Central alpha-2 adrenergic mechanisms affect human sympathetic neuronal discharge strategies

The central neural mechanisms affecting sympathetic neuronal discharge strategies supporting homeostatic adjustments in human muscle sympathetic nerve activity (MSNA) remain unclear. This study investigated the hypothesis that central alpha-2 adrenergic mechanisms affect sympathetic action potential (AP) discharge, recruitment, and latency patterns within human MSNA (peroneal nerve microneurography and continuous wavelet transform). An alpha-2 adrenergic agonist (dexmedetomidine) was infused in eight healthy male and female participants (5 females; 28 ± 7years). Data were recorded during a 5-minute resting control condition, a 10-minute intravenous dexmedetomidine loading dose (0.225 μg/kg), and a dexmedetomidine maintenance dose (~0.1 μg/kg/hr). Data are reported as mean (SD) for the 5-minute control condition and 2-minutes of the dexmedetomidine infusion when MSNA bursting activity was demonstrably reduced. Two-tailed paired t-tests were performed. Dexmedetomidine reduced mean pressure (92 ± 7 to 80 ± 8 mmHg; P < 0.01) but did not affect heart rate (61 ± 13 to 60 ± 14 bpm; P = 0.75). Dexmedetomidine reduced integrated MSNA burst frequency (14 ± 6 to 4 ± 3 bursts/min; P < 0.01), sympathetic AP frequency (75 ± 46 to 16 ± 14 AP/min; P < 0.01), APs per burst (5 ± 1 to 4 ± 1 AP/burst; P = 0.01), AP clusters per burst (3.5 ± 0.7 to 3.0 ± 0.7 clusters/burst; P = 0.03), and de-recruited the largest AP clusters (12 ± 3 to 7 ± 2 AP clusters; P < 0.01). Despite de-recruiting large AP clusters with short conduction times, dexmedetomidine elicited shorter sympathetic AP conduction times (1.18 ± 0.12 to 1.14 ± 0.13 s; P = 0.03). Six participants performed a Valsalva Maneuver (20 s, 40 mmHg mouth pressure) during the control condition and during the dexmedetomidine maintenance dose. Data are reported as mean (SD) change (Δ) from the 2-minute period preceding the Valsalva Maneuver to the Valsalva Maneuver for each condition. During the control condition, Valsalva Maneuver increased AP frequency (Δ 311 ± 317 AP/min; P = 0.048), recruited larger previously silent AP clusters (Δ 2 ± 1 AP clusters; P < 0.01), and elicited shorter AP conduction times (Δ ‑0.09 ± 0.07 s; P = 0.02). Compared to the control condition, Valsalva Maneuver performed during the dexmedetomidine maintenance dose condition elicited lesser increases in AP frequency (Δ 116 ± 189 AP/min; P vs. control = 0.02) and lesser recruitment of larger previously silent AP clusters (Δ 0 ± 1 AP clusters; P vs. control = 0.02). Dexmedetomidine did not affect the latency reduction elicited by Valsalva Maneuver (Δ ‑0.07 ± 0.14 s; P vs. control = 0.61). These data suggest that human central alpha-2 adrenergic mechanisms affect: i) sympathetic postganglionic neuronal discharge and recruitment patterns, and ii) synaptic delays within the sympathetic neurocircuitry influencing AP latency. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the National Institutes of Health (NIH). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.