Arousal state vs. temperature effects on neuronal activity in subcoeruleus area.

It is believed that thermoafferent neurons synapse in the subcoeruleus area (SC). To assess the effect of arousal state on thermoafferent information processing, we recorded SC unit activity in unanesthetized rats. No responses to changes in ambient temperature were observed within an arousal state, but 85% of the units dramatically altered their firing rates as the arousal state changed. We replicated previous experiments on anesthetized animals which were the basis for ascribing thermoafferent function to the SC. Additionally, we monitored electroencephalographic (EEG) activity. In lightly anesthetized rats, five distinct EEG states could be defined, and the temperature profile of the animal was a primary determinant of the EEG state. No thermoresponses were observed within an EEG state, but 78% of the units dramatically altered their firing rates in synchrony with EEG pattern changes. Transient responses to noxious stimuli were also reflected in both the EEG patterns and single-unit activity. We conclude that SC single-unit activity is not specific for processing of thermal information but is correlated with EEG activity, which in turn is determined by a variety of factors including thermal stimuli.