Isoflurane causes concentration-dependent inhibition of medullary raphé 5-HT neurons in situ

Acute psilocybin increased cortical activities in rats

Psilocybin, a naturally occurring hallucinogenic component of magic mushrooms, has significant psychoactive effects in both humans and rodents. But the underlying mechanisms are not fully understood. Blood-oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is a useful tool in many preclinical and clinical trials to investigate psilocybin-induced changes of brain activity and functional connectivity (FC) due to its noninvasive nature and widespread availability. However, fMRI effects of psilocybin on rats have not been carefully investigated. This study aimed to explore how psilocybin affects resting-state brain activity and FC, through a combination of BOLD fMRI and immunofluorescence (IF) of EGR1, an immediate early gene (IEG) closely related to depressive symptoms. Ten minutes after psilocybin hydrochloride injection (2.0 mg/kg, i.p.), positive brain activities were observed in the frontal, temporal, and parietal cortex (including the cingulate cortex and retrosplenial cortex), hippocampus, and striatum. And a region-of-interest (ROI) -wise FC analysis matrix suggested increased interconnectivity of several regions, such as the cingulate cortex, dorsal striatum, prelimbic, and limbic regions. Further seed-based analyses revealed increased FC of cingulate cortex within the cortical and striatal areas. Consistently, acute psilocybin increased the EGR1 level throughout the brain, indicating a consistent activation thought the cortical and striatal areas. In conclusion, the psilocybin-induced hyperactive state of rats is congruent to that of humans, and may be responsible for its pharmacological effects.

Volatile Anesthetics Activate a Leak Sodium Conductance in Retrotrapezoid Nucleus Neurons to Maintain Breathing during Anesthesia in Mice

BACKGROUND

Volatile anesthetics moderately depress respiratory function at clinically relevant concentrations. Phox2b-expressing chemosensitive neurons in the retrotrapezoid nucleus, a respiratory control center, are activated by isoflurane, but the underlying mechanisms remain unclear. The hypothesis of this study was that the sodium leak channel contributes to the volatile anesthetics-induced modulation of retrotrapezoid nucleus neurons and to respiratory output.

METHODS

The contribution of sodium leak channels to isoflurane-, sevoflurane-, and propofol-evoked activity of Phox2b-expressing retrotrapezoid nucleus neurons and respiratory output were evaluated in wild-type and genetically modified mice lacking sodium leak channels (both sexes). Patch-clamp recordings were performed in acute brain slices. Whole-body plethysmography was used to measure the respiratory activity.

RESULTS

Isoflurane at 0.42 to 0.50 mM (~1.5 minimum alveolar concentration) increased the sodium leak channel-mediated holding currents and conductance from -75.0 ± 12.9 to -130.1 ± 34.9 pA (mean ± SD, P = 0.002, n = 6) and 1.8 ± 0.5 to 3.6 ± 1.0 nS (P = 0.001, n = 6), respectively. At these concentrations, isoflurane increased activity of Phox2b-expressing retrotrapezoid nucleus neurons from 1.1 ± 0.2 to 2.8 ± 0.2 Hz (P < 0.001, n = 5), which was eliminated by bath application of gadolinium or genetic silencing of sodium leak channel. Genetic silencing of sodium leak channel in the retrotrapezoid nucleus resulted in a diminished ventilatory response to carbon dioxide in mice under control conditions and during isoflurane anesthesia. Sevoflurane produced an effect comparable to that of isoflurane, whereas propofol did not activate sodium leak channel-mediated holding conductance.

CONCLUSIONS

Isoflurane and sevoflurane increase neuronal excitability of chemosensitive retrotrapezoid nucleus neurons partly by enhancing sodium leak channel conductance. Sodium leak channel expression in the retrotrapezoid nucleus is required for the ventilatory response to carbon dioxide during anesthesia by isoflurane and sevoflurane, thus identifying sodium leak channel as a requisite determinant of respiratory output during anesthesia of volatile anesthetics.

EDITOR’S PERSPECTIVE

Serotonin Transporter and Plasma Membrane Monoamine Transporter Are Necessary for the Antidepressant-Like Effects of Ketamine in Mice

Major depressive disorder is typically treated with selective serotonin reuptake inhibitors (SSRIs), however, SSRIs take approximately six weeks to produce therapeutic effects, if any. Not surprisingly, there has been great interest in findings that low doses of ketamine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, produce rapid and long-lasting antidepressant effects. Preclinical studies show that the antidepressant-like effects of ketamine are dependent upon availability of serotonin, and that ketamine increases extracellular serotonin, yet the mechanism by which this occurs is unknown. Here we examined the role of the high-affinity, low-capacity serotonin transporter (SERT), and the plasma membrane monoamine transporter (PMAT), a low-affinity, high-capacity transporter for serotonin, as mechanisms contributing to ketamine’s ability to increase extracellular serotonin and produce antidepressant-like effects. Using high-speed chronoamperometry to measure real-time clearance of serotonin from CA3 region of hippocampus in vivo, we found ketamine robustly inhibited serotonin clearance in wild-type mice, an effect that was lost in mice constitutively lacking SERT or PMAT. As expected, in wild-type mice, ketamine produced antidepressant-like effects in the forced swim test. Mapping onto our neurochemical findings, the antidepressant-like effects of ketamine were lost in mice lacking SERT or PMAT. Future research is needed to understand how constitutive loss of either SERT or PMAT, and compensation that occurs in other systems, is sufficient to void ketamine of its ability to inhibit serotonin clearance and produce antidepressant-like effects. Taken together with existing literature, a critical role for serotonin, and its inhibition of uptake via SERT and PMAT, cannot be ruled out as important contributing factors to ketamine’s antidepressant mechanism of action. Combined with what is already known about ketamine’s action at NMDA receptors, these studies help lead the way to the development of drugs that lack ketamine’s abuse potential but have superior efficacy in treating depression.

Type of Anaesthetic Influences [11C]MDL100,907 Binding to 5HT2A Receptors in Porcine Brain

PURPOSE

Anaesthesia routinely is used in animal neuroimaging in order to reduce head motion artefacts and minimize the influence of stress. However, anaesthetics can modify radioligand binding profiles at receptor targets studied by positron emission tomography (PET). Here, we determined the effects of two routine anaesthetics on the binding of a tracer of the serotonin 5HT_2A receptors.

PROCEDURES

Isoflurane- and propofol-anesthetised Göttingen minipigs were imaged with [¹¹C]MDL100,907 PET and analysed using regions of interest and statistical non-parametric mapping.

RESULTS

The binding potentials of the tracer in striatum under isoflurane anaesthesia significantly exceeded those obtained under propofol anaesthesia, an effect we attribute to the higher blood flow in brain induced by the former.

CONCLUSIONS

Interactions between radioligands and anaesthesia must be carefully evaluated in the design of in vivo neuroimaging and interpretation of data.

Towards a better understanding of anesthesia emergence mechanisms: Research and clinical implications

Emergence from anesthesia (AE) is the ending stage of anesthesia featuring the transition from unconsciousness to complete wakefulness and recovery of consciousness (RoC). A wide range of undesirable complications, including coughing, respiratory/cardiovascular events, and mental status changes such as emergence delirium, and delayed RoC, may occur during this critical phase. In general anesthesia processes, induction and AE represent a neurobiological example of “hysteresis”. Indeed, AE mechanisms should not be simply considered as reverse events of those occurring in the induction phase. Anesthesia-induced loss of consciousness (LoC) and AE until RoC are quite distinct phenomena with, in part, a distinct neurobiology. Althoughanaesthetics produce LoC mostly by affecting cortical connectivity, arousal processes at the end of anesthesia are triggered by structures deep in the brain, rather than being induced within the neocortex. This work aimed to provide an overview on AE processes research, in terms of mechanisms, and EEG findings. Because most of the research in this field concerns preclinical investigations, translational suggestions and research perspectives are proposed. However, little is known about the relationship between AE neurobiology, and potential complications occurring during the emergence, and after the RoC. Thus, another scope of this review is to underline why a better understanding of AE mechanisms could have significant clinical implications, such as improving the patients’ quality of recovery, and avoiding early and late postoperative complications.

Serotonin Decreases the Gain of Visual Responses in Awake Macaque V1

Serotonin, an important neuromodulator in the brain, is implicated in affective and cognitive functions. However, its role even for basic cortical processes is controversial. For example, in the mammalian primary visual cortex (V1), heterogenous serotonergic modulation has been observed in anesthetized animals. Here, we combined extracellular single-unit recordings with iontophoresis in awake animals. We examined the role of serotonin on well-defined tuning properties (orientation, spatial frequency, contrast, and size) in V1 of two male macaque monkeys. We find that in the awake macaque the modulatory effect of serotonin is surprisingly uniform: it causes a mainly multiplicative decrease of the visual responses and a slight increase in the stimulus-selective response latency. Moreover, serotonin neither systematically changes the selectivity or variability of the response, nor the interneuronal correlation unexplained by the stimulus ("noise-correlation"). The modulation by serotonin has qualitative similarities with that for a decrease in stimulus contrast, but differs quantitatively from decreasing contrast. It can be captured by a simple additive change to a threshold-linear spiking nonlinearity. Together, our results show that serotonin is well suited to control the response gain of neurons in V1 depending on the animal's behavioral or motivational context, complementing other known state-dependent gain-control mechanisms.SIGNIFICANCE STATEMENT Serotonin is an important neuromodulator in the brain and a major target for drugs used to treat psychiatric disorders. Nonetheless, surprisingly little is known about how it shapes information processing in sensory areas. Here we examined the serotonergic modulation of visual processing in the primary visual cortex of awake behaving macaque monkeys. We found that serotonin mainly decreased the gain of the visual responses, without systematically changing their selectivity, variability, or covariability. This identifies a simple computational function of serotonin for state-dependent sensory processing, depending on the animal's affective or motivational state.

Isoflurane, ketamine-xylazine, and urethane markedly alter breathing even at subtherapeutic doses

Anesthetics are widely used for animal research on respiratory control in vivo, but their effect on breathing and CO₂ chemoreception has not been well characterized in mice, a species now often used for these studies. We previously demonstrated that 1% isoflurane markedly reduces the hypercapnic ventilatory response (HCVR) in adult mice in vivo and masks serotonin [5-hydroxytryptamine (5-HT)] neuron chemosensitivity in vitro. Here we investigated effects of 0.5% isoflurane on breathing in adult mice and also found a large reduction in the HCVR even at this subanesthetic concentration. We then tested the effects on breathing of ketamine-xylazine and urethane, anesthetics widely used in research on breathing. We found that these agents altered baseline breathing and blunted the HCVR at doses within the range typically used experimentally. At lower doses ventilation was decreased, but mice appropriately matched their ventilation to metabolic demands due to a parallel decrease in O₂ consumption. Neither ketamine nor urethane decreased chemosensitivity of 5-HT neurons. These results indicate that baseline breathing and/or CO₂ chemoreception in mice are decreased by anesthetics widely viewed as not affecting respiratory control, and even at subtherapeutic doses. These effects of anesthetics on breathing may alter the interpretation of studies of respiratory physiology in vivo.NEW & NOTEWORTHY Anesthetics are frequently used in animal research, but their effects on physiological functions in mice have not been well defined. Here we investigated the effects of commonly used anesthetics on breathing in mice. We found that all tested anesthetics significantly reduced the hypercapnic ventilatory response (HCVR), even at subtherapeutic doses. In addition, ketamine-xylazine and urethane anesthesia altered baseline breathing. These data indicate that breathing and the HCVR in mice are highly sensitive to anesthetic modulation.

Influence of general anaesthesia on the brainstem

The exact role of the brainstem in the control of body functions is not yet well known and the same applies to the influence of general anaesthesia on brainstem functions. Nevertheless in all general anaesthesia the anaesthesiologist should be aware of the interaction of anaesthetic drugs and brainstem function in relation to whole body homeostasis. As a result of this interaction there will be changes in consciousness, protective reflexes, breathing pattern, heart rate, temperature or arterial blood pressure to name a few. Brainstem function can be explored using three different approaches: clinically, analyzing changes in brain electric activity or using neuroimaging techniques. With the aim of providing the clinician anaesthesiologist with a global view of the interaction between the anaesthetic state and homeostatic changes related to brainstem function, the present review article addresses the influence of anaesthetic drug effects on brainstem function through clinical exploration of cranial nerves and reflexes, analysis of electric signals such as electroencephalographic changes and what it is known about brainstem through the use of imaging techniques, more specifically functional magnetic resonance imaging.

Effect of Portal Glucose Sensing on Systemic Glucose Levels in SD and ZDF Rats

Background

The global epidemic of Type-2-Diabetes (T2D) highlights the need for novel therapeutic targets and agents. Roux-en-Y-Gastric-Bypass (RYGB) is the most effective treatment. Studies investigating the mechanisms of RYGB suggest a role for post-operative changes in portal glucose levels. We investigate the impact of stimulating portal glucose sensors on systemic glucose levels in health and T2D, and evaluated the role of sodium-glucose-cotransporter-3 (SGLT3) as the possible sensor.

Methods

Systemic glucose and hormone responses to portal stimulation were measured. In Sprague-Dawley (SD) rats, post-prandial state was simulated by infusing glucose into the portal vein. The SGLT3 agonist, alpha-methyl-glucopyranoside (αMG), was then added to further stimulate the portal sensor. To elucidate the neural pathway, vagotomy or portal denervation was followed by αMG+glucose co-infusion. The therapeutic potential of portal glucose sensor stimulation was investigated by αMG-only infusion (vs. saline) in SD and Zucker-Diabetic-Fatty (ZDF) rats. Hepatic mRNA expression was also measured.

Results

αMG+glucose co-infusion reduced peak systemic glucose (vs. glucose alone), and lowered hepatic G6Pase expression. Portal denervation, but not vagotomy, abolished this effect. αMG-only infusion lowered systemic glucose levels. This glucose-lowering effect was more pronounced in ZDF rats, where portal αMG infusion increased insulin, C-peptide and GIP levels compared to saline infusions.

Conclusions

The portal vein is capable of sensing its glucose levels, and responds by altering hepatic glucose handling. The enhanced effect in T2D, mediated through increased GIP and insulin, highlights a therapeutic target that could be amenable to pharmacological modulation or minimally-invasive surgery.

Marmoset Serotonin 5-HT1A Receptor Mapping with a Biased Agonist PET Probe 18F-F13714: Comparison with an Antagonist Tracer 18F-MPPF in Awake and Anesthetized States

BACKGROUND

In vivo mapping by positron emission tomography of the serotonin 1A receptors has been hindered by the lack of suitable agonist positron emission tomography probes. ¹⁸F-labeled F13714 is a recently developed biased agonist positron emission tomography probe that preferentially targets subpopulations of serotonin 1A receptors in their "active state," but its brain labeling pattern in nonhuman primate has not been described. In addition, a potential confound in the translatability of PET data between nonhuman animal and human arise from the use of anesthetics that may modify the binding profiles of target receptors.

METHODS

Positron emission tomography scans were conducted in a cohort of common marmosets (n=4) using the serotonin 1A receptor biased agonist radiotracer, ¹⁸F-F13714, compared with a well-characterized ¹⁸F-labeled antagonist radiotracer, ¹⁸F-MPPF. Experiments on each animal were performed under both consciousness and isoflurane-anesthesia conditions.

RESULTS

¹⁸F-F13714 binding distribution in marmosets by positron emission tomography differs markedly from that of the ¹⁸F-MPPF. Whereas ¹⁸F-MPPF showed highest binding in hippocampus and amygdala, ¹⁸F-F13714 showed highest labeling in other regions, including insular and cingulate cortex, thalamus, raphe, caudate nucleus, and putamen. The binding potential values of ¹⁸F-F13714 were about one-third of those observed with ¹⁸F-MPPF, with marked individual- and region-specific differences under isoflurane-anesthetized vs conscious conditions.

CONCLUSIONS

These findings highlight the importance of investigating the brain imaging of serotonin 1A receptors using agonist probes such as ¹⁸F-F13714, which may preferentially target subpopulations of serotonin 1A receptors in specific brain regions of nonhuman primate as a biased agonist.