Vascular and haemodynamic response following chronic hypoperfusion in the developing and mature rat

Abstract

Arterial occlusion is one of the main mechanisms leading to hypoperfusion. Vascular growth and collateral circulation can compensate for arterial occlusion and reduce the effects of hypoperfusion. Aberrations in blood flow due to arterial occlusion have been identified in childhood pathologies, yet there have been limited experimental investigations in the vascular response of the developing brain. Therefore we have investigated the chronic vascular and morphological changes following bilateral common carotid occlusion (BCCAO) in both the mature and developing brain using MRI and histology. Four adult male rats were studied in experiments where CBF was measured before and immediately after BCCAO surgery. For chronic experiments 8 adult rats and 8 newborn pups (3-day old) were subjected to BCCAO or sham surgery. Single slice coronal images were obtained 3.3 mm from bregma on a 2.35 T SMIS system using dedicated sequences to measure CBF, T1, T2 and ADC. A high-resolution SE anatomical scan was obtained with 128 128 pixels, 1 mm slice thickness, 17 slices, FOV 30 mm. MR angiography (MRA) of the neck was done with 128 128 128 pixels, FOV 25 25 30 mm and visualisation by maximal intensity projection (MIP) software. To visualise the cerebro-vascular structure papaverine hydrochloride was injected intravenously to induce maximal dilatation and India ink was subsequently injected. There were significant reductions in CBF immediately after BCCAO surgery in the cortices and hippocampi (50%) and a 20% decrease in the thalami, but no reduction in the apparent diffusion coefficient (ADC) was observed, therefore oligaemic hypoperfusion was established. However, 6 months following surgery, CBF was restored in both adults and pups. No changes were observed in the ADC, T1 or T2 values 6 months post-surgery. To investigate the underlying mechanism for the return of CBF to control values, we performed MRA of the neck. As expected, signal from the common carotid arteries was present in the sham-operated rats, but was absent in the BCCAO animals. Interestingly, signal from the vertebral arteries appeared to be enhanced and the presence of collateral formation was evident. Two observers blinded to the experiment evaluated the MRA images and reported more tortuous vertebral arteries in the BCCAO adults and more midline collaterals in the BCCAO pups indicating different extracerebral modes of adaptation dependent on the age at onset of the insult. Furthermore, the vessel diameters of various arteries which make up the circle of Willis were measured 6 months after surgery. Highly significant (p<0.001) differences were seen between sham-operated and BCCOA animals in the basilar, posterior cerebral, posterior communicating arteries and in the internal carotid and anterior cerebral arteries (p<0.01). Interestingly, no significant differences were observed between adult and neonatal age-groups for any of the vessels measured (2-way ANOVA). Our studies suggest that the developing and mature animal exhibit different patterns of arteriogenesis and that the BCCAO hypoperfusion model will be useful for investigating responses to vaso-occlusive disease. Further, non-invasive MRI measurements of cerebral haemodynamics may provide a useful indicator of the homeostatic responses to chronic hypoperfusion.