Publication Date: 2004.
In the mature cochlea, inner hair cells (IHCs) transduce acoustic signals into receptor potentials, communicating to the brain by synaptic contacts with afferent fibers. Before the onset of hearing, a transient efferent innervation is found on IHCs, mediated by a nicotinic cholinergic receptor that may contain both α9 and α10 subunits. Calcium influx through that receptor activates calcium-dependent (SK2-containing) potassium channels. This inhibitory synapse is thought to disappear after the onset of hearing [after postnatal day 12 (P12)]. We documented this developmental transition using whole-cell recordings from IHCs in apical turns of the rat organ of Corti. Acetylcholine elicited ionic currents in 88-100% of IHCs between P3 and P14, but in only 1 of 11 IHCs at P16-P22. Potassium depolarization of efferent terminals caused IPSCs in 67% of IHCs at P3, in 100% at P7-P9, in 93% at P10-P12, but in only 40% at P13-P14 and in none of the IHCs tested between P16 and P22. Earlier work had shown by in situ hybridization that α9 mRNA is expressed in adult IHCs but that α10 mRNA disappears after the onset of hearing. In the present study, antibodies to α10 and to the associated calcium-dependent (SK2) potassium channel showed a similar developmental loss. The correlated expression of these gene products with functional innervation suggests that Alpha10 and SK2, but not Alpha9, are regulated by synaptic activity. Furthermore, this developmental knock-out of α10, but not α9, supports the hypothesis that functional nicotinic acetylcholine receptors in hair cells are heteromers containing both these subunits.
Author affiliation: Katz, E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
Author affiliation: Gómez-Casati, M.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.
Keywords: α9α10 nAChR; Ca2+-activated K+ channel; Cholinergic; Efferent innervation; IHC; Mammalian cochlea; Neonatal development; Transient synapse; acetylcholine; calcium; calcium activated potassium channel; messenger RNA; nicotine; nicotinic receptor; nicotinic receptor alpha10 subunit; nicotinic receptor alpha9 subunit; potassium; receptor subunit; unclassified drug; Alpha10 gene; Alpha9 gene; animal cell; animal tissue; article; auditory stimulation; calcium transport; cochlea; Corti organ; depolarization; efferent nerve; gene; hair cell; hearing; in situ hybridization; ion current; knockout gene; newborn; nonhuman; postnatal development; priority journal; protein expression; rat; receptor potential; sensory nerve; signal transduction; sk2 gene; synapse; synaptogenesis; Acetylcholine; Action Potentials; Age Factors; Animals; Cochlea; Gene Expression Regulation, Developmental; Hair Cells, Inner; Hearing; Patch-Clamp Techniques; Potassium; Potassium Channels, Calcium-Activated; Protein Subunits; Rats; Rats, Sprague-Dawley; Receptors, Nicotinic; Small-Conductance Calcium-Activated Potassium Channels; Synaptic Transmission.
Repository: Biblioteca Digital (UBA-FCEN). Universidad Nacional de Buenos Aires. Facultad de Ciencias Exactas y Naturales
Publication Date: 2005.
GABA and its receptors show particular ontogenic distributions in different rat brain areas. Recently, GABAB receptors (GBR) have been described to assemble as heterodimers formed by a GBR1a/b and a GBR2 subunit. Here, the ontogeny of rat GBRs and the pattern of subunit expression in both sexes were determined in the hypothalamus, a critical area for homeostatic regulation. Male and female rats were sacrificed at 1, 4, 12, 20, 28, 38 days of life and at adulthood and hypothalami were removed and frozen. Western blots analysis for GBR1 and GBR2 subunits showed that both were expressed in male and female hypothalamic membranes from day 1 to adulthood. In females, both GBR1a and GBR1b were maximally expressed in newborns and decreased towards adulthood. At birth, expression of GBR1a was significantly higher than GBR1b, while at 38 days, GBR1b was more abundant. In males, GBR1a and GBR1b expression was higher in young animals and decreased gradually showing adult levels between the second and third weeks of age without differences between isoforms. Comparing GBR1 variants levels in hypothalamus between sexes, GBR1a was significantly more abundant in females at birth while at 38 days its expression was higher in males; GBR1b showed no sex differences along development. GBR2 was detected in hypothalami of females and males at all ages; maximum levels were observed at 12 days and adult levels were attained at 38 days, without sex differences. This is the first report on the ontogeny of hypothalamic GABAB receptors in male and female rats, with a particular developmental pattern of subunit and isoform expression and presenting some sex differences.
Author affiliation: Bianchi, Maria Silvia. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina
Author affiliation: Lux, Victoria Adela R.. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina
Author affiliation: Bettler, Bernhard. University of Basel; Suiza
Author affiliation: Libertun, Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Universidad de Buenos Aires. Facultad de Medicina; Argentina
Repository: CONICET Digital (CONICET). Consejo Nacional de Investigaciones Científicas y Técnicas