Characteristics of spinal dorsal horn neurons after partial chronic deafferentation by dorsal root transection

Comparison of Different In Vivo Animal Models of Brachial Plexus Avulsion and Its Application in Pain Study

Brachial plexus injuries (BPIs) are high-energy trauma that can result in serious functional problems in the affected upper extremities, and brachial plexus avulsion (BPA) could be considered the most severe type of them. The booming occurrence rate of BPA brings up devastating impact on patients' life. Complications of muscle atrophy, neuropathic pain, and denervation-associated psychological disorders are major challenges in the treatment of BPA. Animal models of BPA are good vehicles for this kind of research. Full understanding of the current in vivo BPA models, which could be classified into anterior approach avulsion, posterior approach avulsion, and closed approach avulsion groups, could help researchers select the appropriate type of models for their studies. Each group of the BPA model has its distinct merits and demerits. An ideal BPA model that can inherit the advantages and make up for the disadvantages is still required for further exploration.

Chondroitinase ABC-mediated plasticity of spinal sensory function

Experimental therapeutics designed to enhance recovery from spinal cord injury (SCI) primarily focus on augmenting the growth of damaged axons by elevating their intrinsic growth potential and/or by nullifying the influence of inhibitory proteins present in the mature CNS. However, these strategies may also influence the wiring of intact pathways. The direct contribution of such effects to functional restoration after injury has been mooted, but as yet not been described. Here, we provide evidence to support the hypothesis that reorganization of intact spinal circuitry enhances function after SCI. Adult rats that underwent unilateral cervical spared-root lesion (rhizotomy of C5, C6, C8, and T1, sparing C7) exhibited profound sensory deficits for 4 weeks after injury. Delivery of a focal intraspinal injection of the chondroitin sulfate proteoglycan-degrading enzyme chondroitinase ABC (ChABC) was sufficient to restore sensory function after lesion. In vivo electrophysiological recordings confirm that behavioral recovery observed in ChABC-treated rats was consequent on reorganization of intact C7 primary afferent terminals and not regeneration of rhizotomized afferents back into the spinal cord within adjacent segments. These data confirm that intact spinal circuits have a profound influence on functional restoration after SCI. Furthermore, comprehensive understanding of these targets may lead to therapeutic interventions that can be spatially tailored to specific circuitry, thereby reducing unwanted maladaptive axon growth of distal pathways.

Animal and cellular models of chronic pain

Chronic pain, especially neuropathic pain and cancer pain, is often not adequately treated by currently available analgesics. Animal models provide pivotal systems for preclinical study of pain. This article reviews some of the most widely used or promising new models for chronic pain. Partial spinal ligation, chronic constriction injury, and L5/L6 spinal nerve ligation represent three of the best characterized rodent models of peripheral neuropathy. Recently, several mouse and rat bone cancer pain models have been reported. Primary or permanent cultures of sensory neurons have been established to study the molecular mechanism of pain, especially for neurotransmitter release and signal transduction. The emerging gene microarray, genomics and proteomics methods may be applied to throughly characterize these cells. Each model is uniquely created with distinct mechanisms, it is therefore essential to report and interpret results in the context of a specific model.

Single-unit analysis of the spinal dorsal horn in patients with neuropathic pain

Despite the key role played by the dorsal horn of the spinal cord in pain modulation, single-unit recordings have only been performed very rarely in this structure in humans. The authors report the results of a statistical analysis of 64 unit recordings from the human dorsal horn. The recordings were done in three groups of patients: patients with deafferentation pain resulting from brachial plexus avulsion, patients with neuropathic pain resulting from peripheral nerve injury, and patients with pain resulting from disabling spasticity. The patterns of neuronal activities were compared among these three groups. Nineteen neurons were recorded in the dorsal horns of five patients undergoing DREZotomy for a persistent pain syndrome resulting from peripheral nerve injury (i.e., nondeafferented dorsal horns), 31 dorsal horn neurons were recorded in nine patients undergoing DREZotomy for a persistent pain syndrome resulting from brachial plexus avulsion (i.e., deafferented dorsal horns), and 14 neurons were recorded in eight patients undergoing DREZotomy for disabling spasticity. These groups were compared in terms of mean frequency, coefficient of variation of the discharge, other properties of the neuronal discharge studied by the nonparametric test of Wald-Wolfowitz, and the possible presence of bursts. The coefficient of variation tended to be higher in the deafferented dorsal horn group than in the other two groups. Two neurons displaying burst activity could be recorded, both of which belonged to the deafferented dorsal horn group. A significant difference was found in term of neuronal behavior between the peripheral nerve trauma group and the other groups: The brachial plexus avulsion and disabling spasticity groups were very similar, including various types of neuronal behavior, whereas the peripheral nerve lesion group included mostly neurons with "nonrandom" patterns of discharge (i.e., with serial dependency of interspike intervals).

Clinical and electrophysiological expression of deafferentation pain alleviated by dorsal root entry zone lesions in rats

OBJECT

The aims of this study were to construct an animal model of deafferentation of the spinal cord by brachial plexus avulsion and to analyze the effects of subsequent dorsal root entry zone (DREZ) lesions in this model. To this end, the authors measured the clinical and electrophysiological effects of total deafferentation of the cervical dorsal horn in rats and evaluated the clinical efficacy of cervical DREZ lesioning.

METHODS

Forty-three Sprague-Dawley rats were subjected to total deafferentation of the right cervical dorsal horn by performing a posterior rhizotomy from C-5 to T-1. The clinical effects of this deafferentation, namely self-directed mutilations consisting of scraping and/or ulceration of the forelimb skin or even autotomy of some forelimb digits, were then evaluated. As soon as some of these clinical signs of pain appeared, the authors performed a microsurgical DREZ rhizotomy ([MDR], microincision along the deafferented DREZ and dorsal horn). Before and after MDR, single-unit recordings were obtained in the deafferented dorsal horn and in the contralateral (healthy) side. The mean frequency of spontaneous discharge from the deafferented dorsal horn neurons was significantly higher than that from the healthy side (36.4 Hz compared with 17.9 Hz, p = 0.03). After deafferentation, 81.4% of the rats developed clinical signs corresponding to pain following posterior rhizotomy. Among these animals, scraping was observed in 85.7% of cases, ulceration (associated with edema) in 37.1%, and autotomy in 8.5%. These signs appeared a mean 5.7 weeks (range 1-12 weeks) after deafferentation. Thirteen rats benefited from an MDR; nine (69%) experienced a complete cure, that is, a total resolution of scraping or ulceration (a mean 4.6 weeks after MDR). In contrast, only one of 11 sham-operated animals showed signs of spontaneous recovery (p = 0.01).

CONCLUSIONS

These results emphasize the role of the spinal dorsal horn in the genesis of deafferentation pain and suggest that dorsal horn deafferentation by cervical posterior rhizotomy in the rat provides a reliable model of chronic pain due to brachial plexus avulsion and its suppression by MDR.

Pathobiology of neuropathic pain

This review deals with physiological and biological mechanisms of neuropathic pain, that is, pain induced by injury or disease of the nervous system. Animal models of neuropathic pain mostly use injury to a peripheral nerve, therefore, our focus is on results from nerve injury models. To make sure that the nerve injury models are related to pain, the behavior was assessed of animals following nerve injury, i.e. partial/total nerve transection/ligation or chronic nerve constriction. The following behaviors observed in such animals are considered to indicate pain: (a) autotomy, i.e. self-attack, assessed by counting the number of wounds implied, (b) hyperalgesia, i.e. strong withdrawal responses to a moderate heat stimulus, (c) allodynia, i.e. withdrawal in response to non-noxious tactile or cold stimuli. These behavioral parameters have been exploited to study the pharmacology and modulation of neuropathic pain. Nerve fibers develop abnormal ectopic excitability at or near the site of nerve injury. The mechanisms include unusual distributions of Na(+) channels, as well as abnormal responses to endogenous pain producing substances and cytokines such as tumor necrosis factor alpha (TNF-alpha). Persistent abnormal excitability of sensory nerve endings in a neuroma is considered a mechanism of stump pain after amputation. Any local nerve injury tends to spread to distant parts of the peripheral and central nervous system. This includes erratic mechano-sensitivity along the injured nerve including the cell bodies in the dorsal root ganglion (DRG) as well as ongoing activity in the dorsal horn. The spread of pathophysiology includes upregulation of nitric oxide synthase (NOS) in axotomized neurons, deafferentation hypersensitivity of spinal neurons following afferent cell death, long-term potentiation (LTP) of spinal synaptic transmission and attenuation of central pain inhibitory mechanisms. In particular, the efficacy of opioids at the spinal level is much decreased following nerve injury. Repeated or prolonged noxious stimulation and the persistent abnormal input following nerve injury activate a number of intracellular second messenger systems, implying phosphorylation by protein kinases, particularly protein kinase C (PKC). Intracellular signal cascades result in immediate early gene (IEG) induction which is considered as the overture of a widespread change in protein synthesis, a general basis for nervous system plasticity. Although these processes of increasing nervous system excitability may be considered as a strategy to compensate functional deficits following nerve injury, its by-product is widespread nervous system sensitization resulting in pain and hyperalgesia. An important sequela of nerve injury and other nervous system diseases such as virus attack is apoptosis of neurons in the peripheral and central nervous system. Apoptosis seems to induce neuronal sensitization and loss of inhibitory systems, and these irreversible processes might be in common to nervous system damage by brain trauma or ischemia as well as neuropathic pain. The cellular pathobiology including apoptosis suggests future strategies against neuropathic pain that emphasize preventive aspects.

A new type of microelectrode for obtaining unitary recordings in the human spinal cord

OBJECT

In this paper the authors report on the conception and adjustment of a microelectrode used to obtain unitary recordings in the human spinal cord.

METHOD

To overcome the difficulties related to intraoperative pulsations of the spinal cord, the authors opted to use a floating microelectrode. Because the recordings are obtained most often from spontaneous activities, it is difficult, with a single microelectrode, to separate spikes from electrical artifacts that are related to the switching of devices. Consequently, the authors designed a dual microelectrode made of two tungsten-in-glass-attached microelectrodes separated by 300 microm. Because the two electrodes cannot obtain recordings in the same neuron, it is possible to distinguish unambiguously spikes (recorded on one tip) from electrical artifacts (recorded simultaneously on the two tips). The dual microelectrode is 2 cm long, with a 20-microm tip length, and 800 to 1200-Ohms impedance. This microelectrode can be implanted "free hand," in the dorsal horn, by using a microsurgical forceps under a surgical microscope. The data analysis is performed off-line with spike sorter hardware. In the dorsal horns in 17 patients who were selected to undergo a dorsal root entry zone (DREZ) rhizotomy to treat various pathological conditions, unitary recordings were obtained using this double microelectrode. The authors recorded 57 neurons in good conditions of stability and isolation.

CONCLUSIONS

The microelectrode described in this paper was successfully used to obtain recordings in neurons in more than 85% of the patients. This simplified, floating double microelectrode can therefore be considered for use in microsurgical DREZ rhizotomy to obtain unitary recordings in the human spinal dorsal horn.

Regional changes in forebrain activation during the early and late phase of formalin nociception: analysis using cerebral blood flow in the rat

This is the first neural imaging study to use regional cerebral blood flow (rCBF) in an animal model to identify the patterns of forebrain nociceptive processing that occur during the early and late phase of the formalin test. We measured normalized rCBF increases by an autoradiographic method using the radiotracer [99mTc]exametazime. Noxious formalin consistently produced detectable, well-localized and typically bilateral increases in rCBF within multiple forebrain structures, as well as the interpeduncular nucleus (Activation Index, AI = 66) and the midbrain periaqueductal gray (AI = 20). Structures showing pain-induced changes in rCBF included several forebrain regions considered part of the limbic system. The hindlimb region of somatosensory cortex was significantly activated (AI = 31), and blood flow increases in VPL (AI = 8.7) and the medial thalamus (AI = 9.0) exhibited a tendency to be greater in the late phase as compared to the early phase of the formalin test. The spatial pattern and intensity of activation varied as a function of the time following the noxious formalin stimulus. The results highlight the important role of the limbic forebrain in the neural mechanisms of prolonged persistent pain and provide evidence for a forebrain network for pain.

Plasticity of cutaneous primary afferent projections to the spinal dorsal horn

Reorganization of the somatotopic map in the spinal dorsal horn may be elicited by a variety of deafferenting lesions, including transection of peripheral nerves or dorsal roots, or the application of neurotoxins. While such lesions give rise to a variety of neurochemical and morphological changes in the dorsal horn, collateral sprouting of intact primary afferents appears to be minimal. Recently, intraaxonal injection of neurobiotin has allowed visualization of the entire spinal arborization of single A beta primary afferent fibers in animals where the somatotopy of the relevant region of dorsal horn has also been mapped. In contrast to the somatotopic precision of the terminal fields of peripheral nerves suggested by transganglionic tracing, these studies have shown that afferents make connections many millimeters rostral and caudal to the region where their receptive field is represented in the somatotopic map. Intracellular recording from dorsal horn neurons has further shown that these long-ranging projections make functional, but weak, synaptic connections. Thus the functional somatotopic reorganization that follows nerve lesions in mature animals might be explained simply by an increased synaptic efficacy of these existing projections. In contrast to the negligible sprouting of intact A beta primary afferents, those undergoing axonal regeneration exhibit dense collateral sprouting into deafferented regions of the dorsal horn, particularly the superficial laminae, where the terminal arbors of many small (A delta and C) nociceptive afferent fibres degenerate following peripheral nerve lesions. The inappropriate connections made by these collateral sprouts may partly underlie the painful sequelae of nerve injury in man.

Anterior pretectal nucleus facilitation of superficial dorsal horn neurones and modulation of deafferentation pain in the rat

1. Functional relationships between the anterior pretectal nucleus (APTN) and nociceptive dorsal horn neurones were investigated electrophysiologically in the anaesthetized rat. The effects of APTN lesions were assessed behaviourally in a model of deafferentation pain. 2. Cells in the dorsal and rostral parts of the APTN were excited orthodromically by electrical stimulation of the ipsilateral dorsolateral funiculus or the contralateral dorsal columns, and by noxious and innocuous cutaneous stimuli. 3. Electrical stimulation of the APTN excited nociceptive lamina I spinal neurones. These cells all projected rostrally in the contralateral dorsolateral funiculus. Identical APTN stimulation also inhibited multireceptive spinal neurones which lay deep in the dorsal horn. These particular cells were shown to project to the brain in the ventrolateral funiculus. 4. It is proposed that noxious stimuli excite spinal lamina I projection neurones which send excitatory axons to the brain, including the APTN. The APTN inhibits deep multireceptive neurones, to reduce the perception of noxious stimuli. The discharge of spinal lamina I neurones, however, will be sustained by the noxious stimulus and by facilitation from the APTN. A sustained descending inhibition of this nature would reduce responses to prolonged injury. 5. The involvement of the APTN in responses to a chronic pain state was examined by comparing the behaviour of animals with bilateral lesions of the APTN with normal controls. Lesions of the APTN strongly enhanced the autotomy behaviour triggered by sectioning of the dorsal roots. 6. These observations support the suggestion that the APTN reduces the debilitating effects of prolonged injury.

The depressive effect of intrathecal clonidine on the spinal flexor reflex is enhanced after sciatic nerve section in rats

The effect of intrathecal (i.t.) alpha 2-adrenoceptor agonist, clonidine, on the spinal nociceptive flexor reflex was studied in decerebrate, spinalized, unanesthetized rats with intact sciatic nerves or in rats in which the sciatic nerve had been ipsilaterally sectioned. In rats with intact nerves i.t. clonidine caused a dose-dependent biphasic effect on flexor reflex excitability. At low dose (10 ng) the effect of clonidine was purely facilitatory, whereas with 50-100 ng clonidine the initial facilitation was often followed by reflex depression. Long-lasting, strong reflex depression was observed after i.t. injection of high doses of clonidine (1 and 10 micrograms). Four to 18 days after sciatic nerve section, the depressive effect of clonidine on the flexor reflex was dramatically enhanced. Depression was frequently observed already with doses of 5 and 10 ng, and maximal depression was reached at 100 ng and 1 micrograms in axotomized rats. The facilitatory effect of low doses of clonidine on the reflex was also observed, although somewhat less frequently than in normals. The depressive effect of clonidine on the flexor reflex was reversed by the selective alpha 2-receptor antagonist, atipamezole (20 micrograms, i.t.), in rats with both intact and sectioned sciatic nerves. The present results revealed an increased sensitivity and effectiveness of the depression of spinal reflex mechanisms by i.t. clonidine after sciatic nerve section, which is opposite to the decreased sensitivity to i.t. morphine after axotomy that we observed previously.(ABSTRACT TRUNCATED AT 250 WORDS)

Fos-like immunoreactivity in the rat superficial dorsal horn induced by formalin injection in the forepaw: effects of dorsal rhizotomies

As previously described at the lumbar spinal level, we found that 2 h after subcutaneous formalin injection in the distal part of the fore-limb, Fos-like immunoreactivity (FLI) was induced in the ipsilateral cervical enlargement. Not surprisingly, as the injection site corresponds to the distal part of the C6-C8 dorsal root dermatomes, maximal labelling which predominated in the superficial laminae, was observed in the C6-C8 segments and to a lesser extent in C5. Similar experiments were performed on rats which underwent various types of unilateral dorsal rhizotomies (DRh) 7 days before formalin injection. In animals with C4, C5, T1 and T2 DRh sparing C6-C8 the rostrocaudal distribution was similar to the intact one. But, in animals having C4-T2 DRh sparing one single root, C7, the segmental FLI distribution was modified: it was slightly increased in C7, decreased in C6 and significantly decreased in C8. As expected, no FLI was found in animals with C4 to T2 DRh. The spared root model provides information about the segmental distribution in the cervical spinal cord of the input brought by a single root following stimulation of the distal forelimb, i.e., maximal distribution in the entry segment, but also in the two rostral and one caudal segments.

Ultrastructural characterization of substance-P-immunoreactive synaptic terminals in the cat's normal and rhizotomized trigeminal subnucleus caudalis

Deafferenting injuries often cause transient or permanent physiological alterations within the central projection field of affected primary afferent fibers. Aberrant sensory perceptions, dysesthesias, and hyperalgesias represent the clinical sequelae of such injuries; however, the results of experimental deafferentations have been subject to a variety of interpretations (Rodin and Kruger, 1984b). Neurochemical studies show an increased sensitivity of partially deafferented neurons to substance P (SP). Our previous studies (Hoffmann et al., 1991) documented, primarily at the light-microscopic level, a moderate transient loss of SP-immunoreactive (SPIR) boutons in the trigeminal subnucleus caudalis (Vc)--a loss that seemed to preferentially affect the slightly larger, possibly complex boutons with multiple contacts. However, despite the elimination of the trigeminal input, the larger boutons reappeared. In the present study, therefore, we examined Vc using electron-microscopic immunocytochemistry, in order to document these changes over time and to clarify the structure and relationships of this population of boutons. SPIR boutons occurred in lamina I and II degrees of the substantia gelatinosa of Vc, ranged in size from 1 to 5 microns in diameter, and displayed mixed populations of clear and dense-core vesicles. Most formed single or multiple axodendritic junctions, but a significant number engaged in axoaxonic contacts with both SPIR-labeled and unlabeled terminals. A small number appeared to be the central element of a typical glomerulus, particularly in lamina II degrees. Three to seven days following an ipsilateral retrogasserian rhizotomy, synaptic degeneration was evident in the substantia gelatinosa and often involved glomerular terminals. However, most of these were SPIR-negative and occurred primarily in lamina II degrees. Those SPIR boutons that displayed degenerative features often made single or multiple axodendritic contacts, and in some instances were scalloped. By 30 days, most remaining SPIR boutons were small, with a lower incidence of contacts; however, some of these were axoaxonic. In addition, many SPIR terminals were only very lightly stained--a feature not encountered to such an extent in the contralateral Vc. At 45 days, complex SPIR boutons were again evident in the field, and some showed densely packed vesicles. An increased incidence of clusters of two to four SPIR axoaxonic contacts was also observed. Finally, almost all SPIR boutons encountered at this stage were intensely stained. It is suggested that these alterations represent a compensatory neuroplastic response on the part of overlapping cervical and cranial primary afferents to the partial deafferentation resulting from the interruption of the trigeminal root.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies in autotomy: its pathophysiology and usefulness as a model of chronic pain

An interesting behavioral syndrome results in animals from the same or similar types of lesions that lead to deafferentation pain in humans; many neurectomized animals begin to scratch, bite, or self-mutilate their denervated limb, a phenomenon termed autotomy. The proposition that this behavior in animals is a response to the chronic pain of peripheral nerve injury has met with considerable controversy. If this issue were resolved, then a better understanding of the neurophysiology of autotomy might help elucidate the mechanisms of the human conditions. To determine the association between deafferentation and the autotomy behavior, we developed a pharmacologically induced functional deafferentation preparation using chronic perineural lidocaine infusion of the sciatic nerve. This 'chronic lidocaine' model's behavior was compared with that of the neurectomy model. While autotomy was noted in 80% of the latter group, no animal undergoing a chronic perineural infusion of lidocaine autotomized. We thus conclude that autotomy is not a response to non-painful sensory deafferentation, but rather that this behavior is a response to pain. We also studied the development of autotomy in a variety of other focal denervation preparations. On the basis of these data, we conclude that autotomy is not due to loss of sensory input on a functional basis nor to an action potential-mediated process. Rather, nerve damage which coincidentally involves sensory loss is necessary and sufficient for the development of this behavior. We suggest that interruption of a humoral feedback process homeostatically operating within the first order sensory neuron with its effect exerted post-synaptically leads to autotomy. The evidence supports the existence of a loss of a transportable, humoral autotomy inhibitory factor.

Chronic functional consequences of adult infraorbital nerve transection for rat trigeminal subnucleus interpolaris

In adult rats, transection of the infraorbital nerve and subsequent regeneration have been shown to result in altered somatotopic organization and changes in response properties of primary afferents within the trigeminal ganglion. The present study examined how these changes affect the postsynaptic targets of these neurons within subnucleus interpolaris of the trigeminal brainstem. Extracellular recordings were made from 330 cells in normal rats and 424 cells in rats surviving 57-290 days after transection of the infraorbital nerve in adulthood. Adult infraorbital nerve transection resulted in significant functional reorganization within subnucleus interpolaris. Relative to normal rats, the major changes can be summarized as follows: (1) a decrease in the dorsoventral extent of infraorbital representation; (2) a disruption of inter- and intradivisional somatotopic organization; (3) an increase in the proportion of cells with no discernible receptive field; (4) an increase in receptive field size for cells with infraorbital receptive field components; (5) the appearance of a significant proportion of cells with discontinuous receptive fields; (6) an increase in the proportion of cells exhibiting interdivisional convergence; (7) significant changes in the types of receptor surfaces activating local-circuit neurons with infraorbital receptive field components; (8) the appearance of a significant proportion of cells exhibiting convergence of different receptor surfaces; (9) significant changes in the dynamic response characteristics of cells with infraorbital receptive field components; and (10) an increase in the proportion of spontaneously active infraorbital-responsive cells. The changes observed were quite similar to those reported in adult subnucleus interpolaris following neonatal infraorbital nerve transection. The majority of changes observed in both studies can be most parsimoniously explained by alterations of primary afferents. However, central mechanisms may be more likely substrates for others. Regardless of the mechanism, the mature rodent trigeminal system appears capable of considerable functional reorganization following peripheral nerve damage.

Characteristics of the bursting pattern of action potentials that occurs in the thalamus of patients with central pain

Neurons in the somatosensory thalamus of patients with central pain following spinal cord injury fire in bursts of action potentials more frequently than do similar neurons in patients without pain. Furthermore, the characteristic firing pattern within these bursts is similar to that which is shown to be associated with the occurrence of calcium spikes in intracellular studies of thalamic nuclei. This finding may have significant implications for the etiology and treatment of central pain states.

Diminished dorsal root GABA sensitivity following chronic peripheral nerve injury

The depolarizing effect of gamma-aminobutyric acid (GABA) on rat lumbar dorsal roots was studied in a sucrose gap chamber following axotomy or crush injury of the sciatic nerve or dorsal root. The mean depolarization elicited by GABA on normal dorsal roots (3.96 +/- 0.71 mV, N = 14) was significantly reduced following chronic sciatic axotomy (2.02 +/- 0.99 mV, N = 15). Chronic sciatic crush injury had no significant effect on dorsal root GABA sensitivity. The amplitudes of the dorsal root compound action potentials were the same from rats with normal and injured sciatic nerves, indicating that axons proximal to the sciatic nerve lesion did not undergo appreciable degeneration. A marked loss of dorsal root GABA sensitivity was also seen following dorsal root axotomy or crush injury (1.02 +/- 0.98 mV (N = 10) and 0.69 +/- 0.70 mV (N = 9), respectively). These results indicate that GABA sensitivity of dorsal roots is attenuated following peripheral nerve lesions in which regeneration and functional reconnection with peripheral targets are prevented. Previous work indicates that the primary afferent depolarization is reduced under similar conditions. The reduction in GABA sensitivity of dorsal root fibers described here may have a contributory role in the reduced primary afferent depolarization that follows peripheral nerve transection, which has pathophysiologic implications in chronic pain syndromes.

Unilateral dorsal rhizotomy decreases monoamine levels in the rat spinal cord

In the rat, unilateral dorsal cervicothoracic rhizotomy (C5-T2) resulted in autotomy of the ipsilateral limb. The onset of self-mutilation was variable and attained the maximum degree 8-9 weeks after the dorsal root section. Fifteen and 60 days after the lesion the monoamines dopamine (DA), noradrenaline (NA) and serotonin (5-HT) as well as the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured by high performance liquid chromatography at different levels of the spinal cord. The lesion induced a significant decrease in the spinal levels of DA and NA, both ipsilateral and contralateral to the lesion, not only in the deafferented region but also from T3 downwards. The changes in DA levels were more marked and of earlier onset than those of NA. The decrease in 5-HT and its metabolite 5-HIAA was only observed in the more caudal regions of the non-deafferented ipsilateral cord. The more conspicuous lessening in spinal monoamine levels coincided approximately with the maximum intensity of autotomy. The present results suggest that dorsal rhizotomy induces distant changes in the brainstem nuclei where descending aminergic pathways originate.

Abnormal single-unit activity recorded in the somatosensory thalamus of a quadriplegic patient with central pain

We have performed single unit analysis of the activity of cells located in the ventral nuclear group of thalamus in a patient with dysesthetic pain below the level of a clinically complete traumatic spinal cord transection at C5. Cells located in the parasagittal plane 14 mm lateral to the midline responded to tactile stimulation in small facial and intraoral receptive fields, which were characteristic of patients without somatosensory abnormality [30]. In this patient the 16 mm lateral parasagittal plane contained cells with receptive fields located on the occiput and neck instead of the upper extremity as would normally be expected. Cells with receptive fields on the neck and occiput had not previously been observed in recordings from single units (n = 531) responding to somatosensory stimulation [30]. Thus, on the basis of their location in a region of thalamus which normally represents parts of the body below the level of the spinal cord transection and their unusual receptive fields adjacent to these same parts of the body, we propose that the cells in the 16 mm lateral plane have lost their normal afferent input. Analysis of the autopower spectra of spike trains indicates that cells in the 16 mm lateral plane exhibited a higher mean firing rate and greater tendency to fire in bursts than cells in the 14 mm lateral plane (P less than 0.005). Finally, electrical stimulation at the recording sites in the 16 mm lateral plane evoked a burning sensation in the occiput, neck and upper extremity. These results suggest that regions of thalamus which have lost their normal somatosensory input contain neurons which exhibit abnormal spontaneous and evoked activity and that electrical stimulation of these regions can produce the sensation of burning dysesthesia.

Deafferentation and chronic pain in animals: an evaluation of evidence suggesting autotomy is related to pain

This paper examines evidence which suggests that the self-mutilation of deafferented limbs exhibited by laboratory animals is a response to pain or dysesthesia and is therefore an adequate model of chronic pain. Evidence from studies using physiological, pharmacological and behavioral methods provides strong support that autotomy reflects chronic pain. New evidence presented in this paper demonstrates that specific treatments can be used to manipulate the extent of autotomy, causing increases or decreases, as well as restricting it to specific parts of a denervated foot. This evidence argues that autotomy scores are an appropriate measure of the degree of pain or dysesthesia which results from the deafferentation of a limb.

Electrical stimulation reveals relatively ineffective sural nerve projections to dorsal horn neurons in the cat

Electrical stimulation of the sural nerve (SN) revealed input from sural nerve afferents to L6 and L7 dorsal horn neurons that were not apparent using natural mechanical stimuli, especially in cells with variable latency responses to SN stimulation. Nearly all (31/32) cells that had reliable, fixed latency responses to SN stimulation also had an excitatory receptive field (RF) in the region of skin innervated by the sural nerve (SN region). About one-third (20/57) of the cells with variable latency responses to SN stimulation, however, had an RF outside the SN region. Most (130/146) cells with no response to SN stimulation had RFs outside the SN region. There were no obvious differences between variable latency cells with RFs in the SN region vs those with RFs outside it in latency of response to SN stimulation, recording depth, RF sizes or modality properties. In a subsample of 31 postsynaptic dorsal column neurons all cells responding to SN stimulation also had an RF in the SN region. Strengthening of relatively ineffective projections from the sural nerve by lesions might be expected to lead to an increase in the proportion of cells responding with impulses to natural stimulation of the skin innervated by the sural nerve, and, hence, to an increase in average RF size.

Dysesthesias and self-mutilation in humans and subhumans: a review of clinical and experimental studies

The chronic deafferentation syndrome includes a complex pattern of abnormal self-directed behavior and a stress response. Subhuman self-mutilation is a secondary consequence of the chronic deafferentation syndrome. The evidence indicates that the chronic deafferentation syndrome in subhumans is a valid model for the induced and the spontaneous dysesthesias in humans. Objective criteria for the definition of subhuman dysesthesias have been derived from independent sources of evidence, in neurally intact subjects; those criteria are then found to match the subhuman syndrome of deafferentation. Support for the validity of the inference of subhuman dysesthesias derives from the parallels with the various facts of the human dysesthesias. The credibility of this argument is significantly strengthened by reports of morphological and excitatory physiological abnormalities, in central somatosensory structures, in response to deafferentation. There is no independent subhuman evidence in support of alternate interpretations of the deafferentation syndrome, and those interpretations seem to be inadequate in several aspects. Doubts concerning the validity of this animal model have been allayed by reports of dysesthesias in humans with spinal posterior rhizotomies or ganglionectomies, and also those with congenital analgesia. Moreover, the occurrence of this syndrome in hypoalgesic areas as a consequence of anterolateral cordotomy in monkeys, can best be interpreted as a reflection of dysesthesias. This syndrome is released by neuropathological or neurosurgical lesions in the peripheral or central nervous system; lesions which involve small caliber peripheral afferents or the spinothalamic tract. Variability in the release of this syndrome has been associated with several different factors. So far, the chronic syndrome is intractable. Evidence relates the abnormalities of this syndrome to pathophysiological foci in central relays of the somatosensory system, and suggests that the chronic abnormalities of this syndrome can be sustained at brain levels.

Centrocentral anastomosis in the prevention and treatment of painful terminal neuroma. An experimental study in the rat

In this experimental study, microsurgical centrocentral anastomosis was applied to an experimental model of painful terminal neuroma resulting from left sciatic nerve section in the rat. The anastomosis consisted of end-to-end suturing of the sciatic nerve fascicles to the tibial branch, with the interposition of a nerve graft taken from the same anastomosed fascicle. As a control parameter for the experiment, the autotomy which follows sciatic nerve section in the rat was evaluated. Autotomy is considered an objective indication of abnormal sensations that are provoked by the formation of a terminal neuroma. Histological study of the proximal stump of the sciatic nerve was also performed. The observation period was 10 weeks. The study demonstrates that centrocentral anastomosis reduces the size of the neuroma formation and the incidence of autotomy.

Deafferentation in animals as a model for the study of pain: an alternative hypothesis

The notion that post-deafferentation autonomy is a pain response is unsupported by the results of studies with neurotoxins. The selective massive destruction of a fiber system considered essential to normal nociception--unmyelinated primary afferent axons--prior to deafferenting nerve lesions did not stop or even significantly impede post-denervation DI despite massive evidence from humans and animals that pain following nerve lesions originates in the periphery and is generated by abnormal discharges in the injured nerve. In addition, when a reduction in abnormal impulse discharges of both large and small injured sensory axons could be inferred following neonatal sympathectomy, DI was not reduced in incidence or severity. This latter observation (1) provides further support for a dissociation between DI and pain, since any contribution of myelinated primary afferent axons to painful pathology probably was substantially reduced by sympathectomy and (2) suggests that DI also may be unrelated to non-painful sensory pathology attributable to abnormal activity in the thick-diameter fiber population. These findings and an evaluation of other relevant observations suggest that DI may not be a manifestation of deafferentation pain and perhaps this animal model for the experimental study of pain should be discarded. An alternative view of DI, reconcilable with known properties of this behavior, is that it reflects a proclivity in some species and circumstances to shed a functionally-impaired insensate appendage.

Recovery from partial deafferentation increases 2-deoxyglucose uptake in distant spinal segments

2-Deoxy[14C]glucose autoradiography was used to study the responsiveness of the partially deafferented rat spinal cord to electrical stimulation of low-threshold afferent fibers. Unilateral extradural dorsal rhizotomies were carried out at L3 to S2, sparing L5. Postoperative sensory deficits were appropriate to the extent of the lesion. Acute, 7 day, and 14 to 20 day postrhizotomy animals and unoperated controls were anesthetized prior to isotope injection and electrical stimulation of Ia fibers in the L5 root. Quantitative densitometry was carried out on enlarged autoradiographs, subdividing the spinal gray matter into laminar divisions drawn from the corresponding stained sections. Optical densities from stimulated and unstimulated sides were compared using paired t tests for each experimental group at each lumbosacral segment (L1 to S2) and at T13 for the day 14 to 20 animals. This procedure provided an objective basis for statistical comparisons between homologous areas even where the differences in density were small. Unoperated animals showed activation in 11 of 23 dorsal horn zones extending to L2 and never involving the base of the dorsal horn (lamina V). Acute and day 7 groups did not appreciably differ from the control group except for activation of lamina V within the L4 segment. In the most delayed group, 18 dorsal horn regions were activated, extending to L1 with an additional zone in T13 . Lamina V contained significant labeling in three segments. In no group was there increased labeling of the ventral horn. The results are interpreted as showing that stimulus-related, neural activity increases after a 2-week delay in regions of spinal cord distant from the normal zone of significant metabolic change. This increase in neural activity during recovery is discussed in relation to time-dependent electrophysiologic, structural, and metabolic responses to deafferentation. The longitudinal spread of dorsal horn activation by preserved afferent fibers in the spared root may facilitate more effective central transmission of sensory information.