Deficiency of the immunoproteasome subunit ß5i/LMP7 supports the anxiogenic effects of mild stress and facilitates cued fear memory in mice.

Proteolysis as mediated by one of the major cellular protein degradation pathways, the ubiquitin-proteasome system (UPS), plays an essential role in learning and memory formation. However, the functional relevance of immunoproteasomes in the healthy brain and especially their impact on normal brain function including processes of learning and memory has not been investigated so far. In the present study, we analyzed the phenotypic effects of an impaired immunoproteasome formation using a ß5i/LMP7-deficient mouse model in different behavioral paradigms focusing on locomotor activity, exploratory behavior, innate anxiety, startle response, prepulse inhibition, as well as fear and safety conditioning. Overall, our results demonstrate no strong effects of constitutive ß5i/LMP7-deficiency on gross locomotor abilities and anxiety-related behavior in general. However, ß5i/LMP7-deficient mice expressed more anxiety after mild stress and increased cued fear after fear conditioning. These findings indicate that the basal proper formation of immunoproteasomes and/or at least the expression of ß5i/LMP7 in healthy mice seem to be involved in the regulation of anxiety and cued fear levels.

Orexin deficiency affects sociability and the acquisition, expression, and extinction of conditioned social fear.

Accumulating evidence indicates that the central orexin (hypocretin) system plays an important role in regulating emotional processes in both humans and rodents. Thus, the orexin system has been repeatedly implicated in the pathophysiology of several neuropsychiatric disorders, such as anxiety disorders. Among others, symptoms like social fear and social withdrawal are frequently observed in these disorders. Based on this, we investigated the role of orexin deficiency in social (fear) behavior. For that, female and male orexin-deficient mice were tested for (1) sociability and social novelty, and (2) acquisition, expression, and extinction of conditioned social fear. We found that female orexin-deficient mice displayed reduced sociability and decreased preference for social novelty compared to their wild-type littermates. These effects of orexin deficiency were not observed in males. Moreover, orexin deficiency facilitated the acquisition and/or expression of conditioned social fear and impaired the extinction of social fear in both sexes. Taken together, our results indicate an important, partly sex-dependent, regulatory role of the orexin system in social (fear) behavior. Our findings support the hypothesis of orexin being an integrator of motivation, affect, and emotion.

Corticosterone Treatment and Incubation Time After Contextual Fear Conditioning Synergistically Induce Fear Memory Generalization in Neuropeptide S Receptor-Deficient Mice.

Fear memory generalization is a learning mechanism that promotes flexible fear responses to novel situations. While fear generalization has adaptive value, overgeneralization of fear memory is a characteristic feature of the pathology of anxiety disorders. The neuropeptide S (NPS) receptor (NPSR) has been shown to be associated with anxiety disorders and has recently been identified as a promising target for treating anxiety disorders. Moreover, stress hormones play a role in regulating both physiological and pathological fear memories and might therefore also be involved in anxiety disorders. However, little is known about the interplay between stress hormone and the NPS system in the development of overgeneralized fear. Here, we hypothesize that NPSR-deficient mice with high corticosterone (CORT) levels during the fear memories consolidation are more prone to develop generalized fear. To address this hypothesis, NPSR-deficient mice were submitted to a contextual fear conditioning procedure. Immediately after conditioning, mice received CORT injections (2.5 or 5 mg/kg). One day and 1 month later, the mice were tested for the specificity and strength of their fear memory, their anxiety level, and their startle response. Moreover, CORT blood levels were monitored throughout the experiment. Using this protocol, a specific contextual fear memory was observed in all experimental groups, despite the 5-mg/kg CORT-treated NPSR-deficient mice. This group of mice showed a generalization of contextual fear memory and a decreased startle response, and the females of this group had significantly less body weight gain. These findings indicate that interplay between CORT and the NPS system during the consolidation of fear memories is critical for the generalization of contextual fear.

Sociability and extinction of conditioned social fear is affected in neuropeptide S receptor-deficient mice.

Being cautious of unfamiliar conspecifics is adaptive because sick or aggressive conspecifics may jeopardize survival and well-being. However, prolonged or excessive caution, i.e. fear related to social situations, is maladaptive and may result in social anxiety disorder. Some anxiety disorders in humans are associated with polymorphisms of the neuropeptide S receptor (NPSR) gene. In line with this finding, animal studies showed an important role of NPS and NPSR in anxiety and fear. The present study investigated the role of NPSR deficiency in social behavior under non-aversive and aversive conditions. For this, female and male NPSR-deficient mice were tested for (1) sociability and social novelty and (2) acquisition, expression, and extinction of conditioned social fear. The present study revealed very particular effects of the NPSR genotype: Sociability was reduced in female heterozygous NPSR-deficient mice, but was unaffected in males and the other genotypes. Furthermore, the NPSR genotype did not affect the acquisition and expression of conditioned social fear, but its extinction was impaired in heterozygous and facilitated in homozygous NPSR-deficient mice. This indicates that the NPS system plays a role in social behavior under non-aversive and aversive conditions, partly in a sex-dependent manner. The present findings may help to explain social symptoms in anxiety disorders associated with the NPSR genotype.