Abstract
Layer V of the entorhinal cortex (EC) receives input from the hippocampus and originates main entorhinal outputs. The deep-sublayer Vb, immunopositive for the transcription factor Ctip2, is thought to be the main recipient of hippocampal projections, whereas the superficial-sublayer LVa, immunonegative for Ctip2, originates the main outputs of EC. This disrupts the proposed role of EC as mediating hippocampal-cortical interactions. With the use of specific (trans)synaptic tracing approaches, we report that, in medial entorhinal cortex, layer Vb neurons innervate neurons in layers Va, II, and III. A similar circuitry exists in the lateral entorhinal cortex. We conclude that EC-layer Vb neurons mediate two circuits in the hippocampus-memory system: (1) a hippocampal output circuit to telencephalic areas by projecting to layer Va and (2) a feedback projection, sending information back to the EC-hippocampal loop via neurons in layers II and III.
Highlights
The entorhinal cortex (EC) constitutes the major gateway between the hippocampus and the neocortex and, together with the hippocampus, plays a critical role in memory and spatial navigation
We examined the telencephalic projections of layer V (LV) neurons by retrograde tracing, confirmed that telencephalic projections originate from layers Va (LVa), but not LVb, in MEC, and showed that this scheme applies to LEC
We conclude that LVb neurons are the key elements of two main circuits in the hippocampus-memory system: a hippocampal output circuit to telencephalic areas by projecting to neurons in LVa and a feedback loop by projecting to neurons in LII and LIII. In Both LEC and MEC, LVa and LVb Differ with Respect to Molecular Identity and Projections To examine whether the EC LV of the rat can be further divided into two sublayers, we examined the distribution of Ctip2- and Purkinje cell protein 4 (PCP4)-positive neurons in both MEC and LEC (Figures 1A and 1B)
Summary
The entorhinal cortex (EC) constitutes the major gateway between the hippocampus and the neocortex and, together with the hippocampus, plays a critical role in memory and spatial navigation. Entorhinal LV neurons project to the superficial layers (Dolorfo and Amaral, 1998; Kohler, 1986, 1988; van Haeften et al, 2003), and it has been shown that the hippocampal information may re-enter the entorhinalhippocampal loop (Iijima et al, 1996; Kloosterman et al, 2003a) This re-entrant activity (reverberation) is one of the mechanisms proposed for temporal storage of information in a neuronal network (Edelman, 1989; Iijima et al, 1996; Kloosterman et al, 2003a). This circuitry is assumed to be present in both the medial (MEC) and lateral subdivision (LEC) of the entorhinal cortex
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