Effect of Painful Stimuli on PVNCRH Neurons: Implications for States of Consciousness Under Isoflurane Anesthesia

BACKGROUND: Many patients undergoing surgery experience accompanying pain symptoms preoperatively. The impact of painful stimuli on general anesthesia remains largely unknown. Corticotrophin-releasing hormone neurons in the paraventricular nucleus of the hypothalamus (PVN CRH neurons) are crucial central stress hubs that respond to painful stimuli. These neurons also participate in regulating processes such as sleep and anesthesia. Natural reward can inhibit PVN CRH neurons to relieve stress-induced behavioral changes, but the effect of natural reward on the anesthesia process in patients with pain is not clear. In this study, we assessed the impact of painful stimuli on isoflurane anesthesia and its potential neural mechanism. We also investigated the potential of natural reward therapy for alleviating the impact of painful stimuli on isoflurane anesthesia. METHODS: The righting reflex test and cortical electroencephalography (EEG) were used as measures of consciousness in complete Freund's adjuvant (CFA)-injected mice during isoflurane anesthesia. EEG and burst-suppression ratios (BSR) were used to assess the depth of anesthesia. The expression of c-Fos, fiber photometry recording, and brain slice electrophysiology were used to determine neuronal activity changes in PVN CRH neurons after CFA injection or 10% sucrose treatment. Finally, chemogenetic technology was used to specifically manipulate PVN CRH neurons. RESULTS: Compared to the saline-injected mice, the CFA-injected mice exhibited an increased the mean[SD] induction time of isoflurane anesthesia (354[48] s vs 258[30] s, P = .0001) and a reduced BSR of isoflurane anesthesia (60.1[10.3] % vs 81.5[9.76] %, P = .002). CFA injection increased PVN c-Fos expression (3667[706] vs 1735[407], P = .0002) and enhanced the population activity of PVN CRH neurons (33.4[13.6] % vs 1.23[3.57] %, P = .0009). Chemogenetic suppression of PVN CRH neurons reversed the anesthesia abnormalities in CFA-injected mice. Natural reward accelerated the induction time of isoflurane anesthesia (252[24] s vs 324[36] s, P = .003) and increased the BSR of isoflurane anesthesia (84.8[5.36] % vs 57.7[14.3] %, P = .0005). Chemogenetic activation of PVN CRH neurons reversed the effect of natural reward on isoflurane anesthesia in CFA-injected mice. CONCLUSIONS: Painful stimuli affect the process of isoflurane anesthesia by activating PVN CRH neurons, which implies that these neurons modulate isoflurane anesthesia. Additionally, natural reward alleviates the impact of painful stimuli on isoflurane anesthesia by inhibiting PVN CRH neurons.