| Anesthesia animals are widely used in scientific experiments, for the stability and controllability during the narcotism. However, anesthesia agents would exert inevitable influences on the functional states and responses of the body, especially in the sensory and nervous systems. Many previous studies have been conducted to explore the changes of auditory processing in anesthesia condition. A functional magnetic resonance imaging study has proved that propofol dose-dependently attenuate responses of the auditory cortex to acoustic stimulation. Electrophysiological experiments have also shown the changes of response characteristics (such as the spontaneous rate, response latency, tuning sharpness, minimum median threshold and so on) of individual auditory neurons under different anesthesia. Evaluating the effects of anesthesia on the auditory system should be paid attention to in hearing researches.Non-volatile anesthetics, such as pentobarbital sodium, urethane, and ketamine, are commonly used in auditory experiments. Pentobarbital sodium has been demonstrated to produce hyperpolarizing effect by activating gamma-aminobutyric acid (GABA) receptor, and therefore reduces the firing rates of neurons in the inferior colliculus (IC) and the auditory cortex (AC). In addition, the response patterns and receptive areas of AC in most neurons also change under barbiturates. Thus, pentobarbital does not seem to be an ideal anesthetic for studies on AC.Ketamine is a rapid-acting anesthetic and used primarily for the induction and maintenance of general anesthesia, usually in combination with a sedative such as xylazine or medetomidine. Meanwhile, ketamine known as a noncompetitive N-methyl-d-aspartate (NMDA) receptors antagonist produces an anesthetic state termed dissociative anesthesia, because it appears to by selectively blocking transmission within the association pathway of the brain. Electroencephalograph (EEG) showed that ketamine, in contrast to the barbiturates, depressed the recruiting response when thalamic-neocortical activation was minimally affected at the same time. It means that ketamine may have less impact on the response properties of AC neurons. However, the disadvantage of ketamine is short-term effect of anesthesia, so that it is necessary to supply the maintenance dose periodically for keeping long-term recording in vivo.Urethane, with a long lasting effect and less influence on breathing, is suitable for long time study. The molecular and cellular mechanisms of urethane are intricate and still poorly understood. Earlier studies have showed that urethane could enhance the inhibitory currents (y-aminobutyric acid type A, GABAA and glycine currents) and the function of neuronal acetylcholine (nACh) receptors, and simultaneously, inhibit current responses of excitatory receptors (NMDA and a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, AMPA receptors). The dual inhibitory effects of urethane on excitatory and inhibitory inputs would consequentially influence the integration of neural outputs. Previous extracellular study has also demonstrated the same effects of urethane on neurons in IC.Aforementioned electrophysiological experiments about anesthetic on auditory responses are mostly performed by taking extracellular recording method. Characteristic properties of individual neurons should be analyzed by sorting single unit responses from multi-units, through the shape features of action potentials. The influence from neighboring neurons could not be eliminated exactly with this way. In recent years, the development of in vivo patch-clamp techniques provides a more direct and accurate means to obtain the activities of single cell. By forming high seal resistance (10MΩ to>5GQ) between the pipette tip and the cell membrane, the method of cell-attached recording overcomes the disadvantage of extracellular sorting, and maintains a stable condition for the attached cell within several hours.Relatively speaking, few reports have discussed the variant effects of different anesthetics on auditory responses of single neurons by in vivo cell-attached recording way. Here, we are interested in the diversities in the tonal receptive fields (TRFs) and response patterns of individual neurons in primary auditory cortex (A1) to tone bursts under urethane or ketamine-xylazine (the mixture of ketamine and xylazine, K-X) respectively. The characteristic frequency (CF), the minimum response threshold (MT), bandwidth (BW) and Q values of TRF, spontaneous rates and response latencies are compared between the two anesthesia conditions. The current study may provide evidence for choice of anesthetics in further electrophysiological study on auditory system.This study aimed to compare tonal response properties of neurons in primary auditory cortex under urethane and ketamine-xylazine anesthetized conditions in Sprague-Dawley rats. Forty-five female Sprague-Dawley rats (200-250g) were separated into two groups randomly, and anesthetized with urethane and ketamine-xylazine respectively. Tone pips were chosen as stimuli to obtain action potentials of single neurons by in vivo cell-attached recording. Features of the action potentials were extracted using Matlab software to analyze acoustic response properties of neurons between the two narcotics dosed groups. It showed that Q values and the characteristic frequencies were independent of types of anesthetic agents, however in urethane anesthesia condition, neurons tended to have higher minimum thresholds, lower spontaneous firing rates, longer response latencies, and more frequent occurrence of tuning with stronger inhibition compared to those in ketamine-xylazine dosed group. Urethane and ketamine might have no obvious impact on the transmission pathway of frequency tuning from periphery to auditory cortex, but neurons in urethane anesthetized rats receive enhanced inhibition mediated by interneurons or get lower intrinsic excitability.Complete data were obtained from117well-isolated single neurons collected from45rats. Among these neurons,62neurons were recorded in25animals anesthetized with urethane, and other55in20rats of K-X group. Five different TRF patterns were recorded from the rat A1:V/U-shape, Multi-peak, Closed, Atypical and Untuned.(1) According to the customized standard of group, neurons with TRF like a single-peak "V" or "U" were assorted into V/U-shape type.(2) Multi-peak neurons had TRFs with a closed bottom and an open top formed multi-peak shape.(3) Neurons showing a non-monotonic response to sound intensity, with a closed TRF shape, were defined as Closed type.(4) Atypical neurons exhibited a visible inhibitory region among a wider excitatory area, also named as inhibitory type by other studies.(5) Neurons which responded to the broadband stimuli but showed no selectivity to any frequency/intensity combination were classed as Untuned type. The distribution of the TRF types of Al neurons showed significant difference between the two anesthesia conditions. The urethane group contained a greater proportion of V/U-shape, Closed and Untuned types compared with the K-X group. Conversely, Multi-peak and Atypical neurons appeared more frequently in the K-X group than urethane group.There was no significant difference of CFs distributions between the two groups. It was suggested that neither of the narcotics may obviously disturb the auditory cortical neurons’frequency selectivity which was inherited from periphery auditory system. Meanwhile, the distributions and means of MTs were significantly different in the two conditions. In order to distinguish the diversity of MT between the two groups precisely, the TRF pattern was taken into consideration as a fixed factor. The results showed that MTs of A1neurons were tended toward higher tone levels in urethane anesthesia than in K-X, which meant that neurons in A1tended to have lower excitability under urethane.For a given CF, Q values (Q=CF/BW) are dependent on BW. Here, we compared the influence on Q values (Q10and Q30) under the two different anesthetic conditions. There was no significant difference of the distribution of CF between the two groups, so we could eliminate the influence of diversity of CF on Q values in different anesthesia. Although neurons in the urethane group showed lower Q values, neither Q10nor Q30displayed any markable difference between the two groups.Spontaneous rate of a neuron could reflect excitability of the neuron. While, anesthetic conditions do have some impact on the excitability. The first10ms discharges before the tone stimuli presentation were chosen as the time window to evaluate the diversity of spontaneous rate of each neuron between the two groups. It turned out to be that neurons in urethane anesthetized animals tended to have lower spontaneous rate and therefore lower excitability. Response latency to acoustic stimulus of an auditory neuron can reflect the integrated outputs of both excitatory and inhibitory inputs of the neuron. First, we calculated Lmin of each neuron in the two groups. It was obviously that Lmin under urethane anesthesia was longer than that in the K-X group.The response temporal pattern of the A1neurons was assorted into different types according to Lmin and Lpeak. In principle, in the urethane group, auditory neurons in A1could be divided into four groups:(1) Onset-type:neurons with Lmin shorter than30ms were categorized into this type.(2) Long-latency-type, referred to neurons responded later than30ms after the tone burst, with Lpeak shorter than80ms.(3) Rebound-type, neurons of which displayed a postinhibitory rebound only, of which Lmin was longer than30ms and Lpeak was longer than80ms.(4) Onset combined with rebound type (Onset-Rebound), were defined as those showed onset responses followed by a postinhibitory rebound. Neurons recorded in the K-X group, also exhibited Onset-type, Rebound-type, Onset combined with rebound-type, but without Long-latency-type. Note that, most neurons revealed onset responses within30ms after tone burst, which agreed with previous data obtained by means of extracellular recording, and the comparison of Lmin of onset compositions showed no obvious difference between the two groups. However, the distribution of response patterns under the two conditions appeared distinct diversity. Neurons recorded under K-X anesthesia contained a greater proportion of Onset-type, and the others appeared sparsely compared to those of the urethane group. It is supposed that both narcotics may have some impact on the temporal patterns of responses for A1neurons to tone stimuli. Neurons of A1in the urethane group seemed to accept depressed excitatory inputs or enhanced inhibitory inputs than K-X anesthesia, resulting in greater probabilities of later responses or postinhibitory rebound. On the whole, TRF patterns, Q values and CF of auditory neurons in A1exhibited no obvious diversities in both narcotics dosed groups, which could demonstrate that urethane and ketamine may have no obvious impact on the transmission pathway of frequency tuning from periphery to auditory cortex. Urethane may execute stronger inhibition to neurons compared with K-X in view of lower spontaneous rate and higher MT of neurons in the urethane group. And these two anesthetics have nonspecific inhibitory effect on auditory cortex. Taken together, the data presented here indicate that when investigating acoustic response characteristics of neurons in auditory pathway, at least in A1, anesthesia factors should be taken into account carefully. |
PDF Full Text Request