Showing papers on "Dentate gyrus published in 1978"

An autoradiographic study of the organization of intrahippocampal association pathways in the rat.

Abstract: The longer associational connections of the hippocampal formation have been studied autoradiographically in a series of adult rats after small injections of 3H-amino acids into each of its various cytoarchitectonic fields. The major findings can be summarized as follows. The dentate gyrus projects in a topographically ordered manner upon the pyramidal cells of the regio inferior by way of the supra- and infrapyramidal bundles of mossy fibers. Certain cells in the hilar region of the dentate gyrus (which operationally may be defined as constituting field CA4 of Ammon's horn) give rise to a hippocampodentate projection to the inner one-quarter of the molecular layer of the dentate gyrus. Either the same or closely related cells give rise to fibers which join the Schaffer collateral system from field CA3 to the stratum radiatum and stratum oriens of the regio superior. The regio inferior is also characterized by a longitudinally directed associational bundle which runs throughout the septo-temporal extent of the hippocampus and is centered in the region of subfield CA3a. The regio superior has no reciprocal projection to the regio inferior but sends a substantial projection back to the subiculum and to the entorhinal area. There is also a projection to the subiculum from the regio inferior, and the subiculum itself probably contributes significantly to the projection to the entorhinal and perirhinal cortices. There is a striking parallelism between certain of these associational connections and the commissural projections to the hippocampus and dentate gyrus. Each cytoarchitectonic field that contributes a commissural projection also gives rise to an ipsilateral associational pathway which in its intrahippocampal course and its mode of termination exactly matches that of the commissural projection, although in general, the associational connections are more extensive in their distribution along the septo-temporal extent of the hippocampus than the corresponding commissural connections. The reverse is not true; there are a number of associational projections which are not paralleled by commissural projection. All of the associational projections are topographically arranged, but those which extend across the transverse axis of the hippocampus usually show considerable divergence so that afferents from different levels overlap fairly considerably within their respective projection fields.

The time of origin of neurons in Ammon's horn and the associated retrohippocampal fields.

Abstract: The time of origin of the neurons in the hilus of the dentate gyrus, in the regio superior and regio inferior of Ammon's horn, and in the following retrohippocampal fields — the subiculum, presubiculum, parasubiculum, medial and lateral entorhinal areas and the perirhinal cortex, has been determined in the rat, by the technique of 3H-thymidine autoradiography. In each field the cells are generated over a limited period of about five days, with the majority being formed in a 24–48 h period. As in the neocortex, the cells in the various hippocampal fields are generated in a distinct “inside-out” sequence with respect to the ependymal zone in which they arise. In addition there are two distinct gradients along the transverse, or dentato-rhinal axis, of the formation. Thus the neurons in the hilar region of the dentate gyrus (including field CA4) tend to arise earlier than the pyramidal cells in the regio inferior, and these in turn, are on average, generated earlier than those in the regio superior or in the subiculum. In the retrohippocampal region there is a comparable gradient extending medially from the perirhinal cortex to the presubiculum, the cells in the lateral entorhinal area being formed, on average, earlier than those in the medial entorhinal cortex, and these, in turn, are generated earlier than those in the para- or presubiculum. There is no evidence for a dorso-ventral (or septotemporal gradient) in any part of the hippocampal formation like that found in the dentate gyrus. Cell counts indicate that there are over 320,000 pyramidal cells in the regio superior (field CA1) and about 150,000 in the regio inferior (fields CA2 and CA3).

An autoradiographic study of the commissural and ipsilateral hippocampo-dentate projections in the adult rat.

Abstract: The distribution of the associational and commissural afferents to the inner one-fourth of the molecular layer of the dentate gyrus of the rat, has been studied autoradiographically following small injections of 3H-proline into the hilar region of the dentate (from which both groups of afferents arise). Different patterns of axonal labeling are observed after injections into the temporal (i.e., caudal), middle, or septal (rostral) thirds of the hippocampus. Thus after temporal injections labeled commissural and associational afferents are found only in the caudal third of the dentate gyrus, and the grain densities observed on the two sides are markedly asymmetrical around the short, or transverse, axis of the dentate. On the other hand, injections into the middle third of the hippocampus lead to extensive labeling of the commissural and associational afferents throughout the rostral two-thirds of the dentate gyrus, and their distribution, as judged by grain density estimates, is symmetrical on the two sides. Septal injections label fibers over the rostral half of the dentate, and again the labeling pattern of the two sides is asymmetrical (but in the reverse pattern from that seen after temporal injections). These distinctive patterns in the distribution of the two classes of afferents can generally be accounted for on the following assumptions: (1) the commissural and associational afferents share a common cytochemical specificity; (2) they compete with each other for the limited number of synaptic sites available upon the proximal portions of the granule cells; (3) the granule cells are generated along two distinct morphogenetic gradients: from the temporal to the septal pole of the dentate gyrus, and from the tip of its dorsal (or external) to the tip of its ventral (internal) blade; and (4) the first fibers to arrive monopolize the majority of the available synaptic sites, and those that reach their target field later, synapse predominantly upon the last-formed granule cell dendrites. To this extent our findings are consonant with the “temporal hypothesis” first formulated by Gottlieb and Cowan ('72). However, to account for the restricted distribution of afferents from the temporal part of the hippocampus, it is necessary to further postulate that there is some degree of topographic (or region-to-region) specificity in the ipsilateral and contralateral hippocampodentate projections.

Biochemical correlates of transmission mediated by glutamate and aspartate.

Abstract: — Glutamate and aspartate probably serve as transmitters of hippocampal perforant path and commissural afferents, respectively. We therefore used slices of hippocampal regions to evaluate certain biochemical properties as markers for sites of transmission mediated by these amino acids. In these studies content and accumulation of glutamate and aspartate were compared with their Ca2+-dependent effluxes. Hippocampal regions varied little in their contents of glutamate and aspartate, but slices of regio superior and dentate gyrus accumulated and released more of each than slices of regio inferior. A commissurotomy or bilateral entorhinal lesion altered Ca2+-dependent efflux and accumulation in the same direction, but did not affect the glutamate or aspartate content of any hippocampal region. Elimination of hippocampal mossy fibers reduced the Ca2+-dependent efflux of glutamate and probably aspartate from slices of dentate gyrus, but not of regio inferior, where most mossy fiber synapses are located. The mossy fibers appeared relatively deficient in aspartate in both strains tested, but only in Purdue-Wistar rats were they enriched in glutamate. Removal of the perforant path input to the fascia dentata did not significantly change the activity of any of the enzymes most actively involved in glutamate synthesis. These results suggest that accumulation or high affinity transport of glutamate or aspartate can be employed to localize afferents which use these amino acids as transmitters, although it is not so reliable or selective a marker as Ca2+-dependent efflux. Enrichment in either glutamate or aspartate content or in the activity of enzymes which synthesize them is not a reliable marker. Neither amino acid is likely to be used as a transmitter by the hippocampal mossy fibers.

Terminal proliferation and synaptogenesis following partial deafferentation: The reinnervation of the inner molecular layer of the dentate gyrus following removal of its commissural afferents

Abstract: The inner one-third of the dendritic region of the dentate gyrus granule cells in adult rats receives projections primarily from the commissural fibers of the contralateral hippocampus and the associational fibers of the ipsilateral hippocampus. At two to four days following the complete removal of the contralateral hippocampus, approximately 25% of the terminals in the inner molecular layer are observed degenerating. This provides an excellent model system to investigate possible terminal proliferation induced by deafferentation since (1) the experimental lesion is easily reproducible, (2) no retrograde reactions occur in the granule cells as a direct result of the lesion, (3) no shrinkage is detected in this region following commissural deafferentation, (4) the same dendritic region can be relocated precisely in each animal, and (5) the synaptic counts are highly consistent between animals. Results from this study and from previous investigations demonstrate that the commissural projection is contained within a 0–80 μ zone directly above the granule cell layer. Complete photomontages of this zone were taken, but only the 40–80 μ zone was quantified for neuronal and glial changes in three normal, five 2- to 4-day, and five 50- to 75-day postlesion animals. The average synaptic count dropped to 64% of control values by 2 to 4 days, but returned to 97% by 50- to 75 days postlesion. The number of terminals showing multiple synaptic contacts increased slightly in the long-term animals. Measurements of average terminal area showed no change between the short- and long-term survival groups. These results indicate that this dendritic region is reinnervated following partial deafferentation and that the reinnervation is due primarily to the formation of new terminals rather than the expansion of pre-existing terminals.

Hypertrophy of astroglial processes in the dentate gyrus of the senescent rat

Abstract: Quantitative electron microscopic analysis of the supragranular zone of the dentate gyrus molecular layer has shown that the number and volume fraction of profiles of astroglial processes are significantly increased in senescent rat relative to young adults. These ultrastructural modifications, which are not associated with significant age-related changes in the number of astrocytes or in the width of the molecular layer, may result from a formation of new astroglial processes and/or elongation of existing ones. In either case, the increase in the number and volume fraction of astroglial process profiles is an indicator of age-related astroglial hypertrophy. Hypertrophy of astroglial procecesses, which seems to develop with advanced age as a response to partial deafferentation of neurons, may compensate for a decrease in the dendritic volume fraction, thereby preventing changes in the dimensions of the dentate gyrus molecular layer in senescence.

Polysynaptic activation of the dentate gyrus of the hippocampal formation: An olfactory input via the lateral entorhinal cortex

Abstract: The possibility that olfactory input is transmitted to specific subregions of the hippocampal formation via the entorhinal cortex was investigated electrophysiologically by analyzing the laminar profiles of potentials evoked in the hippocampal formation by stimulation of the lateral olfactory tract (LOT). LOT stimulation resulted in long latency (14–20 ms) evoked responses in the dentate gyrus of the hippocampal formation ipsilateral to the stimulation. The variable long latency of these responses and their inability to follow stimulus rates of 40/s suggested that these potentials reflected polysynaptic activation. Analysis of the laminar profiles of the evoked potentials indicated that the responses originated from a synaptic field localized in the outer portion of the stratum moleculare of the dentate gyrus, a terminal distribution which overlaps that of the lateral entorhinal cortical (LEC) projection to the dentate gyrus. Lesions of the LEC eliminated the long latency responses in the dentate gyrus evoked by LOT stimulation. In addition, a conditioning pulse delivered either to the LOT or to the LEC produced paired pulse potentiation of the response elicited by subsequent stimulation of the other structure. No evidence was found to indicate that responses were generated in regio superior of the hippocampus proper following LOT stimulation. Taken together, these results suggest that stimulation of the LOT activates the dentate gyrus of the hippocampal formation by multisynaptic pathways which relay through the lateral portion of the entorhinal area. This finding is discussed with regard to entorhinal cortical organization and the known olfactory projections to the LEC.

Dendritic atrophy in the dentate gyrus of the senescent rat

Abstract: Quantitative electron microscopic analysis of the supragranular zone of the dentate gyrus molecular layer has shown that the number, volume fraction and surface area of dendritic shaft profiles are significantly decreased in senescent rats, relative to young adults. These modifications of dendritic morphology, which are not associated with age-related changes in dimensions of the molecular layer or in numbers of granule cells, may result from a decrease in the number and/or length of dendrites. In either case, the decreases in the number, volume fraction and surface area of dendritic shaft profiles found in the dentate gyrus of senescent rats signify an age-related atrophy of dendrites. Comparison of changes in the number and volume fraction of dendritic shaft profiles has demonstrated that age-related dendritic atrophy involves predominantly smaller dendritic branches.

Endogenous norepinephrine and serotonin within the hippocampal formation during the development and recovery from septal hyperreactivity.

Abstract: Fluorometric analysis of serotonin (5-HT) and norepinephrine (NE) content of the hippocampal formation revealed that biogenic amines are distributed heterogeneously in the dorsoventral axis, and that NE also exhibits a heterogeneous distribution in the medial-lateral direction while 5-HT does not. Dissection of the hippocampus into its dorsal and ventral halves shows that both NE and 5-HT exhibit higher concentrations in the ventral hippocampus in comparison to its dorsal counterpart. A dissection which separated the cell fields CA 1 and 2 from CA 3 and 4 and the dentate gyrus showed NE to be the highest in the latter region, while 5-HT was uniformly distributed between the two regions. Taken together, these data indicate that NE is more highly concentrated in the CA 3 and 4 and dentate area of the ventral hippocampus while 5-HT concentration differences are apparent only in a dorsal-ventral dissection. Concentrations of NE and 5-HT in the dorsal and ventral hippocampus were also determined at 1, 3, 6, 11, 16, 24, and 30 days following a lesion to the septal nuclei. The results demonstrate that biogenic amine levels in the dorsal hippocampus achieve maximal depletion earlier than do their ventral counterparts, and that percent depletion is greater for 5-HT than NE in both dorsal and ventral areas. On the first day following septal lesions, 5-HT is increased above normal levels. Sixteen days after septal lesion, 5-HT is substantially depleted below normal levels. In addition, by 30 days, 5-HT shows significant return toward normal levels from its earlier depleted state. Behavioral changes related to sensory reactivity correlate with the relative decreases of NE and 5-HT following septal lesions.

Ontogeny of Glucocorticoid Binding in Rodent Brain

Abstract: Neonatal treatment with corticosterone can differentially and persistently reduce either the basal level of plasma corticosterone or the amplitude of the adult diurnal rhythm in the rat depending on the age at which exposure to the steroid occurs. This alteration of basal secretion by hormonal manipulation during the first postnatal week may be related to the high levels of corticosterone found in pituitary cell nuclei following exposure of the immature rat to exogenous corticoid. The ontogenetic course of cytosol binding in the pituitary suggests a mechanism by which such vulnerability may occur. The hypothalamus was the only brain region found to have a constant level of cell nuclear binding throughout development, although it closely resembles the brain as a whole with regard to the development of cytosol binding sites. The significant postnatal neurogenesis of the hippocampus is reflected in a large postnatal rise in both cytosol and nuclear binding of corticosterone. This increase would appear to be due in part to a delay in binding competence by the pyramidal and granule cells of the hippocampus. The autoradiographic evidence indicates that in the immature rat the retention of steroid by hippocampal pyramidal cells correlates directly with the embryonic age of the neuron. Likewise, the oldest granule cells are the most heavily labelled granule cells in the dentate gyrus, while newly arrived cells do not concentrate corticosterone. This would suggest that an event in cellular differentiation occurs sometime after the cells are “in position,” which permits the binding of glucocorticoids by these hippocampal neurons.

Pyramid-like basket cells in the granular layer of the dentate gyrus in the rat

Abstract: Basket cells of the dentate gyrus were identified using Nissl (cresyl violet) staining. It has been found that the ratio between basket and granule cells is 1:150--210. Only a few glial cells, mainly astroglia, were found in the granular layer of the dentate gyrus. In accordance with earlier data it was found that the granule cells and glial cells originate mainly postnatally, but the basket cells, like the pyramidal cells of the hippocampus, originate prenatally.

Kinetics of cell proliferation in the postnatal rat dentate gyrus

Abstract: Cell cycle parameters in the dentate gyrus of the postnatal rat hippocampus were measured, using the method of percentage labelled mitoses. In animals aged 1, 6 and 12 days, cell cycle times range from 15.1 to 17.7 h, S phase duration from 10.1 to 11.7 h, length of the G2 phase from 2.5 to 3.3 h and G1 from 1.1 to 2.4 h. These figures are similar to those obtained for other precursor cell populations in the brain. On the day after birth the labelling index in the dentate hilus is 4.5% and estimated turnover time 220 h, while on day 6 the labelling index is 3.7% and estimated turnover time 310 h. It is calculated that precursor cells outnumber granule cells by at least 3:1 at birth and 2:1 by day 6.

Synapse Formation and Plasticity in the Developing Dentate Gyrus

Abstract: The dentate gyrus of the hippocampal formation may serve as a key area regulating cortical input to the limbic system. Cortical inputs converge at the entorhinal cortex which sends a massive excitatory projection to the granule cells of the dentate gyrus (Anderson, 1975). Pyramidal cells which receive a much sparser input rarely, if ever, fire. In the operation of hippocampal circuitry, cortical inputs drive granule cells which stimulate CA3 pyramidal cells which in turn diverge and project to other parts of the hippocampus, and ultimately to various limbic system nuclei. Inputs converge at the entorhinal cortex and diverge after the dentate gyrus. It is at the dentate gyrus where the pathway is unbranched, and where the first opportunity exists to regulate the flow of cortical information into the limbic system. In systems as diverse as complex engineering functions and metabolic pathways, the initial unbranched point in the system is usually the key regulatory point. Thus it might be that the transfer of information through the dentate gyrus is finely regulated.