More auditory neurons in female mice that have given birth are excited by vocalizations from mouse pups than are auditory neurons in virgin females, according to research by Emory University scientists. Ultrasonic calls emitted by mouse pups communicate distress and elicit a search and retrieval response from mothers.
The study was conducted by Robert Liu, PhD, assistant professor of biology at Emory University. Dr. Liu presented his findings at the Neuroscience 2007 meeting, the annual meeting of the Society for Neuroscience held in San Diego.
Dr. Liu says the study supports the hypothesis that the sensory cortex is plastic and that learning the communicative significance of a sound can change how it is represented by the cortex.
"Knowledge gained from this study and others like it may help us understand normal brain changes associated with learning the importance of new sounds," says Dr. Liu.
This study extends the researchers' previous work in anesthetized mice, which showed that multi-neuron clusters are more informative about vocalizations when they are behaviorally relevant. This is the first time the single-neuron studies in awake mice have been presented. The main difference observed was in a class of neurons that can be excited by simple tonal sounds, such as those found in whistles. In mothers compared to virgin females, approximately twice as many of these neurons could also respond to the pup calls.
"These findings are new and interesting because they help elucidate the mechanisms by which the auditory system distinguishes important messages from background babble," says Liu.
"To control for communicative significance, we took advantage of the ultrasonic communication system between mouse pups and adult females. Past behavioral experiments showed that mothers recognize the behavioral relevance of ultrasonic calls produced by pups that have been isolated from their nest, whereas virgin females with no pup experience do not," says Dr. Liu.
In nature, mothers seek out lost pups and bring them back to the next. In experiments, mothers show approximately a two-to-one preference to approach a speaker playing pup calls compared to a speaker playing a neutral sound, says Dr. Liu. Virgin mice do not show this preference.
"We believe this mouse system provides a unique opportunity to explore the role of communicative significance in driving brain plasticity using electrophysiology, behavior, and in the future, genetics," says Dr.Liu.
Further experiments are underway to explore whether the hormonal changes associated with pregnancy and lactation are necessary to induce these cortical changes or whether experience alone can drive them.
The researchers' previous study in anesthetized mice was reported earlier this year in a paper published in PLoS Biology.