Learning-dependent activation of Fra-1: involvement of ventral hippocampus and SNc/VTA complex in learning and habit formation

Balanced Activation of Striatal Output Pathways by Faster Off-Rate PDE10A Inhibitors Elicits Not Only Antipsychotic-Like Effects But Also Procognitive Effects in Rodents

BACKGROUND

Faster off-rate competitive enzyme inhibitors are generally more sensitive than slower off-rate ones to binding inhibition by enzyme substrates. We previously reported that the cyclic adenosine monophosphate concentration in dopamine D1 receptor-expressing medium spiny neurons (D1-MSNs) may be higher than that in D2-MSNs. Consequently, compared with slower off-rate phosphodiesterase 10A inhibitors, faster off-rate ones comparably activated D2-MSNs but partially activated D1-MSNs. We further investigated the pharmacological profiles of phosphodiesterase 10A inhibitors with different off-rates.

METHODS

Phosphodiesterase 10A inhibitors with slower (T-609) and faster (T-773) off-rates were used. D1- and D2-MSN activation was assessed by substance P and enkephalin mRNA induction, respectively, in rodents. Antipsychotic-like effects were evaluated by MK-801- and methamphetamine-induced hyperactivity and prepulse inhibition in rodents. Cognition was assessed by novel object recognition task and radial arm maze in rats. Prefrontal cortex activation was evaluated by c-Fos immunohistochemistry in rats. Gene translations in D1- and D2-MSNs were evaluated by translating ribosome affinity purification and RNA sequencing in mice.

RESULTS

Compared with T-609, T-773 comparably activated D2-MSNs but partially activated D1-MSNs. Haloperidol (a D2 antagonist) and T-773, but not T-609, produced antipsychotic-like effects in all paradigms. T-773, but not T-609 or haloperidol, activated the prefrontal cortex and improved cognition. Overall gene translation patterns in D2-MSNs by all drugs and those in D1-MSNs by T-773 and T-609 were qualitatively similar.

CONCLUSIONS

Differential pharmacological profiles among those drugs could be attributable to activation balance of D1- and D2-MSNs. The "balanced activation" of MSNs by faster off-rate phosphodiesterase 10A inhibitors may be favorable to treat schizophrenia.

Up-regulation of FOS-like antigen 1 contributes to neuronal apoptosis in the cortex of rat following traumatic brain injury

Neuronal apoptosis is an important process of secondary brain injury which is induced by neurochemical signaling cascades after traumatic brain injury (TBI). Present study was designed to investigate whether FOS-like antigen 1 (Fra-1) is involved in the neuronal apoptosis. Western blot analysis and immunohistochemistry in a rat TBI model revealed a significant increase in the expression of Fra-1 in the ipsilateral brain cortex, which was in parallel with increase in the expression of active caspase-3. With immunofluorescence double-labeling, Fra-1 was colocalized with active caspase-3 and with NeuN, a neuronal marker. In an in vitro cell injury model, H₂O₂ exposure induced cell apoptosis and reduced cell viability and at the same time, a similar increased expression of active caspase-3, p53 and Fra-1 was found in PC12 cells. Down-regulation of Fra-1 through transfection with Fra-1 siRNA remarkably elevated cell viability, reduced the expression of active caspase-3 and p53, and decreased apoptosis of PC12 cells after H₂O₂ exposure. Taken together, present findings suggest that Fra-1 may be involved in the induction of neuronal apoptosis through up-regulating p53 signaling pathway and that this action may contribute to the secondary neuropathological process after TBI.

Motor skill learning and reward consumption differentially affect VTA activation

Dopamine release from the ventral tegmental area (VTA) terminals in the primary motor cortex (M1) enables motor skill acquisition. Here, we test the hypothesis that dopaminergic VTA neurons projecting to M1 are activated when rewards are obtained during motor skill acquisition, but not during task execution at plateau performance, or by rewards obtained without performing skilled movements. Rats were trained to perform a skilled reaching task for 3 days (acquisition) or 7 days (plateau). In combination with retrograde labelling of VTA-to-M1 projection neurons, double immunofluorescence for c-fos and tyrosine hydroxylase (TH) was used to assess activation of dopaminergic and non-dopaminergic VTA neurons. Dopaminergic VTA-to-M1 projection neurons were indeed activated during successful motor skill acquisition, but not when rats failed to learn or had reached plateau performance, nor by food rewards alone. By contrast, dopaminergic VTA neurons that did not project to M1 were activated by both skilled reaching and food rewards. Non-dopaminergic neurons were found to be activated by motor task performance at plateau, but not during skill acquisition. These results indicate that distinct populations of VTA neurons are activated by motor skill acquisition and task performance. Moreover, this activation is not merely related to consumption of food rewards.

A role for WNT/β-catenin signaling in the neural mechanisms of behavior

Wnt signaling pathways play a role in a variety of cellular processes including development, cell proliferation, cell fate, and motility. The Wnt/β-catenin pathway is among the most studied of the Wnt pathways and is highly conserved throughout evolution. Recent in vitro and slice physiology experiments have shown that this pathway also functions in synaptic transmission and activity-dependent synaptic plasticity. Since it has now been shown that many components of this signaling pathway are found in the adult brain, Wnt/β-catenin signaling may be important for maintaining and protecting neural connections throughout the lifespan. Here we summarize the role of Wnt/β-catenin signaling in the postnatal brain and discuss recent studies suggesting that deregulated Wnt signaling can result in altered behavior and cognitive disorders.

The dynamic role of beta-catenin in synaptic plasticity

In addition to its role in development and cell proliferation, β-catenin has been implicated in neuronal synapse regulation and remodeling. Here we review basic molecular and structural mechanisms of synaptic plasticity, followed by a description of the structure and function of β-catenin. We then describe a role for β-catenin in the cellular processes underlying synaptic plasticity. We also review recent data demonstrating that β-catenin mRNA and protein phosphorylation are dynamically regulated during fear memory consolidation in adult animals. Such alterations are correlated with a change in the association of β-catenin with cadherin, and deletion of the β-catenin gene prevents fear learning. Overall, the extant data suggest that β-catenin may function in mediating the structural changes associated with memory formation. This suggests a general role for β-catenin in synaptic remodeling and stabilization underlying long-term memory in adults, and possible roles for dysfunction in the β-catenin pathway in disorders of memory impairment (e.g. Alzheimer's Disease) and in disturbances in which emotional memories are too strong or resistant to inhibition (e.g. fear learning in Posttraumatic Stress Disorder). Further understanding of the β-catenin pathway may lead to better appreciation for the structural mechanisms underlying learning and memory as well as provide novel therapeutic approaches in memory related disorders. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Autoradiographic study of serotonin transporter during memory formation

Serotonin transporter (SERT) has been associated with drugs of abuse like d-methamphetamine (METH). METH is well known to produce effects on the monoamine systems but it is unclear how METH affects SERT and memory. Here the effects of METH and the serotonin reuptake inhibitor fluoxetine (FLX) on autoshaping and novel object recognition (NOR) were investigated. Notably, both memory tasks recruit different behavioral, neural and cognitive demand. In autoshaping task a dose-response curve for METH was determined. METH (1.0mg/kg) impaired short-term memory (STM; lasting less of 90min) in NOR and impaired both STM and long-term memory (LTM; lasting 24 and 48h) in autoshaping, indicating that METH had long-lasting effects in the latter task. A comparative autoradiography study of the relationship between the binding pattern of SERT in autoshaping new untrained vs. trained treated (METH, FLX, or both) animals was made. Considering that hemispheric dominance is important for LTM, hence right vs. left hemisphere of the brain was compared. Results showed that trained animals decreased cortical SERT binding relative to untrained ones. In untrained and trained treated animals with the amnesic dose (1.0mg/kg) of METH SERT binding in several areas including hippocampus and cortex decreased, more remarkably in the trained animals. In contrast, FLX improved memory, increased SERT binding, prevented the METH amnesic effect and re-established the SERT binding. In general, memory and amnesia seemed to make SERT more vulnerable to drugs effects.

Habit learning dissociation in rats with lesions to the vermis and the interpositus of the cerebellum

After cerebellar tumors resection, patients show motor skill learning impairments but also cognitive deficits. However, their exact origins remain controversial. Using a rat model of cerebellar injury, we assessed the involvement of two structures often damaged during resection (vermis and interpositus nuclei) on habits development. During extended training of an instrumental task, rats develop response routines that are no longer voluntary or goal-directed but habit-based, evidenced by their insensitivity to changes in the value of the reward. Here we showed that, in contrast to sham or vermis lesioned rats, discrete lesions to interpositus nuclei prevented rats from developing habits with overtraining, without motor difficulties, nor alteration of the instrumental task acquisition. Our results suggest that the role of the cerebellum can be extended from motor skill learning to cognitive routines learning. Similar habit impairment could possibly account for some of the long-term outcome difficulties observed in cerebellar-damaged patients.

NMDA-Receptor Activation Induces Calpain-Mediated β-Catenin Cleavages for Triggering Gene Expression

The canonical Wnt-beta-catenin signaling pathway is important for a variety of developmental phenomena as well as for carcinogenesis. Here, we show that, in hippocampal neurons, NMDA-receptor-dependent activation of calpain induced the cleavage of beta-catenin at the N terminus, generating stable, truncated forms. These beta-catenin fragments accumulated in the nucleus and induced Tcf/Lef-dependent gene transcription. We identified Fosl1, one of the immediate-early genes, as a target of this signaling pathway. In addition, exploratory behavior by mice resulted in a similar cleavage of beta-catenin, as well as activation of the Tcf signaling pathway, in hippocampal neurons. Both beta-catenin cleavage and Tcf-dependent gene transcription were suppressed by calpain inhibitors. These findings reveal another pathway for beta-catenin-dependent signaling, in addition to the canonical Wnt-beta-catenin pathway, and suggest that this other pathway could play an important role in activity-dependent gene expression.

Delayed suppression of hippocampal cell proliferation in rats following inescapable shocks

Adult Sprague-Dawley rats were exposed to a single session of 100 inescapable tail shocks (IS). Bromodeoxyuridine (BrdU) was administered 1 h, 2 days or 7 days later and hippocampal cell proliferation (CP) was assessed after a 2-h survival period. Measures of plasma corticosterone (CORT) levels were also obtained. Despite a large increase in CORT immediately following IS, no associated change in CP was observed. In fact, the only significant change in CP was seen 7 days after IS, at a time when CORT was unchanged from control levels. These data raise questions about the general nature of the relationship between CORT and CP. They also suggest that, under some conditions, changes in hippocampal CP may emerge only after an "incubation period".