Calcitonin gene-related peptide suppresses hair cell responses to mechanical stimulation in the Xenopus lateral line organ

The presence of calcitonin gene-related peptide (CGRP) in the efferent fibe rs of virtually every hair cell organ studied suggests it may serve some fu ndamental but heretofore unknown role in control of hair cell function. We examined the effects of CGRP on spontaneous and stimulus-evoked discharge p atterns in an in vitro preparation of the lateral line organ of Xenopus lae vis. Discharge patterns were determined by sinusoidally displacing the cupu la with a glass micropipette driven with a piezoelectric device while recor ding afferent fiber activity. All afferent fibers had characteristic freque ncies of 16-32 Hz. Responses synchronized to cupular displacements as small as 20 nm. CGRP suppressed responses of the lateral line organ to displacem ent while increasing spontaneous discharge rate. In the presence of CGRP, s timulus-response curves were shifted 10 dB toward higher displacement level s. The suppression of stimulus-evoked responses suggests a function for CGR P as an efferent neurotransmitter that is similar to that of cholinergic ef ferent transmission in other hair cell organs. The 10 dB shift toward large r displacements makes it comparable in magnitude with the effects of electr ical stimulation of efferents in the mammalian cochlea. This suggests a sig nificant role for CGRP in efferent modulation of the output of this mechano sensory organ.