GABAergic cells in the dentate gyrus appear to be local circuit and projection neurons.

Immunocytochemical results indicate that GAD-positive neurons are found in the molecular and granule cell layers of the dentate gyrus as well as in the hilar region. GAD-positive cells in the molecular and granule cell layers are identified as various types of local circuit neurons. Most of the GAD-positive puncta found throughout the molecular layer and within the granule cell layer are interpreted as axon terminals of these neurons, including five types of basket cells. This interpretation is based on data that indicate the axons of basket cells form synapses with the somata and proximal dendrites of granule cells. The results in the hilus show that 60% of the hilar neurons are GAD-positive. Since previous studies have indicated that 80% of hilar neurons give rise to both associational and commissural pathways, many GABAergic neurons in the hilus are probably projection neurons. This finding is consistent with recent physiological data which suggest that commissural pathway stimulation directly inhibits granule cells. Therefore, GABAergic cells in the dentate gyrus appear to be both projection and local circuit neurons

Postnatal development of the light and electron microscopic features of basket cells in the hippocampal dentate gyrus of the rat.

Light and electron microscopic preparations were used to analyze the postnatal development of the basket cells of the rat dentate gyrus. The basket cells, located at the hilar border, were recognized in 2-day-old rats in Golgi preparations, where they displayed immature dendrites and a small axon arbor in the granule cell layer. At 5 days, the basket cells were found to have a large perikaryal cytoplasm, a round nucleus, an axon that forms symmetric synapses with granule cells, and dendrites and somata that are contacted by other axon terminals. The 10-day basket cells display more mature features, such as Nissl bodies and well-developed Golgi complexes. The basket cells from 16-day-old rats are mature in terms of their ultrastructural features, in that the nuclei are highly indented and display intranuclear rods or sheets, the perikaryal cytoplasm is packed with organelles, and the axon has developed an extensive arborization with the somata and dendrites of granule cells at the border with the molecular layer. This arborization will continue to expand as more granule cells are generated and added to the hilar border. These data correlate well with the immunocytochemical and biochemical development of GABAergic neurons in the dentate gyrus. Furthermore, the maturation of the structure of basket cells appears to precede the appearance of adult-like electrical activity in the hippocampus

The development of GABAergic neurons in the rat hippocampal formation. An immunocytochemical study.

Recent studies have indicated that hippocampal GABAergic neurons in both the dentate gyrus and Ammon's horn are generated prenatally. Although the adult distribution of GABAergic neurons has been previously described by numerous investigators, the early postnatal appearance of these neurons has not been described. In the present study, immunocytochemical methods were used to localize GABAergic neurons with antisera to both GABA and its synthesizing enzyme, glutamate decarboxylase (GAD). The GABA-positive neurons appeared at the earliest postnatal day (PND) examined, 4 PND. In contrast, GAD-positive cells were not observed until 6 PND, and the number of these neurons remained less than that of the GABA-positive neurons until 14 PND. These findings indicated that immunocytochemically detectable amounts of GAD were not present in many young GABAergic neurons. Both GABA- and GAD-positive hippocampal neurons showed two large increases in number during the 4-8 PND and 12-16 PND time periods, and they reached about 90% of adult levels before 18 PND. The regional distribution of GABA- and GAD-positive neurons throughout the hippocampal formation was homogeneous for all ages examined except 4 PND. At this age, the GABA-positive cells appeared in clusters in the proximal CA3 and the distal CA1 relative to the dentate gyrus. In addition, the number of hippocampal neurons immunostained in adult preparations for both antisera to GABA and GAD showed a similar number and distribution. The data on the developmental appearance of GABA and GAD immunoreactivities are consistent with biochemical data for the development of GABA concentration and GAD activity in the hippocampal formation. Together, these data provide important information about the functional maturation of the hippocampal GABAergic system in the first 3 weeks of rat brain development

Direct commissural connections to the basket cells of the hippocampal dentate gyrus: anatomical evidence for feed-forward inhibition.

After lesions were placed in the hippocampal commissures, degenerating terminals could be localized above, inside and beneath the granule cell layer of the contralateral dentate gyrus. The terminals formed asymmetric synapses with spines, dendritic shafts and somata of granule cells. Degenerating terminals also formed synapses with dendrites and somata of basket cells identified by the Golgi-electron microscope technique. These basket cells were located either at the hilar border of the granule cell layer or in the molecular layer and each formed an axonal plexus around the somata and proximal dendrites of granule cells. These observations provide an anatomical basis for the recently described feed-forward inhibition in this brain region

Five types of basket cell in the hippocampal dentate gyrus: a combined Golgi and electron microscopic study.

Five types of basket cell in the hippocampal denate gyrus of rats were analysed with a combined Golgi and electron microscopic method. Light microscopic observations show that the large somata of these different cell types are located either in the granule cell layer or within 30-50 micron of this layer. The somata of basket cells are pyramidal, horizontal, fusiform or multipolar. Dendrites of basket cells are aspinous or sparsely spinous and are found in all layers of the dentate gyrus. Their axons form an extensive plexus in the granule cell and lower molecular layers. Electron microscopic preparations of Golgi-impregnated, gold-toned basket cells revealed gold-labelled neurons with distinct ultrastructural features. All somata of basket cells displayed an extensive perikaryal cytoplasm with large Nissl bodies and nuclei with infoldings, euchromatin, intranuclear rods and sheets, and large nucleoli. The aspinous dendrites as well as the somata had a mixture of asymmetric and symmetric synapses on their surfaces. Basket cell dendrites located in the hilus were contacted by numerous terminals with characteristics of mossy fibres derived from granule cells. Some of these terminals were identified positively in preparations that also contained impregnated granule cells. The axons of basket cells formed exclusively symmetric synapses. The most common postsynaptic structures to these terminals were the somata and dendrites of granule cells. Dendritic spines were rarely contacted by basket cell axons while the axon hillocks and initial segments of granule cells were never contacted. These findings are consistent with previous immunocytochemical, and physiological data that indicate feedback inhibitory mechanisms in the dentate gyrus are mediated via mossy fibre collaterals which synapse with GABAergic basket cells. In addition, the electron microscopic data for basket cells are similar to those for aspinous stellate cells in the neocortex, another type of cortical, GABAergic local circuit neuron. Thus, the basket cells in the dentate gyrus may have a function similar to other inhibitory, cortical local circuit neurons

The synaptic connections of basket cell axons in the developing rat hippocampal formation.

Recent studies have indicated that hippocampal basket cells in both the dentate gyrus and Ammon's horn develop their somal and dendritic features during the first two postnatal weeks in rats. Their axon terminals form exclusively symmetric synapses that are found as early as 5 postnatal days in both regions. The present study used Golgi-electron microscopic material from 10 and 16 day old rats to demonstrate that the axon terminals of basket cells form synapses not only with somata, dendrites, and dendritic spines as reported for adult material but also with axon initial segments. However, the terminals forming synapses with axon initial segments and dendritic spines represent only a minor portion of the total number of basket cell terminals. Quantitative results indicate that 36-62% of the total number of these terminals form axosomatic synapses and 32-50% form axodendritic synapses depending on the analyzed cell. These data indicate that hippocampal basket cells have an axonal distribution similar to that found for cortical basket cells

A substantial number of asymmetric axosomatic synapses is a characteristic of the granule cells of the hippocampal dentate gyrus.

The types of axosomatic synapses for pyramidal neurons of the Ammon's horn and granule cells of the dentate gyrus were investigated in the rat hippocampus. Local circuit neurons in both of these regions were also included. The mean number of axosomatic synapses was higher for pyramidal neurons than granule cells. Practically all of these synaptic contacts were symmetric for the pyramidal neurons of the CA1 area, whereas the percentage of asymmetric axosomatic synapses for granule cells and local circuit neurons was 21% and 26%, respectively. This finding is interesting because granule cells appear to be the only known projection neuron type to possess such a high percentage of asymmetric axosomatic synapses

Ultrastructural features of primate granule cell bodies show important differences from those of rats: axosomatic synapses, somatic spines and infolded nuclei.

Granule cells of the primate dentate gyrus were examined in the electron microscope where they displayed significantly less axosomatic synapses than granule cells in rodents. In addition, primate granule cells frequently had infolded cell nuclei and somal spines which are features that are both rare in rodents. Since the granule cell body is an important site for gamma-aminobutyric acid (GABA)ergic inhibitory control, the reduced number of axosomatic synapses in monkeys suggests that local inhibitory connections of primate granule cells are less than that of rodents. Together, these differences may indicate that the primate granule cells are physiologically more active than rat granule cells

Ultrastructure of commissural neurons of the hilar region in the hippocampal dentate gyrus.

Previous studies have described the polymorph neurons in the hilus of the dentate gyrus at the light microscopic level and have indicated that many of those neurons are the cells of origin for both ipsilateral associational and commissural projections to the dentate gyrus. Because previous studies have not described the ultrastructural characteristics of the hilar neurons, we identified these features of the commissural neurons in the hilus. The method of retrograde transport of horseradish peroxidase (HRP) was utilized with a silver staining technique for HRP intensification. Two populations of labeled commissural neurons were observed in electron microscopic preparations of the contralateral hilus. One type consisted of cells with somata that exhibited round or oval nuclei with no intranuclear inclusions and formed symmetric axosomatic synapses. The main dendrites of those neurons were thick and tapering. In contrast, the other type of labeled neuronal soma had infolded nuclei containing intranuclear rods or sheets, displayed both symmetric and asymmetric axosomatic synapses, and had dendrites that were less thick and generally aspinous. In those same preparations, labeled commissural axon terminals formed synapses with dendrites and dendritic spines in the hilus and molecular layer and with somata in the granule cell layer. From the results of this study it appears that there are two distinct populations of commissural hilar neurons: one type resembles the morphology of the spiny CA3 pyramidal neuron, a type of excitatory projection cell, and the other type is similar to the dentate gyrus basket cell, a local circuit neuron associated with GABAergic inhibition. This latter cell type provides further support for the notion that some commissural neurons are inhibitory