The output of the hippocampus is inhibited during social behavior in the male rat

The eIF4E homolog 4EHP (eIF4E2) regulates hippocampal long-term depression and impacts social behavior

Background

The regulation of protein synthesis is a critical step in gene expression, and its dysfunction is implicated in autism spectrum disorder (ASD). The eIF4E homologous protein (4EHP, also termed eIF4E2) binds to the mRNA 5′ cap to repress translation. The stability of 4EHP is maintained through physical interaction with GRB10 interacting GYF protein 2 (GIGYF2). Gene-disruptive mutations in GIGYF2 are linked to ASD, but causality is lacking. We hypothesized that GIGYF2 mutations cause ASD by disrupting 4EHP function.

Methods

Since homozygous deletion of either gene is lethal, we generated a cell-type-specific knockout model where Eif4e2 (the gene encoding 4EHP) is deleted in excitatory neurons of the forebrain (4EHP-eKO). In this model, we investigated ASD-associated synaptic plasticity dysfunction, ASD-like behaviors, and global translational control. We also utilized mice lacking one copy of Gigyf2, Eif4e2 or co-deletion of one copy of each gene to further investigate ASD-like behaviors.

Results

4EHP is expressed in excitatory neurons and synaptosomes, and its amount increases during development. 4EHP-eKO mice display exaggerated mGluR-LTD, a phenotype frequently observed in mouse models of ASD. Consistent with synaptic plasticity dysfunction, the mice displayed social behavior impairments without being confounded by deficits in olfaction, anxiety, locomotion, or motor ability. Repetitive behaviors and vocal communication were not affected by loss of 4EHP in excitatory neurons. Heterozygous deletion of either Gigyf2, Eif4e2, or both genes in mice did not result in ASD-like behaviors (i.e. decreases in social behavior or increases in marble burying). Interestingly, exaggerated mGluR-LTD and impaired social behaviors were not attributed to changes in hippocampal global protein synthesis, which suggests that 4EHP and GIGYF2 regulate the translation of specific mRNAs to mediate these effects.

Limitations

This study did not identify which genes are translationally regulated by 4EHP and GIGYF2. Identification of mistranslated genes in 4EHP-eKO mice might provide a mechanistic explanation for the observed impairment in social behavior and exaggerated LTD. Future experiments employing affinity purification of translating ribosomes and mRNA sequencing in 4EHP-eKO mice will address this relevant issue.

Conclusions

Together these results demonstrate an important role of 4EHP in regulating hippocampal plasticity and ASD-associated social behaviors, consistent with the link between mutations in GIGYF2 and ASD.

Lasting changes in social behavior and amygdala function following traumatic experience induced by a single series of foot-shocks

Neuronal plasticity within the amygdala mediates many behavioral effects of traumatic experience, and this brain region also controls various aspects of social behavior. However, the specific involvement of the amygdala in trauma-induced social deficits has never been systematically investigated. We exposed rats to a single series of electric foot-shocks--a frequently used model of trauma--and studied their behavior in the social avoidance and psychosocial stimulation tests (non-contact versions of the social interaction test) at different time intervals. Social interaction-induced neuronal activation patterns were studied in the prefrontal cortex (orbitofrontal and medial), amygdala (central, medial, and basolateral), dorsal raphe and locus coeruleus. Shock exposure markedly inhibited social behavior in both tests. The effect lasted at least 4 weeks, and amplified over time. As shown by c-Fos immunocytochemistry, social interactions activated all the investigated brain areas. Traumatic experience exacerbated this activation in the central and basolateral amygdala, but not in other regions. The tight correlation between the social deficit and amygdala activation patterns suggest that the two phenomena were associated. A real-time PCR study showed that CRF mRNA expression in the amygdala was temporarily reduced 14, but not 1 and 28 days after shock exposure. In contrast, amygdalar NK1 receptor mRNA expression increased throughout. Thus, the trauma-induced social deficits appear to be associated with, and possibly caused by, plastic changes in fear-related amygdala subdivisions.

A peptidergic basis for sexual behavior in mammals

Vasopressin (VP) is a peptide neurotransmitter in the limbic system of rats. It is synthesized in the medial amygdaloid nucleus in the presence of sex steroids, transported to other limbic structures such as the hippocampus and septum and secreted there by a calcium-dependent process. In the hippocampus, VP acts on cerebral microvessels and local circuit interneurons. Its excitatory action on the inhibitory interneurons produces near-total shutdown of electrical activity of the efferent fibers of pyramidal cells, the projection neurons of the hippocampus. Stimulation of the medial amygdala and release of the endogenous VP duplicates these effects and, since they are blocked by ventricular application of a VP antagonist, the effects are almost certainly mediated by endogenous VP. Recording from the VP-containing cell bodies or of the hippocampal action of the peptide indicates that the system is selectively involved with the early stages of sexual behavior, specifically those appetitive behaviors that anticipate coitus. Stimulation of the VP cells produces alterations in sexual behavior in a manner consistent with the hypothesis that the medial amygdala organizes the appetitive phase of recognition of an appropriate partner and sexual arousal. This role for the medial amygdala complements the proposed role of nearby structures in the consummatory, reward and learned aspects of sexual behavior. Association between VP, oxytocin (OT) and homologs with sexual behavior is very widespread among vertebrates, including amphibians, reptiles, primates and humans. Humans and other primates display a phenomenon called 'concealed ovulation' that may have played a role in the evolution of social structures. The review concludes with a discussion of possible experimental strategies for evaluating the possible role of VP in concealed ovulation and other conditions in which sexual behavior occurs outside of estrus.

The medial amygdaloid nucleus modifies social behavior in male rats

Electrical stimulation of the medial amygdaloid nucleus (AME) produces a behavioral state in male rats that resembles the postejaculatory interval, but electrical recording from cells in the AME shows that they become active earlier in sexual behavior, around the time that the male first appears to become aware of estrus in the female. In an attempt to resolve which feature of sexual behavior was mediated by the AME, we stimulated the structure bilaterally in freely behaving males using voltage levels too low to produce the postejaculatory interval. We found that electrical activation of this kind facilitated sexual behavior when it would not otherwise occur (i.e., in the presence of a nonestrous female). However, the stimuli suppressed sexual behavior when it would normally occur (i.e., in the presence of a nonestrous female). We discuss alternative interpretations of the results in the context of a general model for the central organization of sexual behavior in males.

Orthodromic activation of peptidergic cells in the medial amygdala

Electrical recordings from vasopressin-containing cells in the medial amygdala were obtained. Electrical stimulation of one major afferent structure, the accessory olfactory bulb, invariably elicited single unit discharge in the peptidergic cells and set up a field potential indicating widespread excitation in the structure. Pheromonal stimuli, normally borne into the brain by the accessory olfactory bulb, were ineffective in activating the medial amygdala. These results in combination with preexisting research suggest that the accessory olfactory bulb is an important influence, but not the only influence, on the activity of the peptidergic cells.