Florian Freudenberg

Dopamine in the orbitofrontal cortex regulates operant responding under a progressive ratio of reinforcement in rats

Prefrontocortical dopamine (DA) plays an essential role in the regulation of cognitive functions and behavior. The orbitofrontal cortex (OFC) receives a dopaminergic projection from the ventral tegmental area and is particularly important for goal-directed appetitive behaviors and for the neural representation of reward value. We here examined the effects of DA receptor blockers locally infused into the OFC, on instrumental behavior under a progressive schedule of reinforcement. After continuous reinforcement training (lever pressing for casein pellets) rats received bilateral intra-OFC-infusions of the DA D1-receptor antagonist SCH23390 (3 microg/0.5 microl), the DA D2-receptor antagonist sulpiride (3 microg/0.5 microl), or phosphate buffered saline through chronically indwelling cannulae. Immediately after infusion they were tested under a time-constrained progressive ratio schedule of reinforcement (3, 6, 9, 12, ... lever presses for 1 casein pellet within 180 s). Both SCH23390 and sulpiride led to a significant reduction of the break point (cessation to respond to the increasing criterion of instrumental effort) compared to vehicle infusions. A food preference test revealed no drug effects on the amount of consumed pellets and on the preference of casein pellets over laboratory chow. Leftward shifts of the break point in progressive ratio tasks indicate a disturbance of the mechanisms that translate motivation into appetitive behavior under conditions of increasing instrumental effort. Therefore, our data indicate that orbitofrontal dopamine is necessary for reward-related instrumental behavior.

Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory

Long Homer proteins forge assemblies of signaling components involved in glutamate receptor signaling in postsynaptic excitatory neurons, including those underlying synaptic transmission and plasticity. The short immediate-early gene (IEG) Homer1a can dynamically uncouple these physical associations by functional competition with long Homer isoforms. To examine the consequences of Homer1a-mediated “uncoupling” for synaptic plasticity and behavior, we generated forebrain-specific tetracycline (tet) controlled expression of Venus-tagged Homer1a (H1aV) in mice. We report that sustained overexpression of H1aV impaired spatial working but not reference memory. Most notably, a similar impairment was observed when H1aV expression was restricted to the dorsal hippocampus (HP), which identifies this structure as the principal cortical area for spatial working memory. Interestingly, H1aV overexpression also abolished maintenance of CA3-CA1 long-term potentiation (LTP). These impairments, generated by sustained high Homer1a levels, identify a requirement for long Homer forms in synaptic plasticity and temporal encoding of spatial memory.

Disturbed social behavior and motivation in rats selectively bred for deficient sensorimotor gating

Deficient prepulse inhibition (PPI) of startle reflects disturbed sensorimotor gating found in certain neuropsychiatric disorders. We here tested whether rats selectively bred for deficient PPI are deteriorated in behavioral paradigms used to model negative symptoms of schizophrenia. Rats with low PPI preferred standard rat-chow when having the choice between lever-pressing for reward-pellets or freely available rat-chow, suggesting reduced motivation. Additionally, these rats show deteriorated social behavior during interaction with a juvenile rat. Rats selectively bred for low PPI may therefore be used as a model to study the biological mechanisms and therapeutic strategies of negative symptoms of schizophrenia.

The role of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in depression: Central mediators of pathophysiology and antidepressant activity?

Depression is a major psychiatric disorder affecting more than 120 million people worldwide every year. Changes in monoaminergic transmitter release are suggested to take part in the pathophysiology of depression. However, more recent experimental evidence suggests that glutamatergic mechanisms might play a more central role in the development of this disorder. The importance of the glutamatergic system in depression was particularly highlighted by the discovery that N-methyl-D-aspartate (NMDA) receptor antagonists (particularly ketamine) exert relatively long-lasting antidepressant like effects with rapid onset. Importantly, the antidepressant-like effects of NMDA receptor antagonists, but also other antidepressants (both classical and novel), require activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Additionally, expression of AMPA receptors is altered in patients with depression. Moreover, preclinical evidence supports an important involvement of AMPA receptor-dependent signaling and plasticity in the pathophysiology and treatment of depression. Here we summarize work published on the involvement of AMPA receptors in depression and discuss a possible central role for AMPA receptors in the pathophysiology, course and treatment of depression.

GluA1 and its PDZ-interaction: a role in experience-dependent behavioral plasticity in the forced swim test.

Glutamate receptor dependent synaptic plasticity plays an important role in the pathophysiology of depression. Hippocampal samples from clinically depressed patients display reduced mRNA levels for GluA1, a major subunit of AMPA receptors. Moreover, activation and synaptic incorporation of GluA1-containing AMPA receptors are required for the antidepressant-like effects of NMDA receptor antagonists. These findings argue that GluA1-dependent synaptic plasticity might be critically involved in the expression of depression. Using an animal model of depression, we demonstrate that global or hippocampus-selective deletion of GluA1 impairs expression of experience-dependent behavioral despair. This impairment is mediated by the interaction of GluA1 with PDZ-binding domain proteins, as deletion of the C-terminal leucine alone is sufficient to replicate the behavioral phenotype. Our results provide evidence for a significant role of hippocampal GluA1-containing AMPA receptors and their PDZ-interaction in experience-dependent expression of behavioral despair and link mechanisms of hippocampal synaptic plasticity with behavioral expression of depression.

A multi-resource data integration approach: identification of candidate genes regulating cell proliferation during neocortical development

Neurons of the mammalian neocortex are produced by proliferating cells located in the ventricular zone (VZ) lining the lateral ventricles. This is a complex and sequential process, requiring precise control of cell cycle progression, fate commitment and differentiation. We have analyzed publicly available databases from mouse and human to identify candidate genes that are potentially involved in regulating early neocortical development and neurogenesis. We used a mouse in situ hybridization dataset (The Allen Institute for Brain Science) to identify 13 genes (Cdon, Celsr1, Dbi, E2f5, Eomes, Hmgn2, Neurog2, Notch1, Pcnt, Sox3, Ssrp1, Tead2, Tgif2) with high correlation of expression in the proliferating cells of the VZ of the neocortex at early stages of development (E15.5). We generated a similar human brain network using microarray and RNA-seq data (BrainSpan Atlas) and identified 407 genes with high expression in the developing human VZ and subventricular zone (SVZ) at 8–9 post-conception weeks. Seven of the human genes were also present in the mouse VZ network. The human and mouse networks were extended using available genetic and proteomic datasets through GeneMANIA. A gene ontology search of the mouse and human networks indicated that many of the genes are involved in the cell cycle, DNA replication, mitosis and transcriptional regulation. The reported involvement of Cdon, Celsr1, Dbi, Eomes, Neurog2, Notch1, Pcnt, Sox3, Tead2, and Tgif2 in neural development or diseases resulting from the disruption of neurogenesis validates these candidate genes. Taken together, our knowledge-based discovery method has validated the involvement of many genes already known to be involved in neocortical development and extended the potential number of genes by 100s, many of which are involved in functions related to cell proliferation but others of which are potential candidates for involvement in the regulation of neocortical development.

Circuit mechanisms of GluA1-dependent spatial working memory

Spatial working memory (SWM), the ability to process and manipulate spatial information over a relatively short period of time, requires an intact hippocampus, but also involves other forebrain nuclei in both in rodents and humans. Previous studies in mice showed that the molecular mechanism of SWM includes activation of AMPA receptors containing the GluA1 subunit (encoded by gria1) as GluA1 deletion in the whole brain (gria1–/–) results in strong SWM deficit. However, since these mice globally lack GluA1, the circuit mechanisms of GluA1 contribution to SWM remain unknown. In this study, by targeted expression of GluA1 containing AMPA receptors in the forebrain of gria1–/– mice or by removing GluA1 selectively from hippocampus of mice with “floxed” GluA1 alleles (gria1fl/fl), we show that SWM requires GluA1 action in cortical circuits but is only partially dependent on GluA1‐containing AMPA receptors in hippocampus. We further show that hippocampal GluA1 contribution to SWM is temporally restricted and becomes prominent at longer retention intervals (≥30 s). These findings provide a novel insight into the neural circuits required for SWM processing and argue that AMPA mediated signaling across forebrain and hippocampus differentially contribute to encoding of SWM.

Challenges with modelling anxiety disorders: a possible hindrance for drug discovery

The previous decade has witnessed a dramatic withdrawal of pharmaceutical companies from psychiatric-based drug discovery predominantly due to the lack of novel-acting therapies [1]. There are numerous reasons for this including a lack of understanding of the underlying etiology of the disorders, aswell as the necessity for novel and more translationally relevant animal models of anxiety disorders. It is important to state from the outset that animal models, particularly rodent models, can only model specific aspects of anxiety disorders and not their entirety. In this opinion piece, we discuss the limitations of current tests and avenues that shouldbeexploredwhenplanningnovel anxietymodels and tests.

The regulation of tetraspanin 8 gene expression—A potential new mechanism in the pathogenesis of bipolar disorder

In a previous study, we identified the single nucleotide polymorphism (SNP) rs4500567, located in the upstream region of tetraspanin 8 (TSPAN8), to be associated with bipolar disorder (BD). Due to its proximal position, the SNP might have an impact on promoter activity, thus on TSPAN8 gene expression. We investigated the impact of rs4500567 on TSPAN8 expression in vitro with luciferase‐based promoter assays in human embryonic kidney (HEK293) and neuroblastoma cells (SH‐SY5Y), and its effect on expression of downstream associated genes by microarray‐based transcriptome analyses. Immunohistochemical localization studies on murine brain slices served to identify possible target regions of altered TSPAN8 expression in the brain. Promoter assays revealed decreased TSPAN8 expression in presence of the minor allele. Transcriptome analyses of TSPAN8‐knockdown cells, mirroring the effects of putatively reduced TSPAN8 expression in minor allele carriers, resulted in 231 differentially expressed genes with enrichments of relevant signaling pathways for psychiatric disorders and neuronal development. Finally, we demonstrate Tspan8 abundance in mouse cerebellum and hippocampus. These findings point to a role of TSPAN8 in neuronal function or development. Considering a rather protective effect of the minor allele of rs4500567, our findings reveal a possible novel mechanism that contributes to the development of BD.

Hippocampal GluA1 expression in Gria1(-/-) mice only partially restores spatial memory performance deficits

Spatial working memory (SWM) is an essential cognitive function important for survival in a competitive environment. In rodents SWM requires an intact hippocampus and SWM expression is impaired in mice lacking the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluA1 (Gria1-/- mice). Here we used viral gene transfer to show that re-expression of GluA1 in the hippocampus can affect the behavioral performance of GluA1 deficient mice. We found that Gria1-/- mice with hippocampus-specific rescue of GluA1 expression (Gria1Hpc mice) are more anxious, less hyperactive and only partly impaired in SWM expression in the Y-maze spatial novelty preference paradigm compared to Gria1-/- mice. However, Gria1Hpc mice still express SWM performance deficits when tested in the rewarded alternation T-maze task. Thus, the restoration of hippocampal function affects several behaviors of GluA1 deficient mice - including SWM expression - in different tasks. The virus-mediated GluA1 expression in Gria1-/- mice is not sufficient for a comprehensive SWM restoration, suggesting that both hippocampal as well as extra-hippocampal GluA1-containing AMPA receptors contribute to SWM.