The Effect Of Afterdischarge On Tone Evoked Response Of Primary Auditory Cortex Neurons In Anesthetized Mice

PART I COMPARISON OF THE AFTERDISCHARGE BYELECTRICAL STIMULATION OF PREFRONTAL CORTEXIN URETHANE AND KETAMINE ANESTHETIZED MICEObjective: To compare the effects of urethane and ketamine on theafterdischarge (AD) by electrical stimulation of prefrontal cortex.Methods: Twenty4-6weeks old female C57/6J mice were divided totwo groups: one was anesthetized with urethane and the other withketamine. Under anesthesia, a tungsten electrode was placed in the leftprefrontal cortex for electrical stimulation. A1-second-long train consistedof60Hz1-ms-long biphasic pulses, with an initial intensity of700uAwere delivered every1minute through the tungsten electrode until the firstAD appeared. After10-15minutes, identical electrical stimulation wasdelivered every10minutes for20times. EEG was continuously recorded.AD occurrence was considered when the EEG wave amplitude increasedsharply and duration was longer than5sconds. EEG wave frequencycomponents were analyzed by fast fourier transformation (FFT) and amplitude was the root mean square (RMS) value based on EEG waverecord.Results: Of10animals in each group, seven developed initial ADwithin3hours in urethane group averaging39stimulations each, while sixdeveloped initial AD within3hours in ketamine group averaging74stimulations each. As ES number increased, AD duration increased as wellfor both ketamine and urethane groups. In pre AD period, in both urethaneand ketamine groups EEG frequencies consisted a predominantly δ and θcomponents. During AD, γ and β components increased. Of note, the γcomponent increased more significantly in urethane group while βcomponent increased more significantly in ketamine group, the differencewas significant between two groups (p<0.05). In post AD period, theincreased β and γ components fell down while δ, θ ratio increased in bothgroups.Conclusions: The AD can be induced by electrical stimulation ofprefrontal cortex in mice under either urethane or ketamine anesthesiaalthough there were some differences in AD frequency components of. PART II THE EFFECT OF AFTERDISCHARGE ON TONEEVOKED RESPONSE OF PRIMARY AUDITORY CORTEXNEURONS IN ANESTHETIZED MICEObjective: To investigate the effect of afterdischarge on tone evokedresponse of primary auditory cortex neurons in anesthetized mice.Methods: Sixteen4-6weeks old female C57/6J mice wereanesthetized with ketamine. Then a tungsten electrode was placed in theleft primary auditory cortex and a frequency amplitude scan was performedto determine the best frequency (BF) and minimum threshold (MT) of therecorded AI neurons. A20ms single tone at best frequency and10dB SPLabove MT was then delivered and tone evoked neural responses wererecorded. The tone delivery was repeated for225times. After that anothertungsten electrode was placed in the left prefrontal cortex for electricalstimulation and a1-second-long train consisted of60Hz1-ms-longbiphasic pulses, with an initial intensity of700uA were delivered every1minute through the tungsten electrode until an AD appeared. After10-15minutes, identical electrical stimulation was delivered every10minutes for20times. A20ms single tone of same frequency and intensity wasdelivered and repeated for225times just after each electrical stimulation,tone evoked neural responses were recorded continiously. When20thEScompleted, another frequency amplitude scan was performed and the20ms single tone of same parameter was delivered continuously to get the neuralresponses in post-AD period.Results: Of16mice,10developed initial AD within3hours inaveraging70.5±3.81stimulations. As ES number increased, AD durationshowed an increasing trend. Compared with pre-AD period, the averagebest frequency, average minimum threshold, average bandwidth at10dBSPL and30dB SPL above minimum threshold, average spike number at10dB SPL and20dB SPL above minimum threshold of primary auditorycortex neurons after AD did not show siginificant differences. As ADincreased, spike number of tone evoked response of primary auditorycortex neurons showed a rising trend, when AD stopped, the tone evokedresponse droped back to baseline level. Spike number of tone evokedresponse of primary auditory cortex neurons during AD was significantlyhigher than that in pre-AD and post-AD period (p<0.05). While responselatency, time to response peak and spike rising slope of primary auditorycortex neurons during AD showed no significant difference with that inpre-AD and post-AD period.Conclusions: Spike number of tone evoked response of primaryauditory cortex neurons increased significantly during afterdischargeepisode, while frequancy/amplitude tuning curve, response latency, time toresponse peak and spike rising slope did not show significant differences.