ALTERATIONS IN [H-3] MK-801, [H-3] MUSCIMOL, [H-3] CYCLIC-AMP, AND [H-3] ROLIPRAM BINDING IN THE GERBIL HIPPOCAMPUS FOLLOWING REPEATED ISCHEMIC INSULTS

Using [H-3]MK-801, [H-3]muscimol, [H-3]cyclic AMP, and [H-3]rolipram, we performed quantitative in vitro autoradiography to determine sequential alterations in the binding of N-methyl-D-aspartate and GABA(A) receptors, particulate cyclic AMP-dependent protein kinase, and cyclic AMP-selective phosphodiesterase, respectively, in the gerbil hippocampus following repeated brief ischemic insults. Changes from 1 h to 28 days after three 2-min ischemic insults at 1-h intervals were compared with those after 2 and 6 min of ischemia. We observed no alterations in the binding of all the four ligands throughout the observation period following 2 min of ischemia which produced no histological neuronal damage except for transient reductions in [H-3]cyclic AMP binding during the early reperfusion period. [H-3]Cyclic AMP binding in the CA1 subfield decreased one day after 6 min of ischemia and four days after three 2-min ischemic insults, and 62-65% of the binding was lost after 28 days. [H-3]Rolipram binding in the CA1 subfield decreased one day after 6 min of ischemia and the binding was reduced by 45-50% after four and 28 days. Following three 2-min ischemic insults, [H-3]rolipram binding decreased in the CA1 at one day, and decreased by 25-33% after 28 days. Both [H-3]MK-801 and [H-3]muscimol binding was preserved during the early reperfusion period after 6 min of ischemia and three 2-min ischemic insults. Reductions in [H-3]MK-801 binding in CA1 were observed four days after ischemic insults when CA1 neuronal destruction was seen. After one month, approximately 50% of [H-3]MK-801 binding was lost in CA1 in both groups. By contrast, reductions in [H-3]muscimol binding (by 20-30%) were only observed at one month. Thus, alterations in the cyclic AMP-related second messenger system preceded histological neuronal death of CA1 pyramidal cells. The results also suggest that N-methyl-D-aspartate receptors play a role in ischemia-induced CA1 neuronal damage. Furthermore, we observed a different pattern of injury following repeated versus single ischemic episodes. Differential localization of N-methyl-D-asparate and GABA(A) receptors, and of cyclic AMP-dependent protein kinase and phosphodiesterase within different subregions of the hippocampus was also suggested.