DEVELOPMENTAL EXPRESSION OF ALPHA-9 ACETYLCHOLINE-RECEPTOR MESSENGER-RNA IN THE RAT COCHLEA AND VESTIBULAR INNER-EAR

Expression of alpha 9 acetylcholine receptor (AChR) mRNA was studied b y in situ hybridization in the rat adult and developing cochlea and ve stibular inner ear. alpha 9 AChR mRNA was first observed in cochlear h air cells (HCs) at embryonic day 18 (E18), increased markedly after bi rth, stayed high until postnatal day 10 (P10), and decreased to substa ntially lower adult levels by P14. High levels of alpha 9 AChR mRNA ex pression were also noted in the developing nonneuronal structures of t he inner sulcus, chondrocytes, and/or osteoblasts in the cochlear caps ule and interscalar laminae. Both developing and adult bone marrow cel ls also expressed intense alpha 9 AChR mRNA. In the vestibular system, alpha 9 AChR mRNA was first observed in HCs at E16 in all sensory epi thelia, increased to its highest levels by P0-P4, then decreased sligh tly to reach adult levels by P10. The results are consistent with the alpha 9 AChR subserving efferent neurotransmission to both cochlear an d vestibular HCs. The observation of alpha 9 AChR mRNA in cochlear HCs 2 weeks prior to functional onset in the cochlea further suggests tha t expression of this gene is not related to HC activity. The observati on of substantial nonneuronal expression of alpha 9 AChR mRNA suggests that this receptor also has functions separate from its role in neuro transmission. (C) 1998 Wiley-Liss, Inc.