Experimental Stressors Alter Hypertonic Saline-Evoked Masseter Muscle Pain and Autonomic Response

Aims: To test in a randomized controlled trial, if hypertonic saline (HS)-evoked pain and autonomic function are modulated by either a cold pressor test (CPT) or mental arithmetic stress induced by a paced auditory serial addition task (PASAT). Methods: Fourteen healthy women participated in three sessions. Pain was induced by two 5% HS infusions (5 minutes each, 30 minutes apart) infused into the masseter muscle. During the second HS infusion, pain was modulated by PASAT, CPT, or control (HS alone). HS-evoked pain intensity was scored on a 0 to 10 numeric rating scale (NRS). Heart rate variability (HRV) and hemodynamic measures were recorded noninvasively (Task Force Monitor). Data were analyzed using repeated measurements ANOVAs and Spearman correlation analysis. Results: HS-evoked pain was significantly and similarly reduced by both PASAT (30.8 ± 27.6%; P .001) and CPT (35.8 ± 26.6%; P .001) compared with the control session (9.0 ± 30.5%; P > .05). PASAT and CPT increased the heart rate compared with control (P .001). CPT reduced measures of vagal activity: Root mean square successive difference, high-frequency (HF) power, and coefficient of HF component variance compared with an internal control, ie, the first HS infusion (P .05), while PASAT did not alter any of these HRV measures (P > .05). Conclusion: CPT and PASAT reduced HS-evoked masseter muscle pain and altered the autonomic response. The increase in heart rate following CPT and PASAT may be caused by different mechanisms. CPT reduced measures of efferent cardiac vagal (parasympathetic) activity, while the PASAT-induced increase in heart rate, but unchanged HRV, may suggest neurohumoral activation. Keywords: autonomic nervous system, cold pressor test, experimental muscle pain, mental stress, trigeminal nociception