Repeated THC Exposure and Respiration after Subacute Cervical Spinal Cord Injury

Rebecca Harbert, MDS
University of Florida

Co-authors: Rebecca A. Harbert¹, Aaron A. Jones¹, Alyssa Mickle¹, Gabriel Lopez¹, Barry Setlow¹, Erica A. Dale^1
1University of Florida

Background: Cervical spinal cord injury (cSCI) disrupts descending respiratory drive and reduces ventilatory capacity (Berlowitz et al., 2016), among other quality of life altering sequelae. A quarter of individuals with SCI report using cannabis for sensorimotor function, mood, and pain (Acland, 2018). Cannabinoids signal through CB1 and CB2 receptors and can modulate serotonergic signaling (Haj-Dahmane and Shen, 2011; Haynes et al., 2023). Spinal serotonergic signaling contributes to respiratory motor plasticity (Baker-Herman and Mitchell, 2002), and cSCI alone upregulates ventral spinal 5-HT₂A receptors (Fuller et al., 2005). 

Objectives: In rats with cSCI, we hypothesized that THC would further increase spinal 5-HT and 5-HT₂A and, given their role in some forms of respiratory neuroplasticity (Dale-Nagle et al., 2010), we tested whether this: 1) occurs near the phrenic motor nucleus and 2) improves ventilatory capacity.

Methods: Adult male Sprague–Dawley rats (n = 5/group) received a C2 hemisection (C2HS) and then ∆⁹-tetrahydrocannabinol (THC; 50 mg/mL) or vehicle vapor on days 7, 9, and 11 post-C2HS. Plethysmography was conducted before and after exposure under room air and hypercapnic–hypoxic conditions. C4 sections were immunolabeled for 5-HT or processed using multiplex RNAscope for ChAT and 5-HT₂A transcripts around cholera toxin B-labeled phrenic motor neurons. 

Results: THC-treated rats exhibited greater 5-HT immunoreactivity surrounding contralesional C4 phrenic motor neurons (est dif = +0.505, 95% CI [0.180, 0.830], p = 0.011), with no ipsilesional or rostrocaudal differences. Additionally, there were more ChAT-positive motor neurons expressing 5-HT₂A ipsilesionally (95% CI [11.10, 82.39], p = 0.013), contralesionally (95% CI [8.30, 79.59], p = 0.0155), rostrally (95% CI [13.37, 87.88], p = 0.0057), in the middle (95% CI [14.22, 88.74], p = 0.0049), but not caudally (95% CI [−1.81, 72.70], p = 0.0657). However, contrary to our hypothesis, repeated THC exposure did not improve tidal volume or minute ventilation (both p>0.05) but reduced breathing frequency 11d post-C2HS (DID ≈ -22.2, 95% CI [-36.8, -7.7], p = 0.003), prolonged inspiratory time on day 9 (room air: + 39.0 msec, 95% CI [7.6, 70.4], p = 0.0155; hypercapnic–hypoxia: +39.7 msec, 95% CI [10.3, 69.2]) and day 11 (Room Air: + 56.1 msec, 95% CI [26.6, 85.6], FDR p = 0.0003) (Harbert et al., 2026).

Conclusion: Repeated THC increased cervical spinal 5-HT and 5-HT₂A expression without improving tidal volume, suggesting that these adaptations alone are insufficient to enhance spinal respiratory plasticity, within this timeframe and dose, after cSCI.