Latency of auditory evoked potential monitoring the effects of general anesthetics on nerve fibers and synapses

Nanoporous graphene-based thin-film microelectrodes for in vivo high-resolution neural recording and stimulation

One of the critical factors determining the performance of neural interfaces is the electrode material used to establish electrical communication with the neural tissue, which needs to meet strict electrical, electrochemical, mechanical, biological and microfabrication compatibility requirements. This work presents a nanoporous graphene-based thin-film technology and its engineering to form flexible neural interfaces. The developed technology allows the fabrication of small microelectrodes (25 µm diameter) while achieving low impedance (∼25 kΩ) and high charge injection (3-5 mC cm-2). In vivo brain recording performance assessed in rodents reveals high-fidelity recordings (signal-to-noise ratio >10 dB for local field potentials), while stimulation performance assessed with an intrafascicular implant demonstrates low current thresholds (<100 µA) and high selectivity (>0.8) for activating subsets of axons within the rat sciatic nerve innervating tibialis anterior and plantar interosseous muscles. Furthermore, the tissue biocompatibility of the devices was validated by chronic epicortical (12 week) and intraneural (8 week) implantation. This work describes a graphene-based thin-film microelectrode technology and demonstrates its potential for high-precision and high-resolution neural interfacing.

Urethane Improves the Response of Auditory Neurons to Tone

Urethane has little effect on nervous system and is often used in neuroscience studies. However, the effect of urethane in neurons is not thoroughly clear. In this study, we investigated changes in neuron responses to tones in inferior colliculus during urethane anesthesia. As urethane was metabolized, the best and characteristic frequencies did not obviously change, but the minimal threshold (MT) remained relatively stable or was elevated. The frequency tuning bandwidth at 60 dB SPL (BW_60dBSPL) remained unchanged or decreased, and the average evoked spike of effective frequencies at 60 dB SPL (ES_60dBSPL) gradually decreased. Although the average evoked spike of effective frequencies at a tone intensity of 20 dB SPL above MT (ES_{20dBSPLaboveMT}) decreased, the frequency tuning bandwidth at a tone intensity of 20 dB SPL above MT (BW_{20dBSPLaboveMT}) did not change. In addition, the changes in MT, ES_60dBSPL, BW_60dBSPL, and ES_{20dBSPLaboveMT} increased with the MT in pre-anesthesia awake state (MT_{pre−anesthesiaawake}). In some neurons, the MT was lower, BW_60dBSPL was broader, and ES_60dBSPL and ES_{20dBSPLaboveMT} were higher in urethane anesthesia state than in pre-anesthesia awake state. During anesthesia, the inhibitory effect of urethane reduced the ES_{20dBSPLaboveMT}, but did not change the MT, characteristic frequency, or BW_{20dBSPLaboveMT}. In the recording session with the strongest neuron response, the first spike latency did not decrease, and the spontaneous spike did not increase. Therefore, we conclude that urethane can reduce/not change the MT, increase the evoked spike, or broaden/not change the frequency tuning range, and eventually improve the response of auditory neurons to tone with or without “pushing down” the tonal receptive field in thresholding model. The improved effect increases with the MT_{pre−anesthesiaawake} of neurons. The changes induced by the inhibitory and improved effects of urethane abide by similar regularities, but the change directions are contrary. The improvement mechanism may be likely due to the increase in the ratio of excitatory/inhibitory postsynaptic inputs to neurons.

Synaptic excitation underlies processing of paired stimulation in the mouse inferior colliculus

The inferior colliculus (IC) receives inputs from the ascending auditory pathway and helps localize the sound source by shaping neurons' responses. However, the contributions of excitatory or inhibitory synaptic inputs evoked by paired binaural stimuli with different inter-stimulus intervals to auditory responses of IC neurons remain unclear. Here, we firstly investigated the IC neuronal response to the paired binaural stimuli with different inter-stimulus intervals using in vivo loose-patch recordings in anesthetized C57BL/6 mice. It was found that the total acoustic evoked spikes remained unchanged under microsecond interval conditions, but persistent suppression would be observed when the time intervals were extended. We further studied the paired binaural stimuli evoked excitatory/inhibitory inputs using in vivo whole-cell voltage-clamp techniques and blockage of the auditory nerve. The amplitudes of the contralateral excitatory inputs could be suppressed, unaffected or facilitated as the interaural delay varied. In contrast, contralateral inhibitory inputs and ipsilateral synaptic inputs remained almost unchanged. Most IC neurons exhibited the suppression of contralateral excitatory inputs over the interval range of dozens of milliseconds. The facilitative effect was generated by the summation of contralateral and ipsilateral excitation. Suppression and facilitation were completely abolished when ipsilateral auditory nerve was blocked pharmacologically, indicating that these effects were exerted by ipsilateral stimulation. These results suggested that the IC would inherit the binaural inputs integrated at the brainstem as well as within the IC and synaptic excitations, modulated by ipsilateral stimulation, underlie the binaural acoustic response.

Auditory Fear Conditioning Alters Sensitivity of the Medial Prefrontal Cortex but this is not based on Frequency-dependent Integration

Although many studies have shown that the prelimbic (PL) cortex of the mPFC is involved in the formation of conditioned freezing behavior, few have considered the acoustic response characteristics of PL cortex. Importantly, the change in auditory response characteristics of the PL cortex after conditional fear learning is largely unknown. Here we used in vivo cell-attached recordings targeting the mPFC during the waking state. We confirmed that the mPFC of adult C57 mice have neurons that respond to noise and tone in the waking state, especially in the PL cortex. Interestingly, the data also confirmed that these neurons responded well to the intensity of sound but did not have frequency topological distribution characteristics. Furthermore, we found that the number of c-fos positive neurons in the PL cortex increased significantly after auditory fear conditioning. The auditory-induced local field potential recordings and in vivo cell-attached recordings demonstrated that the PL cortex was more sensitive to the auditory conditioned stimulus after the acquisition of conditioned fear. The proportion of neurons responding to noise was significantly increased, and the signal to noise ratio of the spikes were also increased. These data reveal that PL neurons themselves responded to the main information (sound intensity), while the secondary information (frequency) response was almost negligible after auditory fear conditioning. This phenomenon may be the functional basis for handling this type of emotional memory, and this response characteristic is thought to be emotional sensitization but does not change the nature of this response.

Robust and Intensity-Dependent Synaptic Inhibition Underlies the Generation of Non-monotonic Neurons in the Mouse Inferior Colliculus

Intensity and frequency are the two main properties of sound. The non-monotonic neurons in the auditory system are thought to represent sound intensity. The central nucleus of the inferior colliculus (ICC), as an important information integration nucleus of the auditory system, is also involved in the processing of intensity encoding. Although previous researchers have hinted at the importance of inhibitory effects on the formation of non-monotonic neurons, the specific underlying synaptic mechanisms in the ICC are still unclear. Therefore, we applied the in vivo whole-cell voltage-clamp technique to record the excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) in the ICC neurons, and compared the effects of excitation and inhibition on the membrane potential outputs. We found that non-monotonic neuron responses could not only be inherited from the lower nucleus but also be created in the ICC. By integrating with a relatively weak IPSC, approximately 35% of the monotonic excitatory inputs remained in the ICC. In the remaining cases, monotonic excitatory inputs were reshaped into non-monotonic outputs by the dominating inhibition at high intensity, which also enhanced the non-monotonic nature of the non-monotonic excitatory inputs.

Selectivity of Monaural Synaptic Inputs Underlying Binaural Auditory Information Integration in the Central Nucleus of Inferior Colliculus

Neurons in the central nucleus of the inferior colliculus (ICC) receive ascending inputs from the ipsilateral and contralateral auditory pathway. However, the contributions of excitatory or inhibitory synaptic inputs evoked by ipsilateral and contralateral stimuli to auditory responses of ICC neurons remain unclear. Using in vivo whole-cell voltage-clamp recordings, we investigated excitatory and inhibitory synaptic currents in neurons of the ICC in response to binaural stimulation by performing an intensity-intensity scan. To systematically analyze the contribution of the ipsilateral and contralateral ear, the sound intensity was randomly delivered to each side from 0 dB sound pressure level (SPL) to 70 dB SPL. Although the synaptic responses were dominated by contralateral inputs at weak sound intensities, they could be increased (or decreased) by additional ipsilateral stimulation at higher intensities. Interestingly, the synaptic responses to contralateral acoustic inputs were not linearly superimposed with the ipsilateral ones. By contrast, the responses showed either a contralateral or ipsilateral profile, depending on which one was more dominant. This change occurred at a certain intensity “switch” point. Thus, the binaural auditory responses of the ICC neurons were not simply mediated by the summation of the inputs evoked by ipsilateral and contralateral stimulations. This suggested that the ICC might inherit the acoustic information integrated at the brainstem, causing the selectivity of monaural excitation and inhibition to underlie the neuronal binaural acoustic response.

Septal and Hippocampal Neurons Contribute to Auditory Relay and Fear Conditioning

The hippocampus has been thought to process auditory information. However, the properties, pathway, and role of hippocampal auditory responses are unclear. With loose-patch recordings, we found that hippocampal neurons are mainly responsive to noise and are not tonotopically organized. Their latencies are shorter than those of primary auditory cortical (A1) neurons but longer than those of medial septal (MS) neurons, suggesting that hippocampal auditory information comes from MS neurons rather than from A1 neurons. Silencing the MS blocks both hippocampal auditory responses and memory of auditory fear conditioning trained with noise and tone. Auditory fear conditioning was associated with some cues but not with a specific frequency of sound, as demonstrated by animals trained with noise, 2.5-, 5-, 10-, 15-, or 30-kHz tones, and tested with these sounds. Therefore, the noise responses of hippocampal neurons have identified a population of neurons that can be associated with auditory fear conditioning.

The Effect of Sedation on Cortical Activation: A Randomized Study Comparing the Effects of Sedation With Midazolam, Propofol, and Dexmedetomidine on Auditory Processing

BACKGROUND

Every day, millions of people undergo surgical procedures facilitated by anesthesia. Yet, there is no clinically accepted measure to predict the effects of sedation or anesthesia on the central nervous system. Auditory brain activation may provide an objective and quantifiable method to measure of the effects of sedation on neuronal processing.

METHODS

This is a randomized clinical trial. Forty-eight healthy volunteers were randomly assigned to receive 1 of 3 sedative drugs (midazolam [n = 11], propofol [n = 12], or dexmedetomidine [n = 12]) at a concentration adjusted to achieve mild sedation by self-rating, or to a no-drug control group (n = 13). Participants underwent functional magnetic resonance imaging while listening to music in a 5-minute block design experiment. We tested the hypothesis that mild sedation changes the magnitude or extent of cortical activation of an auditory stimulus.

RESULTS

We observed a significant reduction in auditory activation in both the dexmedetomidine (P = .001) and midazolam (P = .029) but not the propofol group (P = .619) when compared with saline control.

CONCLUSIONS

Our findings indicate that, compared with saline control, there is a significant reduction of brain activation in the auditory cortex in response to midazolam and dexmedetomidine but not propofol when given at mildly sedative doses. This method serves as a novel approach to quantify the effects of sedative agents in an objective fashion.