• Eric D Melonakos, Morgan J Siegmann, Charles Rey, Christopher O'Brien, Ksenia K Nikolaeva, Ken Solt, and Christa J Nehs.
• From the Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts; the Department of Anaesthesia, Harvard Medical School, Boston, Massachusetts; the Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts.
• Anesthesiology. 2021 Oct 1; 135 (4): 633-648.

BackgroundParabrachial nucleus excitation reduces cortical delta oscillation (0.5 to 4 Hz) power and recovery time associated with anesthetics that enhance γ-aminobutyric acid type A receptor action. The effects of parabrachial nucleus excitation on anesthetics with other molecular targets, such as dexmedetomidine and ketamine, remain unknown. The hypothesis was that parabrachial nucleus excitation would cause arousal during dexmedetomidine and ketamine anesthesia.MethodsDesigner Receptors Exclusively Activated by Designer Drugs were used to excite calcium/calmodulin-dependent protein kinase 2α-positive neurons in the parabrachial nucleus region of adult male rats without anesthesia (nine rats), with dexmedetomidine (low dose: 0.3 µg · kg-1 · min-1 for 45 min, eight rats; high dose: 4.5 µg · kg-1 · min-1 for 10 min, seven rats), or with ketamine (low dose: 2 mg · kg-1 · min-1 for 30 min, seven rats; high dose: 4 mg · kg-1 · min-1 for 15 min, eight rats). For control experiments (same rats and treatments), the Designer Receptors Exclusively Activated by Designer Drugs were not excited. The electroencephalogram and anesthesia recovery times were recorded and analyzed.ResultsParabrachial nucleus excitation reduced delta power in the prefrontal electroencephalogram with low-dose dexmedetomidine for the 150-min analyzed period, excepting two brief periods (peak median bootstrapped difference [clozapine-N-oxide - saline] during dexmedetomidine infusion = -6.06 [99% CI = -12.36 to -1.48] dB, P = 0.007). However, parabrachial nucleus excitation was less effective at reducing delta power with high-dose dexmedetomidine and low- and high-dose ketamine (peak median bootstrapped differences during high-dose [dexmedetomidine, ketamine] infusions = [-1.93, -0.87] dB, 99% CI = [-4.16 to -0.56, -1.62 to -0.18] dB, P = [0.006, 0.019]; low-dose ketamine had no statistically significant decreases during the infusion). Recovery time differences with parabrachial nucleus excitation were not statistically significant for dexmedetomidine (median difference for [low, high] dose = [1.63, 11.01] min, 95% CI = [-20.06 to 14.14, -20.84 to 23.67] min, P = [0.945, 0.297]) nor low-dose ketamine (median difference = 12.82 [95% CI: -3.20 to 39.58] min, P = 0.109) but were significantly longer for high-dose ketamine (median difference = 11.38 [95% CI: 1.81 to 24.67] min, P = 0.016).ConclusionsThese results suggest that the effectiveness of parabrachial nucleus excitation to change the neurophysiologic and behavioral effects of anesthesia depends on the anesthetic's molecular target.Editor’s PerspectiveCopyright © 2021, the American Society of Anesthesiologists. All Rights Reserved.

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