IMMUNOHISTOCHEMICAL LOCALIZATION OF ADENYLYL-CYCLASE IN RAT-BRAIN INDICATES A HIGHLY SELECTIVE CONCENTRATION AT SYNAPSES

Only three isoforms of adenylyl cyclase (EC 4.6.1.1) mRNAs (AC1, -2, a nd -5) are expressed at high levels in rat brain. AC1 occurs predomina ntly in hippocampus acid cerebellum, AC5 is restricted to the basal ga nglia, whereas AC2 is more widely expressed, but at much lower levels. The distribution and abundance of adenylyl cyclase protein were exami ned by immunohistochemistry with an antiserum that recognizes a peptid e sequence shared by all known mammalian adenylyl cyclase isoforms. Th e immunoreactivity in striatum and hippocampus could be readily interp reted within the context of previous in situ hybridization studies. Ho wever, extending the information that could be gathered by comparisons with in situ hybridization analysis, it was apparent that staining wa s confined to the neuropil-corresponding to immunoreactive dendrites a nd axon terminals. Electron microscopy indicated a remarkably selectiv e subcellular distribution of adenylyl cyclase protein. In the CA1 are a of the hippocampus, the densest immunoreactivity was seen in postsyn aptic densities in dendritic spine heads. Labeled presynaptic axon ter minals were also observed, indicating the participation of adenylyl cy clase in the regulation of neurotransmitter release. The selective con centration of adenylyl cyclases at synaptic sites provides morphologic al data for understanding the pre- and postsynaptic roles of adenylyl cyclase in discrete neuronal circuits in rat brain. The apparent clust ering of adenylyl cyclases, coupled with other data that suggest highe r-order associations of regulatory elements including G proteins, N-me thyl-D-aspartate receptors, and cAMP-dependent protein kinases, sugges ts not only that the primary structural information has been encoded t o render the cAMP system responsive to the Ca2+-signaling system but a lso that higher-order strictures are in place to ensure that Ca2+ sign als are economically delivered and propagated.