A structural basis for memory storage in mammals

Basal Forebrain Impairment: Understanding the Mnemonic Function of the Septal Region Translates in Therapeutic Advances

The basal forebrain is one of the three major brain circuits involved in episodic memory formation together with the hippocampus and the diencephalon. The dysfunction of each of these regions is known to cause anterograde amnesia. While the hippocampal pyramidal neurons are known to encode episodic information and the diencephalic structures are known to provide idiothetic information, the contribution of the basal forebrain to memory formation has been exclusively associated with septo-hippocampal cholinergic signaling. Research data from the last decade broadened our understanding about the role of septal region in memory formation. Animal studies revealed that septal neurons process locomotor, rewarding and attentional stimuli. The integration of these signals results in a systems model for the mnemonic function of the medial septum that could guide new therapeutic strategies for basal forebrain impairment (BFI). BFI includes the disorders characterized with basal forebrain amnesia and neurodegenerative disorders that affect the basal forebrain. Here, we demonstrate how the updated model of septal mnemonic function can lead to innovative translational treatment approaches that include pharmacological, instrumental and behavioral techniques.

Microtubules as Regulators of Neural Network Shape and Function: Focus on Excitability, Plasticity and Memory

Neuronal microtubules (MTs) are complex cytoskeletal protein arrays that undergo activity-dependent changes in their structure and function as a response to physiological demands throughout the lifespan of neurons. Many factors shape the allostatic dynamics of MTs and tubulin dimers in the cytosolic microenvironment, such as protein–protein interactions and activity-dependent shifts in these interactions that are responsible for their plastic capabilities. Recently, several findings have reinforced the role of MTs in behavioral and cognitive processes in normal and pathological conditions. In this review, we summarize the bidirectional relationships between MTs dynamics, neuronal processes, and brain and behavioral states. The outcomes of manipulating the dynamicity of MTs by genetic or pharmacological approaches on neuronal morphology, intrinsic and synaptic excitability, the state of the network, and behaviors are heterogeneous. We discuss the critical position of MTs as responders and adaptative elements of basic neuronal function whose impact on brain function is not fully understood, and we highlight the dilemma of artificially modulating MT dynamics for therapeutic purposes.

P2X2 receptors supply extracellular choline as a substrate for acetylcholine synthesis

Acetylcholine (ACh), an excitatory neurotransmitter, is biosynthesized from choline in cholinergic neurons. Import from the extracellular space to the intracellular environment through the high-affinity choline transporter is currently regarded to be the only source of choline for ACh synthesis. We recently demonstrated that the P2X₂ receptor, through which large cations permeate, functions as an alternative pathway for choline transport in the mouse retina. In the present study, we investigated whether choline entering cells through P2X₂ receptors is used for ACh synthesis using a recombinant system. When P2X₂ receptors expressed on HEK293 cell lines were stimulated with ATP, intracellular ACh concentrations increased. These results suggest that P2X₂ receptors function in a novel pathway that supplies choline for ACh synthesis.

New 3-O-substituted xanthone derivatives as promising acetylcholinesterase inhibitors

A new series of 3-O-substituted xanthone derivatives were synthesised and evaluated for their anti-cholinergic activities against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). The results indicated that the xanthone derivatives possessed good AChE inhibitory activity with eleven of them (5, 8, 11, 17, 19, 21-23, 26-28) exhibited significant effects with the IC₅₀ values ranged 0.88 to 1.28 µM. The AChE enzyme kinetic study of 3-(4-phenylbutoxy)-9H-xanthen-9-one (23) and ethyl 2-((9-oxo-9H-xanthen-3-yl)oxy)acetate (28) showed a mixed inhibition mechanism. Molecular docking study showed that 23 binds to the active site of AChE and interacts via extensive π–π stacking with the indole and phenol side chains of Trp86 and Tyr337, besides the hydrogen bonding with the hydration site and π–π interaction with the phenol side chain of Y72. This study revealed that 3-O-alkoxyl substituted xanthone derivatives are potential lead structures, especially 23 and 28 which can be further developed into potent AChE inhibitors.

Discovery of potent and selective butyrylcholinesterase inhibitors through the use of pharmacophore-based screening

Cholinesterase inhibitors have long been used in the treatment of Alzheimer's Disease (AD) via the protection of acetylcholine levels. However, recent research has shown that the specific inhibition of butyrylcholinesterase (BChE) could better ameliorate symptoms within patients. In addition, it has recently been shown that selective inhibition of BChE can also significantly attenuate the toxicity and physiological effects of heroin. Currently, there are no specific and potent inhibitors of BChE approved for use in AD or heroin abuse. Through a combined use of in silico and in vitro screening, we have found three compounds with sub-50 nM IC50 values that specifically target BChE. These newly discovered BChE inhibitors can act as the lead scaffolds for future development of the desirably potent and selective BChE inhibitors.

Alzheimer's Disease as a Membrane Disorder: Spatial Cross-Talk Among Beta-Amyloid Peptides, Nicotinic Acetylcholine Receptors and Lipid Rafts

Biological membranes show lateral and transverse asymmetric lipid distribution. Cholesterol (Chol) localizes in both hemilayers, but in the external one it is mostly condensed in lipid-ordered microdomains (raft domains), together with saturated phosphatidyl lipids and sphingolipids (including sphingomyelin and glycosphingolipids). Membrane asymmetries induce special membrane biophysical properties and behave as signals for several physiological and/or pathological processes. Alzheimer’s disease (AD) is associated with a perturbation in different membrane properties. Amyloid-β (Aβ) plaques and neurofibrillary tangles of tau protein together with neuroinflammation and neurodegeneration are the most characteristic cellular changes observed in this disease. The extracellular presence of Aβ peptides forming senile plaques, together with soluble oligomeric species of Aβ, are considered the major cause of the synaptic dysfunction of AD. The association between Aβ peptide and membrane lipids has been extensively studied. It has been postulated that Chol content and Chol distribution condition Aβ production and posterior accumulation in membranes and, hence, cell dysfunction. Several lines of evidence suggest that Aβ partitions in the cell membrane accumulate mostly in raft domains, the site where the cleavage of the precursor AβPP by β- and γ- secretase is also thought to occur. The main consequence of the pathogenesis of AD is the disruption of the cholinergic pathways in the cerebral cortex and in the basal forebrain. In parallel, the nicotinic acetylcholine receptor has been extensively linked to membrane properties. Since its transmembrane domain exhibits extensive contacts with the surrounding lipids, the acetylcholine receptor function is conditioned by its lipid microenvironment. The nicotinic acetylcholine receptor is present in high-density clusters in the cell membrane where it localizes mainly in lipid-ordered domains. Perturbations of sphingomyelin or cholesterol composition alter acetylcholine receptor location. Therefore, Aβ processing, Aβ partitioning, and acetylcholine receptor location and function can be manipulated by changes in membrane lipid biophysics. Understanding these mechanisms should provide insights into new therapeutic strategies for prevention and/or treatment of AD. Here, we discuss the implications of lipid-protein interactions at the cell membrane level in AD.

Nonsynaptic plasticity model of long-term memory engrams

Using steady-state electrical properties of non-ohmic dendrite based on cable theory, we derive electrotonic potentials that do not change over time and are localized in space. We hypothesize that clusters of such stationary, local and permanent pulses are the electrical signatures of enduring memories which are imprinted through nonsynaptic plasticity, encoded through epigenetic mechanisms, and decoded through electrotonic processing. We further hypothesize how retrieval of an engram is made possible by integration of these permanently imprinted standing pulses in a neural circuit through neurotransmission in the extracellular space as part of conscious recall that acts as a guiding template in the reconsolidation of long-term memories through novelty characterized by uncertainty that arises when new fragments of memories reinstate an engram by way of nonsynaptic plasticity that permits its destabilization. Collectively, these findings seem to reinforce this hypothesis that electrotonic processing in non-ohmic dendrites yield insights into permanent electrical signatures that could reflect upon enduring memories as fragments of long-term memory engrams.

Developing and applying the adverse outcome pathway concept for understanding and predicting neurotoxicity

The Adverse Outcome Pathway (AOP) concept has recently been proposed to support a paradigm shift in regulatory toxicology testing and risk assessment. This concept is similar to the Mode of Action (MOA), in that it describes a sequence of measurable key events triggered by a molecular initiating event in which a stressor interacts with a biological target. The resulting cascade of key events includes molecular, cellular, structural and functional changes in biological systems, resulting in a measurable adverse outcome. Thereby, an AOP ideally provides information relevant to chemical structure-activity relationships as a basis for predicting effects of structurally similar compounds. AOPs could potentially also form the basis for qualitative and quantitative predictive modeling of the human adverse outcome resulting from molecular initiating or other key events for which higher-throughput testing methods are available or can be developed. A variety of cellular and molecular processes are known to be critical for normal function of the central (CNS) and peripheral nervous systems (PNS). Because of the biological and functional complexity of the CNS and PNS, it has been challenging to establish causative links and quantitative relationships between key events that comprise the pathways leading from chemical exposure to an adverse outcome in the nervous system. Following introduction of the principles of MOA and AOPs, examples of potential or putative adverse outcome pathways specific for developmental or adult neurotoxicity are summarized and aspects of their assessment considered. Their possible application in developing mechanistically informed Integrated Approaches to Testing and Assessment (IATA) is also discussed.

Role of the nicotinic acetylcholine receptor in Alzheimer's disease pathology and treatment

Alzheimer's Disease (AD) is the major form of senile dementia, characterized by neuronal loss, extracellular deposits, and neurofibrillary tangles. It is accompanied by a loss of cholinergic tone, and acetylcholine (ACh) levels in the brain, which were hypothesized to be responsible for the cognitive decline observed in AD. Current medication is restricted to enhancing cholinergic signalling for symptomatic treatment of AD patients. The nicotinic acetylcholine receptor family (nAChR) and the muscarinic acetylcholine receptor family (mAChR) are the target of ACh in the brain. Both families of receptors are affected in AD. It was demonstrated that amyloid beta (Aβ) interacts with nAChRs. Here we discuss how Aβ activates or inhibits nAChRs, and how this interaction contributes to AD pathology. We will discuss the potential role of nAChRs as therapeutic targets. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

The zinc dyshomeostasis hypothesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly. Hallmark AD neuropathology includes extracellular amyloid plaques composed largely of the amyloid-β protein (Aβ), intracellular neurofibrillary tangles (NFTs) composed of hyper-phosphorylated microtubule-associated protein tau (MAP-tau), and microtubule destabilization. Early-onset autosomal dominant AD genes are associated with excessive Aβ accumulation, however cognitive impairment best correlates with NFTs and disrupted microtubules. The mechanisms linking Aβ and NFT pathologies in AD are unknown. Here, we propose that sequestration of zinc by Aβ-amyloid deposits (Aβ oligomers and plaques) not only drives Aβ aggregation, but also disrupts zinc homeostasis in zinc-enriched brain regions important for memory and vulnerable to AD pathology, resulting in intra-neuronal zinc levels, which are either too low, or excessively high. To evaluate this hypothesis, we 1) used molecular modeling of zinc binding to the microtubule component protein tubulin, identifying specific, high-affinity zinc binding sites that influence side-to-side tubulin interaction, the sensitive link in microtubule polymerization and stability. We also 2) performed kinetic modeling showing zinc distribution in extra-neuronal Aβ deposits can reduce intra-neuronal zinc binding to microtubules, destabilizing microtubules. Finally, we 3) used metallomic imaging mass spectrometry (MIMS) to show anatomically-localized and age-dependent zinc dyshomeostasis in specific brain regions of Tg2576 transgenic, mice, a model for AD. We found excess zinc in brain regions associated with memory processing and NFT pathology. Overall, we present a theoretical framework and support for a new theory of AD linking extra-neuronal Aβ amyloid to intra-neuronal NFTs and cognitive dysfunction. The connection, we propose, is based on β-amyloid-induced alterations in zinc ion concentration inside neurons affecting stability of polymerized microtubules, their binding to MAP-tau, and molecular dynamics involved in cognition. Further, our theory supports novel AD therapeutic strategies targeting intra-neuronal zinc homeostasis and microtubule dynamics to prevent neurodegeneration and cognitive decline.

Astrocytes mediate in vivo cholinergic-induced synaptic plasticity

Long-term potentiation (LTP) of synaptic transmission represents the cellular basis of learning and memory. Astrocytes have been shown to regulate synaptic transmission and plasticity. However, their involvement in specific physiological processes that induce LTP in vivo remains unknown. Here we show that in vivo cholinergic activity evoked by sensory stimulation or electrical stimulation of the septal nucleus increases Ca²⁺ in hippocampal astrocytes and induces LTP of CA3-CA1 synapses, which requires cholinergic muscarinic (mAChR) and metabotropic glutamate receptor (mGluR) activation. Stimulation of cholinergic pathways in hippocampal slices evokes astrocyte Ca²⁺ elevations, postsynaptic depolarizations of CA1 pyramidal neurons, and LTP of transmitter release at single CA3-CA1 synapses. Like in vivo, these effects are mediated by mAChRs, and this cholinergic-induced LTP (c-LTP) also involves mGluR activation. Astrocyte Ca²⁺ elevations and LTP are absent in IP₃R2 knock-out mice. Downregulating astrocyte Ca²⁺ signal by loading astrocytes with BAPTA or GDPβS also prevents LTP, which is restored by simultaneous astrocyte Ca²⁺ uncaging and postsynaptic depolarization. Therefore, cholinergic-induced LTP requires astrocyte Ca²⁺ elevations, which stimulate astrocyte glutamate release that activates mGluRs. The cholinergic-induced LTP results from the temporal coincidence of the postsynaptic activity and the astrocyte Ca²⁺ signal simultaneously evoked by cholinergic activity. Therefore, the astrocyte Ca²⁺ signal is necessary for cholinergic-induced synaptic plasticity, indicating that astrocytes are directly involved in brain storage information.

Neural cytoskeleton capabilities for learning and memory

This paper proposes a physical model involving the key structures within the neural cytoskeleton as major players in molecular-level processing of information required for learning and memory storage. In particular, actin filaments and microtubules are macromolecules having highly charged surfaces that enable them to conduct electric signals. The biophysical properties of these filaments relevant to the conduction of ionic current include a condensation of counterions on the filament surface and a nonlinear complex physical structure conducive to the generation of modulated waves. Cytoskeletal filaments are often directly connected with both ionotropic and metabotropic types of membrane-embedded receptors, thereby linking synaptic inputs to intracellular functions. Possible roles for cable-like, conductive filaments in neurons include intracellular information processing, regulating developmental plasticity, and mediating transport. The cytoskeletal proteins form a complex network capable of emergent information processing, and they stand to intervene between inputs to and outputs from neurons. In this manner, the cytoskeletal matrix is proposed to work with neuronal membrane and its intrinsic components (e.g., ion channels, scaffolding proteins, and adaptor proteins), especially at sites of synaptic contacts and spines. An information processing model based on cytoskeletal networks is proposed that may underlie certain types of learning and memory.

Model of ionic currents through microtubule nanopores and the lumen

It has been suggested that microtubules and other cytoskeletal filaments may act as electrical transmission lines. An electrical circuit model of the microtubule is constructed incorporating features of its cylindrical structure with nanopores in its walls. This model is used to study how ionic conductance along the lumen is affected by flux through the nanopores, both with and without an external potential applied across its two ends. Based on the results of Brownian dynamics simulations, the nanopores were found to have asymmetric inner and outer conductances, manifested as nonlinear IV curves. Our simulations indicate that a combination of this asymmetry and an internal voltage source arising from the motion of the C-terminal tails causes cations to be pumped across the microtubule wall and propagate in both directions down the microtubule through the lumen, returning to the bulk solution through its open ends. This effect is demonstrated to add directly to the longitudinal current through the lumen resulting from an external voltage source applied across the two ends of the microtubule. The predicted persistent currents directed through the microtubule wall and along the lumen could be significant in directing the dissipation of weak, endogenous potential gradients toward one end of the microtubule within the cellular environment.

Inhibitory avoidance memory deficit induced by scopolamine: Interaction of cholinergic and glutamatergic systems in the ventral tegmental area

Interaction of cholinergic and glutamatergic inputs in the ventral tegmental area (VTA) influencing a learned behavior is a topic of great interest. In the present study the effect of intra-VTA administration of a nonselective muscarinic acetylcholine antagonist, scopolamine, and N-methyl-d-aspartate (NMDA) receptor agents by themselves as well as their interactions on consolidation and retrieval of inhibitory avoidance (IA) memory have been investigated. A step-through inhibitory avoidance task was used for memory assessment in male Wistar rats. The results showed that intra-VTA administration of scopolamine (1 and 2microg/rat) and NMDA receptor antagonist, MK-801 (0.75 and 1microg/rat) immediately after training, impaired consolidation of IA memory. Interestingly, co-administration of an ineffective dose of MK-801 (0.5microg/rat) with ineffective doses of scopolamine (0.25 and 0.5microg/rat) significantly decreased the consolidation process. Post-training intra-VTA injections of NMDA (0.001 and 0.01microg/rat) had no effects by itself, whereas its co-administration with scopolamine (2microg/rat) prevented the effect of the later drug. The results also showed that pre-test intra-VTA administration of scopolamine (3 and 4microg/rat) and MK-801 (1 and 2microg/rat) impaired retrieval of the IA memory. Moreover, co-administration of an ineffective dose of MK-801 (0.5microg/rat) with ineffective doses of scopolamine (1 and 2microg/rat) increasingly reduced the retrieval of the IA memory. On the contrary to its post-training treatment, pre-test administration of NMDA either alone or in combination with scopolamine caused no significant effect on retrieval of IA memory. It can be concluded that muscarinic acetylcholine and NMDA glutamate receptors in the VTA are involved in the mechanism(s) underlying consolidation and retrieval of the IA memory.

The quantitative evaluation of cholinergic markers in spatial memory improvement induced by nicotine-bucladesine combination in rats

We previously showed that post-training intra-hippocampal infusion of nicotine-bucladesine combination enhanced spatial memory retention in the Morris water maze. Here we investigated the role of cholinergic markers in nicotine-bucladesine combination-induced memory improvement. We assessed the expression of choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) in CA1 region of the hippocampus and medial septal area (MSA) of the brain. Post-training bilateral infusion of a low concentration of either nicotine or bucladesine into the CA1 region of the hippocampus did not affect spatial memory significantly. Quantitative immunostaining analysis of optical density in CA1 regions and evaluation of immunopositive neurons in medial septal area of brain sections from all combination groups revealed a significant increase (P<0.001) in the ChAT and VAChT immunoreactivity. The maximum increase was observed with combination of 10-microM/side bucladesine and 0.5 microg/side nicotine and in a concentration dependent manner. Also, increase in the optical density and amount of ChAT and VAChT immunostaining correlated with the decrease in escape latency and traveled distance in rats treated with nicotine and low dose of bucladesine. Taken together, these results suggest that significant increases of ChAT and VAChT protein expressions in the CA1 region and medial septal area are the possible mechanisms of spatial memory improvement induced by nicotine-bucladesine combination.

The "conscious pilot"-dendritic synchrony moves through the brain to mediate consciousness

Cognitive brain functions including sensory processing and control of behavior are understood as "neurocomputation" in axonal-dendritic synaptic networks of "integrate-and-fire" neurons. Cognitive neurocomputation with consciousness is accompanied by 30- to 90-Hz gamma synchrony electroencephalography (EEG), and non-conscious neurocomputation is not. Gamma synchrony EEG derives largely from neuronal groups linked by dendritic-dendritic gap junctions, forming transient syncytia ("dendritic webs") in input/integration layers oriented sideways to axonal-dendritic neurocomputational flow. As gap junctions open and close, a gamma-synchronized dendritic web can rapidly change topology and move through the brain as a spatiotemporal envelope performing collective integration and volitional choices correlating with consciousness. The "conscious pilot" is a metaphorical description for a mobile gamma-synchronized dendritic web as vehicle for a conscious agent/pilot which experiences and assumes control of otherwise non-conscious auto-pilot neurocomputation.

A novel mechanism of hippocampal LTD involving muscarinic receptor-triggered interactions between AMPARs, GRIP and liprin-alpha

BACKGROUND

Long-term depression (LTD) in the hippocampus can be induced by activation of different types of G-protein coupled receptors, in particular metabotropic glutamate receptors (mGluRs) and muscarinic acetylcholine receptors (mAChRs). Since mGluRs and mAChRs activate the same G-proteins and isoforms of phospholipase C (PLC), it would be expected that these two forms of LTD utilise the same molecular mechanisms. However, we find a distinct mechanism of LTD involving GRIP and liprin-alpha.

RESULTS

Whilst both forms of LTD require activation of tyrosine phosphatases and involve internalisation of AMPARs, they use different molecular interactions. Specifically, mAChR-LTD, but not mGluR-LTD, is blocked by peptides that inhibit the binding of GRIP to the AMPA receptor subunit GluA2 and the binding of GRIP to liprin-alpha. Thus, different receptors that utilise the same G-proteins can regulate AMPAR trafficking and synaptic efficacy via distinct molecular mechanisms.

CONCLUSION

Our results suggest that mAChR-LTD selectively involves interactions between GRIP and liprin-alpha. These data indicate a novel mechanism of synaptic plasticity in which activation of M1 receptors results in AMPAR endocytosis, via a mechanism involving interactions between GluA2, GRIP and liprin-alpha.

Impaired cognition and attention in adults: pharmacological management strategies

Cognitive psychology has provided clinicians with specific tools for analyzing the processes of cognition (memory, language) and executive functions (attention-concentration, abstract reasoning, planning). Neuropsychology, coupled with the neurosciences (including neuroimaging techniques), has authenticated the existence of early disorders affecting the "superior or intellectual" functions of the human brain. The prevalence of cognitive and attention disorders is high in adults because all the diseases implicating the central nervous system are associated with cognitive correlates of variable intensity depending on the disease process and the age of the patient. In some pathologies, cognitive impairment can be a leading symptom such as in schizophrenia, posttraumatic stress disorder or an emblematic stigmata as in dementia including Alzheimer's disease. Paradoxically, public health authorities have only recognized as medications for improving cognitive symptoms those with proven efficacy in the symptomatic treatment of patients with Alzheimer's disease; the other cognitive impairments are relegated to the orphanage of syndromes and symptoms dispossessed of medication. The purpose of this review is to promote a true "pharmacology of cognition" based on the recent knowledge in neurosciences. Data from adult human beings, mainly concerning memory, language, and attention processes, will be reported. "Drug therapeutic strategies" for improving cognition (except for memory function) are currently rather scarce, but promising perspectives for a new neurobiological approach to cognitive pharmacology will be highlighted.

Triple Dissociation in the Medial Temporal Lobes: Recollection, Familiarity, and Novelty

Memory for past events may be based on retrieval accompanied by specific contextual details (recollection) or on the feeling that an item is old (familiarity) or new (novelty) in the absence of contextual details. There are indications that recollection, familiarity, and novelty involve different medial temporal lobe subregions, but available evidence is scarce and inconclusive. Using functional magnetic resonance imaging (MRI), we isolated retrieval-related activity associated with recollection, familiarity, and novelty by distinguishing between linear and nonlinear oldness functions derived from recognition confidence levels. Within the medial temporal lobes (MTLs), we found a triple dissociation among the posterior half of the hippocampus, which was associated with recollection, the posterior parahippocampal gyrus, which was associated with familiarity, and anterior half of the hippocampus and rhinal regions, which were associated with novelty. Furthermore, multiple regression analyses based on individual trial activity showed that all three memory signals, i.e., recollection, familiarity, and novelty, make significant and independent contributions to recognition memory performance. Finally, functional dissociations among recollection, familiarity, and novelty were also found in posterior midline, left parietal cortex, and prefrontal cortex regions. This is the first study to reveal a triple dissociation within the MTL associated with distinct retrieval processes. This finding has direct implications for current memory models.

Food for thought: honeybee foraging, memory, and acetylcholine

Recent research implicates acetylcholine signaling through muscarinic receptors in structural changes that take place in the honeybee brain in response to foraging. These new findings are consistent with research from earlier studies implicating cholinergic signaling in associative learning as well as in the response to an enriched environment in mammals, which suggests that cholinergic signaling may play a critical role in learning and memory mechanisms across phyla.

NMDA receptor blockers prevents the facilitatory effects of post-training intra-dorsal hippocampal NMDA and physostigmine on memory retention of passive avoidance learning in rats

In the present study, the effects of post-training intra-dorsal hippocampal (intra-CA1) injection of an N-methyl-D-aspartate (NMDA) receptor agonist and competitive or noncompetitive antagonists, on memory retention of passive avoidance learning was measured in the presence and absence of physostigmine in rats. Intra-CA1 administration of lower doses of the NMDA receptor agonist NMDA (10(-5) and 10(-4) microg/rat) did not affect memory retention, although the higher doses of the drug (10(-3), 10(-2) and 10(-1) microg/rat) increased memory retention. The greatest response was obtained with 10(-1) microg/rat of the drug. The different doses of the competitive NMDA receptor antagonist DL-AP5 (1, 3.2 and 10 microg/rat) and noncompetitive NMDA receptor antagonist MK-801 (0.5, 1 and 2 microg/rat) decreased memory retention in rats dose dependently. Both competitive and noncompetitive NMDA receptor antagonists reduced the effect of NMDA (10(-2) microg/rat). In another series of experiments, intra-CA1 injection of physostigmine (2, 3 and 4 microg/rat) improved memory retention. Post-training co-administration of lower doses of NMDA (10(-5) and 10(-4) microg/rat) and physostigmine (1 microg/rat), doses which were ineffective when given alone, significantly improved the retention latency. The competitive and noncompetitive NMDA receptor antagonists, DL-AP5 and MK-801, decreased the effect of physostigmine (2 microg/rat). Atropine decreased memory retention by itself and potentiated the response to DL-AP5 and MK-801. In conclusion, it seems that both NMDA and cholinergic systems not only play a part in the modulation of memory in the dorsal hippocampus of rats but also have demonstrated a complex interaction as well.

NMDA receptor antagonists antagonize the facilitatory effects of post-training intra-basolateral amygdala NMDA and physostigmine on passive avoidance learning

In the present study, the effects of post-training intra-basolateral amygdala (BLA) injection of an N-methyl-d-aspartate (NMDA) receptor agonist and competitive or noncompetitive antagonists, on memory retention of passive avoidance learning was measured in the presence and absence of physostigmine in rats. Intra-BLA administration of lower doses of NMDA (10(-5) and 10(-4) microg/rat) did not affect memory retention, although higher doses of the drug (10(-3), 10(-2) and 10(-1) microg/rat) increased memory retention. The greatest response was obtained with 10(-1) microg/rat of the drug. The different doses of the competitive NMDA receptor antagonist DL-AP5 (1, 3.2 and 10 microg/rat) and noncompetitive NMDA receptor antagonist MK-801 (0.5, 1 and 2 microg/rat) decreased memory retention in rats dose dependently. Both competitive and noncompetitive NMDA receptor antagonists reduced the effect of NMDA (10(-2) microg/rat). In another series of experiments, intra-BLA injection of physostigmine (2, 3 and 4 microg/rat) improved memory retention. Post-training co-administration of lower doses of NMDA (10(-5) and 10(-4) microg/rat) and physostigmine (1 microg/rat), doses which were ineffective when given alone, significantly improved the retention latency. The competitive and noncompetitive NMDA receptor antagonists, DL-AP5 and MK-801, decreased the effect of physostigmine (2 microg/rat). Atropine decreased memory retention by itself and potentiated the response to DL-AP5 and MK-801. It may be concluded that amygdalar NMDA receptor mechanisms interact with cholinergic systems in the modulation of memory.

Early changes in Huntington's disease patient brains involve alterations in cytoskeletal and synaptic elements

Huntington's disease (HD) is caused by a polyglutamine repeat expansion in the N-terminus of the huntingtin protein. Huntingtin is normally present in the cytoplasm where it may interact with structural and synaptic elements. The mechanism of HD pathogenesis remains unknown but studies indicate a toxic gain-of-function possibly through aberrant protein interactions. To investigate whether early degenerative changes in HD involve alterations of cytoskeletal and vesicular components, we examined early cellular changes in the frontal cortex of HD presymptomatic (PS), early pathological grade (grade 1) and late-stage (grade 3 and 4) patients as compared to age-matched controls. Morphologic analysis using silver impregnation revealed a progressive decrease in neuronal fiber density and organization in pyramidal cell layers beginning in presymptomatic HD cases. Immunocytochemical analyses for the cytoskeletal markers alpha -tubulin, microtubule-associated protein 2, and phosphorylated neurofilament demonstrated a concomitant loss of staining in early grade cases. Immunoblotting for synaptic proteins revealed a reduction in complexin 2, which was marked in some grade 1 HD cases and significantly reduced in all late stage cases. Interestingly, we demonstrate that two synaptic proteins, dynamin and PACSIN 1, which were unchanged by immunoblotting, showed a striking loss by immunocytochemistry beginning in early stage HD tissue suggesting abnormal distribution of these proteins. We propose that mutant huntingtin affects proteins involved in synaptic function and cytoskeletal integrity before symptoms develop which may influence early disease onset and/or progression.

Learning and memory

Learning and memory processes are thought to underlie a variety of human psychiatric disorders, including generalised anxiety disorder and post-traumatic stress disorder. Basic research performed in laboratory animals may help to elucidate the aetiology of the respective diseases. This chapter gives a short introduction into theoretical and practical aspects of animal experiments aimed at investigating acquisition, consolidation and extinction of aversive memories. It describes the behavioural paradigms most commonly used as well as neuroanatomical, cellular and molecular correlates of aversive memories. Finally, it discusses clinical implications of the results obtained in animal experiments in respect to the development of novel pharmacotherapeutic strategies for the treatment of human patients.

Molecular Mechanisms Underlying Emotional Learning and Memory in the Lateral Amygdala

Fear conditioning is a valuable behavioral paradigm for studying the neural basis of emotional learning and memory. The lateral nucleus of the amygdala (LA) is a crucial site of neural changes that occur during fear conditioning. Pharmacological manipulations of the LA, strategically timed with respect to training and testing, have shed light on the molecular events that mediate the acquisition of fear associations and the formation and maintenance of long-term memories of those associations. Similar mechanisms have been found to underlie long-term potentiation (LTP) in LA, an artificial means of inducing synaptic plasticity and a physiological model of learning and memory. Thus, LTP-like changes in synaptic plasticity may underlie fear conditioning. Given that the neural circuit underlying fear conditioning has been implicated in emotional disorders in humans, the molecular mechanisms of fear conditioning are potential targets for psychotherapeutic drug development.

Molecular signals into the insular cortex and amygdala during aversive gustatory memory formation

In this paper, we will provide evidence of the putative molecular signals and biochemical events that mediate the formation of long-lasting gustatory memory trace. When an animal drinks a novel taste (the conditioned stimulus; CS) and it is later associated with malaise (unconditioned stimulus; US), the animal will reject it in the next presentation, developing a long-lasting taste aversion, i.e., the taste cue becomes an aversive signal, and this is referred to as conditioning taste aversion. Different evidence indicates that the novel stimulus (taste) induces a rapid and strong cortical acetylcholine activity that decreases when the stimulus becomes familiar after several presentations. Cholinergic activation via muscarinic receptors initiates a series of intracellular events leading to plastic changes that could be related to short- and/or long-term memory gustatory trace. Such plastic changes facilitate the incoming US signals carried out by, in part, the glutamate release induced by the US. Altogether, these events could produce the cellular changes related to the switch from safe to aversive taste memory trace. A proposed working model to explain the biochemical sequence of signals during taste memory formation will be discussed.

Impaired and spared cholinergic functions in the hippocampus after lesions of the medial septum/vertical limb of the diagonal band with 192 IgG-saporin

To lesion the cholinergic input to the hippocampus, rats received injections of 192 IgG-saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). The lesions produced near-total loss of choline acetyltransferase (ChAT)-positive neurons in the MS/VDB. The loss was accompanied, however, by only partial decreases (to 40% of control levels) in acetylcholine (ACh) release in the hippocampus. Moreover, ACh release in the hippocampus increased when lesioned and control rats were tested on a spontaneous alternation task, indicating that there was significant residual cholinergic function in the hippocampus. The lesions were sufficient to impair spontaneous alternation scores. However, this impairment could be reversed by either systemic or intra-hippocampal injections of the indirect cholinergic agonist, physostigmine, providing additional evidence of residual and effective cholinergic functions in the hippocampus of lesioned rats. Moreover, systemic injections of physostigmine at doses that produced mild tremors in control rats led to more severe tremors in the lesioned rats, suggesting upregulation of cholinergic mechanisms after saporin lesions, likely in brain areas other than the hippocampus. Thus, these findings provide evidence for decreases in cholinergic input to the hippocampus accompanied by deficits on a spontaneous alternation tasks. The findings also provide evidence for considerable residual cholinergic input to the hippocampus after saporin lesions of the MS/VDB. Together, the results suggest that 192 IgG-saporin lesions of the MS/VDB, using methods often employed, do not fully remove septohippocampal cholinergic input to the hippocampus but are nonetheless sufficient to produce impairments on a task impaired by hippocampal lesions.

Neuronal injury in hippocampus with human immunodeficiency virus transactivating protein, Tat

Patients with human immunodeficiency virus infection may develop a dementing illness. Using both in vitro and in vivo models, we investigated the susceptibility of the hippocampal formation to the Tat protein of human immunodeficiency virus. We also determined the pattern of hippocampal injury in patients with human immunodeficiency virus encephalitis. Following exposure of hippocampal slices to Tat, marked susceptibility of CA3 region with relative insensitivity of the CA1/2 region was observed. Injection of Tat into different regions of the rat hippocampus produced similar neuronal loss in both CA3 region and the dentate gyrus. In animals administered Tat, lesions were dose-dependent and immunohistochemical staining showed marked gliosis and loss of microtubule associated protein-2 in the affected areas at 3 days post-injection. Interestingly, synaptophysin staining was relatively preserved. In hippocampal tissue from patients with human immunodeficiency virus encephalitis, loss of microtubule-associated protein-2 staining was reduced in the molecular layer of the dentate gyrus. The results of our experiments demonstrate a unique pattern of hippocampal injury in organotypic culture and rats exposed to Tat. Our observations that patients with human immunodeficiency virus reveal a similar pattern of damage suggests that Tat protein may be pathophysiological relevant in human immunodeficiency virus encephalitis.

Effects of imidacloprid metabolites on habituation in honeybees suggest the existence of two subtypes of nicotinic receptors differentially expressed during adult development

Habituation of the proboscis extension reflex (PER) in honeybees (Apis mellifera) is age-dependent. Very young bees (< or =7 days old) require significantly less trials to abolish the response to multiple sucrose stimulations than older bees (> or =8 days old). A nicotinic agonist, imidacloprid, modifies this behaviour by increasing the number of trials in < or =7-day-old bees and by decreasing it in older bees [Neurobiol. Learn. Mem. 76 (2001) 183.]. Here we tested our hypothesis that this effect is associated with a differential expression of two subtypes of nicotinic acetylcholine receptors (nAChRs). By testing the effects of six metabolites of imidacloprid, we show that two of them, olefin and 5-hydroxy-imidacloprid, modify the number of trials needed to habituate the PER in a contrasting manner. Olefin increases the number of trials in both age groups, whereas 5-hydroxy-imidacloprid decreases the number of trials, but only in 8-day-old individuals. We conclude that olefin and 5-hydroxy-imidacloprid are specific agonists of two subtypes of an nAChR that are differentially expressed during adult maturation of young honeybees. Olefin is the agonist of an nAChR expressed in both age groups, whereas 5-hydroxy-imidacloprid is the agonist of a late-onset nAChR that is activated in 8-day-old bees. The implications of this finding for the honeybee biology are discussed.

Incremental learning in biological and machine learning systems

Incremental learning concepts are reviewed in machine learning and neurobiology. They are identified in evolution, neurodevelopment and learning. A timeline of qualitative axon, neuron and synapse development summarizes the review on neurodevelopment. A discussion of experimental results on data incremental learning with recurrent artificial neural networks reveals that incremental learning often seems to be more efficient or powerful than standard learning but can produce unexpected side effects. A characterization of incremental learning is proposed which takes the elaborated biological and machine learning concepts into account.

Effects of intrahippocampal injection of GABAergic drugs on memory retention of passive avoidance learning in rats

The effect of post-training intrahippocampal injection of gamma-aminobutyric acid (GABA) receptor agonists and antagonists, immediately after a training session on memory retention of passive avoidance learning in rats, was measured in the presence and absence of physostigmine. Post-training treatments were carried out in all the experiments. The different doses of the GABAA receptor agonist muscimol (2, 4 and 6 microg/rat) decreased memory retention in rats dose-dependently. The higher response was obtained with 6 microg/rat of the drug. When the GABAA receptor antagonist bicuculline (0.5, 1, 2 and 4 microg/rat) was administered, only one dose of the drug (1 microg/rat) increased memory retention; however, the antagonist reduced the effect of muscimol. The GABAB receptor agonist, baclofen (0.25, 0.5, 1 and 2 microg/rat) also reduced memory retention in the animals. Intrahippocampal injection of lower doses of the GABAB receptor antagonist CGP35348 (P-[3-aminopropyl]-p-diethoxymethyl-phosphinic acid) (2.5, 5, 10 microg/rat) did not effect memory retention, although the higher doses of the drug (25 and 50 microg/rat) decreased memory retention. The doses of antagonist (2.5, 5 and 10 microg/rat), which did not elicit any response alone, reduced the effect of baclofen. The inhibitory response of CGP35348 was also decreased by bicuculline. In another series of experiments, physostigmine improved memory retention. The GABA receptor agonists, muscimol and baclofen, as well as the GABA receptor antagonists bicuculline and CGP35348, decreased the effect of physostigmine. Atropine decreased memory retention by itself and potentiated the response of muscimol and baclofen. It is concluded that GABAA and GABAB receptor activation may be involved in the impairment of memory retention.

Glutamatergic activity in the amygdala signals visceral input during taste memory formation

Conditioned taste aversion (CTA) is a learning paradigm in which an animal avoids a taste (conditioned stimulus) previously associated with visceral toxic effects [or unconditioned stimulus (US)]. Although many studies have implicated glutamate-mediated neurotransmission in memory consolidation of different types of learning tasks, including CTA, the exact role of this neurotransmitter system in memory formation is not known. Thus, we set out to determine whether glutamate mediates signaling of the US in CTA. We present evidence obtained by in vivo microdialysis that the US (i.p. injection of lithium chloride) induced a dramatic increase in glutamate release in the amygdala and a modest but significant release in the insular cortex. Moreover, CTA can be elicited by intra-amygdalar microinjections of glutamate; consequently, when glutamate is administered just before the presentation of a weak US, a clear CTA is induced. In contrast, the injection of glutamate alone or glutamate 2 h after the suboptimal US did not have any effect on the acquisition of CTA. These results indicate that glutamate activation of the amygdala can partially substitute the US in CTA, thus providing a clear indication that the amygdala conveys visceral information for this kind of memory.

Nicotinic acetylcholine receptor-mediated release of [3H]norepinephrine from developing and adult rat hippocampus: direct and indirect mechanisms

The primary role of nicotinic acetylcholine receptors in adult and developing brain is to modulate neurotransmission. Using in vitro neurotransmitter release, we have examined mechanisms underlying nicotine-induced [(3)H]norepinephrine release from developing and adult rat hippocampus. At birth, nicotine significantly stimulated hippocampal [(3)H]norepinephrine release with a monotonic increase in maximal drug effect over the first ten postnatal days. No developmental changes in agonist or antagonist potency were observed. Comparison of synaptosomal and slice preparations, as well as examination of the effects of tetrodotoxin, indicated that at least two nicotinic acetylcholine receptor populations regulated [(3)H]norepinephrine release from neonatal and adult hippocampus; one localized on noradrenergic terminals, the other on adjacent cells. To further characterize the indirect mechanism of nicotine action in the adult, we examined the effects of pharmacological blockade of various neurotransmitter systems that provide excitatory input to hippocampal noradrenergic terminals. Whereas glutamate and muscarinic receptor blockade was ineffective, the GABA-A receptor antagonists, bicuculline and picrotoxin, inhibited the indirect component of nicotine-mediated [(3)H]norepinephrine release. Furthermore, pentobarbital, an allosteric effector at GABA-A receptors, potentiated the effect of submaximal concentrations of nicotine. These findings are consistent with the hypothesis that nicotine-induced GABA release serves as an additional stimulus for [(3)H]norepinephrine secretion within rat hippocampus.

Plasticity-driven gene expression in the rat retina

Animals exposed to an enriched environment display features of neural plasticity such as an increased brain volume, enhanced number of dendritic spines, as well as enlarged synapses. Here we report the first description of molecular plasticity in the mammalian retina, as revealed by gene expression. A marked upregulation of both NGFI-A and Arc, two candidate-plasticity genes, was observed in adult rats that had been exposed to an enriched environment for 3 weeks. This increase was paralleled by an increase in the expression of the late genes GAP-43 and Synapsin I, which also indicated changes in retinal connectivity. Our results suggest that both NGFI-A and Arc may regulate mechanisms of plasticity that had been invoked by heightened complexity of the visual environment.

Oleamide modulates memory in rats

Oleamide is a recently described lipid, obtained from the cerebrospinal fluid of sleep-deprived cats. It has been observed that oleamide possesses several biological effects, such as sleep induction, and immunological suppression as well as serotonin and gamma-aminobutyric acid receptors activation. In addition, oleamide also binds to the cannabinoid receptors. In this study, we have observed that oleamide facilitates memory extinction in a passive avoidance paradigm, reduces core temperature and pain perception, but does not affect significantly locomotion. These results suggest that oleamide modulates memory processes. However, we do not know if oleamide impairs the retrieval of the memory associated to the "not go" behavior, or facilitates the fast re-learning of the "go" behavior. In addition, since these effects are also induced by marijuana and anandamide, it is very likely that oleamide may be affecting the cerebral cannabinoid system to induce its effects.

Spatial long-term memory is related to mossy fiber synaptogenesis

Structural synaptic changes have been suggested to underlie long-term memory formation. In this work, we investigate if hippocampal mossy fiber synaptogenesis induced by water maze overtraining can be related with long-term spatial memory performance. Rats were trained in a Morris water maze for one to five identical daily sessions and tested for memory retrieval 1 week and 1 month after training. After the last test session, the rat brains were obtained and processed for Timm's staining to analyze mossy fiber projection. The behavioral results showed that with more training, animals showed a better performance in the memory tests, and this performance positively correlates with Timm's staining in the stratum oriens. Furthermore, with the use of the NMDA antagonist MK801 before, but not after acquisition, water maze spatial memory was impaired. Increased Timm's staining in the stratum oriens was observed in the animals treated with MK801 after acquisition but not in those treated before. Finally, we observed that mossy fiber synaptogenesis occurs mainly in the septal region of the dorsal hippocampus, supporting the idea that this anterior region is important for spatial memory. Altogether, these results suggest that mossy fiber synaptogenesis can be related with spatial long-term memory formation.

Contrasting effects of imidacloprid on habituation in 7- and 8-day-old honeybees (Apis mellifera)

We examined the effects of sublethal doses (0.1, 1, and 10 ng per animal) of a new neonicotinoid insecticide, Imidacloprid, on habituation of the proboscis extension reflex (PER) in honeybees (Apis mellifera) reared under laboratory conditions. In untreated honeybees, the habituation of the proboscis extension reflex is age-dependent and there is a significant increase in the number of trials required for habituation in older bees (8-10 days old) as compared to very young bees (4-7 days old). Imidacloprid alters the number of trials needed to habituate the honeybee response to multiple sucrose stimulation. In 7-day-old bees, treatment with Imidacloprid leads to an increase in the number of trials necessary to abolish the response, whereas in 8-day-old bees, it leads to a reduction in the number of trials for habituation (15 min and 1 h after treatment), and to an increase 4 h after treatment. The temporal effects of Imidacloprid in both 7- and 8-day-old bees suggest that 4h after treatment the observed effects are due to a metabolite of Imidacloprid, rather than to Imidacloprid itself. Our results suggest the existence of two distinct subtypes of nicotinic receptors in the honeybee that have different affinities to Imidacloprid and are differentially expressed in 7- and 8-day-old individuals.

Elevation of nerve growth factor and antisense knockdown of TrkA receptor during contextual memory consolidation

We report here a series of experiments establishing a role for nerve growth factor and its high-affinity receptor TrkA in contextual memory consolidation. In all experiments, we trained rats in a novel chamber using tone and shock. Our first experiment revealed that endogenous nerve growth factor (NGF) increases in the hippocampus at a critical time during consolidation that occurs 1 week after training. NGF levels at other intervals (24 hr and 2 and 4 weeks after training) did not differ from those of naive control animals. In our second experiment, we blocked effects that NGF has at 1 week after training by infusing antisense TrkA phosphorothioate DNA oligonucleotide. Reduction of septohippocampal TrkA receptor expression selectively impaired memory consolidation for context but not for tone. Animals with antisense TrkA oligonucleotide infused into the medial septal area or CA1 of the hippocampus froze less when placed in the training chamber than did animals infused with inactive randomized oligonucleotide. At 4 weeks after training, antisense TrkA oligonucleotide had no effect on freezing. Third, we correlated levels of freezing with choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) immunohistochemistry. Antisense TrkA infused into CA1 of the hippocampus reduced cell body cross-sectional area for cholinergic cells in the medial septal area and decreased the density of hippocampal terminals labeled for ChAT and VAChT proteins. Cholinergic cell body measurements were significantly correlated with freezing. Taken together, these results indicate a role for nerve growth factor acting via the TrkA receptor on ChAT and VAChT proteins in contextual memory consolidation.

Cholinergic basal forebrain is critical for social transmission of food preferences

Studies using selective lesions of basal forebrain cholinergic neurons suggest that these neurons play a role in attentional processing, but not learning and memory. However, the tests of learning and memory used thus far have been restricted largely to spatial tasks. In the present study, we examined whether the cholinergic basal forebrain plays a role in a form of nonspatial associative memory, the social transmission of food preferences. Sham-operated control rats were compared to rats with 192 IgG-saporin lesions of the medial septum/diagonal band cholinergic projections to hippocampus or nucleus basalis magnocellularis/substantia innominata cholinergic projections to neocortex. Both lesions impaired 24-h retention of a learned social food preference relative to controls, despite performance on an immediate retention trial that was indistinguishable from controls. Moreover, 24-h retention of the socially learned food preference correlated strongly with cholinergic enzymatic activity in the neocortex, but not in the hippocampus. Immunohistochemical data confirmed significant and selective lesion-induced cholinergic depletions in the intended brain regions. These data provide evidence that the cholinergic basal forebrain, particularly the cholinergic projection to neocortex, is involved in the formation and/or retrieval of social memories related to food preference, and suggest a role for cortical acetylcholine in consolidation of associative memory processes.

Potentiation of cholinergic transmission in the rat hippocampus by angiotensin IV and LVV-hemorphin-7

Recent evidence demonstrates that the fragment of angiotensin II, angiotensin II (3-8) termed angiotensin IV, binds with high affinity to a specific binding site, the AT(4) receptor. Intracerebroventricular injection of AT(4) receptor agonists improves the performance of rats in passive avoidance and spatial learning paradigms. AT(4) receptors and cholinergic neurons are closely associated in regions involved in cognitive processing, such as the hippocampus and neocortex. We therefore postulated that AT(4) receptors affect cognitive processing by modulating cholinergic neurotransmission. To test this, we examined the effect of AT(4) receptor ligands, angiotensin IV and LVV-hemorphin-7, on potassium-evoked [(3)H]acetylcholine ([(3)H]ACh) release from rat hippocampal slices. Hippocampal slices from male Sprague--Dawley rats were incubated with [(3)H]choline chloride, perfused with Krebs--Henseleit solution and [(3)H]ACh release was determined. Angiotensin IV and LVV-hemorphin-7 both potentiated depolarisation-induced [(3)H]ACh release from the rat hippocampus in a concentration-dependent manner with the maximal dose (10(-7)M) of each inducing an increase of 45+/-7.5% (P<0.01) and 95.8+/-19% (P<0.01) above control, respectively. Potentiation of release by both agonists was attenuated by the AT(4) receptor antagonist, divalinal-Ang IV. Angiotensin IV-induced potentiation was not affected by AT(1) and AT(2) receptor antagonists. These results indicate that stimulation of AT(4) receptors can potentiate depolarisation-induced release of ACh from hippocampal slices and suggest that potentiation of cholinergic transmission may be a mechanism by which AT(4) receptor ligands enhance cognition.

GSK3beta-mediated phosphorylation of the microtubule-associated protein 2C (MAP2C) prevents microtubule bundling

A major determinant of neuronal morphology is the cytoskeleton. And one of the main regulatory mechanisms of cytoskeletal proteins is the modification of their phosphorylation state via changes in the relative activities of protein kinases and phosphatases in neurons. In particular, the microtubule-associated protein 2 (MAP2) family of proteins are abundant cytoskeletal components predominantly expressed in neurons and have been found to be substrates for most of protein kinases and phosphatases present in neurons, including glycogen-synthase kinase 3 (GSK3). It has been suggested that changes in GSK3-mediated MAP phosphorylation may modify MT stability and could control neuronal development. We have previously shown that MAP2 is phosphorylated in vitro and in situ by GSK3 at Thr1620 and Thr1623, located in the proline-rich region of MAP2 and recognized by antibody 305. However, the function of the phosphorylation of this site of MAP2 is still unknown. In this study, non-neuronal COS-1 cells have been co-transfected with cDNAs encoding MAP2C and either wild type or mutated GSK3beta to analyze possible effects on microtubule stability and on the association of MAP2 with microtubules. We have found that GSK3beta phosphorylates MAP2C in co-transfected cells. Moreover, this phosphorylation is inhibited by the specific GSK3 inhibitor lithium chloride. Additionally, the formation of microtubule bundles, which is observed after transfection with MAP2C, was decreased when MAP2C was co-transfected with GSK3beta wild type. Microtubule bundles were not observed in cells expressing MAP2C phosphorylated at the site recognized by antibody 305. The absence of microtubule bundles was reverted after treatment of MAP2C/GSK3beta wild type transfected cells with lithium chloride. Highly phosphorylated MAP2C species, which were phosphorylated at the site recognized by antibody 305, appeared in cells co-transfected with MAP2C and GSK3beta wild type. Interestingly, these MAP2C species were enriched in cytoskeleton-unbound protein preparations. These data suggests that GSK3-mediated phosphorylation of MAP2 may modify its binding to microtubules and regulate microtubule stability.