Morphology of the dorsal cochlear nucleus in C57BL/6J and CBA/J mice across the life span.

The morphology of the dorsal cochlear nucleus (DCN) was evaluated across the life span in inbred C57BL/6J (C57) and CBA/J (CBA) mice using 5 age groups (young adult to very old). C57 mice exhibit progressive cochlear sensorineural pathology and hearing loss during middle age; CBA mice have only modest sensorineural pathology late in life. DCN layers I, II, and III were evaluated histologically with serial sections stained for Nissl and fibers. DCN volume decreased with age in C57 mice, but the change began earliest and was most pronounced in layer III. In CBA mice, volume increased during the first year of life and decreased only in the oldest mice. All major DCN cell types were found in both strains at all ages. There was an age-related decrease in the mean size of neurons in C57 mice that was first observed in layer III. In CBA mice, only a nonsignificant trend toward smaller neurons was observed in the oldest mice. An age-related decline in the number of neurons in layer III (but not in layers I and II) occurred in C57 mice. Aged CBA mice exhibited no significant loss of DCN neurons. Thus, age-related changes in the DCN were much more pronounced in C57 mice than in CBA mice, and the changes in C57 mice were most pronounced in layer III. Because layer III receives most of the DCN's primary auditory input, it would be directly affected by age-related hearing loss and degeneration of spiral ganglion cells in C57 mice. This suggests that the age-related changes observed in DCN layer III of C57 mice are affected by progressive peripheral degenerative changes; when peripheral loss is minimal (CBA mice), less substantial age-related changes are observed.     

Morphological changes in the anteroventral cochlear nucleus that accompany sensorineural hearing loss in DBA/2J and C57BL/6J mice

Morphological measurements were made on histological sections of the anteroventral cochlear nucleus (AVCN) in mice of the DBA/2J and C57BL/6J strains to determine the effects of sensorineural cochlear pathology on the number, packing density, and size of neurons and on AVCN volume. Both strains possess alleles that cause progressive cochlear pathology initially affecting the organ of Corti: in DBA mice, hearing loss is evident at 4 weeks of age and progresses rapidly; in C57 mice, hearing loss begins after 2 months of age and progresses more slowly. In both strains AVCN volume decreased, some loss of neurons occurred, and these changes paralleled the progression of peripheral hearing loss. Central changes were rapid in DBA mice, but the ultimate magnitude of the changes in 1-year-old mice did not differ between strains. Both strains differed from well-hearing CBA/J mice which exhibited no changes in the AVCN measures. The findings indicate that pathology of the organ of Corti in adult mice results in degenerative changes in the cochlear nucleus. The data also support earlier findings indicating that, if cochlear pathology does not begin prior to young adulthood, the age of onset and duration of sensorineural impairment have little effect on the ultimate magnitude of central effects.     

Glycine immunoreactivity and receptor binding in the cochlear nucleus of C57BL/6J and CBA/CaJ mice: Effects of cochlear impairment and aging

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 (B_max) 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. J. Comp. Neurol. 385:405–414, 1997. © 1997 Wiley-Liss, Inc.     

Morphometric study of the anteroventral cochlear nucleus of two mouse models of presbycusis

The dimensions and volume of the anterior ventral cochlear nucleus (AVCN), the density and number of AVCN neurons, and the size of neuronal somata nuclei (in Nissl-stained tissue) were determined in two mouse models of age-related hearing loss: the C57BL/6J strain, which undergoes progressive chronic sensorineural hearing loss with onset during young adulthood, and the CBA/J, which demonstrates only moderate hearing loss with onset late in life. Frontal and horizontal AVCN sections, as well as cochleas, were analyzed in 4 C57 age-groups (1, 7, 12, 19+ months) and in 3 CBA groups (1, 10, 22 months). Within each strain no significant changes in AVCN dimensions or volume occur with aging. In C57 mice, packing density and cell number decrease between 1 and 7 months, but remain stable thereafter, despite chronic severe hearing impairment. CBA mice show a reduction in AVCN cell number and packing density only during the second year of life. In aging C57 mice, the size of spherical and perhaps globular cells increases, whereas the size of multipolar cells tends to decrease slightly. In CBA mice, all three AVCN cell types tend to decrease in size with aging. The early cell loss and cell size increases in C57 mice are most consistent in the dorsal (high frequency) region of the AVCN. Likewise, loss of cochlear spiral ganglion cells is most pronounced in the base of the cochlea, which provides input to this region. The data indicate that aging is associated with rather different central effects, depending on AVCN cell type, cochleotopic organization, genotype, and/or the type of peripheral hearing loss involved. The C57 and CBA AVCNs also differ in several aspects irrespective of age. The volume of AVCN and number of AVCN neurons are significantly greater in C57 mice.     

Apical-to-basal gradients in age-related cochlear degeneration and their relationship to "primary" loss of cochlear neurons

The predominant conceptual framework for understanding human age-related hearing loss (ARHL, or presbycusis) holds that three different cochlear elements (organ of Corti, afferent neurons, and stria vascularis) can degenerate independently, and exert independent influences on hearing. Within this framework, temporal bones from subjects with ARHL may be classified as exemplifying sensory (referring to organ of Corti), "primary" neural (loss of afferent neurons without loss of their hair cell targets), strial, or mixed ARHL. While there is general agreement as to the types of cochlear cells most affected by aging, there is less agreement about how to classify ARHL, and whether contributions of particular structures to hearing loss can be isolated. The cochlear apex of humans and animals is particularly prone to apparent primary loss of neurons that may represent an aspect of neural ARHL. We recently reported that in 129S6/SvEv mice apical neuronal loss is often accompanied by abnormalities of spiral limbus, pillar cells, and Reissner's membrane (Ohlemiller and Gagnon [2004] J Comp Neurol 469:377-390). We proposed that the initial pathology occurs within limbus, leading to disruption of perilymphatic ion homeostasis, and eventual loss of neurons as one consequence. We have now examined this issue quantitatively in young and old mice of four different strains (129S6/SvEv, CBA/J, C57BL/6, and BALB/c). Abnormalities of apical spiral limbus were found to correlate only weakly with neuronal loss. Strong correlations were found between neuronal loss and abnormalities of both pillar cells and Reissner's membrane, however. Apical neuronal loss and apical-to-basal progression of pathology of limbus, pillar cells, and Reissner's membrane run counter to most reported age-related cochlear trends. Our findings suggest that these changes share a common triggering influence.     

Effects of prolonged exposure to an augmented acoustic environment on the auditory system of middle-aged C57BL/6J mice: cochlear and central histology and sex differences

Genetic progressive sensorineural hearing loss in mice of the C57BL/6J (B6) inbred strain begins at high frequencies during young adulthood and is severe by 12 months (middle age). Nightly treatment with an augmented acoustic environment (AAE)--12-hour periods of exposure to repetitive noise bursts of moderate intensity, begun at age 25 days--resulted in less severe hearing loss compared with control mice. Cochlear histopathological correlates of AAE treatment, assessed at 12-14 months of age, included lessened severity of progressive loss of outer hair cells in both sexes as well as small savings of spiral ganglion cells in females and inner hair cells in males. AAE effects on the number of surviving neurons (age 12-14 months) in the anterior ventral cochlear nucleus (AVCN) depended on sex. Compared with controls, the loss of AVCN neurons that typically accompanies the initial period of hearing loss (between 2 and 7 months of age) was not significantly affected by AAE treatment in females. In contrast, males treated with the AAE exhibited more severe loss of neurons in the dorsal and ventral extremes of the AVCN than male controls of the same age. AAE treatment begun at age 3-5 months resulted in significant but less severe loss of AVCN neurons in 1-year-old male mice.     

Cellular correlates of progressive hearing loss in 129S6/SvEv mice

Several strains of mice hear well initially but show progressive sensorineural hearing loss. Affected cochlear cell types include all those known to be affected in human age-related hearing loss (ARHL), or presbycusis. Thus these mice have been offered as models of human ARHL. At present, however, few mouse ARHL models are sufficiently well described to serve as the basis for specific hypotheses about human ARHL. We examined 1-month-old and 15-month-old 129S6/SvEv (129S6) mice and compared them with BALB/cJ and CBA/J mice. Age-related elevation of compound action potential thresholds was interpreted in the light of endocochlear potentials and changes in hair cells, afferent neurons, fibrocytes in spiral limbus and ligament, and supporting cells within the organ of Corti. Aging in 129S6 mice was associated with high-frequency hearing loss. Four components of age-related cochlear degeneration emerged from quantitative analyses, including 1) basal loss of outer hair cells; 2) basal loss of type IV fibrocytes in the spiral ligament; 3) apical loss of fibrocytes in spiral limbus, and 4) anomalies of supporting cells in the cochlear base. Although neuronal loss was not consistently found, two mice showed loss of afferent dendrites and cell bodies in the cochlear apex without inner hair cell loss. Despite multifaceted degeneration, hearing loss in 129S6 mice appears to be best explained by degenerative changes in outer hair cells and in the organ of Corti, conforming to human sensory ARHL. Age-related changes in the apical spiral limbus may promote pathology of the medial organ of Corti and eventual loss of afferent neurons, with possible implications for human neural ARHL.     

Two types of afferent terminals innervate cochlear inner hair cells in C57BL/6J mice

Afferent synapses on inner hair cells (IHC) transfer auditory information to the central nervous system (CNS). Despite the importance of these synapses for normal hearing, their response to cochlear disease and dysfunction is not well understood. The C57BL/6J mouse is a model for presbycusis and noise-induced hearing loss because of its age-related hearing loss and susceptibility to acoustic over-exposure. In this context, we sought to establish normal synaptic structure in order to better evaluate synaptic changes due to presbycusis and noise exposure. Ultrastructural analysis of IHCs and afferent terminals was performed in a normal hearing 3-month-old C57BL/6J mouse at cochlear sites corresponding to 8, 16 and 32 kHz using semi-serial sections. A stereologic survey of random sections was conducted of IHCs in 11 additional mice. Two morphologically distinct groups of afferent terminals were identified at all 3 frequency locations in 11 out of 12 animals. "Simple" endings demonstrated classic features of bouton terminals, whereas "folded" endings were larger in size and exhibited a novel morphologic feature that consisted of a fully internalized double membrane that partially divided the terminal into two compartments. In many cases, the double membrane was continuous with the outer terminal membrane as if produced by an invagination. We still must determine the generality of these observations with respect to other mouse strains.     

Age-related changes in calbindin D-28k and calretinin immunoreactivity in the inferior colliculus of CBA/CaJ and C57Bl/6 mice

This study examines calbindin D-28k and calretinin immunoreactivity in the inferior colliculus (IC) of young and old mice of two strains. The CBA/CaJ mouse maintains good hearing until very late in life, whereas the C57Bl/6 strain exhibits severe sensorineural hearing loss at an early age. Young and old mice of both strains were selected with matching auditory brainstem response audiograms and gap detection thresholds. Brain sections were reacted with anti-calbindin D-28k (CB) and anti-calretinin (CR). Staining patterns were characterized and cell counts performed. CB immunoreactivity was high only in the nucleus of the commissure (NCO); counts revealed a 22.3% decrease in the number of CB+ cells in old CBA mice and a 25.1% decrease in old C57 mice. Calretinin immunoreactivity was high in the pericentral regions of the IC, but the central nucleus was devoid of CR+ cells. The dorsal cortex, lateral nucleus, and NCO showed increases of 42.3, 49.0, and 61%, respectively, in the number of CR+ cells, but only in the old CBA mice. No significant change was observed in the old C57 mice. Whereas decreases in CB immunoreactivity are common with age, this study is the first to report an age-related increase in CR immunoreactivity in the auditory system. The increase in CR+ cells is a possible compensatory adaptation to the decrease in CB+ cells. That the number of CR+ cells remains constant with age in C57 mice suggests this compensation may depend upon stimulus-driven activity, but this requires further study. J. Comp. Neurol. 386:92–110, 1997. © 1997 Wiley-Liss, Inc.     

Effects of aging and sensory loss on glial cells in mouse visual and auditory cortices

Normal aging is often accompanied by a progressive loss of receptor sensitivity in hearing and vision, whose consequences on cellular function in cortical sensory areas have remained largely unknown. By examining the primary auditory (A1) and visual (V1) cortices in two inbred strains of mice undergoing either age-related loss of audition (C57BL/6J) or vision (CBA/CaJ), we were able to describe cellular and subcellular changes that were associated with normal aging (occurring in A1 and V1 of both strains) or specifically with age-related sensory loss (only in A1 of C57BL/6J or V1 of CBA/CaJ), using immunocytochemical electron microscopy and light microscopy. While the changes were subtle in neurons, glial cells and especially microglia were transformed in aged animals. Microglia became more numerous and irregularly distributed, displayed more variable cell body and process morphologies, occupied smaller territories, and accumulated phagocytic inclusions that often displayed ultrastructural features of synaptic elements. Additionally, evidence of myelination defects were observed, and aged oligodendrocytes became more numerous and were more often encountered in contiguous pairs. Most of these effects were profoundly exacerbated by age-related sensory loss. Together, our results suggest that the age-related alteration of glial cells in sensory cortical areas can be accelerated by activity-driven central mechanisms that result from an age-related loss of peripheral sensitivity. In light of our observations, these age-related changes in sensory function should be considered when investigating cellular, cortical, and behavioral functions throughout the lifespan in these commonly used C57BL/6J and CBA/CaJ mouse models.     

Differential effects of advancing age on neurotransmitter cell loss in the substantia nigra and striatum of C57BL/6N mice

The present study was carried out to examine the extrapyramidal motor system of C57BL/6N mice for age-related cell loss in cholinergic neurons of the striatum (ST) and dopaminergic (DA) neurons of the substantia nigra (SN). Immunocytochemistry using antibodies against tyrosine hydroxylase (TH) or choline acetyltransferase (CAT) were used to identify DA or cholinergic neurons of the SN and ST in 6 age groups of young (3 months), middle (6, 10, 20 months) and old (25, 30 months) aged mice. We found that while there was a small decline (11%) in the total number of DA neurons of the SN with age, this decrease did not reach statistical significance. In contrast, the total number of cholinergic neurons of the ST significantly decreased between 25 and 30 months of age with the largest cell loss (38%) found in the rostral ST. In addition, the loss of cholinergic neurons in 30-month-old mice was paralleled by a decline in the mean cross-sectional area of the cell soma and nucleus of remaining cholinergic neurons. These data suggest that advancing age has a differential effect on neurotransmitter neurons of the SN and ST and supports the notion that cell loss is not an inevitable characteristic of senescence but is brain region- and cell type-specific. In addition, these data are consistent with the hypothesis that the proliferation of striatal dendrites described previously in aged C57BL/6N mice may result, in part, from a compensatory growth of these processes secondary to age-related cell loss of striatal neurons.     

Projections from the anterior ventral cochlear nucleus to the central nucleus of the inferior colliculus in young and aging C57BL/6 mice

Projections from the anterior ventral cochlear nucleus (AVCN) to the central nucleus of the inferior colliculus (ICC) were studied in young and aging C57BL/6 mice. The latter animals demonstrate progressive loss of hearing. Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected into the inferior colliculus and retrograde transport to the AVCN sections, quality of labelling, number of labelled neurons adjusted for injection size, or topographic organization of projections. Thus, despite progressing loss of auditory sensitivity, chronic profound hearing loss (oldest animals), and aging, projections from AVCN to ICC remain stable.     

Morphometric analysis of obesity (ob/ob)- and diabetes (db/db)-associated hypothalamic neuronal degeneration in C57BL/KsJ mice

The influence of the obese (ob/ob) and diabetes (db/db) genetic mutations on hypothalamic structure was investigated in C57BL/KsJ and C57BL/6J mice strains by morphometric analysis of medial basal nuclei which are recognized to possess glucoregulatory neurons. Brains were collected and prepared for histomorphometric analysis at selected times following the development of expressed obesity and diabetes (Type II, non-insulin dependent) syndromes in order to compare both the strain and genomic influences on neuronal viability in the hypothalamic ventromedial (VMH) and arcuate (ARC) nuclei of mutant and age-matched control mice. The severity of each syndrome was determined by monitoring the concomitant changes in body weight and blood glucose levels in all groups. Both (db/db) and (ob/ob) mutant C57BL/KsJ mice exhibited an increase in the number and distribution of degenerated neurons in the VMH and ARC nuclei relative to corresponding controls. The mutation-associated exacerbation of the normal age-related neuronal loss, as observed in control MBH nuclei, was temporally associated with the overt expression of the hyperglycemic component of the obese and diabetes syndromes in aging C57BL/KsJ mice. No temporal or causal relationships were noted between the enhanced rate of premature neuronal degeneration, and either body weight or blood glucose levels, in either (db/db) or (ob/ob) C57BL/6J mice relative to controls. These data suggest that the hyperglycemic condition which characterizes the (ob/ob) and (db/db) mutant C57BL/KsJ mice is causally associated with the pronounced, premature MBH neuronal degeneration in these mouse strains. Neuronal changes were not pronounced when the genetic mutations were expressed in C57BL/6J mice. The accompanying alterations in brain glucose metabolism, hormone sensitivity, bioamine content and function which are recognized to occur in these mutant C57BL/KsJ mice may be causally associated consequences of the observed changes in MBH structural integrity and neuronal competence, with the severity of the mutation-associated changes being related to genetic background of the murine strain.     

Plasticity of auditory cortex associated with sensorineural hearing loss in adult C57BL/6J mice

The representation of frequency was mapped in the primary auditory cortex (AI) of C57BL/6J (C57) mice during young adulthood (1.5–2 months) when hearing is optimal, and at 3, 6, and 12 months of age, a period during which progressive, high frequency, sensorineural hearing loss occurs in this strain. Maps were also obtained from CBA/CaJ mice which retain good hearing as they age. In AI of young adult C57 mice, and CBA mice, characteristic frequencies (CFs) of multiple-unit clusters were easily identified with extracellular recordings, and a general tonotopic organization was observed from dorsal (high frequency) to ventral and caudal (low frequency). In individual cases there appeared to be deviations from the above tonotopic organization, despite the fact that inbred mice are genetically invariant. As progressive loss of high frequency sensitivity ensued peripherally, a substantially increased representation of middle frequencies was observed in AI. There was no apparent change in the surface area of the auditory cortex despite the elimination of high frequencies, and virtually the entire auditory cortex became devoted to the middle frequencies (especially 10–13 kHz) for which sensitivity remained high. Similar age-related changes were not observed in normal-hearing CBA mice. These findings indicate that plasticity in the representation of frequency in AI is associated with high frequency hearing loss in C57 mice. © 1993 Wiley-Liss, Inc.     

Auditory efferent feedback system deficits precede age-related hearing loss: contralateral suppression of otoacoustic emissions in mice

The C57BL/6J mouse has been a useful model of presbycusis, as it displays an accelerated age-related peripheral hearing loss. The medial olivocochlear efferent feedback (MOC) system plays a role in suppressing cochlear outer hair cell (OHC) responses, particularly for background noise. Neurons of the MOC system are located in the superior olivary complex, particularly in the dorsomedial periolivary nucleus (DMPO) and in the ventral nucleus of the trapezoid body (VNTB). We previously discovered that the function of the MOC system declines with age prior to OHC degeneration, as measured by contralateral suppression (CS) of distortion product otoacoustic emissions (DPOAEs) in humans and CBA mice. The present study aimed to determine the time course of age changes in MOC function in C57s. DPOAE amplitudes and CS of DPOAEs were collected for C57s from 6 to 40 weeks of age. MOC responses were observed at 6 weeks but were gone at middle (15-30 kHz) and high (30-45 kHz) frequencies by 8 weeks. Quantitative stereological analyses of Nissl sections revealed smaller neurons in the DMPO and VNTB of young adult C57s compared with CBAs. These findings suggest that reduced neuron size may underlie part of the noteworthy rapid decline of the C57 efferent system. In conclusion, the C57 mouse has MOC function at 6 weeks, but it declines quickly, preceding the progression of peripheral age-related sensitivity deficits and hearing loss in this mouse strain.     

Aging and presbycusis: effects on 2-deoxy-D-glucose uptake in the mouse auditory brain stem in quiet

Autoradiography was used to assess the incorporation of [2-14C]deoxy-D-glucose by the auditory brain stem of young and aging mice of the C57BL/6 strain (which demonstrates progressive chronic sensorineural hearing loss) and the CBA strain (which maintains good hearing until late in life). Animals were injected with labeled 2-deoxyglucose and placed in quiet for 45 min; brain stem sections were prepared for autoradiography. The amounts of 2-deoxyglucose incorporated into the anterior ventral cochlear nucleus (AVCN), inferior colliculus (IC), and trigeminal nerve (TN) were densitometrically analyzed. Within each subject, the densities of the three structures were statistically compared. In every mouse, inferior colliculus density was greater than that of the anterior ventral cochlear nucleus, which was greater than trigeminal nerve density. To compare subject groups, relative densities (inferior colliculus and anterior ventral cochlear nucleus re: trigeminal nerve) were used; no significant differences were found between groups. Thus, aging, with or without severe loss of hearing, is not associated with altered incorporation of 2-deoxyglucose (and presumably glucose) in quiet.     

Effects of age and caloric intake on glutathione redox state in different brain regions of C57BL/6 and DBA/2 mice

The main purpose of the present study was to determine whether specific regions of the mouse brain exhibit different age-related changes in oxidative stress, as indicated by glutathione redox state and the level of protein-glutathionyl mixed disulfides. Comparison of 3- and 21-month-old mice indicated an age-related decrease in the ratio of reduced to oxidized glutathione (GSH/GSSG) as well as a pro-oxidizing shift in the calculated redox potential (ranging from 6 to 15 mV) in the cortex, hippocampus, striatum and cerebellum, whereas there was little change in the brainstem. This pro-oxidizing shift in redox state was due to a modest decrease in GSH content occurring in all the brain regions examined, and elevations in GSSG amount that were most pronounced in the striatum and cerebellum. The regional changes in glutathione redox state were paralleled by increases in the amounts of protein-mixed disulfides. A reduction of caloric intake by 40% for a short period (7 weeks), implemented in relatively old mice (17 months), increased the GSH/GSSG ratio and redox potential at 19 months in the same brain regions that exhibited age-related decreases. The effects of age and caloric restriction were qualitatively similar in C57BL/6 and DBA/2 mice. However, young DBA/2 mice, which do not show extension of life span in response to long-term caloric restriction, had lower GSH/GSSG ratios and higher protein-mixed disulfides than age-matched C57BL/6 mice. The current findings demonstrate that oxidative stress, as reflected by glutathione redox state, increases in the aging brain in regions linked to age-associated losses of function and neurodegenerative diseases.     

Immunocytochemical detection of synaptophysin in C57BL/6 mice cochlea during aging process

Aged mammals frequently exhibit a bilateral, progressive, and symmetric deafness related to the degeneration of auditory receptor. However, little is still known about aging effects on synapses in this receptor. Synaptophysin (Syp) is a 38 kDa Ca²⁺ binding glycoprotein widely found in presynaptic membrane and vesicles. The Syp has been found in presynaptic buttons of efferent auditory fibers, within the developing and adult auditory receptor. The detection of Syp in aged cochleae could provide relevant information about synaptic changes and receptor degeneration process observed in old animals. This paper focuses on aging linked changes related to the presence of Syp in cochleae of C57BL/6J mice (from 1 to 24 months old). Results showed that during the first months of age, no significant changes were observed in the Syp distribution under the basal pole of inner (IHCs) neither the outer (OHCs) hair cells. At six months of age, a significant decrease of Syp immunocytochemical detection appeared in fibers under the most external row of OHCs, but restricted to the cochlear basal coil. Only a very scarce reduction of Syp was noted under the IHC and the other OHC rows, also at the basal coil. From mice 9 months old on, a progressive decrease of the presence of Syp was found under IHC and all OHC rows starting at the basal coil and reaching the apical coil in the oldest mice. All these data could indicate that the cochlea aging process early affects to presynaptic membrane proteins of efferent endings fibers. This early alteration of cochleae efferent synapses could be involved in the whole degeneration of the Corti's organ.     

The absence of the calcium-buffering protein calbindin is associated with faster age-related decline in hippocampal metabolism

Although reductions in the expression of the calcium-buffering proteins calbindin D-28K (CB) and parvalbumin (PV) have been observed in the aging brain, it is unknown whether these changes contribute to age-related hippocampal dysfunction. To address this issue, we measured basal hippocampal metabolism and hippocampal structure across the lifespan of C57BL/6J, calbindin D-28k knockout (CBKO) and parvalbumin knockout (PVKO) mice. Basal metabolism was estimated using steady state relative cerebral blood volume (rCBV), which is a variant of fMRI that provides the highest spatial resolution, optimal for the analysis of individual subregions of the hippocampal formation. We found that like primates, normal aging in C57BL/6J mice is characterized by an age-dependent decline in rCBV-estimated dentate gyrus (DG) metabolism. Although abnormal hippocampal fMRI signals were observed in CBKO and PVKO mice, only CBKO mice showed accelerated age-dependent decline of rCBV-estimated metabolism in the DG. We also found age-independent structural changes in CBKO mice, which included an enlarged hippocampus and neocortex as well as global brain hypertrophy. These metabolic and structural changes in CBKO mice correlated with a deficit in hippocampus-dependent learning in the active place avoidance task. Our results suggest that the decrease in CB that occurs during normal aging is involved in age-related hippocampal metabolic decline. Our findings also illustrate the value of using multiple MRI techniques in transgenic mice to investigate mechanisms involved in the functional and structural changes that occur during aging.     

FOS and FOSB expression in the medial preoptic nucleus pars compacta of maternally active C57BL/6J and DBA/2J mice

C57BL/6J and DBA/2J inbred mice differ in aspects of maternal behavior and in the morphology of the medial preoptic nucleus (MPO), suggesting a possible association. DBA/2J mice have a compact subnucleus in the MPO, the MPOpc, that is sexually dimorphic and absent in C57BL/6J mice. To determine whether MPOpc cells are activated by maternal behavior, FOS and FOSB immunohistochemistry was performed on brain sections of C57BL/6J and DBA/2J mothers following the return of their pups after a separation of 2 days. In both light and dark phases of the daily cycle, stimulation of DBA/2J mothers evoked an increase in FOS- and FOSB-immunoreactivity in the MPOpc. Stimulated C57BL/6J mice, which lack the MPOpc, did not show an increase in cellular activity in the corresponding MPO region. Cells immediately lateral to the MPOpc were activated by pup stimulation, in both strains. These results suggest that MPOpc cells are active during maternal behavior, and that strain differences in maternal behavior are related to anatomical differences in the MPO.