Development of acetylcholine receptors in cultured outer hair cells

Efferents, originating in the superior olivary complex, preferentially syna pse with cochlear outer hair cells (OHCs), with acetylcholine (ACh) as thei r primary neurotransmitter. The OHC ACh receptors (AChRs), which have unusu al pharmacology, have been cloned and identified as a new subunit (alpha9) of the nicotinic AChR family. The expression of alpha9 AChRs is first detec ted before birth and peaks between 6 and 10 days after birth (DAB) in devel oping mice and rats, while functional maturation of the receptor, as determ ined by measuring the ACh-induced currents, takes place between 6 and 12 DA B. In this study we attempted to examine the development of AChRs in OHCs g rown in explanted cultures, deprived of efferent innervation. ACh-induced c urrents were used as an assay. Reverse transcription-PCR analysis was also performed to detect the expression of alpha9 subunit from cultured OHCs. PC R study indicates that mRNA of the alpha9 subunit was expressed in primary cochlear cultures, similar to that seen in the cochleae of developing anima ls. Measurement of whole-cell currents showed that ACh-induced outward curr ent was first detected around 5 days in a fraction of cultured OHCs. The nu mber of responsive cells increased between 5 and 12 days in culture. The si ze of ACh-induced currents also increased during this period. These results suggest that the development of AChRs in cultured OHCs is not affected by removal of efferent innervation. (C) 2001 Elsevier Science B.V. All rights reserved.