Firing features and potassium channel content of murine spiral ganglion neurons vary with cochlear location

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.

Expression of connexin 30 in the developing mouse cochlea

Mutations in the GJB6 gene encoding connexin 30 (Cx30) can cause dominant forms of nonsyndromic deafness. By studying immunohistochemical localization of Cx30 in the mouse cochlea at different ages from 0 to 30 days after birth, we found that the expression of Cx30 is nearly the same as that of Cx26. These findings suggest that as well as Cx26, Cx30 may also contribute to the generation and maturation of endocochlear potential.

The response of transgenic mice to beta-adrenergic agonist administration is different from that of normal mice

Eighteen transgenic mice carrying an ovine metallothionein la-ovine growth hormone (oMTla-oGH) transgene and 18 littermate normal mice were used to investigate the effects of transgene expression and clenbuterol administration on growth performance and skeletal muscle characteristics. The oGH transgene was activated from 21 d of age, and half of the mice were fed 15 ppm clenbuterol from 42 to 70 d of age. All mice were killed at 70 d of age after 4 wk of treatment, and organs and muscles were dissected, weighted, and analyzed. Transgenic mice (TM) gained 2.6 times more than normal mice (NM). However, TM had a significantly lower (-20%, P < .01) proportion of muscle, expressed as percentages of body weights, and a higher percentage of heart (+10%), liver (+26%, P < .01) and spleen (+64%, P < .01) than NM. Clenbuterol improved the weight gain of TM by 20%, compared with 10% for NM. The growth-promoting effect of clenbuterol was almost exclusively confined to skeletal muscle (24% increase) in NM, in contrast to a more generalized growth increase in all tissues including skeletal muscle (11% increase) in TM. The skeletal muscles of TM were longer but smaller in diameter due to 30% smaller muscle fiber cross-sectional area. Clenbuterol increased the muscle fiber size of all fiber types by 60% in NM, compared to 30% in TM. Muscle DNA concentrations and content were higher (P < .05) in TM than in NM, and clenbuterol administration decreased DNA concentrations but not total DNA content for both genotypes. Cathepsin B, C, and H activities were higher (P < .01) in TM muscle, but the significance is not clear at the present time, although it points to a potential for greater protein degradation and(or) turnover rates as suggested by smaller muscle weights.

Development of calretinin immunoreactivity in the mouse inner ear

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.

[An electron microscopic study of retinal development and pathogenesis in mutant CBA/J mice with hereditary retinal degeneration]

Electron microscopic study of the neural retina pathogenesis was carried out on mice CBA/J in comparison with corresponding process in substrain CBA/Ki (Caley et al., 1972). A difference in terms of the pathology development was found between substrains CBA/J and CBA/Ki. It was shown that, in CBA/J, the retinal degeneration begins later than in CBA/Ki. As distinct from the latter, CBA/J proceeds to increase the thickness of outer nuclear and plexiform layers as well as the number of membranous discs of rod outer segments (ROS) between P10 and P12. In the retina of mouse CBA/J, pathologic alterations of the outer nuclear layer and the inner segments of photoreceptors precede the ROS destruction. It was found that the pigment epithelium is capable to phagocytize membranous discs of ROS in mouse CBA/J at P10 and P12. In CBA/J, the mitochondria in inner segments of photoreceptors begin to decay at P10; by P12, the amount of such segments reach 40%. At P12, the number of pycnotic nuclei (6%) in the outer nuclear layer coincides with the number of inner segments with dense degenerative cytoplasm. At P15, the state of retina as evaluated by the extent of far advanced degeneration of the outer nuclear layer is equalized in mice of both substrains. From P10 to P15, macrophages which phagocytize membranous discs of ROS are present in the interphotoreceptor space of CBA/J. The administration of PABA solution (7.5 x 10(-3) mg/g) daily from P1 to P9 or from P1 to P11 and with subsequent fixation at P10 and P12 respectively exerted no effect on the rate of retinal pathogenesis in mouse CBA/J. The causes of revealed differences in the development of retinal pathology between CBA/Ki and CBA/J are discussed as well as the causes of the absence of PABA effect on the retina of mutant mouse CBA/J.

Development of glutamic acid decarboxylase-immunoreactive elements in the cerebellar cortex of normal and lurcher mutant mice

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.

Antigen-independent selection of T15 idiotype during B-cell ontogeny in mice

Precursors of B cells capable of responding to a T-independent form of phosphorylcholine (PC) in splenic focus assays were detected in the spleens of neonatal mice as early as 4 days after birth. The earliest anti-PC B cells were T15-. T15+ foci-forming B cells were first detected 6 days after birth and expanded rapidly to constitute greater than 80% of the total PC-specific foci by day 10. Injection of heat-killed S. pneumoniae (R36A) into neonatal mice resulted in priming of the antibody response to PC, with an idiotype profile reflecting that of precursors of foci-forming B cells at the time of antigen administration. Priming of 2-day-old mice with 2 x 10(6) and 2 x 10(7) R36A induced a five- and ten-fold increase in the antibody response to phosphorylcholine 6 to 8 weeks later. However, only 10 to 15% of the serum antibodies expressed the normally dominant T15 idiotype. Doses below 2 x 10(5) R36A showed no detectable priming activity. PC-specific hybridomas derived from mice injected with 2 x 10(7) R36A 2 days after birth lacked the idiotypic and molecular characteristics typical of T15+ antibodies. Antibodies to phosphorylcholine, raised by immunization of 6-week-old mice are normally protective against pneumococcal infection. However, serum antibodies from mice treated with R36A 2 days after birth and responding to phosphorylcholine following challenge with R36A at 6 weeks of age failed to protect against deliberate infection with virulent S. pneumoniae. These observations imply that the antigen phosphorylcholine does not play a role in the selective expansion and dominant expression of the T15 idiotype.

Early development in mice. VI: Additive and interactive effects of offspring genotype and maternal environments

Ovarian transplantation (OT) and fostering (F) methods were used concomitantly to test for genotype effects and cytoplasmic, uterine and postnatal maternal effects on nine sensori-motor responses employed for measuring development in two inbred stains of mice: NZB (N) and CBA/H (H). Parental strain differences were observed for six responses: forelimb placing, hindlimb placing, visual placing, crossed extensor, vibrissae placing and bar holding. Reciprocal F1 comparison alone indicates a single H vs. N global maternal effect (visual placing), whereas the joint use of the OT and F methods shows a parental vs. F1 global maternal effect on parental pups for three responses (hindlimb placing, grasping and visual placing) and on F1 pups for two responses (forelimb placing and visual placing). Moreover, a postnatal maternal effect is reported for five responses; righting, forelimb placing, grasping, crossed extensor and visual placing. The effect of the maternal genotype via the uterine environment is observed for four responses: righting, fore- and hindlimb placing and visual placing. Genotypic effects are also demonstrated on fore-and hindlimb placing, bar holding and visual placing. These results confirm the absence of a general genetic factor and demonstrate the absence of a general environmental factor the rate of the early sensori-motor development.

Changes of organ indices of CBA/Ca inbred mice as function of age

Weight changes of seven organs: brain, heart, lungs, liver, spleen, small intestine, and kidneys, were analyzed during aging of male CBA/Ca inbred mice. Data were collected as absolute organ weights, organ weights related to body, as well as brain weights. It was established that most organs increase their weight up to nearly two years of age, after which there is a dramatic decline. Body weight shows a distinct plateau or constancy along the lifespan of the animals; consequently, body weight seems to be generally the most favorable reference point to express some of the physiological changes and parameters of the aging organism. Variation coefficients of the organ weights generally become higher with age.

Uterine and cytoplasmic effects on pup eyelid opening in two inbred strains of mice

Age at Eyelid Opening (AEO) differs in two inbred strains of laboratory mice: NZB and CBA/H. Ovarian transplantation and fostering methods were employed to test for variation of genotypic expression in different environments. (1) A global maternal effect was demonstrated for AEO in parental and F1 pups. (2) AEO of H and N pups is not affected by the parental vs. F1 modification of post-natal maternal environment. (3) The genotype of the mother, via uterine environment, interacts with pup genotype: only the CBA/H strain reacts to the parental vs. F1 modification in uterine environment. (4) A cytoplasmic effect for AEO was successfully replicated. Additional analyses demonstrate that AEO is not cytoplasmically inherited but rather modulated by the cytoplasmic environment.

A critical period during postnatal auditory development of mice

CBA/J mice were given a 50 dB unilateral conductive hearing loss by removal of the left cartilaginous external auditory meatus. When the conductive loss extended from 12 to 24 days after birth, the left globular cells and left large spherical cells of the ventral cochlear nucleus were significantly smaller (P less than 0.01) than comparable cells on the right side. Right medial nucleus of the trapezoid body cells and right inferior colliculus cells were significantly smaller (P less than 0.01) than comparable left side cells. These same effects were found with conductive losses of 4-24, 4-45, and 4-90 days after birth. There were no significant differences (P greater than 0.05) between right and left cell sizes with conductive losses of 4-12 or 24-45 days after birth. There were significant right/left cell size differences (P less than 0.01) when the conductive loss was 4-18, 12-18, or 18-24 days, but these differences were of lesser magnitude than when the conductive loss included the full 12-24 day period. Since normally all these neurons have adult soma size by postnatal day 12, it is evident that adequate acoustic stimulation is necessary between 12 and 24 days postnatally in order to maintain normal brainstem auditory neuronal size in CBA/J mice.

[Changes in the intercellular contacts of hepatocytes in the ontogeny of inbred mouse strains with a high (CBA) and low (C57BL) frequency of spontaneous hepatomas]

Some parameters of cellular contacts between hepatocytes were studied in ontogenesis of the inbred mouse lines with the high (CBA) and low (C57BL) frequency of spontaneous hepatomas. In both the lines the force of intercellular linkage of the hepatocytes drops down by the end of the animal's life. But in the C57BL mice this characteristics increases sharply between the 5th and 10th days after birth, whereas in the CBA mice no such increase is observed. During the whole postnatal development the force of linkage in the CBA mouse liver is thrice lower than in the C57BL mouse liver. The stability of contacts between the hepatocytes in the C57BL mice is not affected during ontogenesis, as well as in the CBA mice, except in the males at the age of 15-24 months when the stability of contacts decreases. This parameter decreases still more in the tumour. Since the CBA males are characterized by a higher frequency of spontaneous hepatomas, a suggestion is put forward to the effect that the primary disturbance of the stability of contacts reflects the early changes characteristic for the spontaneous liver carcinogenesis.