Cerebral blood flow alteration in neuroprotection following cerebral ischaemia

Abstract

The best neuroprotectant for acute ischaemic stroke would always be the rapid return of oxygen and glucose to physiological levels. This is currently provided by thrombolysis which restores blood flow to the ischaemic region. The attempt to confer neuroprotection by targeting the brain parenchyma has shown promise in experimental stroke models, but has unequivocally failed to translate to the clinic. Neuroprotective therapy primarily targets the biochemical cascade that produces cell death following cerebral ischaemia. However, these agents may also alter signal transduction that controls cerebral blood flow, for example glutamate, which may affect the outcome after ischaemia. In these cases, neuroprotection may potentially be due to the improved access to oxygen and glucose rather than biochemical prevention of cell death. Improvement in cerebral blood flow is an important but often overlooked effect of neuroprotective therapy, analogous to the protective effects of drug-induced hypothermia. This short review will discuss cerebral blood flow alteration and protection of the brain in the context of ischaemic preconditioning, oxygen sensing and thrombolysis. Future neuroprotection studies in cerebral ischaemia require stringent monitoring of cerebral blood flow, plus other physiological parameters. This will increase the chances that any protection observed may be able to translate to human therapy.

Figure 1. CBF during ischaemia and reperfusion and subsequent injury in the rat model of MCAO

A, methodology of the MCAO technique and ethical regulations associated with these experiments have been previously described (Nagel et al. 2011). Laser Doppler flowmetry was used to measure relative CBF over the right somatosensory cortex of a male Wistar rat. Baseline CBF was normalized to 100% blood flow units (BFU). Upon temporary common carotid artery (CCA) ligation, CBF was reduced to 60% BFU. A silicon-coated 4-0 monofilament was then inserted through the external carotid artery and advanced up the internal carotid artery to occlude the right middle cerebral artery (MCA). MCAO was confirmed by a sharp decrease in CBF to < 20% BFU, and this was maintained for 90 min. Reperfusion of the MCA was achieved through retraction of the monofilament, when a sharp increase in CBF and a small hyperaemia was observed. After 5 min of MCA reperfusion (to allow removal of the monofilament), the CCA was unclamped to allow full reperfusion to the ischaemic area, which produced a further increase in CBF and hyperaemia lasting approximately 10 min. This was followed by a post-ischaemic hypoperfusion at 50% BFU during the next 60 min. B, cerebral injury was observed 24 h post-ischaemia onset in the striatum and the cortex (arrows) from the same animal as A using triphenyl-tetrazolium staining.

Figure 2. The control of CBF by glutamate through astrocytes and neurons

Glutamate released from synapses can modulate vascular tone and the subsequent supply of oxygen and glucose through neurons and astrocytes. In neurons, glutamate activates NMDA receptors increasing [Ca²⁺]_i and either producing NO through neuronal nitric oxide synthase (nNOS) or prostaglandins (PG) through phospholipase A₂ (PLA₂) and cyclooxygenase-2 to dilate vessels. In astrocytes, glutamate acts on metabotropic glutamate receptors (mGluR) which increases [Ca²⁺]_i and generates arachidonic acid (AA). Three metabolites are derived from AA: PG and EETs in astrocytes which dilate vessels, and 20-HETE in smooth muscle which constricts vessels. Ca²⁺-gated K⁺ channels (g_K(Ca)) on astrocytic endfeet are also activated which releases K⁺ to dilate vessels. This figure has been reproduced with permission from Attwell et al. (2010) and Nature Publishing Group.

Figure 3. The effects of dimethyloxalylglycine (DMOG) on cerebral injury and CBF following permanent MCAO

A, one representative animal per group is presented by diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps. In DWI, infarcts are detected as areas of hyperintensity; in ADC maps, infarcts are detected as areas of hypointensity. B, quantification of ADC infarct volumes 24 h post-MCAO onset show that both 40 mg kg^-1 DMOG and 200 mg kg^-1 DMOG reduced infarct volume compared to control. *P < 0.05 versus control. C, arterial spin labelling: perfusion-weighted imaging was used to create rCBF maps for each group 1 h, 3 h and 24 h post-MCAO. Areas with reduced blood flow are detected as areas of hypointensity. D, rCBF values decreased significantly for all groups following occlusion. During occlusion, rCBF values were not significantly different between groups, but at 1 and 24 h there was a tendency towards higher rCBF values in the 40 mg kg^-1 DMOG group compared with the other groups. #P < 0.1 versus control. E, mean rCBF over time was inversely correlated (P < 0.001) with final infarct volume, with the 40 mg kg^-1 DMOG group producing higher rCBF (50% of animals were above the critical threshold of 30% rCBF of baseline) associated with lower infarcts. This figure has been adapted and reproduced with permission from Nagel et al. (2011) and Nature Publishing Group.