The spatiotemporal expression changes of CB2R in the hippocampus of rats following pilocarpine-induced status epilepticus

Objective: To investigate the spatiotemporal expression of cannabinoid receptor type 2 (CB2R) in the hippocampus of pilocarpine-treated rats experiencing a status epilepticus (SE). Methods: A total of 112 male Sprague-Dawley rats at three weeks of age, weighing 43 g-68.5 g, were used for this study. Rats were randomly divided into a control group (n = 16) and an epilepsy a SE group. The epilepsy SE group was further divided into 6 sub-groups (2 h, 1, 3, 7, 14, and 21 days after status epilepticus (SE)). A SE model was induced with lithium chloride and pilocarpine by intraperitoneal injection. Hematoxylin-Eosin staining (H&E staining) and TdT-mediated dUTP nick end labeling (TUNEL staining) were used to observe neuronal necrosis and apoptosis pyroptosis in the hippocampus. Double-label immunofluorescence and Western blotting were used to detect the distribution and expression of CB2R in the rat hippocampus exposed to SE. Results: According to the Racine scale, nearly 15% of the pilocarpine-induced rats showed Racine stages 1 and 2 in the epilepsy group, almost 74% of the rats showed Racine stages 3 to 5, and 11% of the rats died in the SE or post SE. Morphologically, compared with control group, the number of neurons remarkably decreased while apoptotic pyroptotic neurons in number significantly increased in the CA1, CA3 and DG regions from 2 h to 3 days post SE by H&E and TUNEL staining, respectively (p < 0.05). Double immunofluorescence staining revealed that CB2R was mainly expressed in the CA1, CA3 and DG neuronal regions of the hippocampus and the number of CB2R-positive neurons significantly increased within these regions post SE as compared with the control group, particularly in the DG region. Furthermore, the protein expression of CB2R began to be elevated from 2 h post SE (p < 0.05) and peaked at 1, 3 and 7 days (p < 0.01), respectively peaked at 1 day and high levels were maintained at 3 and 7 days (p < 0.01 for all of them). After that, the protein expression of CB2R gradually declined with by time interval. Conclusions: We demonstrate that CB2R is expressed in hippocampal neurons and time-dependently expressed changed post SE, suggesting that the role of CB2R is mainly through hippocampal neurons at the early stage of pilocarpine-induced SE. CB2R may play a role at the early stage of pilocarpine-induced SE and its role is mainly through the hippocampal neurons.