Adenylyl Cyclases: mRNA and Characteristics of Enzyme Activity in Three Areas of Brain

Abstract: RNase protection assays were used in a comparative analysis of the quantities of mRNA for five “calcium‐sensitive” (types I, III, V, VI, and VIII) adenylyl cyclases and one “calcium‐insensitive” (type II) adenylyl cyclase in mouse cerebral cortex, cerebellum, and nucleus accumbens. The mRNA levels for type V adenylyl cyclase were dominant in the nucleus accumbens. Type V adenylyl cyclase mRNA was also found in the cerebral cortex and at low levels in the cerebellum. Type I adenylyl cyclase mRNA was the major form in the cerebellum with 15–50‐fold higher levels compared with other adenylyl cyclase mRNAs. Type I adenylyl cyclase mRNA was also the most prominent adenylyl cyclase mRNA in the cerebral cortex, although the mRNA levels of other adenylyl cyclase forms were more comparable to those of the type I enzyme in this brain area. The mRNA levels for adenylyl cyclase types II, III, VI, and VIII were intermediate to low depending on the brain area. Cell membranes from the nucleus accumbens demonstrated adenylyl cyclase activity that was synergistically activated by concomitant addition of GTP and forskolin to assay mixtures, reflecting a characteristic of type V adenylyl cyclase protein. Calcium/calmodulin stimulated adenylyl cyclase activity in membranes from all three brain areas. However, synergistic activation of adenylyl cyclase activity by GTP and calcium/calmodulin was noted only with cortical membranes, and this characteristic may reflect the presence of type VIII adenylyl cyclase mRNA in the cortex. Although mRNA for type VIII adenylyl cyclase was almost equivalent in the cortex and cerebellum, the lack of a synergistic effect of GTP plus calcium/calmodulin on the cerebellar enzyme activity may be a result of the significant dominance of type I adenylyl cyclase mRNA (and protein) in the cerebellum. In general, the mRNA levels for the various adenylyl cyclases were predictive of the regulatory characteristics of adenylyl cyclase activity in membranes of the brain areas studied.