Altered behavior and long-term potentiation in type I adenylyl cyclase mutant mice

Distinct roles of adenylyl cyclases 1 and 8 in opiate dependence: behavioral, electrophysiological, and molecular studies

BACKGROUND

Opiate dependence is a result of adaptive changes in signal transduction networks in several brain regions. Noradrenergic neurons of the locus coeruleus (LC) have provided a useful model system in which to understand the molecular basis of these adaptive changes. One of most robust signaling adaptations to repeated morphine exposure in this brain region is upregulation of adenylyl cyclase (AC) activity. Earlier work revealed the selective induction of two calmodulin-dependent AC isoforms, AC1 and AC8, after chronic morphine, but their role in opiate dependence has remained unknown.

METHODS

Whole cell recordings from LC slices, behavioral paradigms for dependence, and gene array technology have been used to dissect the role of AC1 and AC8 in chronic morphine responses.

RESULTS

Both AC1 and AC8 knockout mice exhibit reduced opiate dependence on the basis of attenuated withdrawal; however, partially distinct withdrawal symptoms were affected in the two lines. Loss of AC1 or AC8 also attenuated the electrophysiological effects of morphine on LC neurons: knockout of either cyclase attenuated the chronic morphine-induced enhancement of baseline firing rates as well as of regulation of neuronal firing by forskolin (an activator of ACs). The DNA microarray analysis revealed that both AC1 and AC8 affect gene regulation in the LC by chronic morphine and, in addition to common genes, each cyclase influences the expression of a distinct subset of genes.

CONCLUSIONS

Together, these findings provide fundamentally new insight into the molecular and cellular basis of opiate dependence.

Role of phosphodiesterase 5 in synaptic plasticity and memory

Phosphodiesterases (PDEs) are enzymes that break down the phosphodiesteric bond of the cyclic nucleotides, cAMP and cGMP, second messengers that regulate many biological processes. PDEs participate in the regulation of signal transduction by means of a fine regulation of cyclic nucleotides so that the response to cell stimuli is both specific and activates the correct third messengers. Several PDE inhibitors have been developed and used as therapeutic agents because they increase cyclic nucleotide levels by blocking the PDE function. In particular, sildenafil, an inhibitor of PDE5, has been mainly used in the treatment of erectile dysfunction but is now also utilized against pulmonary hypertension. This review examines the physiological role of PDE5 in synaptic plasticity and memory and the use of PDE5 inhibitors as possible therapeutic agents against disorders of the central nervous system (CNS).

Regulation of hippocampus-dependent memory by cyclic AMP-dependent protein kinase

The hippocampus is crucial for the consolidation of new declarative long-term memories. Genetic and behavioral experimentation have revealed that several protein kinases are critical for the formation of hippocampus-dependent long-term memories. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of hippocampus-dependent memory. We review evidence that PKA is required for hippocampus-dependent memory in mammals, and we highlight some of the proteins that have been implicated as targets of PKA. Future directions and open questions regarding the role of PKA in memory storage are also described.

Requirement of extracellular signal-regulated kinase/mitogen-activated protein kinase for long-term potentiation in adult mouse anterior cingulate cortex

Long-term potentiation (LTP) in the anterior cingulate cortex (ACC) is believed to be critical for higher brain functions including emotion, learning, memory and chronic pain. N-methyl-D-aspartate (NMDA) receptor-dependent LTP is well studied and is thought to be important for learning and memory in mammalian brains. As the downstream target of NMDA receptors, the extracellular signal-regulated kinase (ERK) in the mitogen-activated protein kinase (MAPK) cascade has been extensively studied for its involvement in synaptic plasticity, learning and memory in hippocampus. By contrast, the role of ERK in cingulate LTP has not been investigated. In this study, we examined whether LTP in ACC requires the activation of ERK. We found that P42/P44 MAPK inhibitors, PD98059 and U0126, suppressed the induction of cingulate LTP that was induced by presynaptic stimulation with postsynaptic depolarization (the pairing protocol). We also showed that cingulate LTP induced by two other different protocols was also blocked by PD98059. Moreover, we found that these two inhibitors had no effect on the maintenance of cingulate LTP. Inhibitors of c-Jun N-terminal kinase (JNK) and p38, other members of MAPK family, SP600125 and SB203850, suppressed the induction of cingulate LTP generated by the pairing protocol. Thus, our study suggests that the MAPK signaling pathway is involved in the induction of cingulate LTP and plays a critical role in physiological conditions.

Transfection of mutant CREB in the striatum, but not the hippocampus, impairs long-term memory for response learning

Extensive research has shown that the striatum is necessary for response learning. We reported previously that rats using a response strategy to solve a cross maze task showed sustained phosphorylation of striatal CREB [Colombo, P. J., Brightwell, J. J., & Countryman, R. A. (2003). Cognitive strategy-specific increases in phosphorylated cAMP response element-binding protein and c-Fos in the hippocampus and dorsal striatum. Journal of Neuroscience, 23(8), 3547-3554], a transcription factor implicated in long-term memory formation. In the current study, we used viral vector-mediated gene transfer to test the hypothesis that CREB function in the dorsolateral striatum is necessary for the formation of long-term memory for a response strategy. In addition, we tested the hypothesis that the striatum and the hippocampus interact in a cooperative or competitive manner during memory formation. CREB function was blocked in the dorsolateral striatum by overexpression of a mutant form of CREB in which Ser133 was replaced with Ala (HSV-mCREB). CREB function was increased or decreased in the dorsal hippocampus by overexpressing wild-type CREB (HSV-CREB) or mutant CREB. Rats were trained to make a consistent turning response in one session to a criterion of 9 out of 10 correct trials in a water version of the cross maze. Experimental subjects and controls were trained 3 days following infusion into the hippocampus or striatum and tested for memory of the strategy 6 days later. There were no significant differences between treatment groups in acquisition of the task. At test, controls showed significant savings whereas rats infused with HSV-mCREB in the striatum did not. Rats receiving intrahippocampal overexpression of HSV-CREB, HSV-mCREB, or vehicle all showed significant savings between training and test. The present results show that long-term memory of a response strategy requires CREB function in the dorsolateral striatum and is independent of CREB function in the dorsal hippocampus.

A CE assay for the detection of agonist-stimulated adenylyl cyclase activity

A CE assay was developed for the detection of adenylyl cyclase (AC) activity stimulated at the AC and G protein-coupled receptor (GPCR) level. In the assay, cell membranes overexpressing GPCR and/or AC were incubated with modulators and substrate ATP to produce cAMP in a dose-dependent manner. In both the CE-UV and a radiochemical assay, the addition of forskolin (FSK) resulted in a two- to three-fold maximum increase in AC activity with EC50s of 4.2 +/- 0.7 and 2.4 +/- 0.7 microM, respectively, demonstrating that similar results were obtained by both assays. GPCR activation was also detected using cell membranes overexpressing AC and the beta2-adrenergic receptor (beta2AR) fused to the stimulatory G protein. Terbutaline (beta2AR agonist) increased the basal rate of cAMP formation 1.7 +/- 0.1-fold resulting in an EC50 of 62 +/- 10 nM. The assay's ability to detect antagonists is demonstrated by the expected right-shifted EC50 of terbutaline by the beta2AR antagonist propranolol. The CE-UV assay offers advantages over the traditional radioactivity assay in terms of safety and labor.

Synaptic signalling in cerebellar plasticity

A major goal of learning and memory research is to correlate the function of molecules with the behaviour of organisms. The beautiful laminar structure of the cerebellar cortex lends itself to the study of synaptic plasticity, because its clearly defined patterns of neurons and their synapses form circuits that have been implicated in simple motor behaviour paradigms. The best understood in terms of molecular mechanism is the parallel fibre-Purkinje cell synapse, where presynaptic long-term potentiation and postsynaptic long-term depression and potentiation finely tune cerebellar output. Our understanding of these forms of plasticity has mostly come from the electrophysiological and behavioural analysis of knockout mutant mice, but more recently the knock-in of synaptic molecules with mutated phosphorylation sites and binding domains has provided more detailed insights into the signalling events. The present review details the major forms of plasticity in the cerebellar cortex, with particular attention to the membrane trafficking and intracellular signalling responsible. This overview of the current literature suggests it will not be long before the involvement of the cerebellum in certain motor behaviours is fully explained in molecular terms.

Organization and Ca2+Regulation of Adenylyl Cyclases in cAMP Microdomains

The adenylyl cyclases are variously regulated by G protein subunits, a number of serine/threonine and tyrosine protein kinases, and Ca2+. In some physiological situations, this regulation can be readily incorporated into a hormonal cascade, controlling processes such as cardiac contractility or neurotransmitter release. However, the significance of some modes of regulation is obscure and is likely only to be apparent in explicit cellular contexts (or stages of the cell cycle). The regulation of many of the ACs by the ubiquitous second messenger Ca2+provides an overarching mechanism for integrating the activities of these two major signaling systems. Elaborate devices have been evolved to ensure that this interaction occurs, to guarantee the fidelity of the interaction, and to insulate the microenvironment in which it occurs. Subcellular targeting, as well as a variety of scaffolding devices, is used to promote interaction of the ACs with specific signaling proteins and regulatory factors to generate privileged domains for cAMP signaling. A direct consequence of this organization is that cAMP will exhibit distinct kinetics in discrete cellular domains. A variety of means are now available to study cAMP in these domains and to dissect their components in real time in live cells. These topics are explored within the present review.

Ca2+ -stimulated adenylyl cyclases regulate ERK-dependent activation of MSK1 during fear conditioning

The cAMP and ERK/MAP kinase (MAPK) signal transduction pathways are critical for hippocampus-dependent memory, a process that depends on CREB-mediated transcription. However, the extent of crosstalk between these pathways and the downstream CREB kinase activated during memory formation has not been elucidated. Here we report that PKA, MAPK, and MSK1, a CREB kinase, are coactivated in a subset of hippocampal CA1 pyramidal neurons following contextual fear conditioning. Activation of PKA, MAPK, MSK1, and CREB is absolutely dependent on Ca(2+)-stimulated adenylyl cyclase activity. We conclude that adenylyl cyclase activity supports the activation of MAPK, and that MSK1 is the major CREB kinase activated during training for contextual memory.

Role of the calcium modulated cyclases in the development of the retinal projections

Transmembrane isoforms of adenylate cyclases (AC) integrate a wide variety of extracellular signals from neurotransmitters to morphogens and can also regulate cAMP production in response to calcium entry. Based on observations in the barrelless mouse strain, the Adcy1 gene (AC1) was involved in the segregation of binocular retinal inputs. To determine the potential role of other AC isoforms we localized the Adcy genes in the visual centres during development, using in situ hybridization. Six different AC subtypes were found in the developing retinal ganglion cell layer (RGC; AC1, AC2, AC3, AC5, AC8, and AC9), and three AC subtypes were expressed in the central brain targets, the dorsal lateral geniculate nucleus (AC1 and AC8), the ventral lateral geniculate nucleus (AC2 and AC8) and the superior colliculus (AC1, AC2, AC8). Using a genetic approach we tested the role of the calcium modulated cyclases AC1, AC5 and AC8 for the segregation retinal fibres. Ipsilateral retinal axons remained exuberant in the AC1(-/-) mice, with overlapping retinal projections from both eyes in the superior colliculus and the visual thalamus. These abnormalities were similar to those of barrelless mouse mutants. No abnormalities were detectable in the AC5(-/-) or the AC8(-/-) mice. Similar abnormalities were noted in the single AC1(-/-) and the AC1/AC8 double-knockout mice (DKO). Thus, only AC1 is required for the maturation of the retinal axon terminals whereas AC5 and AC8 are not needed. The specificity of AC1's action is linked to its cellular localization in the RGCs and to its distinctive functional profile, compared with the other cyclases expressed in the same cells.

Calmodulin-Stimulated Adenylyl Cyclase Gene Deletion Affects Morphine Responses

To define the roles of the calmodulin-stimulated adenylyl cyclases (AC1 and AC8) in morphine-induced analgesia, tolerance, physical dependence, and conditioned place preference, we used mice having targeted disruptions of either the AC1 or AC8 genes or both genes [double knockout mice (DKO)]. Mice lacking either AC1 or AC8 genes or DKO did not differ from wild-type mice in short-term antinociceptive responses to morphine measured in the tail-flick analgesia assay. Morphine tolerance that developed immediately within 3 h of morphine administration (10 mg/kg s.c.) was significantly attenuated in DKO mice and AC8 single knockout mice. Tolerance induced continually by daily injections of morphine (10 mg/kg s.c.) was also reduced in DKO mice. In DKO mice continually treated with morphine, there was a significant reduction in withdrawal behaviors, including reduced wet-dog shakes and forepaw tremor after naloxone injection (10 mg/kg i.p.). Morphine produced hyperlocomotion and conditioned place preference in wild-type mice, whereas DKO mice displayed significantly less hyperlocomotion and conditioned place preference. Furthermore, the significant increase in phosphorylated cAMP-response element binding protein (CREB) staining in ventral tegmental area induced by long-term morphine treatment was not evident in DKO mice, suggesting that CREB activation by morphine requires cAMP generated by AC1 and AC8. These results support the hypothesis that calmodulin-stimulated adenylyl cyclases are important mediators of the neuronal responses to morphine.

Molecular details of cAMP generation in mammalian cells: a tale of two systems

The second messenger cAMP has been extensively studied for half a century, but the plethora of regulatory mechanisms controlling cAMP synthesis in mammalian cells is just beginning to be revealed. In mammalian cells, cAMP is produced by two evolutionary related families of adenylyl cyclases, soluble adenylyl cyclases (sAC) and transmembrane adenylyl cyclases (tmAC). These two enzyme families serve distinct physiological functions. They share a conserved overall architecture in their catalytic domains and a common catalytic mechanism, but they differ in their sub-cellular localizations and responses to various regulators. The major regulators of tmACs are heterotrimeric G proteins, which transduce extracellular signals via G protein-coupled receptors. sAC enzymes, in contrast, are regulated by the intracellular signaling molecules bicarbonate and calcium. Here, we discuss and compare the biochemical, structural and regulatory characteristics of the two mammalian AC families. This comparison reveals the mechanisms underlying their different properties but also illustrates many unifying themes for these evolutionary related signaling enzymes.

Stimulation of hippocampal adenylyl cyclase activity dissociates memory consolidation processes for response and place learning

Procedural and declarative memory systems are postulated to interact in either a synergistic or a competitive manner, and memory consolidation appears to be a highly critical stage for this process. However, the precise cellular mechanisms subserving these interactions remain unknown. To investigate this issue, 24-h retention performances were examined in mice given post-training intrahippocampal injections of forskolin (FK) aiming at stimulating hippocampal adenylyl cyclases (ACs). The injection was given at different time points over a period of 9 h following acquisition in either an appetitive bar-pressing task or water-maze tasks challenging respectively "response memory" and "place memory." Retention testing (24 h) showed that FK injection altered memory formation only when given within a 3- to 6-h time window after acquisition but yielded opposite memory effects as a function of task demands. Retention of the spatial task was impaired, whereas retention of both the cued-response in the water maze and the rewarded bar-press response were improved. Intrahippocampal injections of FK produced an increase in pCREB immunoreactivity, which was strictly limited to the hippocampus and lasted less than 2 h, suggesting that early effects (0-2 h) of FK-induced cAMP/CREB activation can be distinguished from late effects (3-6 h). These results delineate a consolidation period during which specific cAMP levels in the hippocampus play a crucial role in enhancing memory processes mediated by other brain regions (e.g., dorsal or ventral striatum) while eliminating interference by the formation of hippocampus-dependent memory.

Calcium-stimulated adenylyl cyclases modulate ethanol-induced neurodegeneration in the neonatal brain

Fetal alcohol exposure results in cognitive and neurobehavioral deficits, but the effects of modifying genetic loci on the severity of these sequelas have not been well characterized. Although the cAMP signaling pathway has been shown to be an important modulator of ethanol sensitivity in adult mice, its potential role in modulating ethanol-induced neurodegeneration has not been examined. Adenylyl cyclases (ACs) 1 and 8 produce cAMP in response to intracellular calcium elevation and modulate several aspects of neuronal function, including ethanol sensitivity. AC1 and AC8 are expressed widely throughout the brain of neonatal mice, and genetic deletion of both AC1 and AC8 in double-knock-out (DKO) mice enhances ethanol-induced neurodegeneration in the brains of neonatal mice. In addition, ethanol treatment induces significantly greater levels of caspase-3 activation in the brains of DKO mice compared with wild-type (WT) mice, reflecting higher numbers of apoptotic neurons. Administration of the NMDA receptor antagonist MK801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine hydrogen maleate] or the GABA(A) receptor potentiator phenobarbital, which mimics components of the effects of ethanol on neurons, results in significantly greater neurodegeneration in the brains of neonatal DKO mice than WT mice. Furthermore, loss of a single calcium-stimulated AC isoform potentiates neurodegeneration after administration of ethanol, MK801, or phenobarbital. In contrast, the levels of physiological cell death, death after hypoxia/ischemia, and excitotoxic cell death are not increased in the brains of DKO mice. Thus, AC1 and AC8 are critical modulators of neurodegeneration induced by activity blockade in the neonatal brain and represent genetic loci that may potentially modify the severity of fetal alcohol syndrome.

The slow afterhyperpolarization in hippocampal CA1 neurons covaries with spatial learning ability in aged Fisher 344 rats

Rodents commonly exhibit age-related impairments in spatial learning tasks, deficits widely thought to reflect cellular or synaptic dysfunction in the hippocampus. Using whole-cell recordings, we examined the afterhyperpolarization (AHP) in CA1 pyramidal cells in hippocampal slices from young (4-6 months of age) and aged (24-26 months of age) Fisher 344 male rats that had been behaviorally characterized in the Morris water maze. The slow AHP (sAHP) recorded from learning-impaired aged rats (AI) was significantly larger than that seen in either age-matched unimpaired rats or young controls. Among aged rats, sAHP amplitude was inversely correlated with both acquisition and probe performance in the water maze. Action potential parameters among the three groups were similar, except for spike accommodation, which was more pronounced in the AI group. Intracellular application of the cAMP analog 8-CPT-cAMP suppressed the sAHP but failed to reveal any age- or performance-related differences in the medium AHP. 8-CPT-cAMP abolished the age-related difference in spike accommodation, whereas instantaneous firing frequency was unchanged. Calcium spikes were of similar amplitude in all three groups but were broader and had significantly larger tails in aged rats; these age-related changes could be mimicked in young neurons after exposure to BayK8644. The calcium spike among aged rats correlated with task acquisition in the maze but, unlike the sAHP, failed to correlate with probe performance. This is the first demonstration that sAHP amplitude covaries with spatial learning ability in aged rats, implying that CA1 excitability strongly influences certain aspects of cognitive function. Our findings also indicate that multiple processes, in addition to elevated calcium influx, conspire to induce cognitive decline during aging.

Calcium-stimulated adenylyl cyclases are critical modulators of neuronal ethanol sensitivity

The importance of the cAMP signaling pathway in the modulation of ethanol sensitivity has been suggested by studies in organisms from Drosophila melanogaster to man. However, the involvement of specific isoforms of adenylyl cyclase (AC), the molecule that converts ATP to cAMP, has not been systemically determined in vivo. Because AC1 and AC8 are the only AC isoforms stimulated by calcium, and ethanol modulates calcium flux by the NMDA receptor, we hypothesized that these ACs would be important in the neural response to ethanol. AC1 knock-out (KO) mice and double knock-out (DKO) mice with genetic deletion of both AC1 and AC8 display substantially increased sensitivity to ethanol-induced sedation compared with wild-type (WT) mice, whereas AC8 KO mice are only minimally more sensitive. In contrast, AC8 KO and DKO mice, but not AC1 KO mice, demonstrate decreased voluntary ethanol consumption compared with WT mice. DKO mice do not display increased sleep time compared with WT mice after administration of ketamine or pentobarbital, indicating that the mechanism of enhanced ethanol sensitivity in these mice is likely distinct from the antagonism of ethanol of the NMDA receptor and potentiation of the GABA(A) receptor. Ethanol does not enhance calcium-stimulated AC activity, but the ethanol-induced phosphorylation of a discrete subset of protein kinase A (PKA) substrates is compromised in the brains of DKO mice. These results indicate that the unique activation of PKA signaling mediated by the calcium-stimulated ACs is an important component of the neuronal response to ethanol.

Molecular identification and functional characterization of an adenylyl cyclase from the honeybee

Cyclic AMP (cAMP) serves as an important messenger in virtually all organisms. In the honeybee (Apis mellifera), cAMP-dependent signal transduction has been implicated in behavioural processes as well as in learning and memory. Key components of cAMP-signalling cascades are adenylyl cyclases. However, the molecular identities and biochemical properties of adenylyl cyclases are completely unknown in the honeybee. We have cloned a cDNA (Amac3) from honeybee brain that encodes a membrane-bound adenylyl cyclase. The Amac3 gene is an orthologue of the Drosophila ac39E gene. The corresponding proteins share an overall amino acid similarity of approximately 62%. Phylogenetically, AmAC3 belongs to group 1 adenylyl cyclases. Heterologously expressed AmAC3 displays basal enzymatic activity and efficient coupling to endogenous G protein signalling pathways. Stimulation of beta-adrenergic receptors induces AmAC3 activity with an EC(50) of about 3.1 microm. Enzymatic activity is also increased by forskolin (EC(50) approximately 15 microm), a specific agonist of membrane-bound adenylyl cyclases. Similar to certain biogenic amine receptor genes of the honeybee, Amac3 transcripts are expressed in many somata of the brain, especially in mushroom body neurones. These results suggest that the enzyme serves in biogenic amine signal transduction cascades and in higher brain functions that contribute to learning and memory of the bee.

Genetic dissociation of acquisition and memory strength in the heat-box spatial learning paradigm in Drosophila

Memories can have different strengths, largely dependent on the intensity of reinforcers encountered. The relationship between reinforcement and memory strength is evident in asymptotic memory curves, with the level of the asymptote related to the intensity of the reinforcer. Although this is likely a fundamental property of memory formation, relatively little is known of how memory strength is determined. Memory performance at different levels in Drosophila can be measured in an operant heat-box conditioning paradigm. In this spatial learning paradigm, flies learn and remember to avoid one-half of a dark chamber associated with a temperature outside of the preferred range. The reinforcement temperature has a strong effect on the level of learning in wild-type flies, with higher temperatures inducing stronger memories. Additionally, two mutations alter memory-acquisition curves, either changing acquisition rate or asymptotic memory level. The rutabaga mutation, affecting a type-1 adenylyl cyclase, decreases the acquisition rate. In contrast, the white mutation, modifying an ABC transporter, limits asymptotic memory. The white mutation does not negatively affect classical olfactory conditioning but actually improves performance at low reinforcement levels. Thus, memory acquisition/memory strength and classical olfactory/operant spatial memories can be genetically dissociated. A conceptual model of operant conditioning and the levels at which rutabaga and white influence conditioning is proposed.

Melatonin inhibits hippocampal long-term potentiation

The goal of this study is to investigate the effect of the hormone melatonin on long-term potentiation and excitability measured by stimulating the Schaffer collaterals and recording the field excitatory postsynaptic potential from the CA1 dendritic layer in hippocampal brain slices from mice. Application of melatonin produced a concentration-dependent inhibition of the induction of long-term potentiation, with a concentration of 100 nm producing an approximately 50% inhibition of long-term potentiation magnitude. Long-duration melatonin treatments of 6 h were also effective at reducing the magnitude of long-term potentiation. Melatonin (100 nm) did not alter baseline evoked responses or paired-pulse facilitation recorded at this synapse. The inhibitory actions of melatonin were prevented by application of the melatonin (MT) receptor antagonist luzindole as well as the MT2 receptor subtype antagonist 4-phenyl-2-propionamidotetraline. These inhibitory actions of melatonin were lost in mice deficient in MT2 receptors but not those deficient in MT1 receptors. In addition, application of the protein kinase A inhibitor H-89 both mimicked the effects of melatonin and precluded further inhibition by melatonin. Finally, the application an activator of adenylyl cyclase, forskolin, overcame the inhibitory effects of melatonin on LTP without affecting the induction of long-term potentiation on its own. These results suggest that hippocampal synaptic plasticity may be constrained by melatonin through a mechanism involving MT2-receptor-mediated regulation of the adenylyl cyclase-protein kinase A pathway.

Hippocampal neurons express a calcineurin-activated adenylyl cyclase

Ca2+-stimulated adenylyl cyclases are important for several forms of neuroplasticity because they couple activity-dependent Ca2+ increases to cAMP in neurons. For example, the calmodulin-stimulated adenylyl cyclases, AC1 and AC8, are required for hippocampus-dependent memory and long-lasting long-term potentiation. To identify other mechanisms for Ca2+ stimulation of adenylyl cyclases, cultured hippocampal neurons from transgenic mice lacking both AC1 and AC8 [double knock-out (DKO) mice] were analyzed for Ca2+ stimulation of intracellular cAMP. Surprisingly, neurons from DKO mice showed significant Ca2+-stimulated cAMP accumulation that was blocked by inhibitors of calcineurin [PP2B (protein phosphatase 2B)], a Ca2+-activated protein phosphatase. Analysis of cultured neurons from calcineurin(-/-) mice confirmed that hippocampal neurons exhibit a calcineurin-dependent cAMP increase, which may contribute to some forms of neuroplasticity.

The immunoreactivity and activity of adenylate cyclase type I are changed in the hippocampal CA1 region after transient forebrain ischemia in gerbils

Adenylate cyclase (AC) has a specific sensitivity to Ca2+/calmodulin. AC-I, one of the mediator of learning and memory, plays an important role in signal transduction underlying learning and memory function. In the present study, we found ischemia-related changes of AC-I in the hippocampal CA1 region, but not in the CA2/3 region, after 5 min of transient forebrain ischemia in gerbils. In the sham-operated group, AC-I immunoreactive neurons were detected in pyramidal and non-pyramidal cells in the hippocampus proper. AC-I immunoreactivity was significantly increased at 3 h in the CA1 region after ischemic insult. Thereafter, AC-I immunoreactivity was gradually decreased. Four days after ischemic insult, AC-I-immunoreactive CA1 pyramidal cells in the stratum pyramidale were very few due to delayed neuronal death. The results of Western blot analysis showed that changes of AC-I protein contents were similar to immunohistochemical data after ischemic insult. Gpp(NH)p-dependent AC-I activity in hippocampal CA1 region was not changed in all groups, while Ca2+/calmodulin-dependent AC-I activity in hippocampal CA1 region was significantly decreased 24 h after ischemia-reperfusion. These results suggest that the decrease of AC-I activity may be associated with impairment of neurodevelopment and neuroplasticity including learning and memory although the AC-I immunoreactivity was maintained 24 h postischemic group compared to that of the sham-operated group.

Comprehensive analysis of the expression patterns of the adenylate cyclase gene family in the developing and adult mouse brain

Adenylate cyclases (Adcys) are components of several developmentally, neurophysiologically, and pharmacologically relevant signaling pathways. A prominent feature of Adcys is their ability to integrate multiple signaling pathways into a single second messenger pathway, the production of cAMP. Nine isoforms of membrane-bound Adcys are known, each encoded by a distinct gene. These isoforms differ in their response to regulatory upstream pathways as well as in their distribution in the brain and elsewhere. Use of various detection methods and animal species has, however, hampered a direct comparison of expression patterns, so the potential contribution of single isoforms to Adcy activity in different brain regions remains unclear. We have determined the expression patterns of all nine Adcy genes in the embryonic, postnatal day 7, and adult mouse brain by nonradioactive robotic in situ hybridization (ISH). Here we describe the salient features of these patterns. Regional colocalization of Adcy transcripts encoding isoforms with different regulatory properties was detected in the cortex, subregions of the hippocampus, olfactory bulb, thalamus, and striatum. Hence, our expression data support models for modulation of cAMP signaling by combinatorial action of multiple Adcy isoforms. However, in several instances, the expression domains of genes encoding isoforms with similar regulatory properties spatially exclude each other, which is most evident in not previously described expression domains of the embryonic midbrain roof. This is suggestive of functional specialization.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

Gene targeting of presynaptic proteins in synaptic plasticity and memory: across the great divide

The past few decades have seen an explosion in our understanding of the molecular basis of learning and memory. The majority of these studies in mammals focused on post-synaptic signal transduction cascades involved in post-synaptic long-lasting plasticity. Until recently, relatively little work examined the role of presynaptic proteins in learning and memory in complex systems. The synaptic cleft figuratively represents a "great divide" between our knowledge of post- versus presynaptic involvement in learning and memory. While great strides have been made in our understanding of presynaptic proteins, we know very little of how presynaptically expressed forms of short- and long-term plasticity participate in information processing and storage. The paucity of cognitive behavioral research in the area of presynaptic proteins, however, is in stark contrast to the plethora of information concerning presynaptic protein involvement in neurotransmitter release, in modulation of release, and in both short- and long-term forms of presynaptic plasticity. It is now of great interest to begin to link the extensive literature on presynaptic proteins and presynaptic plasticity to cognitive behavior. In the future there is great promise with these approaches for identifying new targets in the treatment of cognitive disorders. This review article briefly surveys current knowledge on the role of presynaptic proteins in learning and memory in mammals and suggests future directions in learning and memory research on the presynaptic rim of the "great divide."

Olfactory memory formation in Drosophila: from molecular to systems neuroscience

The olfactory nervous system of insects and mammals exhibits many similarities, which suggests that the mechanisms for olfactory learning may be shared. Molecular genetic investigations of Drosophila learning have uncovered numerous genes whose gene products are essential for olfactory memory formation. Recent studies of the products of these genes have continued to expand the range of molecular processes known to underlie memory formation. Recent research has also broadened the neuroanatomical areas thought to mediate olfactory learning to include the antennal lobes in addition to a previously accepted and central role for the mushroom bodies. The roles for neurons extrinsic to the mushroom body neurons are becoming better defined. Finally, the genes identified to participate in Drosophila olfactory learning have conserved roles in mammalian organisms, highlighting the value of Drosophila for gene discovery.

Removal of G(ialpha1) constraints on adenylyl cyclase in the hippocampus enhances LTP and impairs memory formation

Stimulation of adenylyl cyclase in the hippocampus is critical for memory formation. However, generation of cAMP signals within an optimal range for memory may require a balance between stimulatory and inhibitory mechanisms. The role of adenylyl cyclase inhibitory mechanisms for memory has not been addressed. One of the mechanisms for inhibition of adenylyl cyclase is through activation of G(i)-coupled receptors, a mechanism that could serve as a constraint on memory formation. Here we report that ablation of G(ialpha1) by gene disruption increases hippocampal adenylyl cyclase activity and enhances LTP in area CA1. Furthermore, gene ablation of G(ialpha1) or antisense oligonucleotide-mediated depletion of G(ialpha1) disrupted hippocampus-dependent memory. We conclude that G(ialpha1) provides a critical mechanism for tonic inhibition of adenylyl cyclase activity in the hippocampus. We hypothesize that loss of G(ialpha1) amplifies the responsiveness of CA1 postsynaptic neurons to stimuli that strengthen synaptic efficacy, thereby diminishing synapse-specific plasticity required for new memory formation.

Phosphodiesterase 4B (PDE4B) and cAMP-level regulation within different tissue fractions of rat hippocampal slices during long-term potentiation in vitro

Molecular events associated with mnemonic processes and neuronal plasticity are postulated to result in functional changes in synaptic structure. One possible site is the post-synaptic density, where activity-dependent changes modulate signal transduction cascades. In this report, we detail spatial-temporal changes for phosphodiesterase 4B (PDE4B) proteins and their substrate cAMP within three neuronal fractions during early and late long-term potentiation (LTP). The cAMP-dependent protein kinase A cascade--which can be regulated by distinct PDE4B activity--is required for mnemonic processes as well as mechanisms of neuronal plasticity, such as those during the maintenance or late-LTP. Fluorescence in situ hybridization studies (FISH) identified no translocation of PDE4B3 from the soma after late-LTP induction indicating a subtle, local control of PDE4B activity. Protein changes were detected within the PSD-enriched fraction. From these results, we conclude that either the changes in PDE4B are due to modulation of pre-existing mRNA, or that the protein is specifically translocated to activated synaptic structures. Furthermore, we report late changes in cAMP levels in the somato-dendritic fraction and discuss this result with the increased PDE4B1/3 doublet in the PSD-enriched fraction.

Activation of a nitric-oxide-sensitive cAMP pathway with phencyclidine: elevated hippocampal cAMP levels are temporally associated with deficits in prepulse inhibition

RATIONALE

Schizophrenic patients show deficits in pre-attentive information processing as evidenced, for example, by disrupted prepulse inhibition, a measure of sensorimotor gating. A similar disruption can be observed in animals treated with the psychotomimetic agent, phencyclidine (PCP). However, the mechanism by which PCP alters brain function has not been fully elucidated. Recent studies have demonstrated that certain behavioural and neurochemical effects of PCP in rats and mice are blocked by nitric oxide (NO) synthase inhibition, suggesting an important role for NO in the effects of PCP.

OBJECTIVE

The aim of the present study was to investigate the effects of PCP on cAMP production in the ventral hippocampus and the role of NO in these effects using in vivo microdialysis in rats. Furthermore, the effects of PCP on acoustic startle reactivity and prepulse inhibition of acoustic startle were compared with changes in cAMP levels in the ventral hippocampus.

RESULTS

Significant increases in cAMP levels were observed in the ventral hippocampus following both local infusion (10(-4) mol/l and 10(-3) mol/l) and systemic administration (2 mg/kg) of PCP. The PCP-induced changes in prepulse inhibition and startle reactivity were associated in magnitude and duration with the increase in cAMP levels in the hippocampus. Furthermore, systemic administration of the NO synthase inhibitor, L: -NAME (10 mg/kg), blocked both the changes in cAMP levels and the behavioural responses induced by PCP.

CONCLUSIONS

These findings indicate that the effects of PCP on prepulse inhibition and startle reactivity are associated with an increase in cAMP levels in the ventral hippocampus, and that this change in cAMP response may be linked to the production of NO.

The role of calmodulin as a signal integrator for synaptic plasticity

Excitatory synapses in the brain show several forms of synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD), which are initiated by increases in intracellular Ca(2+) that are generated through NMDA (N-methyl-D-aspartate) receptors or voltage-sensitive Ca(2+) channels. LTP depends on the coordinated regulation of an ensemble of enzymes, including Ca(2+)/calmodulin-dependent protein kinase II, adenylyl cyclase 1 and 8, and calcineurin, all of which are stimulated by calmodulin, a Ca(2+)-binding protein. In this review, we discuss the hypothesis that calmodulin is a central integrator of synaptic plasticity and that its unique regulatory properties allow the integration of several forms of signal transduction that are required for LTP and LTD.

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Active zones for presynaptic plasticity in the brain

Some of the most abundant synapses in the brain such as the synapses formed by the hippocampal mossy fibers, cerebellar parallel fibers and several types of cortical afferents express presynaptic forms of long-term potentiation (LTP), a putative cellular model for spatial, motor and fear learning. Those synapses often display presynaptic mechanisms of LTP induction, which are either NMDA receptor independent of dependent of presynaptic NMDA receptors. Recent investigations on the molecular mechanisms of neurotransmitter release modulation in short- and long-term synaptic plasticity in central synapses give a preponderant role to active zone proteins as Munc-13 and RIM1-alpha, and point toward the maturation process of synaptic vesicles prior to Ca(2+)-dependent fusion as a key regulatory step of presynaptic plasticity.

Requirement for the RIIbeta isoform of PKA, but not calcium-stimulated adenylyl cyclase, in visual cortical plasticity

The cAMP-dependent protein kinase (PKA) signaling pathway plays a key role in visual cortical plasticity. Inhibitors that block activation of all PKA regulatory subunits (RIalpha,RIbeta, RIIalpha, RIIbeta) abolish long-term potentiation (LTP) and long-term depression (LTD) in vitro and ocular dominance plasticity (ODP) in vivo. The details of this signaling cascade, however, including the source of PKA signals and which PKA subunits are involved, are unknown. To investigate these issues we have examined LTP, LTD, and ODP in knock-out mice lacking either the two cortically expressed Ca2+-stimulated adenylyl cyclases (AC1 and AC8) or the predominant neocortical subunit of PKA (RIIbeta). Here we show that plasticity remains intact in AC1/AC8-/- mice, whereas ODP and LTD, but not LTP, are absent in RIIbeta-/- mice. We conclude that (1) plasticity in the visual cortex does not require the activity of known Ca2+-stimulated adenylyl cyclases, (2) the PKA dependence of ODP and LTD, but not LTP, is mediated by RIIbeta-PKA, and (3) multiple isoforms of PKA contribute to LTD.

Effects of age and spatial learning on adenylyl cyclase mRNA expression in the mouse hippocampus

Adenylyl cyclase (AC) subtypes have been implicated in memory processes and synaptic plasticity. In the present study, the effects of aging and learning on Ca2+/calmodulin-stimulable AC1, Ca2+-insensitive AC2 and Ca2+/calcineurin-inhibited AC9 mRNA level were compared in the dorsal hippocampus of young-adult and aged C57BL/6 mice using in situ hybridization. Both AC1 and AC9 mRNA expression were downregulated in aged hippocampus, whereas AC2 mRNA remained unchanged, suggesting differential sensitivities to the aging process. We next examined AC mRNA expression in the hippocampus after spatial learning in the Morris water maze. Acquisition of the spatial task was associated with an increase of AC1 and AC9 mRNA levels in both young-adult and aged groups, suggesting that Ca2+-sensitive ACs are oppositely regulated by aging and learning. However, aged-trained mice had reduced AC1 and AC9, but greater AC2, mRNA levels relative to young-trained mice and age-related learning impairments were correlated with reduced AC1 expression in area CA1. We suggest that reduced levels of hippocampal AC1 mRNA may greatly contribute to age-related defects in spatial memory.

Why Calcium-Stimulated Adenylyl Cyclases?

The Ca2+/calmodulin-stimulated adenylyl cyclases, AC1 and AC8, play a critical role in several forms of neuroplasticity, including long-lasting long-term potentiation (L-LTP) and long-term memory (LTM). By coupling neuronal activity and Ca2+increases to the production of cAMP, AC1 and AC8 activate cAMP-dependent signal transduction and transcriptional pathways critical for L-LTP and LTM.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

The presynaptic active zone protein RIM1alpha is critical for normal learning and memory

The active zone protein RIM1alpha is required both for maintaining normal probability of neurotransmitter release and for long-term presynaptic potentiation at brain synapses. We now demonstrate that RIM1alpha(-/-) mice exhibit normal coordination and anxiety-related behaviors but display severely impaired learning and memory. Mice with a synaptotagmin 1 mutation, which selectively lowers release probability, and mice with Rab3A deletion, which selectively abolishes presynaptic long-term potentiation, do not exhibit this abnormality. Our data suggest that a decrease in release probability or a loss of presynaptic LTP alone is not sufficient to cause major behavioral alterations, but the combination of presynaptic abnormalities in RIM1alpha(-/-) mice severely alters learning and memory.

Overexpression of type-1 adenylyl cyclase in mouse forebrain enhances recognition memory and LTP

Cyclic AMP is a positive regulator of synaptic plasticity and is required for several forms of hippocampus-dependent memory including recognition memory. The type I adenylyl cyclase, Adcy1 (also known as AC1), is crucial in memory formation because it couples Ca(2+) to cyclic AMP increases in the hippocampus. Because Adcy1 is neurospecific, it is a potential pharmacological target for increasing cAMP specifically in the brain and for improving memory. We have generated transgenic mice that overexpress Adcy1 in the forebrain using the Camk2a (also known as alpha-CaMKII) promoter. These mice showed elevated long-term potentiation (LTP), increased memory for object recognition and slower rates of extinction for contextual memory. The increase in recognition memory and lower rates of contextual memory extinction may be due to enhanced extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK) signaling, which is elevated in mice that overexpress Adcy1.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Type 8 adenylyl cyclase is targeted to excitatory synapses and required for mossy fiber long-term potentiation

Mossy fiber/CA3 long-term potentiation (LTP) is hypothesized to depend on cAMP signals generated by Ca2+-stimulated adenylyl cyclases AC1 or AC8. AC1 gene knock-out mice (AC1-/-) show a partial reduction in mossy fiber LTP, suggesting that either AC8 activity is also critical for mossy fiber LTP or that there is a component of mossy fiber LTP that is independent of CaM-activated adenylyl cyclases. To address this issue, mossy fiber LTP was examined in hippocampal slices from AC8-/- and AC1-/- x AC8-/- double knock-out mice (DKO). Despite the fact that AC8 contributes only a small fraction of the Ca2+-stimulated adenylyl cyclase activity in the hippocampus and is less sensitive to Ca2+ than AC1, AC8-/- mice exhibited mossy fiber LTP defects comparable with AC1-/- and DKO mice. Furthermore, short-term plasticity was disrupted in AC8-/- mice but not in AC1-/- mice. Because AC1 is not localized at the excitatory synapses in hippocampal neurons, we hypothesized that AC8 may be targeted to synapses, in which higher synaptic-specific Ca2+ increases occur. Here, we report that AC8 accumulates in puncta of dendrites and axons in hippocampal neurons and colocalizes with synaptic marker proteins. These data indicate that both synaptic and nonsynaptic cAMP signals, generated by different Ca2+-stimulated adenylyl cyclases, are required for mossy fiber LTP.

Regulation and organization of adenylyl cyclases and cAMP

Adenylyl cyclases are a critically important family of multiply regulated signalling molecules. Their susceptibility to many modes of regulation allows them to integrate the activities of a variety of signalling pathways. However, this property brings with it the problem of imparting specificity and discrimination. Recent studies are revealing the range of strategies utilized by the cyclases to solve this problem. Microdomains are a consequence of these solutions, in which cAMP dynamics may differ from the broad cytosol. Currently evolving methodologies are beginning to reveal cAMP fluctuations in these various compartments.

Type 8 adenylyl cyclase is targeted to excitatory synapses and required for mossy fiber long-term potentiation

Mossy fiber/CA3 long-term potentiation (LTP) is hypothesized to depend on cAMP signals generated by Ca2+-stimulated adenylyl cyclases AC1 or AC8. AC1 gene knock-out mice (AC1-/-) show a partial reduction in mossy fiber LTP, suggesting that either AC8 activity is also critical for mossy fiber LTP or that there is a component of mossy fiber LTP that is independent of CaM-activated adenylyl cyclases. To address this issue, mossy fiber LTP was examined in hippocampal slices from AC8-/- and AC1-/- x AC8-/- double knock-out mice (DKO). Despite the fact that AC8 contributes only a small fraction of the Ca2+-stimulated adenylyl cyclase activity in the hippocampus and is less sensitive to Ca2+ than AC1, AC8-/- mice exhibited mossy fiber LTP defects comparable with AC1-/- and DKO mice. Furthermore, short-term plasticity was disrupted in AC8-/- mice but not in AC1-/- mice. Because AC1 is not localized at the excitatory synapses in hippocampal neurons, we hypothesized that AC8 may be targeted to synapses, in which higher synaptic-specific Ca2+ increases occur. Here, we report that AC8 accumulates in puncta of dendrites and axons in hippocampal neurons and colocalizes with synaptic marker proteins. These data indicate that both synaptic and nonsynaptic cAMP signals, generated by different Ca2+-stimulated adenylyl cyclases, are required for mossy fiber LTP.

Type VIII adenylyl cyclase in rat beta cells: coincidence signal detector/generator for glucose and GLP-1

AIMS/HYPOTHESIS

The secretory function of pancreatic beta cells is synergistically stimulated by two signalling pathways which mediate the effects of nutrients and hormones such as glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic peptide (GIP) or glucagon. These hormones are known to activate adenylyl cyclase in beta cells. We examined the type of adenylyl cyclase that is associated with this synergistic interaction.

METHODS

Insulin release, cAMP production, adenylyl cyclase activity, mRNA and protein expression were measured in fluorescence-activated cell sorter-purified rat beta cells and in the rat beta-cell lines RINm5F, INS-1 832/13 and INS-1 832/2.

RESULTS

In primary beta cells, glucagon and GLP-1 synergistically potentiate the stimulatory effect of 20 mmol/l glucose on insulin release and cAMP production. Both effects are abrogated in the presence of the L-type Ca(2+)-channel blocker verapamil. The cAMP-producing activity of adenylyl cyclase in membranes from RINm5F cells is synergistically increased by Ca(2+)-calmodulin and recombinant GTP(gamma)S-activated G(s alpha)-protein subunits. This type of regulation is characteristic for type I and type VIII AC isoforms. Consistent with this functional data, AC mRNA analysis shows abundant expression of type VI AC, four splice variants of type VIII AC and low expression level of type I AC in beta cells. Type VIII AC expression at the protein level was observed using immunoblots of RINm5F cell extracts.

CONCLUSION/INTERPRETATION

This study identifies type VIII AC in insulin-secreting cells as one of the potential molecular targets for synergism between GLP-1 receptor mediated and glucose-mediated signalling.

Central galanin administration blocks consolidation of spatial learning

Galanin is a neuropeptide that inhibits the evoked release of several neurotransmitters, inhibits the activation of intracellular second messengers, and produces deficits in a variety of rodent learning and memory tasks. To evaluate the actions of galanin on encoding, consolidation, and storage/retrieval, galanin was acutely administered to Sprague-Dawley rats at time points before and after training trials in the Morris water maze. Intraventricular administration of galanin up to 3h after subjects had completed daily training trials in the Morris water task impaired performance on the probe trial, indicating that galanin-blocked consolidation. Pretreatment with an adenylate cyclase activator, forskolin, prevented the deficits in distal cue learning produced by galanin. Di-deoxyforskolin, an inactive analog of forskolin, had no effect. These results provide the first evidence that galanin interferes with long-term memory consolidation processes. A potential mechanism by which galanin produces this impairment may involve the inhibition of adenylate cyclase activity, leading to inhibition of downstream molecular events that are necessary for consolidation of long-term memory.

Expression of the specific type IV phosphodiesterase gene PDE4B3 during different phases of long-term potentiation in single hippocampal slices of rats in vitro

Hippocampal long-term potentiation (LTP), the most prominent cellular model for learning and memory formation, consists of phases: early-LTP (<4 h) and late-LTP (>4 h), with the latter dependent upon protein translation and transcription. To explore the molecular processes that might be specifically regulated during late-LTP, we have modified standard electrophysiological and molecular biological methods, which allowed the cloning of activated genes and their products from single hippocampal slices in vitro 8 h after LTP induction. From one such screen we identified a specific type IV phosphodiesterase gene, PDE4B3, the first cAMP-specific phosphodiesterase to be associated with LTP. Previous studies documented an integral role for the cAMP-PKA system in late-LTP and recently, inhibition of cAMP degradation facilitates LTP and ameliorates mnemonic deficits. We now report that PDE4B3 is modulated during LTP phases. Its activation is NMDA-receptor dependent and its transcription is transiently up-regulated 2 h after tetanization. Protein expression peaks 6 h after LTP induction and is rapidly down-regulated at 8 h, whereas cAMP levels decrease during LTP phases. Immunohistochemical studies identified that the majority of type IV phosphodiesterase protein staining is localized to the cell bodies and dendrites of neurones in hippocampal CA1.

Spatial learning induces differential changes in calcium/calmodulin-stimulated (ACI) and calcium-insensitive (ACII) adenylyl cyclases in the mouse hippocampus

Several lines of evidence indicate that Ca2+/calmodulin-stimulated isoforms of adenylyl cyclase (AC) are involved in long-term potentiation and in certain forms of learning. Recently, we found that training in different types of learning task differentially activates Ca2+-sensitive versus Ca2+-insensitive AC activities in certain brain regions, indicating that AC species other than those stimulated by Ca2+/calmodulin may play an important role in learning processes (Guillou, Rose, & Cooper, 1999). Here, we report the effects of spatial reference memory training in a radial arm maze on the levels of AC1 and AC2 mRNA in the dorsal hippocampus of C57BL/6 mice. Acquisition of the task was associated with a learning-specific and time-dependent increase of AC1 mRNA expression selectively in subfields CA1-CA2. In contrast, AC2 mRNA levels were either reduced or not reliably affected depending on the stage of acquisition. Moreover, no significant changes in AC expression were observed either in the dorsal hippocampus of mice trained in a non-spatial (procedural) version of the task or in cortical regions of mice learning the spatial or procedural task. The regional specificity of these effects indicates that the formation of spatial and non-spatial memory requires distinct contributions from Ca2+-sensitive and Ca2+-insensitive AC in the hippocampus. It is suggested that downregulation of AC2 throughout all hippocampal subfields may play a permissive role during the acquisition of spatial learning whereas an upregulation of AC1 specifically in subfield CA1, may be critical to accurately encode, store or use spatial information.

Calmodulin-regulated adenylyl cyclases: cross-talk and plasticity in the central nervous system

Gene disruption studies have shown that the Ca(2+)-stimulated adenylyl cyclases, AC1 and AC8, are critical for some forms of synaptic plasticity, including long-term potentiation as well as long-term memory formation (LTM). It is hypothesized that these enzymes are required for LTM to support the increased expression of a family of genes regulated through the cAMP/Ca(2+) response element-binding protein/cAMP response element transcriptional pathway. In contrast to AC1 and AC8, AC3 is a Ca(2+)-inhibited adenylyl cyclase that plays an essential role in olfactory signal transduction. Coupling of odorant receptors to AC3 stimulates cAMP transients that function as the major second messenger for olfactory signaling. These cAMP transients are caused, at least in part, by Ca(2+) inhibition of AC3, which is mediated through calmodulin-dependent protein kinase II. The unique structure and regulatory properties of these adenylyl cyclases make them attractive drug target sites for modulation of a number of physiological processes including memory formation and olfaction.

A possible mechanism for improvement by a cognition-enhancer nefiracetam of spatial memory function and cAMP-mediated signal transduction system in sustained cerebral ischaemia in rats

1. Accumulated evidence indicates that the adenylyl cyclase (AC)/cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)/cAMP-responsive element binding protein (CREB) signal transduction system may be linked to learning and memory function. 2. The effects of nefiracetam, which has been developed as a cognition enhancer, on spatial memory function and the AC/cAMP/PKA/CREB signal transduction system in rats with sustained cerebral ischaemia were examined. 3. Microsphere embolism (ME)-induced sustained cerebral ischaemia was produced by injection of 700 microspheres (48 micro m in diameter) into the right hemisphere of rats. Daily oral administration of nefiracetam (10 mg kg(-1) day(-1)) was started from 15 h after the operation. 4. The delayed treatment with nefiracetam attenuated the ME-induced prolongation of the escape latency in the water maze task that was examined on day 7 to 9 after ME, but it did not reduce the infarct size. 5. ME decreased Ca(2+)/calmodulin (CaM)-stimulated AC (AC-I) activity, cAMP content, cytosolic PKA Cbeta level, nuclear PKA Calpha and Cbeta levels, and reduced the phosphorylation and DNA-binding activity of CREB in the nucleus in the right parietal cortex and hippocampus on day 3 after ME. The ME-induced changes in these variables did not occur by the delayed treatment with nefiracetam. 6. These results suggest that nefiracetam preserved cognitive function, or prevented cognitive dysfunction, after sustained cerebral ischaemia and that the effect is, in part, attributable to the prevention of the ischaemia-induced impairment of the AC/cAMP/PKA/CREB signal transduction pathway.

Spatial, contextual and working memory are not affected by the absence of mossy fiber long-term potentiation and depression

The mossy fibers of the hippocampus display NMDA-receptor independent long-term plasticity. A number of studies addressed the role of mossy fiber long-term plasticity in memory, but have provided contrasting results. Here, we have exploited a genetic model, the rab3A null-mutant, which is characterized by the absence of both mossy fiber long-term potentiation and long-term depression. This mutant was backcrossed to 129S3/SvImJ and C57Bl/6J to obtain standardized genetic backgrounds. Spatial working memory, assessed in the eight-arm radial maze, was unchanged in rab3A null-mutants. Moreover, one-trial cued and contextual fear conditioning was normal. Long-term spatial memory was tested in the Morris water maze. Two different versions of this task were used, an 'easy' version and a 'difficult' one. On both versions, no differences in search time and quadrant preferences were observed. Thus, despite the elimination of mossy fiber long-term plasticity, these tests revealed no impairments in mnemonic capabilities. We conclude that spatial, contextual and working memory do not depend on mossy fiber plasticity.

Synaptic concentration of type-I adenylyl cyclase in cerebellar neurons

Specific subcellular targeting and spatial arrangement of signaling molecules are important for efficient signal transduction. The neuro-specific type-I adenylyl cyclase (AC1) is stimulated by Ca2+, and plays an essential role in neurodevelopment and neuroplasticity. We generated hemagglutinin (HA)-tagged AC1 to study its subcellular localization in cultured neurons. The HA-tagged AC1 has similar enzymatic activity and regulatory properties to that of non-tagged protein. HA-AC1 targeted to both apical and basolateral domains in the epithelial Madin-Darby canine kidney (MDCK) cells, and it was found in both axons and dendrites in cultured hippocampal neurons as well as in cerebellar granule neurons. Interestingly, AC1 showed a distinct punctate form of immunostaining in MDCK cells and transfected neurons, suggesting it targets to specific subcellular domains. By immunostaining with different synaptic markers, we found that AC1 puncta were located at the excitatory synapses in cerebellar granule neurons. Our data provide a possible cellular mechanism for the physiological role of AC1 in neuroplasticity.