Adenylyl cyclase (AC) signal transduction has been shown to be affected in Alzheimer's disease (AD). Deficits have been described in different components of the system, from the receptor to the effector level. [3H]forskolin is a diterpene that binds with high affinity to AC. In the present report, we used autoradiography to study [3H]forskolin binding to sections of entorhinal cortex and hippocampus from 23 cases staged for AD pathology according to Braak and Braak [Acta Neuropathol. 82 (1991) 239-259]. This protocol defines six stages according to neurofibrillary changes, which start in the entorhinal region (stages I-II), spread to the hippocampus (stages III-IV) and finally to the isocortical areas (stages V-VI). The amyloid classification includes three stages in which the basal isocortex is first affected (stage A), followed by other isocortical association areas (stage B) and finally the primary isocortical areas (stage C). We also studied the effects of the GTP-analogue Gpp[NH]p on binding, in order to detect changes in G-protein-AC coupling. We used two different concentrations of Gpp[NH]p, that were previously reported to inhibit and stimulate [3H]forskolin binding via Gi and Gs, respectively. Results showed that [3H]forskolin binding declined significantly with staging for neurofibrillary changes only in the entorhinal region (P < 0.05, ANOVA). In addition, the decrease in [3H]forskolin binding observed in the presence of 1 microM Gpp[NH]p diminished significantly with staging in the entorhinal region (P < 0.05, ANOVA). No significant changes were seen with amyloid staging, with the exception of the CA1 subfield of the hippocampus, where [3H]forskolin binding in the absence of Gpp[NH]p was significantly decreased at stage B compared with all other stages (P < 0.05, ANOVA). In conclusion, our results showed a very limited decrease in [3H]forskolin binding with the progression of AD pathology, suggesting that the AC levels may be largely preserved in the disease. The specific change in the effect of a low concentration of Gpp[NH]p on the binding could indicate the loss of Ca2+/calmodulin-sensitive AC isoforms in AD.