The Role of Vesicular Glutamate Transporter Type 3 in Social Behavior, with a Focus on the Median Raphe Region.

Social behavior is important for our well-being, and its dysfunctions impact several pathological conditions. Although the involvement of glutamate is undeniable, the relevance of vesicular glutamate transporter type 3 (VGluT3), a specific vesicular transporter, in the control of social behavior is not sufficiently explored. Since midbrain median raphe region (MRR) is implicated in social behavior and the nucleus contains high amount of VGluT3+ neurons, we compared the behavior of male VGluT3 knock-out (KO) and VGluT3-Cre mice, the latter after chemogenetic MRR-VGluT3 manipulation. Appropriate control groups were included. Behavioral test battery was used for social behavior (sociability, social discrimination, social interaction, resident intruder test) and possible confounding factors (open field, elevated plus maze, Y-maze tests). Neuronal activation was studied by c-Fos immunohistochemistry. Human relevance was confirmed by VGluT3 gene expression in relevant human brainstem areas. VGluT3 KO mice exhibited increased anxiety, social interest, but also aggressive behavior in anxiogenic environment and impaired social memory. For KO animals, social interaction induced lower cell activation in the anterior cingulate, infralimbic cortex, and medial septum. In turn, excitation of MRR-VGluT3+ neurons was anxiolytic. Inhibition increased social interest 24 h later but decreased mobility and social behavior in aggressive context. Chemogenetic activation increased the number of c-Fos+ neurons only in the MRR. We confirmed the increased anxiety-like behavior and impaired memory of VGluT3 KO strain and revealed increased, but inadequate, social behavior. MRR-VGluT3 neurons regulated mobility and social and anxiety-like behavior in a context-dependent manner. The presence of VGluT3 mRNA on corresponding human brain areas suggests clinical relevance.

The Dopaminergic Cells in the Median Raphe Region Regulate Social Behavior in Male Mice.

According to previous studies, the median raphe region (MRR) is known to contribute significantly to social behavior. Besides serotonin, there have also been reports of a small population of dopaminergic neurons in this region. Dopamine is linked to reward and locomotion, but very little is known about its role in the MRR. To address that, we first confirmed the presence of dopaminergic cells in the MRR of mice (immunohistochemistry, RT-PCR), and then also in humans (RT-PCR) using healthy donor samples to prove translational relevance. Next, we used chemogenetic technology in mice containing the Cre enzyme under the promoter of the dopamine transporter. With the help of an adeno-associated virus, designer receptors exclusively activated by designer drugs (DREADDs) were expressed in the dopaminergic cells of the MRR to manipulate their activity. Four weeks later, we performed an extensive behavioral characterization 30 min after the injection of the artificial ligand (Clozapine-N-Oxide). Stimulation of the dopaminergic cells in the MRR decreased social interest without influencing aggression and with an increase in social discrimination. Additionally, inhibition of the same cells increased the friendly social behavior during social interaction test. No behavioral changes were detected in anxiety, memory or locomotion. All in all, dopaminergic cells were present in both the mouse and human samples from the MRR, and the manipulation of the dopaminergic neurons in the MRR elicited a specific social response.

Median raphe region GABAergic neurons contribute to social interest in mouse.

Gamma-aminobutyric acid (GABA) is a well-known inhibitory neurotransmitter implicated in numerous physiological and pathological behaviors including social interest. Dysregulation of the median raphe region (MRR), a main serotoninergic nucleus, is also characterized by increased social problems. As the majority of MRR cells are GABAergic, we aimed to reveal the social role of these cells. Chemogenetic techniques were used in vesicular GABA transporter Cre mice and with the help of adeno-associated virus vectors artificial receptors (DREADDs, stimulatory, inhibitory or control, containing only a fluorophore) were expressed in MRR GABAergic cells confirmed by immunohistochemistry. Four weeks after viral injection a behavioral test battery (sociability; social interaction; resident-intruder) was conducted. The artificial ligand (clozapine-N-oxide, 1 mg/10 ml/kg) was administrated 30 min before the tests. As possible confounding factors, locomotion (open field/OF), anxiety-like behavior (elevated plus maze/EPM), and short-term memory (Y-maze) were also evaluated. Stimulation of the GABAergic cells in MRR had no effect on locomotion or working and social memory; however, it increased social interest during sociability and social interaction but not in resident-intruder tests. Accordingly, c-Fos elevation in MRR-GABAergic cells was detected after sociability, but not resident-intruder tests. In the EPM test, the inhibitory group entered into the open arms later, suggesting an anxiogenic-like tendency. We confirmed the role of MRR-GABAergic cells in promoting social interest. However, different subpopulations (e.g. long vs short projecting, various neuropeptide containing) might have divergent roles, which might remain hidden and requires further studies.

Structural and functional alterations in the prefrontal cortex after post-weaning social isolation: relationship with species-typical and deviant aggression.

Although the inhibitory control of aggression by the prefrontal cortex (PFC) is the cornerstone of current theories of aggression control, a number of human and laboratory studies showed that the execution of aggression increases PFC activity; moreover, enhanced activation was observed in aggression-related psychopathologies and laboratory models of abnormal aggression. Here, we investigated these apparently contradictory findings in the post-weaning social isolation paradigm (PWSI), an established laboratory model of abnormal aggression. When studied in the resident-intruder test as adults, rats submitted to PWSI showed increased attack counts, increased share of bites directed towards vulnerable body parts of opponents (head, throat, and belly) and reduced social signaling of attacks. These deviations from species-typical behavioral characteristics were associated with a specific reduction in the thickness of the right medial PFC (mPFC), a bilateral decrease in dendritic and glial density, and reduced vascularization on the right-hand side of the mPFC. Thus, the early stressor interfered with mPFC development. Despite these structural deficits, aggressive encounters enhanced the activation of the mPFC in PWSI rats as compared to controls. A voxel-like functional analysis revealed that overactivation was restricted to a circumscribed sub-region, which contributed to the activation of hypothalamic centers involved in the initiation of biting attacks as shown by structural equation modeling. These findings demonstrate that structural alterations and functional hyperactivity can coexist in the mPFC of rats exposed to early stressors, and suggest that the role of the mPFC in aggression control is more complex than suggested by the inhibitory control theory.

Proinflammatory adhesion molecules facilitate polychlorinated biphenyl-mediated enhancement of brain metastasis formation.

Polychlorinated biphenyls (PCBs) are environmental toxicants that cause vascular inflammation and facilitate the development of brain metastases. The crucial event in metastasis formation is adhesion of blood-borne tumor cells to the vascular endothelium, followed by their transcapillary migration. The aim of the present study was to examine the mechanisms of PCB118-induced brain metastasis formation at the blood-brain barrier level with the focus on tumor cell adhesion to the brain endothelium. PCB118 was administered orally to wild-type or intercellular cell adhesion molecule-1 (ICAM-1)-deficient mice, followed by an injection of Lewis lung carcinoma cells into the carotid artery. Treatment with PCB118 resulted in enhanced development of brain metastases. Injection of tumor cells induced overexpression of ICAM-1 and vascular endothelial cell adhesion molecule-1 (VCAM-1) in brain endothelium that was further potentiated in mice exposed to PCB118. PCB118 did not affect the number of adhered and extravasated tumor cells in ICAM-1-deficient mice. Additional in vitro studies indicated that VCAM-1-neutralizing antibody protected against PCB118-induced adhesion of tumor cells to cultured brain endothelial cells. These results indicate that exposure to selected PCB congeners, such as PCB118, induces adhesion and transcapillary migration of tumor cells. This process is facilitated by proinflammatory adhesion molecules and results in potentiation of brain metastasis formation.