Hippocampal long-term potentiation in adult mice after recovery from ketamine anesthesia

Ketamine induced synaptic plasticity operates independently of long-term potentiation

Synaptic plasticity occurs via multiple mechanisms to regulate synaptic efficacy. Homeostatic and Hebbian plasticity are two such mechanisms by which neuronal synapses can be altered. Although these two processes are mechanistically distinct, they converge on downstream regulation of AMPA receptor activity to modify glutamatergic neurotransmission. However, much remains to be explored regarding how these two prominent forms of plasticity interact. Ketamine, a rapidly acting antidepressant, increases glutamatergic transmission via pharmacologically-induced homeostatic plasticity. Here, we demonstrate that Hebbian plasticity mechanisms are still intact in synapses that have undergone homeostatic scaling by ketamine after either systemic injection or perfusion onto hippocampal brain slices. We also investigated this relationship in the context of stress induced by chronic exposure to corticosterone (CORT) to better model the circumstances under which ketamine may be used as an antidepressant. We found that CORT induced an anhedonia-like behavioral phenotype in mice but did not impair long-term potentiation (LTP) induction. Furthermore, corticosterone exposure does not impact the intersection of homeostatic and Hebbian plasticity mechanisms, as synapses from CORT-exposed mice also demonstrated intact ketamine-induced plasticity and LTP in succession. These results provide a mechanistic explanation for how ketamine used for the treatment of depression does not impair the integrity of learning and memory processes encoded by mechanisms such as LTP.

Esketamine Anesthetizes Mice With a Similar Potency to Racemic Ketamine

Esketamine, the right-handed optical isomer of racemic ketamine, has recently become widely used for anesthesia and analgesia as a replacement for racemic ketamine. However, there are limited studies comparing the anesthetic and analgesic effects of esketamine and racemic ketamine in mice. This research was conducted to analyze the dose-dependent anesthetic and analgesic efficacy of esketamine in mice and to compare its potency with that of the racemate. We tested the anesthetic effects of different doses of esketamine and compared its potency with that of the racemate using righting reflex tests. Then, the acetic acid-induced pain model and formalin-induced pain model were used to investigate the analgesic effect. Compared with racemic ketamine, an equivalent dose of esketamine at 100 mg/kg was required to induce stable anesthesia. In contrast, 5 mg/kg esketamine was sufficient to provide analgesic effects similar to those of 10 mg/kg ketamine. Together, esketamine had a similar potency to racemic ketamine for anesthesia and a stronger potency for analgesia in mice.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Effect of Ketamine on LTP and NMDAR EPSC in Hippocampus of the Chronic Social Defeat Stress Mice Model of Depression

Depression is a common mental disorder that is associated with memory dysfunction. Ketamine has recently been demonstrated to be a rapid antidepressant. The mechanisms underlying how depression induces memory dysfunction and how ketamine relieves depressive symptoms remain poorly understood. This work compared three groups of male C57BL/6J mice: mice exposed to chronic social defeat stress (CSDS) to induce a depression-like phenotype, depression-like mice treated with ketamine, and control mice that were not exposed to CSDS or treated with ketamine. Spatial working memory and long term memory were assessed by spontaneous alternation Y-maze and fear conditioning tests, respectively. We used western blot to analyze the density of N-methyl-D-aspartate receptor (NMDAR) subunits in the hippocampus. We recorded long term potentiation (LTP) and NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) in hippocampal slices. We observed that compared with control mice, depression-like mice had significant reductions in spatial working memory and contextual fear memory. The level of NR2B, LTP and NMDA receptor-mediated EPSCs of depression-like mice were decreased. Ketamine treatment attenuated the memory impairment, and increased the density of NR2B and the amplitude of LTP and NMDA receptor-mediated EPSCs in the hippocampus of depression-like mice. In conclusion, depression-like mice have deficits in working memory and contextual fear memory. The decrease of NR2B, LTP induction and NMDA receptor-mediated EPSCs in the hippocampus may be involved in this process. Ketamine can improve expression of NR2B, LTP induction and NMDA receptor-mediated EPSCs in the hippocampus of depression-like mice, which might be part of the reason why ketamine can alleviate the memory dysfunction induced by depression.

Neonatal exposure of ketamine inhibited the induction of hippocampal long-term potentiation without impairing the spatial memory of adult rats

Ketamine is one of general anesthetics and has been commonly used in obstetric and pediatric anesthesia. However, effects of exposure to ketamine on neonatal brain are largely unknown. In this study, we aim to investigate the effect of neonatal exposure of ketamine on spatial memory and long-term potentiation (LTP) in the hippocampus of adult rats. One-week-old neonatal rats were separated into ketamine group and control group. Neonatal rats in ketamine group were received intraperitoneal injection of 25 mg/kg (low-dose group, N = 8) or 50 mg/kg ketamine (high-dose group, N = 8). Neonatal Rats in control group received saline injection (N = 8). After 10 weeks, the spatial memory of adult rats was examined by using Morris Water Maze, and LTP in the hippocampus of adult rats was assessed by electrophysiological experiment. We found that exposure of ketamine to neonatal rats, either low-dose or high-dose, had not induced alteration on their adulthood's escape latency, swimming speed and the percentage of time spent in original quadrant compared with the control. The electrophysiological examination showed that the induction of LTP in hippocampus was significantly reduced in adult rats of ketamine group (either low-dose or high-dose). Our study showed that neonatal exposure of ketamine inhibited the induction of hippocampal LTP without impairing the spatial memory of adult rats.