COMPARATIVE ANALYSIS OF NEURONAL ACTIVITY IN FIRST- AND HIGHER-ORDER THALAMIC NUCLEI ACROSS MULTIPLE ANESTHETIC AGENTS

General anesthesia is a reversible state characterized by loss of consciousness, analgesia, amnesia, and immobility. Although key molecular targets of anesthetics are known, the cellular and circuit mechanisms underlying their effects remain poorly understood. Here, we investigated how different anesthetic agents shape thalamic neuronal activity in vivo. Using high-density Neuropixels probes, we simultaneously recorded spontaneous single-unit activity from first- and higher-order thalamic nuclei in the rat somatosensory system. Recordings were performed under four anesthetics: ketamine/xylazine (KX), fentanyl/medetomidine/midazolam (FMM), urethane, and isoflurane. We compared multiple features of neuronal activity across thalamic nuclei and anesthetics, including firing rate, burst frequency, burstiness, intraburst frequency, and intraburst spike count. In addition, we analyzed the phase relationship between thalamic firing and anesthetic-induced slow oscillations. Distinct and anesthetic-specific thalamic activity patterns were observed. KX induced highly synchronized activity with coordinated population dynamics and propagation of up-states across nuclei, while FMM produced similar but less synchronized patterns. Isoflurane (1.5%) strongly suppressed thalamic activity, with only sparse neuronal firing. Urethane generated two distinct activity states resembling natural non-REM–like synchronized and REM-like desynchronized thalamocortical activity. We also found marked differences in the number of active neurons, firing rate distributions and burst properties of thalamic neurons across anesthetics. Isoflurane yielded the fewest active neurons, whereas urethane and FMM showed the highest. Firing rate distributions differed systematically between anesthetic agents, reflecting distinct modes of thalamic network engagement. Together, these findings demonstrate that different anesthetics induce qualitatively distinct thalamic activity patterns, providing insight into the network mechanisms underlying general anesthesia.