Identification of auditory neurons by retrograde labelling for patch-clamp recordings in a mixed culture of chick brainstem

Activity-dependent and activity-independent development of the axon initial segment

The axon initial segment (AIS) is the site of spike initiation in neurons. Previous studies revealed that spatial distribution of the AIS varies greatly among neurons to meet their specific needs. However, when and how this differentiation arises is unknown. Neurons in the avian nucleus laminaris (NL) are binaural coincidence detectors for sound localization and show differentiation in the distribution of the AIS, with shorter length and a more distal position from the soma with an increase in tuning frequency. We studied these characteristics of the AIS in NL neurons of the chicken during development and found that the AIS differentiates in its distribution after initial formation, and this is driven by activity-dependent and activity-independent mechanisms that differentially regulate distal and proximal boundaries of the AIS. Before hearing onset, the ankyrinG-positive AIS existed at a wide stretch of proximal axon regardless of tuning frequency, but Na+ channels were only partially distributed within the AIS. Shortly after hearing onset, Na+ channels accumulated along the entire AIS, which started shortening and relocating distally to a larger extent in neurons with higher tuning frequencies. Ablation of inner ears abolished the shortening of the AIS without affecting the position of its proximal boundary, indicating that both distal and proximal AIS boundaries are disassembled during development, and the former is dependent on afferent activity. Thus, interaction of these activity-dependent and activity-independent mechanisms determines the cell-specific distribution of the AIS in NL neurons and plays a critical role in establishing the function of sound localization circuit.

Molecular cloning and characterization of chicken neuronal intermediate filament protein α-internexin

α-Internexin is one of the neuronal intermediate filament (IF) proteins, which also include low-, middle-, and high-molecular-weight neurofilament (NF) triplet proteins, designated NFL, NFM, and NFH, respectively. The expression of α-internexin occurs in most neurons as they begin differentiation and precedes the expression of the NF triplet proteins in mammals. However, little is known about the gene sequence and physiological function of α-internexin in avians. In this study we describe the molecular cloning of the mRNA sequence encoding the chicken α-internexin (chkINA) protein from embryonic brains. The gene structure and predicted amino acid sequence of chkINA exhibited high similarity to those of its zebrafish, mouse, rat, bovine, and human homologs. Data from transient-transfection experiments show that the filamentous pattern of chkINA was found in transfected cells and colocalized with other endogenous IFs, as demonstrated via immunocytochemistry using a chicken-specific antibody. The expression of chkINA was detected at the early stage of development and increased during the developmental process of the chicken. chkINA was expressed widely in chicken brains and colocalized with NF triplet proteins in neuronal processes, as assessed using immunohistochemistry. We also found that chkINA was expressed abundantly in the developing cerebellum and was the major IF protein in the parallel processes of granule neurons. Thus, we suggest that chkINA is a neuron-specific IF protein that may be a useful marker for studies of chicken brain development.