Michael Feyder

Mice lacking the AMPA GluR1 receptor exhibit striatal hyperdopaminergia and 'schizophrenia-related' behaviors.

There is growing evidence implicating dysfunctional glutamatergic neurotransmission and abnormal interactions between the glutamate and dopamine (DA) systems in the pathophysiology of various neuropsychiatric disorders including schizophrenia. The present study evaluated knockout (KO) mice lacking the L-α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) GluR1 receptor subunit for a range of behaviors considered relevant to certain symptoms of schizophrenia. KO showed locomotor hyperactivity during exposure to open field and in response to a novel object, but normal activity in a familiar home cage. Open field locomotor hyperactivity in KO was effectively normalized to WT levels by treatment with the DA antagonist and neuroleptic haloperidol, while locomotor stimulant effects of the NMDA receptor antagonist MK-801 were absent in KO. Social behaviors during a dyadic conspecific encounter were disorganized in KO. KO showed deficits in prepulse inhibition of the acoustic startle response. In vivo chronoamperometric measurement of extracellular DA clearance in striatum demonstrated retarded clearance in KO. These data demonstrate behavioral abnormalities potentially pertinent to schizophrenia in GluR1 KO, together with evidence of dysregulated DA function. Present findings provide novel insight into the potential role of GluR1, AMPA receptors and glutamate × DA interactions in the pathophysiology of schizophrenia and other neuropsychiatric conditions.

Impaired discrimination learning in mice lacking the NMDA receptor NR2A subunit

N-Methyl-D-aspartate receptors (NMDARs) mediate certain forms of synaptic plasticity and learning. We used a touchscreen system to assess NR2A subunit knockout mice (KO) for (1) pairwise visual discrimination and reversal learning and (2) acquisition and extinction of an instrumental response requiring no pairwise discrimination. NR2A KO mice exhibited significantly retarded discrimination learning. Performance on reversal was impaired in NR2A KO mice during the learning phase of the task; with no evidence of heightened perseverative responses. Acquisition and extinction of an instrumental behavior requiring no pairwise discrimination was normal in NR2A KO mice. The present findings demonstrate a significant and selective deficit in discrimination learning following loss of NR2A.

Paradoxical reversal learning enhancement by stress or prefrontal cortical damage: rescue with BDNF

Stress affects various forms of cognition. We found that moderate stress enhanced late reversal learning in a mouse touchscreen-based choice task. Ventromedial prefrontal cortex (vmPFC) lesions mimicked the effect of stress, whereas orbitofrontal and dorsolateral striatal lesions impaired reversal. Stress facilitation of reversal was prevented by BDNF infusion into the vmPFC. These findings suggest a mechanism by which stress-induced vmPFC dysfunction disinhibits learning by alternate (for example, striatal) systems.

Does gene deletion of AMPA GluA1 phenocopy features of schizoaffective disorder

Glutamatergic dysfunction is strongly implicated in schizophrenia and mood disorders. GluA1 knockout (KO) mice display schizophrenia- and depression-related abnormalities. Here, we asked whether GluA1 KO show mania-related abnormalities. KO were tested for behavior in approach/avoid conflict tests, responses to repeated forced swim exposure, and locomotor responses under stress and after psychostimulant treatment. The effects of rapid dopamine depletion and treatment with lithium or GSK-3β inhibitor on KO locomotor hyperactivity were tested. Results showed that KO exhibited novelty- and stress-induced locomotor hyperactivity, reduced forced swim immobility and alterations in approach/avoid conflict tests. Psychostimulant treatment and dopamine depletion exacerbated KO locomotor hyperactivity. Lithium, but not GSK-3β inhibitor, treatment normalized KO anxiety-related behavior and partially reversed hyperlocomotor behavior, and also reversed elevated prefrontal cortex levels of phospho-MARCKS and phospho-neuromodulin. Collectively, these findings demonstrate mania-related abnormalities in GluA1 KO and, combined with previous findings, suggest this mutant may provide a novel model of features of schizoaffective disorder.

Impaired Pavlovian fear extinction is a common phenotype across genetic lineages of the 129 inbred mouse strain.

Fear extinction is impaired in psychiatric disorders such as post‐traumatic stress disorder and schizophrenia, which have a major genetic component. However, the genetic factors underlying individual variability in fear extinction remain to be determined. By comparing a panel of inbred mouse strains, we recently identified a strain, 129S1/SvImJ (129S1), that exhibits a profound and selective deficit in Pavlovian fear extinction, and associated abnormalities in functional activation of a key prefrontal‐amygdala circuit, as compared with C57BL/6J. The first aim of the present study was to assess fear extinction across multiple 129 substrains representing the strains four different genetic lineages (parental, steel, teratoma and contaminated). Results showed that 129P1/ReJ, 129P3/J, 129T2/SvEmsJ and 129X1/SvJ exhibited poor fear extinction, relative to C57BL/6J, while 129S1 showed evidence of fear incubation. On the basis of these results, the second aim was to further characterize the nature and specificity of the extinction phenotype in 129S1, as an exemplar of the 129 substrains. Results showed that the extinction deficit in 129S1 was neither the result of a failure to habituate to a sensitized fear response nor an artifact of a fear response to (unconditioned) tone per se. A stronger conditioning protocol (i.e. five × higher intensity shocks) produced an increase in fear expression in 129S1, relative to C57BL/6J, due to rapid rise in freezing during tone presentation. Taken together, these data show that impaired fear extinction is a phenotypic feature common across 129 substrains, and provide preliminary evidence that impaired fear extinction in 129S1 may reflect a pro‐fear incubation‐like process.

Do GluA1 knockout mice exhibit behavioral abnormalities relevant to the negative or cognitive symptoms of schizophrenia and schizoaffective disorder

The glutamate system has been strongly implicated in the pathophysiology of psychotic illnesses, including schizophrenia and schizoaffective disorder. We recently found that knockout (KO) mice lacking the AMPA GluA1 subunit displayed behavioral abnormalities relevant to some of the positive symptoms of these disorders. Here we phenotyped GluA1 KO mice for behavioral phenotypes pertinent to negative and cognitive/executive symptoms. GluA1 KO mice were tested for conspecific social interactions, the acquisition and extinction of an operant response for food-reward, operant-based pairwise visual discrimination and reversal learning, and impulsive choice in a delay-based cost/benefit decision-making T-maze task. Results showed that GluA1 KO mice engaged in less social interaction than wildtype (WT) controls when tested in a non-habituated, novel environment, but, conversely, displayed more social interaction in a well habituated, familiar environment. GluA1 KO mice were faster to acquire an operant stimulus-response for food reward than WT and were subsequently slower to extinguish the response. Genotypes showed similar pairwise discrimination learning and reversal, although GluA1 KO mice made fewer errors during early reversal. GluA1 KO mice also displayed increased impulsive choice, being less inclined to choose a delayed, larger reward when given a choice between this and a smaller, immediate reward, compared to WT mice. Finally, sucrose preference did not differ between genotypes. Collectively, these data add to the growing evidence that GluA1 KO mice display at least some phenotypic abnormalities mimicking those found in schizophrenia/schizoaffective disorder. Although these mice, like any other single mutant line, are unlikely to model the entire disease, they may nevertheless provide a useful tool for studying the role of GluA1 in certain aspects of the pathophysiology of major psychotic illness.

Durable fear memories require PSD-95.

Traumatic fear memories are highly durable but also dynamic, undergoing repeated reactivation and rehearsal over time. Although overly persistent fear memories underlie anxiety disorders, such as posttraumatic stress disorder, the key neural and molecular mechanisms underlying fear memory durability remain unclear. Postsynaptic density 95 (PSD-95) is a synaptic protein regulating glutamate receptor anchoring, synaptic stability and certain types of memory. Using a loss-of-function mutant mouse lacking the guanylate kinase domain of PSD-95 (PSD-95GK), we analyzed the contribution of PSD-95 to fear memory formation and retrieval, and sought to identify the neural basis of PSD-95-mediated memory maintenance using ex vivo immediate-early gene mapping, in vivo neuronal recordings and viral-mediated knockdown (KD) approaches. We show that PSD-95 is dispensable for the formation and expression of recent fear memories, but essential for the formation of precise and flexible fear memories and for the maintenance of memories at remote time points. The failure of PSD-95GK mice to retrieve remote cued fear memory was associated with hypoactivation of the infralimbic (IL) cortex (but not the anterior cingulate cortex (ACC) or prelimbic cortex), reduced IL single-unit firing and bursting, and attenuated IL gamma and theta oscillations. Adeno-associated virus-mediated PSD-95 KD in the IL, but not the ACC, was sufficient to impair recent fear extinction and remote fear memory, and remodel IL dendritic spines. Collectively, these data identify PSD-95 in the IL as a critical mechanism supporting the durability of fear memories over time. These preclinical findings have implications for developing novel approaches to treating trauma-based anxiety disorders that target the weakening of overly persistent fear memories.

Reward-Related Behavioral Paradigms for Addiction Research in the Mouse: Performance of Common Inbred Strains

The mouse has emerged as a uniquely valuable species for studying the molecular and genetic basis of complex behaviors and modeling neuropsychiatric disease states. While valid and reliable preclinical assays for reward-related behaviors are critical to understanding addiction-related processes, and various behavioral procedures have been developed and characterized in rats and primates, there have been relatively few studies using operant-based addiction-relevant behavioral paradigms in the mouse. Here we describe the performance of the C57BL/6J inbred mouse strain on three major reward-related paradigms, and replicate the same procedures in two other commonly used inbred strains (DBA/2J, BALB/cJ). We examined Pavlovian-instrumental transfer (PIT) by measuring the ability of an auditory cue associated with food reward to promote an instrumental (lever press) response. In a separate experiment, we assessed the acquisition and extinction of a simple stimulus-reward instrumental behavior on a touch screen based task. Reinstatement of this behavior was then examined following either continuous exposure to cues (conditioned reinforcers, CRs) associated with reward, brief reward and CR exposure, or brief reward exposure followed by continuous CR exposure. The third paradigm examined sensitivity of an instrumental (lever press) response to devaluation of food reward (a probe for outcome insensitive, habitual behavior) by repeated pairing with malaise. Results showed that C57BL/6J mice displayed robust PIT, as well as clear extinction and reinstatement, but were insensitive to reinforcer devaluation. DBA/2J mice showed good PIT and (rewarded) reinstatement, but were slow to extinguish and did not show reinforcer devaluation or significant CR-reinstatement. BALB/cJ mice also displayed good PIT, extinction and reinstatement, and retained instrumental responding following devaluation, but, unlike the other strains, demonstrated reduced Pavlovian approach behavior (food magazine head entries). Overall, these assays provide robust paradigms for future studies using the mouse to elucidate the neural, molecular and genetic factors underpinning reward-related behaviors relevant to addiction research.

Impaired associative fear learning in mice with complete loss or haploinsufficiency of AMPA GluR1 receptors

There is compelling evidence that l-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) glutamate receptors containing the GluR1 subunit contribute to the molecular mechanisms associated with learning. AMPA GluR1 glutamate receptor knockout mice (KO) exhibit abnormal hippocampal and amygdala plasticity, and deficits on various assays for cognition including Pavlovian fear conditioning. Here we examined associative fear learning in mice with complete absence (KO) or partial loss (heterozygous mutant, HET) of GluR1 on multiple fear conditioning paradigms. After multi-trial delay or trace conditioning, KO displayed impaired tone and context fear recall relative to WT, whereas HET were normal. After one-trial delay conditioning, both KO and HET showed impaired tone and context recall. HET and KO showed normal nociceptive sensitivity in the hot plate and tail flick tests. These data demonstrate that the complete absence of GluR1 subunit-containing receptors prevents the formation of associative fear memories, while GluR1 haploinsufficiency is sufficient to impair one-trial fear learning. These findings support growing evidence of a major role for GluR1-containing AMPA receptors in amygdala-mediated forms of learning and memory.

A novel role for PSD-95 in mediating ethanol intoxication, drinking and place preference.

The synaptic signaling mechanisms mediating the behavioral effects of ethanol (EtOH) remain poorly understood. Post‐synaptic density 95 (PSD‐95, SAP‐90, Dlg4) is a key orchestrator of N‐methyl‐D‐aspartate receptors (NMDAR) and glutamatergic synapses, which are known to be major sites of EtOHs behavioral actions. However, the potential contribution of PSD‐95 to EtOH‐related behaviors has not been established. Here, we evaluated knockout (KO) mice lacking PSD‐95 for multiple measures of sensitivity to the acute intoxicating effects of EtOH (ataxia, hypothermia, sedation/hypnosis), EtOH drinking under conditions of free access and following deprivation, acquisition and long‐term retention of EtOH conditioned place preference (CPP) (and lithium chloride‐induced conditioned taste aversion), and intoxication‐potentiating responses to NMDAR antagonism. PSD‐95 KO exhibited increased sensitivity to the sedative/hypnotic, but not ataxic or hypothermic, effects of acute EtOH relative to wild‐type controls (WT). PSD‐95 KO consumed less EtOH than WT, particularly at higher EtOH concentrations, although increases in KO drinking could be induced by concentration‐fading and deprivation. PSD‐95 KO showed normal EtOH CPP 1 day after conditioning, but showed significant aversion 2 weeks later. Lithium chloride‐induced taste aversion was impaired in PSD‐95 KO at both time points. Finally, the EtOH‐potentiating effects of the NMDAR antagonist MK‐801 were intact in PSD‐95 KO at the dose tested. These data reveal a major, novel role for PSD‐95 in mediating EtOH behaviors, and add to growing evidence that PSD‐95 is a key mediator of the effects of multiple abused drugs.