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Neurons from varied regions of the central nervous system can show widely divergent responses to electrical stimuli that are determined by cell-specific differences in ion channel composition. The well-ordered and highly characterized peripheral auditory system allows one to explore the significance of this diversity during the final stages of postnatal development. We examined the electrophysiological features of murine spiral ganglion neurons in vitro at a time when recordings could be made from the cell bodies before myelination. These cells carry information about sound stimuli from hair cell receptors in the basilar membrane and are arranged tonotopically. Spiral ganglion neuron responses to depolarizing current injection were assessed with whole-cell current clamp recordings from cells that were isolated separately from the apical and basal thirds of the mouse cochlea. These cells displayed systematic variation in their firing. Apex neurons (low frequency coding) showed longer latency, slowly adapting responses, whereas base neurons (high frequency coding) showed short latency, rapidly adapting responses to the same stimuli. This physiological diversity was mirrored by regional differences in ion channel content assessed immunohistochemically. Apex neurons had a preponderance of Kv4.2 subunits, whereas base neurons possessed greater levels of K(Ca), Kv1.1, and Kv3.1 subunits. Taken together, these results indicate that the distribution of a set of voltage-gated potassium channels may relate specifically to a particular range of coding frequencies. These studies also suggest that intrinsic properties of spiral ganglion neurons can contribute to the characteristic responses of the peripheral auditory system. Their potential role in development and adult function is discussed., (Copyright 2002 Wiley-Liss, Inc.)

Glycinergic neurons in the cochlear nucleus (CN) of C57BL/6J (C57) and CBA/CaJ (CBA) mice were studied by using immunocytochemical and receptor-binding techniques. Adult C57 mice exhibit progressive cochlear pathology as they age, whereas aging CBA mice retain good hearing. In the CN of old C57 mice (18 months) with severe hearing loss, the number of glycine-immunoreactive neurons decreased significantly. The number (Bmax) of strychnine-sensitive glycine receptors (GlyR) decreased significantly in the dorsal CN of old C57 mice. Significant effects were not observed in the CN of middle-aged C57 mice (with less-severe hearing loss) or in very old CBA mice (which do not exhibit severe hearing loss). The data suggest that the combination of severe hearing loss and old age results in deficits in one or more inhibitory glycinergic circuits in the CN.

Calretinin is a calcium-binding protein of the EF-hand family. It has been previously identified in particular cell types of adult guinea pig, rat, and chinchilla inner ear. Development of calretinin immunoreactivity in the mouse inner ear was investigated from embryonic day 13 (E13) to the adult stage. In the adult mouse vestibule, calretinin immunoreactivity was present in the same structures as described for the rat and guinea pig: the population of afferent fibers forming calyx units and a small number of ganglion neurons. The earliest immunoreactivity was found at E17 in vestibular hair cells (VHCs), then, at E19, in afferent fibers entering the sensory epithelia and in rare ganglion neurons. At postnatal day 4 (P4), a few vestibular nerve fibers and ganglion neurons were reactive. From this stage until P14, immunoreactivity developed in the calyx units and disappeared from VHCs. At P14, immunostaining was adult-like. In the adult mouse cochlea, immunoreactivity was present in the same cell populations as described in the rat: the inner hair cells (IHCs) and most of Corti's ganglion neurons. Calretinin immunoreactivity appeared at E19-P0 in IHCs and ganglion neurons of the basal turn. At P1, outer hair cells (OHCs) of the basal turn were positive. Calretinin immunoreactivity then appeared in IHCs, OHCs, and ganglion neurons of the medial turn, then of the apical turn. At P4, all IHCs and OHCs and most of the ganglion neurons were immunostained. Immunoreactivity gradually disappeared from the OHCs starting at P10 and, at P22, only IHCs and ganglion neurons were positive. The sequences of appearance of calretinin were specific to each cell type of the inner ear and paralleled their respective maturation. Calretinin was transiently expressed in VHCs and OHCs.

The development of glutamic acid decarboxylase-immunoreactivity (GAD-IR) in cells, fibers, and varicosities of the cerebellar cortex has been examined by light microscopy in normal and lurcher mutant mice between postnatal day 3 and 30 (P3-P30). Purkinje cell morphology was demonstrated in adjacent sections by using an antiserum to the 28Kd vitamin D-dependent calcium binding protein (CaBP). In early postnatal lurcher mice, but not in normal littermates, GAD-IR fibers, presumably Purkinje cell pseudopodia, invade the external granular layer. The plexus of CaBP-IR axons in the internal granular layer is much less complex in lurcher mice than in normal littermates, even before the onset of lurcher Purkinje cell degeneration at P8. In normal mice, GAD-IR fibers encapsulate Purkinje cell somata by P15. Lurcher Purkinje cells, in contrast, receive scattered contacts by GAD-IR puncta and possess a "cap" of such elements surrounding the primary dendrite and apical soma. Pinceau formations, visible as a knot of GAD-IR puncta hanging from the base of Purkinje cells in normal P15 mice, are not present in lurcher littermates. "Empty baskets" or collapsed pinceau formations in regions devoid of Purkinje cells are not revealed by anti-GAD immunohistochemistry in the P17-P30 lurcher cerebellar cortex.

Preincubation of C57BL adult marrow cells or CBA fetal liver cells with a 250-fold excess concentration of purified GM-CSF failed to reduce the frequency of cells forming eosinophil, megakaryocyte or erythroid colonies in subsequent agar cultures. When excess concentrations of purified GM-CSF were added to agar cultures stimulated by pokeweed mitogen-stimulated spleen conditioned medium (SCM), no reduction was observed in the frequency of eosinophil, megakaryocyte or erythroid colonies. Addition of 4 units of purified erythropoietin (EPO) to cultures of fetal liver or adult marrow cells stimulated by SCM increased the number of erythroid colonies but did not reduce the number of non-erythroid colonies or the non-erythroid content of mixed erythroid colonies. Although neither GM-CSF nor EPO alone was able to stimulate erythroid colony formation in agar cultures of fetal liver cells, small numbers of large erythroid colonies were stimulated to develop in cultures containing both purified regulators. Purified GM-CSF was also able to support the survival in vitro of a small proportion of erythroid colony-forming cells in fetal liver populations cultured initially in the absence of SCM and the survival of some eosinophil and megakaryocyte colony-forming cells in similar cultures of adult marrow cells. The results do not support the hypothesis that GM-CSF and EPO compete for a common pool of uncommitted progenitor cells. On the contrary, the data indicate that GM-CSF und EPO are able to collaborate in stimulating the proliferation of some erythropoietic cells. Furthermore, purified GM-CSF appears to be able to support temporarily the survival and/or initial proliferation of at least some cells forming erythroid, eosinophil and megakaryocyte colonies, even though GM-CSF is unable to stimulate the formation of colonies of these types.