Effects of dizocilpine on autotomy behavior after nerve section in mice

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Correlation between autotomy-behavior and current theories of neuropathic pain

The past 10 years have brought several new experimental models with which to study chronic neuropathic pain in animals. Consequently, our knowledge about the mechanisms subserving neuropathic pain in humans has improved. However, the first animal model that was used for studying this type of chronic pain was the autotomy-model which can still be considered as a useful tool for pain studies. The present review assesses some of the similarities and differences between autotomy-model and more recent models of experimental traumatic mononeuropathy. In addition, it considers some of the similarities between the results obtained in clinical studies and in autotomy studies.

Substantia nigra lesion suppresses the antagonistic effects of N-methyl-D-aspartate receptor antagonist (MK-801) on the autotomy in the rat

Rats received right dorsal root ganglionectomy (DRGn) to induce autotomy, and were treated with MK-801 and/or left substantia nigra (SN) lesion after DRGn. The behavior was quantified using an autotomy grading scale. All the rats in the control groups manifested autotomy from 4 to 19 days after DRGn and attained the highest autotomy score. The group treated with MK-801 immediately after DRGn showed suppression of the development of autotomy. The groups receiving left SN lesion with 6-hydroxydopamine immediately, 2, or 4 days after DRGn showed similar patterns of autotomy as the control groups. However, when combined with the administration of MK-801 immediately after DRGn, SN lesion done immediately or 2 days after DRGn suppressed the antagonistic effect of MK-801 (P < 0.01). When the SN lesion was delayed by 4 days, the suppression effect disappeared. These data suggest that the action of the NMDA receptor antagonist on the autotomy within 4 days after DRGn depend on the integrity of the dopaminergic system.

Suppression of autotomy by N-methyl-D-aspartate receptor antagonist (MK-801) in the rat

The effects of different doses and time of administration of the N-methyl-D-aspartate (NMDA) receptor antagonist (MK-801) on the development of the autotomy (self-mutilation) were studied in the rats receiving dorsal root ganglionectomy (DRGn). The rats without any treatment and those treated with normal saline immediately after DRGn were the control groups. Three groups of rats were treated with 0.1, 0.5 or 1.0 mg/kg of MK-801 immediately after DRGn, and another three were treated with 1.0 mg/kg of MK-801 2, 4, or 7 days after DRGn. The behavioral observations of these rats were quantified using an autotomy grading scale ranging from 0 to 19, and the scores were compared among these groups. The rats in the control groups manifested autotomy from 5 to 17 days after DRGn and all of them (100%) attained the highest autotomy score. Lower doses (0.1 or 0.5 mg/kg) of MK-801 had no effect on the development of the autotomy. In contrast, higher dose (1.0 mg/kg) of MK-801 administered immediately after DRGn significantly suppressed the autotomy as compared to the control groups (P < 0.01) and only 17% of the rats in this group attained the highest score. The antagonistic effect was retained when the treatment of MK-801 was delayed to 2 days after DRGn, however, it disappeared when the treatment was delayed to 4 or 7 days after DRGn. Thus, the antagonistic effect of MK-801 on the autotomy induced by DRGn was dose-related and time-dependent. The main role of the NMDA receptor in the development of the autotomy was within several days after DRGn.

Strain differences in autotomy levels in mice: relation to spinal excitability

The consequences of combined transection of the sciatic and femoral nerves were investigated in mice of the ICR and C3HEB strains. Whereas all the animals of the C3HEB strain showed very clear self mutilatory behavior of the denervated limb (autotomy) none of the ICR mice showed autotomy. Further tests, using the hot plate and tail flick methods, show that C3HEB mice were more sensitive to noxious thermal stimuli than ICR mice. Finally, spinalization at the lumbar level revealed a markedly higher level of spinal excitability in C3HEB mice as evident from a marked decrease in nociceptive thresholds in these animals. No such threshold decrease was observed in spinalized ICR mice. The results suggest that different levels of spinal excitability underlie the susceptibility for the emergence of autotomy in mice. It is proposed that such different levels of excitability may also underlie the susceptibility for the emergence of neuropathic pain.

Attenuation of NMDA-induced spinal hypersensitivity by adrenal medullary transplants

Abnormal sensory hyperexcitability consequent to peripheral injury most likely involves activation of N-methyl-D-aspartate (NMDA) receptors in the spinal cord. This activation may lead to a cascade of neuroplastic events resulting in the exaggeration of sensory responses and the persistence of pathological pain states. Recent studies in our laboratory have demonstrated that the transplantation of adrenal medullary cells into the spinal subarachnoid space can alleviate pathological pain symptoms, possibly by reducing spinal hyperexcitability. The purpose of this study was to assess spinal NMDA activation-induced hypersensitivity to noxious and innocuous stimuli, and determine whether adrenal medullary transplants can intervene favorably to reduce these responses. Animals with either adrenal medullary or control transplants were injected intrathecally with several doses of NMDA, and responses to sensory stimuli were determined over time. NMDA at all doses tested (1-50 nmol) produced significant thermal and mechanical hyperalgesia and tactile allodynia in control transplanted animals, with peak severity at 30 min post-injection. In contrast, both the severity and duration of these exaggerated sensory responses were markedly reduced in animals with adrenal medullary transplants. To assess a possible contribution of released opioid peptides and catecholamines from the transplanted chromaffin cells, animals were pretreated with opiate antagonist naloxone or alpha-adrenergic antagonist phentolamine. While naloxone was ineffective, the phentolamine partially attenuated, but did not completely abolish, the antinociceptive effects of the transplants. The results of these studies demonstrate that adrenal medullary grafts can reduce hypersensitivity responses to NMDA-mediated activation via alpha-adrenergic modulation in addition to other neuroprotective mechanisms.