Functional hyperemic response in the rat visual cortex under halothane anesthesia

The Effect of Low-Intensity Transcranial Ultrasound Stimulation on Neural Oscillation and Hemodynamics in the Mouse Visual Cortex Depends on Anesthesia Level and Ultrasound Intensity

OBJECTIVE

Low-intensity transcranial ultrasound stimulation (TUS) can induce motor responses, neural oscillation and hemodynamic responses. Early studies demonstrated that the motor responses evoked by TUS critically depend on anesthesia levels and ultrasound intensity. However, the neural mechanism of how anesthesia levels and ultrasound intensity influence on brain responses during TUS has never been explored yet. To investigate this question, we applied different anesthesia levels and ultrasound intensities on the visual cortex of mouse and observed neural oscillation change and hemodynamic responses during TUS.

METHODS

low-intensity ultrasound was delivered to mouse visual cortex under different anesthesia levels, and simultaneous recordings for local field potentials (LFPs) and hemodynamic responses were carried out to measure and analyze the changes quantitatively.

RESULTS

(i) The change of mean amplitude and mean relative power of sharp wave-ripple (SPW-R) in LFPs induced by TUS decreased as the anesthesia level increased (from awake to 1.5% isoflurane). (ii) The hemodynamic response level induced by TUS decreased as the anesthesia level increased (from awake to1.5% isoflurane). (iii) The coupling strength between neural activities and hemodynamic responses was dependent on anesthesia level. (iv) The neural activities and hemodynamic responses increase as a function of ultrasound intensity.

CONCLUSION

These results support that the neural activities and hemodynamic response of the mouse visual cortex induced by TUS are related to the anesthesia level and ultrasound intensity.

SIGNIFICANCE

This finding suggests that careful maintenance of anesthesia level and ultrasound intensity is required to acquire accurate LFP and hemodynamic data from samples with TUS.

Comparison of stimulus-evoked cerebral hemodynamics in the awake mouse and under a novel anesthetic regime

Neural activity is closely followed by a localised change in cerebral blood flow, a process termed neurovascular coupling. These hemodynamic changes form the basis of contrast in functional magnetic resonance imaging (fMRI) and are used as a correlate for neural activity. Anesthesia is widely employed in animal fMRI and neurovascular studies, however anesthetics are known to profoundly affect neural and vascular physiology, particularly in mice. Therefore, we investigated the efficacy of a novel ‘modular’ anesthesia that combined injectable (fentanyl-fluanisone/midazolam) and volatile (isoflurane) anesthetics in mice. To characterize sensory-evoked cortical hemodynamic responses, we used optical imaging spectroscopy to produce functional maps of changes in tissue oxygenation and blood volume in response to mechanical whisker stimulation. Following fine-tuning of the anesthetic regime, stimulation elicited large and robust hemodynamic responses in the somatosensory cortex, characterized by fast arterial activation, increases in total and oxygenated hemoglobin, and decreases in deoxygenated hemoglobin. Overall, the magnitude and speed of evoked hemodynamic responses under anesthesia resembled those in the awake state, indicating that the novel anesthetic combination significantly minimizes the impact of anesthesia. Our findings have broad implications for both neurovascular research and longitudinal fMRI studies that increasingly require the use of genetically engineered mice.

Anesthesia and the quantitative evaluation of neurovascular coupling

Anesthesia has broad actions that include changing neuronal excitability, vascular reactivity, and other baseline physiologies and eventually modifies the neurovascular coupling relationship. Here, we review the effects of anesthesia on the spatial propagation, temporal dynamics, and quantitative relationship between the neural and vascular responses to cortical stimulation. Previous studies have shown that the onset latency of evoked cerebral blood flow (CBF) changes is relatively consistent across anesthesia conditions compared with variations in the time-to-peak. This finding indicates that the mechanism of vasodilation onset is less dependent on anesthesia interference, while vasodilation dynamics are subject to this interference. The quantitative coupling relationship is largely influenced by the type and dosage of anesthesia, including the actions on neural processing, vasoactive signal transmission, and vascular reactivity. The effects of anesthesia on the spatial gap between the neural and vascular response regions are not fully understood and require further attention to elucidate the mechanism of vascular control of CBF supply to the underlying focal and surrounding neural activity. The in-depth understanding of the anesthesia actions on neurovascular elements allows for better decision-making regarding the anesthetics used in specific models for neurovascular experiments and may also help elucidate the signal source issues in hemodynamic-based neuroimaging techniques.

Flash visual evoked potentials in mice can be modulated by transcranial direct current stimulation

Transcranial direct current stimulation (tDCS) in humans has been shown to affect the size of visual evoked potentials (VEPs) in a polarity-dependent way. VEPs have been widely employed in mice to study the visual system in physiological and pathological conditions and are extensively used as animal models of neurological and visual disorders. The present study was performed to evaluate whether mice VEPs could be modulated by tDCS in the same manner as in humans. We describe here the effects of 10 min tDCS (anodal, cathodal or no stimulation) on flash-VEPs in C57BL/6 mice under sevoflurane anesthesia. VEP amplitudes of the first major peak (P1) were analyzed before, at 0, 5 and 10 min after tDCS. Compared with no stimulation condition, anodal tDCS increased P1 amplitude slightly more than 25%, while cathodal stimulation had opposite effects, with a decrease of P1 amplitude by about 30%. After-effects tended to reverse toward basal levels within 10 min after tDCS. These results, suggesting polarity-dependent modulation similar to what described in humans of tDCS effects on VEPs, encourage the use of mice models to study tDCS mechanisms of action and explore therapeutic applications on neurological models of disease.

Dose-dependent effect of isoflurane on neurovascular coupling in rat cerebral cortex

Neurovascular coupling studies are widely conducted in anesthetized animals using functional magnetic resonance imaging (fMRI). In this study, the dose-dependent effects of isoflurane on neurovascular coupling were examined with concurrent recordings of the local field potential (FP) and cerebral blood flow (CBF) in the rat somatosensory cortex. Electrical forepaw stimulation was used, and consisted of either a single pulse or 10 pulses at various frequencies. We observed that the FP response to single-pulse stimulation remained unaffected across the different levels of isoflurane tested (1.1-2.1%), whereas the CBF response to single-pulse stimulation increased dose-dependently (7 +/- 3% to 17 +/- 4%). The isoflurane dose did not affect the vascular reactivity induced by a hypercapnic challenge. These findings suggest that the action of isoflurane affects the neurovascular mechanisms. For 10-pulse stimulation, the summation of the evoked FP responses monotonically decreased with an increase in the isoflurane dose, possibly due to enhancement of the neural adaptation. In contrast, the dose-dependent effect on the CBF response varied with the stimulus frequency; a dose-dependent decrease in the CBF response was observed for high-frequency stimulation, whereas a dose-dependent increase was observed for low-frequency stimulation. Furthermore, a linear time-invariant model consisting of the single-pulse hemodynamic impulse response convoluted with 10-pulse FP recordings showed that the neurovascular transfer function was altered by the isoflurane dose for high-frequency stimulation. These results indicate that careful and consistent maintenance of the depth of anesthesia is required when comparing fMRI data obtained from different animals or physiological and pharmacological manipulations.

A protocol for use of medetomidine anesthesia in rats for extended studies using task-induced BOLD contrast and resting-state functional connectivity

The alpha-2-adrenoreceptor agonist, medetomidine, which exhibits dose-dependent sedative effects and is gaining acceptance in small-animal functional magnetic resonance imaging (fMRI), has been studied. Rats were examined on the bench using the classic tail-pinch method with three infusion sequences: 100 microg/kg/h, 300 microg/kg/h, or 100 microg/kg/h followed by 300 microg/kg/h. Stepping the infusion rate from 100 to 300 microg/kg/h after 2.5 h resulted in a prolonged period of approximately level sedation that cannot be achieved by a constant infusion of either 100 or 300 microg/kg/h. By stepping the infusion dosage, experiments as long as 6 h are possible. Functional MRI experiments were carried out on rats using a frequency dependent electrical stimulation protocol-namely, forepaw stimulation at 3, 5, 7, and 10 Hz. Each rat was studied for a four-hour period, divided into two equal portions. During the first portion, rats were started at a 100 microg/kg/h constant infusion. During the second portion, four secondary levels of infusion were used: 100, 150, 200, and 300 microg/kg/h. The fMRI response to stimulation frequency was used as an indirect measure of modulation of neuronal activity through pharmacological manipulation. The frequency response to stimulus was attenuated at the lower secondary infusion dosages 100 or 150 microg/kg/h but not at the higher secondary infusion dosages 200 or 300 microg/kg/h. Parallel experiments with the animal at rest were carried out using both electroencephalogram (EEG) and functional connectivity MRI (fcMRI) methods with consistent results. In the secondary infusion period using 300 microg/kg/h, resting-state functional connectivity is enhanced.

FTY720 sustains and restores neuronal function in the DA rat model of MOG-induced experimental autoimmune encephalomyelitis

FTY720 (fingolimod) is an oral sphingosine 1-phosphate (S1P) receptor modulator under development for the treatment of multiple sclerosis (MS). To elucidate its effects in the central nervous system (CNS), we compared functional parameters of nerve conductance in the DA rat model of myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE) after preventive and therapeutic treatment. We demonstrate that prophylactic therapy protected against the emergence of EAE symptoms, neuropathology, and disturbances to visual and somatosensory evoked potentials (VEP, SEP). Moreover, therapeutic treatment from day 25 to 45 markedly reversed paralysis in established EAE and normalized the electrophysiological responses, correlating with decreased demyelination in the brain and spinal cord. The effectiveness of FTY720 in this model is likely due to several contributing factors. Evidence thus far supports its role in the reduction of inflammation and preservation of blood-brain-barrier integrity. FTY720 may also act via S1P receptors in glial cells to promote endogenous repair mechanisms that complement its immunomodulatory action.

Cortical spreading depression causes and coincides with tissue hypoxia

Cortical spreading depression (CSD) is a self-propagating wave of cellular depolarization that has been implicated in migraine and in progressive neuronal injury after stroke and head trauma. Using two-photon microscopic NADH imaging and oxygen sensor microelectrodes in live mouse cortex, we find that CSD is linked to severe hypoxia and marked neuronal swelling that can last up to several minutes. Changes in dendritic structures and loss of spines during CSD are comparable to those during anoxic depolarization. Increasing O2 availability shortens the duration of CSD and improves local redox state. Our results indicate that tissue hypoxia associated with CSD is caused by a transient increase in O2 demand exceeding vascular O2 supply.