The immunocytochemical localization of substance P (SP) in vestibular endorgans of various mammals (rat, guinea pig, rabbit, squirrel monkey, and man) was investigated. SP-like immunoreactivity was found in all the vestibular endorgans studied. Most SP immunoreactive fibers were thick and distributed around or beneath sensory hair cells. A smaller group of SP immunoreactive varicose thin fibers was present in the stroma of vestibular endorgans, nerve trunks, and along blood vessels around the vestibular nerves. The former, probably corresponding to the afferent nerve endings of hair cells, was found predominantly in the peripheral region of each endorgan. There were no great species differences in staining patterns, except in rat vestibular endorgans, where numerous thin SP immunoreactive fibers were distributed beneath the sensory epithelium. SP immunoreactive fibers were also found in human vestibular endorgans, suggesting that SP may play an important role in human vestibular function.
