Effects of low-dose lipopolysaccharide and age on spatial learning in different Morris water maze protocols

Mitochondrial dysfunction precedes hippocampal IL-1β transcription and cognitive impairments after low-dose lipopolysaccharide injection in aged mice

Acute cognitive impairments termed delirium often occur after inflammatory insults in elderly patients. While previous preclinical studies suggest mitochondria as a target for reducing neuroinflammation and cognitive impairments after LPS injection, fewer studies have evaluated the effects of a low-grade systemic inflammation in the aged brain. Thus, to identify the significance of mitochondrial dysfunction after a clinically relevant systemic inflammatory stimulus, we injected old-aged mice (18-20 months) with low-dose lipopolysaccharide (LPS, 0.04 mg/kg). LPS injection reduced mitochondrial respiration in the hippocampus 24 h after injection (respiratory control ratio [RCR], state3u/state4o; control = 2.82 ± 0.19, LPS = 2.57 ± 0.08). However, gene expression of the pro-inflammatory cytokine IL-1β was increased (RT-PCR, control = 1.00 ± 0.30; LPS = 2.01 ± 0.67) at a more delayed time point, 48 h after LPS injection. Such changes were associated with cognitive impairments in the Barnes maze and fear chamber tests. Notably, young mice were unaffected by low-dose LPS, suggesting that mitochondrial dysfunction precedes neuroinflammation and cognitive decline in elderly patients following a low-grade systemic insult. Our findings highlight mitochondria as a potential therapeutic target for reducing delirium in elderly patients.

The effect of acute and chronic formaldehyde exposure on learning and memory in male and female rats

AIM

Formaldehyde is a chemical that lies behind the various systemical failures in organism. Many products that people use contain formaldehyde. Owing to its tissue fixative properties, scientists who work in life sciences are exposed to this substance more than others. Several studies have shown that formaldehyde affects the CA1 and CA3 regions of the hippocampus, which play crucial roles in memory consolidation. In this study, we aimed to investigate anxiety levels and indicate the short and long term effects of formaldehyde and sex-related differences by exposing formaldehyde to male and female rats.

MATERIALS AND METHODS

Formaldehyde (10 mg/kg) was administered intraperitoneally for 7 days for acute exposure and 30 days for chronic exposure. Cognitive assessment was performed using fear conditioning, elevated plus maze, and Morris water maze tests. TUNEL staining was used to identify apoptosis in the brains obtained after decapitation.

RESULTS

Exposure to intraperitoneal formaldehyde does not impair learning and memory in acute and chronic periods and has no effect on depression or anxiety. After acute exposure, apoptosis was observed in the hippocampal CA1 and CA3 regions in males. When the cognitive test results were examined, no differences were found between the experimental and control groups. There was also no significant difference between males and females.

Characterization of Spatial Learning and Sickness Responses in Aging Rats Following Recurrent Lipopolysaccharide Administration

Infections in older individuals can result in cognitive function decline, yet research is limited on how recurrent infections affect cognitive responses. Activation of the immune system results in sickness responses mediated by cytokines. This pilot study examined effects of a model of recurrent infection in aged, male Brown Norway rats on sickness responses, including spatial learning, and cytokine levels. To model initial and recurrent infection, 300 μg/kg lipopolysaccharide (LPS) or saline was administered 1/day for 2 consecutive days during 2 weeks separated by 16 days. Testing occurred for 6 days during each LPS injection week using the Morris water maze, a measure used to evaluate spatial learning. Directional heading error (DHE) and swim time latency served as spatial learning indices. Retention tests and probe trials assessed memory. Plasma cytokine levels were assessed 5 and 24 hr after each LPS injection during Week 2. While food intake and weight decreased significantly in LPS-injected rats compared to controls during Week 1, both displayed increased DHE. Despite exhibiting lessened sickness behaviors during Week 2, experimental animals still displayed spatial learning deficits. Probe trials revealed memory deficits in LPS-injected animals. Interleukin 6 level was higher in the experimental group 5 and 24 hr after LPS injection on Day 1 compared to Day 2 and higher in the experimental compared to the control group at 5 hr on Day 1. Cognitive effects were dissociated from metabolic effects in aged rats, with recurring LPS exposure resulting in persistent cognitive impairment despite decreased sickness responses. Further research with older individuals is warranted.