LOCALIZATION OF PREPROENKEPHALIN MESSENGER RNA-EXPRESSING CELLS IN RAT AUDITORY BRAIN-STEM WITH IN-SITU HYBRIDIZATION

Hair cells and auditory nerve dendrites in the inner ear are innervate d by pontine neurons that have been demonstrated by immunochemical tec hniques to contain several neurotransmitters, including acetylcholine and the opioid peptide enkephalins and dynorphins. The functions of th ese nerve fibers are not known, but may involve modifying auditory sen sitivity to low intensity stimuli. In the guinea pig the opioid pathwa ys originate in the lateral superior olivary region. A recent study in the gerbil has reported cells expressing preproenkephalin mRNA presen t only in the ventral nucleus of the trapezoid body, and not in the su perior olivary region. In the present study, a non-radioisotopically l abeled in situ hybridization method was used to identify cells express ing mRNA coding for preproenkephalin in rat pontine neurons, specifica lly in the ventral nucleus of the trapezoid body. These cells may repr esent an enkephalin-containing medial olivocochlear system in the rat, the origin of the lateral system in the rat that differs markedly fro m the better-studied guinea pig and cat, or a non-olivocochlear enkeph alin-containing system.