. The residual pain sensitivity in the peroneal innervation field in the instep was documented using the skin pinch test in three of 11 animals. The area of sensitivity encompassed 19 to 40% of the maximum nociceptive innervation area of the normal peroneal nerve. The nerve pinch test revealed functional sensory axons in all communicating peroneal nerves, in which 277+/-119 myelinated axons (mean +/- standard deviation) were found by histological investigation. CONCLUSIONS: The authors conclude that at least partial recovery of sensory function due to collateral sprouting of axons after end-to-side nerve repair is possible in principle. However, the presence of functional sensory axons in the peroneal nerve stumps did not guarantee the recovery of skin sensitivity to pain in all animals. No functional or morphological evidence of an untoward effect of collateral sprouting into the end-to-side communicating nerve was detected in the axons of the donor nerve itself.

Myosin heavy chain profiles in regenerated fast and slow muscles innervated by the same motor nerve become nearly identical.

Plasticity of mature muscles exposed to different activation patterns is limited, probably due to restricted adaptive range of their muscle fibres. In this study, we tested whether satellite cells derived from slow muscles can give rise to a normal fast muscle, if transplanted to the fast muscle bed. Marcaine-treated rat soleus and extensor digitorum longus (EDL) muscles were transplanted to the EDL muscle bed and innervated by the 'EDL' nerve. Six months later expression of myosin heavy chain isoforms was analysed by areal densities of fibres, binding specific monoclonal antibodies, and by SDS gel electrophoresis. Both regenerated muscles closely resembled each other. Their myosin heavy chain profiles were similar to those in fast muscles although they were not identical to that in the control EDL muscle. Since not even regenerated EDL was able to reach the myosin heavy chain isoform profile of mature EDL muscle, our experimental model did not permit studying the adaptive capacity of satellite cells in different muscles in its whole extent. However, the results favour the multipotential myoblast stem cell population in rat muscles and underline the importance of the extrinsic regulation of muscle phenotype.

Acetylcholinesterase in the neuromuscular junction.

New findings regarding acetylcholinesterase (AChE) in the neuromuscular junction (NMJ), obtained in the last decade, are briefly reviewed. AChE is highly concentrated in the NMJs of vertebrates. Its location remains stable after denervation in mature rat muscles but not in early postnatal muscles. Agrin in the synaptic basal lamina is able to induce sarcolemmal differentiations accumulating AChE even in the absence of a nerve ending. Asymmetric A12 AChE form is the major molecular form of AChE in vertebrate NMJs. Extrajunctional suppression of this form is a developmental phenomenon. Motor nerve is able to reinduce expression of the A12 AChE form in the ectopic NMJs even in muscles with complete extrajunctional suppression of this form. The 'tail' of the A12 AChE form is made of collagen Q. It contains domains for binding AChE to basal lamina with ionic and covalent interactions. Muscle activity is required for normal AChE expression in muscles and its accumulation in the NMJs. In addition, the pattern of muscle activation also regulates AChE activity in the NMJs, demonstrating that the pattern of synaptic transmission is able to modulate one of the key synaptic components.

Prior collateral sprouting enhances elongation rate of sensory axons regenerating through acellular distal segment of a crushed peripheral nerve.

Regenerating axons in crushed peripheral nerves grow through their distal nerve segments even in the absence of Schwann cell support, but their elongation rate is reduced by 30%. We examined whether prior exposure of sensory neurons to trophic factors achieved either by collateral sprouting or regeneration after conditioning lesion could enhance subsequent regeneration of their axons after crush, and compensate for loss of cell support. Collateral sprouting of the peroneal cutaneous sensory axons in the rat was evoked by transection of adjacent peripheral nerves in the hind leg. The segment of the peroneal nerve distal to the crush was made acellular by repeated freezing. Sensory axon elongation rate during regeneration was measured by the nerve pinch test. Prior axonal sprouting for two weeks increased the elongation rate of sensory axons through the acellular distal nerve segment back to normal value observed in control crushed nerves. The number of axons in the acellular distal segment at a fixed distance from the crush site was about 50% greater in sprouting than in control non-sprouting nerves. However, prior sprouting caused no further increase of axon elongation rate in control crushed nerves. Prior collateral sprouting, therefore, could in some respect compensate for loss of cell support in the distal nerve segment after crush lesion. This suggests that loss of cell-produced trophic factors is probably responsible for slower elongation rate through the acellular distal nerve segment. Surprisingly, prior conditioning lesion caused no enhancement of elongation rate of the sensory axons regenerating in the absence of cell support.

Acetylcholinesterase mRNA level and synaptic activity in rat muscles depend on nerve-induced pattern of muscle activation.

Acetylcholinesterase (AChE) mRNA levels are severalfold higher in fast rat muscles compared with slow. We hypothesized that AChE mRNA levels and AChE activity in the neuromuscular junction depend on a specific nerve-induced pattern of motor unit activation. Chronic low-frequency stimulation, mimicking the activation pattern in slow muscles, was applied to fast muscles in rats. Molecular forms of AChE were analyzed by velocity sedimentation, and AChE mRNA levels were analyzed by Northern blots. AChE mRNA levels in stimulated fast muscles dropped to 10-20% of control after 1 week and became comparable to those in slow soleus muscles. The activity of the junctional A12 AChE form in 35 d stimulated fast muscles decreased to 56% of control value, reaching that in the soleus muscle. Therefore, synaptic AChE itself depends on the muscle activation pattern. Complete inactivity after denervation also decreased the AChE mRNA level in fast muscles to <10% in 48 hr. In contrast, profuse fibrillations observed in noninnervated immature regenerating muscles maintain AChE mRNA levels at 80% of that in the innervated fast muscles. If protein synthesis was inhibited by cycloheximide, AChE mRNA levels in 3-d-old regenerating muscle, still containing myoblasts, increased approximately twofold. No significant increase after cycloheximide application was observed either in denervated mature fast muscles or in normal slow muscles. Low AChE mRNA levels observed in those muscles are probably not caused by decreased stability of AChE mRNA as demonstrated in myoblasts.

Extent of nociceptive dermatomes in adult rats is not primarily maintained by axonal competition.

Nociceptive innervation territories of individual peripheral and spinal nerves in the skin of the rat hind paw were investigated. In addition, the hypothesis that competitive interactions among the axons from adjacent dorsal root ganglia (DRG) play an important role in maintenance of dermatomal extent in adult animals was tested. The area of innervation territories of individual spinal and peripheral nerves was determined by nociceptive pinch test of the skin after extirpation of adjacent DRGs or transection of adjacent peripheral nerves, respectively. Positions of nociceptive dermatomes and innervation territories of peripheral nerves were similar to the territories innervated by the C-fibers described earlier by dye extravasation technique. In contrast, our results convincingly demonstrated substantial overlap of nociceptive (probably A delta) fibers from adjacent dermatomes in which the autonomous innervation areas were only about one-half of the maximal areas. Nociceptive territories of peripheral nerves overlapped, too. Accordingly, we could find no autonomous innervation area of the sural nerve. Two weeks after extirpation of adjacent DRGs, the area of each of the isolated dermatomes L3, L4, and L5 increased only by about 10%, and it did not change detectably during the next 6 months. The results of our study (a) support the view that innervation fields supplied by the nociceptive (probably A delta) fibers are greater and display more overlap than those supplied by the C-fibers of the same nerve and (b) suggest that axonal competition for innervation territory is not decisive for maintenance of dermatomal borders in the adult rat.