Genetic variation in the histoarchitecture of the hippocampal region of mice

Genetic Background Influence on Hippocampal Synaptic Plasticity: Frequency-Dependent Variations between an Inbred and an Outbred Mice Strain

For almost half a century, acute hippocampal slice preparations have been widely used to investigate anti-amnesic (or promnesic) properties of drug candidates on long-term potentiation (LTP)-a cellular substrate that supports some forms of learning and memory. The large variety of transgenic mice models now available makes the choice of the genetic background when designing experiments crucially important. Furthermore, different behavioral phenotypes were reported between inbred and outbred strains. Notably, some differences in memory performance were emphasized. Despite this, investigations, unfortunately, did not explore electrophysiological properties. In this study, two stimulation paradigms were used to compare LTP in the hippocampal CA1 area of both inbred (C57BL/6) and outbred (NMRI) mice. High-frequency stimulation (HFS) revealed no strain difference, whereas theta-burst stimulation (TBS) resulted in significantly reduced LTP magnitude in NMRI mice. Additionally, we demonstrated that this reduced LTP magnitude (exhibited by NMRI mice) was due to lower responsiveness to theta-frequency during conditioning stimuli. In this paper, we discuss the anatomo-functional correlates that may explain such hippocampal synaptic plasticity divergence, although straightforward evidence is still lacking. Overall, our results support the prime importance of considering the animal model related to the intended electrophysiological experiments and the scientific issues to be addressed.

The dentate mossy fibers: structural organization, development and plasticity

Hippocampal mossy fibers are the axons of the dentate granule cells and project to hippocampal CA3 pyramidal cells and mossy cells of the dentate hilus (CA4) as well as a number of interneurons in the two areas. Besides their role in hippocampal function, studies of which are still evolving and taking interesting turns, the mossy fibers display a number of unique features with regard to axonal projections, terminal structures and synaptic contacts, development and variations among species and strains, as well as to normal occurring and lesion-induced plasticity and neural transplantation. These features are the topic of this review, which will use the mossy fiber system of the rat as basis and reference in its aim to provide an up-to-date, yet historically based guide to students in the field.

The entorhinal cortex of the mouse: organization of the projection to the hippocampal formation

The origin and the terminations of the projections from the entorhinal cortex to the hippocampal formation of the mouse (C57BL/6J strain) have been studied using anterogradely and retrogradely transported tracers. The entorhinal cortex is principally divided into two areas, the lateral entorhinal area (LEA) and the medial entorhinal area (MEA). LEA is the origin of the lateral perforant path that terminates in the outer one-third of the molecular layer of the dentate gyrus, and MEA is the origin of the medial perforant path that ends in the middle one-third of the molecular layer of the dentate gyrus. This projection is mostly to the ispsilateral dentate gyrus; only a few labeled axons and terminals are found in the contralateral dentate gyrus. The projection to the dentate gyrus originates predominantly from neurons in layer II of the entorhinal cortex. The entorhinal cortex also projects to CA3 and CA1 and to subiculum; in both CA3 and CA1, the terminals are present in stratum lacunosum-moleculare, whereas in the subiculum the terminals are in the outer part of the molecular layer. The projection from the entorhinal cortex to CA3, CA1, and subiculum is bilateral, and it originates predominantly from neurons in layer III, but a small number of neurons in the deeper layers of the entorhinal cortex contributes to this projection. The projection of entorhinal cortex to the hippocampus is topographically organized, neurons in the lateral part of both LEA and MEA project to the dorsal part (i.e., septal pole) of the hippocampus, whereas the projection to the ventral (i.e., temporal pole) hippocampus originates from neurons in medial parts of the entorhinal cortex.

Strain differences in urocortin expression in the Edinger-Westphal nucleus and its relation to alcohol-induced hypothermia

The Edinger-Westphal nucleus is the primary source of urocortin in rodent brain. Mapping of inducible transcription factors has shown that the Edinger-Westphal nucleus is preferentially sensitive to ethanol self-administration. In the present study we have immunohistochemically compared expression of urocortin and c-Fos in naive and ethanol-treated C57BL/6J and DBA/2J mouse inbred strains. We found that C57BL/6J mice possess significantly higher numbers of urocortin-expressing cells in the Edinger-Westphal compared to DBA/2J mice. Subsequent histological analysis confirmed a lower number of large neurons in the DBA/2J Edinger-Westphal nucleus. Surprisingly, despite the differences in structure, no strain differences were observed in the number of c-Fos-containing cells after acute (0.6-4.8 g/kg, i.p.) and repeated (2.4 g/kg, 14 days, one injection/day) administration of ethanol. Double-label immunohistochemistry showed that ethanol-induced c-Fos expression is present in different sets of Edinger-Westphal cells between the strains. Specifically, expression of c-Fos in C57BL/6J mice is preferentially induced in urocortin cells, while c-Fos in DBA/2J mice occurs in a mixed population of cells. Behavioral analysis of the B6D2 F2 intercross, a heterogeneous mouse strain, showed that the number of urocortin cells is positively correlated with basal temperatures and ethanol-induced hypothermia. Involvement of the Edinger-Westphal in alcohol-induced hypothermia is further confirmed by analysis of urocortin cells in the HOT/COLD selected lines. These results provide evidence that C57BL/6J and DBA/2J mice have structural differences in the Edinger-Westphal that can result in activation of different populations of neurons upon alcohol intoxication contributing to differential thermoregulation between these inbred strains.

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 sub-regions, 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. We found significant strain differences for the number of ChAT-positive neurons in the MS/vDB; in the numbers of cholinergic septo-hippocampal projection neurons; in the density of cholinergic fibers in hippocampal subfields CA3c, CA1, and in the dentate gyrus; and in the density of muscarinic receptors in the hippocampus. In contrast the GABAergic component of the septo-hippocampal projection did not differ between the strains investigated. The number of ChAT-reactive neurons in the MS/vDB was not correlated with either hippocampal cholinergic markers. This might be attributed to different collateralization of cholinergic neurons or to different projections of these neurons to other brain regions. These results show a strong hereditary variability within the septo-hippocampal cholinergic system in mice. In view of the role of the cholinergic system in learning and memory processes, strain differences in cholinergic markers might be helpful in explaining behavioral variation.

Correlations between radial-maze learning and structural variations of septum and hippocampus in rodents

Large, but non-pathological, individual differences in neuroanatomy of the brain exist in rodents, which have been shown to covary with behavioral traits. In the present review, we explore the relationship between variations in the extent of the intra- and infrapyramidal mossy fiber projection of the hippocampus and spatial and non-spatial learning capacities in mice and rats. Preliminary data concerning anatomical variation in the septo-hippocampal cholinergic system and its consequences for individual behavior are also presented. We conclude that the hippocampal intra- and infrapyramidal mossy fiber projection is intimately involved in the regulation of spatial, but not of non-spatial learning capabilities. Although lesion studies have shown that a well-functioning cholinergic system is a prerequisite for performance in spatial learning tasks, our preliminary data suggest that individual differences in the cholinergic system do not explain individual differences in learning.

Are the fine-structural characteristics of mouse hippocampal mossy fiber synapses determined by the density of mossy fiber axons?

Heritable variation of mossy fiber synapses in hippocampal region CA3 was studied in the two inbred mouse strains C3H and CPB-K. Previous Timm studies had shown a larger mossy fiber projection in C3H mice. In contrast, synaptic boutons of CPB-K mice were larger in size and perimeter and were contacted by more dendritic spines than in C3H mice. These results point to an inverse relationship between the size of the mossy fiber projection and the number of spine synapses formed by an individual mossy fiber bouton. Thus, the fiber density of a projection may be crucial for the actual morphology of the synaptic contacts formed.

Learning in inbred mice: strain-specific abilities across three radial maze problems

Mice belonging to the C57BL/6, DBA/2 (DBA), and C3H/He (C3H) strains were compared in three different eight-arm radial maze tasks requiring various degrees of spatial and nonspatial information processing. The results show that, on the standard radial maze task, C57 performed better than DBA, which, in turn, performed better than C3H. Fewer differences in the four-baited arm task and no difference in the cued version task were found between C57 and DBA, while C3H still performed more poorly. The high performance shown by C57 mice in all problems seems to be related to their ability to build up maze-running patterns based upon an optimal proportion of 45 degrees angle turns, according to the demand of the situation. The cognitive and discriminative mechanisms involved in the solving of each task, the sensorial characteristics of the three strains, and the limits of an approach based upon neuroanatomical-behavioral correlations are discussed.

Distribution of acetylcholinesterase in the hippocampal region of the mouse. III. The area dentata

The distribution of acetylcholinesterase (AChE) was examined in the area dentata of the adult mouse (Mus musculus domesticus). A distinctly stratified distribution of the enzyme was observed and was compared in detail with cytoarchitectural fields and layers. In the stratum moleculare, bands of relatively high AChE activity were seen immediately beneath the pia, at the borders between the outer, middle, and deep portions of the stratum moleculare, and superficial to the granule cell layer. AChE activity was low in the intervening parts of the stratum moleculare. In contrast to the rat, three sublaminae could be discerned in the hilus of the mouse at most septotemporal levels: a limiting subzone, a hilar plexiform layer, and a deep hilar cell mass. Deep to the granule cell layer, AChE activity was high in the limiting subzone and, septally, in the hilar plexiform layer. The deep hilar cell mass stained lightly towards the septal pole of the region but darker at more temporal levels. Numerous AChE-stained cells were seen in the hilus, with the exception of the most temporal levels. A comparative analysis of the AChE pattern of the area dentata reveals that 1) AChE-intense supra- and infragranular bands are found in all mammals, whereas 2) considerable difference between various strains of mice and between species are seen in the stratum moleculare. The functional significance of the AChE pattern is discussed in relation to species differences and connectivity and also with respect to possible activities of the enzyme other than hydrolysis of ACh, which may be involved in growth-related functions and in the plastic and degenerative processes observed in Alzheimer's disease.

Distribution of acetylcholinesterase in the hippocampal region of the mouse: II. Subiculum and hippocampus

The distribution of acetylcholinesterase (AChE) was examined in the subiculum and hippocampus of the adult mouse (Mus musculus domesticus). A distinctly stratified AChE pattern was observed in both areas and was compared in detail with cytoarchitectural fields and layers. In the subiculum, the lateral plexiform layer was lightly stained superficially and moderately stained at depth, where it abutted the lateral, moderately stained cell layer. Medially, a moderately stained deep plexiform layer separated the darkly stained superficial plexiform layer from the equally AChE-intense cell layer. At depth, the subicular cell layer was delimited by a band of very high AChE activity. In regio superior of the hippocampus, AChE-intense bands delimited the moderately stained strata moleculare, radiatum, and oriens toward the subjacent layers. In the stratum pyramidale, precipitate insinuated between the cell bodies gave a dark appearance to the deep part of the layer. The homologous strata of regio inferior appeared darker, but the relative staining intensities corresponded largely to those in regio superior. AChE activity in the layer of mossy fibers was almost absent septally but increased gradually to very high levels temporally. The AChE staining pattern, in conjunction with cytochemical and morphological evidence, strongly suggests a division of the pyramidal cell layer of the mouse and rat into superficial and deep substrata and discourages the definition of a prosubiculum in rodents. A comparative analysis of the AChE pattern reveals that: 1) in the subiculum, differences between species are observed within a generalized pattern of medial darkly staining and lateral lightly staining portions; 2) in the hippocampus, a conservation of the AChE pattern is seen in strata associated with intrinsic hippocampal connection; while 3) numerous interspecific differences are found in the stratum moleculare.

Distribution of acetylcholinesterase in the hippocampal region of the mouse: I. Entorhinal area, parasubiculum, retrosplenial area, and presubiculum

The distribution of acetylcholinesterase (AChE) was examined in the multilayered posterior part of the hippocampal region of the adult mouse (Mus musculus domesticus), namely, the entorhinal area, the parasubiculum, the presubiculum, and those parts of the retrosplenial cortex that extend into the posterior hippocampal region (area retrosplenialis 29d and 29e). A modification of the Koelle copper thiocholine method was employed for the histochemical demonstration of AChE. The AChE staining resulted in a distinctly stratified pattern, which has been compared in detail with the fields and layers defined by cyto- and fibro-architecture. Most of the enzyme activity was located in the neuropil, but both moderately and intensely stained nerve cell bodies were observed too. In the entorhinal area two main subfields were identified, which have been designated pars medialis and pars lateralis. In pars medialis, the superficial two thirds of layer I, the interstices between the stellate cell bodies in layer II, and layers IV and VI showed moderate to high content of AChE, whereas layer V and, especially, layer III were poor in enzyme activity. A particular feature was the occurrence of cone-shaped, darkly stained areas within layer II and, occasionally, the deep part of layer I. The staining of pars laterais differed in several respects from that of pars medialis, the most prominent feature being a less conspicuous stratification. In addition, intensely stained somata occurred more frequently than in pars medialis, although they still constituted only a very small minority of the total number of nerve cell bodies. In the parasubiculum, a clear cytoarchitectural subdivision into a posterolateral parasubiculum a and an anteromedial parasubiculum b was observed. These subfields showed, however, only minor differences in AChE staining. Thus, in both subfields, layers I and IV stained intensely, whereas layers II and III showed moderate to intense staining. Layers V and VI did not differ in appearance from the corresponding layers of the entorhinal area. The retrosplenial areas 29d and 29e appeared very light in the AChE pattern, area 29e being the better stained. The presubiculum was very rich in AChE, with layers, I, III and IV being particularly intensely stained. The small nerve cell bodies of layer II were unstained, whereas the intervening neuropil was intensely stained. The distribution of AChE in the mouse was compared with that in the rat, guinea pig, and rabbit, described previously. The staining pattern is largely similar in all four species, but striking species-specific differences do exist.(ABSTRACT TRUNCATED AT 400 WORDS)

Genetic control of hippocampal cholinergic and dynorphinergic mechanisms regulating novelty-induced exploratory behavior in house mice

Neurobehavioral genetics endeavors to trace the pathways from genetic and environmental determinants to neuroanatomical and neurophysiological systems and, thence, to behavior. Exploiting genetic variation as a tool, the behavioral sequelae of manipulating these neuronal systems by drugs and antisera are analyzed. Apart from research in rats, this paper deals mainly with the genetically-influenced regulation in mice of exploratory behaviors that are adaptive in novel surroundings and are hippocampally-mediated. Special attention is paid to neuropeptidergic, GABAergic, and cholinergic synaptic functions in the mouse hippocampus. The behaviorally different inbred mouse strains C57BL/6 and DBA/2 show opposite reactions (reductions and increases, respectively, in exploration rates) to peripheral and intrahippocampal injections with agents that interfere with peptidergic, cholinergic, and GABAergic neurotransmission. These findings can be explained by an interdependent over-release of opioids, arrested GABA release, and excess acetylcholine in the hippocampal neuronal network of DBA/2 mice, as compared to C57BL/6 mice where these systems are functionally well balanced. Very similar results have been obtained with the lines SRH and SRL, derived from C57BL/6 and DBA/2, and genetically selected for rearing behavior. Most probably, the opioids act to disinhibit exploratory responses. An additional genetic approach is mentioned, in which four inbred mouse strains and one derived heterogeneous stock are used for estimating genetic correlations between structural properties of the hippocampal mossy fibers and levels of hippocampal dynorphin B, on the one hand, and frequencies of exploratory responses to environmental novelty, on the other.

Comparative quantitative study of the hippocampal region of two closely related species of wild mice: interspecific and intraspecific variations in volumes of hippocampal components

To investigate the structural changes in the hippocampal region (subiculum, Ammon's horn, and area dentata) associated with speciation, the volumes of homologous components of this region of the forebrain were compared in the two closely related murine rodent species, Apodemus flavicollis (yellow-necked wood mice) and A. sylvaticus (long-tailed field mice), and in two geographically separated groups of A. sylvaticus. With the exception of the mossy fiber zone, no significant differences were found in the relative sizes of the components of Ammon's horn. Significant interspecific differences were found in the deep subiculum, in the intermediate (medial perforant path) and deep (commissural-associational) zones of the molecular layer of the area dentata, and in the hilus. A significant intraspecific difference was found in the combined intermediate and superficial (medial and lateral perforant path) zone of the dentate molecular layer. Differences found in the relative size of the hilus and the mossy fiber zone of Ammon's horn were complementary in that the combined volumes of these zones, which are both terminal fields of dentate granule cells, did not differ in the species. This finding provides evidence that the distribution of the terminal field of a projection system can be altered while the size of the terminal field is maintained. Within the hippocampal region, components of Ammon's horn appear well suited for quantitative comparative studies that span taxonomic units beyond the species level. In agreement with previous quantitative studies, differences in the volumes of components of Ammon's horn found in species of different orders are more likely to reflect phylogenetic trends rather than changes resulting from specializations of the particular species used. This is not true for the subiculum and the components of the area dentata. Experimentally induced changes in the connectivity of the hippocampal region are discussed in terms of the structural changes which may be responsible for the quantitative differences observed between the two species studied here.

Xenografting of fetal mouse hippocampal tissue to the brain of adult rats: effects of cyclosporin A treatment

This study examines the effect of the immunosuppressive drug Cyclosporin A (CyA) on the survival and differentiation of solid grafts of fetal (E16-17) mouse hippocampi transplanted to the brain of adult rats. The CyA was given as daily subcutaneous injections of 20 mg/kg from the day before transplantation with reduction of the dose to 15 mg/kg after 14 days. Five weeks after transplantation neuron containing xenografts were recovered in 11 out of 17 CyA-treated recipients (65%). After 8 weeks 9 out of 21 grafts were found (43%). In the control groups, treated only with the vehicle olive oil, 8 out of 14 xenografts were recovered after 5 weeks survival (36%) and only 3 out of 17 after 8 weeks (18%). All xenografts were infiltrated with mononuclear lymphocytic-like cells, but the infiltration was least extensive and least dense in the CyA treated animals. An observed correlation between this cellular infiltration and the gliosis in the xenografts suggested that CyA also directly or indirectly influenced the glial reaction. Most surviving xenografts were located next to the lateral ventricles or the choroid fissure. They were organotypically organized with identifyable cell and neuropil layers, and their connectional organization was similar to rat and mouse allografts grafted to adult recipients. In the absence of major extrinsic afferents the intrinsic pathways observed with Timm staining had reorganized according to known principles for aberrant growth and collateral sprouting. Ingrowth of extrinsic host afferents was only demonstrated for AChE positive host fibers. We conclude that CyA treatment of adult rat recipients can increase the survival of intracerebral fetal mouse hippocampal xenografts and reduce the histological signs of rejection. Xenografting combined with CyA treatment thereby permits the use of a wider spectrum of donor neurons for studies of neuronal interaction and repair.

Prenatal development of the hippocampus in two strains of inbred mice

In this paper we present some morphological observations on the morphogenesis of the hippocampus in two inbred strains of mice, C57BL/6J (C57) and DBA/2J (DBA), from day 16 (E16) to day 17 (E17) of prenatal life. At E16 some differences in the histoarchitecture of the hippocampal anlage begin to be detectable. At E17 the differences become more relevant, particularly those concerning the lamination pattern. The more relevant differences concern the differentiation of the pyramidal layer, which appears well defined in C57 mice, while in DBA mice there is no clearcut delimitation between the pyramidal layer and the intermediate zone, where numerous migrating neurons are present. These observations may be interpreted in terms of different migration rates for the neurons in the two strains.

Nerve connections between mouse and rat hippocampal brain tissue: ultrastructural observations after intracerebral xenografting

Fascia dentata tissue from embryonic mice was grafted to the hippocampal region of newborn Kyoto rats. After 1-7 months the recipients received lesions of the entorhinal cortex on the side of transplantation. Three days later their brains were processed for electron microscopy. The xenografts were identified by their content of mouse dentate granule cells which have smaller cell nuclei with more nucleoli than the corresponding host rat granule cells. Electron dense, degenerating host rat entorhinal fibers terminated in the outer parts of the mouse dentate molecular layer corresponding to the normal perforant path zones. They formed the normal type of synaptic contacts with dendritic spines. The findings demonstrate that precise synaptic contacts can be made across a species barrier.

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.

Localization of cholecystokinin in the dentate commissural-associational system of the mouse and rat

Cholecystokinin immunoreactivity (CI) was demonstrated in the inner one third of the molecular layer of area dentata in the rat and 3 strains of mice. This part of the molecular layer coincides with the terminal field of the commissural-associational system (C/A-system). In rats there is also CI in the middle third of this layer where the medial perforant path terminates, but there is no CI in this area in any of the 3 strains of mice. Different selective lesions except those involving the hilus, did not change this staining pattern. Electrolytic lesions of the hilus removed the CI in the terminal field of the C/A-system. Similarly, after kainic acid injections, the CI in the C/A-zone is reduced to a degree that is proportional to the amount of damage of the hilus. In the rat these lesions produced virtually no change in the CI of the medial perforant path terminal zone. The predominant type of cell within the hilus is the mossy cell and the cells of this type contain CI. The mossy cells have both commissural and ipsilateral projections that terminate in the inner one third of the molecular layer. Other cell types within the hilus contain CI, such as basket cells, oviform-like and fusiform-like cells. However, there was no evidence that any of these cells had commissural projections. Apart from the mossy cells there are other cells with commissural projections such as those in regio inferior. However, there was no evidence that any of them contained CI or also gave rise to an associational projection to the inner one third of the molecular layer. Moreover, there was a difference in the distribution of commissurally projecting cells that contained CI in the rat as compared to the mouse brain. This difference is the same as that occurring in the distribution of mossy cells where in the rat cells are found throughout the hilus, with most close to, but never inside the granule cell layer as compared to the mouse where virtually all are at a central part of the hilus. Thus these results are consistent with the hypothesis that all mossy cells contain CI and are the sole source of the C/A-system. After X-ray irradiation of the neonatal rat it is known that only part of the infrapyramidal blade of the granule cell layer is formed.(ABSTRACT TRUNCATED AT 400 WORDS)

The organization and development of the hippocampal mossy fiber system

The anatomical organization and development of the hippocampal mossy fiber system has been reviewed with special reference to its organization in the common laboratory rat. The mossy fibers originate from the granule cells of the dentate granular layer and the few granule cells found scattered in the dentate molecular layer and hilus. Via a complex system of collaterals the mossy fibers terminate on several types of neurons in the hilus, e.g. the basket cells and the mossy cells. Upon leaving the hilus to pass into Ammon's horn, the mossy fibers converge to form a distinct band of fibers that terminates on the proximal part of the apical and basal dendrites of the pyramidal and basket cells of the regio inferior. In some mammalian species the mossy fibers may continue into the adjacent part of the regio superior. Despite differences in the number of granule cells and pyramidal cells at different septotemporal levels this organization is relatively uniform along the septotemporal extent of the hippocampus. During development the mossy fibers grow out in a sequential manner that matches the pattern of neurogenesis and the aggregation of the cells of origin. From the level at which they originate, the fibers diverge along the septotemporal axis in such a way that the oldest granule cells have the most extensive projections. The adult topographic organization, which is already apparent at the earliest developmental stages, is thus formed in a stepwise fashion. It is concluded that the organization of the hippocampal mossy fibers indicates that neuronal specificity should not be explained by cellular recognition alone, but rather as the cumulated product of the preceding sequence of developmental events that include neurogenesis, migration, aggregation and directed axonal outgrowth.

Timm staining of hippocampal nerve cell bodies in the Kyoto rat. A cell marker in allo- and xenografting of rat and mouse brain tissue, revealing neuronal migration

Using the histochemical Timm sulphide silver method, a strain-specific, increased stainability of the cell bodies of the dentate granule cells and the hippocampal pyramidal cells was observed in the inbred Kyoto rat. The resulting dense staining which also includes neurons in the neocortex and cerebellum can be used as a cell marker in studies of intracerebral allo- and xenotransplantation of rat and mouse hippocampal tissue. Used in such experiments the marker revealed migration of dentate granule cells from Kyoto transplants into the Wistar host fascia dentata.

Selective attention and the brain: a hypothesis concerning the hippocampal--ventral striatal axis, the mediation of selective attention, and the pathogenesis of attentional disorders

The mechanisms mediating selective attention are not currently known. Dysfunctional selective attention is a common and prominent finding in a variety of medical and psychiatric conditions. A hypothesis is developed that efferents from the hippocampal formation are the final common pathway of processes which determine the noteworthiness of both exteroceptive and interoceptive stimuli, and that dysfunction of these efferents is a common pathway for a variety of anatomical, electrophysiological, and neurochemical lesions. This hypothesis suggests that clinical syndromes of disordered attention may be caused by various lesions of efferent connections from hippocampal formation to nucleus accumbens. The hypothesis further addresses the possibility that the threshold of hippocampus to various classes of stimuli may change on a diurnal and phasic basis. Experimental evidence that bears on the hypothesis is reviewed and experimental implications of the hypothesis are explored.

Basal forebrain innervation of rodent neocortex: studies using acetylcholinesterase histochemistry, Golgi and lesion strategies

Acetylcholinesterase (AChE)-rich projections from basal forebrain to neocortex cerebri were characterized in the present study. The purpose was to investigate 3 aspects of these projections in rats and mice that have been incompletely described in previous work: intracortical organization of the fibers, subcortical pathways and axonal branching patterns of individual basal forebrain neurons. AChE histochemistry, lesions and Golgi impregnations were the principal strategies employed in this light microscopic study. The moderately dense, AChE-stained innervation of neocortex can be altered by intracortical lesions. The results depended on the region involved and the orientation of the lesion. Sagittal knife cuts had barely detectable effects, regardless of sites. Coronal knife cut lesions in medial cortex resulted in substantial loss of staining in cingulate and medial occipital fields. In contrast, coronal lesions of lateral or anterior cortex produce only small zonal reductions in staining. The interpretation of the latter findings that we favor is that AChE-rich basal forebrain fibers enter lateral/anterior cortex and branch densely there, but in tangentially limited and overlapping terminal domains. Observations on the topography and targets of AChE-rich basal forebrain cortical afferents revealed that the fibers could be grouped based on certain characteristics. Three sets of fibers were distinguishable: anterior pathway innervating cortex of the frontal pole. These fibers were traceable to the region of the substantia innominata/nucleus basalis. They crossed the neostriatum and external capsule in the sagittal plane, forming in 3 dimensions an orderly sheet-like array of fibers bridging the anteroventral surface of the neostriatum with nearby polar cortex medial pathway innervating cingulate and medial occipital cortex. Emerging predominantly from the region of the diagonal band, the fibers run caudally as a triangular bundle in deep layer VI of cingulate cortex. lateral pathway innervating most of remaining lateral neocortex. The fibers radiate out from substantia innominata/nucleus basalis with a complex 3-dimensional organization. In all pathways, fibers enter and initially run within layer VI before ascending pialward, although the intracortical course in layer VI differs between pathways. These fibers primarily terminate in layer V with a secondary concentration in layer I. However, the latter appears to receive substantial AChE-stained inputs from other sources, possibly intracortical, as well. The pathways overlap at their respective boundary zones. This system is comparably organized in rats and mice.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal developments of AChE activity in the hippocampus of the reeler mutant mouse

The acetylcholinesterase (AChE) activity of the hippocampus and the dentate gyrus in the reeler mutant mouse was studied histochemically by the method of Karnovsky and Roots. Developmental studies were done on days 0,3,6,9,12,15,18 and 21 postnatally, and in adults. The adult reeler mouse differs from the normal mouse in that there is no accumulation of the activity at the junction between the stratum lacunosum-moleculare and the stratum radiatum, weaker and divided activity in CA1, and the translocation of the activity to the granule cell zone of the dentate gyrus. The results obtained are considered to be due to the cells' ectopia. But also, other factors such as genesis of the cells in regard to constructing the cytoarchitecture, may influence them. According to the postnatal observations, the developmental pattern of AChE activity in the reeler mouse is about the same as in the normal mouse. AChE-rich cells in the hippocampus showed maximum activity and number from days 12 to 15. The neuropile reactions increased after day 9 in both kinds of mice. The relationship between the movement of the AChE positive neuropiles and cells and their functions are also discussed.

Hippocampal mossy fibers in the regio superior of the European hedgehog

The hippocampal mossy fiber zone of the European hedgehog has been studied at the light and electron microscopic levels. In contrast to the organization of this zone in other mammals, the mossy fiber zone of the hedgehog is not confined to the region of Ammon's horn that is characterized by large pyramidal cells (i.e. regio inferior) but extends into the region that contains small pyramidal cells (i.e. regio superior). The terminals, throughout the relatively extensive mossy fiber zone of the hedgehog, have the large multivesicular, multicontact morphology that characterizes the mossy fiber terminals observed in other species and can be demonstrated to be the axon terminals of granule cells located in the fascia dentata. In view of the primitive organization of the forebrain of the European hedgehog, which is believed to be representative of that of the first mammals, the boundaries of the mossy fiber zone of these animals suggest that the mossy fibers extended over a larger part of Ammon's horn during the first phases of mammalian evolution and became restricted to the regio inferior during the early stages of mammalian evolution.

An allometric study of the area dentata in the rat and mouse

The volumes of the hilus, stratum granulosum, stratum moleculare and the zones of the stratum moleculare corresponding to the terminal fields of the perforant paths and the ipsilateral-commissural projections have been estimated in the area dentata of Wistar rats and DBA/2J mice, stained according to the Timm technique. The density of granule cells in the stratum granulosum and synaptic contacts in the stratum moleculare have been estimated from low power electron micrographs using stereological techniques. The volume and density parameters have been used to calculate the total number of granule cells and synaptic contacts in the fasciae dentatae of these two species. It is concluded that the number of synaptic contacts is proportional to the volume of the fascia dentata while the number of granule cells is proportional to the surface area of the fascia dentata and that the fasciae dentatae in these two species are isometric forms. The implications of these relationships are discussed with regard to the effect of the size of the fascia dentata on information processing in this structure. A significant difference existed between the ratio of the volumes of the hila and the ratio of the volumes of the fasciae dentatae in the two species studied. However, the volume of the deep zone of the stratum moleculare fasciae dentatae, the terminal zone of the hilar afferents was proportional to the volume of the hilus. The same density of synaptic contacts in the deep zone of the strata molecularia of both species, therefore, indicates a proportionality between the volume of the hilus and the number of synaptic contacts made by the hilar afferents. These observations are discussed with respect to the manner by which the maintenance of the observed proportionalities and the relative differences in the size and number of neurons in the subregions of the area dentata may be involved in the modification of the form and function of this region during phylogenesis.