Auditory cortical responses to neonatal deafening: pyramidal neuron spine loss without changes in growth or orientation.

Neonatal rabbits were unilaterally deafened at birth by surgical removal of the stapes, aspiration of the cochlear lymph, and kanamycin injection into the oval window. At 60 days of age, all rabbits were screened with brain stem evoked response tests in order to establish the efficacy of the deafening procedure. The auditory cortex contralateral to the destroyed cochlea was processed according to Golgi-Cox/Nissl procedures. Temporal bone histology revealed nearly complete outer hair cell loss in the damaged cochlea. The dendritic system of lamina III/IV pyramidal neurons contralateral to the deafened ear was digitized from frontal sections using a computer microscope system. Spine counts were also made along the basal dendrites. Spine counts revealed that neonatally deafened rabbits and 38.7% fewer spines along their basal dendrites. No differences between experimental and control rabbits were found in terms of soma cross-sectional area, total number of basal dendrites, total number of dendritic branches and total basal dendritic length. A fan-in projection of the dendritic system revealed no changes in the radial growth of basal dendrites resulting from the early acoustic trauma. In a prior study, spine-free nonpyramidal neurons in the same sections revealed altered dendritic growth and abnormally recurved dendrites. The separate response of pyramidal and nonpyramidal cell types to early cochlear damage is evidence for the different role of epigenetic determinants of dendritic form and orientation in sensory neocortical neurons.