CHOLINERGIC AGONISTS INCREASE INTRACELLULAR CALCIUM-CONCENTRATION IN FROG VESTIBULAR HAIR-CELLS

Acetylcholine (ACh) is usually considered to be the neurotransmitter o f the efferent vestibular system. The nature and the localization of c holinergic receptors have been investigated on frog isolated vestibula r hair cells (VHCs), by measuring variations of intracellular calcium concentration ([Ca2+](i)), using calcium sensitive dye fura-2. Focal i ontophoretic ACh (1 M, 300 nA.40 ms) application induced a rapid incre ase in [Ca2+](i), reaching a peak in 20 s and representing about 5-fol d the resting level (from 61+/-6 to 320+/-26 nM). Applications of musc arinic agonists as methacholine and carbachol induced weaker calcium r esponses (from 78+/-25 to 238+/-53 nM) than the one obtained with ACh applications. These muscarinic agonists were efficient only in precise zones. Desensitization of muscarinic receptors to successive stimulat ions was significant. Perfusion of nicotine or 1,1-dimethyl-4-phenyl-p iperazinium (DMPP), a nicotinic agonist, induced an increase in [Ca2+] (i) only in some cells (4/28 with DMPP). These results indicated the p resence of cholinergic receptors on frog VHCs: muscarinic receptors we re more responsive than nicotinic receptors. Presence of muscarinic an d nicotinic receptors in the membrane of VHCs could indicate different modulations of VHCs activity mediated by [Ca2+](i) and involving an e fferent control which represents a central regulation of the vestibula r afferent message.