Bilateral central pain sensitization in rats following a unilateral thalamic lesion may be treated with high doses of ketamine

Abstract

Background: Central post-stroke pain is a neuropathic pain condition caused by a vascular lesion, of either ischemic or hemorrhagic origin, in the central nervous system and more precisely involving the spinothalamocortical pathway responsible for the transmission of painful sensations. Few animal models have been developed to study this problem. The objectives of this study were to evaluate different modalities of pain in a central neuropathic pain rat model and to assess the effects of ketamine administered at different doses. Animals were evaluated on the rotarod, Hargreaves, Von Frey and acetone tests. A very small hemorrhage was created by injecting a collagenase solution in the right ventral posterolateral thalamic nucleus. Following the establishment of the neuropathy, ketamine was evaluated as a therapeutic drug for this condition.

Results: Histopathological observations showed a well localized lesion with neuronal necrosis and astrocytosis following the collagenase injection that was localized within the VPL. No significant change in motor coordination was observed following surgery in either the saline or collagensae groups. In the collagenase group, a significant decrease in mechanical allodynia threshold was observed. A sporadic and transient cold allodynia was also noted. No thermal hyperalgesia was seen following the collagenase injection. Ketamine was then tested as a potential therapeutic drug. A significant decrease in motor coordination was seen only following the administration of 25 mg/kg of ketamine in both groups. An alleviation of mechanical allodynia was achieved only with the high ketamine dose. The minimal effective ketamine serum concentration (150 ng/mL) was only achieved in animals that received 25 mg/kg.

Conclusions: An intrathalamic hemorrhage induced a bilateral mechanical allodynia in rats. Cold hyperalgesia was observed in 60% of these animals. Mechanical allodynia was alleviated with high doses of ketamine which corresponded with therapeutic plasmatic concentrations.

Figure 1

Motor coordination evaluated with the rotarod test in Spraque-Dawley rats. Animals received stereotaxically either saline (n = 8) or a collagenase solution (n = 7) in the ventroposterolateral nucleus of the thalamus. No significant difference was seen between sham and collagenase groups at baseline and at different post surgical time points up to 22 days (F_1,13 = 0.18, ns). Only the IP administration of 25 mg/kg of ketamine caused a significant impairment (p < 0.05) in motor coordination seen in both groups.

Figure 2

Heat sensitivity evaluated with Hargreave’s test in Spraque-Dawley rats. Animals received stereotaxically either saline (n = 8) or a collagenase solution (n = 7) in the ventroposterolateral nucleus of the thalamus. No significant difference in the thermal thresholds between both groups was seen at baseline and at different post surgical time points up to 22 days (F_1,13 = 0.23, ns).

Figure 3

Evaluation of mechanical allodynia using Von Frey filaments. No significant difference was seen for both hind limbs at baseline values between both saline and collagenase groups. As early as the second post-surgical day, a significant decrease in the mechanical threshold in both hind limbs was noted for the collagenase group when compared to sham animals in the right (F_1,24.5 = 130.4, p < 0.0001) and left (F_1,28.1 = 207.8, p < 0.0001) hind paws. Only the 25 mg/kg of ketamine dose reduced mechanical allodynia in the contralateral (p < 0.0001) and ipsilateral (p < 0.01) hind paws.

Figure 4

Percentage of Sprague Dawley rats reacting to acetone to determine cold hyperalgesia. Although no significant difference was seen when comparing results either prior or after surgery for either sham or collagenase groups, some animals within the collagenase groups clearly appear to be more sensitive to cold.

Figure 5

Localisation and size of the collagenase lesion. Photomicrograph of a tranverse rat brain section (4 μm) showing a well circumscribed lesion injection in VPL nucleus of the thalamus following a collagenase solution (H&E stain, x12.5). H : hippocampus, LV : lateral ventricle, T : thalamus.

Figure 6

Photomicrograph of the collagenase lesion in VPL nucleus of the thalamus stained with cresyl violet. A marked decrease in the number of normal neurons is appreciated as well as area of neuronal degeneration in the area of the VPL. (x100).

Figure 7

Photomicrograph of the collagenase lesion after GFAP immunohistochemistry in a brain section following a collagenase solution injection in VPL nucleus of the thalamus. Presence of astrocytosis (filamentous structures) is visible in the lesioned area. Mineralization (dark blue granular deposits) can be observed surrounding the lesion (lower part of the photomicrograph). (x60).