CHOLINERGICALLY-INDUCED CHANGES IN OUTWARD CURRENTS IN HAIR-CELLS ISOLATED FROM THE SEMICIRCULAR CANAL OF THE FROG

Two cholinergically-induced modulations of membrane conductances have been identified in hair cells isolated from the crista ampullaris of the leopard frog (Rana pipiens), using the whole cell recording configuration of the patch clamp technique. Of 56 crista hair cells tested, 28 showed drug-induced changes in membrane current or membrane potential which were repeatable and could be reversed with washout of drug. The predominant effect (observed in 20 hair cells) of acetylcholine (Arch, 100μM to 1mM) or carbachol (1μM to 50μM) applied to these hair cells was the reduction of an outward current corresponding to a change in conductance of approximately -0.22 nS. This action by Ach on hair cells has been inferred from previous studies of afferent fiber discharge which reported an increase in firing rate with stimulation of efferent fibers or exogenous application of cholinomimetics (Bernard et al., 1985; Valli et al., 1986; Guth et al., 1986; Norris et al., 1988). The Ach-induced reduction in outward current was associated with a depolarization of the zero-current membrane potential by approximately +2.5 mV. In a total of 8 hair cells, an Ach-induced reversible increase in outward current was recorded. Changes in conductance were approximately +0.13 nS and were associated with a hyperpolarization of the zero-current membrane potential by approximately -2.2 mV. This current increase is likely to be responsible for the inhibitory post-synaptic potentials (IPSPs) which have previously been recorded intracellularly from acoustico-lateralis hair cells during stimulation of the efferent innervation (Flock and Russell, 1976; Ashmore and Russell, 1982; Art et al., 1984, 1985). Of the remaining 28 hair cells, six cells failed to exhibit any change in membrane conductance or membrane potential in the presence of cholinomimetics while an additional 15 cells exhibited decreases, and 7 cells exhibited increases in outward conductance, during application of Ach or carbachol, which were neither reversible with washout nor repeatable. The Ach-induced decrease in outward current could be reversible blocked by removal of Ca2+ from the external solution. The antagonism of the Ach-induced decrease on outward current by atropine (10-5 M) suggests that this current may correspond to a facilitatory become-preferring Ach receptor mediated response previously identified in the isolated semicircular canal (Norris et al., 1988). The enhanced conductance observed in the presence of Ach in some cells may be related to the suppressive actions of Ach in the isolated semicircular canal also reported by us (Norris et al. 1988) and to the inhibitory actions of efferent stimulation and Ach application previously investigated in turtle cochlear hair cells (Art et al., 1984). The possible identities of these conductances are discussed in relation to ionic currents previously identified in hair cells. © 1990.