Synaptic Evoked Potentials from Regenerating Dorsal Root Axons within Fetal Spinal Cord Tissue Transplants

Previously injured dorsal roots were electrically stimulated to determine if regenerating sensory axons can form physiologically active synaptic contacts with neurons within fetal spinal cord tissue transplants. Dorsal rootlets, sectioned at their spinal cord entry zone, were apposed to intraspinal transplants of fetal spinal cord tissue grafted along each side of a nerve growth factor-treated nitrocellulose implant. Two to six months later, the rootlets were transected between the spinal cord and their respective ganglia and electrically stimulated. Evoked potentials were recorded from the dorsal surface of the transplant, but were absent from adjacent ipsilateral and contralateral spinal cord regions. A glass micropipette was advanced through the transplant and used to record intramedullary field potentials evoked by dorsal root stimulation. Maximal negative potentials occurred 400–700 μm below the dorsal surface of the transplant, shifting to positive potentials deeper into the transplant. Additionally, both spontaneous and electrically evoked single neuronal action potentials were observed along the microelectrode track. Evoked potentials were abolished following transection of the rootlets between the stimulation site and the transplant. Immunocytochemical evidence of the production of fos protein following electrical stimulation of the regenerated dorsal rootlets was demonstrated within transplant neurons and some ventrally located host neurons, providing an anatomical correlate to the electrophysiological recordings of synaptic activation. These results provide evidence of the structural and functional integration of regenerated sensory axons with both transplant and host neurons.     

Presynaptic modulation of synaptic effectiveness of afferent and ventrolateral tract fibers in the frog spinal cord

In the isolated frog spinal cord, antidromic stimulation of motor nerves produces intraspinal field potentials with a characteristic spatial distribution. When recording from the ventral horn, there is a short latency (1–2 msec) response corresponding to activity generated by antidromic activation of motoneuron cell bodies and proximal dendrites. In addition, in the dorsal horn, a delayed wave (12–13 msec latency) corresponding in time with the negative dorsal root potential is also recorded. This wave (VR-SFP) is positive at the dorsal surface of the cord and inverts to negativity at more ventral regions. The negative VR-SFP is maximum between 300–500 μm depth from the dorsal surface and decays with increasing depth towards the motor nucleus. Six days after chronic section of the dorsal roots L7 to L9 in one side of the spinal cord, stimulation of the motor nerves on the deafferented side produces only the early response attributable to antidromic activation of motoneurons. No distinctive VR-SFPs are recorded at any depth within the cord. These findings are consistent with the interpretation that afferent fiber terminals are the current generators of the VR-SFP. The presynaptic and postsynaptic focal potentials recorded in the motor nucleus after stimulation of the ventrolateral tract, as well as the corresponding synaptic potentials electrotonically recorded from the ventral roots, are not depressed during conditioning stimulations which produce primary afferent depolarization. This contrasts with the depression of the presynaptic and post-synaptic focal potentials and synaptic potentials produced by stimulation of sensory fibers. It is concluded that, unlike the afferent fiber terminals, the terminals of the ventrolateral tract are not subjected to a presynaptic modulation of the type involving primary afferent depolarization.     

Do interneurones in lower lumbar segments contribute to the presynaptic depolarization of group I muscle afferents in Clarke's column?

Intersegmentally evoked primary afferent depolarization (PAD) was analysed to investigate whether any lower lumbar propriospinal neurones are involved in mediating PAD from group I afferents both in the same segments and in Clarke's column.The intersegmental PAD of lower lumbar afferents, as judged by recording dorsal root potentials, was evoked by stimuli applied in the grey matter of L3 and L4 segments. With intraspinal stimuli of 10 μA or less PAD was evoked from two foci: from within the middle part of the dorsal columns and from the dorsal part of the dorsal horn. Dorsal root potentials evoked from the dorsal horn focus appeared with longer latencies. When the dorsal columns were transected PAD was evoked only from the dorsal horn focus. No PAD appeared upon stimulation of Clarke's column after transection of the dorsal column even with stronger (20 μA) stimuli.Interactions between the actions of the intraspinal stimuli and of different groups of afferents were analysed to define the neuronal pathways via which the intersegmental PAD was evoked. Neurones located within both the lower and the upper lumbar segments were found to be involved. Indications have only been found for a contribution of neurones mediating PAD from afferents other than group I afferents.Lesions of the ipsilateral and contralateral, lateral and ventral funiculi (in addition to the dorsal columns) were made in order to define which of these funiculi are required for the appearance of the intersegmental PAD. The intersegmental PAD could evoked from the dorsal horn when either the contralateral or the ipsilateral funiculi were left intact.     

Hindlimb movement in the cat induced by amplitude-modulated stimulation using extra-spinal electrodes

Hindlimb movement in the cat induced by electrical stimulation with an amplitude-modulated waveform of the dorsal surface of the L5-S1 spinal cord or the L5-S1 dorsal/ventral roots was investigated before and after acute spinal cord transection at the T13-L1 level. Stimulation of the spinal cord or dorsal/ventral root at the same spinal segment induced similar movements including coordinated multi-joint flexion or extension. The induced movements changed from flexion to extension when the stimulation was moved from rostral (L5) to caudal (S1) spinal segments. Stimulation of a dorsal or ventral root on one side induced only ipsilateral hindlimb movement. However, stimulation on the dorsal surface of the spinal cord along the midline or across the spinal cord induced bilateral movements. The extension induced by stimulation of L7 dorsal root produced the largest ground reaction force that was strong enough to support body weight. Dorsal root stimulation induced a larger ground reaction force than ventral root stimulation and produced a more graded recruitment curve. Stepping at different speeds could be generated by combined stimulation of the rostral (L5) and the caudal (L6/L7) spinal segments with an appropriate timing between the different stimulation channels. Acute transection of the spinal cord did not change the responses indicating that the induced movements did not require the involvement of the supraspinal locomotor centers. The methods and the stimulation strategy developed in this study might be utilized to restore locomotor function after spinal cord injury.     

Ventral root potentials of the cat's sacrococcygeal segments evoked by stimulation of intact and transected dorsal roots

The latency and amplitude of reflex-evoked potentials in the sacrococcygeal ventral roots of acute spinalized cats were investigated. The characteristics of the potentials were examined in response to electrical stimulation of intact and acutely transected dorsal roots. We found that: the last sacral and caudal (coccygeal) segments of the cat's spinal cord are endowed with electrophysiologic characteristics that distinguish them from other spinal segments (e.g., L7-S1); afferent stimulation of the corresponding intact dorsal roots evokes in the ventral root of segment S2 a small monosynaptic response, whereas no monosynaptic response is seen in segment Ca6; acute transection of the dorsal roots provokes an increment of the monosynaptic response in all segments studied except for Ca6; rhizotomy provokes in Ca5 the appearance of polysynaptic responses to electrical stimulation of the corresponding dorsal root; and transection of the cutaneous afferent fibers of the coccygeal motoneurons resulted in an increment of monosynaptic and polysynaptic responses, indicating the removal of inhibitory effects.     

Evidence for invasion of regenerated ventral root afferents into the spinal cord of the rat subjected to sciatic neurectomy during the neonatal period

Sectioning the sciatic nerve of experimental animals at the neonatal stage triggers growth of afferent fibers in the ventral root. The present study examined the possibility that the regenerating fiber terminals grow into the spinal cord. The sciatic nerve on one side was cut in neonatal rats. After the rats were fully grown, either an electrophysiological or a histochemical study was performed. The results of electrophysiological experiments showed that stimulation of certain loci in the L5 spinal cord evoked antidromic potentials in the L5 ventral root with a long latency. Various evidence suggests that the long latency potentials are due to activation of C fibers. These C-fiber potentials were on average bigger and were elicited from more numerous loci on the side ipsilateral to the sciatic nerve lesion than on the contralateral side. Furthermore, stimulation of the spinal cord of unoperated normal rats rarely evoked such potentials. For the histochemical study, horseradish peroxidase (HRP) was injected into the L5 spinal cord after cutting the L4-L6 dorsal roots. A lot more cells in the L5 dorsal root ganglion (DRG) on the side ipsilateral to the sciatic nerve lesion were labeled with HRP transported retrogradely through the L5 ventral root than on the contralateral side. Control experiments showed that few DRG cells are labeled with HRP in normal unoperated rats. The combined results of the electrophysiological and histochemical studies suggest invasion of ventral root afferents into the spinal cord, given enough postoperative time. It is not known whether or not these terminals make functional synaptic contacts in the spinal cord.     

Electrophysiological investigation of IDPN neuropathy--initial studies

beta,beta'-iminodipropionitrile (IDPN) causes giant axon swellings in proximal internodes of spinal motor axons and in stem processes of dorsal root ganglia neurons. The electrophysiological consequences of the swellings were investigated in cats given IDPN (50 mg/kg i.p.) weekly for 0 (control), 1 (7 days), 2 (14 days), or 5 (35 days) weeks; some additional animals were studied 15 (50 days) or 65 (100 days) days following the fifth injection. Extracellular recordings of the spinal monosynaptic reflex revealed amplitudes of efferent responses to be equally reduced at 35, 50 and 100 days. Single stimulation of dorsal roots, or soleus or medial gastrocnemius afferents often evoked doublet efferent responses which arose intraspinally. Similar repetitive responses were observed in dorsal root (afferent) input upon single stimulation of peripheral nerves. Latencies to monosynaptic responses became progressively prolonged over 100 days; spinal cord contribution to the increased latencies was maximal by 35 days. Conduction velocities in single soleus and medial gastrocnemius axons declined to 70 and 64% of normal by 50 days and to 56 and 50% of normal by 100 days respectively. Maximal recurrent inhibition was reduced 42-49% by 35 days and quantitatively similar at 50 and 100 days. Intracellular recordings from spinal motoneurons revealed that, at 35 days, action potentials could be elicited by orthodromic but not by antidromic stimulation in 14% of cells tested. This was taken as evidence of block of impulse conduction by the intervening axonal swelling upon antidromic stimulation. The possibility of abnormal electrical interactions between swollen and demyelinated intraparenchymal axons was tested by determining the number of ventral root fibers via which antidromic stimulation would elicit an action potential in the same motoneuron perikaryon. Electrical crosstalk, never seen in normal animals, varied in incidence during the evolution of the neuropathy (7-35 days), reaching a maximum of 30% at 14 days. Many motoneuron action potentials were remarkably similar to those observed in chromatolytic motoneurons, exhibiting, among other features, decreased SD thresholds and IS conduction times, reduced amplitudes and durations of afterhyperpolarization, delayed depolarizations and an enhanced propensity to fire repetitively upon single stimulation. There was no morphological evidence of chromatolysis in the motoneurons. The concept of "functional axotomy" is introduced to accommodate these findings and discussed in terms of altered dendritic excitability. Not all motoneuron types are equally compromised in the neuropathy.(ABSTRACT TRUNCATED AT 400 WORDS)     

Primary motor neurons fail to up-regulate voltage-gated sodium channel Na(v)1.3/brain type III following axotomy resulting from spinal cord injury

Epilepsy occurs in a small proportion of patients with spinal cord injury (SCI), but whether it is due to concomitant traumatic head injury or to changes in cortical motor neurons secondary to axotomy within the spinal cord is not known. Na(v)1.3/brain type III sodium channel expression is up-regulated following peripheral axotomy of dorsal root ganglion (DRG) and facial motor neurons, but, to date, Na(v)1.3 expression has not been examined in upper (cortical) motor neurons following axotomy associated with SCI. In the present study, we examine Na(v)1.3 expression in upper motor neurons within rat primary motor cortex following midthoracic (T9) dorsal column transection, which severs the axons of those cells. Axotomized pyramidal cells were identified by retrograde transport of fluorogold. Immunolabeled cells were confined to layer V of the primary motor cortex and exhibited low levels of Na(v)1.3 staining. After axotomy, no significant changes were detected in Na(v)1.3 density or distribution in injured or uninjured cells, compared with control brains, in contrast to up-regulation of Na(v)1.3 in ipsilateral DRG neurons after sciatic nerve transection. These results do not preclude a role for voltage-gated sodium channels in post-SCI epilepsy but suggest that up-regulated expression of Na(v)1.3 channel is not involved.     

The Effect of Site and Type of Nerve Injury on Spinal Glial Activation and Neuropathic Pain Behavior

A number of rat peripheral neuropathy models have been developed to simulate human neuropathic pain conditions. The current study sought to determine the relative importance of site versus type of peripheral nerve injury in eliciting mechanical allodynia and spinal glial responses. Rats received one of seven different surgical treatments at the L5 spinal level: spinal nerve cryoneurolysis, spinal nerve tight ligation, dorsal root cryoneurolysis, dorsal root tight ligation, dorsal root transection, ventral root tight ligation, or laminectomy/dural incision sham. Foot-lift response frequency to mechanical stimulation of the ipsilateral hindpaw was assessed postlesion on days 1, 3, 5, and 7. L5 spinal cords were retrieved for immunohistochemical analysis of microglial (OX-42) and astrocytic (anti-glial fibrillary acidic protein) responses. Both types of spinal nerve lesion, freeze and tight ligation, produced rapid and profound mechanical allodynia with intense glial responses. Dorsal root lesions also resulted in intense mechanical allodynia; however, glial responses were almost exclusively astrocytic. Ventral root tight ligation and sham provoked no marked behavioral changes and only sporadic glial responses. Direct dorsal horn communication with the dorsal root ganglion was not a crucial factor in the development of mechanical allodynia, since decentralization of the L5 DRG by complete L5 dorsal root lesion produced profound mechanical sensitization. Conversely, microglial activation responses appear to be dependent upon dorsal root ganglion-mediated signals and, contrary to behavioral responses, were robust only when the lesion was made peripheral to the cell body. Astrocytic activation was always observed following axonal injury and reliably coexisted with behavioral responses.     

Increased spike-frequency adaptation and tea sensitivity in dorsal root fibers after sciatic nerve injury

Excitability of rat dorsal root axons were studied 3 weeks after injuryto the sciatic nerve. Whole nerve recordings were obtained from injured andcontrol nerves in a sucrose gap chamber. Constant current depolarization pulses (30–200 ms) applied approximately 50% above the stimulus strength required for maximal amplitude compound action potentials (CAPs) evoked burst of action potentials in the dorsal root which displayed spike adaptation. The depolarization-induced burst response of the dorsal roots was greatly reduced after crush or transection of the sciatic nerve. However, application of the potassium channel blocker, tetraethylammonium (TEA), restored the burst discharge in injured dorsal root axons. Brief tetanic stimulation of the dorsal root also induced an afterhyperpolarization (AHP) that was twice as large in the transection group as compared to the control group, and which was blocked by TEA. There were no changes seen in the amplitude of the compound action potential, frequency-following characteristics, refractory properties, or 4-AP sensitivity in the dorsal roots after peripheral nerve injury. These results suggest that there is enhanced spike adaptation that occurs at the same time as an increase in the sensitivity to the potassium channel blocker, TEA, in axon regions proximal to the site of nerve injury and have implications for the pathophysiology of nerve injury. © 1993 John Wiley & Sons, Inc.     

Effects of a nigral descending pathway on cervical spinal cord afferent fibers and interneurons

Recent neuroanatomical studies have demonstrated caudally directed projections from the pars reticulata region of the substantia nigra (SN_pr) to the tectum and tegmentum. It is known that one of the most predominant effects of basal ganglia activation in the rat is contralateral head turning behavior. Therefore, this study examined the potential effect of such a descending pathway on the cervical spinal cord as a possible substrate for the head turning behavior. Stimulation of the SN_pr evoked a cervical dorsal root potential as well as decreased the amplitude of cervical-evoked dorsal root potentials. This latter decrease in amplitude was suggested to arise from occlusion of the supraspinal-evoked response and the cervical response. Twenty percent of cervical spinal cord interneurons activated by dorsal root stimulation were found to be inhibited by nigral stimulation. Thus long-lasting synaptic depression in some neuronal elements may occur in both the conditioned and test dorsal root potential. These results suggest that nigral descending fibers emanating from the pars reticulata may possibly play a major role in the characteristic head turning behavior.     

Olfactory ensheathing cell grafts have minimal influence on regeneration at the dorsal root entry zone following rhizotomy

The effectiveness of grafts of olfactory ensheathing cells (OECs) as a means of promoting functional reconnection of regenerating primary afferent fibers was investigated following dorsal root injury. Adult rats were subjected to dorsal root section and reanastomosis and at the same operation a suspension of purified OECs was injected at the dorsal root entry zone and/or into the sectioned dorsal root. Regeneration of dorsal root fibers was then assessed after a survival period ranging from 1 to 6 months. In 11 animals, electrophysiology was used to look for evidence of functional reconnection of regenerating dorsal root fibers. However, electrical stimulation of lesioned dorsal roots failed to evoke detectable cord dorsum or field potentials within the spinal cord of any of the animals examined, indicating that reconnection of regenerating fibers with spinal cord neurones had not occurred. In a further 11 rats, immunocytochemical labeling and biotin dextran tracing of afferent fibers in the lesioned roots was used to determine whether regenerating fibers were able to grow into the spinal cord in the presence of an OEC graft. Although a few afferent fibers could be seen to extend for a limited distance into the spinal cord, similar minimal in-growth was seen in control animals that had not been injected with OECs. We therefore conclude that OEC grafts are of little or no advantage in promoting the in-growth of regenerating afferent fibers at the dorsal root entry zone following rhizotomy.     

Sensory interactions with a central motor program in anuran larvae

Swimming in anuran larvae is directed by a central motor program that is modulated by spinal sensory input. Dorsal root stimulation activates the locomotor program in vitro and can produce either an increase or a decrease in the rate of ongoing fictive swimming. Records obtained from the distal stumps of cut dorsal roots during passive tail bending show that receptors respond to tail movement but not to tail position. During episodes of fictive swimming, primary afferent terminals are depolarized and their sensitivity to antidromic stimulation increased, indicating that the motor program exerts presynaptic inhibitory control over spinal sensory transmission. These results suggest that the central program is very sensitive to dorsal root inputs and modulates these inputs during swimming.     

Evidence of facilitatory coerulospinal action in lumbar motoneurons of cats

Functional connectivity of the feline coerulospinal projection was delineated by utilizing the combined approaches of antidromic activation and electrical stimulation. We isolated 25 locus coeruleus (LC) neurons that were electrophysiologically identified and histologically verified and that could be driven by stimulating the spinal cord. Antidromicity of the spike potentials was confirmed by the constant latency, the high frequency (100 Hz) following, fractionation of the initial segment-somatodendritic potential, and collision between the antidromic and the spontaneous orthodromic spikes. The mean conduction speed was20 ± 8m/sec(range= 7to32m/sec). Intracellular studies revealed facilitatory LC actions in 22 lumbar motoneurons (MNs), In 13 MNs, LC activation alone produced slow-rising excitatory postsynaptic potentials (EPSPs) of3 ± 1mV amplitude that lasted 4–30 msec. Six of the 13 MNs discharged action potentials upon LC stimulation. In the remaining 9 MNs, no observable potential change was registered after LC activation. Antecedent LC stimulation consistently potentiated the synaptic efficacy of testing dorsal root shocks. The enhancement of synaptic activation was antagonized by systemic injection of phenoxybenzamine (3 mg/kg). These results suggest that facilitation of MNs by the LC is at least in part mediated by distal dendritic depolarization. Those MNs that exhibited augmented excitability but no demonstrable EPSPs may have been activated by norepinephrine-mediated synaptic modulation.     

Tetrodotoxin-resistant sodium channels Na(v)1.8/SNS and Na(v)1.9/NaN in afferent neurons innervating urinary bladder in control and spinal cord injured rats

Tetrodotoxin-resistant (TTX-R) sodium channels Na(v)1.8/SNS and Na(v)1.9/NaN are preferentially expressed in small diameter dorsal root ganglia (DRG) neurons. The urinary bladder is innervated by small afferent neurons from L6/S1 DRG, of which approximately 75% exhibit high-threshold action potentials that are mediated by TTX-R sodium channels. Following transection of the spinal cord at T8, the bladder becomes areflexic and then gradually hyper-reflexic, and there is an attenuation of the TTX-R sodium currents in bladder afferent neurons. In the present study, we demonstrate that Na(v)1.8 is expressed in both bladder and non-bladder afferent neurons, while Na(v)1.9 is expressed in non-bladder afferent neurons but is rarely observed in bladder afferent neurons. In spinal cord transected rats 28-32 days following transection, there is a decreased expression of Na(v)1.8 sodium channels in bladder afferents, but no change in the expression of Na(v)1.8 in non-bladder afferent neurons. Both bladder and non-bladder afferent neurons exhibit limited increases in Na(v)1.9 expression following spinal cord transection. These results demonstrate that the expression of TTX-R channels in bladder afferent neurons changes after spinal cord transection, and these changes may contribute to the increased excitability of these neurons following spinal cord injury.     

Responses of spinal neurones to cutaneous and dorsal root stimuli in rats with mechanical allodynia after contusive spinal cord injury

The firing of neurones in spinal segments adjacent to a contusive T13 spinal cord injury was characterised in anaesthetised rats. Three groups of rats were examined: (1) allodynic spinally injured, (2) non-allodynic spinally injured and (3) normal, uninjured. Spinal cord field potentials evoked by electrical dorsal root stimulation and the responses of 207 dorsal horn neurones to mechanical stimuli applied to the skin were studied. Within the lesioned spinal segment few active neurones were encountered and field potentials were absent. Depolarising field potentials recorded rostral to the lesion were reduced in both allodynic and non-allodynic animals compared to uninjured controls, while those recorded in caudal segments were enhanced in allodynic animals. Neuronal recordings revealed that allodynia was associated with exaggerated responses, including afterdischarges, to innocuous and noxious mechanical stimuli in a proportion of wide dynamic range, but not low threshold, neurones. These changes were observed both rostral and caudal to the site of injury. The results suggest that an increased responsiveness of some dorsal horn neurones in segments neighbouring a contusive spinal cord injury may contribute to the expression of mechanical allodynia. It is proposed that a relative lack of inhibition underlies altered cell responses.     

Characterization of forms of immunoreactive somatostatin in sensory neuron and normal and deafferented spinal cord

In order to determine the contribution made by primary sensory afferents and supraspinal projections to the immunoreactive somatostatin (IRS) content of the spinal cord, measurements were made of the concentration of IRS in the dorsal and ventral halves of the cord in cats subjected to unilateral lumbosacral dorsal rhizotomy (L1-S3) alone or combined with spinal cord transection. The molecular forms of IRS (characterized by gel chromatography) in L7 lumbar spinal cord, L6-S1 dorsal roots, ventral roots and dorsal root ganglia, and sciatic nerve were also determined. S14 was the predominant form in all tissues examined, but two additional molecular forms corresponding to S28 and S11.5 kdalton were present in dorsal root ganglia and spinal cord; S28 but not S11.5 kdalton was detected in both dorsal roots and sciatic nerves. These results indicate that S14 and S28 and S28 are transported along the central and peripheral processes of dorsal root ganglia, but that spinal cord S11.5 kdalton originates in the central nervous system. IRS in the dorsal horn was reduced by ca. 40% following dorsal root section. Neither disruption of descending pathways by spinal transection nor surgical isolation of the lumbar segements lowered cord somatostatin content below that produced by dorsal root section, indicating that most of the somatostatin within the cord arises from the dorsal root and from neurons in local spinal segments. Although the total content of IRS in the dorsal horn was reduced by ca. 40% following dorsal rhizotomy, the pattern of molecular forms was not changed accordingly. Since S14 and S28 but not S11.5 kdalton are transported via the dorsal root, the dorsal root section would be predicted to produce a relatively greater decrease in S14 and S28 than in S11.5 kdalton. Therefore, failure to find a selective loss of S14 and S28 suggests that dorsal rhizotomy affects dorsal horn IRS content not only by removing afferent input but possibly also by modifyinh the processing of IRS by the remaining somatostatinergic neurons.     

p38 activation in uninjured primary afferent neurons and in spinal microglia contributes to the development of neuropathic pain induced by selective motor fiber injury

Compelling evidence shows that the adjacent uninjured primary afferents play an important role in the development of neuropathic pain after nerve injury. The underlying mechanisms, however, are largely unknown. In the present study, the selective motor fiber injury was performed by L5 ventral root transection (L5 VRT), and p38 activation in dorsal root ganglia (DRG) and L5 spinal dorsal horn was examined. The results showed that phospho-p38 immunoreactivity (p-p38-IR) was increased in both L4 and L5 DRGs, starting on day 1 and persisting for nearly 3 weeks (P<0.05) following L5 VRT and that the activated p38 was confined in neurons, especially in IB4 positive C-type neurons. L5 VRT also induced p38 activation in L5 spinal dorsal horn, occurred at the first day after the lesion and lasted for 2 weeks (P<0.05). The activated p38 is restricted entirely in spinal microglia. In contrast, selective injury of sensory neurons by L5 dorsal root transection (L5 DRT) failed to induce behavioral signs of neuropathic pain and activated p38 only in L5 DRG but not in L4 DRG and L5 spinal dorsal horn. Intraperitoneal injection of thalidomide, an inhibitor of TNF-alpha synthesis, prevented p38 activation in DRG and spinal cord. Intrathecal injection of p38 inhibitor SB203580, starting before L5 VRT, inhibited the abnormal pain behaviors. Post-treatment with SB203580 performed at the 1st day or at the 8th day after surgery also reduced established neuropathic pain. These data suggest that p38 activation in uninjured DRGs neurons and in spinal microglia is necessary for the initiation and maintenance of neuropathic pain induced by L5 VRT.     

The effect of concussive head injury on central cholinergic neurons

This study examined the effect of fluid percussion head injury on the activity of cholinergic neurons in specific brain areas of the rat 12 min, 4 h and 24 h following injury. Acetylcholine (ACh) turnover, used as an index of cholinergic neuronal activity, was determined using a gas chromatographic-mass spectrometric technique. The most striking changes in cholinergic activity were observed in the dorsal pontine tegmentum, where concussive head injury produced an increase in ACh turnover 12 min and 4 h following injury. This area has been previously associated with behavioral changes observed following concussive injury. ACh turnover in the thalamus, a region to which pontine cholinergic neurons project, also tended to increase 4 h following injury. On the other hand, ACh turnover tended to decrease in the amygdala 4 h following injury. Although there were no significant changes in hippocampal ACh content or turnover following injury. ACh content did tend to increase in that brain region 12 min following injury. There were no significant effects of injury on cholinergic neurons in the cingulate/frontal cortex. These changes in cholinergic neuronal activity may contribute to the neurological deficits following concussive injury. In particular, activation of cholinergic neurons in the pontine region may contribute to components of behavioral suppression associated with reversible traumatic unconsciousness. More generalized changes in cholinergic function may lead to the production of more chronic deficits.