Protein kinase A as a therapeutic target for memory disorders: rationale and challenges

Inverted-U dopamine D1 receptor actions on prefrontal neurons engaged in working memory

Dopamine (DA) D1 receptor (D1R) stimulation in prefrontal cortex (PFC) produces an 'inverted-U' dose-response, whereby either too little or too much D1R stimulation impairs spatial working memory. This response has been observed across species, including genetic linkages with human cognitive abilities, PFC activation states and DA synthesis. The cellular basis for the inverted U has long been sought, with in vitro intracellular recordings supporting a variety of potential mechanisms. The current study demonstrates that the D1R agonist inverted-U response can be observed in PFC neurons of behaving monkeys: low levels of D1R stimulation enhance spatial tuning by suppressing responses to nonpreferred directions, whereas high levels reduce delay-related firing for all directions, eroding tuning. These sculpting actions of D1R stimulation are mediated in monkeys and rats by cyclic AMP intracellular signaling. The evidence for an inverted U at the cellular level in behaving animals promises to bridge in vitro molecular analyses with human cognitive experience.

Protective Effects of Chronic Lithium Treatment against Spatial Memory Retention Deficits Induced by the Protein Kinase AII Inhibitor H-89 in Rats

We have previously shown that infusion of the PKAII inhibitor H-89 in the CA1 area of the hippocampus impaired spatial memory retention. There is some evidence suggesting the neuroprotective effects of chronic lithium administration including its ability to attenuate a deleterious effect of chronic stress on spatial memory in rats. In the present study, we investigated whether chronic administration of lithium can improve memory as well as influence the inhibitory effect of H-89 on spatial memory retention. Male albino rats were treated systemically with lithium (600 mg/l) for 4 weeks and then trained for 4 days in the Morris water maze. Testing the animals 48 h later showed a significant reduction in escape latency (p < 0.05) and travel distance (p < 0.05) compared to the controls. In separate experiments, the rats were similarly treated with lithium for 4 weeks, followed by similar training for 4 days and then immediately infused bilaterally with vehicle or 5 micromol/l H-89 into the CA1 region of the hippocampus. Animals were then tested 48 h after H-89 infusion in order to assess their spatial memory retention. The lithium treatment caused a significant reduction in escape latency (p < 0.001) and travel distance (p < 0.001) compared to H-89-treated animals. The data suggest that lithium treatment for 4 weeks improved spatial memory retention and that lithium pretreatment prevented or reversed the H-89-induced spatial memory deficits.

Nucleus accumbens PKA inhibition blocks acquisition but enhances expression of amphetamine-produced conditioned activity in rats

RATIONALE

The nucleus accumbens (NAc) plays a central role in dopamine-produced reward-related learning. In previous studies, the cyclic adenosine monophosphate-dependent protein kinase (PKA) inhibitor Rp-Cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium salt (Rp-cAMPS) blocked the acquisition but not expression of NAc reward-related learning for natural rewards and the acquisition of psychostimulant drug conditioning.

OBJECTIVES

The current study assessed the role of PKA in the expression of NAc amphetamine (amph)-produced conditioning using conditioned activity (CA).

MATERIALS AND METHODS

After 5 days of habituation, a test environment was paired with bilateral NAc injections of amph (0.0 or 25.0 micro g) and the PKA inhibitor Rp-cAMPS (0.0, 5.0, 10.0, or 20.0 micro g) over three 60-min conditioning sessions separated by 48 h. To test for effects on expression, some groups received vehicle or amph alone before conditioning sessions and were injected with 0.0, 0.25, 5.0, or 20.0 mug of Rp-cAMPS before the single 60-min test session.

RESULTS

Amph produced acute increases in locomotion and robust CA. Rp-cAMPS impaired the acquisition of amph-produced CA but not its expression; in fact, it enhanced expression.

CONCLUSIONS

Results show that PKA inhibition blocks the acquisition but not the expression of amph-produced conditioning.

Amnesia and the hippocampus

PURPOSE OF REVIEW

Long-term memory impairments have great medical significance and a considerable health and economic burden. Understanding their cognitive and neuroanatomical underpinnings is of crucial importance. Severe amnesia is usually observed following bilateral hippocampal pathology. This review addresses the precise role of the hippocampus and related medial temporal lobe structures in amnesia.

RECENT FINDINGS

Disagreements exist over whether, following selective hippocampal damage: retrograde amnesia for episodic memories is temporally limited or extensive and ungraded; anterograde amnesia involves both recollective and familiarity processes. It is accepted that material specific impairments follow unilateral medial temporal lobe damage, with verbal and nonverbal memory lateralized to left or right, respectively. Memory for unknown faces, however, may not depend on the hippocampus. Pharmacological studies in animals, with some extension to humans, highlight promising future therapeutic interventions targeting synaptic plasticity modulation.

SUMMARY

Despite considerable progress, some issues remain unresolved. The available evidence favours the view, however, that the hippocampus, in conjunction with other cortical areas, is critical for the retrieval of remote episodic memories and for both recollection and familiarity anterograde memory processes. There are as yet no effective pharmacological treatments for medial temporal lobe amnesia, but various rehabilitative techniques may be useful.

Amnesia or retrieval deficit? Implications of a molecular approach to the question of reconsolidation

Post-retrieval interference with a memory has uncovered a phenomenon known to the field as reconsolidation. In this article, we will review the specific molecular mechanisms that have been implicated in reconsolidation. As a result of numerous studies over the past five years, it can now be said with a fair amount of certainty that reconsolidation is not a recapitulation of the mechanisms underlying consolidation, despite what the term "reconsolidation" may suggest. Therefore, in addition to reviewing the known mechanisms of reconsolidation, we will propose that two experimental approaches involving the targeting of specific molecular mechanisms, and the study of these mechanisms during retrieval, may serve useful to the field as it is now able to advance beyond comparisons between consolidation and reconsolidation.

Alpha2A-adrenoceptor stimulation improves prefrontal cortical regulation of behavior through inhibition of cAMP signaling in aging animals

The working-memory functions of the prefrontal cortex (PFC) are improved by stimulation of postsynaptic, alpha2A-adrenoceptors, especially in aged animals with PFC cognitive deficits. Thus, the alpha2A-adrenoceptor agonist, guanfacine, greatly improves working-memory performance in monkeys and rats following systemic administration or intra-PFC infusion. Alpha2A-adrenoceptors are generally coupled to Gi, which can inhibit adenylyl cyclases and reduce the production of cAMP. However, no study has directly examined whether the working-memory enhancement observed with guanfacine or other alpha2A-adrenoceptor agonists results from cAMP inhibition. The current study confirmed this hypothesis in both rats and monkeys, showing that treatments that increase cAMP-mediated signaling block guanfacine's beneficial effects. In aged rats, guanfacine was infused directly into the prelimbic PFC and was challenged with co-infusions of the cAMP analog, Sp-cAMPS. In aging monkeys, systemically administered guanfacine was challenged with the phosphodiesterase 4 inhibitor, rolipram, using intramuscular doses known to have no effect on their own. In both studies, agents that mimicked the actions of cAMP (rats) or increased endogenous cAMP (monkeys) completely blocked the enhancing effects of guanfacine on working-memory performance. These results are consistent with alpha2A-adrenoceptor stimulation enhancing PFC working-memory function via inhibition of cAMP-mediated signaling.

Stimulants: Therapeutic actions in ADHD

Stimulants such as methylphenidate and amphetamine are currently the most common treatment for attention deficit hyperactivity disorder (ADHD). For years, it was assumed that stimulants had paradoxical calming effects in ADHD patients, whereas stimulating 'normal' individuals and producing locomotor activation in rats. It is now known that low doses of stimulants focus attention and improve executive function in both normal and ADHD subjects. Furthermore, the seminal work of Kuczenski and Segal showed that low, oral doses of methylphenidate reduce locomotor activity in rats as well. Berridge et al have now shown that these low doses produce marked increases in norepinephrine and dopamine release in the prefrontal cortex, whereas having only subtle effects on subcortical catecholamine release. ihe prefrontal cortex regulates behavior and attention using representational knowledge, and imaging and neuropsychological studies have shown that the prefrontal cortex is weaker in subjects with ADHD. This cortical area is very sensitive to levels of catecholamines: moderate levels engage postsynaptic alpha2A-adrenoceptors and D1 receptors and improve prefrontal regulation of behavior and attention, while high levels impair prefrontal function via alpha1-adrenoceptors and excessive D1 receptor stimulation. Administering low doses of methylphenidate to rats improves the working memory and attentional functions of the prefrontal cortex, while high doses impair working memory and produce a perseverative pattern of errors similar to that seen in patients. The low dose improvement is hiocked by either an alpha2-adrenoceptor or Dl receptor antagonist, suggesting that both norepinephrine and dopamine contribute to the beneficial actions of stimulant medications.

A model for agonism and antagonism in an ancient and ubiquitous cAMP-binding domain

The cAMP-binding domain (CBD) is an ancient and conserved regulatory motif that allosterically modulates the function of a group of diverse proteins, thereby translating the cAMP signal into a controlled biological response. The main receptor for cAMP in mammals is the ubiquitous regulatory (R) subunit of protein kinase A. Despite the recognized significant potential for pharmacological applications of CBDs, currently only one group of competitive inhibitor antagonists is known: the (R(p))-cAMPS family of phosphorothioate cAMP analogs, in which the equatorial exocyclic oxygen of cAMP is replaced by sulfur. It is also known that the diastereoisomer (S(p))-cAMPS with opposite phosphorous chirality is a cAMP agonist, but the molecular mechanism of action of these analogs is currently not fully understood. Previous crystallographic and unfolding investigations point to the enhanced CBD dynamics as a key determinant of antagonism. Here, we investigate the (R(p))- and (S(p))-cAMPS-bound states of R(CBD-A) using a comparative NMR approach that reveals a clear chemical shift and dynamic NMR signature, differentiating the (S(p))-cAMPS agonist from the (R(p))-cAMPS antagonist. Based on these data, we have proposed a model for the (R(p)/S(p))-cAMPS antagonism and agonism in terms of steric and electronic effects on two main allosteric relay sites, Ile(163) and Asp(170), respectively, affecting the stability of a ternary inhibitory complex formed by the effector ligand, the regulatory and the catalytic subunits of protein kinase A. The proposed model not only rationalizes the existing data on the phosphorothioate analogs, but it will also facilitate the design of novel cAMP antagonists and agonists.

Biomolecular interaction analysis in functional proteomics

To understand the function of highly complex eukaryotic tissues like the human brain, in depth knowledge about cellular protein networks is required. Biomolecular interaction analysis (BIA), as a part of functional proteomics, aims to quantify interaction patterns within a protein network in detail. We used the cAMP dependent protein kinase (PKA) as a model system for the binding analysis between small natural ligands, cAMP and cAMP analogues, with their physiological interaction partner, the regulatory subunit of PKA. BIA comprises a variety of methods based on physics, biochemistry and molecular biology. Here we compared side by side real time SPR (surface plasmon resonance, Biacore), a bead based assay (AlphaScreen), a fluorescence based method (Fluorescence polarisation) and ITC (isothermal titration calorimetry). These in vitro methods were complemented by an in cell reporter assay, BRET(2) (bioluminescence resonance energy transfer), allowing to test the effects of cAMP analogues in living cells.

Using substrate-binding variants of the cAMP-dependent protein kinase to identify novel targets and a kinase domain important for substrate interactions in Saccharomyces cerevisiae

Protein kinases mediate much of the signal transduction in eukaryotic cells and defects in kinase function are associated with a variety of human diseases. To understand and correct these defects, we will need to identify the physiologically relevant substrates of these enzymes. The work presented here describes a novel approach to this identification process for the cAMP-dependent protein kinase (PKA) in Saccharomyces cerevisiae. This approach takes advantage of two catalytically inactive PKA variants, Tpk1K336A/H338A and Tpk1R324A, that exhibit a stable binding to their substrates. Most protein kinases, including the wild-type PKA, associate with substrates with a relatively low affinity. The binding observed here was specific to substrates and was dependent upon PKA residues known to be important for interactions with peptide substrates. The general utility of this approach was demonstrated by the ability to identify both previously described and novel PKA substrates in S. cerevisiae. Interestingly, the positions of the residues altered in these variants implicated a particular region within the PKA kinase domain, corresponding to subdomain XI, in the binding and/or release of protein substrates. Moreover, the high conservation of the residues altered and, in particular, the invariant nature of the R324 position suggest that this approach might be generally applicable to other protein kinases.

Place preference induced by nucleus accumbens amphetamine is impaired by local blockade of Group II metabotropic glutamate receptors in rats

Background

The nucleus accumbens (NAc) plays a critical role in amphetamine-produced conditioned place preference (CPP). In previous studies, NAc basal and amphetamine-produced DA transmission was altered by Group II mGluR agents. We tested whether NAc amphetamine CPP depends on Group II mGluR transmission.

Results

NAc injections (0.5 μl/side) of the Group II mGluR antagonist (2 S)- a-ethylglutamic acid (EGLU: 0.01–0.8 μg but not 0.001 μg) impaired CPP. The drug did not block the acute locomotor effect of amphetamine.

Conclusion

Results suggest that Group II mGluRs may be necessary for the establishment of NAc amphetamine-produced CPP. These receptors may also mediate other forms of reward-related learning dependent on this structure.

D1 receptor modulation of memory retrieval performance is associated with changes in pCREB and pDARPP-32 in rat prefrontal cortex

We have recently shown a significant role of dopamine D(1) receptors in recognition and temporal order memory retrieval for objects in rodents [Hotte M, Naudon L, Jay TM. Modulation of recognition and temporal order memory retrieval by dopamine D(1) receptor in rats. Neurobiol Learn Mem 2005;84:85-92]. The present study investigates the signal transduction pathways underlying dopamine D(1) receptor modulation of retrieval performance in these memory tasks at different delays. We analyzed the level of phosphorylation of both CREB (cAMP response element binding protein) and DARPP-32 (dopamine and cAMP-regulated phosphoprotein, 32 kDa) in (1) the prefrontal cortex of rats that had performed the object recognition task, (2) the prefrontal and perirhinal cortices of rats that had performed the temporal order memory task for objects. For comparison, we explored the phosphorylation state of CREB and DARPP-32 in the prefrontal cortex, nucleus accumbens and hippocampus of rats having performed badly on the delayed spatial win-shift task after D(1) blockade. The improvement in recognition and temporal order memory performance at a 4h-delay was associated with an increased phosphorylation of both CREB and DARPP-32 in the prefrontal cortex of rats treated with the D(1) agonist SKF 81297. By contrast, the significant impairment of delayed spatial memory retrieval after administration of the selective D(1) antagonist SCH 23390 was associated with decreased phosphorylation of CREB and DARPP-32 in the prefrontal cortex. These results provide insight into molecular mechanisms involved in D(1) receptor-dependent modulation of short- versus long-term memory in prefrontal cortex where DARPP-32 in synergy with CREB may represent a pivotal role.

Opposing effects of AMPA and 5-HT1A receptor blockade on passive avoidance and object recognition performance: correlation with AMPA receptor subunit expression in rat hippocampus

It has been suggested that antagonists at serotonin 5-HT1A receptors may exert a procognitive effect by facilitating glutamatergic neurotransmission. Here we further explored this issue by looking for the ability of a 5-HT1A antagonist to prevent the learning deficit induced by AMPA receptor blockade in two behavioural procedures in rats, and for concomitant molecular changes presumably involved in memory formation in the hippocampus. Pretraining administration of the competitive AMPA receptor antagonist, NBQX, produced a dose-related retention impairment in a passive avoidance task 24h later, and also impaired retention in a novel object recognition test when an intertrial interval of 3h was selected. Pretreatment with the selective 5-HT1A receptor antagonist, WAY-100635, prevented the learning deficit induced by NBQX in the two behavioural procedures. In biochemical studies performed on rat hippocampus after the retention tests, we found that learning increased the membrane levels of AMPA receptor GluR1 and GluR2/3 subunits, as well as the phosphorylated forms of GluR1, effects that were abolished by NBQX administration before the training session. Pretreatment with WAY-100635 counteracted the NBQX effects and restored the initial learning-specific increase in Ca2+/calmodulin-dependent protein kinase II (CaMKII) function and the later increase in GluR2/3 and phosphorylated GluR1 surface expression. Moreover, administration of WAY-100635 before object recognition training improved recognition memory 24h later and potentiated the learning-associated increase in AMPA receptor subunits. The results support the proposed utility of 5-HT1A antagonists in the treatment of cognitive disorders.

Persistent changes in motivation to self-administer cocaine following modulation of cyclic AMP-dependent protein kinase A (PKA) activity in the nucleus accumbens

Drug-induced neuroadaptations within the nucleus accumbens, including activation of cAMP-dependent protein kinase A (PKA), may contribute to the synaptic plasticity and behavioural changes that underlie drug addiction. As a direct test of this hypothesis, we examined the effects in rats of PKA activation (Sp-cAMPS infusions of 10 and 20 nmol/side) and inhibition (Rp-cAMPS infusions of 10 and 20 nmol/side) in the nucleus accumbens on motivation to obtain cocaine as measured by responding under the progressive-ratio schedule. Bilateral infusions of Sp-cAMPS (20 nmol/side) resulted in an increase in progressive-ratio responding for cocaine and this effect persisted for several days. In contrast, Rp-cAMPS (20 nmol/side) produced persistent decreases in progressive-ratio responding for cocaine beginning on the day of administration and lasting for several days. These data suggest that alternations in PKA activity within the nucleus accumbens as a consequence of repeated cocaine exposure may contribute to addiction by producing persistent increases in motivation to obtain cocaine.

Presynaptic mechanism underlying cAMP-induced synaptic potentiation in medial prefrontal cortex pyramidal neurons

cAMP, a classic second messenger, has been proposed recently to participate in regulating prefrontal cortical cognitive functions, yet little is known about how it does so. In this study, we used forskolin, an adenylyl cyclase activator, to examine the effects of cAMP on excitatory synaptic transmission in the medial prefrontal cortex (mPFC) using whole-cell patch-clamp recordings from visually identified layer II-III or V pyramidal cells in vitro. We found that bath application of forskolin significantly increased the amplitude of excitatory postsynaptic currents (EPSCs) in a concentration- and age-dependent manner. This enhancement was completely abolished by coapplication of cAMP-dependent protein kinase (PKA) inhibitor and p42/p44 mitogen-activated protein kinase (MAPK) kinase inhibitor, but not application of either drug alone. The membrane-permeable cAMP analog adenosine 3',5'-cyclic monophosphorothioate, Sp-isomer, triethylammonium salt, or activation of beta-adrenergic receptor by isoproterenol mimicked the effect of forskolin to potentiate EPSCs. However, neither exchange protein activated by cAMP (Epac) inhibitor brefeldin A nor hyperpolarization and cyclic nucleotide-activated channel blocker 4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride (ZD7288) affected forskolin response. The augmentation of EPSCs by forskolin was accompanied by a reduction of the synaptic failure rate, coefficient of variation and paired-pulse ratio of EPSCs, and an increase in release probability and number of releasable synaptic vesicles. Forskolin also significantly increased the frequency of miniature EPSCs without altering their amplitude distribution. These results indicate that cAMP acts presynaptically to elicit a synaptic potentiation on the layer V pyramidal neurons of mPFC through converging activation of PKA and p42/p44 MAPK signaling pathways.