Implicit aversive memory under anaesthesia in animal models: a narrative review

Dexmedetomidine alleviates propofol-induced neural injury in developing rats

Propofol, a commonly used intravenous anesthetic, has been associated with neurodegeneration in the developing brain upon repeated exposure. Dexmedetomidine is an α2 adrenoceptor agonist that was previously reported to possess neuroprotective properties. Here, we confirmed the impacts of dexmedetomidine on propofol-induced neuroapoptosis and subsequent spatial learning and memory deficits in neonatal rats. We found that dexmedetomidine effectively mitigated propofol-induced spatial learning and memory impairments and improved aversive memory in developing rats. Dexmedetomidine reduced propofol-induced cell apoptosis in the hippocampus and modulated the mRNA expression of Bcl-2 and Bax. Additionally, dexmedetomidine attenuated the propofol-induced increase of inflammatory factors IL-6 and TNF-α. The reduced phosphorylation levels of Akt and CREB levels by propofol were re-activated by dexmedetomidine. In conclusion, our findings demonstrated that dexmedetomidine effectively mitigated propofol-induced cognitive and memory impairments in developing rats by modulating apoptosis and reducing inflammation via activating the Akt/CREB/BDNF signaling pathway. These findings suggest potential strategies to protect the developing brain from the adverse effects of anesthetics and improve patient care in pediatric anesthesia practice.

Anesthesia and the neurobiology of fear and posttraumatic stress disorder

PURPOSE OF REVIEW

Dysfunction of fear memory systems underlie a cluster of clinically important and highly prevalent psychological morbidities seen in perioperative and critical care patients, most archetypally posttraumatic stress disorder (PTSD). Several sedative-hypnotics and analgesics are known to modulate fear systems, and it is theoretically plausible that clinical decisions of the anesthesiologist could impact psychological outcomes. This review aims to provide a focused synthesis of relevant literature from multiple fields of research.

RECENT FINDINGS

There is evidence in some contexts that unconscious fear memory systems are less sensitive to anesthetics than are conscious memory systems. Opiates may suppress the activation of fear systems and have benefit in the prevention of PTSD following trauma. There is inconsistent evidence that the use of propofol and benzodiazepines for sedation following trauma may potentiate the development of PTSD relative to other drugs. The benefits of ketamine seen in the treatment of major depression are not clearly replicated in PTSD-cluster psychopathologies, and its effects on fear processes are complex.

SUMMARY

There are multiple theoretical mechanisms by which anesthetic drugs can modulate fear systems and clinically important fear-based psychopathologies. The current state of research provides some evidence to support further hypothesis investigation. However, the absence of effectiveness studies and the inconsistent signals from smaller studies provide insufficient evidence to currently offer firm clinical guidance.

Propofol improves learning and memory in post-traumatic stress disorder (PTSD) mice via recovering hippocampus synaptic plasticity

AIMS

Propofol, the most commonly used intravenous anesthetic, is known for its protective effect in various human and animal disease models such as post-traumatic stress disease (PTSD). However, it still needs efforts to clarify the effect of propofol on fear memory extinction and the relevant mechanisms.

METHODS

Fear memory extinction was examined in PTSD mice model. Thirty-six mice were randomly divided into three groups: a shock + propofol group (sh + Pro), shock + normal saline group (sh + NS), and sham group. The mice were treated with propofol (150 mg/kg) or normal saline (of the same volume) intraperitoneally 30 min after the conditioning. These mice's behavior was analysed with contextual test, sucrose preference test (SPT) and Morris water maze (MWM). Additionally, the synaptic plasticity of the hippocampus was examined by long-term potentiation (LTP) and long-term depression (LTD).

KEY FINDINGS

Compared with the sham group, the sh + NS group showed increased freezing time and depressive behavior, meanwhile the sh + Pro group showed minor behavioral changes. What's more, we found that propofol rescued the impaired long-term potentiation (LTP) and long-term depression (LTD) in hippocampus of PTSD mice. All these suggest that propofol can accelerate fear memory extinction and change synaptic plasticity of PTSD mice.

SIGNIFICANCE

The study proved that propofol can protect the mice from PTSD by reserving synaptic plasticity and brought a new insight into PTSD treatment indicating that propofol maybe a potential cure for PTSD.

Implicit Memory and Anesthesia: A Systematic Review and Meta-Analysis

General anesthesia should induce unconsciousness and provide amnesia. Amnesia refers to the absence of explicit and implicit memories. Unlike explicit memory, implicit memory is not consciously recalled, and it can affect behavior/performance at a later time. The impact of general anesthesia in preventing implicit memory formation is not well-established. We performed a systematic review with meta-analysis of studies reporting implicit memory occurrence in adult patients after deep sedation (Observer's Assessment of Alertness/Sedation of 0-1 with spontaneous breathing) or general anesthesia. We also evaluated the impact of different anesthetic/analgesic regimens and the time point of auditory task delivery on implicit memory formation. The meta-analysis included the estimation of odds ratios (ORs) and 95% confidence intervals (CIs). We included a total of 61 studies with 3906 patients and 119 different cohorts. For 43 cohorts (36.1%), implicit memory events were reported. The American Society of Anesthesiologists (ASA) physical status III-IV was associated with a higher likelihood of implicit memory formation (OR:3.48; 95%CI:1.18-10.25, p < 0.05) than ASA physical status I-II. Further, there was a lower likelihood of implicit memory formation for deep sedation cases, compared to general anesthesia (OR:0.10; 95%CI:0.01-0.76, p < 0.05) and for patients receiving premedication with benzodiazepines compared to not premedicated patients before general anesthesia (OR:0.35; 95%CI:0.13-0.93, p = 0.05).

Depth of Anesthesia Monitoring

The electroencephalogram (EEG) can be analyzed in its raw form for characteristic drug-induced patterns of change or summarized using mathematical parameters as a processed electroencephalogram (pEEG). In this article we aim to summarize the contemporary literature pertaining to the commonly available pEEG monitors including the effects of commonly used anesthetic drugs on the EEG and pEEG parameters, pEEG monitor pitfalls, and the clinical implications of pEEG monitoring for anesthesia, pediatrics, and intensive care.

Mouse Anesthesia: The Art and Science

There is an art and science to performing mouse anesthesia, which is a significant component to animal research. Frequently, anesthesia is one vital step of many over the course of a research project spanning weeks, months, or beyond. It is critical to perform anesthesia according to the approved research protocol using appropriately handled and administered pharmaceutical-grade compounds whenever possible. Sufficient documentation of the anesthetic event and procedure should also be performed to meet the legal, ethical, and research reproducibility obligations. However, this regulatory and documentation process may lead to the use of a few possibly oversimplified anesthetic protocols used for mouse procedures and anesthesia. Although a frequently used anesthetic protocol may work perfectly for each mouse anesthetized, sometimes unexpected complications will arise, and quick adjustments to the anesthetic depth and support provided will be required. As an old saying goes, anesthesia is 99% boredom and 1% sheer terror. The purpose of this review article is to discuss the science of mouse anesthesia together with the art of applying these anesthetic techniques to provide readers with the knowledge needed for successful anesthetic procedures. The authors include experiences in mouse inhalant and injectable anesthesia, peri-anesthetic monitoring, specific procedures, and treating common complications. This article utilizes key points for easy access of important messages and authors’ recommendation based on the authors’ clinical experiences.

Neurons in the Nonhuman Primate Amygdala and Dorsal Anterior Cingulate Cortex Signal Aversive Memory Formation under Sedation

BACKGROUND

Anesthetics aim to prevent memory of unpleasant experiences. The amygdala and dorsal anterior cingulate cortex participate in forging emotional and valence-driven memory formation. It was hypothesized that this circuitry maintains its role under sedation.

METHODS

Two nonhuman primates underwent aversive tone-odor conditioning under sedative states induced by ketamine or midazolam (1 to 8 and 0.1 to 0.8 mg/kg, respectively). The primary outcome was behavioral and neural evidence suggesting memory formation. This study simultaneously measured conditioned inspiratory changes and changes in firing rate of single neurons in the amygdala and the dorsal anterior cingulate cortex in response to an expected aversive olfactory stimulus appearing during acquisition and tested their retention after recovery.

RESULTS

Aversive memory formation occurred in 26 of 59 sessions under anesthetics (16 of 29 and 10 of 30, 5 of 30 and 21 of 29 for midazolam and ketamine at low and high doses, respectively). Single-neuron responses in the amygdala and dorsal anterior cingulate cortex were positively correlated between acquisition and retention (amygdala, n = 101, r = 0.51, P < 0.001; dorsal anterior cingulate cortex, n = 121, r = 0.32, P < 0.001). Neural responses during acquisition under anesthetics were stronger in sessions exhibiting memory formation than those that did not (amygdala median response ratio, 0.52 versus 0.33, n = 101, P = 0.021; dorsal anterior cingulate cortex median response ratio, 0.48 versus 0.32, n = 121, P = 0.012). The change in firing rate of amygdala neurons during acquisition was correlated with the size of stimuli-conditioned inspiratory response during retention (n = 101, r = 0.22 P = 0.026). Thus, amygdala and dorsal anterior cingulate cortex responses during acquisition under anesthetics predicted retention. Respiratory unconditioned responses to the aversive odor anesthetics did not differ from saline controls.

CONCLUSIONS

These results suggest that the amygdala-dorsal anterior cingulate cortex circuit maintains its role in acquisition and maintenance of aversive memories in nonhuman primates under sedation with ketamine and midazolam and that the stimulus valence is sufficient to drive memory formation.

EDITOR’S PERSPECTIVE

Anesthesia analgesia in the amygdala

General anesthetics during surgery are presumed to block pain by dampening brain activity and promoting loss-of-consciousness. A new study shows that anesthetics activate an endogenous analgesia neural ensemble in the central nucleus of the amygdala.

Dissociable learning processes, associative theory, and testimonial reviews: A comment on Smith and Church (2018)

Smith and Church (Psychonomic Bulletin & Review, 25, 1565-1584 2018) present a "testimonial" review of dissociable learning processes in comparative and cognitive psychology, by which we mean they include only the portion of the available evidence that is consistent with their conclusions. For example, they conclude that learning the information-integration category-learning task with immediate feedback is implicit, but do not consider the evidence that people readily report explicit strategies in this task, nor that this task can be accommodated by accounts that make no distinction between implicit and explicit processes. They also consider some of the neuroscience relating to information-integration category learning, but do not report those aspects that are more consistent with an explicit than an implicit account. They further conclude that delay conditioning in humans is implicit, but do not report evidence that delay conditioning requires awareness; nor do they present the evidence that conditioned taste aversion, which should be explicit under their account, can be implicit. We agree with Smith and Church that it is helpful to have a clear definition of associative theory, but suggest that their definition may be unnecessarily restrictive. We propose an alternative definition of associative theory and briefly describe an experimental procedure that we think may better distinguish between associative and non-associative processes.