Inferior colliculus projections to pontine nuclei in guinea pig.

The present study examined the neural projection from the inferior colliculus to the pontine nuclei in guinea pig. This projection has been reported in other animals, and our goal was to establish the projection in guinea pig, a commonly used auditory model. Ultimately, we wanted to determine if the pontine nuclei could be a component of the descending auditory system from the inferior colliculus to the cochlear nucleus. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into one inferior colliculus of 10 animals and the pontine nuclei examined under a light microscope to detect PHA-L-labeled fibers. PHA-L-labeled fibers were observed in the ipsilateral pontine nuclei in 70% of the animals. While the majority of labeled fibers were smooth in appearance, a few fibers with en passant type varicosities (indicating synapses) were observed in the dorsolateral area of the pontine nuclei, adjacent to the lateral lemniscus. These findings do not support a robust projection from the inferior colliculus to the pontine nuclei in guinea pig. This is in opposition to findings in bat in which the projection may play a major role in modulating responses to sound.