Differential activation of adenylyl cyclases by spatial and procedural learning

Vestibular loss promotes procedural response during a spatial task in rats

Declarative memory refers to a spatial strategy using numerous sources of sensory input information in which visual and vestibular inputs are assimilated in the hippocampus. In contrast, procedural memory refers to a response strategy based on motor skills and familiar gestures and involves the striatum. Even if vestibular loss impairs hippocampal activity and spatial memory, vestibular-lesioned rats remain able to find food rewards during complex spatial memory task. Since hippocampal lesions induce a switch from declarative memory to procedural memory, we hypothesize that vestibular-lesioned rats use a strategy other than that of hippocampal spatial response to complete the task and to counterbalance the loss of vestibular information. We test, in a reverse T-maze paradigm, the types of strategy vestibular-lesioned rats preferentially uses in a spatial task. We clearly demonstrate that all vestibular-lesioned rats shift to a response strategy to solve the spatial task, while control rats use spatial and response strategies equally. We conclude that the loss of vestibular informations leading to spatial learning impairments is not offset at the hippocampus level by integration process of other sense mainly visual informations; but favors a response strategy through procedural memory most likely involving the striatum, cerebellum, and motor learning.

Developmental molecular and functional cerebellar alterations induced by PCP4/PEP19 overexpression: implications for Down syndrome

PCP4/PEP19 is a modulator of Ca(2+)-CaM signaling. In the brain, it is expressed in a very specific pattern in postmitotic neurons. In particular, Pcp4 is highly expressed in the Purkinje cell, the sole output neuron of the cerebellum. PCP4, located on human chromosome 21, is present in three copies in individuals with Down syndrome (DS). In a previous study using a transgenic mouse model (TgPCP4) to evaluate the consequences of 3 copies of this gene, we found that PCP4 overexpression induces precocious neuronal differentiation during mouse embryogenesis. Here, we report combined analyses of the cerebellum at postnatal stages (P14 and adult) in which we identified age-related molecular, electrophysiological, and behavioral alterations in the TgPCP4 mouse. While Pcp4 overexpression at P14 induces an earlier neuronal maturation, at adult stage it induces increase in cerebellar CaMK2alpha and in cerebellar LTD, as well as learning impairments. We therefore propose that PCP4 contributes significantly to the development of Down syndrome phenotypes through molecular and functional changes.

Adenylyl cyclases as innovative therapeutic goals

Pharmacological modulation of intracellular cyclic AMP (cAMP) signalling could provide new therapeutic and experimental tools. Although drugs interfering with this pathway have traditionally targeted membrane receptors, the effector enzyme adenylyl cyclase (AC), which functions as a signalling catalyst, also presents an interesting target. Thus, development of isoform-selective stimulator and/or inhibitor compounds for AC could lead to organ-specific pharmacotherapeutics for treating heart failure, cancer and neurodegenerative diseases. In this review, the potential of AC as the object of drug therapy is discussed.

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

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.

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.

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.

A constraint on cAMP signaling

Studies in invertebrates and vertebrates have demonstrated a critical role for cAMP signaling and adenylyl cyclase (AC) activity in learning and memory. In this issue of Neuron, Pineda et al. show that in the hippocampus, reduction of AC activity via the inhibitory G protein G(i) is critical for memory formation, suggesting that a balance of inhibitory and stimulatory regulators of AC is required for optimal cAMP signaling.

Multi-functional drugs for various CNS targets in the treatment of neurodegenerative disorders

Individuals with neurodegenerative diseases such as Parkinson's disease or Alzheimer's disease are benefiting from drugs developed to act on a single molecular target. However, current pharmacological approaches are limited in their ability to modify significantly the course of the disease, and offer incomplete and transient benefit to patients. New therapeutic strategies comprise drug candidates designed specifically to act on multiple neural and biochemical targets for the treatment of cognition impairment, motor dysfunction, depression and neurodegeneration. Examples include the development of single molecular entities that combine two or more of the following properties: (i) cholinesterase inhibition; (ii) activation or inhibition of specific subtypes of acetylcholine receptors or alpha-adrenoceptors; (iii) anti-inflammatory activity; (iv) monoamine oxidase inhibition; (v) catechol-O-methyl transferase inhibition; (vi) nitric oxide production; (vii) neuroprotection; (viii) anti-apoptotic activity; and (ix) activation of mitochondrial-dependent cell-survival genes and proteins. These bi- or multi-functional compounds might provide greater symptomatic efficacy, and better utility as potential neuroprotective disease-modifying drugs.

Spatiotemporal localization of the calcium-stimulated adenylate cyclases, AC1 and AC8, during mouse brain development

Type 1 and type 8 adenylate cyclases, AC1 and AC8, are membrane bound enzymes that produce cAMP in response to calcium entry and could thus control a large number of developmental processes. We provide a detailed spatiotemporal localization of these genes in the mouse brain during embryonic and postnatal life using in situ hybridization. AC1 gene expression begins early in embryonic life (before E13), and its expression is much more widespread than in adults. Transient expression of AC1 is found in the striatum, the dorsal thalamus, the trigeminal nerve nuclei, the Purkinje cells of the cerebellum, the interneurons of the hippocampus, and the retinal ganglion cells. In all these structures, the peak of AC1 gene expression occurs during early postnatal life, decreasing by P10. After P15, AC1 expression is confined to the hippocampus, the cerebral cortex, and to the granule cells of the cerebellum. AC8 gene expression also begins early in embryonic life (E12)--but in a more limited number of regions than in adults. AC8 expression is initially restricted to the epithalamus, the hypothalamus, the superior colliculus, the cerebellar anlage the proliferative zone of the rhombic lip, and the spinal cord. The expression increases and broadens during postnatal life, particularly in the thalamus and the cerebral cortex. A transient peak of AC8 expression is found in layer IV of the somatosensory cortex. Thus, AC1 and AC8 have an early developmental onset with complementary spatiotemporal distribution patterns: AC1 is most broadly distributed in embryonic life, whereas AC8 is most broadly expressed in adulthood. Transient expression of these genes designate areas that may be particularly sensitive to neural activity/calcium-modulated cAMP responses during development.

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.

Cerebellar contribution to spatial event processing: do spatial procedures contribute to formation of spatial declarative knowledge?

Spatial knowledge of an environment involves two distinct competencies: declarative spatial knowledge, linked to where environmental cues are and where the subject is with respect to the cues, and, at the same time, procedural spatial knowledge, linked to how to move into the environment. It has been previously demonstrated that hemicerebellectomized (HCbed) rats are impaired in developing efficient exploration strategies, but not in building spatial maps or in utilizing localizing cues. The aim of the present study was to analyse the relationships between spatial procedural and declarative knowledge by using the open field test. HCbed rats have been tested in two different protocols of the open field task. The results indicate that HCbed animals succeeded in moving inside the arena, in contacting the objects and in habituating to the new environment. However, HCbed animals did not react to environmental changes, when their impaired explorative pattern was inappropriate to the environment, suggesting that they were not able to represent a new environment because they were not able to explore it appropriately. Nevertheless, when their altered procedures were favoured by object arrangement, they detected environmental changes as efficiently as did normal rats. This finding suggests that no declarative spatial learning is possible without appropriate procedural spatial learning.

Dissociation of visual associative and motor learning in Drosophila at the flight simulator

Ever since operant conditioning was studied experimentally, the relationship between associative learning and possible motor learning has become controversial. Although motor learning and its underlying neural substrates have been extensively studied in mammals, it is still poorly understood in invertebrates. The visual discriminative avoidance paradigm of Drosophila at the flight simulator has been widely used to study the flies' visual associative learning and related functions, but it has not been used to study the motor learning process. In this study, newly-designed data analysis was employed to examine the flies' solitary behavioural variable that was recorded at the flight simulator-yaw torque. Analysis was conducted to explore torque distributions of both wild-type and mutant flies in conditioning, with the following results: (1) Wild-type Canton-S flies had motor learning performance in conditioning, which was proved by modifications of the animal's behavioural mode in conditioning. (2) Repetition of training improved the motor learning performance of wild-type Canton-S flies. (3) Although mutant dunce(1) flies were defective in visual associative learning, they showed essentially normal motor learning performance in terms of yaw torque distribution in conditioning. Finally, we tentatively proposed that both visual associative learning and motor learning were involved in the visual operant conditioning of Drosophila at the flight simulator, that the two learning forms could be dissociated and they might have different neural bases.

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.

Effects of nefiracetam on cerebral adenylyl cyclase activity in rats with microsphere embolism-induced memory dysfunction

The effects of nefiracetam on the cerebral adenylyl cyclase (AC) activity of animals with microsphere embolism-induced memory dysfunction were examined. Sustained cerebral ischemia in the right cerebral hemisphere was induced by an injection of microspheres into the right internal carotid artery of rats. To examine learning and memory function, the water maze test was performed from day 7 to day 10 after the operation. The escape latency of the microsphere-embolized (ME) rat in the water maze task was longer than that of the sham-operated (Sham) rat, suggesting that spatial memory dysfunction occurred in the ME rat. Gsalpha and Gi(1/2)alpha protein levels in the cerebral cortex, striatum and hippocampus of the ME rat, when determined on day 11, were similar to those of the Sham rats. The basal AC activity in the striatum, but not in the other two regions, of the ME rat decreased. The AC activity in the presence of 10 microM colforsin daropate (Col), a direct stimulator of AC, was increased by approximately 20-fold in sham animals and 7- to 10-fold in the ME rat. Treatment of the ME rat with 10 mg/kg/d nefiracetam p.o. from day 1 to day 10 after the operation shortened the escape latency, restored the basal AC activity in the striatum, and reversed the Col-induced increases in AC in these three regions without any changes in the cerebral Gsalpha and Gi(1/2)alpha protein levels. These results suggest that nefiracetam-mediated activation of AC activity may contribute to the improvement of memory and learning function in sustained cerebral ischemia.

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.

Persistent effects of delayed treatment with nefiracetam on the water maze task in rats with sustained cerebral ischemia

The present study was aimed at determining whether nefiracetam might have a persistent cognition-enhancing effect in animals with sustained cerebral ischemia. Sustained cerebral ischemia was induced by injecting 700 microspheres into the right internal carotid artery of rats [microsphere-embolized (ME) rats]. The ME and sham-operated rats were treated with 10 mg/kg/day nefiracetam p.o. from the first to the 9th day after the operation. The escape latency of the ME rat in the water maze test, when performed on days 7 to 9 after the operation, was lengthened. This effect was attenuated by the delayed treatment with nefiracetam. The nefiracetam-treated ME rat showed a shortened escape latency in the retention test on day 17 as well as in the contraposition test on day 18. These results indicate that a persistent improvement of the spatial memory function impaired by sustained cerebral ischemia was achieved even after cessation of treatment with nefiracetam. The functional damage to learning and memory was associated with decreases in the membranous adenylyl cyclase I and cytosolic protein kinase A (PKA) catalytic subunit and regulatory subunit proteins in the right hippocampus and cerebral cortex. The delayed treatment with nefiracetam appreciably prevented the decreases in these proteins. The present study suggests that nefiracetam may have an ability to cause persistent improvement of learning and memory function, possibly through protection against the ischemia-induced impairment to the adenylyl cyclase/cAMP/PKA signal transduction pathway.

Effects of hippocampal lesions on patterned motor learning in the rat

Motor skill learning in rats has been linked to cerebellar function as well as to cortical and striatal influences. The present study evaluated the contribution of the hippocampus to motor learning. Adult male rats received electrolytic lesions designed to selectively destroy the hippocampus; a sham-lesioned group of animals served as a control. The animals with hippocampal lesions acquired a patterned motor learning task as well as sham controls. In contrast, rats with hippocampal lesions were impaired in spatial, but not cued, learning in the Morris water maze. In addition, lesioned rats showed profound impairment in the novel object recognition memory task, when a 1-h delay was used between training and testing. Taken together, these results suggest that the hippocampus is not necessary during acquisition of the motor learning task.

Impairment of adenylyl cyclase and of spatial memory function after microsphere embolism in rats

The purpose of the present study was to characterize alterations in the adenylyl cyclase (AC), cyclic adenosine 3',5'-monophosphate (cAMP), and spatial memory function after sustained cerebral ischemia. Sustained cerebral ischemia was induced by injection of 900 microspheres (48 microm in diameter) into the right (ipsilateral) hemisphere of rats. Alterations in the AC and cAMP in the cerebral cortex and hippocampus were examined up to 7 days after the embolism. A decrease in the cAMP content was seen in the ipsilateral hemisphere throughout the experiment. Microsphere embolism (ME) decreased the activity of Ca(2+)/calmodulin (CaM)-sensitive AC in the ipsilateral hemisphere throughout the experiment, whereas the basal and 5'-guanylyl imidodiphosphate (Gpp(NH)p)-sensitive AC activities were not altered. Immunoblotting analysis of AC subtypes with specific antibodies showed a decrease in the immunoreactivity of AC-I in the ipsilateral hemisphere during these periods. No significant differences in the immunoreactivity of AC-V/VI and AC-VIII were observed after ME. The levels of GTP-binding proteins Galpha(s), Galpha(i), and Gbetawere unchanged. Furthermore, microsphere-embolized rats showed prolongation of the escape latency in the water maze task determined on the seventh to ninth day after the operation. These results suggest that sustained cerebral ischemia may induce the impairment of the AC, particularly a selective reduction in the AC-I level and activity, coupled with the decrease in cAMP content. This reduction may play an appreciable role in the disturbance in cAMP-mediated signal transduction system, possibly leading to learning and memory dysfunction.

Two critical periods for cAMP-dependent protein kinase activity during long-term memory consolidation in the crab Chasmagnathus

Activation of the cAMP pathway was found to be implicated in the memory process. In the context-signal learning paradigm of the crab Chasmagnathus, the protein kinase (PKA) activator Sp-5,6-DCl-cBIMPS facilitated long-term memory (LTM) induced by spaced training while the PKA inhibitor 8-chloroadenosine-3', 5'-monophosphorothioate, Rp-isomer (Rp-8-Cl-cAMPS) produced amnesia. In the present report the effect of the PKA inhibitor on long-term retention was assessed when administered (systemic injection of 2 microg/animal) at various times after training. According to previous results obtained with a lower dose, retention is impaired when the drug is administered immediately pretraining. An effect on acquisition was ruled out considering that the drug did not affect the performance during training. On the contrary, no effect of the PKA inhibitor was found with an immediately posttraining injection and amnesia was observed only when training was shortened from 15 to 12 trials (training duration from 45 to 36 min). At 2 and 12 h posttraining Rp-8-Cl-cAMPS injection failed to impair retention, but amnesia was found when the drug was injected at 4 and 8 h after training. In order to assess a possible effect of the drug in retrieval, the PKA inhibitor was administered 15 min before testing, and no amnestic effect was observed. These results suggest that two phases of PKA activity are required during consolidation of LTM, one during training and the other between 4 and 8 h after training. The link between these two periods of PKA activation and the two phases of the transcription factor NF-kappaB activation previously found in this model, as well as the similar time course found in rodents, is discussed. An amnestic effect of the drug was not found when administered immediately before a massed training protocol that yielded an intermediate-term memory, suggesting that in this type of memory PKA activation is not required.

Isoform-specific changes of adenylate cyclase mRNA expression in rat brains following chronic electroconvulsive shock

1. Electroconvulsive shock (ECS) has been reported to regulate the cAMP signaling system at various levels, suggesting that the cAMP system is involved in the therapeutic mechanism. 2. Chronic ECS has been suggested to change the expressions of adenylate cyclase (AC) genes, which constitute at least 9 families. However, little is known about its effect on the expression of AC. Therefore, to understand how chronic ECS alters the expression of AC genes in the brain, the authors analyzed the expression of 9 AC isoforms at the transcriptional level in rat hippocampus and cerebellum by quantitative RT-PCR following chronic ECS treatment. 3. Chronic ECS treatment was found to induce differential changes in the expression of AC isoforms in an isoform- and brain region-specific manner in the rat hippocampus and cerebellum. 4. Thus, it is concluded that chronic ECS induces differential changes in the expression of AC isoform mRNA in an isoform- and brain region-specific manner in the rat hippocampus and cerebellum. This suggests that the differential expression of AC isoforms might be an important mechanism by which chronic ECS treatment regulates the cAMP signaling system in rat brains.

Inositol hexakisphosphate increases L-type Ca2+ channel activity by stimulation of adenylyl cyclase

Inositol hexakisphosphate (InsP6) is a most abundant inositol polyphosphate that changes simultaneously with inositol 1,4,5-trisphosphate in depolarized neurons. However, the role of InsP6 in neuronal signaling is unknown. Mass assay reveals that the basal levels of InsP6 in several brain regions tested are similar. InsP6 mass is significantly elevated in activated brain neurons and lowered by inhibition of neuronal activity. Furthermore, the hippocampus is most sensitive to electrical challenge with regard to percentage accumulation of InsP6. In hippocampal neurons, InsP6 stimulates adenylyl cyclase (AC) without influencing cAMP phosphodiesterases, resulting in activation of protein kinase A (PKA) and thereby selective enhancement of voltage-gated L-type Ca2+ channel activity. This enhancement was abolished by preincubation with PKA and AC inhibitors. These data suggest that InsP6 increases L-type Ca2+ channel activity by facilitating phosphorylation of PKA phosphorylation sites. Thus, in hippocampal neurons, InsP6 serves as an important signal in modulation of voltage-gated L-type Ca2+ channel activity.

Intrahippocampal injections of somatostatin dissociate acquisition from the flexible use of place responses

Previous studies showed that injections of somatostatin (SS-14) into the hippocampus facilitate the acquisition of spatial tasks in mice. The present study was aimed at better understanding the learning and memory processes that could be affected by hippocampal SS-14 stimulation. Balb/c mice were submitted to a two-stage learning paradigm. In stage 1, they were trained for acquisition of a spatial discrimination task in a radial maze and, in stage 2, were submitted to a probe test aiming at evaluating their ability to use flexibly their previously acquired knowledge. Injections of vehicle or SS-14 were given during the acquisition phase and/or before the probe test using a 2 x 2 factorial design. Mice receiving SS-14 during acquisition failed to succeed in the probe test despite showing a trend to faster acquisition of the initial spatial discrimination task. By contrast, when given only prior to probe trials, SS-14 did not yield any behavioural effects. Thus, SS-14 interfered with the establishment of a flexible form of memory, not with its expression per se, and therefore dissociated the acquisition of place responses from their flexible use. The theoretical issues raised by the present findings are discussed.

Task-dependent role for dorsal striatum metabotropic glutamate receptors in memory

The effect of post-training intradorsal striatal infusion of metabotropic glutamate receptor (mGluR) drugs on memory consolidation processes in an inhibitory avoidance (IA) task and visible/hidden platform water maze tasks was examined. In the IA task, adult male Long-Evans rats received post-training intracaudate infusions of the broad spectrum mGluR antagonist alpha-methyl-4-carboxyphenylglycine (MCPG; 1.0, 2.0 mM/0.5 microL), the group I/II mGluR agonist 1-aminocyclopentane-1,3-carboxylic acid (ACPD; 0.5 or 1.0 microM/0.5 microL), or saline immediately following footshock training, and retention was tested 24 h later. In the visible- and hidden-platform water maze tasks, rats received post-training intracaudate infusions of ACPD (1.0 microM), MCPG (2.0 mM), or saline immediately following an eight-trial training session, followed by a retention test 24 h later. In the IA task, post-training infusion of ACPD (0.5 and 1.0 microM) or MCPG (1.0 and 2.0 mM) impaired retention. In the IA and visible-platform water maze tasks, post-training infusion of ACPD (1.0 microM), or MCPG (2.0 mM) impaired retention. In contrast, neither drug affected retention when administered post-training in the hidden-platform task, consistent with the hypothesized role of the dorsal striatum in stimulus-response habit formation. When intradorsal striatal injections were delayed 2 h post-training in the visible-platform water maze task, neither drug affected retention, indicating a time-dependent effect of the immediate post-training injections on memory consolidation. It is hypothesized that MCPG impaired memory via a blockade of postsynaptic dorsal striatal mGluR's, while the impairing effect of ACPD may have been caused by an influence of this agonist on presynaptic "autoreceptor" striatal mGluR populations.

Regulation of adenylyl cyclase in the central nervous system

Adenylyl cyclases (ACs) are a family of enzymes that synthesize one of the major second messengers, cAMP, upon stimulation. Since the report of the first adenylyl cyclase (AC) gene in 1989, tremendous efforts have been devoted to identifying and characterizing more AC isozymes. In the past decade, significant knowledge regarding the basic structure, tissue distribution, and regulation of AC isozymes has been accumulated. Because members of the AC superfamily are tightly controlled by various signals, one of the most important impacts of these AC isozymes is their contribution to the complexity and fine-tuning of cellular signalling, especially in the central nervous system (CNS) where multiple signals constantly occur. This review focuses on recent progress toward understanding the physiological roles of ACs in the CNS.