Terminal types on ipsilaterally and contralaterally projecting lateral superior olive cells

The lateral superior olive (LSO) in ferret contains two distinct populations of principal cells, one population projecting to the ipsilateral and the other projecting to the contralateral inferior colliculus. In addition, these populations have been shown to be distinct from each other on the basis of tonotopic and isofrequency distribution within LSO, of dendritic morphology, and of neurotransmitter within the somata. The present study compared the two populations on the basis of the type of synaptic input. Laterality of projection was established using horseradish peroxidase histochemistry. Synaptic terminals contacting LSO somata identified as projecting ipsilaterally or contralaterally were quantified as round (R) or nonround (NR), representing presumptive excitatory and inhibitory input, respectively. Results indicate that the vast majority of somatic terminals contacting both projection populations are NR and, furthermore, that R terminals are significantly more likely to contact ipsilaterally than contralaterally projecting LSO cells. There is no significant difference in number of NR terminals or total number of terminals between ipsilaterally and contralaterally projecting LSO cells. These findings provide additional support to the notion that the LSO is comprised of two distinct cell populations. Moreover, they indicate a difference in the balance of somatic inhibition and excitation which may have an impact on the nature of the response properties of the two populations.