Recovery from focal brain ischemia induced by extradural compression in diabetic and non-diabetic rats

Abstract

Cerebrovascular disease and head trauma are among the leading causes of death and disability. In addition to severe motor disturbances, ischemia resulting from stroke or traumatic brain injury (TBI) can cause mild to severe cognitive deficits. One-third of all patients with stroke show hyperglycemia upon hospital admission, and the majority of these are diabetic. The combination of diabetes and stroke was reported to worsen recovery.

In agreement with clinical evidence, larger infarct sizes and neurodegeneration were observed in rats following hyperglycemic brain ischemia. The Goto-Kakizaki (GK) rat is a non-insulin-dependent, spontaneously diabetic animal and, to our knowledge, few studies of recovery from focal brain ischemia were performed in this rat strain. In our laboratory, we have developed a model of experimental brain injury in the Sprague-Dawley rat produced by transient, short-lasting (30 min) unilateral extradural compression (EC) of the right sensorimotor cortex. EC produces neurological manifestations (contralateral fore- and hind-limb paresis) and selective neuronal death in the cortex, hippocampus and thalamus, resembling clinical cortical stroke.

The aims of the present thesis were to (1) characterize motor and cognitive deficits in both non-diabetic Wistar and diabetic GK rats following EC, (2) to study compression induced-neurodegeneration and potential correlations with the behavioural findings in non-diabetic Wistar and diabetic GK rats, (3) to compare glucose levels and cerebral blood flow (CBF) upon EC and reperfusion in both strains and (4) to investigate strain differences in the expression of several antioxidant and heme-degrading enzymes.

Recovery of motor and cognitive functions following EC were assessed with the lever-press task (LPT) and locomotor activity (LMA) monitoring in a novel environment, in parallel with the beam walking and the rotarod. Neurodegeneration induced by EC was concomitantly characterized in several brain regions by using Fluoro-Jade (FJ) as a marker of neurodegeneration and GFAP as marker of reactive astrocytosis at 2, 5, 7, 10 and 14 days in Wistar rats and at 2, 7 and 14 days in GK rats. Cortical CBF upon EC and during reperfusion was measured with Laser-Doppler flowmetry. At 48 h post-EC, mRNA expression of heme-degrading enzymes (HO-1, HO-2), biliverdin reductase (BVR), superoxide dismutases (SOD-1, SOD-2), inflammatory and proapoptotic markers (iNOS, TNFalpha, Bax) were compared between strains with real time RT-PCR. HO-1 expression at 48 h post-EC was studied using double-fluorescence immunohistochemistry for neurons (Fluorescent Nissl staining), astrocytes (GFAP) and microglia (OX-42).

Locomotor and exploratory activities of compressed Wistar rats were reduced, in parallel with hemiparesis, detected on the beam walking and on the rotarod on day 1. The LMA parameters normalized on day 2, whereas a phase of increased locomotor activity coupled with deficient habituation to the environment was observed on day 3. Importantly, the deficient habituation was no longer attributable to the motor impairments. The learning of the LPT was delayed in naive-to-task Wistar rats up to 10 days after EC. Fluoro-Jade/GFAP staining demonstrated a consistent pattern of cortical, striatal and thalamic degeneration but revealed variable degrees of degeneration in hippocampal areas. The improvement in LPT performance of naïve-compressed rats was followed by a reduction of damage in cortical associative areas. Additional lesion-effects from damaged hippocampii may have overlapped in a minority of subjects, while the subcortical lesions provoked by EC were unlikely to explain the behavioural findings.

GK but not control Wistar rats showed a pronounced hyperglycemic response upon EC, a lower degree of cortical CBF recovery during reperfusion, impaired behavioural habituation to a novel environment on the first five days post-ischemia, impaired learning of a LPT two weeks after EC and a higher degree of neurodegeneration labelled by FJ in the cortex, hippocampus and thalamus at virtually all time-points post-ischemia. Under basal conditions, GK rats exhibited higher mRNA expression of heme degrading, antioxidant and pro-inflammatory genes such as HO-1, iNOS and TNFalpha under basal conditions. At 48 h post-ischemia, HO- 1 was one of the main upregulated genes in the ipsilateral cortex of both diabetic and non-diabetic rats. HO-1 secretion was localized in peri-lesional astrocytes and few microglial cells.

Previous experience with the task and familiarity with the environment appear to accelerate recovery from brain ischemia and may initiate compensatory mechanisms at early stages of recovery, with emphasis on the associative cortical areas. The GK rat consistently showed aggravated hyperglycemia, worsened cortical reperfusion and longer-lasting impairments of motor and cognitive functions encouraging further brain injury studies in this rat strain. Counteracting oxidative stress caused by heme degradation and neuroinflammation following normo- and hyperglycemic brain ischemia may thus provide an effective therapy for focal brain ischemia due to a potentially extended therapeutic window.