Functional perturbation of forebrain principal neurons reveals differential effects in novel and well-learned tasks

Propionic acid ameliorates cognitive function through immunomodulatory effects on Th17 cells in perioperative neurocognitive disorders

Background

Elderly patients undergoing surgery are prone to cognitive decline known as perioperative neurocognitive disorders (PND). Several studies have shown that the microglial activation and the decrease of short-chain fatty acids (SCFAs) in gut induced by surgery may be related to the pathogenesis of PND. The purpose of this study was to determine whether microglia and short-chain fatty acids were involved in cognitive dysfunction in aged rats.

Methods

Male wild-type Wistar rats aged 11-12 months were randomly divided into control group (Ctrl: Veh group), propionic acid group (Ctrl: PA group), exploratory laparotomy group (LP: Veh group) and propionic acid + exploratory laparotomy group (LP: PA group) according to whether exploratory laparotomy (LP) or PA pretreatment for 21 days was performed. The motor ability of the rats was evaluated by open field test on postoperative day 3 (POD3), and then the cognitive function was evaluated by Y-maze test and fear conditioning test. The expression of IL-1β, IL-6, RORγt and IL-17A mRNA in hippocampus was detected by RT-qPCR, the expression of IL-17A and IL-17RA in hippocampus was detected by Western blot, and the activation of microglia was detected by immunofluorescence.

Results

The PND rat model was successfully established by laparotomy. Compared with Ctrl: Veh group, the body weight of LP: Veh group decreased, the percentage of spontaneous alternations in Y maze decreased (P < 0.001), and the percentage of freezing time in contextual fear test decreased (P < 0.001). Surgery triggers neuroinflammation, manifested as the elevated levels of the inflammatory cytokines IL-1β (P < 0.001) and IL-6 (P < 0.001), the increased expression of the transcription factor RORγt (P = 0.0181, POD1; P = 0.0073, POD5)and major inflammatory cytokines IL-17A (P = 0.0215, POD1; P = 0.0071, POD5), and the increased average fluorescence intensity of Iba1 (P < 0.001, POD1; P < 0.001, POD5). After PA preconditioning, the recovery of rats in LP: PA group was faster than that in LP: Veh group as the body weight lost on POD1 (P = 0.0148) was close to the baseline level on POD5 (P = 0.1846), and they performed better in behavioral tests. The levels of IL-1β (P < 0.001) and IL-6 (P = 0.0035) inflammatory factors in hippocampus decreased on POD1 and the average fluorescence intensity of Iba1 decreased (P = 0.0024, POD1; P < 0.001, POD5), representing the neuroinflammation was significantly improved. Besides, the levels of RORγt mRNA (P = 0.0231, POD1; P = 0.0251, POD5) and IL-17A mRNA (P = 0.0208, POD1; P = 0.0071, POD5) in hippocampus as well as the expression of IL-17A (P = 0.0057, POD1; P < 0.001, POD5) and IL-17RA (P = 0.0388) decreased.

Conclusion

PA pretreatment results in reduced postoperative neuroinflammation and improved cognitive function, potentially attributed to the regulatory effects of PA on Th17-mediated immune responses.

The dysfunction of parvalbumin interneurons mediated by microglia contributes to cognitive impairment induced by lipopolysaccharide challenge

BACKGROUND

The mechanisms underlying cognitive impairments induced by systemic inflammation remain unclear. Increasing evidence has suggested that parvalbumin (PV) interneurons play an important role in regulating cognitive behaviors and its dysfunction is implicated in many neurological disorders. Thus, the present study was aimed to detect whether the destruction of PV interneurons mediates cognitive impairment associated with systemic inflammation.

METHODS

Male wild-type C57BL/6J mice (12-14 weeks old) received lipopolysaccharide (LPS 2 mg/kg i.p.) injection to establish the systemic inflammation model. For the suppression of microglial activation, minocycline (50 mg/kg i.p.) was applied. Animal behavior tests were conducted on day 3 post-LPS injection including the open field test, fear conditioning test and Y maze test. The PV expression in hippocampus was detected by Western blot and immunofluorescence. The number of perisomatic boutons around the NeuN-positive cells and microglia in hippocampus was detected by immunofluorescence.

RESULTS

LPS induced hippocampus-dependent memory and working memory impairment, coinciding with decreased PV expression, reduced perisomatic boutons around the NeuN-positive cells and activated microglia in the hippocampus. Notably, the treatment of minocycline suppressed the microglial activation and rescued the PV expression as well as the perisomatic boutons around the NeuN-positive cells in the hippocampus, contributing to improved cognitive function.

CONCLUSION

Our study suggests that the dysfunction of parvalbumin interneurons mediated by microglia plays a key role in LPS-induced cognitive impairments, which may serve a therapeutic strategy for cognitive disorders associated with systemic inflammation.

The puzzle of spontaneous alternation and inhibition of return: How they might fit together

Two isolated spatial phenomena share a similar "been there; done that" effect on spatial behavior. Originally discovered in rodent learning experiments, spontaneous alternation is a tendency for the organism to visit a different arm in a T-maze on subsequent trials. Originally discovered in human studies of attention, inhibition of return is a tendency for the organism to orient away from a previously attended location. Whereas spontaneous alternation was identified by O'Keefe & Nadel as dependent on an intact hippocampus, inhibition of return is dependent on neural structures that participate in oculomotor control (the superior colliculus, parietal and frontal cortex). Despite the isolated literatures, each phenomenon has been assumed to reflect a basic novelty-seeking process, avoiding places previously visited or locations attended. In this commentary, we explore and compare the behavioral manifestations and neural underpinnings of these two phenomena, and suggest what is still needed to determine whether they operate in parallel or serial.