Herbert Schwegler

Radial-maze performance and structural variation of the hippocampus in mice: a correlation with mossy fibre distribution.

Twenty-four male mice, belonging to 8 different inbred strains, were tested in an 8-arm radial maze. Clear strain differences were found for performance on the third day of training, which correlated very strongly with the size of the hippocampal intra- and infrapy ramidal mossy fibre (iip-MF) terminal fields. These results, combined with those from earlier experiments, indicate that genetic variations of the iip-MF projection influence processes that determine behavioural abilities of mice.

Temporal dynamics of mouse hippocampal clock gene expression support memory processing

Hippocampal plasticity and mnemonic processing exhibit a striking time‐of‐day dependence and likely implicate a temporally structured replay of memory traces. Molecular mechanisms fulfilling the requirements of sensing time and capturing time‐related information are coded in dynamics of so‐called clock genes and their protein products, first discovered and described in the hypothalamic suprachiasmatic nucleus. Using real‐time PCR and immunohistochemical analyses, we show that in wildtype mice core clock components (mPer1/PER1, mPer2/PER2, mCry1/CRY1, mCry2/CRY2, mClock/CLOCK, mBmal1/BMAL1) are expressed in neurons of all subregions of the hippocampus in a time‐locked fashion over a 24‐h (diurnal) day/night cycle. Temporal profiling of these transcriptional regulators reveals distinct and parallel peaks, at times when memory traces are usually formed and/or consolidated. The coordinated rhythmic expression of hippocampal clock gene expression is greatly disordered in mice deficient for the clock gene mPer1, a key player implicated in both, maintenance and adaptative plasticity of circadian clocks. Moreover, Per1‐knockout animals are severely handicapped in a hippocampus‐dependent long‐term spatial learning paradigm. We propose that the dynamics of hippocampal clock gene expression imprint a temporal structure on memory processing and shape at the same time the efficacy of behavioral learning.

Anxiety-related behavior and densities of glutamate, GABAA, acetylcholine and serotonin receptors in the amygdala of seven inbred mouse strains

The amygdala is a brain region involved in the regulation of anxiety-related behavior. The purpose of this study was to correlate anxiety-related behavior of inbred mouse strains (BA//c, BALB/cJ, C3H/HeJ, C57BL/6J, CPB-K, DBA/2J, NMRI) to receptor binding in the amygdala. Binding site densities of receptors (NMDA, AMPA, kainate, GABA(A), serotonin, muscarinergic M(1)-M(2)) were measured with quantitative receptor autoradiography using tritiated ligands. Measurements of fear-sensitized acoustic startle response (ASR; induced by footshocks), elevated plus maze (EPM) behavior and receptor binding studies showed differences between the strains except for AMPA and muscarinergic M(2) receptors. Factor analysis revealed a Startle Factor with positive loadings of the density of serotonin and kainate receptors, and the amplitudes of the baseline and fear-sensitized ASRs. A second Anxiety-related Factor only correlated with the fear-sensitized ASR and anxiety parameters on the EPM but not receptor densities. There were also two General Activity Factors defined by (negative) correlations with entries to closed arms of the EPM. Because the density of NMDA and muscarinergic M(1) receptors also correlated negatively with the two factors, these receptors had a positive effect on general activity. In contrast, correlations of GABA(A), serotonin, and kainate receptors had the opposite sign as compared to closed arm entries. It is concluded that hereditary variations in the amygdala, particularly in kainate and serotonin receptors, play a role for the baseline and fear-sensitized ASR, whereas the general activity is influenced by many neurotransmitter receptor systems.

Contribution of amygdala neurons containing peptides and calcium-binding proteins to fear-potentiated startle and exploration-related anxiety in inbred Roman high- and low-avoidance rats

The purpose of this study was to investigate amygdala‐related fear and anxiety in two inbred rat lines differing in emotionality (RHA/Verh and RLA/Verh), and to relate the behaviour of the animals to neuronal types in different nuclei of the amygdala. The behavioural tests used were the motility test, elevated plus‐maze and fear‐potentiated startle response. The neurons investigated were immunoreactive for the anxiogenic peptide corticotropin‐releasing factor (CRF‐ir), the anxiolytic peptide neuropeptide Y (NPY‐ir), and the calcium‐binding proteins parvalbumin (PARV‐ir) and calbindin (CALB‐ir). The NPY‐ir, PARV‐ir and CALB‐ir neurons studied were subpopulations of GABAergic neurons. RLA/Verh rats, which showed a significant fear‐potentiation of the acoustic startle response, had more CRF‐ir projection neurons in the central nucleus of the amygdala. The same RLA/Verh rats were either less or equally anxious in the motility test (similar to open field) and elevated plus‐maze as compared with RHA/Verh rats. In accordance with this behaviour, the RLA/Verh rats had more NPY‐ir neurons in the lateral, and more PARV‐ir neurons in basal nuclei of the amygdala than RHA/Verh rats, but no differences were detected in the number of CRF‐ir and CALB‐ir neurons of the basolateral complex. In conclusion, the RLA/Verh rats displayed an opposite behaviour in the fear‐potentiated startle model and the exploratory tests measuring anxiety based on choice behaviour. Thus, the anxiogenic systems in the central nucleus and anxiolytic systems in the basolateral complex of the amygdala might be differentially involved in the fear‐potentiated startle paradigm and exploratory tests in the Roman rat lines.

Learning spatial orientation tasks in the radial-maze and structural variation in the hippocampus in inbred mice

In the present paper we review a series of experiments showing that heritable variations in the size of the hippocampal intra- and infrapyramidal mossy fiber (IIPMF) terminal fields correlate with performance in spatial, but not non-spatial radial-maze tasks. Experimental manipulation of the size of this projection by means of early postnatal hyperthyroidism produces the effects predicted from the correlations obtained with inbred mouse strains. Although the physiological mechanisms behind these correlations are unknown as yet, several lines of evidence indicate that these correlations are causal.

Water maze and radial maze learning and the density of binding sites of glutamate, GABA, and serotonin receptors in the hippocampus of inbred mouse strains.

Correlations between the densities of ionotropic glutamate, GABAA, and serotonin binding sites in the hippocampus of seven inbred mouse strains and strain‐specific learning capacities in two types of maze were studied. Binding site densities were measured with quantitative receptor autoradiography. Learning capacities were determined in a water maze task as well as in spatial and nonspatial versions of an eight‐arm radial maze. The densities of most binding sites differed significantly between the strains in the subfields of Ammons horn (CA1 and CA3) and the dentate gyrus, except for serotonin binding sites in CA1. By comparing the different strains, significant receptor–behavioral correlations between the densities of the GABAA receptors and the activity‐dependent behavior in the water maze as well as the spatial learning in the radial maze were found. The densities of D,L‐α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxalone propionate (AMPA) and kainate receptors correlated positively with learning capacity in the spatial eight‐arm radial maze. We conclude that hereditary variations mainly in AMPA, kainate, and GABAA receptor densities are involved in behavioral variations in spatial and nonspatial learning tasks. Hippocampus 10:213–225, 2000

Dissociation of spatial reference memory, spatial working memory, and hippocampal mossy fiber distribution in two rat strains differing in emotionality

Rats of the inbred strains DA/Han and BDE/Han were compared on two complex spatial learning tasks, a spatial reference memory task in a 16-unit multiple T-maze and a spatial working memory task in an eight-arm radial-maze. In addition, sizes of hippocampal mossy fiber terminal fields were measured. BDE rats showed marked superiority in multiple T-maze learning whereas DA rats outperformed BDE rats on the radial-maze task. DA rats had significantly larger intra- and infrapyramidal mossy fiber terminal fields (IIP-MF). This is consistent with findings from other studies suggesting that large IIP-MF are related to excellent spatial radial-maze learning, but it also indicates that size of IIP-MF is correlated with processing of a specific type of spatial information rather than with overall spatial abilities. BDE rats had more extended suprapyramidal mossy fiber projections (SP-MF) and a larger hilus. Rats of both strains differed in exploratory behaviour and emotionality: DA rats revealed little freezing and had a high rearing activity, whereas BDE rats showed frequent freezing and reared rarely. Results suggest that IIP-MF are involved with flexible expression of memory, updating environmental information and parallel processing whereas SP-MF might be linked to processing of familiar information. Presumably, emotional factors contribute to performance differences.

Modulation of mRNA expression of the neurotrophins of the nerve growth factor family and their receptors in the septum and hippocampus of rats after transient postnatal thyroxine treatment I. Expression of nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin 4 mRNA

Abstract Early postnatal application of thyroid hormones to rats results in morphological changes of the septo-hippocampal cholinergic and the hippocampal mossy fiber systems. Modulation in the expression of either neurotrophins and/or their receptors is postulated to be involved in these effects. In a recent study, we showed that, after thyroxine application, the mRNA expression of neurotrophins of the nerve-growth-factor (NGF) family is significantly upregulated both in septum and hippocampus. To test whether the neurotrophin receptors (the low-affinity neurotrophin receptor p75 and the specific high-affinity receptors trkA, trkB, and trkC) were also affected by hormone administration, newborn rats were treated daily with subcutaneous injections of thyroxine until postnatal day 12 (P12) at latest. Control animals received corresponding injections of saline. The pups were sacrificed at defined intervals from P9 to P14. The septal areas and the hippocampi were analyzed using the reverse-transcription polymerase chain reaction (RT-PCR) method for quantification of p75, trkA, trkB, and trkC mRNA levels. Analysis of variance over the total investigation period revealed no significant general increases of the gene expressions of either neurotrophin receptor, neither in the septum nor in the hippocampus, although previous results have shown marked changes in neurotrophin levels. On particular postnatal days, significant upregulation could be observed in hippocampus for trkB and trkC. From these and recent data, we conclude that modulation of neurotrophin expression rather than neurotrophin-receptor expression contributes to the morphological modifications within the hippocampal mossy fiber system and the septo-hippocampal cholinergic system.

A dual function of Bcl11b/Ctip2 in hippocampal neurogenesis

The development of the dentate gyrus is characterized by distinct phases establishing a durable stem‐cell pool required for postnatal and adult neurogenesis. Here, we report that Bcl11b/Ctip2, a zinc finger transcription factor expressed in postmitotic neurons, plays a critical role during postnatal development of the dentate gyrus. Forebrain‐specific ablation of Bcl11b uncovers dual phase‐specific functions of Bcl11b demonstrated by feedback control of the progenitor cell compartment as well as regulation of granule cell differentiation, leading to impaired spatial learning and memory in mutants. Surprisingly, we identified Desmoplakin as a direct transcriptional target of Bcl11b. Similarly to Bcl11b, postnatal neurogenesis and granule cell differentiation are impaired in Desmoplakin mutants. Re‐expression of Desmoplakin in Bcl11b mutants rescues impaired neurogenesis, suggesting Desmoplakin to be an essential downstream effector of Bcl11b in hippocampal development. Together, our data define an important novel regulatory pathway in hippocampal development, by linking transcriptional functions of Bcl11b to Desmoplakin, a molecule known to act on cell adhesion.

Genetic variation in the morphology of the septo-hippocampal cholinergic and GABAergic system in mice. I. Cholinergic and GABAergic markers.

In the present study, variations of cholinergic and GABAergic markers in the medial septum/vertical limb of the diagonal band of Broca (MS/vDB) and the hippocampus of eight different inbred mouse strains were investigated. By means of immunocytochemistry against the acetylcholine‐synthesizing enzyme choline acetyltransferase (ChAT), the cholinergic neurons were visualized and the number of ChAT‐positive neuronal profiles in the MS/vDB was counted. Cholinergic and GABAergic septo‐hippocampal projection neurons were detected with a combined retrograde tracing and immunocytochemical approach. In order to quantify the cholinergic innervation of various hippocampal subregions, we estimated the density of acetylcholinesterase (AChE)‐containing fibers as visualized by AChE histochemistry. Additionally, the densities of muscarinic receptors (mainly the subtypes M1 and M2) in different hippocampal areas of seven inbred strains were measured by means of quantitative receptor autoradiography.