Gene expression differences over a critical period of afferent-dependent neuron survival in the mouse auditory brainstem.

Deprivation of auditory nerve input in young mice results in dramatic neuron death in the anteroventral cochlear nucleus (CN), while the same manipulation performed in older mice does not result in significant neuronal loss. The molecular basis underlying this critical period of susceptibility to loss of afferent input remains largely unknown. One possibility is that developmental differences in baseline mRNA expression of specific genes could predispose CN neurons to either death or survival after deafferentation. We used two microarray platforms to identify differentially expressed genes in the CN during and after this critical period. Results across platforms were also compared to each other. Three ages were examined; during the critical period (postnatal day (P)7), at the closing of the critical period (P14), and 1 week after the critical period (P21). Of all the genes surveyed (22,690 or 15,247), 1,082 were identified as significantly changed in expression during the critical period relative to after. Real-time reverse transcription polymerase chain reaction and immunohistochemistry confirmed and extended the microarray results for a subset of these genes. Further analysis of genes related to apoptotic pathways showed 6 out of 7 differentially expressed known pro-apoptotic genes had higher expression during the critical period. In contrast, 9 out of 11 differentially expressed known pro-survival genes increased after the critical period when CN neurons survive deprivation. This finding supports the concept that multiple neuroprotective mechanisms increase and pro-apoptotic factors decrease over development to protect mature neurons from stressful insults, making them less dependent on afferent input for survival.

A new method for imaging and 3D reconstruction of mammalian cochlea by fluorescent confocal microscopy.

Traditional methods for anatomical and morphometric studies of cochlear tissues have relied upon either microdissection of the organ of Corti or the generation of serial sections of the cochlea. Such methods are time-consuming, disruptive to three-dimensional relationships and often restrict sampling to very limited numbers of cells. We have found that cells and tissue components of the cochlear duct may be labelled by fluorescent markers within intact cochleae, which are then embedded in epoxy resin for subsequent viewing by fluorescent microscopy methods. This approach allows imaging through thick optical volumes with preservation of three-dimensional relationships. Unlike sectioned tissue, alignment of the sample relative to the focal axis may be easily corrected by re-orientation of the optical volume with common image processing software. Fluorescently labelled cochleae embedded in epoxy can be viewed by most fluorescent microscopy methods including laser scanning confocal microscopy, multi-photon confocal microscopy and widefield epi-fluorescence microscopy with deconvolution. Furthermore, semi-thin sections made from these preparations are compatible with traditional histological stains, as well as allowing brightly labelled epi-fluorescent images.

Chronic placental insufficiency has long-term effects on auditory function in the guinea pig.

Very low birth weight and growth-restricted infants have an increased risk of auditory impairments. It is uncertain whether these impairments are related to adverse pre-, peri- or postnatal events. We aimed to determine whether a period of chronic placental insufficiency (CPI) in the guinea pig results in long-term alterations to auditory function. Near mid-gestation, CPI was induced via unilateral ligation of the uterine artery. At 8 weeks of age, auditory brainstem responses (ABRs) were recorded in response to unilateral acoustic stimulation in prenatally-compromised (PC, n=8) and control animals (n=8). Stimuli consisted of 100 micros clicks, presented at 33 pulses per second (pps) and tone pip stimuli at frequencies of 2, 4, 8, 16 and 32 kHz. To examine temporal response properties, click stimuli were also presented at rates of 66, 132 and 200 pps. Normal ABR waveforms were elicited by both click and tone pip stimuli in all animals. Moreover, there was no difference between control and PC animals in stimulus detection thresholds across the frequencies examined. Using high rate click stimuli, PC animals demonstrated a significant increase in both the latency of wave III (normalised to 33 pps) and the wave I-III inter-peak interval compared to the controls. We hypothesise that these functional changes reflect alterations in myelination of the auditory brainstem and/or changes in synaptic efficacy. The results suggest subtle deficits in neural conduction in the PC guinea pig at maturity, and may have implications for speech perception abilities of low birth weight or prenatally affected infants.