Receptor-mediated release of inositol phosphates in the cochlear and vestibular sensory epithelia of the rat

Various neurotransmitters, hormones and other modulators involved in intercellular communication exert their biological action at receptors coupled to phospholipase C (PLC). This enzyme catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP₂) to inositol 1,4,5-trisphosphate (InsP₃) and 1,2-diacylglycerol (DG) which act as second messengers. In the organ of Corti of the guinea pig, the InsP₃ second messenger system is linked to muscarinic cholinergic and P₂y purinergic receptors. However, nothing is known about the the InsP₃ second messenger system in the vestibule. In this study, the receptor-mediated release of inositol phosphates (InsPs) in the vestibular sensory epithelia was compared to that in the cochlear sensory epithelia of Fischer-344 rats. After preincubation of the isolated intact tissues with myo-[³H] in-ositol, stimulation with the cholinergic agonist carbamylcholine or the P₂ purinergic agonist ATP-γ-S resulted in a concentration-dependent increase in the formation of [³H]InsPs in both epithelia. Similarly, the muscarinic cholinergic agonist muscarine enhanced InsPs release in both organs, while the nicotinic cholinergic agonist dimethylphenylpiperadinium (DMPP) was ineffective. The muscarinic cholinergic antagonist atropine completely suppressed the InsPs release induced by carbamylcholine, while the nicotinic cholinergic antagonist mecamylamine was ineffective. Potassium depolarization did not alter unstimulated or carbamylcholine-stimulated release of InsPs in either organ. In both tissues, the P₂ purinergic agonist α,β-methylene ATP also increased InsPs release, but the P₁ purinergic agonist adenosine did not. These results extend our previous observations in the organ of Corti of the guinea pig to the rat and suggest a similar control of the InsP₃ second messenger system in the vestibular sensory epithelia. In contrast to the cochlear sensory epithelia, atropine also significantly suppressed unstimulated InsPs release in the vestibular sensory epithelia. This suggests that the physiological mechanisms of the efferent nervous systems involving InsP₃ second messenger system might be different in vestibular versus cochlear sensory epithelia.