Dopaminergic psychostimulants cause arousal from isoflurane-induced sedation without reversing memory impairment in rats.

BACKGROUND: Dopaminergic psychostimulants can restore arousal in anaesthetised animals, and dopaminergic signalling contributes to hippocampal-dependent memory formation. We tested the hypothesis that dopaminergic psychostimulants can antagonise the amnestic effects of isoflurane on visuospatial working memory. METHODS: Sixteen adult Sprague-Dawley rats were trained on a trial-unique nonmatching-to-location (TUNL) task which assessed the ability to identify a novel touchscreen location after a fixed delay. Once trained, the effects of low-dose isoflurane (0.3 vol%) on task performance and activity, assessed by infrared beam breaks, were assessed. We attempted to rescue deficits in performance and activity with a dopamine D1 receptor agonist (chloro-APB), a noradrenergic reuptake inhibitor (atomoxetine), and a mixed dopamine/norepinephrine releasing agent (dextroamphetamine). Anaesthetic induction, emergence, and recovery from anaesthesia were also investigated. RESULTS: Low-dose isoflurane impaired working memory in a sex-independent and intra-trial delay-independent manner as assessed by task performance, and caused an overall reduction in activity. Administration of chloro-APB, atomoxetine, or dextroamphetamine did not restore visuospatial working memory, but chloro-APB and dextroamphetamine recovered arousal to levels observed in the baseline awake state. Performance did not differ between induction and emergence. Animals recovered to baseline performance within 15 min of discontinuing isoflurane. CONCLUSIONS: Low-dose isoflurane impairs visuospatial working memory in a nondurable and delay-independent manner that potentially implicates non-hippocampal structures in isoflurane-induced memory deficits. Dopaminergic psychostimulants counteracted sedation but did not reverse memory impairments, suggesting that isoflurane-induced amnesia and isoflurane-induced sedation have distinct underlying mechanisms that can be antagonised independently.

Methylphenidate Reversal of Dexmedetomidine-Induced Versus Ketamine-Induced Sedation in Rats.

BACKGROUND: Dexmedetomidine and ketamine have long elimination half-lives in humans and have no clinically approved reversal agents. Methylphenidate enhances dopaminergic and noradrenergic neurotransmission by inhibiting reuptake transporters for these arousal-promoting neurotransmitters. Previous studies in rats demonstrated that intravenous methylphenidate induces emergence from isoflurane and propofol general anesthesia. These 2 anesthetics are thought to act primarily through enhancement of inhibitory Gamma-aminobutyric acid type A (GABAA) receptors. In this study, we tested the behavioral and neurophysiological effects of methylphenidate in rats after low and high doses of dexmedetomidine (an alpha-2 adrenergic receptor agonist) and ketamine (an N-methyl-D-aspartate [NMDA] receptor antagonist) that induce sedation and unconsciousness, respectively. METHODS: All experiments used adult male and female Sprague-Dawley rats (n = 32 total) and all drugs were administered intravenously in a crossover, blinded experimental design. Locomotion after sedating doses of dexmedetomidine (10 µg/kg) or ketamine (10 mg/kg) with and without methylphenidate (5 mg/kg) was tested using the open field test (n = 16). Recovery of righting reflex after either high-dose dexmedetomidine (50 µg/kg) or high-dose ketamine (50 mg/kg) with and without methylphenidate (1-5 mg/kg) was assessed in a second cohort of rats (n = 8). Finally, in a third cohort of rats (n = 8), frontal electroencephalography (EEG) was recorded for spectral analysis under both low and high doses of dexmedetomidine and ketamine with and without methylphenidate. RESULTS: Low-dose dexmedetomidine reduced locomotion by 94% in rats. Methylphenidate restored locomotion after low-dose dexmedetomidine (rank difference = 88.5, 95% confidence interval [CI], 70.8-106) and the effect was blocked by coadministration with a dopamine D1 receptor antagonist (rank difference = 86.2, 95% CI, 68.6-104). Low-dose ketamine transiently attenuated mobility by 58% and was not improved with methylphenidate. Methylphenidate did not affect the return of righting reflex latency in rats after high-dose dexmedetomidine nor ketamine. Frontal EEG analysis revealed that methylphenidate reversed spectral changes induced by low-dose dexmedetomidine (F [8,87] = 3.27, P = .003) but produced only transient changes after high-dose dexmedetomidine. Methylphenidate did not induce spectral changes in the EEG after low- or high-dose ketamine. CONCLUSIONS: Methylphenidate reversed behavioral and neurophysiological correlates of sedation, but not unconsciousness, induced by dexmedetomidine. In contrast, methylphenidate did not affect sedation, unconsciousness, nor EEG signatures in rats after ketamine. These findings suggest that methylphenidate may be efficacious to reverse dexmedetomidine sedation in humans.

Oestrous cycle affects emergence from anaesthesia with dexmedetomidine, but not propofol, isoflurane, or sevoflurane, in female rats.

BACKGROUND: Although sex differences in anaesthetic sensitivity have been reported, what underlies these differences is unknown. In rodents, one source of variability in females is the oestrous cycle. Here we test the hypothesis that the oestrous cycle impacts emergence from general anaesthesia. METHODS: Time to emergence was measured after isoflurane (2 vol% for 1 h), sevoflurane (3 vol% for 20 min), dexmedetomidine (50 µg kg-1 i.v., infused over 10 min), or propofol (10 mg kg-1 i.v. bolus) during proestrus, oestrus, early dioestrus, and late dioestrus in female Sprague-Dawley rats (n=24). EEG recordings were taken during each test for power spectral analysis. Serum was analysed for 17ß-oestradiol and progesterone concentrations. The effect of oestrous cycle stage on return of righting latency was assessed using a mixed model. The association between righting latency and serum hormone concentration was tested by linear regression. Mean arterial blood pressure and arterial blood gases were assessed in a subset of rats after dexmedetomidine and compared in a mixed model. RESULTS: Oestrous cycle did not affect righting latency after isoflurane, sevoflurane, or propofol. When in the early dioestrus stage, rats emerged more rapidly from dexmedetomidine than in the proestrus (P=0.0042) or late dioestrus (P=0.0230) stage and showed reduced overall power in frontal EEG spectra 30 min after dexmedetomidine (P=0.0049). 17ß-Oestradiol and progesterone serum concentrations did not correlate with righting latency. Oestrous cycle did not affect mean arterial blood pressure or blood gases during dexmedetomidine. CONCLUSIONS: In female rats, the oestrous cycle significantly impacts emergence from dexmedetomidine-induced unconsciousness. However, 17ß-oestradiol and progesterone serum concentrations do not correlate with the observed changes.

Loss of lumbar disc height with age and its impact on pain and sensitivity associated behaviors in mice.

PURPOSE: Aging is a risk factor for several debilitating conditions including those related to chronic back pain and intervertebral disc degeneration, both of which have no cure. Mouse models are useful tools for studying disc degeneration and chronic back pain in a tightly controlled and clinically relevant aging environment. Moreover, mice offer the advantage of carrying out longitudinal studies to understand the etiology and progression of disc pathology induced by genetic or surgical strategies. Previously, age-related behavioral trends of discomfort and enhanced nociception in mice were reported; however, whether these measures are mediated by structural and pathological changes in the disc is unknown. METHODS: The goal of the present observational study was to identify behavioral correlates of age-related degenerative changes in the disc. Towards this, we collected radiographs from 150 mice (77 females) between three and 23 months of age and measured the disc height index for each level of lumbar disc. Behavioral measures were collected on several of these mice which included rearing and distance travelled in an open field test; time spent in rearing, reaching, immobile, and self-suspended in the tail suspension test; bilateral hind paw licking in response to cold allodynia using acetone; and unilateral hind paw licking in response to heat hyperalgesia using capsaicin. RESULTS: Results show that the lower lumbar discs lose height with age and these changes are independent of body composition measures including body weight, bone mineral density, fat mass, lean weight mass, percent fat mass, and percent lean mass. Disc height positively correlates with rearing and mobility in the open field test, immobility in the tail suspension test, and thermal hyperalgesia. Disc height negatively correlates with cold allodynia and rearing in the tail suspension test. Furthermore, mediation analysis shows that the lumbosacral disc significantly mediates the effect of age on rearing in the open field test, but not cold allodynia, suggesting this behavior is a useful measure of age-related axial discomfort due to disc degeneration. CONCLUSION: In summary, the findings from the current study show that disc height are associated with measures of axial discomfort and nociception in mice.

Modulating anesthetic emergence with pathway-selective dopamine signaling.

PURPOSE OF REVIEW: To summarize the recent preclinical findings investigating dopaminergic circuits for their involvement in reversing anesthetic-induced unconsciousness. RECENT FINDINGS: The release of dopamine from the ventral tegmental area onto dopamine D1 receptor-expressing neurons in the nucleus accumbens promotes emergence following general anesthesia. Two relevant targets of dopamine D1 receptor-expressing neurons in the nucleus accumbens include the lateral hypothalamus and ventral pallidum. Activating mesocortical dopaminergic projections from the ventral tegmental area to the prelimbic cortex has also been shown to hasten emergence from general anesthesia. In contrast, the nigrostriatal dopamine pathway is not involved in regulating anesthetic emergence. The role of the tuberoinfundibular endocrine dopamine pathway remains to be tested; however, recent studies have identified an important function of neuroendocrine signaling on modulating general anesthesia. SUMMARY: Potential avenues for accelerating anesthetic emergence may be found through targeting specific arousal-promoting pathways in the brain. Accumulating evidence from rodent studies manipulating cell type- and circuit-specific signaling pathways have identified dopamine as a potent modulator of general anesthesia. Specifically, dopamine signaling along the mesolimbic and mesocortical pathways plays a fundamental role in regulating consciousness.

Sex, drugs, and anaesthesia research.

In this issue of the British Journal of Anaesthesia, Joksimovic and colleagues report significant sex differences in sensitivity to the behavioural and neurophysiological effects of 3ß-OH, a novel neurosteroid anesthetic. Female rats were more sensitive to the effects of 3ß-OH than male rats, although the mechanims remain unclear. Sex differences have been understudied in anaesthesia research, and this article by Joksimovic and colleagues emphasizes the need to devote more effort to understanding these differences.

The Neural Circuits Underlying General Anesthesia and Sleep.

General anesthesia is characterized by loss of consciousness, amnesia, analgesia, and immobility. Important molecular targets of general anesthetics have been identified, but the neural circuits underlying the discrete end points of general anesthesia remain incompletely understood. General anesthesia and natural sleep share the common feature of reversible unconsciousness, and recent developments in neuroscience have enabled elegant studies that investigate the brain nuclei and neural circuits underlying this important end point. A common approach to measure cortical activity across the brain is electroencephalogram (EEG), which can reflect local neuronal activity as well as connectivity among brain regions. The EEG oscillations observed during general anesthesia depend greatly on the anesthetic agent as well as dosing, and only some resemble those observed during sleep. For example, the EEG oscillations during dexmedetomidine sedation are similar to those of stage 2 nonrapid eye movement (NREM) sleep, but high doses of propofol and ether anesthetics produce burst suppression, a pattern that is never observed during natural sleep. Sleep is primarily driven by withdrawal of subcortical excitation to the cortex, but anesthetics can directly act at both subcortical and cortical targets. While some anesthetics appear to activate specific sleep-active regions to induce unconsciousness, not all sleep-active regions play a significant role in anesthesia. Anesthetics also inhibit cortical neurons, and it is likely that each class of anesthetic drugs produces a distinct combination of subcortical and cortical effects that lead to unconsciousness. Conversely, arousal circuits that promote wakefulness are involved in anesthetic emergence and activating them can induce emergence and accelerate recovery of consciousness. Modern neuroscience techniques that enable the manipulation of specific neural circuits have led to new insights into the neural circuitry underlying general anesthesia and sleep. In the coming years, we will continue to better understand the mechanisms that generate these distinct states of reversible unconsciousness.

Electrical stimulation of the ventral tegmental area restores consciousness from sevoflurane-, dexmedetomidine-, and fentanyl-induced unconsciousness in rats.

BACKGROUND: Dopaminergic neurons in the ventral tegmental area (VTA) are crucially involved in regulating arousal, making them a potential target for reversing general anesthesia. Electrical deep brain stimulation (DBS) of the VTA restores consciousness in animals anesthetized with drugs that primarily enhance GABAA receptors. However, it is unknown if VTA DBS restores consciousness in animals anesthetized with drugs that target other receptors. OBJECTIVE: To evaluate the efficacy of VTA DBS in restoring consciousness after exposure to four anesthetics with distinct receptor targets. METHODS: Sixteen adult Sprague-Dawley rats (8 female, 8 male) with bipolar electrodes implanted in the VTA were exposed to dexmedetomidine, fentanyl, ketamine, or sevoflurane to produce loss of righting, a proxy for unconsciousness. After receiving the dopamine D1 receptor antagonist, SCH-23390, or saline (vehicle), DBS was initiated at 30 µA and increased by 10 µA until reaching a maximum of 100 µA. The current that evoked behavioral arousal and restored righting was recorded for each anesthetic and compared across drug (saline/SCH-23390) condition. Electroencephalogram, heart rate and pulse oximetry were recorded continuously. RESULTS: VTA DBS restored righting after sevoflurane, dexmedetomidine, and fentanyl-induced unconsciousness, but not ketamine-induced unconsciousness. D1 receptor antagonism diminished the efficacy of VTA stimulation following sevoflurane and fentanyl, but not dexmedetomidine. CONCLUSIONS: Electrical DBS of the VTA restores consciousness in animals anesthetized with mechanistically distinct drugs, excluding ketamine. The involvement of the D1 receptor in mediating this effect is anesthetic-specific.

Return of the Righting Reflex Does Not Portend Recovery of Cognitive Function in Anesthetized Rats.

As the number of individuals undergoing general anesthesia rises globally, it becomes increasingly important to understand how consciousness and cognition are restored after anesthesia. In rodents, levels of consciousness are traditionally captured by physiological responses such as the return of righting reflex (RORR). However, tracking the recovery of cognitive function is comparatively difficult. Here we use an operant conditioning task, the 5-choice serial reaction time task (5-CSRTT), to measure sustained attention, working memory, and inhibitory control in male and female rats as they recover from the effects of several different clinical anesthetics. In the 5-CSRTT, rats learn to attend to a five-windowed touchscreen for the presentation of a stimulus. Rats are rewarded with food pellets for selecting the correct window within the time limit. During each session we tracked both the proportion of correct (accuracy) and missed (omissions) responses over time. Cognitive recovery trajectories were assessed after isoflurane (2% for 1 h), sevoflurane (3% for 20 min), propofol (10 mg/kg I.V. bolus), ketamine (50 mg/kg I.V. infusion over 10 min), and dexmedetomidine (20 and 35 µg/kg I.V. infusions over 10 min) for up to 3 h following RORR. Rats were classified as having recovered accuracy performance when four of their last five responses were correct, and as having recovered low omission performance when they missed one or fewer of their last five trials. Following isoflurane, sevoflurane, and propofol anesthesia, the majority (63-88%) of rats recovered both accuracy and low omission performance within an hour of RORR. Following ketamine, accuracy performance recovers within 2 h in most (63%) rats, but low omission performance recovers in only a minority (32%) of rats within 3 h. Finally, following either high or low doses of dexmedetomidine, few rats (25-32%) recover accuracy performance, and even fewer (0-13%) recover low omission performance within 3 h. Regardless of the anesthetic, RORR latency is not correlated with 5-CSRTT performance, which suggests that recovery of neurocognitive function cannot be inferred from changes in levels of consciousness. These results demonstrate how operant conditioning tasks can be used to assess real-time recovery of neurocognitive function following different anesthetic regimens.

D-Amphetamine Rapidly Reverses Dexmedetomidine-Induced Unconsciousness in Rats.

D-amphetamine induces emergence from sevoflurane and propofol anesthesia in rats. Dexmedetomidine is an α2-adrenoreceptor agonist that is commonly used for procedural sedation, whereas ketamine is an anesthetic that acts primarily by inhibiting NMDA-type glutamate receptors. These drugs have different molecular mechanisms of action from propofol and volatile anesthetics that enhance inhibitory neurotransmission mediated by GABAA receptors. In this study, we tested the hypothesis that d-amphetamine accelerates recovery of consciousness after dexmedetomidine and ketamine. Sixteen rats (Eight males, eight females) were used in a randomized, blinded, crossover experimental design and all drugs were administered intravenously. Six additional rats with pre-implanted electrodes in the prefrontal cortex (PFC) were used to analyze changes in neurophysiology. After dexmedetomidine, d-amphetamine dramatically decreased mean time to emergence compared to saline (saline:112.8 ± 37.2 min; d-amphetamine:1.8 ± 0.6 min, p < 0.0001). This arousal effect was abolished by pre-administration of the D1/D5 dopamine receptor antagonist, SCH-23390. After ketamine, d-amphetamine did not significantly accelerate time to emergence compared to saline (saline:19.7 ± 18.0 min; d-amphetamine:20.3 ± 16.5 min, p = 1.00). Prefrontal cortex local field potential recordings revealed that d-amphetamine broadly decreased spectral power at frequencies <25 Hz and restored an awake-like pattern after dexmedetomidine. However, d-amphetamine did not produce significant spectral changes after ketamine. The duration of unconsciousness was significantly longer in females for both dexmedetomidine and ketamine. In conclusion, d-amphetamine rapidly restores consciousness following dexmedetomidine, but not ketamine. Dexmedetomidine reversal by d-amphetamine is inhibited by SCH-23390, suggesting that the arousal effect is mediated by D1 and/or D5 receptors. These findings suggest that d-amphetamine may be clinically useful as a reversal agent for dexmedetomidine.