A biometrical-genetic analysis of granule cell number in the area dentata of house mice

Neurogenetic basis of cognition: Facts and hypotheses

In the natural setting, cognitive processes direct behavioural adjustments and sometimes result in behavioural novelties which allow the organism to cope with environmental pressures. The resulting behavioural changes exhibit various forms which are dependent upon different causal factors and cognitive processes. Under long-lasting environmental changes, these behavioural adaptations can become hereditary either through the process of cultural transmission or through genetic mechanisms sensitive to selective forces acting on genotypes. In the last few years, neuroethology and behavioural neurosciences have produced an increasing amount of precise knowledge about brain-behaviour relationships, neurobiological bases of cognitive processes and their development. Unfortunately, the approach to these phenomena is basically normative and does not tell us much about non-pathological determinants of individual variation in cognitive and behavioural competences. In contrast, the differential approach has provided some cases of structural variations in the brain which are under genetic control and thus liable to evolve under selective pressures. Brain size, the ratio of various brain structures to the total brain, the number and density of neurons in various parts of the brain and the variations of neuronal circuitry are potential candidates. This paper reviews them and examines their possible behavioural and cognitive outcomes. The issue here is to examine if and where in the brain potential conditions occur that would allow the genetic evolution of cognitive processes.

Insensitivity of the analysis of variance to heredity-environment interaction

It makes sense to attribute a definite percentage of variation in some measure of behavior to variation in heredity only if the effects of heredity and environment are truly additive. Additivity is often tested by examining the interaction effect in a two-way analysis of variance (ANOVA) or its equivalent multiple regression model. If this effect is not statistically significant at the α = 0.05 level, it is common practice in certain fields (e.g., human behavior genetics) to conclude that the two factors really are additive and then to use linear models, which assume additivity. Comparing several simple models of nonadditive, interactive relationships between heredity and environment, however, reveals that ANOVA often fails to detect nonadditivity because it has much less power in tests of interaction than in tests of main effects. Likewise, the sample sizes needed to detect real interactions are substantially greater than those needed to detect main effects. Data transformations that reduce interaction effects also change drastically the properties ofthe causal model and may conceal theoretically interesting and practically useful relationships. If the goal ofpartitioning variance among mutually exclusive causes and calculating “heritability” coefficients is abandoned, interactive relationships can be examined more seriously and can enhance our understanding of the ways living things develop.

Probing mouse brain microstructure using oscillating gradient diffusion MRI

High resolution diffusion tensor images of the mouse brain were acquired using the pulsed gradient spin echo sequence and the oscillating gradient spin echo sequence. The oscillating gradient spin echo tensor images demonstrated frequency-dependent changes in diffusion measurements, including apparent diffusion coefficient and fractional anisotropy, in major brain structures. Maps of the rate of change in apparent diffusion coefficient with oscillating gradient frequency revealed novel tissue contrast in the mouse hippocampus, cerebellum, and cerebral cortex. The observed frequency-dependent contrasts resembled neuronal soma-specific Nissl staining and nuclei-specific 4',6-diamidino-2-phenylindole (DAPI) staining in the mouse brain, which suggests that the contrasts might be related to key features of cytoarchitecture in the brain. In the mouse cuprizone model, oscillating gradient spin echo-based diffusion MRI revealed significantly higher frequency-dependence of perpendicular diffusivity (λ(⊥) ) in the demyelinated caudal corpus callosum at 4 weeks after cuprizone treatment when compared with control mice and mice at 6 weeks after cuprizone treatment. The elevated frequency-dependence of λ(⊥) coincided with the infiltration of activated microglia/macrophages and disruption of axons during acute demyelination in the caudal corpus callosum. The results demonstrate the potential of oscillating gradient spin echo-based diffusion MRI for providing tissue contrasts complimentary to conventional pulsed gradient spin echo-based diffusion MRI.

Smaller hippocampal volume predicts pathologic vulnerability to psychological trauma

In animals, exposure to severe stress can damage the hippocampus. Recent human studies show smaller hippocampal volume in individuals with the stress-related psychiatric condition posttraumatic stress disorder (PTSD). Does this represent the neurotoxic effect of trauma, or is smaller hippocampal volume a pre-existing condition that renders the brain more vulnerable to the development of pathological stress responses? In monozygotic twins discordant for trauma exposure, we found evidence that smaller hippocampi indeed constitute a risk factor for the development of stress-related psychopathology. Disorder severity in PTSD patients who were exposed to trauma was negatively correlated with the hippocampal volume of both the patients and the patients' trauma-unexposed identical co-twin. Furthermore, severe PTSD twin pairs-both the trauma-exposed and unexposed members-had significantly smaller hippocampi than non-PTSD pairs.

Life-long stability of neurons: a century of research on neurogenesis, neuronal death and neuron quantification in adult CNS

In this chapter we provide an extensive review of 100 years of research on the stability of neurons in the mammalian brain, with special emphasis on humans. Although Cajal formulated the Neuronal Doctrine, he was wrong in his beliefs that adult neurogenesis did not occur and adult neurons are dying throughout life. These two beliefs became accepted "common knowledge" and have shaped much of neuroscience research and provided much of the basis for clinical treatment of age-related brain diseases. In this review, we consider adult neurogenesis from a historical and evolutionary perspective. It is concluded, that while adult neurogenesis is a factor in the dynamics of the dentate gyrus and olfactory bulb, it is probably not a major factor during the life-span in most brain areas. Likewise, the acceptance of neuronal death as an explanation for normal age-related senility is challenged with evidence collected over the last fifty years. Much of the problem in changing this common belief of dying neurons was the inadequacies of neuronal counting methods. In this review we discuss in detail implications of recent improvements in neuronal quantification. We conclude: First, age-related neuronal atrophy is the major factor in functional deterioration of existing neurons and could be slowed down, or even reversed by various pharmacological interventions. Second, in most cases neuronal degeneration during aging is a pathology that in principle may be avoided. Third, loss of myelin and of the white matter is more frequent and important than the limited neuronal death in normal aging.

Complex trait analysis of the hippocampus: mapping and biometric analysis of two novel gene loci with specific effects on hippocampal structure in mice

Notable differences in hippocampal structure are associated with intriguing differences in development and behavioral capabilities. We explored genetic and environmental factors that modulate hippocampal size, structure, and cell number using sets of C57BL/6J (B6) and DBA/2J (D2) mice; their F1 and F2 intercrosses (n = 180); and 35 lines of BXD recombinant inbred (RI) strains. Hippocampal weights of the parental strains differ by 20%. Estimates of granule cell number also differ by approximately 20%. Hippocampal weights of RI strains range from 21 to 31 mg, and those of individual F2 mice range from 23 to 36 mg (bilateral weights). Volume and granule cell number are well correlated (r = 0.7-0.8). Significant variation is associated with differences in age and sex. The hippocampus increases in weight by 0.24 mg per month, and those of males are 0.55 mg heavier (bilateral) than those of females. Heritability of variation is approximately 50%, and half of this genetic variation is generated by two quantitative trait loci that map to chromosome 1 (Hipp1a: genome-wide p < 0.005, between 65 and 100 cM) and to chromosome 5 (Hipp5a, p < 0.05, between 15 and 40 cM). These are among the first gene loci known to produce normal variation in forebrain structure. Hipp1a and Hipp5a individually modulate hippocampal weight by 1.0-2.0 mg, an effect size greater than that generated by age or sex. The Hipp gene loci modulate neuron number in the dentate gyrus, collectively shifting the population up or down by as much as 200,000 cells. Candidate genes for the Hipp loci include Rxrg and Fgfr3.

Stereological estimation of the total number of neurons in the murine hippocampus using the optical disector

Using a stereological method, the optical disector, we examined three inbred strains of mice (NZB/BINJ, DBA/2, and C57BL/6J) for morphological differences in volume, neuronal number, and density of the pyramidal cell and dentate gyrus granule cell layers of the hippocampus. We found significant differences in volume and neuronal number for both regions between the three strains at 9 weeks of age, but only modest differences in neuronal density. The left dentate volume was 90% larger in the NZB strain and 70% greater in the DBA strain (P<0.0001), and the left pyramidal cell layer was 144% larger in the NZB strain and 150% larger in the DBA strain, than in the B6 strain (P<0.0001). Neuron number in the left dentate was 81% greater in NZB and 37% greater in DBA (P<0.001), and in the left pyramidal cell layer 118% greater in the NZB and 92% greater in the DBA (P<0.01). Differences in neuronal density of the left dentate were not significant (P = 0.060, ns). For the left pyramidal cell layer, neuronal density was 14% greater in B6 and 34% greater in NZB than the DBA strain (P = 0.016). No significant differences were found in left-right laterality, or according to sex. We found that strain accounted for 60% of the variance in hippocampal volume and 44% of neuron number. These differences thus mainly reflect genetic variation in hippocampal volume and may have important implications for brain evolution, behaviour, and human diseases where hippocampal degeneration is involved.

Age-related morphological changes in the hippocampus in two mouse strains

The granule cell number (nGR) in the dentate gyrus (DG) has been reported to vary considerably among inbred strains of mice, thus providing proof of some genetically associated components to this variation. Furthermore, several authors have described age-related morphological changes in the DG in both humans and animals, but there is no general agreement in the literature about the occurrence of such changes. The purpose of this study was to investigate for strain differences in hippocampal structure changes in old C57BL/6J (B) and DBA/2J (D) mice as compared with younger ones. The nGR in the DG, as well as other structural parameters of the hippocampus, were determined in female B and D mice of 4 and 24 months. The two-way analysis of variance indicated a significant interaction between 'strain' and 'age' for the nGR, suggesting that this parameter changes differently with age in B and D mice. This finding indicates that these strains could present a differential susceptibility in granule cell aging raising the possibility that age effects on the granule cell population in the DG could be influenced by some hereditary factors.

PD 142676 (CI 1002), a novel anticholinesterase and muscarinic antagonist

Inhibition of brain acetylcholinesterase (AChE) can provide relief from the cognitive loss associated with Alzheimer's disease (AD). However, unwanted peripheral side effects often limit the usefulness of the available anticholinesterases. Recently, we identified a dihydroquinazoline compound, PD 142676 (CI 1002) that is a potent anticholinesterase and a functional muscarinic antagonist at higher concentrations. Peripherally, PD 142676, unlike other anticholinesterases, inhibits gastrointestinal motility in rats, an effect consistent with its muscarinic antagonist properties. Centrally, the compound acts as a cholinomimetic. In rats, PD 142676 decreases core body temperature. It also increases neocortical arousal, as measured by quantitative electroencephalography, and cortical acetylcholine levels, measured by in vivo microdialysis. The compound improves the performance of C57/B10j mice in a water maze task and of aged rhesus monkeys in a delayed match-to-sample task involving short-term memory. The combined effect of AChE inhibition and muscarinic antagonism distinguishes PD 142676 from other anticholinesterases, and may be useful in treating the cognitive dysfunction of AD and produce fewer peripheral side effects.

Effects of undernutrition during early life on granule cell numbers in the rat dentate gyrus

Undernutrition during early life is known to affect the morphology of the hippocampal formation. Recent advances in stereological techniques have made it possible to make relatively unbiased estimates of total cell numbers in well-defined brain regions. It was decided to use these methods to determine the effects of different levels of undernutrition during early postnatal life on the granule cells of the rat dentate gyrus. Male hooded Long Evans rats were undernourished between the 16th day of gestation and 30 postnatal days of age to two different levels. The daily food intake of level-1 and level-2 rats represented about 60 and 40%, respectively, of that eaten by well-fed, age-matched controls. Nutritional rehabilitation of the rats was commenced when they had reached 30 days of age by placing them on an ad libitum diet. Groups of control and experimental rats were killed at 70 and 212 days of age. The Cavalieri principle was used to determine the granule cell layer volume within the dentate gyrus, and the "dissector" method was used to determine numerical densities of these granule cells. These estimates were used to calculate the total numbers of granule cells. There were between 260,000 and 320,000 granule cells within the dentate gyrus of 70-day-old control and experimental rats. By 212 days of age, well-fed controls had an average of about 834,000 granule cells. The level-1 and level-2 previously undernourished rats had about 515,000 and 595,000 granule cells, respectively. Two-way analysis of variance procedures showed significant main effects of nutrition and age as well as a significant interaction between them.(ABSTRACT TRUNCATED AT 250 WORDS)

A genetic-correlational study of hippocampal structural variation and variation in exploratory activities of mice

Our previous work provided evidence that hippocampal opioid peptides form an important neurochemical substrate underlying the gene-dependent exploratory behavior of mice. A prominent hippocampal opioid is dynorphin B, which resides in the mossy fibers exclusively. In order to seek support for causal relationships between dynorphinergic hippocampal mechanisms and exploration, a quantitative-genetic method was chosen. For this purpose, mice from the inbred strains C57BL/6, DBA/2, BLN, and CPB-K were used. Their hippocampal mossy fiber projections were visualized by means of immunohistochemistry, using a highly specific anti-dynorphin B antiserum. The additive-genetic correlations that were estimated suggest pleiotropic gene effects on locomotion, rearing-up, wall-leaning, and several intra- and infrapyramidal mossy fiber (iipMF) variables. Long iipMF, in particular, were found to be associated with high exploratory activity.

On the sources of strain and sex differences in granule cell number in the dentate area of house mice

The origins of strain and sex differences in the number of granule cells in the dentate area of hippocampus were examined in a breeding study employing two inbred strains of mice that differ substantially in granule cell number. Sources of hereditary variation analyzed included autosomes, sex chromosomes, and maternal factors, including cytoplasmic and environmental. The results corroborated those of an earlier study in finding that 80% of the strain variation is attributable to autosomal differences. In addition, there appears to be a cytoplasmic factor that results in a strain-dependent sex dimorphism. The autosomal contribution is attributed to mechanisms operating during the primary phase of granule cell genesis. The possibility that the sex difference results from strain differences in mitochondrial DNA affecting rate of cell death is considered.

On the development of strain and sex differences in granule cell number in the area dentata of house mice

Male and female house mice of 6 inbred strains high or low in granule cell number as adults were examined at 3 immature postnatal ages beginning with day 13, and in young adulthood at day 84. The difference between mice of high and of low strains was present by postnatal day 13. Possible contributions of both incremental and decremental developmental events must be considered. Both males and females exhibited a reduction in granule cell number between postnatal days 20 and 27. Competition for efferent target cell sites was considered as a basis for sex-independent granule cell death, but no supporting evidence was obtained. Females displayed a greater reduction in granule cell number than did males. Thus, a sex dimorphism (females lower) appeared at that time. A low-level testosterone effect acting during this period of granule cell death, or a long-term consequence of high perinatal testosterone levels, might be responsible.

Water-maze learning and effects of cholinergic drugs in mouse strains with high and low hippocampal pyramidal cell counts

Morphological differences have been found in inbred strains of mice in the number and volume of pyramidal cells in Ammon's horn of the hippocampus. Among the mouse strains surveyed, NZB/BINJ (NZB) and C57BL/10J (B10) are most divergent in both total volume and total number of neurons. These genetically derived differences were exploited to determine hippocampal involvement in the acquisition of a spatial water maze. Genetic differences in hippocampal cell number were related to the acquisition of this spatial task. Mice with small numbers of hippocampal pyramidal cells, the B10 strain, acquired a water-maze task more slowly than either NZB mice or (NZBxNZW) F1 (NZBWF) animals. In addition, strain differences in responsivity to cholinergic manipulations were found. B10 mice were more sensitive than NZB or NZBWF mice to both the disruptive effects of scopolamine and the facilitory effects of physostigmine on swim maze learning. Although other inherited differences undoubtedly exist between these strains as is apparent in other mouse lines, these data suggest a prominent role for the hippocampus in the learning of spatially oriented behavior. Furthermore, this behavior appears to be responsive to cholinergic manipulations.

On the numbers of neurons in fields CA1 and CA3 of the hippocampus of Sprague-Dawley and Wistar rats

In a previous study it was found that there are significant differences in the numbers of granule cells in the dentate gyrus of adult Sprague-Dawley and Wistar rats and also that the continued postnatal addition of new cells to the dentate gyrus has quite different consequences in the two strains. We have now extended these observations to the two major cytoarchitectonic fields of the hippocampus (the regio superior or field CA1; and the regio inferior or field CA3). The mean number of pyramidal neurons in field CA1 of 1-month-old Sprague-Dawley rats is 420,000 (+/- 60,000 S.E.), while Wistar rats at the same age have 320,000 (+/- 20,000). The numbers of neurons in field CA3 in the two strains are: 330,000 (+/- 30,000) and 210,000 (+/- 20,000), respectively. Whether these strain differences reflect specific differences in the neural organization of the hippocampal formation in the two strains, or are related to more general differences in total body weight or brain weight, is unknown. Since during the first two days postnatally we estimate that there are between 358,000 and 491,000 cells in field CA1 of Sprague-Dawley rats, it would seem that there is no significant naturally-occurring neuronal death in this hippocampal field. This may be due to the extensive collateral projections of the hippocampal pyramidal neurons.

Genetic determination of striatal tyrosine hydroxylase activity in mice

An additive major gene effect is described for tyrosine hydroxylase activity in mouse corpus striatum (CS). Quantitative genetic analysis indicated the presence of a segregating Mendelian factor with robust additive effect in F2 generations derived from crossing two highly inbred mouse strains, C57BL/6ByJ and BALB/cJ, with intermediate (INT) and high (HI) TH activity in CS. Significant positive correlation was found between striatal and mesencephalic TH activity in the segregating generations, raising the possibility that a common single gene may express its effect through pleiotropy or linkage. Genetic preparations taking advantage of the major gene effect should serve well as animal models of DA-mediated neuropsychiatric disorders.

A quantitative-genetic analysis of hippocampal variation in the mouse

This report analyses the genetic underpinnings of the proportions of the hippocampal terminal fields in the mouse at the midseptotemporal level. We used 5 inbred strains and all possible F1 crosses between them (diallel cross). Broad heritabilities ranged from 11 to 53%. Additive genetic variation was present for all phenotypes analyzed. Directional dominance was found for the relative size of the suprapyramidal mossy fiber terminal field only. For the stratum lacunosum-moleculare, ambidirectional dominance emerged. These findings suggest that, in evolutionary history, directional selection has operated for a proportionally large suprapyramidal terminal field. For all other hippocampal variables (viz. the relative sizes for the strata oriens, pyramidale, radiatum, lacunosum-moleculare, CA4, intra- and infrapyramidal mossy fiber terminal field and the absolute size of the regio inferior) past stabilizing selection was inferred.

Hippocampal variation between the inbred mouse strains C3H/HeJ and DBA/2: a quantitative-genetic analysis

A classical cross-breeding study involving the inbred mouse strains DBA/2 and C3H/HeJ revealed a rather complex mode of inheritance for the following hippocampal variables: size of stratum pyramidale, number of supra-, intra- and infrapyramidal mossy fiber synapses, and the size of terminal fields receiving entorhinal input. A polygenic mode of inheritance was inferred for these phenotypes. For the size of the regio inferior a model containing additive genetic effects only was sufficient to explain the variation between generations. The strain difference may be caused by one genetic factor only. In agreement with previous experiments a strong negative correlation between the number of intra- and infrapyramidal mossy fiber synapses and shuttle-box avoidance performance was found in the genetically heterogeneous F2 population.