Role of Adenosine A1 Receptor in Sleep Deprivation-Induced Neuroinflammation: Insights on Rapid Eye Movement Sleep and Fear Extinction Memory Recall in Rats.

Introduction: Sleep deprivation (SD), stemming from a myriad of aetiologies, is a prevalent health condition frequently overlooked. It typically impairs memory consolidation and synaptic plasticity, potentially through neuroinflammatory mechanisms and adenosinergic signalling. It is still unclear whether the adenosine A1 receptor (A1R) modulates SD-induced neurological deficits in the hippocampus.

Objectives: This study aims to evaluate the effects of SD on fear extinction memory recall and emotional behaviour in male Sprague Dawley rats; to investigate the role of A1R antagonism by the administration of 8-cyclopentyltheophylline (8-CPT), an A1R antagonist during 48-hour SD in mitigating neuroinflammation and synaptic plasticity deficits induced by SD; and to assess changes in hippocampal neurogenesis, neuronal cell death, and sleep architecture in response to A1R antagonism during SD.

Methods: A total of 39 animals were used in the study, and they were divided into three experimental groups: 1) cage control (CC; n = 13); 2) SD for 48 hours (SD; n = 13); 3) SD for 48 hours+ 8-CPT (20 mg/kg/day in 20% DMSO divided into two doses, morning and evening, i.p.; n = 13). 'n' refers to the sample size/number of animals in each group. Rats were subjected to SD after cued fear extinction training for 48 hours followed by fear extinction memory recall test, anxious-depressive-like behaviours by open field test (OFT), sucrose preference test, and forced swim test (FST). Levels of adenosine in the hippocampus were quantified by high-performance liquid chromatography. Protein levels of interleukin-6 (IL-6) and IL-10 were quantified by enzyme-linked immunosorbent assay (ELISA). Expression levels of proteins and genes of interest were analysed using immunohistochemistry and real-time polymerase chain reaction (RT-PCR), respectively. Sleep architecture was assessed by recording electroencephalography (EEG), electromyography, and electrooculography from rats.

Results: Administration of CPT during SD reversed extinction recall impairments (p = 0.01), improved line crossings in OFT, sucrose preference (p < 0.01), and reduced immobility during the FST (p < 0.01). Immunohistochemical analysis of DG, CA3, and CA1 regions of the hippocampus revealed a significant upregulation of A1R expression in the SD and SD+CPT groups (p < 0.001, n = 5). Expression of post-synaptic density protein (PSD-95) and synaptophysin increased and a marked reduction in the Toll-like receptor-4 (TLR-4) expression in activated microglia in the SD+CPT group. 8-CPT partially restored SD-induced decline in serotonin and brain-derived neurotrophic factor. SD-induced neuronal apoptosis through caspase-3 and the P-p38 mitogen-activated protein kinase pathway was partially reversed by 8-CPT. RT-PCR results showed that A1R antagonism attenuated gene expression of pro-inflammatory cytokines (IL-1β, TNFα, p-NFκB s536, and IL-6) and increased anti-inflammatory cytokines (IL-1ra, IL-4, IL-10, IL-11, and IL-13) during SD. EEG recordings revealed that A1R antagonism increased REM sleep without affecting non-REM sleep during SD, leaving rebound sleep unaffected.  Conclusion: These findings highlight the role of A1R antagonism in restoring fear extinction memory recall, synaptic plasticity, adult neurogenesis, neuronal cell death, and attenuating neuroinflammation during SD, paving the way for the further exploration of its therapeutic potential in sleep-related cognitive deficits.