State-dependent cortical unit activity reflects dynamic brain state transitions in anesthesia

Abstract

How anesthesia affects cortical neuronal spiking and information transfer could help understand the neuronal basis of conscious state. Recent investigations suggest that global state of the anesthetized brain is not stationary but changes spontaneously at a fixed level of anesthetic concentration. How cortical unit activity changes with dynamically transitioning brain states under anesthesia is unclear. We hypothesized that distinct cortical states are characterized by distinct neuronal spike patterns. Extracellular unit activity was measured with sixty-four-channel silicon microelectrode arrays in cortical layers 5/6 of primary visual cortex of chronically instrumented, freely moving male rats (N = 7) during stepwise reduction of the anesthetic desflurane (6, 4, 2, and 0%). Unsupervised machine learning applied to multi-unit spike patterns revealed five distinct brain states of which four occurred at various anesthetic concentrations and shifted spontaneously. In deeper anesthesia states, the number of active units and overall spike rate decreased while the remaining active units showed increased bursting (excitatory neurons), spike timing variability, unit-to-population correlation and unit-to-unit transfer entropy, especially among putative excitatory units, despite the overall decrease in transfer entropy. A novel desynchronized brain state with increased spike timing variability, entropy and electromyographic activity that occurred mostly in deep anesthesia was discovered. These results provide evidence for distinct unit activity patterns associated with spontaneous changes in local cortical brain states at stationary anesthetic conditions. The appearance of a paradoxical, desynchronized brain state in deep anesthesia contends the prevailing view of monotonic dose-dependent anesthetic effects on the brain. SIGNIFICANCE STATEMENTRecent studies suggest that spontaneous changes in brain state occur under anesthesia. However, the spiking behavior of cortical neurons associated with such state changes has not been investigated. We found that local brain states defined by multi-unit activity had non-unitary relationship with the current anesthetic level. A paradoxical brain state displaying asynchronous firing pattern and high electromyographic activity was found unexpectedly at high-dose anesthesia. In contrast, the synchronous fragmentation of neuronal spiking appeared to be a robust signature of the state of anesthesia. The findings challenge the assumption of monotonic, anesthetic dose-dependent behavior of cortical neuron populations. They enhance the interpretation of neuroscientific data obtained under anesthesia and understanding of the neuronal basis of anesthetic-induced state of unconsciousness.