Immunocytochemical distribution of Met-enkephalin-Arg⁶-Gly ⁷-Leu⁸ (Met-8) in the auditory system of the rat

Methionine-enkephalin-Arg⁶-Gly⁷-Leu⁸ (Met⁸) is known to act as a neurotransmitter or neuromodulator and it has been implicated in pain, cardiovascular and motor mechanisms, but its role in audition is currently unknown. In the present study we have applied an immunocytochemical technique and describe the distribution of cell bodies and fibers containing Met⁸ in the auditory pathway of the rat. The main finding is that we found either Met⁸-immunoreactive fibers or cell bodies or both in virtually all nuclei of the rat auditory system except for the medial superior olive and the ventral division of the medial geniculate body in which we did not find any immunoreactivity for Met⁸. This suggests that the neuropeptide Met⁸ is widely distributed throughout the auditory system of the rat. Our results suggest that Met⁸ could play at least two roles in hearing. It seems to be involved in the processing of the descending auditory pathway, and it may be implicated in the multisensory integration of auditory information that takes place in the non-lemniscal auditory pathway.