Influence of spinalization on spinal withdrawal reflex responses varies depending on the submodality of the test stimulus and the experimental pathophysiological condition in the rat

Reduced mechanical hypersensitivity by inhibition of the amygdala in experimental neuropathy: Sexually dimorphic contribution of spinal neurotransmitter receptors

Here we studied spinal neurotransmitter mechanisms involved in the reduction of mechanical hypersensitivity by inhibition of the amygdaloid central nucleus (CeA) in male and female rats with spared nerve injury (SNI) model of neuropathy. SNI induced mechanical hypersensitivity that was stronger in females. Reversible blocking of the CeA with muscimol (GABAA receptor agonist) induced a reduction of mechanical hypersensitivity that did not differ between males and females. Following spinal co-administration of atipamezole (α2-adrenoceptor antagonist), the reduction of mechanical hypersensitivity by CeA muscimol was attenuated more in males than females. In contrast, following spinal co-administration of raclopride (dopamine D2 receptor antagonist) the reduction of hypersensitivity by CeA muscimol was attenuated more in females than males. The reduction of mechanical hypersensitivity by CeA muscimol was equally attenuated in males and females by spinal co-administration of WAY-100635 (5-HT1A receptor antagonist) or bicuculline (GABAA receptor antagonist). The CeA muscimol induced attenuation of ongoing pain-like behavior (conditioned place preference test) that was reversed by spinal co-administration of atipamezole in both sexes. The results support the hypothesis that CeA contributes to mechanical hypersensitivity and ongoing pain-like behavior in SNI males and females. Disinhibition of descending controls acting on spinal α2-adrenoceptors, 5-HT1A, dopamine D2 and GABAA receptors provides a plausible explanation for the reduction of mechanical hypersensitivity by CeA block in SNI. The involvement of spinal dopamine D2 receptors and α2-adrenoceptors in the CeA muscimol-induced reduction of mechanical hypersensitivity is sexually dimorphic, unlike that of spinal α2-adrenoceptors in the reduction of ongoing neuropathic pain.

Age-related changes in peripheral nociceptor function

Pain and pain management in the elderly population is a significant social and medical problem. Pain sensation is a complex phenomenon that typically involves activation of peripheral pain-sensing neurons (nociceptors) which send signals to the spinal cord and brain that are interpreted as pain, an unpleasant sensory experience. In this work, young (4-5 months) and aged (26-27 months) Fischer 344 x Brown Norway (F344xBN) rats were examined for nociceptor sensitivity to activation by thermal (cold and heat) and mechanical stimulation following treatment with inflammatory mediators and activators of transient receptor potential (TRP) channels. Unlike other senses that decrease in sensitivity with age, sensitivity of hindpaw nociceptors to thermal and mechanical stimulation was not different between young and aged F344xBN rats. Intraplantar injection of bradykinin (BK) produced greater thermal and mechanical allodynia in aged versus young rats, whereas only mechanical allodynia was greater in aged rats following injection of prostaglandin E₂ (PGE₂). Intraplantar injection of TRP channel activators, capsaicin (TRPV1), mustard oil (TRPA1) and menthol (TRPM8) each resulted in greater mechanical allodynia in aged versus young rats and capsaicin-induced heat allodynia was also greater in aged rats. A treatment-induced allodynia that was greater in young rats was never observed. The anti-allodynic effects of intraplantar injection of kappa and delta opioid receptor agonists, salvinorin-A and D-Pen²,D-Pen⁵]enkephalin (DPDPE), respectively, were greater in aged than young rats, whereas mu opioid receptor agonists, [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) and morphine, were not effective in aged rats. Consistent with these observations, in primary cultures of peripheral sensory neurons, inhibition of cAMP signaling in response to delta and kappa receptor agonists was greater in cultures derived from aged rats. By contrast, mu receptor agonists did not inhibit cAMP signaling in aged rats. Thus, age-related changes in nociceptors generally favor increased pain signaling in aged versus young rats, suggesting that changes in nociceptor sensitivity may play a role in the increased incidence of pain in the elderly population. These results also suggest that development of peripherally-restricted kappa or delta opioid receptor agonists may provide safer and effective pain relief for the elderly.

Nociception induces a differential presynaptic modulation of the synaptic efficacy of nociceptive and proprioceptive joint afferents

A previous study has indicated that during the state of central sensitization induced by the intradermic injection of capsaicin, there is a gradual facilitation of the dorsal horn neuronal responses produced by stimulation of the high-threshold articular afferents that is counteracted by a concurrent increase of descending inhibitory actions. Since these changes occurred without significantly affecting the responses produced by stimulation of the low-threshold articular afferents, it was suggested that the capsaicin-induced descending inhibition included a preferential presynaptic modulation of the synaptic efficacy of the slow conducting nociceptive joint afferents (Ramírez-Morales et al., Exp Brain Res 237:1629-1641, 2019). The present study was aimed to investigate more directly the contribution of presynaptic mechanisms in this descending control. We found that in the barbiturate anesthetized cat, stimulation of the high-threshold myelinated afferents in the posterior articular nerve (PAN) produces primary afferent hyperpolarization (PAH) in the slow conducting (25-35 m/s) and primary afferent depolarization (PAD) in the fast conducting (40-50 m/s) articular fibers. During the state of central sensitization induced by capsaicin, there is a supraspinally mediated shift of the autogenic PAH to PAD that takes place in the slow conducting fibers, basically without affecting the autogenic PAD generated in the fast conducting afferents. It is suggested that the change of presynaptic facilitation to presynaptic inhibition induced by capsaicin on the slow articular afferents is part of an homeostatic process aimed to keep the nociceptive-induced neuronal activity within manageable limits while preserving the proprioceptive information required for proper control of movement.

Role of 5-HT receptors in neuropathic pain: potential therapeutic implications

5-HT plays a crucial role in the progress and adjustment of pain both centrally and peripherally. The therapeutic action of the 5-HT receptors` agonist and antagonist in neuropathic pain have been widely reported in many studies. However, the specific roles of 5-HT subtype receptors have not been reviewed comprehensively. Therefore, we summarized the recent findings on multiple subtypes of 5-HT receptors in both central and peripheral nervous system in neuropathic pain, particularly, 5-HT1, 5-HT2, 5-HT3 and 5-HT7 receptors. In addition, 5-HT4, 5-HT5 and 5-HT6 receptors were also reviewed. Most of studies focused on the function of 5-HT subtype receptors in spinal level compared to brain areas. Based on these evidences, the pain process can be facilitated or inhibited that depending on the specific subtypes and the distribution of 5-HT receptors. Therefore, this review may provide potential therapeutic implications in treatment of neuropathic pain.

Mechanisms of Below-Level Pain Following Spinal Cord Injury (SCI)

Mechanisms of below-level pain are discoverable as neural adaptations rostral to spinal injury. Accordingly, the strategy of investigations summarized here has been to characterize behavioral and neural responses to below-level stimulation over time following selective lesions of spinal gray and/or white matter. Assessments of human pain and the pain sensitivity of humans and laboratory animals following spinal injury have revealed common disruptions of pain processing. Interruption of the spinothalamic pathway partially deafferents nocireceptive cerebral neurons, rendering them spontaneously active and hypersensitive to remaining inputs. The spontaneous activity among these neurons is disorganized and unlikely to generate pain. However, activation of these neurons by their remaining inputs can result in pain. Also, injury to spinal gray matter results in a cascade of secondary events, including excitotoxicity, with rostral propagation of excitatory influences that contribute to chronic pain. Establishment and maintenance of below-level pain results from combined influences of injured and spared axons in the spinal white matter and injured neurons in spinal gray matter on processing of nociception by hyperexcitable cerebral neurons that are partially deafferented. A model of spinal stenosis suggests that ischemic injury to the core spinal region can generate below-level pain. Additional questions are raised about demyelination, epileptic discharge, autonomic activation, prolonged activity of C nocireceptive neurons, and thalamocortical plasticity in the generation of below-level pain. PERSPECTIVE: An understanding of mechanisms can direct therapeutic approaches to prevent development of below-level pain or arrest it following spinal cord injury. Among the possibilities covered here are surgical and other means of attenuating gray matter excitotoxicity and ascending propagation of excitatory influences from spinal lesions to thalamocortical systems involved in pain encoding and arousal.

A newly identified nociresponsive region in the transitional zone (TZ) in rat sensorimotor cortex

The primary somatosensory cortex (S1) comprises a number of functionally distinct regions, reflecting the diversity of somatosensory receptor submodalities innervating the body. In particular, two spatially and functionally distinct nociceptive regions have been described in primate S1 (Vierck et al., 2013; Whitsel et al., 2019). One region is located mostly in Brodmann cytoarchitectonic area 1, where a subset of neurons exhibit functional characteristics associated with myelinated Aδ nociceptors and perception of 1st/sharp, discriminative pain. The second region is located at the transition between S1 and primary motor cortex (M1) in area 3a, where neurons exhibit functional characteristics associated with unmyelinated C nociceptors and perception of 2nd/slow, burning pain. To test the hypothesis that in rats the transitional zone (TZ) - which is a dysgranular region at the transition between M1 and S1 - is the functional equivalent of the nociresponsive region of area 3a in primates, extracellular spike discharge activity was recorded from TZ neurons in rats under general isoflurane anesthesia. Thermonoxious stimuli were applied by lowering the contralateral forepaw or hindpaw into a 48-51 °C heated water bath for 5-10 s. Neurons in TZ were found to be minimally affected by non-noxious somatosensory stimuli, but highly responsive to thermonoxious skin stimuli in a slow temporal summation manner closely resembling that of nociresponsive neurons in primate area 3a. Selective inactivation of TZ by topical lidocaine application suppressed or delayed the nociceptive withdrawal reflex, suggesting that TZ exerts a tonic facilitatory influence over spinal cord neurons producing this reflex. In conclusion, TZ appears to be a rat homolog of the nociresponsive part of monkey area 3a. A possibility is considered that this region might be primarily engaged in autonomic aspects of nociception.

Chronic orofacial pain animal models - progress and challenges

INTRODUCTION

Chronic orofacial pain is one of the most common pain conditions experienced by adults. Animal models are often selected as the most useful scientific methodology to explore the pathophysiology of the disorders that cause this disabling pain to facilitate the development of new treatments. The creation of new models or the improvement of existing ones is essential for finding new ways to approach the complex neurobiology of this type of pain. Areas covered: The authors describe and discuss a variety of animal models used in chronic orofacial pain (COFP). Furthermore, they examine in detail the mechanisms of action involved in orofacial neuropathic pain and orofacial inflammatory pain. Expert opinion: The use of animal models has several advantages in chronic orofacial pain drug discovery. Choosing an animal model that most closely represents the human disease helps to increase the chances of finding effective new therapies and is key to the successful translation of preclinical research to clinical practice. Models using genetically modified animals seem promising but have not yet been fully developed for use in chronic orofacial pain research. Although animal models have provided significant advances in the pharmacological treatment of orofacial pain, several barriers still need to be overcome for better treatment options.

Involvement of the periaqueductal gray in the descending antinociceptive effect induced by the central nucleus of amygdala

Here we studied whether descending control of mechanical nociception by glutamate in the central nucleus of the amygdala (CeA) of healthy control animals is induced by amygdaloid NMDA receptors and relayed through the midbrain periaqueductal gray (PAG). Mechanical nociception in the hind paws was assessed in rats with chronic guide cannulae for glutamate administration in the right CeA and for inducing local anesthesia in the PAG. In a separate electrophysiological study, ON-like PAG neurons giving an excitatory response to noxious pinch of the tail were recorded in anesthetized rats following glutamate administration into the CeA. A high dose of glutamate (100 microg) in the CeA induced mechanical antinociception in the contra- but not ipsilateral hind limb. Antinociception was prevented by an NMDA receptor antagonist in the CeA or local anesthesia of the PAG. Discharge rate of ON-like PAG neurons was increased by a high dose of glutamate (100 microg) in the CeA and this increase was prevented by an NMDA receptor antagonist in the CeA. The results indicate that amygdaloid NMDA receptors in the CeA may induce contralaterally mechanical antinociception through a circuitry relaying in the PAG. Activation of ON-like PAG neurons is associated with the descending antinociceptive effect. Mechanisms and causality of this association still remain to be studied.

Neuropathic Pain After Spinal Cord Injury: Challenges and Research Perspectives

Neuropathic pain is a debilitating consequence of spinal cord injury (SCI) that remains difficult to treat because underlying mechanisms are not yet fully understood. In part, this is due to limitations of evaluating neuropathic pain in animal models in general, and SCI rodents in particular. Though pain in patients is primarily spontaneous, with relatively few patients experiencing evoked pains, animal models of SCI pain have primarily relied upon evoked withdrawals. Greater use of operant tasks for evaluation of the affective dimension of pain in rodents is needed, but these tests have their own limitations such that additional studies of the relationship between evoked withdrawals and operant outcomes are recommended. In preclinical SCI models, enhanced reflex withdrawal or pain responses can arise from pathological changes that occur at any point along the sensory neuraxis. Use of quantitative sensory testing for identification of optimal treatment approach may yield improved identification of treatment options and clinical trial design. Additionally, a better understanding of the differences between mechanisms contributing to at- versus below-level neuropathic pain and neuropathic pain versus spasticity may shed insights into novel treatment options. Finally, the role of patient characteristics such as age and sex in pathogenesis of neuropathic SCI pain remains to be addressed.

Hyperalgesia and sensitization of dorsal horn neurons following activation of NK-1 receptors in the rostral ventromedial medulla

Neurons in the rostral ventromedial medulla (RVM) project to the spinal cord and are involved in descending modulation of pain. Several studies have shown that activation of neurokinin-1 (NK-1) receptors in the RVM produces hyperalgesia, although the underlying mechanisms are not clear. In parallel studies, we compared behavioral measures of hyperalgesia to electrophysiological responses of nociceptive dorsal horn neurons produced by activation of NK-1 receptors in the RVM. Injection of the selective NK-1 receptor agonist Sar9,Met(O2)11-substance P (SSP) into the RVM produced dose-dependent mechanical and heat hyperalgesia that was blocked by coadministration of the selective NK-1 receptor antagonist L-733,060. In electrophysiological studies, responses evoked by mechanical and heat stimuli were obtained from identified high-threshold (HT) and wide dynamic range (WDR) neurons. Injection of SSP into the RVM enhanced responses of WDR neurons, including identified neurons that project to the parabrachial area, to mechanical and heat stimuli. Since intraplantar injection of capsaicin produces robust hyperalgesia and sensitization of nociceptive spinal neurons, we examined whether this sensitization was dependent on NK-1 receptors in the RVM. Pretreatment with L-733,060 into the RVM blocked the sensitization of dorsal horn neurons produced by capsaicin. c-Fos labeling was used to determine the spatial distribution of dorsal horn neurons that were sensitized by NK-1 receptor activation in the RVM. Consistent with our electrophysiological results, administration of SSP into the RVM increased pinch-evoked c-Fos expression in the dorsal horn. It is suggested that targeting this descending pathway may be effective in reducing persistent pain.NEW & NOTEWORTHY It is known that activation of neurokinin-1 (NK-1) receptors in the rostral ventromedial medulla (RVM), a main output area for descending modulation of pain, produces hyperalgesia. Here we show that activation of NK-1 receptors produces hyperalgesia by sensitizing nociceptive dorsal horn neurons. Targeting this pathway at its origin or in the spinal cord may be an effective approach for pain management.

Baclofen reversed thermal place preference in rats with chronic constriction injury

Chronic constriction injury to the sciatic nerve was used as an animal model of neuropathic pain. Instead of frequently used reflex-based tests we used an operant thermal place preference test to evaluate signs of neuropathic pain and the effect of baclofen administration in rats with neuropathy. Chronic constriction injury was induced by four loose ligations of the sciatic nerve. Thermal place preference (45 °C vs. 22 °C and 45 °C vs. 11 °C) was measured after the ligation and after the administration of baclofen in sham and experimental rats. Rats with the chronic constriction injury spent significantly less time on the colder plate compared to sham operated animals at the combination 45 °C vs. 11 °C. After administration of baclofen (10 mg/kg s.c.), the aversion to the colder plate in rats with chronic constriction injury disappeared. At the combination 45 °C vs. 22 °C, no difference in time spent on colder and/or warmer plate was found between sham and experimental animals. These findings show the importance of cold allodynia evaluation in rats with chronic constriction injury and the effectiveness of baclofen in this neuropathic pain model.

Comparison of operant escape and reflex tests of nociceptive sensitivity

Testing of reflexes such as flexion/withdrawal or licking/guarding is well established as the standard for evaluating nociceptive sensitivity and its modulation in preclinical investigations of laboratory animals. Concerns about this approach have been dismissed for practical reasons - reflex testing requires no training of the animals; it is simple to instrument; and responses are characterized by observers as latencies or thresholds for evocation. In order to evaluate this method, the present review summarizes a series of experiments in which reflex and operant escape responding are compared in normal animals and following surgical models of neuropathic pain or pharmacological intervention for pain. Particular attention is paid to relationships between reflex and escape responding and information on the pain sensitivity of normal human subjects or patients with pain. Numerous disparities between results for reflex and operant escape measures are described, but the results of operant testing are consistent with evidence from humans. Objective reasons are given for experimenters to choose between these and other methods of evaluating the nociceptive sensitivity of laboratory animals.

The degree of acute descending control of spinal nociception in an area of primary hyperalgesia is dependent on the peripheral domain of afferent input

Descending controls of spinal nociceptive processing play a critical role in the development of inflammatory hyperalgesia. Acute peripheral nociceptor sensitization drives spinal sensitization and activates spino–supraspinal–spinal loops leading to descending inhibitory and facilitatory controls of spinal neuronal activity that further modify the extent and degree of the pain state. The afferent inputs from hairy and glabrous skin are distinct with respect to both the profile of primary afferent classes and the degree of their peripheral sensitization. It is not known whether these differences in afferent input differentially engage descending control systems to different extents or in different ways. Injection of complete Freund's adjuvant resulted in inflammation and swelling of hairy hind foot skin in rats, a transient thermal hyperalgesia lasting <2 h, and longlasting primary mechanical hyperalgesia (≥7 days). Much longer lasting thermal hyperalgesia was apparent in glabrous skin (1 h to >72 h). In hairy skin, transient hyperalgesia was associated with sensitization of withdrawal reflexes to thermal activation of either A- or C-nociceptors. The transience of the hyperalgesia was attributable to a rapidly engaged descending inhibitory noradrenergic mechanism, which affected withdrawal responses to both A- and C-nociceptor activation and this could be reversed by intrathecal administration of yohimbine (α-2-adrenoceptor antagonist). In glabrous skin, yohimbine had no effect on an equivalent thermal inflammatory hyperalgesia. We conclude that acute inflammation and peripheral nociceptor sensitization in hind foot hairy skin, but not glabrous skin, rapidly activates a descending inhibitory noradrenergic system. This may result from differences in the engagement of descending control systems following sensitization of different primary afferent classes that innervate glabrous and hairy skin.

A modified Hargreaves' method for assessing threshold temperatures for heat nociception

This study describes a modified Hargreaves' method for assessing paw withdrawal threshold temperatures for heat (PWT-H) nociception in the hind paws of rats. This method utilises radiant heat to maintain controlled lamp temperatures (CLTs) on a glass floor beneath the rat hind paw. An ascending series of CLTs were applied for 10s, with 5-10min intervals between applications, until characteristic withdrawal behaviour was observed or a cutoff CLT was reached. The average plantar epicutaneous temperatures measured from anaesthetised rats corresponding to CLTs and withdrawal latencies were used for determining PWT-H. The mean PWT-H in 2-month-old (mo) naïve Sprague-Dawley rats (n=38) was 47.6±0.2°C, which is comparable to the noxious threshold temperature for human glabrous skin (46.5±0.5°C). The PWT-H was consistent between trials and daily assessments over four consecutive days. No significant differences were observed between the PWT-H in 2-, 6- to 8-, and >24-mo F344 rats, but the PWT-H in 1-mo rats was significantly reduced. Three hours following plantar incision, the PWT-H decreased to 37.5±0.2°C, consistent with previous observations of C-fibre afferents from incised glabrous skin firing at 36.7±3.6°C. Parallel testing, using the current method and an electronic von Frey device, illustrated similar degrees of incision-induced hyperalgesia, gradual improvements in hyperalgesia, and reversals induced through morphine and gabapentin. In conclusion, the present method facilitates a comparison of PWT-H using electrophysiological and human psychophysical studies involving thermosensation, and as a behavioural assay identical to von Frey testing, this method also measures the threshold for nociception.

Peripheral neuropathy induces cutaneous hypersensitivity in chronically spinalized rats

BACKGROUND/OBJECTIVES

The present study was aimed at the issue of whether peripheral nerve injury-induced chronic pain is maintained by supraspinal structures governing descending facilitation to the spinal dorsal horn, or whether altered peripheral nociceptive mechanisms sustain central hyperexcitability and, in turn, neuropathic pain. We examined this question by determining the contribution of peripheral/spinal mechanisms, isolated from supraspinal influence(s), in cutaneous hypersensitivity in an animal model of peripheral neuropathy.

METHODS

Adult rats were spinalized at T8-T9; 8 days later, peripheral neuropathy was induced by implanting a 2-mm polyethylene cuff around the left sciatic nerve. Hind paw withdrawal responses to mechanical or thermal plantar stimulation were evaluated using von Frey filaments or a heat lamp, respectively.

RESULTS

Spinalized rats without cuff implantation exhibited a moderate decrease in mechanical withdrawal threshold on ~day 10 (P < 0.05) and in thermal withdrawal threshold on ~day 18 (P < 0.05). However, cuff-implanted spinalized rats developed a more rapid and significant decrease in mechanical (~day 4; P < 0.001) and thermal (~day 10; P < 0.05) withdrawal thresholds that remained significantly decreased through the duration of the study.

CONCLUSIONS

Our findings demonstrate an aberrant peripheral/spinal mechanism that induces and maintains thermal and to a greater degree tactile cutaneous hypersensitivity in the cuff model of neuropathic pain, and raise the prospect that altered peripheral/spinal nociceptive mechanisms in humans with peripheral neuropathy may have a pathologically relevant role in both inducing and sustaining neuropathic pain.

Adaptation of a novel operant orofacial testing system to characterize both mechanical and thermal pain

Mechanical pain sensitivity is characteristic of many orofacial pain conditions; however, few models exist to quantify this pain. Here we evaluated a novel adaptation of our existing operant system to characterize orofacial pain following mechanical and thermal stimuli. We demonstrate that the operant system is able to detect painful and analgesic responses to mechanical stimuli. These findings allow comparison of both mechanical and thermal stimuli using the same outcome measures.

Roles of the rostroventromedial medulla and the spinal 5-HT(1A) receptor in descending antinociception induced by motor cortex stimulation in the neuropathic rat

Electric stimulation of the primary motor cortex (M1) has been effective in suppressing pain-related responses in neuropathic as well as healthy control animals. We studied whether the rostroventromedial medulla (RVM) or the spinal 5-HT(1A) receptor contributes to antinociception induced by stimulation of M1 in neuropathic animals. Assessments of the noxious heat-evoked limb withdrawal reflecting spinal nociception was performed in rats with spinal nerve ligation-induced peripheral neuropathy under light pentobarbital anesthesia. Spinal antinociception induced by electric stimulation of M1 was reduced following block of the RVM with intramedullary injection of muscimol, a GABA(A) receptor agonist, or following intrathecal administration of WAY-100635, a 5-HT(1A) receptor antagonist. The results indicate that the RVM and the descending serotonergic pathway acting on the spinal 5-HT(1A) receptor contribute to spinal antinociception induced by M1 stimulation in neuropathic animals.

Behavioral and anatomical characterization of the bilateral sciatic nerve chronic constriction (bCCI) injury: correlation of anatomic changes and responses to cold stimuli

Background

Unilateral constrictive sciatic nerve injury (uCCI) is a common neuropathic pain model. However, the bilateral constrictive injury (bCCI) model is less well studied, and shows unique characteristics. In the present study, we sought to correlate effects of bCCI on nocifensive responses to cold and mechanical stimuli with selected dorsal horn anatomic markers. bCCI or sham ligation of both rat sciatic nerves were followed up to 90 days of behavioural testing. Additional rats sacrificed at 15, 30 and 90 days were used for anatomic analyses. Behavioural tests included hindpaw withdrawal responses to topical acetone, cold plate testing, an operant thermal preference task and hindpaw withdrawal thresholds to mechanical probing.

Results

All nocifensive responses to cold increased and remained enhanced for >45 days. Mechanical withdrawal thresholds decreased for 25 days only. Densitometric analyses of immunoperoxidase staining in the superficial dorsal horn at L4-5 revealed decreased cholecystokinin (CCK) staining at all times after bCCI, decreased mu opiate receptor (MOR) staining, maximal at 15 days, increased neuropeptide Y (NPY) staining only at days 15 and 30, and increased neurokinin-1 receptor (NK-1R) staining at all time points, maximal at 15 days. Correlation analyses at 45 days post-bCCI, were significant for individual rat nocifensive responses in each cold test and CCK and NK-1R, but not for MOR or NPY.

Conclusions

These results confirm the usefulness of cold testing in bCCI rats, a new approach using CCI to model neuropathic pain, and suggest a potential value of studying the roles of dorsal horn CCK and substance P in chronic neuropathic pain. Compared to human subjects with neuropathic pain, responses to cold stimuli in rats with bCCI may be a useful model of neuropathic pain.

Animal models of pain: progress and challenges

Many are frustrated with the lack of translational progress in the pain field, in which huge gains in basic science knowledge obtained using animal models have not led to the development of many new clinically effective compounds. A careful re-examination of animal models of pain is therefore warranted. Pain researchers now have at their disposal a much wider range of mutant animals to study, assays that more closely resemble clinical pain states, and dependent measures beyond simple reflexive withdrawal. However, the complexity of the phenomenon of pain has made it difficult to assess the true value of these advances. In addition, pain studies are importantly affected by a wide range of modulatory factors, including sex, genotype and social communication, all of which must be taken into account when using an animal model.

Governing role of primary afferent drive in increased excitation of spinal nociceptive neurons in a model of sciatic neuropathy

Previously we reported that the cuff model of peripheral neuropathy, in which a 2 mm polyethylene tube is implanted around the sciatic nerve, exhibits aspects of neuropathic pain behavior in rats similar to those in humans and causes robust hyperexcitation of spinal nociceptive dorsal horn neurons. The mechanisms mediating this increased excitation are not known and remain a key unresolved question in models of peripheral neuropathy. In anesthetized adult male Sprague-Dawley rats 2-6 weeks after cuff implantation we found that elevated discharge rate of single lumbar (L(3-4)) wide dynamic range (WDR) neurons persists despite acute spinal transection (T9) but is reversed by local conduction block of the cuff-implanted sciatic nerve; lidocaine applied distal to the cuff (i.e. between the cuff and the cutaneous receptive field) decreased spontaneous baseline discharge of WDR dorsal horn neurons approximately 40% (n=18) and when applied subsequently proximal to the cuff, i.e. between the cuff and the spinal cord, it further reduced spontaneous discharge by approximately 60% (n=19; P<0.05 proximal vs. distal) to a level that was not significantly different from that of naive rats. Furthermore, in cuff-implanted rats WDR neurons (n=5) responded to mechanical cutaneous stimulation with an exaggerated afterdischarge which was reversed entirely by proximal nerve conduction block. These results demonstrate that the hyperexcited state of spinal dorsal horn neurons observed in this model of peripheral neuropathy is not maintained by tonic descending facilitatory mechanisms. Rather, on-going afferent discharges originating from the sciatic nerve distal to, at, and proximal to the cuff maintain the synaptically-mediated gain in discharge of spinal dorsal horn WDR neurons and hyperresponsiveness of these neurons to cutaneous stimulation. Our findings reveal that ectopic afferent activity from multiple regions along peripheral nerves may drive CNS changes and the symptoms of pain associated with peripheral neuropathy.

Nociceptive behavior in animal models for peripheral neuropathy: spinal and supraspinal mechanisms

Since the initial description by Wall [Wall, P.D., 1967. The laminar organization of dorsal horn and effects of descending impulses. J. Neurophysiol. 188, 403-423] of tonic descending inhibitory control of dorsal horn neurons, several studies have aimed to characterize the role of various brain centers in the control of nociceptive input to the spinal cord. The role of brainstem centers in pain inhibition has been well documented over the past four decades. Lesion to peripheral nerves results in hypersensitivity to mild tactile or cold stimuli (allodynia) and exaggerated response to nociceptive stimuli (hyperalgesia), both considered as cardinal signs of neuropathic pain. The increased interest in animal models for peripheral neuropathy has raised several questions concerning the rostral conduction of the neuropathic manifestations and the role of supraspinal centers, especially brainstem, in the inhibitory control or in the abnormal contribution to the maintenance and facilitation of neuropathic-like behavior. This review aims to summarize the data on the ascending and descending modulation of neuropathic manifestations and discusses the recent experimental data on the role of supraspinal centers in the control of neuropathic pain. In particular, the review emphasizes the importance of the reciprocal interconnections between the analgesic areas of the brainstem and the pain-related areas of the forebrain. The latter includes the cerebral limbic areas, the prefrontal cortex, the intralaminar thalamus and the hypothalamus and play a critical role in the control of pain considered as part of an integrated behavior related to emotions and various homeostatic regulations. We finally speculate that neuropathic pain, like extrapyramidal motor syndromes, reflects a disorder in the processing of somatosensory information.

Distinct roles of the anterior cingulate cortex in spinal and supraspinal bee venom-induced pain behaviors

A wide variety of human and animal experiments suggest that the anterior cingulate cortex (ACC) is one of the key brain substrates subserving higher order processing of noxious information. However, no sufficient data are now available regarding the mediation by ACC of different levels of pain processing as well as its potential descending modulation of spinal nociception. Using the well-developed rat bee venom (BV) model, the present study evaluated the effect of lesions of bilateral ACC on two levels of spontaneous nociceptive behaviors (spinally-processed persistent paw flinching reflex and supraspinally-processed paw lifting/licking) and heat or mechanical hypersensitivity under the inflammatory pain state. In contrast to the sham lesion group (saline microinjection into the ACC), bilateral complete ACC chemical lesions (kainic acid microinjection into the ACC) significantly decreased the BV-induced paw lifting and licking behavior (less time spent by the animal in paw lifting/licking) but produced no influence upon spinally-processed spontaneous paw flinching reflex (no change in number of paw flinches following subcutaneous BV injection). Moreover, the bilateral ACC lesions relieved the BV-evoked primary thermal or mechanical hypersensitivity compared with the sham control group. However, incomplete lesions of bilateral ACC failed to affect the abovementioned pain-related behaviors. No effects were seen on basal pain sensitivity in either group of rats. Motor coordination, as measured by Rota-Rod treadmill test, was not impaired by bilateral ACC lesions. These results implicate that the ACC area of the brain plays differential roles in the mediation of different levels of spontaneous pain-related behaviors. The present study also provides additional evidence for the ACC-mediated descending facilitation of primary hyperalgesia (pain hypersensitivity) identified in the injured area under inflammatory pain state.

Pelvic nerve input mediates descending modulation of homovisceral processing in the thoracolumbar spinal cord of the rat

BACKGROUND & AIMS

Colonic afferents project to the lumbosacral and thoracolumbar spinal cord via the pelvic and hypogastric/lumbar colonic nerves, respectively. Both spinal regions process inflammatory colonic stimuli. The role of thoracolumbar segments in processing acute colorectal pain is questionable, however, because the lumbosacral spinal cord appears sufficient to process reflex responses to acute pain. Here, we show that activity in pelvic nerve colonic afferents actively modulates thoracolumbar dorsal horn neuron processing of the same colonic stimulus through a supraspinal loop: homovisceral descending modulation.

METHODS

Dorsal horn neurons were recorded in the rat thoracolumbar spinal cord after acute or chronic pelvic neurectomy and cervical cold block.

RESULTS

Acute pelvic neurectomy or lidocaine inhibition of lumbosacral dorsal roots facilitated the excitatory response of thoracolumbar dorsal horn neurons to colorectal distention (CRD) and decreased the percentage of neurons inhibited by CRD, suggesting colonic input over the pelvic nerve inhibits thoracolumbar processing of the same stimulus. Ectopic activity developed in the proximal pelvic nerve after chronic neurectomy reactivating the inhibitory circuit, inhibiting thoracolumbar neurons. Cervical cold block alleviated the inhibition in intact or chronic neurectomized rats. However, the facilitated response after acute pelvic neurectomy was inhibited by cervical cold block, exposing an underlying descending facilitation. Inhibiting pelvic nerve input after cervical cold block had minimal effect.

CONCLUSIONS

These data demonstrate that input over the pelvic nerve modulates the response of thoracolumbar spinal neurons to CRD by a supraspinal loop and that increasing thoracolumbar processing increases visceral hyperalgesia.

Neuronal activation at the spinal cord and medullary pain control centers after joint stimulation: a c-fos study in acute and chronic articular inflammation

Chronic inflammatory pain induces short- and long-term central changes, which have been mainly studied at the spinal cord level. Supraspinal pain control centers intrinsically connected with the dorsal horn are also prone to be affected by chronic inflammatory pain. C-fos expression was used as a neuronal activation marker at spinal and supraspinal levels to i) compare acute and chronic articular inflammation, and ii) analyze the effects of brief innocuous or noxious stimulation of a chronically inflamed joint. Acute articular inflammation was induced by an inflammatory soup with prostaglandin E(2) and bradykinin, both at 10(-5) M. Chronic articular inflammation consisted of 14 days of monoarthritis. Early c-fos expression was studied 4 min after inflammatory soup injection or stimulation of the arthritic joint whereas late c-fos expression was evaluated 2 h after those stimuli. At the spinal cord, the analysis was focused on the dorsal horn (laminae I-V) and supraspinally, five major regions of the endogenous pain control system were considered: the caudal ventrolateral medulla (VLM), the dorsal reticular nucleus (DRt), the ventral reticular nucleus (VRt), the nucleus of the solitary tract (Sol) and the rostroventromedial medulla (RVM). Acute articular inflammation induced early and late increases in c-fos expression at the spinal level and late increases supraspinally whereas the effects of monoarthritis were more moderate and restricted to the spinal cord. When monoarthritic animals were subjected to gentle touch or bending of the joint, early increases in c-fos expression were detected supraspinally, but not at the spinal level. In this region, noxious mechanical stimulation induced late increases in non-inflamed animals and both early and late increases in monoarthritic rats. Supraspinally, noxious stimulation induced only late increases in c-fos expression. The present results show complex differences in the patterns of c-fos expression between the spinal cord and medullary areas of the pain control system during articular inflammation, which indicate that the somatosensory system is differentially affected by the installation of chronic pain.

Differential effect of supraspinal modulation on the nociceptive withdrawal reflex and pain sensation

OBJECTIVE

To study whether the nociceptive withdrawal reflex (WR) and pain sensation are differentially affected by supraspinal modulation and to determine the nature of this modulation.

METHODS

The WR and pain sensation elicited by electrical stimulation were measured in complete spinal cord injury (SCI) subjects and in intact controls under two different experimental conditions; "facilitation" and "neutral" control.

RESULTS

Pain sensation was the same under both conditions, whereas the characteristics of the WR were highly dependent on them. In intact body regions the WR threshold was similar to pain threshold under facilitation but was near pain tolerance in neutral conditions. Furthermore, WR was elicited in 100% of trials under facilitation but only in 57% of trials in neutral conditions. Thresholds of WR in paralyzed regions were significantly higher than in intact regions (p<0.001). The former showed a clear stimulus-response relationship as did pain sensation whereas the WR in intact regions did not.

CONCLUSIONS

The WR and pain sensation are differentially affected by supraspinal modulation. The WR is subject to both excitatory and inhibitory influences, depending on the instructions subjects receive.

SIGNIFICANCE

The experimental setup and subjects' mental state should be considered when interpreting changes in the WR. Extreme caution should be employed when utilizing reflexive indices as a measure of pain. Verbal report seems a more suitable tool to evaluate pain since it is relatively stable with repeated measurements and in accordance with stimulation intensity.

Spinal and pontine α2-adrenoceptors have opposite effects on pain-related behavior in the neuropathic rat

Descending noradrenergic pathways contribute to feedback inhibition of pain by releasing norepinephrine in the spinal cord. Noradrenergic nuclei in the pons contain abundant alpha(2)-adrenoceptors. We assessed the contribution of pontine alpha(2)-adrenoceptors to endogenous regulation of pain in nerve-injured rats. Tactile allodynia and mechanical hyperalgesia were assessed in the injured dermatome and heat nociception in an uninjured dermatome. Atipamezole, an alpha(2)-adrenoceptor antagonist, or saline was administered systemically or microinjected into the locus coeruleus, the lateral parabrachial nucleus, the central nucleus of the amygdala, the midbrain periaqueductal gray, and/or through an intrathecal (i.t.) catheter to the spinal cord. Atipamezole administered systemically, into the amygdala or the periaqueductal gray had no significant effects on pain behavior. Atipamezole (0.3-5 microg) microinjected into the pons, the locus coeruleus or the lateral parabrachial nucleus, produced a selective and dose-related antiallodynia, which was reversed by i.t. administration of atipamezole (5 microg). I.t. administration of atipamezole alone (5 microg) produced thermal hypersensitivity in the non-neuropathic segment (tail) of nerve-injured animals. In sham-operated controls, i.t. administration of atipamezole had no effect. Suppression of heat nociception in uninjured dermatomes of nerve-injured but not the control animals following i.t. administration of atipamezole indicates that nerve injury produced a tonic activation of noradrenergic feedback inhibition acting on spinal alpha(2)-adrenoceptors. In parallel, antiallodynia induced by pontine administration of atipamezole indicates that nerve injury induces a tonic activation of pontine alpha(2)-adrenoceptors that promotes neuropathic hypersensitivity by attenuating descending inhibition. Thus, spinal and pontine alpha(2)-adrenoceptors have opposite effects on pain-related behavior in neuropathic animals.

Nociceptive threshold and analgesic response to morphine in aged and young adult rats as determined by thermal radiation and intracerebral electrical stimulation

The present experiment compared the nociceptive threshold and analgesic response to morphine in young (4-5 months) and aged (24 months) rats using peripheral thermal stimulation and intracerebral electrical stimulation. Responses to thermal stimuli were assessed using both the classical tail-flick procedure in which latency of response is the dependent variable and a new method in which threshold in calories of heat is the dependent variable. In the intracerebral nociceptive threshold procedure, electrical stimuli were delivered via an electrode implanted in the mesencephalic reticular formation (MRF), a pain pathway, and the animals were trained to terminate the stimulation by turning a cylindrical manipulandum embedded in one wall of the experimental chamber. For the classical tail-flick method, the aged rats required a greater intensity of stimulation to produce a basal response latency that was between 2.5 and 3.5 s. Using the new psychophysical method for determining the tail-flick threshold, the aged rats' basal thresholds were significantly higher than that of the young rats. However, the basal thresholds obtained by direct stimulation of the MRF failed to show a significant age effect, suggesting that the registration of pain is not different between young and aged rats. These age-related differences in baseline tail-flick response may be due to changes in the spinal reflex associated with aging. Although, there was no difference in the analgesic effects of morphine between young and aged rats using the latency of the tail-flick response, evidence for decreased analgesic response was seen using the tail-flick threshold measure and the intracerebral stimulation threshold method.

A PET activation study of brush-evoked allodynia in patientswith nerve injury pain

Acute experimental brush-evoked allodynia induces a cortical activation pattern that differs from that typically seen during experimental nociceptive pain. In this study, we used positron emission tomography to measure changes in regional cerebral blood flow (rCBF) in patients with clinical allodynia. Nine patients with peripheral nerve injury were scanned during rest, brush-evoked allodynia, and brushing of normal contralateral skin. PET data were analyzed for the whole group and for single subjects. Allodynic stimulation activated the contralateral orbitofrontal cortex (BA 11) in every patient. Whereas normal brushing activated most strongly the contralateral insular cortex, allodynic brushing produced an ipsilateral activation in this area. Another important difference between normal and allodynic brushing was the absence of a contralateral primary somatosensory cortex (SI) activation during allodynic brushing. No thalamic activation was observed during allodynic or control brushing. Although no anterior cingulate cortex (ACC) activation could be demonstrated in the group analysis, single subject analysis revealed that four patients activated this region during brush-evoked allodynia. A direct post hoc comparison of brush -and allodynia-induced rCBF changes showed that allodynia was associated with significantly stronger activations in orbitofrontal cortex and ipsilateral insula whereas non-painful brushing more strongly activated SI and BA 5/7. These findings indicate that activity in the cortical network involved in the sensory-discriminative processing of nociceptive pain is downregulated in neuropathic pain. Instead, there is an upregulation of activity in the orbitofrontal and insular cortices, which is probably due to the stronger emotional load of neuropathic pain and higher computational demands of processing a mixed sensation of brush and pain.

Bilateral chronic constriction of the sciatic nerve: a model of long-term cold hyperalgesia

Effects of chronic constriction injury (CCI) and sham surgery of both sciatic nerves were evaluated for reflex lick/guard (L/G) and operant escape responses to thermal stimulation of rats. Experiment 1 compared L/G and escape responses to 0.3 degrees C, 43 degrees C, and 47 degrees C stimulation during a period of 60 days after CCI. Experiment 2 evaluated escape from 44 degrees C, 47 degrees C, and 10 degrees C for 100 days after CCI. The rats escaped from heat or cold stimulation of the paws in a dark compartment by climbing on a thermally neutral platform in a brightly lit compartment. For reflex testing, a single compartment provided no escape option. There was no significant effect of bilateral CCI on reflex or escape responses to nociceptive heat. However, there were long-term increases in the duration of L/G responding during trials of 0.3 degrees C stimulation and in the duration of escape responding to 10 degrees C. Hyperalgesia for cold was confirmed by a preference test, with a 2-compartment shuttle box with one floor heated (45 degrees C) and the other floor cooled (10 degrees C). Occupancy of the heated compartment was significantly increased by CCI (indicating a relative aversion for cold).

PERSPECTIVE

For preclinical testing of treatments for allodynia/hyperalgesia after nerve injury, it is crucial to use methods of testing that are sensitive to effects on nociception throughout the neuraxis. Operant escape testing satisfies this criterion and is sensitive to bilateral CCI of rats, which avoids asymmetric postural/motor influences of unilateral CCI.

Use of a novel thermal operant behavioral assay for characterization of orofacial pain sensitivity

Orofacial pain has been well-characterized clinically, but evaluation of orofacial pain in animals has not kept pace. The objective of this study was to describe behavioral responses to facial thermal stimulation and inflammation with/without an analgesic using a novel operant paradigm. Animals were trained to voluntarily place their face against a stimulus thermode (37.7-57.2 degrees C) providing access to positive reinforcement. These contingencies present a conflict between positive reward and tolerance for nociceptive stimulation. Inflammation was induced and morphine was provided as an analgesic in a subset of animals. Six outcome measures were determined: reward intake, reward licking contacts, stimulus facial contacts, facial contact duration, ratio of reward/stimulus contacts, and ratio of facial contact duration/event. Animals displayed aversive behaviors to the higher temperatures, denoted by a significant decrease in reward intake, total facial contact duration, and reward licking events. The number of facial contacts increased with increasing temperature, replacing long drinking bouts with more frequent short drinks, as reflected by a low ratio of facial contact duration/event. The number of reward licking/facial contact events was significantly decreased as the thermal stimulus intensity increased, providing another pain index derived from this operant method. These outcomes were significantly affected in the direction of increased nociception following inflammation, and these indices of hyperalgesia were reversed with morphine administration. These data reflect an orofacial pain behavior profile that was based on an animal's responses in an operant escape paradigm. This technique allows evaluation of nociceptive processing and modulation throughout the neuraxis.

Electrophysiological study on primary afferent properties of a chronic constriction nerve injury model in spinal rats

A chronic constriction nerve injury (CCI) model of the rat sciatic nerve is known to exhibit neuropathic pain behavior. The authors conducted electrophysiological analysis for the primary afferent properties of this model in a decerebrate spinal preparation. In the CCI model, electrical transcutaneous stimulation for A-delta and C-fibers showed a low current threshold to elicit the flexion withdrawal reflex. The antidromic ectopic firing activity recorded from the sural nerve showed abnormal firing patterns, which were not seen in normal rats, as follows: (1) an increase of spontaneous firing frequency, (2) development of an on-off pattern that consisted of cyclic burst spikes, and (3) increased firing number under the hypoxic condition. The amplitude of the A-delta component in the antidromic sensory nerve-evoked potential was lower than that in normal rats. The current study clarified the electrophysiological parameters reflecting pathological hypersensivity and excitability of primary afferents in the CCI model, which could not found by behavioral analysis. These results may be useful in future studies evaluating possible treatments of neuropathic pain.

Descending control of persistent pain: inhibitory or facilitatory?

The periaqueductal gray matter (PAG) and the nucleus raphe magnus and adjacent structures of the rostral ventromedial medulla (RVM), with their projections to the spinal dorsal horn, constitute the "efferent channel" of a pain-control system that "descends" from the brain onto the spinal cord. Considerable evidence has recently emerged regarding participation of this system in persistent pain conditions such as inflammation and neuropathy. Herein, this evidence is reviewed and organized to support the idea that persistent nociception simultaneously triggers descending facilitation and inhibition. In models of inflammation, descending inhibition predominates over facilitation in pain circuits with input from the inflamed tissue, and thus attenuates primary hyperalgesia, while descending facilitation predominates over inhibition in pain circuits with input from neighboring tissues, and thus facilitates secondary hyperalgesia. Both descending facilitation and inhibition mainly stem from RVM. The formalin-induced primary hyperalgesia, although considered a model for inflammation, is mainly facilitated from RVM. Also, formalin-induced secondary hyperalgesia is facilitated by RVM. Again, formalin triggers a concomitant but concealed descending inhibition. The (primary) hyperalgesia and allodynia of the neuropathic syndrome are also facilitated from RVM. Simultaneously, there is an inhibition of secondary neuronal pools that is partly supported from the PAG. Because in all these models of peripheral damage descending facilitation and inhibition are triggered simultaneously, it will be important to elucidate why inhibition predominates in some neuronal pools and facilitation in others. Therapies that enhance descending inhibition and/or attenuate descending facilitation are furthermore an important target for research in the future.

Descending facilitatory control of mechanically evoked responses is enhanced in deep dorsal horn neurones following peripheral nerve injury

Pain resulting from peripheral nerve injury, characterised by ongoing pain, hyperalgesia and allodynia arises from peripheral and central processes. Here, we studied the potential role of central facilitations in nerve injury by investigating the effect of blocking the excitatory 5HT3 receptor with ondansetron. 5HT3 receptors play a pronociceptive role in the spinal cord and ondansetron has previously been shown to produce antinociception in behavioural studies. We investigated the effects of spinally administered ondansetron (10, 50 and 100 microg) on the responses of deep dorsal horn neurones, evoked by peripheral electrical stimuli and a range of natural (mechanical punctate and heat) stimuli, 2 weeks after nerve injury induced through tight ligation of L5/6 spinal nerves (SNL). Comparisons were made between SNL rats and a sham-operated group. Ondansetron produced little effect on the electrically evoked responses (Abeta-, Adelta- and C-fibre-evoked responses, postdischarge); however, responses to mechanical punctate stimuli (von Frey filaments 1-75 g) were markedly reduced in both SNL and control groups. Furthermore, the drug effect was significantly enhanced after SNL (p<0.05). In particular, the lowest dose (10 microg) now became effective after SNL. Ondansetron produced less marked effects on thermal responses. Our results demonstrate that neuropathic pain states are associated with an enhanced descending facilitatory control of mechanical responses of spinal neurones, mediated through the activation of spinal 5HT3 receptors. These excitatory influences are likely to contribute to the development and maintenance of central sensitisation in the spinal cord, and furthermore, to the behavioural manifestation of tactile allodynia.

Pain thresholds in alcohol preferring and non-preferring rats: diurnal and repeated trial line differences

BACKGROUND

Studies in humans and animals suggest that those with a genetic predisposition toward high levels of alcohol consumption have decreased pain thresholds. The present study explores this association in rat lines selectively bred for alcohol preference or nonpreference.

METHODS

13 HAD1, 11 LAD1, 16 iP5C, and 16 iNP1 rats were tested in the Ugo Basile Plantar Analgesiometer and the IITC Tail Flick device. Five trials were conducted in each test session with one session per device conducted in the light and one in the dark. Trial one latency represents baseline latency. Decline in Latency over Repeated Trials (DLRT) was analyzed using linear regression.

RESULTS

Testing in the Plantar Analgesiometer revealed no significant line differences in baseline latency (iP5C versus iNP1; HAD1 vs. LAD1). The alcohol preferring lines (iP5C and HAD1), however, both demonstrated an increase in baseline latency in the dark phase compared to the light phase (p < 0.05). The iNP1 line demonstrated highly significant DLRT in both the light phase (p < 0.001) and dark phase (p < 0.01) while the iP5C line demonstrated a significant DLRT only at night (p < 0.01). In the tail flick apparatus, the HAD1 line demonstrated a significantly increased baseline latency compared to the LAD1 line in both the light (p < 0.05) and dark (p < 0.01) phases. The HAD1 line also demonstrated a significant increase in baseline latency in the dark compared to the light phase (p < 0.05) and a significant DLRT in the light (p < 0.01) and the dark phase (p < 0.001).

CONCLUSIONS

The alcohol preferring HAD1 line demonstrates a significantly increased baseline tail latency compared to the nonpreferring LAD1 line in both light and dark phases. There is a significant diurnal rhythm of paw and tail latency in both of the alcohol preferring lines (iP5C and HAD1) but not in the nonpreferring lines (iNP1 and LAD1). A novel finding of Decline in Latency with Repeated Trials (DLRT) was found in both alcohol preferring and nonpreferring lines; degree of DLRT differed as a function of line and circadian period.

Descending serotonergic facilitation mediated through rat spinal 5HT3 receptors is unaltered following carrageenan inflammation

A descending facilitatory drive originating from superficial dorsal horn NK1-expressing neurones and relaying through parabrachial and rostroventral medial medulla to act on deep dorsal horn neurones, mediated through spinal 5HT3 receptors, was recently documented. To determine if this pathway plays a role in the pathophysiology of inflammation, we investigated the effects of spinally administered ondansetron (a selective 5HT3 receptor antagonist) on deep dorsal horn neuronal responses in carrageenan inflamed and naïve animals using in vivo electrophysiology. The mechanical and thermal evoked responses of spinal neurones were dose dependently attenuated by ondansetron to a similar degree in both groups. In contrast, the electrically evoked responses (Abeta-, Adelta-, C-fibre evoked response and post-discharge) remained unaltered in both groups. Thus 5HT3 receptor mediated descending facilitation remains unaltered at this stage after tissue injury.

Nociceptive response to innocuous mechanical stimulation is mediated via myelinated afferents and NK-1 receptor activation in a rat model of neuropathic pain

Peripheral nerve injury in humans can produce a persistent pain state characterized by spontaneous pain and painful responses to normally innocuous stimuli (allodynia). Here we attempt to identify some of the neurophysiological and neurochemical mechanisms underlying neuropathic pain using an animal model of peripheral neuropathy induced in male Sprague-Dawley rats by placing a 2-mm polyethylene cuff around the left sciatic nerve according to the method of Mosconi and Kruger. von Frey hair testing confirmed tactile allodynia in all cuff-implanted rats before electrophysiological testing. Rats were anesthetized and spinalized for extracellular recording from single spinal wide dynamic range neurons (L(3-4)). In neuropathic rats (days 11-14 and 42-52 after cuff implantation), ongoing discharge was greater and hind paw receptive field size was expanded compared to control rats. Activation of low-threshold sensory afferents by innocuous mechanical stimulation (0.2 N for 3 s) in the hind paw receptive field evoked the typical brief excitation in control rats. However, in neuropathic rats, innocuous stimulation also induced a nociceptive-like afterdischarge that persisted 2-3 min. This afterdischarge was never observed in control rats, and, in this model, is the distinguishing feature of the spinal neural correlate of tactile allodynia. Electrical stimulation of the sciatic nerve at 4 and at 20 Hz each produced an initial discharge that was identical in control and in neuropathic rats. This stimulation also produced an afterdischarge that was similar at the two frequencies in control rats. However, in neuropathic rats, the afterdischarge produced by 20-Hz stimulation was greater than that produced by 4-Hz stimulation. Given that acutely spinalized rats were studied, only peripheral and/or spinal mechanisms can account for the data obtained; as synaptic responses from C fibers begin to fail above approximately 5-Hz stimulation [Pain 46 (1991) 327], the afterdischarge in response to 20-Hz stimulation suggests a change mainly in myelinated afferents and a predominant role of these fibers in eliciting this afterdischarge. These data are consistent with the suggestion that peripheral neuropathy induces phenotypic changes predominantly in myelinated afferents, the sensory neurons that normally respond to mechanical stimulation. The NK-1 receptor antagonist, CP-99,994 (0.5 mg/kg, i.v.), depressed the innocuous pressure-evoked afterdischarge but not the brief initial discharge of wide dynamic range neurons, and decreased the elevated ongoing rate of discharge in neuropathic rats. These results support the concept that following peripheral neuropathy, myelinated afferents may now synthesize and release substance P. A result of this is that tonic release of substance P from the central terminals of these phenotypically altered neurons would lead to ongoing excitation of NK-1-expressing nociceptive spinal neurons. In addition, these spinal neurons would also exhibit exaggerated responses to innocuous pressure stimulation. The data in this study put forth a possible neurophysiological and neurochemical basis of neuropathic pain and identify substance P and the NK-1 receptor as potential neurochemical targets for its management.

Spinal shock revisited: a four-phase model

Spinal shock has been of interest to clinicians for over two centuries. Advances in our understanding of both the neurophysiology of the spinal cord and neuroplasticity following spinal cord injury have provided us with additional insight into the phenomena of spinal shock. In this review, we provide a historical background followed by a description of a novel four-phase model for understanding and describing spinal shock. Clinical implications of the model are discussed as well.

Long-lasting changes in rostral ventral medulla neuronal activity after inflammation

Activity-dependent synaptic plasticity occurs in many regions of the central nervous system. It is known that spinal sensory transmission undergoes long-lasting changes after tissue injury and inflammation, but much less is known about descending modulation. In the present study, we demonstrate that tissue injury causes long-lasting changes within the rostral ventral medulla, a region critical for descending modulation of spinal sensory transmission. Subcutaneous formalin injections induced changes in the activity of rostral ventral medulla neurons, particularly during phase 2 (10 to 55 minutes after injection). The activity of neutral cells, which showed no response to acute noxious stimuli, was significantly decreased after formalin injection. Furthermore, 2 "silent" cells became active after the formalin injection. To demonstrate directly that descending biphasic modulation from the rostral ventral medulla was affected after the inflammation, we investigated descending modulation of a spinal nociceptive reflex produced by focal electrical stimulation in the rostral ventral medulla and found that both facilitation and inhibition were significantly decreased. These data suggest that rostral ventral medulla circuits modulating spinal sensory transmission undergo profound and long-lasting changes after tissue injury and inflammation. This may contribute to the pathological modification of nociceptive processing in chronic pain states.

Low dose systemic morphine attenuates operant escape but facilitates innate reflex responses to thermal stimulation

Effects of systemic morphine on operant escape responses of rats to thermal stimulation were compared directly with effects on innate licking and guarding responses. For these independent tests, thermal stimulation was delivered via the floor of testing chambers with or without platforms that provided an escape option. The principal findings were (1) administration of 0.5 to 1.5 mg/kg morphine attenuated escape from nociceptive heat and (2) in distinct contrast, licking and guarding responses to heat were enhanced by these doses. When escape responding was calculated as time on the heated plate without licking or guarding, sensitivity to morphine was greater for 44 degrees C than for 47 degrees C or 50 degrees C. Also, escape responses to cold (0 degrees C or 10 degrees C) were unaffected by 1.5 mg/kg morphine. The preferential reduction of heat nociception by morphine was demonstrated also by an operant preference task that gave the animals the option of standing on a cold (10 degrees C) or a hot (45 degrees C) surface. Administration of 0.5 mg/kg morphine increased occupancy of the hot surface. Platform time during operant tests was low and variable for warm stimulation (36 degrees C) and was significantly increased by each level of heat, showing that platform occupancy represented escape from nociception rather than avoidance responses. A lack of significant effects of 1.5 mg/kg morphine on operant performance during cold or warm stimulation controls for effects of systemic morphine other than antinociception.

Peripheral and central sites of action for A-85380 in the spinal nerve ligation model of neuropathic pain

Neuronal nicotinic receptor (NNR) agonists such as ABT 594 have been shown to be effective in a wide range of preclinical models of acute and neuropathic pain. The present study, using the NNR agonist A-85380, sought to determine if NNR agonists are acting via similar or differing mechanisms to induce anti-nociception and anti-allodynia. A systemic administration of the quaternary NNR antagonist chlorisondamine (0.4 micromol/kg, intraperitoneal (i.p.)) did not alter A-85380-induced (0.75 micromol/kg, i.p.) anti-nociception in the rat paw withdrawal model of acute thermal pain. In contrast, previous studies have demonstrated that blockade of central NNRs by prior administration of chlorisondamine (10 microg i.c.v.) prevents A-85380 induced anti-nociception indicating a predominantly central site of action of NNR agonists in relieving acute pain. In the rat spinal nerve ligation model of neuropathic pain, A-85380 induced a dose-dependent anti-allodynia (0.5-1.0 micromol/kg) that was blocked by pretreatment with mecamylamine (1 micromol/kg). Interestingly, unlike acute pain, both systemic and central administration of chlorisondamine blocked A-85380-induced anti-allodynia, an effect that was determined not to be due to a non-specific effect of chlorisondamine or to chlorisondamine crossing the blood-brain barrier. The peripheral site of action was shown not to be the primary receptive field, since A-85380 had equally potent anti-allodynic effects when it was injected into either the affected or unaffected paw. In contrast, infusion of A-85380 directly onto the L5 dorsal root ganglion on the affected side resulted in a dose-dependent and marked anti-allodynia (10-20 microg) at doses that had no effect when injected systemically. This effect was blocked by pretreatment with chlorisondamine. Together these data further support the idea that different mechanisms underlie different pain states and suggest that the effects of NNR agonists in neuropathic pain may be due in part to peripheral actions of the compounds.

Differential recruitment of endogenous pain inhibitory systems in neuropathic pain patients

Neuronal hyperexcitability is a key finding in patients with neuropathic pain. Contributing to hyperexcitability may be decreased activity in the endogenous pain inhibitory systems. The present study aimed at recruiting descending inhibition, by the use of painful heterotopic stimulation (HTS), in 16 patients with peripheral chronic neuropathic pain and associated brush-evoked allodynia. Two experiments were performed: one examined the effect of HTS on ongoing pain and intensity of brush-evoked allodynia and the other tested the effect of HTS on ongoing pain and area of brush-evoked allodynia. Both experiments consisted of two sessions, one with painful cold HTS (1 degrees C water bath) another with non-painful neutral HTS (32 degrees C water bath). The area of brush-evoked allodynia was significantly reduced (P=0.003) during painful HTS, as compared to non-painful HTS. In contrast, neither the intensity of brush-evoked allodynia nor the ongoing pain was significantly changed. The results indicate that endogenous pain modulating systems can alter some aspects of chronic neuropathic brush-evoked allodynia. The differential effect of painful HTS on ongoing pain and area of brush-evoked allodynia suggest that separate mechanisms are involved.

Comparison of the pain suppressive effects of clinical and experimental painful conditioning stimuli

Studies in healthy volunteers suggested that the classical counterirritation phenomenon (i.e. pain inhibits pain effect) might depend on diffuse noxious inhibitory controls (DNIC), which modulate the spinal transmission of nociceptive signals. In the present study, we sought to determine whether similar mechanisms were at play in patients with different subtypes of neuropathic pain. Ten patients presenting with a traumatic peripheral nerve injury associated with dynamic mechano-allodynia (i.e. pain triggered by brushing) or static mechano-allodynia (i.e. pain triggered by light pressure stimuli) were included in this study. To investigate counterirritation mechanisms in these patients, we analysed the RIII nociceptive flexion reflex and concomitant painful sensation elicited by electrical stimulation of the sural nerve. We compared the effects of heterotopic 'clinical' conditioning stimuli (i.e. pain evoked by brushing or pressure within the allodynic area located in the upper limb or chest) to those of experimental heterotopic noxious stimuli (HNCS) consisting of a cold pressor test or tourniquet test applied to the normal upper limb. Static mechano-allodynia induced inhibitions of both the RIII reflex and the concomitant painful sensation. These effects were similar to those induced by HNCS and were probably due to an increased activation of DNIC. In contrast, in patients with dynamic allodynia, brushing within the allodynic area reduced the pain sensation at the foot, but did not inhibit the electrophysiological responses, suggesting that in this case the counterirritation effect may take place at the supraspinal level. Thus, the mechanisms of counterirritation are not univocal, but depend on the pathophysiological mechanisms of clinical pain.