Effects of Mild Closed-Head Injury and Subanesthetic Ketamine Infusion on Microglia, Axonal Injury, and Synaptic Density in Sprague-Dawley Rats.

Mild traumatic brain injury (mTBI) affects millions of people in the U.S. Approximately 20-30% of those individuals develop adverse symptoms lasting at least 3 months. In a rat mTBI study, the closed-head impact model of engineered rotational acceleration (CHIMERA) produced significant axonal injury in the optic tract (OT), indicating white-matter damage. Because retinal ganglion cells project to the lateral geniculate nucleus (LGN) in the thalamus through the OT, we hypothesized that synaptic density may be reduced in the LGN of rats following CHIMERA injury. A modified SEQUIN (synaptic evaluation and quantification by imaging nanostructure) method, combined with immunofluorescent double-labeling of pre-synaptic (synapsin) and post-synaptic (PSD-95) markers, was used to quantify synaptic density in the LGN. Microglial activation at the CHIMERA injury site was determined using Iba-1 immunohistochemistry. Additionally, the effects of ketamine, a potential neuroprotective drug, were evaluated in CHIMERA-induced mTBI. A single-session repetitive (ssr-) CHIMERA (3 impacts, 1.5 joule/impact) produced mild effects on microglial activation at the injury site, which was significantly enhanced by post-injury intravenous ketamine (10 mg/kg) infusion. However, ssr-CHIMERA did not alter synaptic density in the LGN, although ketamine produced a trend of reduction in synaptic density at post-injury day 4. Further research is necessary to characterize the effects of ssr-CHIMERA and subanesthetic doses of intravenous ketamine on different brain regions and multiple time points post-injury. The current study demonstrates the utility of the ssr-CHIMERA as a rodent model of mTBI, which researchers can use to identify biological mechanisms of mTBI and to develop improved treatment strategies for individuals suffering from head trauma.

Effects of a Subanesthetic Ketamine Infusion on Inflammatory and Behavioral Outcomes after Closed Head Injury in Rats.

Traumatic brain injury (TBI) affects millions of people annually, and most cases are classified as mild TBI (mTBI). Ketamine is a potent trauma analgesic and anesthetic with anti-inflammatory properties. However, ketamine's effects on post-mTBI outcomes are not well characterized. For the current study, we used the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA), which replicates the biomechanics of a closed-head impact with resulting free head movement. Adult male Sprague-Dawley rats sustained a single-session, repeated-impacts CHIMERA injury. An hour after the injury, rats received an intravenous ketamine infusion (0, 10, or 20 mg/kg, 2 h period), during which locomotor activity was monitored. Catheter blood samples were collected at 1, 3, 5, and 24 h after the CHIMERA injury for plasma cytokine assays. Behavioral assays were conducted on post-injury days (PID) 1 to 4 and included rotarod, locomotor activity, acoustic startle reflex (ASR), and pre-pulse inhibition (PPI). Brain tissue samples were collected at PID 4 and processed for GFAP (astrocytes), Iba-1 (microglia), and silver staining (axonal injury). Ketamine dose-dependently altered locomotor activity during the infusion and reduced KC/GRO, TNF-α, and IL-1ß levels after the infusion. CHIMERA produced a delayed deficit in rotarod performance (PID 3) and significant axonal damage in the optic tract (PID 4), without significant changes in other behavioral or histological measures. Notably, subanesthetic doses of intravenous ketamine infusion after mTBI did not produce adverse effects on behavioral outcomes in PID 1-4 or neuroinflammation on PID 4. A further study is warranted to thoroughly investigate beneficial effects of IV ketamine on mTBI given multi-modal properties of ketamine in traumatic injury and stress.

Military-related mild traumatic brain injury: clinical characteristics, advanced neuroimaging, and molecular mechanisms.

Mild traumatic brain injury (mTBI) is a significant health burden among military service members. Although mTBI was once considered relatively benign compared to more severe TBIs, a growing body of evidence has demonstrated the devastating neurological consequences of mTBI, including chronic post-concussion symptoms and deficits in cognition, memory, sleep, vision, and hearing. The discovery of reliable biomarkers for mTBI has been challenging due to under-reporting and heterogeneity of military-related mTBI, unpredictability of pathological changes, and delay of post-injury clinical evaluations. Moreover, compared to more severe TBI, mTBI is especially difficult to diagnose due to the lack of overt clinical neuroimaging findings. Yet, advanced neuroimaging techniques using magnetic resonance imaging (MRI) hold promise in detecting microstructural aberrations following mTBI. Using different pulse sequences, MRI enables the evaluation of different tissue characteristics without risks associated with ionizing radiation inherent to other imaging modalities, such as X-ray-based studies or computerized tomography (CT). Accordingly, considering the high morbidity of mTBI in military populations, debilitating post-injury symptoms, and lack of robust neuroimaging biomarkers, this review (1) summarizes the nature and mechanisms of mTBI in military settings, (2) describes clinical characteristics of military-related mTBI and associated comorbidities, such as post-traumatic stress disorder (PTSD), (3) highlights advanced neuroimaging techniques used to study mTBI and the molecular mechanisms that can be inferred, and (4) discusses emerging frontiers in advanced neuroimaging for mTBI. We encourage multi-modal approaches combining neuropsychiatric, blood-based, and genetic data as well as the discovery and employment of new imaging techniques with big data analytics that enable accurate detection of post-injury pathologic aberrations related to tissue microstructure, glymphatic function, and neurodegeneration. Ultimately, this review provides a foundational overview of military-related mTBI and advanced neuroimaging techniques that merit further study for mTBI diagnosis, prognosis, and treatment monitoring.

Association of Traumatic Brain Injury Severity and Self-Reported Neuropsychiatric Symptoms in Wounded Military Service Members.

The impact of traumatic brain injury (TBI) severity and loss of consciousness (LOC) on the development of neuropsychiatric symptoms was studied in injured service members (SMs; n = 1278) evacuated from combat settings between 2003 and 2012. TBI diagnoses of mild TBI (mTBI) or moderate-to-severe TBI (MS-TBI) along with LOC status were identified using International Classification of Diseases, Ninth Revision (ICD-9) codes and the Defense and Veterans Brain Injury Center Standard Surveillance Case Definition for TBI. Self-reported psychiatric symptoms were evaluated for post-traumatic stress disorder (PTSD) with the PTSD Checklist, Civilian Version for PTSD, the Patient Health Questionnaire-9 for major depressive disorder (MDD), and the Patient Health Questionnaire-15 for somatic symptom disorder (SSD) in two time periods post-injury: Assessment Period 1 (AP1, 0.0-2.5 months) and Assessment Period 2 (AP2, 3-12 months). mTBI, but not MS-TBI, was associated with increased neuropsychiatric symptoms: PTSD in AP1 and AP2; MDD in AP1; and SSD in AP2. A subgroup analysis of mTBI with and without LOC revealed that mTBI with LOC, but not mTBI without LOC, was associated with increased symptoms as compared to non-TBI: PTSD in AP1 and AP2; MDD in AP1; and SSD in AP1 and AP2. Moreover, mTBI with LOC was associated with increased MDD symptoms in AP2, and SSD symptoms in AP1 and AP2, compared to mTBI without LOC. These findings reinforce the need for the accurate characterization of TBI severity and a multi-disciplinary approach to address the devastating impacts of TBI in injured SMs.

Effects of an intravenous ketamine infusion on inflammatory cytokine levels in male and female Sprague-Dawley rats.

BACKGROUND: Ketamine, a multimodal dissociative anesthetic drug, is widely used as an analgesic following traumatic injury. Although ketamine may produce anti-inflammatory effects when administered after injury, the immunomodulatory properties of intravenous (IV) ketamine in a non-inflammatory condition are unclear. In addition, most preclinical studies use an intraperitoneal (IP) injection of ketamine, which limits its clinical translation as patients usually receive an IV ketamine infusion after injury. METHODS: Here, we administered sub-anesthetic doses of a single IV ketamine infusion (0, 10, or 40 mg/kg) to male and female Sprague-Dawley rats over a 2-h period. We collected blood samples at 2- and 4-h post-ketamine infusion to determine plasma inflammatory cytokine levels using multiplex immunoassays. RESULTS: The 10 mg/kg ketamine infusion reduced spontaneous locomotor activity in male and female rats, while the 40 mg/kg infusion stimulated activity in female, but not male, rats. The IV ketamine infusion produced dose-dependent and sex-specific effects on plasma inflammatory cytokine levels. A ketamine infusion reduced KC/GRO and tumor necrosis factor alpha (TNF-α) levels in both male and female rats, interleukin-6 (IL-6) levels in female rats, and interleukin-10 (IL-10) levels in male rats. However, most cytokine levels returned to control levels at 4-h post-infusion, except for IL-6 levels in male rats and TNF-α levels in female rats, indicating a different trajectory of certain cytokine changes over time following ketamine administration. CONCLUSIONS: The current findings suggest that sub-anesthetic doses of an IV ketamine infusion may produce sex-related differences in the effects on peripheral inflammatory markers in rodents, and further research is warranted to determine potential therapeutic effects of an IV ketamine infusion in an inflammatory condition.

Enhanced Fear Memories and Altered Brain Glucose Metabolism (18F-FDG-PET) following Subanesthetic Intravenous Ketamine Infusion in Female Sprague-Dawley Rats.

Although women and men are equally likely to receive ketamine following traumatic injury, little is known regarding sex-related differences in the impact of ketamine on traumatic memory. We previously reported that subanesthetic doses of an intravenous (IV) ketamine infusion following fear conditioning impaired fear extinction and altered regional brain glucose metabolism (BGluM) in male rats. Here, we investigated the effects of IV ketamine infusion on fear memory, stress hormone levels, and BGluM in female rats. Adult female Sprague-Dawley rats received a single IV ketamine infusion (0, 2, 10, or 20 mg/kg, over a 2-h period) following auditory fear conditioning (three pairings of tone and footshock). Levels of plasma stress hormones, corticosterone (CORT) and progesterone, were measured after the ketamine infusion. Two days after ketamine infusion, fear memory retrieval, extinction, and renewal were tested over a three-day period. The effects of IV ketamine infusion on BGluM were determined using 18F-fluoro-deoxyglucose positron emission tomography (18F-FDG-PET) and computed tomography (CT). The 2 and 10 mg/kg ketamine infusions reduced locomotor activity, while 20 mg/kg infusion produced reduction (first hour) followed by stimulation (second hour) of activity. The 10 and 20 mg/kg ketamine infusions significantly elevated plasma CORT and progesterone levels. All three doses enhanced fear memory retrieval, impaired fear extinction, and enhanced cued fear renewal in female rats. Ketamine infusion produced dose-dependent effects on BGluM in fear- and stress-sensitive brain regions of female rats. The current findings indicate that subanesthetic doses of IV ketamine produce robust effects on the hypothalamic-pituitary-adrenal (HPA) axis and brain energy utilization that may contribute to enhanced fear memory observed in female rats.

Potentiation of glutamatergic synaptic transmission onto lateral habenula neurons following early life stress and intravenous morphine self-administration in rats.

Early life stress presents an important risk factor for drug addiction and comorbid depression and anxiety through persistent effects on the mesolimbic dopamine pathways. Using an early life stress model for child neglect (a single 24 h episode of maternal deprivation, MD) in rats, recent published works from our lab show that MD induces dysfunction in the ventral tegmental area and its negative controller, the lateral habenula (LHb). MD-induced potentiation of glutamatergic synaptic transmission onto LHb neurons shifts the coordination of excitation/inhibition (E/I) balance towards excitation, resulting in an increase in the overall spontaneous neuronal activity with elevation in bursting and tonic firing, and in the intrinsic excitability of LHb neurons in early adolescent male rats. Here, we explored how MD affects intravenous morphine self-administration (MSA) acquisition and sucrose preference as well as glutamatergic synaptic function in LHb neurons of adult male rats self-administering morphine. We found that MD-induced increases in LHb neuronal and glutamatergic synaptic activity and E/I ratio persisted into adulthood. Moreover, MD significantly reduced morphine intake, triggered anhedonia-like behaviour in the sucrose preference test and was associated with persistent glutamatergic potentiation 24 h after the last MSA session. MSA also altered the decay time kinetics of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR) currents in LHb neurons of control rats during this time period. Our data highlight that early life stress-induced glutamatergic plasticity in LHb may dampen the positive reinforcing and motivational properties of both natural rewards and opioids, and may contribute to the development of anhedonia and dysphoric states associated with opioids.

The Newest Battlefield Opioid, Sublingual Sufentanil: A Proposal to Refine Opioid Usage in the U.S. Military.

There is an ongoing opioid epidemic in the USA, and the U.S. military is not immune to the health threat. To combat the epidemic, the Department of Defense (DoD) and Department of Veterans' Affairs (DVA) issued new clinical practice guidelines and launched the Opioid Safety Initiative aimed at reducing opioid prescriptions. Furthermore, the DoD continually refined opioid protocols for acute pain on the battlefield, evolving from intramuscular morphine to intravenous morphine administration to oral transmucosal fentanyl citrate lollipops (Actiq) to finally sublingual sufentanil tablets (SSTs, Dsuvia). Interestingly, the newest introduction of SSTs into the military sparked great controversy, as there are concerns over the drug's potential for misuse. However, although the opioid crisis may understandably foster an aversion to new candidate opioids, the therapeutic benefits of effective opioids in acute trauma settings should not be overlooked. SSTs may offer an improved analgesic option to meet the battlefield's unmet needs with its non-invasive, sublingual delivery system and favorable pharmacologic properties that mitigate the risk for side effects, addiction, and adverse outcomes. Accordingly, this commentary aims to (1) review the evolution of opioid use on the battlefield and discuss the medical benefits and limitations of SSTs in acute trauma settings, (2) highlight the importance of chronic pain management post-deployment through evidence-based non-opioid modalities, and (3) explore avenues of future research. Ultimately, we propose that SSTs are an important improvement from existing battlefield opioids and that refining, not abandoning, opioid usage will be key to effectively managing pain in the military.

Somatic Symptom Severity, Not Injury Severity, Predicts Probable Posttraumatic Stress Disorder and Major Depressive Disorder in Wounded Service Members.

Although previous studies have reported an association between patient-reported somatic symptom severity and the development of posttraumatic stress disorder (PTSD) or major depressive disorder (MDD) in injured military service members (SMs), conclusions from other studies regarding the association between clinician-determined injury severity and PTSD or MDD remain unclear. The present study investigated whether somatic symptoms or injury severity predict the development of probable PTSD or MDD in wounded SMs medically evacuated from combat areas. Data including SM demographic characteristics, clinician-determined injury severity (i.e., Injury Severity Score [ISS] and Abbreviated Injury Scale [AIS] values), and self-report assessments of PTSD (PTSD Checklist-Civilian Version), MDD (Patient Health Questionnaire [PHQ]-9), and somatic symptoms (PHQ-15) were analyzed. A total of 2,217 SMs completed at least one self-assessment between 2003 and 2014, with 425 having completed assessments at each assessment period (AP), conducted 1-75 (AP1), 76-165 (AP2), and 166-255 (AP3) days postinjury. Between AP1 and AP3, the rates of probable PTSD and MDD increased from 3.0% to 11.7% and from 2.8% to 9.2%, respectively. Somatic symptom severity at AP1 predicted probable PTSD and MDD at all three APs, odds ratios (ORs) = 3.5-11.5; however, ISS values did not predict probable PTSD or MDD at any AP, ORs = 0.6-0.9. This suggests that the initial severity of self-reported somatic symptoms rather than clinician-determined injury severity predicts the development of probable PTSD and MDD in wounded SMs.

Hospitalization for physical injury may contribute to recovery of invisible war wounds: Response to Peterson's (2021) commentary on Soumoff et al. (2021).

Peterson's (2021) commentary on our recently published manuscript (Soumoff et al., 2021) suggests that our findings are an example of visible, physical injuries of war facilitating communication with others, which, in turn, fosters recovery from invisible war wounds. We agree that in the proper context, the retelling of one's traumatic story can be important for recovery from and, perhaps, even the prevention of posttraumatic stress disorder (PTSD) and major depressive disorder (MDD). Participants in our study cohort differed from most others who experienced combat trauma in that while they were hospitalized, they experienced nearly daily visits from a behavioral health provider to address traumatic stress-related symptoms. It is likely that individuals who sustained more severe physical injury (i.e., higher Injury Severity Score [ISS] ratings) had longer hospital stays, received more support, and had more opportunities to retell their stories than those with less severe injuries, leading to decreases in PTSD and MDD symptoms. To note support of this supposition, in Table 5 of Soumoff et al. (2021), although not significant, the adjusted odds ratios (aORs) for PTSD and MDD were below 1 for service members with high (i.e., above 16) ISS ratings. The physical injury-related hospitalizations participants in our sample experienced fostered activities described by Peterson (2021) that likely contributed to the prevention and resolution of PTSD and MDD symptoms, benefits not received by most individuals who suffer only invisible wounds of war.

Effects of subanesthetic intravenous ketamine infusion on neuroplasticity-related proteins in male and female Sprague-Dawley rats.

Although ketamine, a multimodal dissociative anesthetic, is frequently used for analgesia and treatment-resistant major depression, molecular mechanisms of ketamine remain unclear. Specifically, differences in the effects of ketamine on neuroplasticity-related proteins in the brains of males and females need further investigation. In the current study, adult male and female Sprague-Dawley rats with an indwelling jugular venous catheter received an intravenous ketamine infusion (0, 10, or 40 mg/kg, 2-h), starting with a 2 mg/kg bolus for ketamine groups. Spontaneous locomotor activity was monitored by infrared photobeams during the infusion. Two hours after the infusion, brain tissue was dissected to obtain the medial prefrontal cortex (mPFC), hippocampus including the CA1, CA3, and dentate gyrus, and amygdala followed by Western blot analyses of a transcription factor (c-Fos), brain-derived neurotrophic factor (BDNF), and phosphorylated extracellular signal-regulated kinase (pERK). The 10 mg/kg ketamine infusion suppressed locomotor activity in male and female rats while the 40 mg/kg infusion stimulated activity only in female rats. In the mPFC, 10 mg/kg ketamine reduced pERK levels in male rats while 40 mg/kg ketamine increased c-Fos levels in male and female rats. Female rats in proestrus/estrus phases showed greater ketamine-induced c-Fos elevation as compared to those in diestrus phase. In the amygdala, 10 and 40 mg/kg ketamine increased c-Fos levels in female, but not male, rats. In the hippocampus, 10 mg/kg ketamine reduced BDNF levels in male, but not female, rats. Taken together, the current data suggest that subanesthetic doses of intravenous ketamine infusions produce differences in neuroplasticity-related proteins in the brains of male and female rats.

Translational relevance of fear conditioning in rodent models of mild traumatic brain injury.

Mild traumatic brain injury (mTBI) increases the risk of posttraumatic stress disorder (PTSD) in military populations. Utilizing translationally relevant animal models is imperative for establishing a platform to delineate neurobehavioral deficits common to clinical PTSD that emerge in the months to years following mTBI. Such platforms are required to facilitate preclinical development of novel therapeutics. First, this mini review provides an overview of the incidence of PTSD following mTBI in military service members. Secondly, the translational relevance of fear conditioning paradigms used in conjunction with mTBI in preclinical studies is evaluated. Next, this review addresses an important gap in the current preclinical literature; while incubation of fear has been studied in other areas of research, there are relatively few studies pertaining to the enhancement of cued and contextual fear memory over time following mTBI. Incubation of fear paradigms in conjunction with mTBI are proposed as a novel behavioral approach to advance this critical area of research. Lastly, this review discusses potential neurobiological substrates implicated in altered fear memory post mTBI.

Effects of Ketamine on Rodent Fear Memory.

Ketamine, a multimodal anesthetic drug, has become increasingly popular in the treatment of pain following traumatic injury as well as treatment-resistant major depressive disorders. However, the psychological impact of this dissociative medication on the development of stress-related disorders such as post-traumatic stress disorder (PTSD) remains controversial. To address these concerns, preclinical studies have investigated the effects of ketamine administration on fear memory and stress-related behaviors in laboratory animals. Despite a well-documented line of research examining the effects of ketamine on fear memory, there is a lack of literature reviews on this important topic. Therefore, this review article summarizes the current preclinical literature on ketamine and fear memory with a particular emphasis on the route, dose, and timing of ketamine administration in rodent fear conditioning studies. Additionally, this review describes the molecular mechanisms by which ketamine may impact fear memory and stress-related behaviors. Overall, findings from previous studies are inconsistent in that fear memory may be increased, decreased, or unaltered following ketamine administration in rodents. These conflicting results can be explained by factors such as the route, dose, and timing of ketamine administration; the interaction between ketamine and stress; and individual variability in the rodent response to ketamine. This review also recommends that future preclinical studies utilize a clinically relevant route of administration and account for biological sex differences to improve translation between preclinical and clinical investigations.

Sex-related differences in intravenous ketamine effects on dissociative stereotypy and antinociception in male and female rats.

Ketamine, a multimodal dissociative anesthetic drug, is widely used to treat various conditions including acute pain and treatment-resistant depression. We previously reported that subanesthetic doses of intravenous (i.v.) ketamine produced transient dissociative stereotypy and antinociception in male rats. However, sex-related differences in the effects of i.v. ketamine on these measures are not well characterized. Adult male and female Sprague-Dawley rats (10 weeks old) received an i.v. bolus saline or ketamine (2 and 5 mg/kg), and dissociative stereotypy (head weaving, ataxia, and circling) and natural behaviors (horizontal activity, rearing, and grooming) were quantified over a 10-min period. Ten minutes after the behavioral observation, antinociception was measured using a tail flick test. The i.v. ketamine administration increased head weaving, ataxia, circling, and horizontal activity while decreasing rearing and grooming behaviors in male and female rats. Following 5 mg/kg ketamine administration, ataxia was greater in female rats, while head weaving was greater in male rats. Among the female rats, head weaving was greater in the low estrogen group (diestrus phase) as compared to the high estrogen group (proestrus/estrus phase). Ketamine doses (2 and 5 mg/kg) produced antinociception in male and female rats, and female rats were more sensitive to the antinociceptive effects of 2 mg/kg ketamine. The current findings suggest that i.v. ketamine administration, a clinically relevant route of administration, may produce sex-related differences in dissociative behaviors and analgesia between males and females.

Association between intravenous ketamine-induced stress hormone levels and long-term fear memory renewal in Sprague-Dawley rats.

Ketamine is a multimodal dissociative anesthetic and analgesic that is widely used after traumatic injury. We previously reported that an analgesic dose of intravenous (IV) ketamine infusion (10 mg/kg, 2-h) after fear conditioning enhanced short-term fear memory in rats. Here, we investigated the effects of the same dose of an IV ketamine infusion on plasma stress hormone levels and long-term fear memory in rats. Adult male Sprague-Dawley rats (9-week-old with an average weight of 308 g upon arrival) received a ketamine infusion (0 or 10 mg/kg, 2-h) immediately after auditory fear conditioning (three auditory tone and footshock [0.6 mA, 1-s] pairings) on Day 0. After the infusion, a blood sample was collected from a jugular vein catheter for corticosterone and progesterone assays, and each animal was tested on tail flick to measure thermal antinociception. One week later, animals were tested on fear extinction acquisition (Day 7), fear extinction retrieval (Day 8), and fear renewal (Day 9). The IV ketamine infusion, compared to the saline infusion, reduced locomotor activity (sedation), increased tail flick latency (antinociception), and elevated plasma corticosterone and progesterone levels. The ketamine infusion did not alter long-term fear memory extinction or fear renewal. However, elevated corticosterone and progesterone levels resulting from the ketamine infusion were correlated with sedation, antinociception, and long-term fear memory renewal. These results suggest that individual differences in sensitivity to acute ketamine may predict vulnerability to develop fear-related disorders.

Examination of the Gateway Hypothesis in a rat model.

The Gateway Hypothesis is based on epidemiological data and states there is a progression of drug use from use of a softer drug (e.g., nicotine) to use of a harder drug (e.g., morphine). It has been suggested that this sequence is causal and is relevant to drug prevention policies and programs. The present experiment used an animal model to investigate whether the Gateway Hypothesis involves a causal progression. Subjects were 16 female and 16 male Sprague-Dawley rats with ages comparable to late adolescence/emerging adulthood in humans. Subjects received nicotine (6 mg/kg/day) or saline for 21 days SC via osmotic minipump and subsequently were allowed to self-administer IV morphine (0.5 mg/kg/injection, 3 h/day) for 10 days. Results did not confirm the Gateway Hypothesis. In fact, rats pre-exposed to nicotine self-administered significantly less morphine than did rats pre-exposed to saline. These findings may be relevant to future drug use prevention policies and programs.

Effects of subanesthetic intravenous ketamine infusion on neuroplasticity-related proteins in the prefrontal cortex, amygdala, and hippocampus of Sprague-Dawley rats.

Ketamine, a multimodal dissociative anesthetic, is a powerful analgesic administered following trauma due to its hemodynamic and respiratory stability. However, ketamine can cause hallucination and dissociation which may adversely impact traumatic memory after an injury. The effects of ketamine on proteins implicated in neural plasticity are unclear due to different doses, routes, and timing of drug administration in previous studies. Here, we investigated the effects of a single intravenous (IV) ketamine infusion on protein levels in three brain regions of rats. Adult male Sprague-Dawley rats with indwelling IV catheters underwent an auditory fear conditioning (three pairings of tone and mild footshock 0.8 mA, 0.5 s) and received a high dose of IV ketamine (0 or 40 mg/kg/2 h) infusion (Experiment 1). In a follow-up study, animals received a low dose of IV ketamine (0 or 10 mg/kg/2 h) infusion (Experiment 2). Two hours after the infusion, brain tissue from the medial prefrontal cortex (mPFC), hippocampus, and amygdala were collected for western blot analyses. Protein levels of a transcription factor (c-Fos), brain-derived neurotrophic factor (BDNF), and phosphorylated extracellular signal-regulated kinase (pERK) were quantified in these regions. The 40 mg/kg ketamine infusion increased c-Fos levels in the mPFC and amygdala as well as pERK levels in the mPFC and hippocampus. The 10 mg/kg ketamine infusion increased BDNF levels in the amygdala, but decreased pERK levels in the mPFC and hippocampus. These findings suggest that a clinically relevant route of ketamine administration produces dose-dependent and brain region-specific effects on proteins involved in neuroplasticity.

Enhanced fear memories and brain glucose metabolism (18F-FDG-PET) following sub-anesthetic intravenous ketamine infusion in Sprague-Dawley rats.

Ketamine is a multimodal dissociative anesthetic, which provides powerful analgesia for victims with traumatic injury. However, the impact of ketamine administration in the peri-trauma period on the development of post-traumatic stress disorder (PTSD) remains controversial. Moreover, there is a major gap between preclinical and clinical studies because they utilize different doses and routes of ketamine administration. Here, we investigated the effects of sub-anesthetic doses of intravenous (IV) ketamine infusion on fear memory and brain glucose metabolism (BGluM) in rats. Male Sprague-Dawley rats received an IV ketamine infusion (0, 2, 10, and 20 mg/kg, 2 h) or an intraperitoneal (IP) injection (0 and 10 mg/kg) following an auditory fear conditioning (3 pairings of tone and foot shock [0.6 mA, 1 s]) on day 0. Fear memory retrieval, fear extinction, and fear recall were tested on days 2, 3, and 4, respectively. The effects of IV ketamine infusion (0 and 10 mg/kg) on BGluM were measured using 18F-fluoro-deoxyglucose positron emission tomography (FDG-PET) and computed tomography (CT). The IV ketamine infusion dose-dependently enhanced fear memory retrieval, delayed fear extinction, and increased fear recall in rats. The IV ketamine (10 mg/kg) increased BGluM in the hippocampus, amygdala, and hypothalamus, while decreasing it in the cerebellum. On the contrary, a single ketamine injection (10 mg/kg, IP) after fear conditioning facilitated fear memory extinction in rats. The current findings suggest that ketamine may produce differential effects on fear memory depending on the route and duration of ketamine administration.

Effects of Subanesthetic Intravenous Ketamine Infusion on Corticosterone and Brain-Derived Neurotrophic Factor in the Plasma of Male Sprague-Dawley Rats.

Posttrauma anesthetic agents influence neuroendocrine responses that may affect fear memory. The effects of a subanesthetic intravenous (IV) ketamine infusion on mediators of stress and memory in rodents are unknown. Therefore, we used a clinically relevant method to administer a 2-hour subanesthetic IV ketamine infusion following a rodent fear-conditioning paradigm (paired tone plus foot shock) to evaluate the effects on corticosterone and brain-derived neurotrophic factor in the plasma of male Sprague-Dawley rats. We found that subanesthetic ketamine infusions (5 and 20 mg/kg/h) dose-dependently increased plasma corticosterone levels. Ketamine at 20 mg/kg/h significantly reduced plasma brain-derived neurotrophic factor measured 2 hours after the conclusion of the ketamine infusion. These results demonstrate that a subanesthetic IV ketamine infusion maintained a heightened neuroendocrine stress response after fear conditioning and reduced levels of a neurotrophin associated with memory, which may influence fear memory processing. The behavioral outcomes of these effects are unknown and warrant future investigation.