The murine Ca2+-stimulated adenylyl cyclase (type I) (EC 4.6.1.1), which is expressed predominantly in brain, was inactivated by targeted mutagenesis. Ca2+-stimulated adenylyl cyclase activity was reduced 40-60% in the hippocampus, neocortex, and cerebellum. Long-term potentiation in the CAI region of the hippocampus from mutants was perturbed relative to controls. Both the initial slope and maximum extent of changes in synaptic response were reduced. Although mutant mice learned to find a hidden platform in the Morris water task normally, they did not display a preference for the region where the platform had been when it was removed. These results indicate that disruption of the gene for the type I adenylyl cyclase produces changes in behavior and that the cAMP signal transduction pathway may play an important role in synaptic plasticity.There is considerable interest in the molecular mechanisms underlying neuroplasticity. Gene disruption studies have demonstrated that calmodulin (CaM) kinase II-a, fyn-tyrosine kinase, and protein kinase C--y are important for spatial learning and long-term potentiation (LTP) in the hippocampus (1-5). The cAMP-dependent protein kinase (PKA) has also been implicated in the regulation of synaptic plasticity and may be required for some forms of LTP (6-9). However, the role of adenylyl cyclases (ACs; EC 4.6.1.1) in learning and memory in vertebrates is undefined. The importance of the type I adenylyl cyclase (I-AC) for learning and memory is of particular interest because it is neurospecific and expressed in areas of brain that show LTP (10, 11). Furthermore, the Drosophila mutant rutabaga has a learning defect and is deficient in a Ca2+/CaM-sensitive AC that is similar to . In this study, the role of I-AC in learning and memory was evaluated after the gene was inactivated by targeted mutagenesis. MATERIALS AND METHODSDisruption of the I-AC Gene in Mice. A mouse I-AC genomic clone, ZX3 (14.5 kb), encoding the N-terminal 210 amino acids of I-AC was isolated from a genomic library (OLA 129) provided by Anton Berns (Netherlands Cancer Institute, Amsterdam). The gene-targeting vector, 1126, was constructed by replacing a 0.5-kb Nar I-Pst I fragment of ZX3, which includes the coding region for the first 140 amino acids of I-AC, with neor cassette (15 (18). Free Ca2+ was determined using fura-2 fluorescence (18). The data are presented as mean ± SEM of triplicate assays. Primary cultures of cerebellar neurons were prepared from 6-day-old postnatal mice as described (19). Changes in intracellular cAMP levels were measured as described (20).Morris Water Task A circular pool (1.2 m in diameter) was filled with opaque water. Naive 7-week-old mice were subjected to an initial training session in with they swam in the pool for 60 sec, after which time they were placed on the platform for 45 sec. Mice were given two training sessions (three trials per session, with a 1-h interim between trials) per day separated by 3 h. During each trial, the mice were allowed to search for a visible ...

The granule cell population response to perforant path stimulation decreased significantly within seconds following release of endogenous dynorphin from dentate granule cells. The depression was blocked by the opioid receptor antagonists naloxone and norbinaltorphimine, suggesting that the effect was mediated by dynorphin activation of kappa 1 type opioid receptors. Pharmacological application of dynorphin B in the molecular layer was effective at reducing excitatory synaptic transmission from the perforant path, but application in the hilus had no significant effect. These results suggest that endogenous dynorphin peptides may be released from a local source within the dentate molecular layer. By light microscopy, dynorphin-like immunoreactivity (dynorphin-LI) was primarily found in granule cell axons in the hilus and stratum lucidum with only a few scattered fibers evident in the molecular layer. At the extreme ventral pole of the hippocampus, a diffuse band of varicose processes was also seen in the molecular layer, but this band was not present in more dorsal sections similar to those used for the electrophysiological studies. Electron microscopic analysis of the molecular layer midway along the septotemporal axis revealed that dynorphin-LI was present in dense-core vesicles in both spiny dendrites and unmyelinated axons with the majority (74%) of the dynorphin-LI-containing dense-core vesicles found in dendrites. Neuronal processes containing dynorphin-LI were observed throughout the molecular layer. The results suggest that dynorphin release from granule cell processes in the molecular layer regulates excitatory inputs entering the hippocampus from cerebral cortex, thus potentially counteracting such excitation-induced phenomena as epileptogenesis or long-term potentiation.

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Granule cells in the guinea pig dentate gyrus release kappa opioid neuropeptides, dynorphins, from dendrites as well as from axon terminals. We have found that both L- and N-type calcium channel antagonists inhibited dendritic dynorphin release. In contrast, N-type but not L-type calcium channel antagonists inhibited axonal dynorphin release. Neither L- nor N-type channel antagonists directly altered the effects of kappa opioid receptor activation. By inhibiting dynorphin release, L-type channel antagonists also facilitated the induction of long-term potentiation of the perforant path-granule cell synapse. These studies establish that a single cell type can release a transmitter from two different cellular domains and provide new distinction between axonal and dendritic transmitter release mechanisms.