Age-dependent Changes Of Isolation Calls In Infant Rats And The Encoding Of The Isolation Calls By The Auditory Cortex Neurons In Adult Rats

Isolation calls generated by infant rats under the conditions of isolation from their mothers have been extensively studied.These isolation calls are generally ultrasonic vocalizations(USVs).An increase in the number of isolation-induced ultrasonic calls indicates increased anxiety.Previous studies have found that the frequency and duration of the isolation calls vary with age.However,there is still a lack of systematic research on how the other acoustic parameters of the isolated calls emitted by infant rats vary with age under the condition of mother-infant separation.In addition,the neural mechanism of the encoding of the isolated calls by the neurons in the auditory cortex of adult rats is still unclear.In this study,we recorded the isolation calls of neonatal rats from day 1 to 21 after birth,and analyzed the changes of various acoustic parameters of the isolation calls with age.We further recorded the auditory responses of auditory cortex neurons in adult rats to isolated sounds(natural calls)and the time-reversed calls,and determine the preference of auditory cortex neurons for natural calls and reversed calls,as well as the dependence of the responses of the auditory cortex neurons on the level of the isolated calls,and compared the gender differences in the response strength of adult rat auditory cortex neurons to the isolated calls.We recorded the isolation calls emitted by infant rats under the conditions of maternal and child separation from postnatal day 1 to days 21.The results showed age-related changes in the types of isolation calls,the call rates,the call frequency,the call duration,and the time interval between two adjacent calls.We also found age-related changes in the number of call trains,the duration of call trains,the number of calls in a train,and the time interval between two adjacent call trains.We used the recorded isolated sounds as acoustic stimuli,and recorded the auditory responses of neurons in the primary auditory cortex of adult rats to natural and reversed calls under three stimuli levels(75 d B SPL,65 d B SPL and 55 d B SPL).The results showed that most neurons in the primary auditory cortex had higher response strength to natural calls than to reverse calls,and showed a preference for natural sounds.Only a few neurons had preference for reversed calls,or had no significant preference for both the natural and reversed calls.In addition,the response of most auditory neurons to natural sound is level-dependent,and the strength of the auditory response to natural sound increases with the increase of sound levels.However,we did not find significant differences in the response strength of neurons in the auditory cortex of adult rats to the isolated calls of infant rats.The results of this study provide experimental evidence for understanding the development of isolated calls in young rats and the encoding mechanism of intraspecific calls in adult rats under the condition of mother-infant separation.