BACKGROUND
neuropathic pain is associated with exaggerated responses to painful stimuli , pain provoked by normally innocuous stimulation , abnormal spontaneous sensations  and a spontaneous burning pain  <cit> .
this type of chronic pain remains generally undertreated, perhaps at least partially due to a lack of mechanism-based treatments.
currently, the mechanisms of neuropathic pain following peripheral nerve injury remain unresolved.
the physiological consequences of peripheral nerve damage associated with neuropathic pain readouts in animal models have provided detailed information suggesting an involvement of c-fiber sensory neurons in mediating the functional changes in these models  <cit> .
this is consistent with the classic concept that pain and central sensitization are largely due to sensory input from c-fiber afferents.
however, compelling evidence from animal models suggests a role of large a-fiber, heavily myelinated neurons in mediating neuropathic pain.
for example, intracellular recordings in vitro from dorsal root ganglia  in an axotomy model, where the l <dig> spinal nerve was cut, showed enhanced responses of a-type neurons to intracellular injection of depolarizing current  <cit>  and exhibited spontaneous discharge, which was not observed in c neurons  <cit>
spontaneous activity and increased excitability have also been demonstrated in a-type neurons in vitro in the dorsal root ganglion  compression model  <cit>  and in the chronic constriction injury model  <cit>  of neuropathic pain, although this does not seem to be restricted only to a-type neurons  <cit> .
classification of neurons in in vitro studies can be only on the basis of conduction velocity or soma size, but classification cannot be based on functional criteria.
full functional classification is available in in vivo studies.
in axotomy models in vivo extracellular recordings from sensory nerve fibers  <cit>  and from drg neurons  <cit>  have demonstrated an increase in excitability of a-fiber neurons.
however, in such studies, where peripheral nerves are cut, functional classification of sensory neurons is also inaccessible.
most types of neuropathic pain do not arise from cut nerves but from physically or metabolically induced neurotrauma.
there have been some in vivo studies based on nerve intact models of peripheral neuropathic pain.
extracellular recordings from dorsal root fibers in the cci model have shown a general increase in excitability of all neuron types  <cit> .
khan et al.
<cit>  have reported, on the other hand, that in the streptozotocin model of diabetic neuropathy single aδ-fiber and aβ-fiber sensory axons in the tibial nerve develop ectopic discharge and higher spontaneous activity, and exhibit lower activation threshold to mechanical stimuli.
the changes in sensitivity and in axon excitability were unaltered by treatment with i.p.
resiniferatoxin to deplete c-fiber neurons.
we have also been investigating mechanisms of nociceptive pain in an intact nerve animal model of peripheral neuropathy, in our case by inserting a polyethylene cuff around one sciatic nerve  <cit> .
our previous studies demonstrated that myelinated afferents mediate the increased responses of spinal dorsal horn nociceptive neurons to noxious  <cit>  and innocuous stimulation  <cit>  and that the peripheral drive exciting these neurons is due to ectopic discharge arising from multiple sites along the sensory nerve  <cit>
little is known, however, regarding the specific functional type of myelinated neuron undergoing changes in excitability.
for example, the studies on the cci  <cit> , the steptozotocin  <cit>  and the drg compression  <cit>  models did not classify neurons other than on the basis of conduction velocity.
further, the studies on the drg compression model did not differentiate between different types of a neuron.
studies on axotomy models lose the ability to classify neurons functionally.
tal et al.,  <cit>  reported that spontaneous discharge and increased mechanosensitivity varies depending on the type of peripheral tissues innervated.
in view of the contradictory evidence regarding the specific neuron types undergoing the changes in excitability in in vivo electrophysiological studies on nerve intact models of neuropathic pain, it was considered important to understand the specific population of aβ-fiber drg neurons possibly implicated in mechanisms of neuropathic pain.
clear classification criteria are available in the literature based on responses to natural stimulation  <cit> .
although earlier in vivo studies on sensory neurons in neuropathic pain models had access to stimulation of peripheral receptive fields this was not done in classification of neuron types; rather, classification was done on the basis of conduction velocity.
as a result, we set out to record in vivo from single aβ-fiber drg neurons that were functionally classified on the basis of their responses to natural stimulation of peripheral fields, comparing excitability of neurons in cuff-implanted model animals with excitability of neurons in surgically naive animals.
classification was also based on conduction velocity and other criteria defined by fang et al.
<cit> .
as our aim was to gain information on changes in excitability of these classified neurons, we recorded intracellularly in order to determine excitability by intracellular injection of depolarizing current, by application of von frey filaments to the peripheral receptive field and by electrical stimulation of dorsal roots.
RESULTS
withdrawal response in the behavioral von frey test
stimulation of the plantar surface of the hind paw with von frey filaments evoked a withdrawal response in control animals, with hairs exerting pressures of 10- <dig> g. three weeks after cuff implantation on the sciatic nerve on the right side, the rats fully developed behavioral signs of mechanical hypersensitivity on the affected hind limb, a parallel to the tactile allodynia reported by people with neuropathic pain.
neuropathic animals responded to filaments that the control animals ignored, i.e.
<dig> - <dig>  g, which evoked a clear withdrawal on the nerve-injured side.
furthermore, in neuropathic rats the withdrawal was exaggerated in amplitude and duration, and it was frequently accompanied by licking of the paw.
withdrawal thresholds were  <dig>  ±  <dig>  g in control animals  and  <dig>  ±  <dig>  g in neuropathic animals .
comparison of the data indicated a difference between these groups p <  <dig> .
the data are shown in figure  <dig>
excitability of the receptive field measured by responses to application of von frey filaments
to determine whether changes in properties of peripheral receptors might contribute to the mechanical hypersensitivity that characterizes this model, von frey filaments were applied to the peripheral receptive fields of neurons studied in the electrophysiological recordings.
calibrated von frey filaments were applied to the identified receptive field areas as a tactile stimulation and the minimum filament that elicited an ap in the soma was recorded.
the mechanical thresholds with von frey filaments of all aβ drg neurons during electrophysiology recording are shown in figure  <dig>  the mechanical thresholds of gf , ra , sa , ms  and aβ htm  neurons in control rats were within the range  <dig> - <dig> g,  <dig> - <dig> g,  <dig> - <dig> g,  <dig> - <dig> g and 4- <dig> g, respectively.
in neuropathic rats, the thresholds of gf , ra  neurons were the same as in control animals.
however, in neuropathic animals the thresholds of sa , ms  and htm  neurons were shifted lower, to  <dig> - <dig> g,  <dig> - <dig> g and 2- <dig> g, respectively.
the mean mechanical threshold of the ms neurons was significantly lower in neuropathic rats than in control rats:  <dig>  ±  <dig>  g in control vs.  <dig>  ±  <dig>  g in neuropathic neurons .
the other subtypes of aβ-fiber neurons showed no significant difference.
thresholds of hair neurons were  <dig>  ±  <dig>  g in control and  <dig>  ±  <dig>  g in neuropathic neurons .
those in ra neurons were- <dig>  ±  <dig>  g vs.  <dig>  ±  <dig>  g in control and neuropathic animals, respectively .
those in sa neurons were  <dig>  ±  <dig>  g in control animals and  <dig>  ±  <dig>  g in neuropathic animals .
those in htm neurons in control animals were  <dig>  ±  <dig>  g vs. <dig>  ±  <dig>  g in neuropathic animals .
some neurons in neuropathic rats exhibited ongoing discharge consisting of fast excitatory aps.
for example, c-fiber cool neurons usually discharge at room temperature and ms neurons can discharge when the leg is fixed in an extended position in both control and neuropathic rats.
in the present study, ongoing discharge was recorded from c-fiber cool neurons and aβ-ms neurons in both control  and neuropathic rats .
as this study focused on differences in responses evoked by applied stimuli between control and model animals these neurons were excluded from the present analysis.
in neuropathic rats,  <dig> of  <dig> aβ htm,  <dig> of  <dig> hair aβ ltm neurons demonstrated certain electrical signs that are reasonably interpreted as an increase in excitability .
an increased number of action potentials could be evoked by stimulation of the receptive field, and the discharge persisted for several minutes after removal of the stimulus from the receptive field.
it is important to note that in control animals none of the  <dig> aβ htm or of the  <dig> aβ ltm neurons demonstrated any prolonged discharge following application of peripheral stimuli.
excitability of the soma measured by responses to injection of depolarizing current
the ap responses to intracellular depolarizing current pulse injection were tested to determine whether there is a difference in soma excitability induced by the peripheral neuropathy.
figure  <dig> illustrates the threshold currents that elicited aps in control and neuropathic animals.
compared to the control group, aβ-fiber htm neurons and aβ-fiber ltm ms neurons in neuropathic animals showed a significant difference from controls; activation threshold of htm neurons was  <dig>  ±  <dig>  na  vs.  <dig>  ±  <dig>  na  in control and neuropathic rats, respectively .
activation threshold of ms neurons was  <dig>  ±  <dig>  na in control  and  <dig>  ±  <dig>  g in neuropathic  rats .
there was no significant difference in aβ-fiber ltm cutaneous neurons in control vs. model animals .
the effects of peripheral neuropathy on repetitive discharge were analyzed quantitatively by frequency-current analysis.
figure 5a shows the number of aps elicited with different current strengths, comparing control vs. neuropathic rats.
compared to the control group, aβ-fiber htm neurons and aβ-fiber ltm ms neurons in the neuropathic rats showed a significant difference, while aβ-fiber ltm cutaneous neurons showed no difference.
subthreshold oscillations and bursting patterns of discharge, which have been implicated in generating ectopic discharge in an axotomy model of neuropathic pain , were not observed in this intact model.
for aβ htm neurons, the results were based on  <dig> neurons in control rats and  <dig> neurons in neuropathic rats.
with a  <dig>  na,  <dig> ms current injection, the number of aps in control rats was  <dig>  ±  <dig>  and in neuropathic rats it was  <dig>  ±  <dig>  .
with a  <dig> na,  <dig> ms current injection, the number of aps in control rats was  <dig>  ±  <dig>  while neuropathic rats this was  <dig>  ±  <dig>  .
with a  <dig>  na,  <dig> ms current injection, the number of aps in control rats was  <dig>  ±  <dig>  while neuropathic rats it was  <dig>  ±  <dig>  .
for ms neurons, the results were based on  <dig> neurons in control rats and  <dig> neurons in neuropathic rats.
with a  <dig>  na,  <dig> ms current injection, the number of aps in control rats was  <dig>  ±  <dig>  while in neuropathic rats it was  <dig>  ±  <dig>  .
with a  <dig> na,  <dig> ms current injection, the number of aps in control rats was  <dig>  ±  <dig>  and  <dig>  ±  <dig>  in neuropathic rats .
with a  <dig>  na,  <dig> ms current injection, the number of aps in control rats was  <dig>  ±  <dig>  while it was  <dig>  ±  <dig>  in neuropathic rats .
for aβ-fiber ltm cutaneous neurons, the results were based on  <dig> neurons in control animals and  <dig> in neuropathic animals.
with the  <dig>  na,  <dig> ms current injection, the number of spikes in control rats was  <dig>  ±  <dig>  and in neuropathic rats it was  <dig>  ±  <dig>  .
with the  <dig> na,  <dig> ms current injection, the number of spikes in control rats was  <dig>  ±  <dig>  and  <dig>  ±  <dig>  in neuropathic rats .
with the  <dig>  na,  <dig> ms current injection, the number of aps in control rats was  <dig>  ±  <dig>  and neuropathic rats it was  <dig>  ±  <dig>  .
excitability of the axon measured by responses to dorsal root stimulation
the responses to axonal stimulation by delivering single current pulses of  <dig> ma to the dorsal root were examined.
dorsal root excitability was determined as the chronaxie curve , which was derived by determining the minimum current applied to the dorsal root that just evoked a soma ap with pulse durations of  <dig>  ms,  <dig> ms,  <dig> ms,  <dig> ms and  <dig> ms. the data are shown in figure  <dig>  ms neurons showed significantly lower current intensity threshold with  <dig>  ms stimulation .
to investigate the fast process of recovery from an ap in each subtype of afferent neuron, paired-pulse stimuli were delivered to the dorsal root.
the ri of aβ neurons showed a significant difference between neuropathic and control rats.
figure 7a displays the ri distributions for individual neurons in each aβ neuron subtype in control and neuropathic rats.
in control rats, aβ-fiber htm neurons had the longest ri .
ms neurons had the shortest ri .
other types of aβ ltm neurons had an intermediate ri .
in neuropathic animals, under the same stimulation conditions, it was found that ri in all aβ neurons was significantly greater compared with neurons in control animals.
ri in ms neurons in neuropathic rats was  <dig>  ±  <dig>  ms, n =  <dig> .
ri in hair neurons was  <dig>  ±  <dig>  ms n =  <dig> .
aβ htm neurons also showed significantly greater ri  <dig>  ±  <dig>  ms, n =  <dig> .
discussion
general characteristics of neuropathic afferent neurons
sensitization is often considered to play a major role in the neural basis of hyperalgesia and allodynia associated with peripheral neuropathy.
the present study determined the excitability and patterns of discharge in functionally defined drg neurons, including electrophysiological properties under different conditions of neuronal activation.
in addition, some neurons were provoked to display a discharge, including otherwise quiescent neurons.
a systematic correlation of neuronal type with expression of discharge indicated that ab neurons, including ltm and htm neurons, exhibited increased activity in neuropathic rats, whereas these neurons did not display such activity in control rats.
this principle conclusion is consistent with our previous studies on this model  <cit> .
these data are also consistent with previous reports on neuropathic rat models.
for example, after peripheral nerve injury both axotomized as well as intact afferent neurons supplying skeletal muscle but not skin afferents generate ongoing activity within the drg  <cit> .
it has also been shown that after nerve injury sensitization of primary sensory neurons is characterized not only by abnormal discharges but also by having a lowered activation threshold and by exhibiting exaggerated responses to various stimuli  <cit> .
here we report several signs of altered excitability, specifically of ab drg neurons.
for example, persisting activity was generated specifically in both aβ htm and aβ ltm neurons.
further, a reduction was also observed in the mechanical response threshold of some subtypes of aβ-fiber neuron in the neuropathic rats when the distribution of mechanical thresholds receptive fields of the individual neurons was compared.
this suggests a change in excitability of peripheral terminal receptors.
intracellular current injection into the soma of ab drg neurons generated significantly more aps in neuropathic rats than in controls, which suggests that there is a change in excitability of ab drg neuron somata as well.
while the difference in the soma ap response in ab drg neurons between control and neuropathic rats suggests that a constitutively enhanced hyperexcitability occurs in these neurons, it is important to point out that in contrast there was an increased current threshold for activation of dorsal roots as well as a delay in the latency of the second ap in response to paired-pulse dorsal root stimulation at shorter inter-pulse intervals.
this suggests an attenuated excitability of aβ axons in the dorsal root.
this appears to be a novel finding.
to our knowledge previous studies have not compared changes in excitability in soma, axon and peripheral receptor in this or in other animal models of chronic pain.
possible mechanisms underlying changes in excitability in different parts of aβ drg neurons
the cellular mechanisms underlying generation of abnormal discharge and the changes in activation threshold of the soma and peripheral sensory receptor in aβ drg neurons are not clear.
it is generally presumed that altered discharge results from the classical hodgkin-huxley repetitive firing processes.
the ability of theses neurons to sustain repetitive discharge depends on all three major neuronal ion channels, na+, ca2+ and k+.
alterations in the level of expression, cellular localization, distribution and activation kinetics of each of these channel types might be involved in determination of these changes in excitability.
thus, they may have contributed to the difference in excitability between soma, axon and peripheral receptor.
it is also worth considering how injury to the axon may lead to changes in excitability in the somata and the peripheral endings.
while no clear answers immediately come to mind, some studies on models of peripheral neuropathy might offer some insight.
for example, walters and ambron  <cit>  hypothesized that axonal injury might unmask nuclear localization signals in certain axoplasm proteins at the injured site.
it was suggested that this may cause the transport of cytokines, nerve growth  or glial cell line-derived neurotrophic factor , tumor necrosis factor  or other inflammatory mediators released by immune cells and schwann cells  <cit> , to be transported retrogradely to the somata and activate transcription factors, which then induce the alternation in expression of neuropeptides, receptors and ion channels in the somata.
this in turn may induce increased excitability in the drg somata.
in fact, a previous study provided evidence that drg somata exhibit de novo expression of ttx-sensitive type iii na+ channels, a reduction of ttx-resistant na+ current, and a change in k+ and ca+ currents in different neuropathic animal models  <cit> .
the mechanisms underlying the changes in the activation threshold of the peripheral terminals might be related also to alterations in the release of cytokines and growth factors.
previous studies have shown that tnf-α is unregulated during wallerian degeneration  <cit>  and that exogenous administration of tnf-α produces a direct activation and sensitization of injured afferent fibers  <cit> .
when exposed to ngf sensory neurons undergo an increase in either the firing of aps evoked by a ramp of the depolarizing current  <cit>  or in currents evoked by the application of capsaicin  <cit> .
there is no clear explanation for the changes in excitability of the ms neurons.
muscle spindles are the sensory endings primarily responsible for initiation of proprioception.
these receptors can be exquisitely sensitive, responding to light tapping, vibration or pressure applied to the skin overlying the muscle spindle within the muscle belly  <cit> .
however, unless there are adequate changes in muscle length, the muscle spindle will not be excited.
in the present study we observed a significantly decreased mechanical threshold of ms neurons.
however, the mechanisms underlying this change in excitability remain to be determined.
although there was no direct evidence to explain the attenuated excitability of dorsal root axons in response to direct electrical stimulation, previous studies have shown that the behavioral and cellular effects of nerve injury to the central branches, such as partial dorsal rhizotomy, are different from those induced by injury to the peripheral branches, such as chronic constriction injury of the sciatic nerve.
for example, sheen et al.
failed to observe a neuropathic pain behavior after a dorsal rhizotomy  <cit> , while song et al.
found that partial dorsal rhizotomy produced significantly less severe hyperalgesia  <cit> .
these studies suggest that the dorsal root might generate different injury signals and might transport such signals to the spinal cord rather than to drg somata  <cit> .
based on our results, we further suggest that the attenuated excitability of dorsal root axons might act as a "negative injury signal", while the increased excitability of somata and peripheral receptors might act as a "positive injury signal" to the central nervous system in this model of peripheral neuropathic pain.
future studies are necessary to test this suggestion.
consequences of activity of aβ neurons
in this model, a significant correlation was found between several variables of discharge and pain behaviors.
the "positive" correlation along with the "negative" correlation, as shown in this study, suggest that altered excitability may be important for maintaining neuropathic pain behavior.
although not each nerve injury necessarily leads to pain, our previous studies in this model have shown that light tactile stimulation induces brisk paw withdrawal  <cit> .
further, innocuous touch stimulation of the cutaneous receptive field induces an excessively prolonged afterdischarge in spinal wide dynamic range neurons and this unusually prolonged afterdischarge of dorsal horn neurons is reduced by peripheral nerve block  <cit> .
this present study has shown that ab drg neurons exhibit increased discharge and excitability, which includes a prolonged discharge firing pattern in response to direct soma excitation and receptive field stimulation.
thus, it seems reasonable to suggest that the prolonged nature of the afterdischarge in ab neurons in this model of peripheral neuropathy may lead to greater peripheral drive and that mechanical allodynia, and perhaps also the burning pain, associated with peripheral neuropathy may be due to this increased activity in ab neurons delivering excitation to nociceptive neurons in the spinal cord.
thus, based on previous evidence from other laboratories as well as our findings on functionally-identified drg neurons in this intact nerve model of peripheral neuropathy, we propose that the hyperalgesia and dysesthesia experienced by people with peripheral neuropathy might be at least partially due to sustained discharge of ab primary afferents in response to light tactile or noxious stimulation of the skin.
we further postulate that the altered excitability of ab neurons that develops in neuropathic rat models, as cited above, may be a critical factor in signaling mechanical allodynia and may contribute to allodynia and spontaneous pain following peripheral nerve injury in humans.
overall, our data provide specific evidence for a major contribution of altered excitability of ab neurons deriving from multiple regions of these neurons, including the receptive field and the somata, possibly modified by altered excitability of dorsal root axons, resulting in altered spinal nociceptive mechanisms in this animal model of painful peripheral neuropathy.
CONCLUSIONS
this study has demonstrated changes in excitability of ab htm and ltm primary sensory neurons in the nerve-intact cuff model of peripheral neuropathic pain and that different sites of these sensory neurons exhibit different changes in excitability after sciatic nerve injury.
these differences consisted of a reduced mechanical activation threshold and abnormal prolonged discharges following receptive field stimulation, a reduced current activation threshold of the cell body and a greater number of aps evoked by intracellular current injection into the soma, yet an increased current activation threshold and delayed recovery kinetics in dorsal root axons.
this study provides new evidence to support a role for injury-induced plasticity of primary sensory neurons in this model of neuropathic pain and also indicates that changes to different regions of the sensory neurons might have different effects on peripheral drive to central mechanisms and thus possibly also hyperalgesia or allodynia.

 neuropathic pain is associated with exaggerated responses to painful stimuli, pain provoked by normally innocuous stimulation, abnormal spontaneous sensations and a spontaneous burning pain. this type of chronic pain remains generally undertreated, perhaps at least partially due to a lack of mechanism-based treatments. currently, the mechanisms of neuropathic pain following peripheral nerve injury remain unresolved. the physiological consequences of peripheral nerve damage associated with neuropathic pain readouts in animal models have provided detailed information suggesting an involvement of c-fiber sensory neurons in mediating the functional changes in these models. this is consistent with the classic concept that pain and central sensitization are largely due to sensory input from c-fiber afferents. however, compelling evidence from animal models suggests a role of large a-fiber, heavily myelinated neurons in mediating neuropathic pain. for example, intracellular recordings in vitro from dorsal root ganglia in an axotomy model, where the l spinal nerve was cut, showed enhanced responses of a-type neurons to intracellular injection of depolarizing current and exhibited spontaneous discharge, which was not observed in c neurons spontaneous activity and increased excitability have also been demonstrated in a-type neurons in vitro in the dorsal root ganglion compression model and in the chronic constriction injury model of neuropathic pain, although this does not seem to be restricted only to a-type neurons. classification of neurons in in vitro studies can be only on the basis of conduction velocity or soma size, but classification cannot be based on functional criteria. full functional classification is available in in vivo studies. in axotomy models in vivo extracellular recordings from sensory nerve fibers and from drg neurons have demonstrated an increase in excitability of a-fiber neurons. however, in such studies, where peripheral nerves are cut, functional classification of sensory neurons is also inaccessible. most types of neuropathic pain do not arise from cut nerves but from physically or metabolically induced neurotrauma. there have been some in vivo studies based on nerve intact models of peripheral neuropathic pain. extracellular recordings from dorsal root fibers in the cci model have shown a general increase in excitability of all neuron types. khan et al. have reported, on the other hand, that in the streptozotocin model of diabetic neuropathy single aδ-fiber and aβ-fiber sensory axons in the tibial nerve develop ectopic discharge and higher spontaneous activity, and exhibit lower activation threshold to mechanical stimuli. the changes in sensitivity and in axon excitability were unaltered by treatment with i.p. resiniferatoxin to deplete c-fiber neurons. we have also been investigating mechanisms of nociceptive pain in an intact nerve animal model of peripheral neuropathy, in our case by inserting a polyethylene cuff around one sciatic nerve. our previous studies demonstrated that myelinated afferents mediate the increased responses of spinal dorsal horn nociceptive neurons to noxious and innocuous stimulation and that the peripheral drive exciting these neurons is due to ectopic discharge arising from multiple sites along the sensory nerve little is known, however, regarding the specific functional type of myelinated neuron undergoing changes in excitability. for example, the studies on the cci, the steptozotocin and the drg compression models did not classify neurons other than on the basis of conduction velocity. further, the studies on the drg compression model did not differentiate between different types of a neuron. studies on axotomy models lose the ability to classify neurons functionally. tal et al., reported that spontaneous discharge and increased mechanosensitivity varies depending on the type of peripheral tissues innervated. in view of the contradictory evidence regarding the specific neuron types undergoing the changes in excitability in in vivo electrophysiological studies on nerve intact models of neuropathic pain, it was considered important to understand the specific population of aβ-fiber drg neurons possibly implicated in mechanisms of neuropathic pain. clear classification criteria are available in the literature based on responses to natural stimulation. although earlier in vivo studies on sensory neurons in neuropathic pain models had access to stimulation of peripheral receptive fields this was not done in classification of neuron types; rather, classification was done on the basis of conduction velocity. as a result, we set out to record in vivo from single aβ-fiber drg neurons that were functionally classified on the basis of their responses to natural stimulation of peripheral fields, comparing excitability of neurons in cuff-implanted model animals with excitability of neurons in surgically naive animals. classification was also based on conduction velocity and other criteria defined by fang et al.. as our aim was to gain information on changes in excitability of these classified neurons, we recorded intracellularly in order to determine excitability by intracellular injection of depolarizing current, by application of von frey filaments to the peripheral receptive field and by electrical stimulation of dorsal roots.  withdrawal response in the behavioral von frey test stimulation of the plantar surface of the hind paw with von frey filaments evoked a withdrawal response in control animals, with hairs exerting pressures of 10- g. three weeks after cuff implantation on the sciatic nerve on the right side, the rats fully developed behavioral signs of mechanical hypersensitivity on the affected hind limb, a parallel to the tactile allodynia reported by people with neuropathic pain. neuropathic animals responded to filaments that the control animals ignored, i.e. - g, which evoked a clear withdrawal on the nerve-injured side. furthermore, in neuropathic rats the withdrawal was exaggerated in amplitude and duration, and it was frequently accompanied by licking of the paw. withdrawal thresholds were ± g in control animals and ± g in neuropathic animals. comparison of the data indicated a difference between these groups p <. the data are shown in figure excitability of the receptive field measured by responses to application of von frey filaments to determine whether changes in properties of peripheral receptors might contribute to the mechanical hypersensitivity that characterizes this model, von frey filaments were applied to the peripheral receptive fields of neurons studied in the electrophysiological recordings. calibrated von frey filaments were applied to the identified receptive field areas as a tactile stimulation and the minimum filament that elicited an ap in the soma was recorded. the mechanical thresholds with von frey filaments of all aβ drg neurons during electrophysiology recording are shown in figure the mechanical thresholds of gf, ra, sa, ms and aβ htm neurons in control rats were within the range - g, - g, - g, - g and 4- g, respectively. in neuropathic rats, the thresholds of gf, ra neurons were the same as in control animals. however, in neuropathic animals the thresholds of sa, ms and htm neurons were shifted lower, to - g, - g and 2- g, respectively. the mean mechanical threshold of the ms neurons was significantly lower in neuropathic rats than in control rats: ± g in control vs. ± g in neuropathic neurons. the other subtypes of aβ-fiber neurons showed no significant difference. thresholds of hair neurons were ± g in control and ± g in neuropathic neurons. those in ra neurons were- ± g vs. ± g in control and neuropathic animals, respectively. those in sa neurons were ± g in control animals and ± g in neuropathic animals. those in htm neurons in control animals were ± g vs. ± g in neuropathic animals. some neurons in neuropathic rats exhibited ongoing discharge consisting of fast excitatory aps. for example, c-fiber cool neurons usually discharge at room temperature and ms neurons can discharge when the leg is fixed in an extended position in both control and neuropathic rats. in the present study, ongoing discharge was recorded from c-fiber cool neurons and aβ-ms neurons in both control and neuropathic rats. as this study focused on differences in responses evoked by applied stimuli between control and model animals these neurons were excluded from the present analysis. in neuropathic rats, of aβ htm, of hair aβ ltm neurons demonstrated certain electrical signs that are reasonably interpreted as an increase in excitability. an increased number of action potentials could be evoked by stimulation of the receptive field, and the discharge persisted for several minutes after removal of the stimulus from the receptive field. it is important to note that in control animals none of the aβ htm or of the aβ ltm neurons demonstrated any prolonged discharge following application of peripheral stimuli. excitability of the soma measured by responses to injection of depolarizing current the ap responses to intracellular depolarizing current pulse injection were tested to determine whether there is a difference in soma excitability induced by the peripheral neuropathy. figure illustrates the threshold currents that elicited aps in control and neuropathic animals. compared to the control group, aβ-fiber htm neurons and aβ-fiber ltm ms neurons in neuropathic animals showed a significant difference from controls; activation threshold of htm neurons was ± na vs. ± na in control and neuropathic rats, respectively. activation threshold of ms neurons was ± na in control and ± g in neuropathic rats. there was no significant difference in aβ-fiber ltm cutaneous neurons in control vs. model animals. the effects of peripheral neuropathy on repetitive discharge were analyzed quantitatively by frequency-current analysis. figure 5a shows the number of aps elicited with different current strengths, comparing control vs. neuropathic rats. compared to the control group, aβ-fiber htm neurons and aβ-fiber ltm ms neurons in the neuropathic rats showed a significant difference, while aβ-fiber ltm cutaneous neurons showed no difference. subthreshold oscillations and bursting patterns of discharge, which have been implicated in generating ectopic discharge in an axotomy model of neuropathic pain, were not observed in this intact model. for aβ htm neurons, the results were based on neurons in control rats and neurons in neuropathic rats. with a na, ms current injection, the number of aps in control rats was ± and in neuropathic rats it was ±. with a na, ms current injection, the number of aps in control rats was ± while neuropathic rats this was ±. with a na, ms current injection, the number of aps in control rats was ± while neuropathic rats it was ±. for ms neurons, the results were based on neurons in control rats and neurons in neuropathic rats. with a na, ms current injection, the number of aps in control rats was ± while in neuropathic rats it was ±. with a na, ms current injection, the number of aps in control rats was ± and ± in neuropathic rats. with a na, ms current injection, the number of aps in control rats was ± while it was ± in neuropathic rats. for aβ-fiber ltm cutaneous neurons, the results were based on neurons in control animals and in neuropathic animals. with the na, ms current injection, the number of spikes in control rats was ± and in neuropathic rats it was ±. with the na, ms current injection, the number of spikes in control rats was ± and ± in neuropathic rats. with the na, ms current injection, the number of aps in control rats was ± and neuropathic rats it was ±. excitability of the axon measured by responses to dorsal root stimulation the responses to axonal stimulation by delivering single current pulses of ma to the dorsal root were examined. dorsal root excitability was determined as the chronaxie curve, which was derived by determining the minimum current applied to the dorsal root that just evoked a soma ap with pulse durations of ms, ms, ms, ms and ms. the data are shown in figure ms neurons showed significantly lower current intensity threshold with ms stimulation. to investigate the fast process of recovery from an ap in each subtype of afferent neuron, paired-pulse stimuli were delivered to the dorsal root. the ri of aβ neurons showed a significant difference between neuropathic and control rats. figure 7a displays the ri distributions for individual neurons in each aβ neuron subtype in control and neuropathic rats. in control rats, aβ-fiber htm neurons had the longest ri. ms neurons had the shortest ri. other types of aβ ltm neurons had an intermediate ri. in neuropathic animals, under the same stimulation conditions, it was found that ri in all aβ neurons was significantly greater compared with neurons in control animals. ri in ms neurons in neuropathic rats was ± ms, n =. ri in hair neurons was ± ms n =. aβ htm neurons also showed significantly greater ri ± ms, n =. discussion general characteristics of neuropathic afferent neurons sensitization is often considered to play a major role in the neural basis of hyperalgesia and allodynia associated with peripheral neuropathy. the present study determined the excitability and patterns of discharge in functionally defined drg neurons, including electrophysiological properties under different conditions of neuronal activation. in addition, some neurons were provoked to display a discharge, including otherwise quiescent neurons. a systematic correlation of neuronal type with expression of discharge indicated that ab neurons, including ltm and htm neurons, exhibited increased activity in neuropathic rats, whereas these neurons did not display such activity in control rats. this principle conclusion is consistent with our previous studies on this model. these data are also consistent with previous reports on neuropathic rat models. for example, after peripheral nerve injury both axotomized as well as intact afferent neurons supplying skeletal muscle but not skin afferents generate ongoing activity within the drg. it has also been shown that after nerve injury sensitization of primary sensory neurons is characterized not only by abnormal discharges but also by having a lowered activation threshold and by exhibiting exaggerated responses to various stimuli. here we report several signs of altered excitability, specifically of ab drg neurons. for example, persisting activity was generated specifically in both aβ htm and aβ ltm neurons. further, a reduction was also observed in the mechanical response threshold of some subtypes of aβ-fiber neuron in the neuropathic rats when the distribution of mechanical thresholds receptive fields of the individual neurons was compared. this suggests a change in excitability of peripheral terminal receptors. intracellular current injection into the soma of ab drg neurons generated significantly more aps in neuropathic rats than in controls, which suggests that there is a change in excitability of ab drg neuron somata as well. while the difference in the soma ap response in ab drg neurons between control and neuropathic rats suggests that a constitutively enhanced hyperexcitability occurs in these neurons, it is important to point out that in contrast there was an increased current threshold for activation of dorsal roots as well as a delay in the latency of the second ap in response to paired-pulse dorsal root stimulation at shorter inter-pulse intervals. this suggests an attenuated excitability of aβ axons in the dorsal root. this appears to be a novel finding. to our knowledge previous studies have not compared changes in excitability in soma, axon and peripheral receptor in this or in other animal models of chronic pain. possible mechanisms underlying changes in excitability in different parts of aβ drg neurons the cellular mechanisms underlying generation of abnormal discharge and the changes in activation threshold of the soma and peripheral sensory receptor in aβ drg neurons are not clear. it is generally presumed that altered discharge results from the classical hodgkin-huxley repetitive firing processes. the ability of theses neurons to sustain repetitive discharge depends on all three major neuronal ion channels, na+, ca2+ and k+. alterations in the level of expression, cellular localization, distribution and activation kinetics of each of these channel types might be involved in determination of these changes in excitability. thus, they may have contributed to the difference in excitability between soma, axon and peripheral receptor. it is also worth considering how injury to the axon may lead to changes in excitability in the somata and the peripheral endings. while no clear answers immediately come to mind, some studies on models of peripheral neuropathy might offer some insight. for example, walters and ambron hypothesized that axonal injury might unmask nuclear localization signals in certain axoplasm proteins at the injured site. it was suggested that this may cause the transport of cytokines, nerve growth or glial cell line-derived neurotrophic factor, tumor necrosis factor or other inflammatory mediators released by immune cells and schwann cells, to be transported retrogradely to the somata and activate transcription factors, which then induce the alternation in expression of neuropeptides, receptors and ion channels in the somata. this in turn may induce increased excitability in the drg somata. in fact, a previous study provided evidence that drg somata exhibit de novo expression of ttx-sensitive type iii na+ channels, a reduction of ttx-resistant na+ current, and a change in k+ and ca+ currents in different neuropathic animal models. the mechanisms underlying the changes in the activation threshold of the peripheral terminals might be related also to alterations in the release of cytokines and growth factors. previous studies have shown that tnf-α is unregulated during wallerian degeneration and that exogenous administration of tnf-α produces a direct activation and sensitization of injured afferent fibers. when exposed to ngf sensory neurons undergo an increase in either the firing of aps evoked by a ramp of the depolarizing current or in currents evoked by the application of capsaicin. there is no clear explanation for the changes in excitability of the ms neurons. muscle spindles are the sensory endings primarily responsible for initiation of proprioception. these receptors can be exquisitely sensitive, responding to light tapping, vibration or pressure applied to the skin overlying the muscle spindle within the muscle belly. however, unless there are adequate changes in muscle length, the muscle spindle will not be excited. in the present study we observed a significantly decreased mechanical threshold of ms neurons. however, the mechanisms underlying this change in excitability remain to be determined. although there was no direct evidence to explain the attenuated excitability of dorsal root axons in response to direct electrical stimulation, previous studies have shown that the behavioral and cellular effects of nerve injury to the central branches, such as partial dorsal rhizotomy, are different from those induced by injury to the peripheral branches, such as chronic constriction injury of the sciatic nerve. for example, sheen et al. failed to observe a neuropathic pain behavior after a dorsal rhizotomy, while song et al. found that partial dorsal rhizotomy produced significantly less severe hyperalgesia. these studies suggest that the dorsal root might generate different injury signals and might transport such signals to the spinal cord rather than to drg somata. based on our results, we further suggest that the attenuated excitability of dorsal root axons might act as a "negative injury signal", while the increased excitability of somata and peripheral receptors might act as a "positive injury signal" to the central nervous system in this model of peripheral neuropathic pain. future studies are necessary to test this suggestion. consequences of activity of aβ neurons in this model, a significant correlation was found between several variables of discharge and pain behaviors. the "positive" correlation along with the "negative" correlation, as shown in this study, suggest that altered excitability may be important for maintaining neuropathic pain behavior. although not each nerve injury necessarily leads to pain, our previous studies in this model have shown that light tactile stimulation induces brisk paw withdrawal. further, innocuous touch stimulation of the cutaneous receptive field induces an excessively prolonged afterdischarge in spinal wide dynamic range neurons and this unusually prolonged afterdischarge of dorsal horn neurons is reduced by peripheral nerve block. this present study has shown that ab drg neurons exhibit increased discharge and excitability, which includes a prolonged discharge firing pattern in response to direct soma excitation and receptive field stimulation. thus, it seems reasonable to suggest that the prolonged nature of the afterdischarge in ab neurons in this model of peripheral neuropathy may lead to greater peripheral drive and that mechanical allodynia, and perhaps also the burning pain, associated with peripheral neuropathy may be due to this increased activity in ab neurons delivering excitation to nociceptive neurons in the spinal cord. thus, based on previous evidence from other laboratories as well as our findings on functionally-identified drg neurons in this intact nerve model of peripheral neuropathy, we propose that the hyperalgesia and dysesthesia experienced by people with peripheral neuropathy might be at least partially due to sustained discharge of ab primary afferents in response to light tactile or noxious stimulation of the skin. we further postulate that the altered excitability of ab neurons that develops in neuropathic rat models, as cited above, may be a critical factor in signaling mechanical allodynia and may contribute to allodynia and spontaneous pain following peripheral nerve injury in humans. overall, our data provide specific evidence for a major contribution of altered excitability of ab neurons deriving from multiple regions of these neurons, including the receptive field and the somata, possibly modified by altered excitability of dorsal root axons, resulting in altered spinal nociceptive mechanisms in this animal model of painful peripheral neuropathy.  this study has demonstrated changes in excitability of ab htm and ltm primary sensory neurons in the nerve-intact cuff model of peripheral neuropathic pain and that different sites of these sensory neurons exhibit different changes in excitability after sciatic nerve injury. these differences consisted of a reduced mechanical activation threshold and abnormal prolonged discharges following receptive field stimulation, a reduced current activation threshold of the cell body and a greater number of aps evoked by intracellular current injection into the soma, yet an increased current activation threshold and delayed recovery kinetics in dorsal root axons. this study provides new evidence to support a role for injury-induced plasticity of primary sensory neurons in this model of neuropathic pain and also indicates that changes to different regions of the sensory neurons might have different effects on peripheral drive to central mechanisms and thus possibly also hyperalgesia or allodynia. 
thus, the present study aimed at recording intracellularly from aβ-fiber dorsal root ganglion neurons and determining excitability of the peripheral receptive field, of the cell body and of the dorsal roots. in addition, conduction velocity of the dorsal roots, configuration of the action potential and rate of adaptation to stimulation were also criteria for classification. excitability was measured as the threshold to activation of the peripheral receptive field, the response to intracellular injection of depolarizing current into the soma and the response to electrical stimulation of the dorsal roots. aβ drg neurons in neuropathic rats demonstrated a mean mechanical threshold to receptive field stimulation that were significantly lower than in control rats, a prolonged discharge following this stimulation, a decreased activation threshold and a greater response to depolarizing current injection into the soma, as well as a longer refractory interval and delayed response to paired pulse electrical stimulation of the dorsal roots. however, the dorsal roots exhibit a decrease in excitability. in control animals mechanical thresholds of all neurons were within normal ranges. thus, if these neurons participate in neuropathic pain this differential change in excitability may have implications in the peripheral drive that induces central sensitization, at least in animal models of peripheral neuropathic pain, and aβ sensory neurons may thus contribute to allodynia and spontaneous pain following peripheral nerve injury in humans. a peripheral neuropathy was induced in sprague dawley rats by inserting two thin polyethylene cuffs around the right sciatic nerve. the present study has demonstrated changes in functionally classified aβ low threshold and high threshold drg neurons in a nerve intact animal model of peripheral neuropathy that demonstrates nociceptive responses to normally innocuous cutaneous stimuli, much the same as is observed in humans with neuropathic pain. animal models have provided some directions, but little is known about the specific sensory neurons that undergo changes in such a way as to induce and maintain activation of sensory pain pathways. our previous studies implicated changes in the aβ, normally non-nociceptive neurons in activating spinal nociceptive neurons in a cuff-induced animal model of neuropathic pain and the present study was directed specifically at determining any change in excitability of these neurons. causes of neuropathic pain following nerve injury remain unclear, limiting the development of mechanism-based therapeutic approaches. neuropathic painprimary afferent neuronhyperexcitabilityectopic dischargemuscle spindle neurondorsal root ganglion we demonstrate further that the peripheral receptive fields of these neurons are more excitable, as are the somata. all animals were confirmed to exhibit tactile hypersensitivity to von frey filaments three weeks later, before the acute electrophysiological experiments. under stable intracellular recording conditions neurons were classified functionally on the basis of their response to natural activation of their peripheral receptive field.
 causes of neuropathic pain following nerve injury remain unclear, limiting the development of mechanism-based therapeutic approaches. animal models have provided some directions, but little is known about the specific sensory neurons that undergo changes in such a way as to induce and maintain activation of sensory pain pathways. our previous studies implicated changes in the aβ, normally non-nociceptive neurons in activating spinal nociceptive neurons in a cuff-induced animal model of neuropathic pain and the present study was directed specifically at determining any change in excitability of these neurons. thus, the present study aimed at recording intracellularly from aβ-fiber dorsal root ganglion neurons and determining excitability of the peripheral receptive field, of the cell body and of the dorsal roots. methods a peripheral neuropathy was induced in sprague dawley rats by inserting two thin polyethylene cuffs around the right sciatic nerve. all animals were confirmed to exhibit tactile hypersensitivity to von frey filaments three weeks later, before the acute electrophysiological experiments. under stable intracellular recording conditions neurons were classified functionally on the basis of their response to natural activation of their peripheral receptive field. in addition, conduction velocity of the dorsal roots, configuration of the action potential and rate of adaptation to stimulation were also criteria for classification. excitability was measured as the threshold to activation of the peripheral receptive field, the response to intracellular injection of depolarizing current into the soma and the response to electrical stimulation of the dorsal roots.  in control animals mechanical thresholds of all neurons were within normal ranges. aβ drg neurons in neuropathic rats demonstrated a mean mechanical threshold to receptive field stimulation that were significantly lower than in control rats, a prolonged discharge following this stimulation, a decreased activation threshold and a greater response to depolarizing current injection into the soma, as well as a longer refractory interval and delayed response to paired pulse electrical stimulation of the dorsal roots.  the present study has demonstrated changes in functionally classified aβ low threshold and high threshold drg neurons in a nerve intact animal model of peripheral neuropathy that demonstrates nociceptive responses to normally innocuous cutaneous stimuli, much the same as is observed in humans with neuropathic pain. we demonstrate further that the peripheral receptive fields of these neurons are more excitable, as are the somata. however, the dorsal roots exhibit a decrease in excitability. thus, if these neurons participate in neuropathic pain this differential change in excitability may have implications in the peripheral drive that induces central sensitization, at least in animal models of peripheral neuropathic pain, and aβ sensory neurons may thus contribute to allodynia and spontaneous pain following peripheral nerve injury in humans. neuropathic painprimary afferent neuronhyperexcitabilityectopic dischargemuscle spindle neurondorsal root ganglion 