Hippocampal and lateral entorhinal cortex physiological activity during trace conditioning under urethane anesthesia.

Significant evidence shows that the acquisition of delay conditioning can occur in out-of-awareness states, such as under anesthesia. However, it is unclear to what extent and what type of conditioning animals may achieve during nonawake states. Trace conditioning is an appealing protocol to study under anesthesia, given the long empty gap separating the conditioned and unconditioned stimuli, which must be bridged for acquisition to happen. Here, we show evidence that rats develop physiological responses during the trace conditioning paradigm under anesthesia. We recorded the activity of the hippocampus (HPC) and lateral entorhinal cortex (LEC) in urethane-anesthetized rats, along with an electromyogram and an electrocardiogram. The protocol consisted of randomly presenting two distinct sound stimuli (CS- and CS+), where only one stimulus (CS+) was assigned to be trace-paired with a footshock. A trial-average analysis revealed that animals developed significant climbing heart rate activity initiating at the CS onset and persisting during the trace period. Such climbing arose for both CS- and CS+ with similar slopes but different intercepts, suggesting CS+ heart rates were typically above CS-. The power and coherence of HPC and LEC high-frequency bands (>100 Hz) significantly increased during CS presentation and trace, similarly to CS- and CS+ and insensitive to either activated or deactivated states. To the best of our knowledge, this is the first attempt to perform a trace conditioning protocol under anesthesia. Confirmation of this procedure acquisition can allow a new preparation for the exploration of brain mechanisms that bind time-discontinuous events.NEW & NOTEWORTHY Some forms of learning, such as some types of conditioning, can occur in anesthetized states. However, the extent to which memories can be formed in these states is still an open question. Here, we investigated the trace conditioning under urethane anesthesia and found heart rate, hippocampus, and lateral entorhinal cortex physiological changes to stimuli presentation. This new preparation may allow for exploration of memory acquisition of time-discontinuous events in the nonawake brain.