Adenosine A1 Receptor Activation as a Brake on the Microglial Response after Experimental Traumatic Brain Injury in Mice

Abstract

We reported that adenosine A1 receptor (A1AR) knockout (KO) mice develop lethal status epilepticus after experimental traumatic brain injury (TBI), which is not seen in wild-type (WT) mice. Studies in epilepsy, multiple sclerosis, and neuro-oncology suggest enhanced neuro-inflammation and/or neuronal death in A1AR KO. We hypothesized that A1AR deficiency exacerbates the microglial response and neuronal damage after TBI. A1AR KO and WT littermates were subjected to mild controlled cortical impact (3 m/sec; 0.5 mm depth) to left parietal cortex, an injury level below the acute seizure threshold in the KO. At 24 h or 7 days, mice were sacrificed and serial sections prepared. Iba-1 immunostaining was used to quantify microglia at 7 days. To assess neuronal injury, sections were stained with Fluoro-Jade C (FJC) at 24 h to evaluate neuronal death in the hippocampus and cresyl violet staining at 7 days to analyze cortical lesion volumes. We also studied the effects of adenosine receptor agonists and antagonists on 3H-thymidine uptake (proliferation index) by BV-2 cells (immortalized mouse microglial). There was no neuronal death in CA1 or CA3 quantified by FJC. A1AR KO mice exhibited enhanced microglial response; specifically, Iba-1 + microglia were increased 20–50% more in A1AR KO versus WT in ipsilateral cortex, CA3, and thalamus, and contralateral cortex, CA1, and thalamus (p < 0.05). However, contusion and cortical volumes did not differ between KO and WT. Pharmacological studies in cultured BV-2 cells indicated that A1AR activation inhibits microglial proliferation. A1AR activation is an endogenous inhibitor of the microglial response to TBI, likely via inhibition of proliferation, and this may represent a therapeutic avenue to modulate microglia after TBI