Ontogenetic development of the G protein-mediated adenylyl cyclase signalling in rat brain

Maturation of the brain adenylyl cyclase (AC) signalling system was investigated in the developing rat cortex, thalamus and hippocampus. Expression of AC type II, IV and VI measured by Western blot dramatically increased in all tested brain regions during the first 3 weeks after birth and these levels were maintained in adulthood. AC type I did not change during ontogenesis. In parallel, AC enzyme activities were determined in order to obtain the functional correlates to the preceding structural (immunoblot) analyses of trimeric G proteins [Ihnatovych et al., Dev. Brain Res. (2002) in press]. Surprisingly, basal, manganese-, fluoride-, forskolin- and GTP-stimulated adenylyl cyclase developed similarly. The relatively low enzyme activities, which were determined at birth, progressively increased (about four times) to a clear maximum around postnatal day PD 12. This was followed by a progressive regression to adulthood so that activity of AC at PD 90 was comparable with the low neonatal level. The peak of AC activities at PD 12 was detected in all tested brain regions. Stimulatory (isoproterenol) effect on basal AC activity as well as inhibitory (baclofen) effect on forskolin-stimulated AC activity were unchanged between PD 12 and PD 90. Thus, comparison of results of the structural and functional analyses of adenylyl cyclase signalling system revealed a clear dissociation between the increase in the amount protein of various AC isoforms and the decrease of total G-protein mediated enzyme activities between PD 12 and adulthood. As none of the complex changes in trimeric G protein levels can explain this difference, the future research has to be oriented to identification of potential negative regulators of AC in the course of brain development. Among these, the newly discovered group of GTPase activating proteins, RGS, appears to be of primary importance because these proteins represent potent negative regulators of any G protein-mediated signalling in brain.