Int. J. Devl Neuroscience 44 (2015) 92–101 Contents lists available at ScienceDirect International Journal of Developmental Neuroscience journal homepage: www.elsevier.com/locate/ijdevneu Firing properties of auditory primary afferents from the basilar papilla in the chick Salvador Galicia a , Celso Cortes b , Jorge Cebada b,c , Ignacio Méndez-Balbuena d , Amira Flores e,∗ a Escuela de Biología, Benemérita Universidad Autónoma de Puebla (BUAP), Blvd. Valsequillo y Av. San Claudio, Edif. 112-A C.U. Colonia Jardines de San Manuel CP 72570 Puebla, Pue., Mexico b Facultad de Medicina, BUAP, Av. 13 Sur 2702 Colonia Volcanes CP 72410 Puebla, Pue., Mexico c Hospital Universitario de Puebla, BUAP, Av. 25 poniente 1301Colonia Volcanes CP 72410 Puebla, Pue., Mexico d Facultad de Psicología, BUAP, Av. 3 oriente 403 Colonia Centro CP 72000 Puebla, Pue., Mexico e Instituto de Fisiología, BUAP, Av. 14 Sur 6301 Colonia Jardines de San Manuel CP 72570 Puebla, Pue., Mexico a r t i c l e i n f o Article history: Received 10 February 2015 Received in revised form 31 May 2015 Accepted 31 May 2015 Available online 9 June 2015 Keywords: Basilar papilla Development Action potential Cochlea Inner ear Chicken a b s t r a c t We performed intracellular and single-unit extracellular recordings of neurons from different regions of the basilar papilla in the isolated chicken inner ear. We compared the spontaneous activity and the response properties of these neurons in embryos at E15 versus posthatching animals at P1. The recordings were carried out from the apical (position 0) to the basal extension at three positions of the basilar papilla, at 5%, 10% and 40% of the entire length of the cochlea. We found that the neurons at E15 recorded from these three regions exhibited a significant higher coefficient of variation compared with those neurons at P1 recorded in the same positions. This shows that in the posthatching age P1 the neurons from the whole basilar papilla become less irregular. We found that the intracellular action potential waveforms generated at E15 had small amplitudes and small depolarization slopes in comparison to those recorded at P1, respectively (53 ± 1 mV vs. 62 ± 2 mV; 66 ± 12 mV/msec vs. 166 ± 23 mV/msec). Furthermore, we also found that the response patterns to injection of current steps were phasic, tonic, or in the form of a not yet reported “burst” pattern. Our study shows that the low irregular discharge, the immature action potential waveforms, and the differences in the response patterns to current injection, highlights the important differences between neurons at E15 and P1, consistent with the incapacity of auditory neurons at embryonic age E16, to respond at sound levels <100 decibels. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The first evoked responses to electrical and sound stimulation in the chick’s auditory system can be recorded between days 10 and 12 of the embryonic development (E10-E12); however, it is up for debate the idea that hearing is possible during this period of time (Saunders et al., 1973; Jackson et al., 1982; Lu and Trussell, 2007 Gao and Lu, 2008). The main counterexample is that the matura- tion of the tectorial membrane and the transduction apparatus in the hair cells achieves a mature profile only at around E18-19 (Si et al., 2003). Moreover, it is unlikely that cochlear primary afferent ∗ Corresponding author at: Laboratorio de Neurobiología, Instituto de Fisiología – Benemérita Universidad Autónoma de Puebla, 14 sur 6301, Col. Jardines de San Manuel, Puebla, Pue. CP 72570 México. Fax: +52 222 2295500x7323. E-mail addresses: amira.flores@correo.buap.mx, amrayo@yahoo.com.mx (A. Flores). neurons at E16 encode frequency information about sounds of nat- ural environment (Rubel and Rebillard, 1981; Manley et al., 1991; Jones and Jones 2000). Consistent with this idea, Jones et al., (2006) found that in ages younger than E16, cochlear ganglion neurons exhibit a profound insensitivity to sound, given that most cells are unresponsive at sound levels <100 dB SPL. Accordingly with Jones and Jones, 2000 (and consistent with Si et al., 2003) it is not until E19, when cochlear ganglion neurons are capable to develop most of the basic types of spontaneous primary afferent activity found in mature postnatal animals. Therefore in terms of spontaneous discharge, auditory neurons are relatively mature at E19, as it is reflected also in the slow rate of changes observed in the firing rate and the coefficient of variation (CV) of discharge regularity (Manley et al., 1991; Jones and Jones, 2000). The studies by Manley et al. (1991) and Jones and Jones (2000) are also consistent with other reports in which the pattern of spontaneous activity of afferent neurons is considered as a good http://dx.doi.org/10.1016/j.ijdevneu.2015.05.007 0736-5748/© 2015 Elsevier Ltd. All rights reserved.