SEQUENTIAL-CHANGES IN MUSCARINIC ACETYLCHOLINE, ADENOSINE-A1 AND CALCIUM-ANTAGONIST BINDING-SITES IN THE GERBIL HIPPOCAMPUS FOLLOWING REPEATED BRIEF ISCHEMIA

We performed quantitative autoradiography to determine sequential alterations in the binding of muscarinic cholinergic and adenosine A, receptors and of an L-type Calcium channel blocker in the gerbil hippocampus following repeated brief ischemic insults. [H-3]Quinuclidinyl benzilate (QNB), [H-3]cyclohexyladenosine (CHA) and [H-3]PN200-110 were used to label muscarinic and adenosine A1 receptors and L-type calcium channels, respectively. Changes at 1 h, 6 h, 1 day, 4 days and 1 month after three 2-min ischemic insults were compared with changes after single 2- or 6-min ischemia. Two-minute ischemia, which causes no histopathological neuronal damage, produced no persistent alterations in binding sites. We observed a transient and mild increase in binding activities, especially in [H-3]CHA binding, at 1 h of recirculation. Following 6-min ischemia and three 2-min ischemic insults, [H-3]QNB and [H-3]PN200-110 binding decreased by more than 50% in the CA1 subfield by 1 month, but [H-3]CHA binding decreased transiently by 20-30% at 4 days when delayed neuronal death of hippocampal CA1 pyramidal cells took place. Reductions in binding, especially in [H-3]QNB binding, following three 2-min ischemic insults were greater and appeared earlier than those after 6-min ischemia. Furthermore, alterations extended to the CA3 subfield and the dentate gyrus following repeated insults. Thus, alterations in receptor binding after repeated ischemic insults were greater than those after equivalent single period of ischemia.