Permeability changes of the blood-labyrinth barrier measured in vivo during experimental treatments

The communication between blood and cochlear perilymph was investigated using the tracer ion trimethylphenylammonium (TMPA). TMPA can be detected in micromolar concentrations by ion-selective microelectrodes, allowing it to be used as an almost ideal marker to study intercommunication between fluid compartments. TMPA-sensitive electrodes were sealed into the cochlear scalae, using procedures which avoided the artifactual displacement of perilymph by cerebrospinal fluid (CSF). TMPA was infused intravenously at a low rate to produce a plasma concentration of approximately 0.5,mM. The time course of entry into perilymph of scala tympani (ST), scala vestibuli (SV) and into CSF were compared. After 90 min infusion, the mean CSF concentration reached 14.3% of that measured in plasma. The TMPA concentrations measured in ST and SV perilymph were significantly lower than those recorded in CSF, only reaching an average of 6.5% and 3.7% of the plasma concentration respectively after 90 min. The slow entry of TMPA confirms the existence of a tight blood-labyrinth barrier, equivalent to the blood-brain or blood-CSF barriers. The rate of TMPA entry into perilymph was increased by epinephrine-induced hypertension or by simultaneous administration of histamine and prostaglandin E₂. These treatments are presumed to increase the permeability of the blood-labyrinth barrier. Characterization and manipulation of blood-labyrinth barrier permeability could be important to our understanding cochlear pathology.