Activation of the canonical nuclear factor-κB pathway is involved in isoflurane-induced hippocampal interleukin-1β elevation and the resultant cognitive deficits in aged rats

Highlights

• Isoflurane induces hippocampal IL-1β elevation and cognitive deficits in aged rats.

• Isoflurane transiently activates the canonical NF-κB pathway in aged rat hippocampus.

• NF-κB inhibitor mitigates isoflurane-induced IL-1β elevation and cognitive deficits.

• We report a linkage between NF-κB signaling, IL-1β expression, and cognitive changes.

Abstract

Although much recent evidence has demonstrated that neuroinflammation contributes to volatile anesthetic-induced cognitive deficits, there are few existing mechanistic explanations for this inflammatory process. This study was conducted to investigate the effects of the volatile anesthetic isoflurane on canonical nuclear factor (NF)-κB signaling, and to explore its association with hippocampal interleukin (IL)-1β levels and anesthetic-related cognitive changes in aged rats. After a 4-h exposure to 1.5% isoflurane in 20-month-old rats, increases in IκB kinase and IκB phosphorylation, as well as a reduction in the NF-κB inhibitory protein (IκBα), were observed in the hippocampi of isoflurane-exposed rats compared with control rats. These events were accompanied by an increase in NF-κB p65 nuclear translocation at 6 h after isoflurane exposure and hippocampal IL-1β elevation from 1 to 6 h after isoflurane exposure. Nevertheless, no significant neuroglia activation was observed. Pharmacological inhibition of NF-κB activation by pyrrolidine dithiocarbamate markedly suppressed the IL-1β increase and NF-κB signaling, and also mitigated the severity of cognitive deficits in the Morris water maze task. Overall, our results demonstrate that isoflurane-induced cognitive deficits may stem from upregulation of hippocampal IL-1β, partially via activation of the canonical NF-κB pathway, in aged rats.

Introduction

Postoperative cognitive dysfunction (POCD) in the elderly has emerged as a major health concern [1]. It is associated with premature departure from the workforce, increased disability, and early mortality [2]. Unfortunately, the pathophysiology of POCD remains elusive.

The potential risk factors for POCD can be classified into three categories from the patient, the surgery or the anesthesia [1]. Many recent animal studies [3], [4], [5], [6] and clinical observations [7], [8] have supplied evidence that anesthetics, particularly inhalational anesthetics, may play a role in cognitive deficits. Isoflurane, an inhalation anesthetic that is widely used clinically, has been shown to induce POCD through cytokine-dependent neuroinflammatory mechanisms, in which isoflurane increases the production of interleukin 1 (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α in a rat or mouse brain [3], [9]. Nevertheless, the exact signaling mechanisms by which isoflurane mediates increases in these proinflammatory cytokines remain to be elucidated.

Nuclear factor (NF)-κB is one of the most critical transcription factors involved in inflammation [10]. The NF-κB family comprises of RelA/p65, RelB, c-Rel, p50, and p52, RelA/p50 heterodimer being the most abundant and widely expressed [11]. Multiple proinflammatory signals activate NF-κB, mostly through inhibitor of NF-κB protein (IκB) kinase (IKK)-dependent phosphorylation of IκB, leading to IκB ubiquitination and degradation. This ultimately leads to nuclear translocation of NF-κB and induction of transcription of the target genes [11], including the proinflammatory cytokine IL-1β [10]. NF-κB signaling can be regulated by IκBs and IKKs through various routes. The most frequently observed is the canonical pathway, which is characterized by phosphorylation of IκBα on serine residues 32 and 36 and proteasome degradation, and nuclear translocation of RelA/p65 depending on the catalytic subunits IKKα/β [12].

Recent in vivo and in vitro studies have revealed that NF-κB activation may be involved in mediating the neuroinflammation in models of Alzheimer’s disease [13], [14], which has a similar molecular pathological mechanism to POCD. Therefore, the present study aimed to determine whether NF-κB signaling is involved in isoflurane-induced neuroinflammation and cognitive impairment in vivo. Specifically, we hypothesized that isoflurane would induce neuroinflammation through activation of the canonical NF-κB pathway in aged rats.