Social propinquity in rodents as measured by tube cooccupancy differs between inbred and outbred genotypes.

Existing assays of social interaction are suboptimal, and none measures propinquity, the tendency of rodents to maintain close physical proximity. These assays are ubiquitously performed using inbred mouse strains and mutations placed on inbred genetic backgrounds. We developed the automatable tube cooccupancy test (TCOT) based on propinquity, the tendency of freely mobile rodents to maintain close physical proximity, and assessed TCOT behavior on a variety of genotypes and social and environmental conditions. In outbred mice and rats, familiarity determined willingness to cooccupy the tube, with siblings and/or cagemates of both sexes exhibiting higher cooccupancy behavior than strangers. Subsequent testing using multiple genotypes revealed that inbred strain siblings do not cooccupy at higher rates than strangers, in marked contrast to both outbred and rederived wild mice. Mutant mouse strains with "autistic-like" phenotypes (Fmr1-/y and Eif4e Ser209Ala) displayed significantly decreased cooccupancy.

Loss of eIF4E Phosphorylation Engenders Depression-like Behaviors via Selective mRNA Translation.

The MAPK/ERK (mitogen-activated protein kinases/extracellular signal-regulated kinase) pathway is a cardinal regulator of synaptic plasticity, learning, and memory in the hippocampus. One of major endpoints of this signaling cascade is the 5' mRNA cap binding protein eIF4E (eukaryotic Initiation Factor 4E), which is phosphorylated on Ser 209 by MNK (MAPK-interacting protein kinases) and controls mRNA translation. The precise role of phospho-eIF4E in the brain is yet to be determined. Herein, we demonstrate that ablation of eIF4E phosphorylation in male mice (4Eki mice) does not impair long-term spatial or contextual fear memory, or the late phase of LTP. Using unbiased translational profiling in mouse brain, we show that phospho-eIF4E differentially regulates the translation of a subset of mRNAs linked to inflammation, the extracellular matrix, pituitary hormones, and the serotonin pathway. Consequently, 4Eki male mice display exaggerated inflammatory responses and reduced levels of serotonin, concomitant with depression and anxiety-like behaviors. Remarkably, eIF4E phosphorylation is required for the chronic antidepressant action of the selective serotonin reuptake inhibitor fluoxetine. Finally, we propose a novel phospho-eIF4E-dependent translational control mechanism in the brain, via the GAIT complex (gamma IFN activated inhibitor of translation). In summary, our work proposes a novel translational control mechanism involved in the regulation of inflammation and depression, which could be exploited to design novel therapeutics.SIGNIFICANCE STATEMENT We demonstrate that downstream of the MAPK (mitogen-activated protein kinase) pathway, eukaryotic Initiation Factor 4E (eIF4E) Ser209 phosphorylation is not required for classical forms of hippocampal LTP and memory. We reveal a novel role for eIF4E phosphorylation in inflammatory responses and depression-like behaviors. eIF4E phosphorylation is required for the chronic action of antidepressants, such as fluoxetine in mice. These phenotypes are accompanied by selective translation of extracellular matrix, pituitary hormones, and serotonin pathway genes, in eIF4E phospho-mutant mice. We also describe a previously unidentified translational control mechanism in the brain, whereby eIF4E phosphorylation is required for inhibiting the translation of gamma IFN activated inhibitor of translation element-containing mRNAs. These findings can be used to design novel therapeutics for depression.

Role of α4- and α6-containing nicotinic receptors in the acquisition and maintenance of nicotine self-administration.

Tobacco smoking is a major cause of death and disease and as such there is a critical need for the development of new therapeutic approaches to treat nicotine addiction. Here, we utilize genetic and pharmacological tools to further investigate the nicotinic acetylcholine receptor (nAChR) subtypes that support intravenous self-administration of nicotine. α4-S248F mice contain a point mutation within the α4 nAChR subunit which confers increased sensitivity to nicotine and resistance to mecamylamine. Here, we show that acute administration of mecamylamine (2 mg/kg, i.p.) reduces established nicotine self-administration (0.05 mg/kg/infusion) in wild-type (WT), but not in α4-S248F heterozygous mice, demonstrating a role for α4* nAChRs in the modulation of ongoing nicotine self-administration. Administration of N,N-decane-1,10-diyl-bis-3-picolinium diiodide (bPiDI), a selective α6ß2* nAChR antagonist, dose dependently (5 and 10 mg/kg, i.p.) impairs the acquisition of nicotine self-administration and reduces established nicotine self-administration in WT mice when administered acutely (10 mg/kg, i.p.). This was not due to a general reduction in locomotor activity and the same dose of bPiDI did not affect operant responding for sucrose. bPiDI treatment (10 mg/kg, i.p.) also impaired both the acquisition and maintenance of nicotine self-administration in α4-S248F heterozygous mice. This provides further evidence for the involvement of α6ß2* nAChRs in the reinforcing effects of nicotine that underlies its ability to support ongoing self-administration. Taken together, selective targeting of α6ß2* or α4α6ß2* nAChRs may prove to be an effective strategy for the development of smoking cessation therapies.

Comparison of the spatial-cognitive functions of dorsomedial striatum and anterior cingulate cortex in mice.

Neurons in anterior cingulate cortex (aCC) project to dorsomedial striatum (DMS) as part of a corticostriatal circuit with putative roles in learning and other cognitive functions. In the present study, the spatial-cognitive importance of aCC and DMS was assessed in the hidden-platform version of the Morris water maze (MWM). Brain lesion experiments that focused on areas of connectivity between these regions indicated their involvement in spatial cognition. MWM learning curves were markedly delayed in DMS-lesioned mice in the absence of other major functional impairments, whereas there was a more subtle, but still significant influence of aCC lesions. Lesioned mice displayed impaired abilities to use spatial search strategies, increased thigmotaxic swimming, and decreased searching in the proximity of the escape platform. Additionally, aCC and DMS activity was compared in mice between the early acquisition phase (2 and 3 days of training) and the over-trained high-proficiency phase (after 30 days of training). Neuroplasticity-related expression of the immediate early gene Arc implicated both regions during the goal-directed, early phases of spatial learning. These results suggest the functional involvement of aCC and DMS in processes of spatial cognition that model associative cortex-dependent, human episodic memory abilities.

Metformin ameliorates core deficits in a mouse model of fragile X syndrome.

Fragile X syndrome (FXS) is the leading monogenic cause of autism spectrum disorders (ASD). Trinucleotide repeat expansions in FMR1 abolish FMRP expression, leading to hyperactivation of ERK and mTOR signaling upstream of mRNA translation. Here we show that metformin, the most widely used drug for type 2 diabetes, rescues core phenotypes in Fmr1-/y mice and selectively normalizes ERK signaling, eIF4E phosphorylation and the expression of MMP-9. Thus, metformin is a potential FXS therapeutic.

Inability to acquire spatial information and deploy spatial search strategies in mice with lesions in dorsomedial striatum.

Dorsal striatum has been shown to contribute to spatial learning and memory, but the role of striatal subregions in this important aspect of cognitive functioning remains unclear. Moreover, the spatial-cognitive mechanisms that underlie the involvement of these regions in spatial navigation have scarcely been studied. We therefore compared spatial learning and memory performance in mice with lesions in dorsomedial (DMS) and dorsolateral striatum (DLS) using the hidden-platform version of the Morris water maze (MWM) task. Compared to sham-operated controls, animals with DMS damage were impaired during MWM acquisition training. These mice displayed delayed spatial learning, increased thigmotaxis, and increased search distance to the platform, in the absence of major motor dysfunction, working memory defects or changes in anxiety or exploration. They failed to show a preference for the target quadrant during probe trials, which further indicates that spatial reference memory was impaired in these animals. Search strategy analysis moreover demonstrated that DMS-lesioned mice were unable to deploy cognitively advanced spatial search strategies. Conversely, MWM performance was barely affected in animals with lesions in DLS. In conclusion, our results indicate that DMS and DLS display differential functional involvement in spatial learning and memory. Our results show that DMS, but not DLS, is crucial for the ability of mice to acquire spatial information and their subsequent deployment of spatial search strategies. These data clearly identify DMS as a crucial brain structure for spatial learning and memory, which could explain the occurrence of neurocognitive impairments in brain disorders that affect the dorsal striatum.

Telencephalic neurocircuitry and synaptic plasticity in rodent spatial learning and memory.

Spatial learning and memory in rodents represent close equivalents of human episodic declarative memory, which is especially sensitive to cerebral aging, neurodegeneration, and various neuropsychiatric disorders. Many tests and protocols are available for use in laboratory rodents, but Morris water maze and radial-arm maze remain the most widely used as well as the most valid and reliable spatial tests. Telencephalic neurocircuitry that plays functional roles in spatial learning and memory includes hippocampus, dorsal striatum and medial prefrontal cortex. Prefrontal-hippocampal circuitry comprises the major associative system in the rodent brain, and is critical for navigation in physical space, whereas interconnections between prefrontal cortex and dorsal striatum are probably more important for motivational or goal-directed aspects of spatial learning. Two major forms of synaptic plasticity, namely long-term potentiation, a lasting increase in synaptic strength between simultaneously activated neurons, and long-term depression, a decrease in synaptic strength, have been found to occur in hippocampus, dorsal striatum and medial prefrontal cortex. These and other phenomena of synaptic plasticity are probably crucial for the involvement of telencephalic neurocircuitry in spatial learning and memory. They also seem to play a role in the pathophysiology of two brain pathologies with episodic declarative memory impairments as core symptoms, namely Alzheimer's disease and schizophrenia. Further research emphasis on rodent telencephalic neurocircuitry could be relevant to more valid and reliable preclinical research on these most devastating brain disorders. This article is part of a Special Issue entitled SI: Brain and Memory.

NMDA receptor binding is reduced within mesocorticolimbic regions following chronic inhalation of toluene in adolescent rats.

The purposeful inhalation of volatile solvents, such as toluene, to induce self-intoxication is prevalent, particularly within adolescent populations. Chronic misuse results in cognitive and neurobiological impairments, as well as an increased risk for addictive behaviours in adulthood. Toluene-induced neuroadaptations within mesocorticolimbic circuitry are thought, in part, to mediate some of the adverse outcomes of toluene misuse, however our understanding of the neuroadaptive processes remains equivocal. An understanding of these processes is particularly important relative to exposure that occurs during adolescence and at concentrations that reflect various patterns of use. Therefore, we exposed male adolescent Wistar rats (postnatal day [PN] 27) to either air or low or high concentrations of inhaled toluene in a chronic and intermittent fashion (CIT, 3,000 or 10,000ppm) for 1 h/day, 3-5 times per week for 4 weeks to model different patterns of human inhalant abuse. Brains were subsequently analysed using autoradiography, qPCR and immunohistochemistry 3 days following the exposure period to investigate toluene-induced neuroadaptations within mesocorticolimbic circuitry. In CIT-exposed rats binding to N-methyl-D-aspartate (NMDA) receptors containing the GluN2B subunit, as determined using [(3)H]-ifenprodil, was decreased in a concentration-related manner in the caudal cingulate cortex, dorsal striatum and accumbens; however, this was not associated with changes in GluN2B protein expression. There were no differences in [(3)H]-epibatidine binding to heteromeric neuronal nicotinic acetylcholine (nACh) receptors. Relative expression of mRNA transcripts encoding NMDA, nACh, γ-aminobutyric acid type-A (GABAA) and dopamine receptor subunits was unchanged in all regions assessed following CIT. Our data suggest that adolescent CIT exposure impacts NMDA receptors within regions of corticostriatal circuitry, possibly via post-translational mechanisms. Dysfunctional glutamatergic signalling within corticostriatal regions may contribute to the adverse outcomes observed following adolescent toluene abuse.

Increased gait variability in mice with small cerebellar cortex lesions and normal rotarod performance.

The physiological and pathophysiological role of the cerebellum in neuromotor performance and gait is a prominent research topic in contemporary brain research. However, it has proven difficult to measure subtle neuromotor changes and cerebellar dysfunction in laboratory rodents with some of the common behavioural assays. Rotarod assays and gait analyses have been used extensively as indicators of neuromotor performance, and more specifically, cerebellar function. Standard rotarod procedures fail to reveal subtle motor alterations, whereas automated gait analysis could be more sensitive in this respect. In the present study, we compared detailed treadmill gait analysis to the standard accelerating rotarod assay in its ability to reveal neuromotor alterations in mice with small bilateral lesions in the cerebellar cortex. This small lesion model showed no readily observable signs of ataxia or abnormal activity. In the rotarod test, cerebellar-lesioned mice performed at the level of control animals, and basic gait parameters were not altered. However, cerebellar-lesioned mice did show increased front base-width and hind stride length variability, as well as increased stride length incongruity between different paws. We conclude that small cerebellar lesions lead to increased gait variability as it does in humans with cerebellar dysfunction. Treadmill gait analysis is better suited than accelerating rotarod assays to measure such subtle neuromotor defects.

Chronic administration of AFQ056/Mavoglurant restores social behaviour in Fmr1 knockout mice.

Fragile X syndrome is caused by lack of FMR1 protein (FMRP) leading to severe symptoms, including intellectual disability, hyperactivity and autistic-like behaviour. FMRP is an RNA binding protein involved in the regulation of translation of specific target mRNAs upon stimulation of metabotropic glutamate receptor 5 (mGluR5) at the synapse. The absence of FMRP leads to enhanced activity of mGluR5 signal transduction pathways. Many conflicting results have been reported regarding social behaviour deficits in Fmr1 knockout mice, and little is known about the involvement of mGluR5 pathways on social behaviour. In this study, a three-chambered task was used to determine sociability and preference for social novelty in Fmr1 knockout mice. Disruption of Fmr1 functioning resulted in enhanced interaction with stranger mouse during sociability while no significant changes were observed during preference for social novelty assay. Chronic administration of a specific mGluR5 antagonist, AFQ056/Mavoglurant, was able to restore sociability behaviour of Fmr1 knockout mice to levels of wild type littermates. These results support the importance of mGluR5 signalling pathways on social interaction behaviour and that AFQ056/Mavoglurant might be useful as potential therapeutic intervention to rescue various behavioural aspects of the fragile X phenotype.