Consequences of Transient Focal Cerebral Ischaemia for Second Messenger and Neurotransmitter Binding in the Rat: Quantitative Autoradiographic Analysis of Forskolin, Dopamine D1 Receptor Binding and Cerebral Blood Flow Changes

In order to study the consequences of reperfusion for ischaemic brain injury, quantitative ligand binding autoradiography was carried out in a model of reversible focal cerebral ischaemia. Endothelin-1 applied to the abluminal surface of the middle cerebral artery in anaesthetized Sprague-Dawley rats induced severe focal ischaemia and subsequent reperfusion (assessed by blood flow tracers ^99mTc]HMPAO and ¹⁴C]iodoantipyrine respectively) by 2 h after insult. Ligand binding autoradiography on consecutive sections demonstrated these blood flow changes to be associated with a significant reduction in forskolin binding throughout the middle cerebral artery territory (e.g. 25% in parietal cortex, 11% in dorsolateral caudate nucleus). The most marked losses in forskolin binding were in areas where ischaemia was severe and reperfusion was poor. However, the same changes in cerebral blood flow had no significant effect on D₁ dopamine receptor binding (e.g. >2% reduction in the caudate nucleus). These data demonstrate that ligand binding characteristics are significantly affected as early as 2 h after insult, with evidence of differential sensitivity for forskolin and D₁ dopamine binding. With regard to the consequences of reperfusion, comparison with our previous study of 2 h maintained ischaemia demonstrates reperfusion-related salvage of dopamine and forskolin binding in the caudate nucleus but possible exacerbation of forskolin binding loss in the cortex.