A 2-year longitudinal study of swimming navigation in mice devoid of the prion protein: no evidence for neurological anomalies or spatial learning impairments

Uncontrolled accumulation of a conformationally distorted protein (PrP(Sc)) is supposed to be the pathological process leading to spongiform encephalopathy. Targeted disruptions of the Prn-P gene in the mouse have resulted in animals that did not show anomalies in spatial and avoidance learning and were resistant to experimental infections. However, another Prn-P knockout mouse was reported to show ataxia and Purkinje cell degeneration developing after 70 weeks of age. In this study the initial observations are confirmed on swimming navigation of PrP-null mutant mice using an enlarged sample of 58 mice. A representative subsample of 16 mice was then followed up for their ability of swimming navigation up to an age of two years (104 weeks). Surviving PrP-null mutants (n = 4) and controls (n = 6) did not differ in any measure, nor were there indications of ataxia and Purkinje cell degeneration. It was concluded that the PrP-knockout mice used by Büeler et al. were probably normal with respect to aging processes and that resistance to scrapie is not necessarily paid for by late neuronal degeneration. The reasons for the discrepancy between different knockout experiments require experimental clarification, however.

Deficits in memory tasks of mice with CREB mutations depend on gene dosage

Studies in Aplysia, Drosophila, and mice have shown that the transcription factor CREB is involved in formation and retention of long-term memory. To analyze the impact of differential CREB levels on learning and memory, we varied the gene dosage of CREB in two strains of mutant mice: (1) CREBalphadelta mice, in which the alpha and delta isoforms are disrupted, but a third isoform beta is strongly up-regulated; (2) CREBcomp, a compound strain with one alphadelta allele and one CREBnull allele in which all CREB isoforms are disrupted. To minimize genetic background effects, CREB mutations were backcrossed into a C57BL/6 and a FVB/N strain, respectively, and studies were performed in F1 hybrids from these lines. CREBcomp but not CREBalphadelta F1 hybrids were impaired in water maze learning and fear conditioning, demonstrating a CREB gene dosage effect. However, analysis of the platform searching strategies in the water maze task suggested that CREBcomp mutants are impaired in behavioral flexibility rather than in spatial memory. In contrast to previous experiments using CREBalphadelta mice with different genetic background, the F1 hybrid CREBalphadelta and CREBcomp mice did not show deficits in a social transmission of food preference task nor in dentate gyrus and CA1 LTP as recorded from slice preparations. These data indicate that the hybrid vigor typical for F1 hybrids may compensate for a reduction in CREB levels in some tests. On the other hand, the persistence of clear behavioral deficits as shown by the F1 hybrid CREBcomp mice in water maze and fear conditioning indicates a robust and repeatable phenomenon that will permit further functional analysis of CREB.

Neurobehavioral development, adult openfield exploration and swimming navigation learning in mice with a modified beta-amyloid precursor protein gene

The processing of beta-amyloid precursor protein (betaAPP) and its metabolites plays an important role in the pathogenesis of Alzheimer's disease (AD) and Down's syndrome. The authors have reported elsewhere that a targeted mutation resulting in low expression of a shortened betaAPP protein (betaAPP(delta/delta)) entails reduced learning abilities. Here the authors investigate whether these effects were caused by postnatal developmental actions of the altered protein. The authors examined 35 mice carrying the betaAPP(delta/delta) mutation for somatic growth and sensorimotor development during the first 4 postnatal weeks (pw) and compared them with 31 wildtype litter-mates. Thereafter, the same mice were tested at about 10 weeks of age for openfield behavior and for swimming navigation learning. Mutant mice showed both transient and long-lasting deficits in development. Body weight deficit started to emerge at postnatal day (pd) 12, peaked with a 15.1% deficit at pd 27 and lasted until pw 33-37. Significant transient deficits in mutant mice during sensorimotor development were observed in three time windows (pd 3-10, pd 11-19 and pd 20-27), long-lasting effects, manifest at pw 8-12 and pw 33-37, emerged at any of the three periods. In the adult mice, exploratory activity of betaAPP mutants in the openfield arena was severely reduced. In the Morris water maze task, mutant mice showed moderate escape performance deficits during the acquisition period but no impairment in spatial memory. The authors conclude that a defective betaAPP gene impairs postnatal somatic development, associated with transient as well as long-lasting neurobehavioral retardation and muscular weakness. Comparison with earlier data suggests that early postnatal handling may attenuate some of the non-cognitive performance deficits in the water maze. Further, the manifestation and time course of behavioral yet not neuropathological symptoms in betaAPP mutant mice resemble in some aspects those of the human Down's syndrome.

Deficits in Memory Tasks of Mice with CREB Mutations Depend on Gene Dosage

Studies in Aplysia, Drosophila, and mice have shown that the transcription factor CREB is involved in formation and retention of long-term memory. To analyze the impact of differential CREB levels on learning and memory, we varied the gene dosage of CREB in two strains of mutant mice: (1) CREBαΔ mice, in which the α and Δ isoforms are disrupted, but a third isoform β is strongly up-regulated; (2) CREBcomp, a compound strain with one αΔ allele and one CREBnull allele in which all CREB isoforms are disrupted. To minimize genetic background effects, CREB mutations were backcrossed into a C57BL/6 and a FVB/N strain, respectively, and studies were performed in F1 hybrids from these lines. CREBcomp but not CREBαΔ F1 hybrids were impaired in water maze learning and fear conditioning, demonstrating a CREB gene dosage effect. However, analysis of the platform searching strategies in the water maze task suggested that CREBcomp mutants are impaired in behavioral flexibility rather than in spatial memory. In contrast to previous experiments using CREBαΔ mice with different genetic background, the F1 hybrid CREBαΔ and CREBcomp mice did not show deficits in a social transmission of food preference task nor in dentate gyrus and CA1 LTP as recorded from slice preparations. These data indicate that the hybrid vigor typical for F1 hybrids may compensate for a reduction in CREB levels in some tests. On the other hand, the persistence of clear behavioral deficits as shown by the F1hybrid CREBcomp mice in water maze and fear conditioning indicates a robust and repeatable phenomenon that will permit further functional analysis of CREB.

Spatial Memory and Learning in Transgenic Mice: Fact or Artifact?

Spatial learning of transgenic mice is often assessed in the Morris watermaze, where mice must use distant cues to locate a submerged platform. Such learning is confounded by species-specific noncognitive swimming strategies. Factor analysis permits cognitive and noncognitive strategies to be disentangled and their association with electrophysiological phenomena to be investigated.

Genetically modified mice and cognition

Cognition in transgenic and knockout mice is preferentially assessed by spatial learning in the Morris water maze. Awareness is growing, however, that the putative cognitive deficits observed using such a paradigm may be biased by the genetic background and behavioral peculiarities of the specific animals used. Recent progress in cognitive research includes new behavioral tests and refined analysis of performance impairments. Advances in our understanding of memory and learning are being made possible through use of transgenic rescue of disrupted genes, inducible and reversible gene targeting in selected brain regions, and single-cell recordings of hippocampal place cells in mutant mice.

Expression of the axon growth-related neural adhesion molecule TAG-1/axonin-1 in the adult mouse brain

TAG-1/axonin-1 is a neuronal cell adhesion molecule of the immunoglobulin superfamily. It is predominantly expressed during neural development and has been reported to be involved in axonal growth and pathfinding. Here, the expression of TAG-1/axonin-1 was investigated anatomically in the adult mouse brain by in situ hybridization using digoxigenin-labeled cRNA probes. Low levels of TAG-1/axonin-1 could be detected in cerebellar granule cells, in tufted and mitral cells of the olfactory bulb, and in pyramidal cells of area CA1 and CA3 of the hippocampus. We suspect that the expression of TAG-1/axonin-1 in these structures of the adult brain may serve neural plasticity.

Increased flexibility and selectivity in spatial learning of transgenic mice ectopically expressing the neural cell adhesion molecule L1 in astrocytes

The expression of the neural cell adhesion molecule L1 is altered by neuronal activity and promotes neurite outgrowth in vitro. To study the effects of L1 on learning and synaptic plasticity, transgenic mice have been created which express L1 ectopically in glial fibrillary acidic protein (GFAP) expressing astrocytes. Ninety mice, including GFAP-L1-transgenic mice from two genetic backgrounds and their littermates, were tested for swimming navigation learning in the Morris water maze according to a standardized protocol. While learning the position of an invisible target platform and also relearning its position after relocation, GFAP-L1-transgenic mice spent a greater fraction of their swim time in the target quadrant. Moreover, they showed a more rapid improvement of escape performance during the first day of training. Factor analysis revealed that this difference in swimming pattern could not be explained by non-cognitive factors. Factor analysis also revealed that, during a probe trial, the GFAP-L1-transgenic mice spent comparatively less time in the old target quadrant than predicted by the increased searching they had shown during acquisition learning. Hence, ectopic expression of L1 by astrocytes in mice appears to be linked to a factor which increases behavioural flexibility and selectivity while learning and relearning, but concomitantly may lead to a relative reduction of spatial retention.

Defective limbic system in mice lacking the tailless gene

The gene tailless is a member of the superfamily of genes that encode transcription factors of the ligand-activated nuclear receptor type, and is expressed in the invertebrate and vertebrate brain. In mice, its transcripts are restricted to the periventricular zone of the forebrain, the site of origin of neurons and glia. Here we use homologous recombination to generate mice that lack a functional tailless protein. Homozygous mutant mice are viable at birth, indicating that tailless is not required for prenatal survival; however, adult mutant mice show a reduction in the size of rhinencephalic and limbic structures, including the olfactory, infrarhinal and entorhinal cortex, amygdala and dentate gyrus. Both male and female mice are more aggressive than usual and females lack normal maternal instincts. These animals therefore enable a molecular approach to be taken towards understanding the genetic architecture and morphogenesis of the forebrain.

Expression of neuroserpin, an inhibitor of tissue plasminogen activator, in the developing and adult nervous system of the mouse

Neuroserpin is a serine protease inhibitor of the serpin family that has been identified as an axonally secreted glycoprotein in neuronal cultures of chicken dorsal root ganglia. To obtain an indication for possible functions of neuroserpin, we analyzed its expression in the developing and the adult CNS of the mouse. In the adult CNS, neuroserpin was most strongly expressed in the neocortex, the hippocampal formation, the olfactory bulb, and the amygdala. In contrast, most thalamic nuclei, the caudate putamen, and the cerebellar granule cells were devoid of neuroserpin mRNA. During embryonic development, neuroserpin mRNA was not detectable in neuroepithelia, but it was expressed in the differentiating fields of most CNS regions concurrent with their appearance. In the cerebellum, the granule cells and a subgroup of Purkinje cells were neuroserpin-positive during postnatal development. As a further step toward the elucidation of neuroserpin function, we performed a study to identify potential target proteases. In vitro, neuroserpin formed SDS-stable complexes and inhibited the amidolytic activity of tissue plasminogen activator, urokinase, and plasmin. In contrast, no complex formation with or inhibition of thrombin was found. Expression pattern and inhibitory specificity implicate neuroserpin as a candidate regulator of plasminogen activators, which have been suggested to participate in the modulation or reorganization of synaptic connections in the adult. During development, neuroserpin may attenuate extracellular proteolysis related to processes such as neuronal migration, axogenesis, or the formation of mature synaptic connections.

A role for the Ras signalling pathway in synaptic transmission and long-term memory

Members of the Ras subfamily of small guanine-nucleotide-binding proteins are essential for controlling normal and malignant cell proliferation as well as cell differentiation. The neuronal-specific guanine-nucleotide-exchange factor, Ras-GRF/CDC25Mm, induces Ras signalling in response to Ca2+ influx and activation of G-protein-coupled receptors in vitro, suggesting that it plays a role in neurotransmission and plasticity in vivo. Here we report that mice lacking Ras-GRF are impaired in the process of memory consolidation, as revealed by emotional conditioning tasks that require the function of the amygdala; learning and short-term memory are intact. Electrophysiological measurements in the basolateral amygdala reveal that long-term plasticity is abnormal in mutant mice. In contrast, Ras-GRF mutants do not reveal major deficits in spatial learning tasks such as the Morris water maze, a test that requires hippocampal function. Consistent with apparently normal hippocampal functions, Ras-GRF mutants show normal NMDA (N-methyl-D-aspartate) receptor-dependent long-term potentiation in this structure. These results implicate Ras-GRF signalling via the Ras/MAP kinase pathway in synaptic events leading to formation of long-term memories.

Assessing the effects of the 129/Sv genetic background on swimming navigation learning in transgenic mutants: a study using mice with a modified beta-amyloid precursor protein gene

The Morris water maze is frequently used to screen mutant mice generated by gene targeting. Targeted ES-cells are often derived from 129/Sv or BALB/c mice, known as poor swimming navigation learners. After mating the founders with C57BL/6 mice, the F2 or F3 hybrid generation is typically used for behavioral testing. In hybrid 129/Sv x C57BL/6 mice, a modification of the betaAPP gene entails impaired swimming navigation learning. This is readily detected despite behavioral variability, because wild-type 129/Sv x C57BL/6 hybrids outperform either of the parental strains and provide a control sample with good baseline performance. However, after backcrossing to the 129/Sv(ev) strain, the mutation effects are no longer detectable, masked by the very poor performance of wild-type 129/Sv(ev) mice. We conclude that F2 and F3 generations of 129/Sv x C57BL/6 crosses provide a suitable genetic background for behavioral testing of transgenic mice, provided that the samples are large enough to compensate for genetic and epigenetic variability and provided that normal performance in the control group is verified by comparison against a large database of mice tested under identical conditions. Creating congenic lines by backcrossing to an inbred strain is unlikely to enhance the sensitivity of the Morris test. Backcrossing to 129/Sv(ev) may even reduce it.

Neurotrypsin, a novel multidomain serine protease expressed in the nervous system

We have cloned a novel murine cDNA encoding a multidomain serine protease, termed neurotrypsin, which exhibits an unprecedented domain composition. The deduced amino acid sequence defines a mosaic protein of 761 amino acids consisting of a kringle domain, followed by three scavenger receptor cysteine-rich repeats, and a serine protease domain. Based on comparisons of the primary structure, the protease domain belongs to the subfamily of trypsin-like serine proteases. In situ hybridization revealed that the expression of neurotrypsin in the adult murine nervous system is confined to distinct subsets of neurons. The most prominent expression was found in the cerebral cortex, the hippocampus, and the amygdala. Le., structures engaged in the processing and storage of learned behaviors and memories. Together with the recently obtained evidence that extracellular serine proteases play a role in neural plasticity, this expression pattern suggests that the extracellular proteolytic action of neurotrypsin subserves structural reorganizations associated with learning and memory operations.

Anatomy of rat semaphorin III/collapsin-1 mRNA expression and relationship to developing nerve tracts during neuroembryogenesis

Semaphorin III/collapsin-1 (semaIII/coll-1) is a chemorepellent that exhibits a repulsive effect on growth cones of dorsal root ganglion neurons. To identify structures that express semaIII/coll-1 in developing mammals, we cloned the rat homologue and performed in situ hybridization on embryonic, neonatal, and adult rats. The relationship between semaIII/coll-1 mRNA distribution and developing nerve tracts was studied by combining in situ hybridization with immunohistochemistry for markers of growing nerve fibers. At embryonic day 11, semaIII/coll-1 expression was restricted to the olfactory pit, the basal and rostral surface of the telencephalic vesicle, the anlage of the eye, the epithelium of Rathke's pouch, and the somites. At later developmental stages, semaIII/coll-1 mRNA was found to be widely distributed in neuronal as well as in mesenchymal and epithelial structures outside the nervous system. Strong expression was found in the olfactory bulb, retina, lens, piriform cortex, amygdalostriatal area, pons, cerebellar anlage, motor nuclei of cranial nerves, and ventral spinal cord. After birth, mesenchymal staining decreased rapidly and expression became progressively restricted to specific sets of neurons in the central nervous system (CNS). In the mature CNS, semaIII/coll-1 mRNA remains detectable in mitral cells, neurons of the accessory bulb and cerebral cortex, cerebellar Purkinje cells, as well as a subset of cranial and spinal motoneurons. The temporal and spatial expression pattern of semaIII/coll-1 mRNA and its relationship to emerging nerve tracts suggests that semaIII/coll-1 is involved in guiding growing axons towards their targets by forming a molecular boundary that instructs axons to engage in the formation of specific nerve tracts.

Mice lacking the gene encoding tissue-type plasminogen activator show a selective interference with late-phase long-term potentiation in both Schaffer collateral and mossy fiber pathways

The gene encoding tissue-type plasminogen activator (t-PA) is an immediate response gene, downstream from CREB-1 and other constitutively expressed transcription factors, which is induced in the hippocampus during the late phase of long-term potentiation (L-LTP). Mice in which the t-PA gene has been ablated (t-PA-/-) showed no gross anatomical, electrophysiological, sensory, or motor abnormalities but manifest a selective reduction in L-LTP in hippocampal slices in both the Schaffer collateral-CA1 and mossy fiber-CA3 pathways. t-PA-/- mice also exhibit reduced potentiation by cAMP analogs and D1/D5 agonists. By contrast, hippocampal-dependent learning and memory were not affected in these mice, whereas performance was impaired on two-way active avoidance, a striatum-dependent task. These results provide genetic evidence that t-PA is a downstream effector gene important for L-LTP and show that modest impairment of L-LTP in CA1 and CA3 does not result in hippocampus-dependent behavioral phenotypes.

Paw preference and intra-/infrapyramidal mossy fibers in the hippocampus of the mouse

The size of the intra-/infrapyramidal mossy fiber projections (IIP-MF) and their left/right asymmetry were assessed in 86 mice of either sex, including 26 animals from two mouse lines bred for strong or weak paw preference, 38 mice of a randomly bred F3 generation derived from an eight-way cross, and 22 mice with variably sized corpora callosa in which only the left hippocampus was measured. Prior to morphometry, all mice were tested for paw preference. In addition, we compared the strain means in paw preference as observed in nine inbred mouse strains with known differences in their IIP-MF distribution. Mice bred for strong paw preference had a 70% larger IIP-MF projection than weakly lateralized and dyscallosal mice; random-bred mice fell in-between the extremes. The individual scores of the strength of paw preference were positively correlated with the extent of the IIP-MF. Among the inbred strains, the extent of the IIP-MF was similarly correlated with the strength of paw preference. The acallosal mice showed a significant negative correlation between extent of the IIP-MF projection and test-retest reliability of paw use. The left-right asymmetry of the IIP-MF was significantly and positively correlated with the direction of paw preference in the entire sample. We conclude that size and asymmetry of the IIP-MF projection are some of the many factors influencing the direction of paw preference and its strength, albeit moderately. We hypothesize that mice with larger IIP-MF projections use a given paw more consistently, being perhaps more resistant to interferences, and that left-right asymmetries of the IIP-MF may bias and/or reinforce an initial choice of a paw. In addition, the data provide another example of correlations between IIP-MF variations and nonspatial behavior.

Selective breeding for extremes in open-field activity of mice entails a differentiation of hippocampal mossy fibers

The brains of mice selectively bred for differential locomotor activity in an open field (DeFries et al., Behav. Genet. 8:3-13, 1978) were analyzed for selection-dependent changes in the size of synaptic fields at the midseptotemporal level for the hippocampus. Timm-stained areas of all hippocampal fields from both left and right hippocampi were measured on five horizontal sections from the midseptotemporal level. The sample included 25 mice from two replicate lines, each one consisting of a high (HI); a low (LO), and a control line (CTL). The main selection effect was an enlargement of the intra-infrapyramidal mossy fiber (IIP-MF) projection in both HI lines by about 70% compared to LO and CTL mice (p < .0001), while other mossy fiber fields did not show differences. These findings confirm that genetic variations of the IIP-MF projection influence hippocampal processes mediating exploratory activities.

Mice homozygous for a modified beta-amyloid precursor protein (beta APP) gene show impaired behavior and high incidence of agenesis of the corpus callosum

The amyloid precursor protein (beta APP) gene of the mouse was disrupted by homologous recombination; however, contrary to expectation, brain and other tissues still contained beta APP-specific RNA, albeit at a level 5-10 fold lower than wild-type and lacking the disrupted exon, which had been spliced out. The brain contained shortened beta APP-specific protein at a low level. Mutant mice were severely impaired in spatial learning and exploratory behavior and showed increased incidence of agenesis of the corpus callosum.

The gene of chicken axonin-1. Complete structure and analysis of the promoter

We have isolated and characterised the gene encoding the chicken axonal cell adhesion molecule axonin-1. This gene comprises 23 exons distributed over approximately 40 kb. Each of the six immunoglobulin-like domains and the four fibronectin-type-III-like domains of axonin-1 is encoded by two exons. The introns between two domains are exclusively phase I. Their exon/intron borders correspond to the domain borders of the protein, suggesting that the gene of axonin-1 had been generated by exon shuffling. Three transcripts with a length of 4.3 kb, 5 kb, and 8 kb are found, and we provide evidence that they result from alternative use of polyadenylation signals. In situ hybridization revealed co-localisation of these transcripts in time and space in the developing chicken retina. Several identical transcription initiation sites were found in retina, brain, and cerebellum by RNase protection assay and anchored polymerase chain reaction. By transfection of HeLa cells, rat PC-12 phaeochromocytoma cells, and chicken embryonic fibroblasts with serially truncated segments of the 5'-flanking region linked to a luciferase reporter gene, we have found that the sequence from -91 to +56 relative to the transcription initiation site is sufficient to promote efficient gene expression. Tissue-specific expression of the axonin-1 gene seems to be regulated in part by sequences more than 1 kb upstream of the transcription initiation site. As revealed by computer analysis, the sequence immediately upstream of exon 1 contains an AP-2 binding site, a tumor phorbol-ester-responsive element, and a homeodomain protein binding site, but no canonical TATA box. A second AP-2 binding site and a homeodomain protein binding site are located within exon 1.

Evidence for physiological growth of hippocampal mossy fiber collaterals in the guinea pig during puberty and adulthood

By means of Timm's procedure and computer-assisted morphometry, the left and right hippocampi of 69 hybrid guinea pigs from nine age levels (P5, P10, P20, P40, P80, P160, P320, and P610, and P1100) were analyzed for postnatal growth of recurrent hippocampal mossy fiber collaterals (RMFC) terminating below, within, and above the dentate granule cell layer. Postnatal growth of RMFCs showed, in both sexes, a first peak at P40, with stainable mossy fiber boutons covering the cell bodies of large neurones, some of which were reminiscent of basket cells. No significant changes of the density of mossy fiber collaterals were noticed from P40 to P160. At P320 a remarkable expansion of RMFCs was noted in a few animals, and by P610 all animals showed highly proliferated RMFCs which densely covered cell bodies and dendrites of target cells. The oldest group (P1100) showed an equal or slightly lowered density of RMFCs. We conclude that the growth of recurrent mossy fiber collaterals occurs in two spurts. The first completes just before sexual maturity. The second spurt occurs in the mid-life period, between P160 and P610.

Behavioral and anatomical deficits in mice homozygous for a modified beta-amyloid precursor protein gene

The beta-amyloid precursor protein (beta APP) gene of the mouse was disrupted by inserting into exon 2 a cassette containing a neomycin resistance gene and a putative transcription termination sequence. Contrary to expectation, brain and other tissues from mice homozygous for the insertion still contained beta APP-specific RNA, albeit at a level 5- to 10-fold lower than wild type and lacking the disrupted exon, which had been spliced out. The brain contained shortened beta APP-specific protein at a low level. Mutant mice were severely impaired in spatial learning and exploratory behavior and showed increased incidence of agenesis of the corpus callosum.

Distribution of TAG-1/axonin-1 in fibre tracts and migratory streams of the developing mouse nervous system

The axonal cell adhesion molecule, TAG-1/axonin-1, stimulates axonal growth and supports neurite fasciculation in vitro. Using a polyclonal antiserum raised against chick axonin-1, which shares 75% of its sequence with TAG-1 of the rat, we have mapped the distribution of TAG-1/axonin-1 throughout the developing nervous system of the mouse. Although absent from proliferating neuroepithelia and from non-neuronal cells, immunoreactivity for TAG-1/axonin-1 is expressed by stage-specific subpopulations of differentiating neurons from embryonic day 10 to postnatal day 15. It stains their axons and the surface of their parent somata during the early phases of axogenesis. In agreement with a putative role of TAG-1/axonin-1 as an axon-bound growth substrate, immunoreactivity is found in developing spinal and cranial nerves, in corticothalamic projections, as well as in subsets of fasciculating long projecting tracts of the central nervous system, such as the dorsal funiculi of the spinal cord, the lateral olfactory and optic tracts, the fasciculus retroflexus, and the predorsal bundle. High levels of immunoreactivity characterise the development of the cerebellar molecular layer, the corpus callosum, anterior and hippocampal commissure, and of crossed projections in the spinal cord and at several levels of the brainstem. Intense immunoreactivity in fine collaterals of cutaneous afferents, including their growth cones that are in contact with the embryonic skin, suggests a role of TAG-1/axonin-1 in target recognition. While staining is weak on the somata of radially migrating neurons such as cortical neurons and cerebellar granule cells, strong immunoreactivity is associated with neural somata and processes of the three tangential migrations that form the precerebellar nuclei, indicating a possible involvement of TAG-1/axonin-1 in contacts between these neurons and the processes they migrate upon.

Swimming navigation, open-field activity, and extrapolation behavior of two inbred mouse strains with Robertsonian translocation of chromosomes 8 and 17

Female mice from inbred strains carrying a Robertsonian translocation (nine CBARb and eight C57BL/6Rb) were compared with animals from their respective strains (seven CBA and nine C57BL/6) first in open-field activity (two exposures of 10-min duration), then during 5 days (with six trials each) in Morris' swimming navigation test, and finally, in their ability to extrapolate the future position of a food reward being moved slowly out of their reach. ANOVA (strain and translocation) revealed significant effects of Robertsonian translocations (Rb) in swimming navigation, Rb mice being impaired primarily in the initial phases of acquisition and during the first trials of platform reversal and the impairment being stronger in C57BL/6 mice. In the open field, Rb mice were as active as the normal strains but showed significantly increased path tortuosity and moved slightly faster. In the extrapolation task, Rb mice showed above-chance levels in moving to the target indicated by the disappearance of the stimulus, while normal mice chose at chance levels, but the translocation effects were not statistically significant. These data indicate that telocentric fusion of chromosomes may entail behavioral alterations, perhaps by subtle changes in neurotransmitters or limbic circuitry. The expression of such alterations, however, can be remarkably strain dependent.

Hippocampal mossy fibers and swimming navigation in mice: correlations with size and left-right asymmetries

Individual differences in the extent of the infrapyramidal mossy fiber projection (IIP-MF) correlate with performance in tasks sensitive to hippocampal lesions, notably two-way avoidance, radial maze learning, and swimming navigation. Previous studies of swimming navigation suggested that the capacity of reversal learning and measures of directionality might also be related to asymmetries in the distribution of the IIP-MF. In order to verify these findings, the authors crossed the Collins High- and Low-lateralized mice (known to differ in mossy fiber morphology and brain asymmetries) and obtained a F2-generation characterized by strong individual differences in these traits. Twenty-three (13 females, 10 males) mice were tested during 3 days for acquisition of swimming navigation (16 trials) toward a central platform, and during two days (12 trials) for their capacity of reversal learning toward a shifted platform. Morphometry of Timm-stained hippocampi revealed several, partially independent correlations: Larger IIP-MF projections were associated with prolonged crossing over the former platform position during the entire reversal learning; larger IIP-MF projections on the left were correlated with more precise crossing of the former platform position during the first 45 seconds of reversal learning; both extent and asymmetry of IIP-MF correlated positively with overnight improvement of reversal learning; the size of the entire mossy fiber projection (CA4, suprapyramidal and IIP-MF) correlated positively with the time spent in the platform quadrant and measures of initial orientation during acquisition of the task; and the mice showed an ipsilateral turning bias (spin) toward the side with the larger mossy fiber projection. The authors conclude that an intact hippocampus mediates differential processes underlying swimming navigation, and that left and right subfields may have differential functions.

CBP-18, a Ca(2+)-binding protein in rat brain: tissue distribution and localization

The distribution of a novel calcium-binding protein with a molecular mass of 18 kDa (CBP-18) in the rat brain was studied by means of biochemical methods and immunohistochemistry on cryostat-sectioned tissue and compared with staining patterns of parvalbumin on adjacent sections. The biochemical analysis revealed high levels of CPB-18 in cortex and cerebellum, low levels in the lungs, and undetectable levels in all other tissues tested. Immunohistochemically, the polyclonal rabbit-derived antibody for CPB-18 showed selective affinity with periglomerular cells and dendrites in the olfactory bulb. Distinct immunostaining of scattered cells and their proximal dendrites was found in the anterior olfactory nuclei and in the perirhinal and entorhinal cortex. Strong staining of neuropil with recognizable but diffusely outlined cells was observed in the retrosplenial cortex, central amygdala, hippocampal rudiment, septum, area preoptica, hypothalamus, colliculus superior, and parabrachial nuclei. The cerebellum showed strong neuropil staining of both the molecular and the granule cell layer. Less intense neuropil staining and a few scattered cells were found in the neocortex, the remaining basal forebrain, and in the entire brainstem. Immunoreactivity was barely detectable or missing in the striatum, the hippocampus, the thalamus, and in the colliculus inferior. Thus, CPB-18 shows a unique staining pattern in the CNS, different from all other Ca(2+)-binding proteins studied so far.

A new computer program for detailed off-line analysis of swimming navigation in the Morris water maze

The program TRACK-ANALYZER runs on AT-compatible microcomputers and performs off-line analysis of data recorded from Morris water-maze experiments by means of a video tracking system or digitizing tablet. Raw data must be available on disk as ASCII-files listing position coordinates sampled at a constant frequency. Automatic recognition and correction of artifacts and missing data is a key feature of the program, as well as the option to combine commands to user-defined macros. TRACK-ANALYZER offers maximal flexibility regarding experimental schedule, maze geometry and recording parameters and may also be used to analyze open-field activity. In addition to calculating basic parameters of the swim path, such as path length and time, the program counts crossings and hits of goal platform and four virtual reference annuli, calculates search times in five different maze fields, determines directionality, tortuosity and turning preferences of swimming behavior and allows the viewing of any number of trials simultaneously on screen. ASCII-formated data output may easily be exported to commercial statistics and graphics software.

Weak or missing paw lateralization in a mouse strain (I/LnJ) with congenital absence of the corpus callosum

Ward et al. (Brain Research 424 (1987) 84-88) have reported that reduced size of the corpus callosum (CC) was associated with a lower degree of paw preference in the mouse strain 129/J but not in the strain BALB/cCF. Both strains show individually different degrees of development of the CC but mice completely lacking CC occur rarely. The mouse strain I/LnJ shows complete agenesis of the CC. Thus, we have compared the degree of paw lateralization by means of a food reaching task in two samples of I/LnJ mice (n1 = 81, n2 = 93) with that of two common mouse strains which show a normal CC (C57BL/6JIbm, n = 44; DBA/2JZur, n = 48). The two samples of I/LnJ mice were tested in different laboratories. The first sample of I/LnJ mice had a mean age of 36 weeks. As compared to the control mice, the males but not the females showed a significantly reduced degree of paw preference. Both, callosal and acallosal mice showed a preference for left choices. The replication sample of I/LnJ mice contained only animals between 6 and 8 weeks old. All of them were ambilateral. There was no side preference and no gender difference. We conclude that congenital absence of the CC is a factor which may substantially interfere with the development of paw lateralization. However, depending on age and gender, about half of the acallosal mice develop a paw preference.

Swimming navigation and structural variations of the infrapyramidal mossy fibers in the hippocampus of the mouse

The extent of the infrapyramidal mossy fiber projection in CA3 (IIP-MF) at the midseptotemporal level correlates negatively with two-way avoidance learning and positively with performance in the radial maze, both tasks known to be sensitive to hippocampal lesions. If hippocampal structural variations are causing behavioral variations, one must predict positive correlations between the extent of the IIP-MF and performance in swimming navigation. Thus, the authors studied learning and reversal learning of swimming navigation in mice in which the size of the IIP-MF had been randomized by means of systematic crosses and in 2 mouse strains known for differential infrapyramidal projections (C57BL/6 and DBA/2). In 19 random-bred mice (9 male, 10 female), the extent of the IIP-MF showed negative correlations with swimming time after platform reversal (day 4: r = -0.50, P < .03; day 5 r = -0.73, P < .001), but none during acquisition of the task. In addition, statistical analysis suggested an influence of asymmetrically distributed mossy fiber projections during reversal learning. The strain comparison between 18 DBA/2 and 16 C57BL/6 male mice confirmed these results: no strain difference during days 1-3, and a significantly faster swimming time in the strain C57BL/6 (with large IIP-MF) at day 5 (second day of reversal), associated with significantly more crossings of the former platform location during the early phases of reversal learning. This latter measure was also negatively correlated with asymmetry of the IIP-MF in both strains. Finally, variations of the IIP-MF were correlated partially with adjustment of swimming speed that appeared to depend on size and asymmetry of CA4 as well. Thus, natural variations in the size of the IIP-MF distribution, and, perhaps, of CA4, appear to linearly influence processes directly involved in complex spatial learning.

Using genetically-defined rodent strains for the identification of hippocampal traits relevant for two-way avoidance behavior: a non-invasive approach

Genetically-defined rodent strains permit the identification of hippocampal traits which are of functional relevance for the performance of two-way avoidance behavior. This is exemplified here by analyzing the relationship between infrapyramidal mossy fibers (a tiny projection terminating upon the basal dendrites of hippocampal pyramidal neurons) and two-way avoidance learning in about 800 animals. The necessary steps include 1) identification of structural traits sensitive to selective breeding for extremes in two-way avoidance, 2) testing the robustness of the associations found by studying individual and genetical correlations between hippocampal traits and behavior, 3) establishing causal relationships by Mendelian crossing of strains with extreme structural traits and studying the behavioral consequences of such structural 'randomization', 4) confirming causal relationships by manipulating the structural variable in inbred (isogenic) strains, thereby eliminating the possibility of genetic linkage, and 5) ruling out the possibility of spurious associations by studying the correlations between the hippocampal trait and other behaviors known to depend on hippocampal functioning. In comparison with the classical lesion approach for identifying relationships between brain and behavior, the present procedure appears to be superior in two aspects: it is non-invasive, and it focuses automatically on those brain traits which are used by natural selection to shape behaviorally-defined animal populations, i.e., it reveals the natural regulators of behavior.