Projections of the lateral reticular nucleus to the cochlear nucleus in rats

The lateral reticular nucleus (LRN) resides in the rostral medulla and caudal pons, is implicated in cardiovascular regulation and cranial nerve reflexes, and gives rise to mossy fibers in the cerebellum. Retrograde tracing data revealed that medium-sized multipolar cells from the magnocellular part of the LRN project to the cochlear nucleus (CN). We sought to characterize the LRN projection to the CN using BDA injections. Anterogradely labeled terminals in the ipsilateral CN appeared as boutons and mossy fibers, and were examined with light and electron microscopy. The terminal field in the CN was restricted to the granule cell domain (GCD), specifically in the superficial layer along the anteroventral CN and in the granule cell lamina. Electron microscopy showed that the smallest LRN boutons formed 1-3 synapses, and as boutons increased in size, they formed correspondingly more synapses. The largest boutons were indistinguishable from the smallest mossy fibers, and the largest mossy fiber exhibited 15 synapses. Synapses were asymmetric with round vesicles and formed against thin dendritic profiles characterized by plentiful microtubules and the presence of fine filopodial extensions that penetrated the ending. These structural features of the postsynaptic target are characteristic of the terminal dendritic claw of granule cells. LRN projections are consistent with known organizational principles of non-auditory inputs to the GCD.