Brain immune cells characterization in UCMS exposed P2X7 knock-out mouse.

BACKGROUND: Several lines of evidence suggest that neuroinflammation might be a key neurobiological mechanism of depression. In particular, the P2X7 receptor (P2X7R), an ATP-gated ion channel involved in activation of the pro-inflammatory interleukin IL-1ß, has been shown to be a potential new pharmacological target in depression. The aim of this study was to explore the impact of unpredictable chronic mild stress (UCMS) on behavioural changes, hippocampal neurogenesis, and cellular characterisation of brain immune cells, in P2X7R Knock-Out (KO) mice. METHODS: P2X7R KO and wild-type (WT) mice were subjected to a 6-week UCMS protocol and received a conventional oral antidepressant (15 mg.kg-1 fluoxetine) or water per os. The mice then underwent behavioural tests consisting of the tail suspension test (TST), the elevated plus maze (EPM) test, the open field test, the splash test and the nest building test (week 7). Doublecortin immunostaining (DCX) of brain slices was used to assess neurogenesis in the dentate gyrus. Iba1 and TMEM119 immunostaining was used to characterise brain immune cells, Iba1 as a macrophage marker (including microglial cells) and TMEM119 as a potential specific resident microglial cells marker. RESULTS: After a 6-week UCMS exposure, P2X7R KO mice exhibited less deterioration of their coat state, spent a significantly smaller amount of time immobile in the TST and spent a larger amount of time in the open arms of the EPM. As expected, adult ventral hippocampal neurogenesis was significantly decreased by UCMS in WT mice, while P2X7R KO mice maintained ventral hippocampal neurogenesis at similar levels in both control and UCMS conditions. In stress-related brain regions, P2X7R KO mice also exhibited less recruitment of Iba1+/TMEM119+ and Iba1+/TMEM119- cells in the brain. The ratio between these two staining patterns revealed that brain immune cells were mostly composed of Iba1+/TMEM119+ cells (87 to 99%), and this ratio was affected neither by P2X7R genetic depletion nor by antidepressant treatment. DISCUSSION: Behavioural patterns, neurogenesis levels and density of brain immune cells in P2X7R KO mice after exposure to UCMS significantly differed from control conditions. Brain immune cells were mostly increased in brain regions known to be sensitive to UCMS exposure in WT but not in P2X7R KO mice. Considering Iba1+/TMEM119- staining might characterize peripheral immune cells, the ratio between Iba1+/TMEM119+ cells and IBA1+/TMEM119- cells, suggests that the rate of peripheral immune cells recruitment may not be modified neither by P2X7R gene expression nor by antidepressant treatment.

Neuroinflammation and depression: A review.

Some recent clinical and preclinical evidence suggests that neuroinflammation is a key factor that interacts with the three neurobiological correlates of major depressive disorder: depletion of brain serotonin, dysregulation of the hypothalamus-pituitary-adrenal (HPA) axis and alteration of the continuous production of adult-generated neurons in the dentate gyrus of the hippocampus. This review discusses the main players in brain immunity as well as how inflammation interacts with the above three mechanisms. It is reported that kynurenine (KYN) pathway alteration in favour of its excitotoxic component and HPA axis dysregulation have the common effect of increasing extracellular glutamate levels and glutamate neurotransmission, which can impact hippocampal neurogenesis. This pathophysiological cascade appears to be triggered or sustained and reinforced by any chronic inflammatory condition involving increased circulating markers of inflammation that are able to cross the blood-brain barrier and activate microglia; it can also be the consequence of primary brain neuroinflammation, such as in neurodegenerative disorders with early manifestations that are frequently depressive symptoms. Further recent data indicate that primary microglial activation may also result from a direct impact of chronic stress on vascular function. The intricated dynamic crosstalk between neuroinflammation and other relevant neurobiological correlates of depression add to evidence that neuroinflammation may be a key therapeutic target for future therapeutic strategies in major depressive disorder.

Prefrontal cortex rTMS reverses behavioral impairments and differentially activates c-Fos in a mouse model of post-traumatic stress disorder.

BACKGROUND: Post-traumatic stress disorder (PTSD) is a severe mental illness correlated with alterations in fear extinction neurocircuits that involve prefrontal, amygdala and hippocampal structures. Current treatments indirectly restore prefrontal control of fear responses, but still cannot achieve full remission in all patients. OBJECTIVE/HYPOTHESIS: Repetitive TMS (rTMS) can directly and chronically act on subparts of the prefrontal cortex (PFC) as a potential alternative treatment. However, preclinical studies are needed to further the comprehension of its mechanisms and thus enhance its efficacy. METHODS: A 40-mm coil is used on a stereotaxic frame to apply 12-Hz high-intensity rTMS of the ventromedial PFC (vmPFC) in a foot-shock mouse model of PTSD. Chronic rTMS treatment was applied 7 days after the shocks every day up to day 12 (5 sessions, 3750 pulses). RESULTS: One session of rTMS (750 pulses) was able to precisely evoke immediate c-Fos activity in an area of the vmPFC (0.5 mm2) in preliminary control mice. When used in the foot-shock model, chronic rTMS treatment (n = 19) counteracted short-term episodic memory deficits at day 18, and enhanced extinction dynamics when reexposed to the shocking chamber at day 22. Associated c-Fos activity was found increased in the rodent's vmPFC (infralimbic cortex), the basolateral amygdala and the ventral CA1 (hippocampal output). CONCLUSIONS: This study is the first to use prefrontal cortex rTMS in a mouse model of PTSD. Chronic rTMS of the vmPFC reversed stress-induced behavioral impairments and acted on distributed networks of fear extinction up to 10 days after treatment.