Influence of social isolation on serotonergic fiber density in the auditory midbrain

Abstract

Animal communication involves both a “sender”, which emits a signal, and a “receiver”, which processes the signal. Signal processing can be shaped by past social experience. In the auditory midbrain, serotonin is a potential mechanism for linking social experience with auditory processing for three reasons: 1) the serotonergic system is sensitive to social experience in other brain regions; 2) changes in serotonin concentration reflect contextual features of social encounters; and 3) serotonin influences single auditory neurons’ response to sounds, such as conspecific vocalizations. Laboratory mice (Mus musculus) provide an excellent model for addressing this potential mechanism, because mice demonstrate sensitivity to social experience via a simple manipulation of social housing conditions, and because laboratory mice are being increasingly studied as a model of auditory communication. In this study, we investigated how social isolation affects serotonergic fiber density in the inferior colliculus (IC), an auditory midbrain region that demonstrates selectivity for species‐specific vocalizations. Male CBA/J laboratory mice were given different social experiences by housing mice for four weeks either individually or in a social group of three mice. Subsequently, immunohistochemistry with florescence was used to label the serotonin transporter (SERT) as a marker for serotonergic fibers. Coronal sections of brain tissue were imaged, and ImageJ was used to quantify fiber density as the percent area of the IC covered by fibers. Mice from both social treatments demonstrated dense coverage (around 30%) of the IC by serotonergic fibers, corroborating with previous studies that demonstrate innervation of the IC by the serotonergic system. Individually housed mice had qualitatively lower percent coverage of the IC by serotonergic fibers than socially housed mice, particularly in sections of the IC that were more caudal; however, there were no statistically significant differences between social versus isolated treatment groups. Whether the smaller qualitative differences over the entire IC could be driven by disparities in the effects of social isolation among subregions of the IC has not yet been explored. These preliminary results suggest that social experience does not affect serotonergic fiber density in the IC. However, this does not rule out influences of social experience on other aspects of serotonergic signaling in the auditory midbrain, including socially induced serotonin release and profiles of serotonin receptors.Support or Funding InformationNSF grants: IOS 1456298 and DBI 1460949