CNS O2 toxicity and Seizure Genesis: Hyperbaric radio telemetric recordings of activity in the motor cortex and dorsal medulla oblongata or cerebellum during exposure to hyperbaric O2 ± CO2 in freely behaving Sprague‐Dawley rats

Abstract

We have adapted a radio telemetry method developed in mice (Weiergräber et al. 2005, Brain Res Protocols 14:154‐164; and Papazoglou et al. 2016, jove doi:10.3791/54216) for use in unrestrained rats during exposure to hyperbaric atmospheres, including hyperoxia (HBO2), hypercapnic hyperoxia (HBO2CO2), and Heliox (HBHeO2). We used this method to test the hypothesis that seizures (Sz) induced during exposure to HBO2 ± CO2 (CNS oxygen toxicity, CNS‐OT) originate in subcortical areas before onset of ictal activity in the motor cortex and visible behavioral Sz. A 4ET 4‐channel telemetry module plus battery (DSI) were implanted in the abdominal cavity of male Sprague‐Dawley rats (284‐396g) under isoflurane anesthesia using sterile surgical methods. Telemetry leads were secured as follows: in the chest wall at the insertion of the diaphragm with the intercostal muscles (rmEMG); in the muscles over the heart (ECG); the 3rd & 4th pairs of leads were tunneled sc towards the head. The 3rd pair of leads was placed caudal to Bregma (motor cortex, ECoG) and the negative lead of the 4th pair was placed over the cerebellum (reference electrode). The deep intracerebral electrode was fashioned from tungsten wire (FHC, Inc.), bent to a 90°angle. Insulation was removed from the horizontal arm of the tungsten wire and the vertical arm was cut to a length required to penetrate the skull, epidural space, and dorsal medulla oblongata (8.5‐9mm, EBulboG) or cerebellum (5‐5.2mm, ECerG). The horizontal arm of the tungsten wire was slid between the insulation and wire of the positive 4th telemetry lead and secured in place with a knotted suture and Vetbond glue. The horizontal arm of the tungsten electrode was held using a bulldog clip attached to the manipulator of a stereotaxic apparatus and lowered into position through a predrilled hole in the skull. All leads were fixed in place using dental cement and the incision closed with wound clips. At least 1 week later, after removing the wound clips, the rat was placed in an animal chamber inside a hyperbaric chamber; the animal chamber was ventilated continuously with air, 100% O2, or 1.75‐2.5% CO2 in O2 and pressed in parallel with the air‐filled hyperbaric chamber (1‐5 ATA). For HBHeO2, 4% O2 in He was used to produce normoxia at 5 ATA. Our initial findings show that ECoG activity remains relatively quiet until onset of generalized Sz. In contrast, activity in the cerebellum (Figs. 1 & 2) and dorsal medulla, once activated, is intermittent and begins during compression, later on during HBO2 exposure, or just prior to activation of ECoG activity and visible motor Sz. During and after Sz, ECoG and ECerG/EBulboG activities appear synchronized (Fig. 2). Exposure to HBO2CO2 accelerates Sz genesis and increases the intensity and duration of the Sz. To identify the position of the deep electrode, the anesthetized rat was perfused with PFA fixative (after removal of the telemetry module). The head and deep electrode were imaged via CT scan to capture the internal skull geometry, which is then co‐registered with rat brain templates (Barriere et al., 2019 SIGMA rat brain atlas; atlases for multimodal MRI data analysis). Our initial findings suggest that subcortical regions of the CNS are activated early on during exposure to HBO2 and presumably function as “ox tox trigger zones” during the early phase of Sz genesis (Ciarlone et al., 2019 Redox Biol., doi:10.1016/j.redox.2019.101159).