The Effect of Amitriptyline on Ectopic Discharge of Primary Afferent Fibers in the L5 Dorsal Root in a Rat Model of Neuropathic Pain

Contact hypersensitivity to oxazolone provokes vulvar mechanical hyperalgesia in mice

The interplay among pain, allergy and dysregulated inflammation promises to yield significant conceptual advances in immunology and chronic pain. Hapten-mediated contact hypersensitivity reactions are used to model skin allergies in rodents but have not been utilized to study associated changes in pain perception in the affected skin. Here we characterized changes in mechanical hyperalgesia in oxazolone-sensitized female mice challenged with single and repeated labiar skin exposure to oxazolone. Female mice were sensitized with topical oxazolone on their flanks and challenged 1-3 times on the labia. We then measured mechanical sensitivity of the vulvar region with an electronic pressure meter and evaluated expression of inflammatory genes, leukocyte influx and levels of innervation in the labiar tissue. Oxazolone-sensitized mice developed vulvar mechanical hyperalgesia after a single labiar oxazolone challenge. Hyperalgesia lasted up to 24 hours along with local influx of neutrophils, upregulation of inflammatory cytokine gene expression, and increased density of cutaneous labiar nerve fibers. Three daily oxazolone challenges produced vulvar mechanical hyperalgesic responses and increases in nerve density that were detectable up to 5 days post-challenge even after overt inflammation resolved. This persistent vulvar hyperalgesia is resonant with vulvodynia, an understudied chronic pain condition that is remarkably prevalent in 18-60 year-old women. An elevated risk for vulvodynia has been associated with a history of environmental allergies. Our pre-clinical model can be readily adapted to regimens of chronic exposures and long-term assessment of vulvar pain with and without concurrent inflammation to improve our understanding of mechanisms underlying subsets of vulvodynia and to develop new therapeutics for this condition.

Long-term changes in trigeminal ganglionic and thalamic neuronal activities following inferior alveolar nerve transection in behaving rats

The transection of the inferior alveolar nerve (IANx) produces allodynia in the whisker pad (V2 division) of rats. Ectopic discharges from injured trigeminal ganglion (TG) neurons and thalamocortical reorganization are possible contributors to the sensitization of uninjured V2 primary and CNS neurons. To test which factor is more important, TG and ventroposterior medial nucleus (VPM) neurons were longitudinally followed before, during, and after IANx for up to 80 d. Spontaneous discharges and mechanical stimulation-evoked responses were recorded in conscious and in anesthetized states. Results show (1) a sequential increase in spontaneous activities, first in the injured TG neurons of the IAN (2-30 d), followed by uninjured V2 ganglion neurons (6-30 d), and then VPM V2 neurons (7-30 d) after IANx; (2) ectopic discharges included burst and regular firing patterns in the IAN and V2 branches of the TG neurons; and (3) the receptive field expanded, the modality shifted, and long-lasting after-discharges occurred only in VPM V2 neurons. All of these changes appeared in the late or maintenance phase (7-30 d) and disappeared during the recovery phase (40-60 d). These observations suggest that ectopic barrages in the injured IAN contribute more to the development of sensitization, whereas the modality shift and evoked after-discharges in the VPM thalamic neurons contribute more to the maintenance phase of allodynia by redirecting tactile information to the cortex as nociceptive.

Segmental spinal root avulsion in the adult rat: a model to study avulsion injury pain

Road traffic accidents are the most common cause of avulsion injury, in which spinal roots are torn from the spinal cord. Patients suffer from a loss of sensorimotor function, intractable spontaneous pain, and border-zone hypersensitivity. The neuropathic pains are particularly difficult to treat because the lack of a well-established animal model of avulsion injury prevents identifying the underlying mechanisms and hinders the development of efficacious drugs. This article describes a hindlimb model of avulsion injury in adult rats where the L5 dorsal and ventral spinal root are unilaterally avulsed (spinal root avulsion [SRA]), leaving the adjacent L4 spinal root intact. SRA produced a significant ipsilateral hypersensitivity to mechanical and thermal stimulation by 5 days compared with sham-operated or naïve rats. This hypersensitivity is maintained for up to 60 days. No autotomy was observed and locomotor deficits were minimal. The hypersensitivity to peripheral stimuli could be temporarily ameliorated by administration of amitriptyline and carbamazepine, drugs that are currently prescribed to avulsion patients. Histological assessment of the L4 ganglion cells revealed no significant alterations in calcitonin gene-related peptide (CGRP), IB4, transient receptor potential cation channel subfamily V member 1 (TrpV1), or N52 staining across groups. Immunohistochemistry of the spinal cord revealed a localized glial response, phagocyte infiltration, and neuronal loss within the ipsilateral avulsed segment. A comparable response from glia and phagocytes was also found in the intact L4 spinal cord, supporting the role for central mechanisms within the L4-5 spinal cord in contributing to the generation of the pain-related behavior. The SRA model provides a platform to investigate possible new pharmacological treatments for avulsion injuries.

Pretreatment with intrathecal amitriptyline potentiates anti-hyperalgesic effects of post-injury intra-peritoneal amitriptyline following spinal nerve ligation

Background

Amitriptyline, a tricyclic antidepressant and potent use-dependent blocker of sodium channels, has been shown to attenuate acute and chronic pain in several preclinical modes. The purpose of this study was to investigate whether intrathecal pretreatment with amitriptyline combined with post-injury intra-peritoneal amitriptyline is more effective than post-injury treatment alone on L5 spinal nerve ligation (SNL)-induced neuropathic pain.

Methods

96 adult male Sprague–Dawley rats were allocated into 4 groups: group S, Sham; group L, L5 spinal nerve Ligation with vehicle treatment; group A, SNL and post-injury intra-peritoneal (Abdominal) amitriptyline twice daily × 3 days; group P, intrathecal Pretreatment with amitriptyline, SNL and intra-peritoneal amitriptyline twice daily × 3 days. Responses to thermal and mechanical stimuli, as well as sodium channel expression in injured dorsal root ganglion (DRG) and activated glial cells in spinal dorsal horn (SDH) were measured pre-operatively and on post-operative day (POD) 4, 7, 14, 21 and 28.

Results

SNL-evoked hyper-sensitivity responses to thermal and mechanical stimuli, up-regulated Nav1.3 and down-regulated Nav1.8 expression in DRG, and activated microglia and astrocytes in SDH. In group A, intra-peritoneal amitriptyline alone alleviated thermal hypersensitivity on POD7, reversed Nav1.8 and reduced activated microglia on POD14. In group P, intrathecal pretreatment with amitriptyline not only potentiated the effect of intra-peritoneal amitriptyline on thermal hypersensitivity and Nav1.8, but attenuated mechanical hypersensitivity on POD7 and reduced up-regulated Nav1.3 on POD14. Furthermore, this treatment regimen reduced astrocyte activation on POD14.

Conclusions

Concomitant intrathecal pretreatment and post-injury intra-peritoneal amitriptyline was more effective than post-injury treatment alone on attenuation of behavioral hypersensitivity, decrease of activated microglia and astrocytes and dysregulated Nav1.3 and 1.8.

Amitriptyline does not block the action of ATP at human P2X4 receptor

BACKGROUND AND PURPOSE

Amitriptyline is a tricyclic antidepressant that is also widely used to treat neuropathic pain in humans, but the mechanism of this anti-hyperalgesic effect is unknown. Microglia in the mouse spinal cord become activated in neuropathic pain, and expression of P2X4 receptors by these microglia is increased. Antisense RNA targeting P2X4 receptors suppresses the development of tactile allodynia in rats. This suggests that blockade of P2X4 receptors might be the mechanism by which amitriptyline relieves neuropathic pain.

EXPERIMENTAL APPROACH

We expressed human, rat and mouse P2X receptors (P2X2, P2X4, P2X7) in human embryonic kidney cells and evoked inward currents by applying ATP. We compared the action of ATP on control cells and cells treated with amitriptyline.

KEY RESULTS

Amitriptyline (10 microM), either applied acutely or by pre-incubation for 2-6 h, had no effect on inward currents evoked by ATP (0.3-100 microM) at human P2X4 receptors. At rat and mouse receptors, amitriptyline (10 microM) caused a modest reduction in the maximum responses to ATP, without changes in EC(50) values, but it had no effect at 1 microM. Amitriptyline also had no effects on currents evoked by ATP at rat P2X2 receptors, or at rat or human P2X7 receptors.

CONCLUSION AND IMPLICATIONS

The results do not support the view that amitriptyline owes its pain-relieving actions in man to the direct blockade of P2X4 receptors.