Search for a gene that may result in fewer neurons in the adult mouse hippocampus.

Experimentally produced C57BL/6J-Pdebrd1 congenic mice carrying this gene for retinal degeneration were previously found to have fewer hippocampal neurons than partner strain C57BL/6J+Pdeb-rd1 mice possessing normal retinas. A linked passenger gene on the inserted chromosome segment containing the Pdebrd1 gene might have been responsible. An inbred strain segregating at the Pdebrd1 locus and with a genetic background on which the gene is normally present has now been examined. No neuron loss was observed. These new results indicate that, while Pdebrd1 was necessary for the occurrence of fewer hippocampal neurons in C57BL/6J-Pdebrd1 mice, it alone was not sufficient to produce this effect. The Pdebrd1 gene was acting in combination with at least one other gene not on the introduced chromosome segment. The results also provide evidence that genetic background effects can minimize or eliminate hippocampal neuron loss in some strains that normally carry the Pdebrd1 gene.

The mouse gene retinal degeneration (rd) may reduce the number of neurons present in the adult hippocampal dentate gyrus.

C57BL/6J and C57BL/6J-rd le Gus-sh mice, congenics at the rd locus, were compared with respect to number of granule cells and presumed pyramidal basket cells in the hippocampal dentate gyrus. Number of both types of neurons were less in rd/rd mice than in +/-/+/- mice. The rd gene may be responsible.

New strains of seizure-prone mice.

The origins of two new strains of seizure-prone mice are provided, and some of their behavioral characteristics are described. Comparison of the hippocampal granule cell layer of one of the new strains with the two inbred strains from which it was derived revealed strain differences in the diameter of granule cell nuclei and in the number of granule cells in the suprapyramidal blade. Basket cell counts did not differ between the strains, but both basket cell and granule cell number were consistently higher for the suprapyramidal blade than for the infrapyramidal blade. The existence of these and other blade differences suggests that the two blades will prove to be functionally distinctive neuronal systems.

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.