Illustration of rat inner ear anatomy and transportation routes of substances to be gained in the inner ear after different administrations. After intratympanic delivery, agents may enter the inner ear through the round window membrane (1) and annular ligament (2), which surrounds the stapes footplate within the oval window and comprises the stapediovestibular joint (3). Stapedial artery (4) is a prominant structure in the rat middle ear, which runs through the obturator foramen of the stapes and inferior to the round window niche. The inner ear is composed of the cochlea (5) and vestibule. The vestibule contains three semicircular canals including the lateral (6), posterior (7) and superior canals (8), and the saccule (9) and utricle (10). The crista (11), which is located in each ampulla, is the sensory structure of the semicircular canals. The macula (12) is the sensory structure of both the saccule and utricle. The cochlea contains three chambers, which are the scala tympani (13), the scala vestibuli (14), and the scala media (15). The scala tympani and vestibuli are filled by perilymph (high sodium-containing fluid). The scala media is filled with endolymph (high potassium-containing fluid) and also contains the sensory structure, the organ of Corti (16). The basilar membrane (17), which extends from the osseous spiral lamina (18), separates the scala media from scala tympani. Reissner's membrane (19) separates the scala media from scala vestibuli. Spiral ganglion cells (20), which are located in the modiolus, send peripheral processes to the hair cells and central processes to the cochlear nuclei via cochlear nerve. Capillaries in the modiolus (21) and spiral ligament (22) contribute to the blood-perilymph barrier, which is the transportation pathway of agents to reach the inner ear after intravenous or oral administration. Capillaries in the stria vascularis (23) are attributable to the blood-endolymph barrier.