Spinal Cord Injury Leads to Reductions in Central Nervous System 5HT Coupled with Blunting of Arousal and Genioglossus Muscle Responses to Hypercapnia in Mice

Abstract

PurposeThe present investigation was designed to examine if spinal cord injury (SCI) leads to reductions in central nervous system serotonin (5HT). The investigation was also designed to investigate if this reduction was coupled to blunting of arousal responses (AR) and genioglossus muscle sensitivity (GS) to hypercapnia.MethodsTelemetry transmitters were surgically implanted in wild type mice (Tph2+/+) (n = 4) and tryptophan hydroxylase 2 knockout mice (Tph2−/−) (n = 4) to measure electroencephalography, genioglossus muscle activity, core body temperature and gross motor activity. Following recovery, the Tph2+/+ and Tph2−/− mice were placed in whole body plethysmographs and exposed to episodes of 7 % carbon dioxide to determine the AR and GS to hypercapnia during non‐rapid eye movement (NREM) sleep. Following these measures a C2 hemisection of the spinal cord was completed and AR and GS were measured subsequently. In addition, high‐performance liquid chromatography was performed to measure 5HT levels in three brainstem sections (i.e. dorsal respiratory group and hypoglossal nucleus; raphe magnus, obscurus and pallidus; raphe median and paramedian) obtained from 4 intact TPH2+/+ mice and 4 TPH2−/− mice fourteen days after SCI.ResultsTotal 5HT levels in the brainstem were reduced following SCI (3.2 ± 0.1 vs. 1.5 ± 0.3 ng/mg; P < 0.001). In particular, the levels of 5HT in the dorsal respiratory group and hypoglossal nucleus (1.2 ± 0.2 vs. 0.4 ± 0.1 ng/mg; P < 0.01) and in the raphe median and paramedian (1.3 ± 0.2 vs. 0.8 ± 0.1 ng/mg; P < 0.08) were reduced following SCI. In conjunction with the reduction in 5HT the AR (Before SCI: 8.3 ± 4.4 vs. After SCI: 33.8 ± 8.7 s; P < 0.01) and the GS (Before SCI: 180 ± 58 vs. After SCI: 33 ± 10 microvolts; P < 0.02) to hypercapnia was blunted following SCI in the Tph2+/+ but not the Tph2−/− mice.ConclusionsSCI in a spontaneously breathing non‐anesthetized sleeping mouse model was accompanied by a reduction in central nervous system 5HT. This reduction could be responsible for the blunted AR and GS to hypercapnia during NREM sleep. Ultimately, depletion of 5HT could result in an increase in apnea frequency and blunting of the AR and GS could lead to increases in apnea duration following SCI.