H. Schwegler

Behavioral responses to novelty and structural variation of the hippocampus in mice. II: Multivariate genetic analysis

On the basis of results from lesion studies in rodents, covariations are expected to exist between naturally-occurring heritable variations in hippocampal morphology and exploratory behavior elicited by novel surroundings. For this reason, we set up a full diallel cross between five inbred mouse strains and analyzed the behavioral and the hippocampal anatomical variation in male animals from this cross. Employing a bivariate extension of the diallel-cross analysis, estimates were obtained for the phenotypical, environmental, and genetical correlations between the phenotypes studied. A factor analysis performed on the matrix of additive-genetic correlations revealed that variations in the size of the intra- and infrapyramidal mossy fiber terminal fields (iip-MF) are negatively related to open-field exploration and novelty-induced fear. These results indicate that having larger iip-MF projections promotes the collection and processing of information about a novel environment, entailing lower levels of exploration and fear.

Water-maze learning in the mouse correlates with variation in hippocampal morphology

We studied mouse learning performance in a water-maze task in order to explore the relations between heritable structural variation in the hippocampus and spatial learning abilities. Based on previous findings we hypothesized a relation between spatial learning abilities and the size of the intra- and infrapyramidal mossy fiber (iip-MF) terminal field. Factor analysis revealed that learning in this water maze has two components: one related to apparatus-induced activity and one related to maze-learning ability. The size of the iip-MF terminal field covaried negatively with the activity-related component but positively with the spatial-learning component. These findings are in agreement with previous results obtained for shuttle-box learning.

Behavioral responses to novelty and structural variation of the hippocampus in mice. I: Quantitative-genetic analysis of behavior in the open-field

As a first step towards a multivariate quantitative-genetic analysis of covariations between heritable variation in hippocampal structure and mouse behavior, a univariate analysis of the genetic architecture of behavioral responses to novelty is presented. For several components of exploratory behavior considerable amounts of genetic variation were found and an evolutionary history of stabilizing selection for intermediate levels of exploration was inferred. Comparison of these results with those from a previous study indicated that even a relatively small diallel cross, involving 4-5 inbred strains, may provide useful genetic information on a specific sample of animals. Larger numbers of strains are needed to provide precise estimates of genetic parameters in a population.

A QUANTITATIVE-GENETIC ANALYSIS OF HIPPOCAMPAL VARIATION IN THE MOUSE

SUMMARY 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.

Strain-specific correlations between hippocampal structural traits and habituation in a spatial novelty situation

The rat strains Naples high-excitable (NHE) and Naples low-excitable (NLE) have been selectively bred since 1976 for behavioral arousal in a spatial novelty situation. The Timm-stained hippocampi of 20 NHE and 18 NLE rats were examined morphometrically for differences in the proportions (volume percentages) of terminal fields in the fascia dentata and CA3/CA4 at the mid-septotemporal level. Prior to histology animals were tested on 2 days for exploratory activity in a square alley system (Làt box). Overall, the two strains differed significantly in the percentage of stratum lacunosum molecular (NHE greater than NLE, P less than 0.001), of the intra/infrapyramidal mossy fiber (IIP-MF) projection (NLE greater than NHE, P less than 0.001), of the granule cell layer (NLE greater than NHE, P less than 0.05) and of the outer molecular layer (NHE greater than NLE, P less than 0.05). Both strains had an IIP-MF projection smaller than in any other rat strain. Most of the strain differences, however, appear to reflect genetic drift rather than a response to selective breeding. Unexpectedly, the highly active line (NHE) showed a strong correlation between the IIP-MF and overnight habituation; the larger the IIP-MF projection, the lesser the long-term habituation (r = -0.70, P less than 0.001). In the NLE rats, the IIP-MF correlated positively with overnight habituation (though not significantly, because of an outlier). In both lines, stratum oriens was negatively correlated with short-term habituation in the second exposure (NHE: r = -0.68, P less than 0.001; NLE: r = -0.66, P less than 0.01). Thus, hippocampal variability of non-genetic origin appears to be correlated with processes which modulate strain-characteristic responses to a spatial novelty situation.

Genetic Selection for Novelty-Induced Rearing Behavior in Mice Produces Changes in Hippocampal Mossy Fiber Distributions

Previous investigations in mice revealed the existence of a set of genes that influence variations in hippocampal anatomy as well as variations in behavioral responses to novelty. In particular, a positive genetic correlation was found between the size of the intra- and infrapyramidal mossy fiber (iip-MF) projection and rearing frequency in an open-field. On the basis of these findings, we hypothesized that genetic selection for rearing would entail correlated changes in hippocampal morphology. This was tested in the inbred selection lines SRH (selection for rearing: high) and SRL (selection for rearing: low). As expected, the SRH mice appeared to possess iip-MF terminal fields that were larger than those of the SRL mice. Because the behavioral difference between the two lines is most probably caused by a single genetic unit, these animals represent valuable material for molecular-genetic investigations into the mechanisms that control behavioral and neuroanatomical variation.

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.

Strain-specific development of the mossy fiber system in organotypic cultures of the mouse hippocampus

The postnatal development of the hippocampus of the inbred mice strains BALB/c, C57BL/6, and DBA/2 was studied in organotypic explant cultures using the roller-tube technique. In vivo, mice exhibit strain-specific mossy fiber distribution patterns. As a main result we found, that after cultivation of 3-4 weeks, similar strain-specific patterns became apparent in vitro, as visualized by a modified Timm staining. From this experiment we can conclude that a postnatal extrinsic influence cannot be the cause of the strain-specific hippocampal features.

Substrain divergence in C3H inbred mice

To investigate substrain divergence in the highly inbred C3H strain, we studied the C3H/HeJ, C3H/HeJ//Bkl, C3H/He//Han, and C3H/HeJ//Hd strains. The Hd subline differed from the other three in its lower performance on a two-way active-avoidance learning task, larger infra- and intrapyramidal mossy fiber terminal fields, and rejection of skin grafts from other lines. There were no differences among the other three strains. This substrain divergence is explained by differences in animal husbandry techniques.