The CA3 "backprojection" to the dentate gyrus

Abstract

The hippocampus is typically described in the context of the trisynaptic circuit, a pathway that relays information from the perforant path to the dentate gyrus, dentate to area CA3, and CA3 to area CA1. Associated with this concept is the assumption that most hippocampal information processing occurs along the trisynaptic circuit. However, the entorhinal cortex may not be the only major extrinsic input to consider, and the trisynaptic circuit may not be the only way information is processed in hippocampus. Area CA3 receives input from a variety of sources, and may be as much of an "entry point" to hippocampus as the dentate gyrus. The axon of CA3 pyramidal cells targets diverse cell types, and has commissural projections, which together make it able to send information to much more of the hippocampus than granule cells. Therefore, CA3 pyramidal cells seem better designed to spread information through hippocampus than the granule cells. From this perspective, CA3 may be a point of entry that receives information which needs to be "broadcasted," whereas the dentate gyrus may be a point of entry that receives information with more selective needs for hippocampal processing. One aspect of the argument that CA3 pyramidal cells have a widespread projection is based on a part of its axonal arbor that has received relatively little attention, the collaterals that project in the opposite direction to the trisynaptic circuit, "back" to the dentate gyrus. The evidence for this "backprojection" to the dentate gyrus is strong, particularly in area CA3c, the region closest to the dentate gyrus, and in temporal hippocampus. The influence on granule cells is indirect, through hilar mossy cells and GABAergic neurons of the dentate gyrus, and appears to include direct projections in the case of CA3c pyramidal cells of ventral hippocampus. Physiological studies suggest that normally area CA3 does not have a robust excitatory influence on granule cells, but serves instead to inhibit it by activating dentate gyrus GABAergic neurons. Thus, GABAergic inhibition normally controls the backprojection to dentate granule cells, analogous to the way GABAergic inhibition appears to control the perforant path input to granule cells. From this perspective, the dentate gyrus has two robust glutamatergic inputs, entorhinal cortex and CA3, and two "gates," or inhibitory filters that reduce the efficacy of both inputs, keeping granule cells relatively quiescent. When GABAergic inhibition is reduced experimentally, or under pathological conditions, CA3 pyramidal cells activate granule cells reliably, and do so primarily by disynaptic excitation that is mediated by mossy cells. We suggest that the backprojection has important functions normally that are dynamically regulated by nonprincipal cells of the dentate gyrus. Slightly reduced GABAergic input would lead to increased polysynaptic associative processing between CA3 and the dentate gyrus. Under pathological conditions associated with loss of GABAergic interneurons, the backprojection may support reverberatory excitatory activity between CA3, mossy cells, and granule cells, possibly enhanced by mossy fiber sprouting. In this case, the backprojection could be important to seizure activity originating in hippocampus, and help explain the seizure susceptibility of ventral hippocampus.

Figure 1. The “trisynapto-centric” vs. “CA3-centric” view of hippocampal information processing

A. The trisynaptic circuit is diagrammed schematically for a horizontal section through the adult rodent hippocampus. Cell layers are in gray. B. The axonal arbor of CA3 pyramidal cells is schematically presented to illustrate the perspective that these neurons may be a central point of information processing in the hippocampus.

Figure 2. Recordings in hippocampal slices show evidence of trisynaptic and backprojecting pathways

A. Extracellular recordings of evoked responses to 5 stimulation sites (1–5) and 4 recording sites (A–D). Recordings were made sequentially in the same slice in response to a fixed stimulus for each stimulus site. B. A schematic is used to allow better comparisons of fimbria-evoked and outer molecular layer-evoked responses recorded at four sites in the same slices: the outer molecular layer, granule cell layer, CA3c cell layer, and CA3b cell layer. X marks recording site locations. C. Comparison of evoked responses to fimbria stimulation in the same slice demonstrate the responses in the granule cell layer following pyramidal cell activation by the fimbria, but do so with a delay. Superimposition of responses illustrates that the onset of the granule cell layer field potential begins approximately 1–2 ms after the antidromic population spike recorded in area CA3b. This delay suggests an intermediary synapse, probably in CA3c or the hilus, between CA3b and granule cells. Indeed, the granule cell field potential begins immediately after the CA3c orthodromic population spike, which probably is due to CA3b recurrent excitation of CA3c pyramidal cells. At the same time, hilar cells are also activated and innervate granule cells (see later figures), although CA3c pyramidal cells could innervate granule cells also (Li et al., 1994). The bottom trace is an IPSP recorded intracellularly from a granule cell in the same slice in response to the same stimulus. It shows that the onset of the IPSP is similar to the onset of the granule cell layer field potential, which likely reflects the average of many IPSPs in granule cells situated around the extracellular electrode.

Figure 3. Physiological evidence for a CA3 backprojection mediated by hilar neurons

A. A schematic illustrates the backprojection supported by physiological evidence collected to date. It shows that CA3 pyramidal cells innervate GABAergic and glutamatergic mossy cells of the hilus, which in turn innervate granule cells. B. Recordings illustrate the physiological correlates of the schematic in A. Intracellular recordings from granule cells in response to fimbria stimulation in an adult male rat slice illustrate an evoked IPSP that reverses at −70 mV, indicated a GABA_A receptor-mediated IPSP is primarily evoked under normal conditions. After the GABA_A receptor antagonist bicuculline was bath-applied, the evoked response was an EPSP followed by an IPSP that reversed at approximately −80 mV, suggesting an EPSP followed by a GABA_B receptor-mediated IPSP is normally masked by GABA_A receptor-mediated inhibition. From Scharfman (1994a).

Figure 4. The CA3 excitatory backprojection is dependent on hilar mossy cells

A. Schematic illustrating the experimental approach for results shown in B–C. A site in CA3c was used for extracellular recording while recording intracellularly from a granule cell. Pressure application of CNQX was used at two distinct sites in the hilus or in the inner molecular layer. CNQX was applied in microdrops that were barely detectable by eye, allowing preferential application to select areas of the slice. B. In the presence of bicuculline, CA3 epileptiform discharges were evoked by fimbria stimulation, and following the onset of the burst discharge, a granule cell depolarized and discharged 2 action potentials. After CNQX was pressure-applied to the hilus, the granule cell response decreased but the CA3 discharge remained unaffected. C. In a different slice, CNQX pressure-application to the inner molecular layer, near the recorded granule cell, reversibly decreased the EPSP of the granule cell, suggesting an inner molecular layer glutamatergic synapse was necessary for the EPSP. From Scharfman (1994a).

Figure 5

The trisynaptic circuit and CA3 backprojection supports a bi-directional gate to the dentate gyrus granule cells.

Figure 6. Evidence for associative networks and reverberatory circuits in the dentate gyrus under control of GABAergic inhibition

A. A schematic illustrates electrode locations for the recordings shown in B. B. Stimulation of specific sites in the slice evoked bursts of action potentials in CA3 pyramidal cells, and simultaneous intracellular recordings from a granule cell show bidirectional activation of CA3 and the granule cell. C. In a different slice from B where CA3 epileptiform bursts were followed by numerous afterdischarges, CA3 appeared to precede activation of the simultaneously-recorded granule cell, but during the afterdischarges, the converse appeared to develop. From Scharfman (1994a).