Memory & Conditioning Under Anesthesia

Anesthesia, Pain

propofol, dexmedetomidine, fentanyl, functional MRI, electric nerve stimulation, sedation, functional connectivity

The purpose of this study is to determine the effects of pain on long-term memory and conditioned physiologic responses in the presence and absence of distinct intravenous anesthetics. Functional magnetic resonance imaging will be used to identify the neural correlates of these phenomena The study will occur over 5 visits and involves no long-term follow up.

Purpose: Sedative-hypnotic and analgesic agents (termed anesthetics) are routinely used during medical procedures to prevent or ease suffering, suppressing the conscious experience of pain and its encoding into memory. While overt awareness under general anesthesia is a rare clinical event, implicit memory may still form. Further, at sub-hypnotic anesthetic doses, animals show enhanced fear conditioning and humans may have enhanced amygdala activity. This motivates the investigator's study, as poorly-contextualized aversive memories are theorized to initiate anxiety-spectrum disorders, which may explain the high incidence of post-traumatic stress disorder after anesthetic awareness. Objective: How anesthetics facilitate or inhibit poorly-contextualized aversive memories is incompletely understood, with little mechanistic work done in human subjects. Thus, there is a critical need to understand how anesthetics modulate the memory and threat response systems during painful stimulation. The overall scientific objective is to determine the memory-modulating effects of propofol, dexmedetomidine, and fentanyl in the context of periodic painful stimulation. Aim 1: Determine how behavioral and physiologic measures of memory are modulated by pain and the individual effects of three pharmacologically distinct drugs: propofol, dexmedetomidine, and fentanyl. Hypotheses: Based on previous results, 1a) explicit memory will be significantly reduced by propofol and dexmedetomidine, but only modestly by fentanyl. Consistent with my preliminary data, 1b) priming effects will be seen for pain-paired words under all drugs. Electrodermal activity changes still occur with opioids and propofol, thus 1c) pain-related physiologic responses will persist with these two drugs but be blunted by the anti-adrenergic effect of dexmedetomidine. Aim 2: Determine the brain structures differentially engaged in memory encoding under pain and drug conditions. Task-related functional magnetic resonance imaging (MRI) activity for behavioral measures of explicit or implicit memory will be determined, comparing pain-paired vs non-pain items across drug and no-drug datasets. Functional connectivity (FC) MRI (fcMRI) will be compared between task and drug conditions. The entire brain will be explored, but predictions for key structures follow. Hypotheses: 2a) Hippocampal activity, will be blunted by propofol and dexmedetomidine, while fentanyl will have minimal effect. 2b) Amygdala activity, responsible for physiologic responses, will parallel the predictions in 1c across drug and pain conditions. 2c) Insula activity will be greater for pain-paired items, and this will be attenuated by fentanyl \> dexmedetomidine \> propofol, corresponding to their anticipated analgesic effect. 2d) Pain has been shown to affect fcMRI during a cognitive task, and thus FC between the key regions in 2a-c will be reduced by all three drugs, in characteristic patterns.

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