New strains of seizure-prone mice

Late maturation of adult-born neurons in the temporal dentate gyrus

Hippocampal function varies along its septotemporal axis, with the septal (dorsal) pole more frequently involved in spatial learning and memory and the temporal (ventral) pole playing a greater role in emotional behaviors. One feature that varies across these subregions is adult neurogenesis. New neurons are more numerous in the septal hippocampus but are more active in the temporal hippocampus during water maze training. However, many other aspects of adult neurogenesis remain unexplored in the context of septal versus temporal subregions. In addition, the dentate gyrus contains another functionally important anatomical division along the transverse axis, with the suprapyramidal blade showing greater experience-related activity than the infrapyramidal blade. Here we ask whether new neurons differ in their rates of survival and maturation along the septotemporal and transverse axes. We found that neurogenesis is initially higher in the infrapyramidal than suprapyramidal blade, but these cells are less likely to survive, resulting in similar densities of neurons in the two blades by four weeks. Across the septotemporal axis, neurogenesis was higher in septal than temporal pole, while the survival rate of new neurons did not differ. Maturation was assessed by immunostaining for the neuronal marker, NeuN, which increases in expression level with maturation, and for the immediate-early gene, Arc, which suggests a neuron is capable of undergoing activity-dependent synaptic plasticity. Maturation occurred approximately 1-2 weeks earlier in the septal pole than in the temporal pole. This suggests that septal neurons may contribute to function sooner; however, the prolonged maturation of new temporal neurons may endow them with a longer window of plasticity during which their functions could be distinct from those of the mature granule cell population. These data point to subregional differences in new neuron maturation and suggest that changes in neurogenesis could alter different hippocampus-dependent behaviors with different time courses.

Patterning of retinoic acid signaling and cell proliferation in the hippocampus

The nuclear receptor ligand retinoic acid (RA) has been identified as an endogenous regulatory factor in the hippocampus, acting on pyramidal neurons and granule neuron progenitors, but almost nothing is known about the distribution of RA itself in the hippocampus. This study describes the source of RA for the rodent hippocampus in the meninges via the key RA synthetic enzyme retinaldehyde dehydrogenase 2 (RALDH2). Diffusion of RA from the meninges potentially creates a gradient of RA across the infrapyramidal and suprapyramidal blades of the dentate gyrus, enhanced by the expression of the RA catabolic enzyme Cyp26B1 between the blades, and an infrapyramidal and suprapyramidal blade difference is evident in RA-regulated transcription. This asymmetry may contribute to some of the physiological and molecular differences between the blades, including a disparity in the rates of cell proliferation in the subgranular zone of the two blades through RA inhibition of cell proliferation. Such differences can be altered by either the application of excess RA, its effect dependent on the relative position along the septotemporal axis, or change in RA signaling through mutation of retinol binding protein, while the capacity of RA to inhibit proliferation of cells in the dentate gyrus is demonstrated using in vitro slice culture. Use of synthetic and catabolic enzymes in the hippocampus to create differing zones of RA concentration parallels the mechanisms used in the developing brain to generate patterns of RA-regulated transcription. © 2012 Wiley Periodicals, Inc.

Sex differences in the hilar mossy cells of the guinea-pig before puberty

Numerous sex differences have been detected in the morphology of the dentate and hippocampal neurons and hippocampus-dependent memory functions. The aim of the present study was to ascertain whether the mossy cells, an interneuron population forming a recurrent excitatory circuit with the dentate granule cells, are sexually dimorphic. The brains of juvenile (15-16 days old) and peripubescent (45-46 days old) male and female guinea-pigs were Golgi-Cox stained. Mossy cells were sampled from the hilus in the septal third of the dentate gyrus and their dendritic tree and somata were analyzed. The analysis was separately conducted on mossy cells with soma located in the portions of the hilus that face the upper blade (upper hilus) and lower blade (lower hilus), respectively. The mossy cells in the upper hilus were found to be sexually dimorphic in both juvenile and peripubescent animals. At both ages females had a larger dendritic tree than males. This difference was due to a greater mean branch length and, in peripubescent animals, also to a greater number of branches. In juvenile males, the spines on the proximal dendrites (thorny excrescences) had a greater density than in females. No differences in spine density were present in peripubescent animals. Unlike the mossy cells in the upper hilus, the mossy cells in the lower hilus showed very few sex differences in juvenile animals and no differences in peripubescent animals. The few differences favored females, that had more proximal branches and a greater spine density on the distal dendrites than males. The results show that the mossy cells of the guinea-pig are sexually dimorphic prior to puberty. Extending a previous investigation, the present data provide evidence that sex differences are mainly confined to the dentate region corresponding to the upper blade and upper hilus. The observed segregation of the sexual dimorphism in the upper blade/upper hilus suggests that this region might underlie the sexual dimorphism in hippocampus-dependent memory functions.

Sex differences in the hippocampal dentate gyrus of the guinea-pig before puberty

The aim of the present research was to ascertain the presence of sex differences in the hippocampal dentate gyrus of the guinea-pig, a long-gestation rodent which gives birth to mature young and whose brain is at a more advanced stage of maturation at birth than that of the rat and mouse. The brains of neonatal (15-16 days old) and prepubescent (45-46 days old) male and female guinea pigs were Golgi-Cox stained. Granule cells were sampled from the upper (suprapyramidal) and lower (infrapyramidal) blade of the septal dentate gyrus and their dendritic tree and soma were measured. The analysis was conducted separately on granule cells with soma in the superficial (superficial granule cells) and deep (deep granule cells) half of the granule cell layer. Numerous sex differences were found in the upper blade of the dentate gyrus. Neonatal males had more dendritic branches than females in the innermost dendritic tree of both superficial and deep granule cells, but females had more branches over the middle/outer dendritic tree and a longer dendritic length. In prepubescent animals, the sex difference in the middle dendritic tree of the superficial granule cells changed direction, with males having more branches than females. In the deep granule cells, the sex differences were similar to those in neonatal animals. In both granule cell types, the dendritic length was similar in the two sexes. While no sex differences were found in dendritic spine density in neonatal animals, in prepubescent animals spine density was greater in females. In the lower blade the granule cells showed very few sex differences in both neonatal and prepubescent animals. This study shows wide dynamically changing sex differences in the granule cells located in the upper blade of the septal dentate gyrus, but almost no differences in the lower blade. These results demonstrate that sex differences are not ubiquitous in the dentate gyrus and suggest that the lower blade, unlike the upper blade, might be involved in non-sexually dimorphic behaviors.

Genetic selection of mouse lines differing in sensitivity to a benzodiazepine receptor inverse agonist

Mice were selectively bred according to their sensitivity or their resistance to the convulsive effects of a 4-mg/kg dose of methyl beta-carboline-3-carboxylate (beta-CCM), a benzodiazepine (BZ) receptor inverse agonist. The selection proved to be easy, with a clear separation of the two lines, convulsing with short latencies or resistant, already at the first generation of selection. Selection of a third line of animals convulsing with long latencies did not succeed. 3H-Ro 15-1788 binding analysis provided evidence for a strong decrease in Bmax in the resistant line.

New seizure frequency QTL and the complex genetics of epilepsy in EL mice

EL/Suz (EL) mice experience recurrent seizures that are similar to common partial complex epilepsy in humans. In the mice, seizures occur naturally at 90-100 days of age, but can be induced in younger mice and analyzed as a semi-quantitative trait after gentle rhythmic stimulation. A previous genetic mapping study of EL backcrosses to the strains ABP/LeJ or DBA/2J showed two quantitative trait loci (QTL) with large effects on seizure frequency (El1, Chr 9; El2, Chr 2) and implied the existence of other QTL with lesser effects. To further the understanding of EL-derived seizure alleles, we examined intercross progeny of EL and the strains ABP/LeJ and DDY/Jcl, and also a backcross of (EL x DDY)F1 hybrids to DDY. A new large-effect seizure frequency QTL was found (El5, Chr 14), a more minor QTL confirmed (El3, Chr 10), and two additional QTL proposed (El4, Chr 9; El6, Chr 11). The serotonin receptor gene, Htr2a, maps near and is a candidate for El5, and linkages of other serotonin receptor genes to seizure frequency QTL are noted. In addition, a strong gender effect was revealed, and epistasis was found between Chr 9 and Chr 14 markers. Despite this progress, however, our results revealed a more complex determinism of epilepsy in EL mice than previously described. In particular, no single El locus or pair was essential for frequent seizures, as QTL with large effects, such as El5, El2, and El1, were highly dependent on genetic context. Our studies highlight the importance of gene interaction in some complex mammalian traits defined by natural variation.

Analysis of two measures of paw preference in a large population of inbred mice

Two measures of lateralized forelimb usage, the Collins paw preference test and the Lateral Paw Preference test (LPP), were tested in 693 mice of 29 inbred strains and F1 crosses, and 2 embryo transfer groups. These strains included NZB, SM, and the NXSM recombinant inbred (RI) strains; RF and the NXRF RI strains; BXSB and the Y-consomic BXSB-Yaa+; DBA/2 and F1s of DBA/2 and BXSB and BXSB-Yaa; and NZB x NZW F1s. The findings indicated that (1) the Collins and LPP tests were independent in terms of direction of lateralization, (2) there were significant population biases of 60.96% rightward on the LPP test, and 54.39% leftward on the Collins test, (3) there were significant strain differences on measures of absolute asymmetry on both tests, (4) there were strain differences for direction of asymmetry on the Collins test, but not on LPP, (5) on the basis of the NXSM RI strains, 3 genetic loci contribute to strength of laterality on the LPP test, and (6) there was a strong correlation among strain means for strength of lateralization on the two tests. These results extend previous findings on the strength and direction of laterality, showing that two independent systems for direction of laterality may depend on the same mechanism for magnitude, and establish that multiple factors of handedness, previously identified in humans and other primates, also exist in mice.

Morphology and distribution of astrocytes in the molecular layer of the dentate gyrus in NZB/BlNJ, Dreher, and C57BL/6J mice

Numerous ectopic granule cells are found in the molecular layer of the dentate gyrus in NZB/BlNJ and dreher mutant mice. These ectopic neurons occur either singly or in small clusters. In contrast, few ectopic granule cells are seen in the dentate molecular layer of C57BL/6J mice and dreher control littermates. In this investigation we have examined the morphology, number, and distribution of molecular layer astrocytes in NZB/BlNJ, dreher homozygotes, dreher littermate controls, and C57BL/6J mice to determine the effect of the presence of ectopic granule cells. In the molecular layer of C57BL/6J mice and dreher control littermates, astrocytes have a typical stellate appearance with processes emanating in all directions. The arborization of astrocytes in areas devoid of ectopic granule cells in NZB/BlNJ mice and dreher homozygotes was similar to that in C57BL/6J mice and dreher control littermates. In contrast, the morphology of astrocytes in the immediate vicinity of ectopic granule cells or ectopic clusters was distinctly non-stellate. Furthermore, the somata and processes of these astrocytes occasionally appeared to make intimate contact with the ectopic granule cells. A quantitative analysis of the number and distribution of astrocytes in NZB/BlNJ vs C57BL/6J mice and dreher vs. control littermates indicated that these parameters were not altered by the presence of the ectopic neurons. We conclude that the trophic effects of ectopic neurons on glial cells can affect the growth and orientation of astrocytic processes without a concomitant effect on glial cell number.

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.