Cortico‐hippocampal communication by way of parallel parahippocampal‐subicular pathways

Abstract

The hippocampal memory system, consisting of the hippocampal formation and the adjacent parahippocampal region, is known to play an important role in learning and memory processes. In recent years, evidence from a variety of experimental approaches indicates that each of the constituting fields of the hippocampal memory system may serve functionally different, yet complementary roles. Understanding the anatomical organization of cortico-parahippocampal-hippocampal connectivity may lead to a further understanding of these potential functional differences. In the present paper we present the two main conclusions of experiments in which we studied the anatomical organization of the hippocampal memory system of the rat in detail, with a focus on the pivotal position of the entorhinal cortex. We first conclude that the simple traditional view of the entorhinal cortex as simply the input and output structure of the hippocampal formation needs to be modified. Second, our data indicate the existence of two parallel pathways through the hippocampal memory system, arising from the perirhinal and postrhinal cortex. These two parallel pathways may be involved in separately processing functionally different types of sensory information. This second proposition will be subsequently evaluated on the basis of series of electrophysiological studies we carried out in rats and some preliminary functional brain imaging studies in humans.

A review on the neural bases of episodic odor memory: from laboratory-based to autobiographical approaches

“…The involvement of the PC in episodic odor memory is modality-specific (Gottfried et al, 2004; Lehn et al, 2013), it is independent of odor valence (Gottfried et al, 2004; Yeshurun et al, 2009), and it tends to be lateralized to the right (vs. left) hemisphere (Savic et al, 2000; Dade et al, 2002; Gottfried et al, 2004; Plailly et al, 2005; Cerf-Ducastel and Murphy, 2006). The hippocampus is also consistently observed in both approaches, which is consistent with a large amount of literature that stresses the importance of this brain region in episodic memory (e.g., Suzuki and Amaral, 1994; Burwell and Amaral, 1998; Witter et al, 2000; Squire et al, 2004; Davachi, 2006; Diana et al, 2007; Eichenbaum et al, 2007). The literature involving the olfactory modality further shows that hippocampal activation reflects the memory performance (Royet et al, 2011; Lehn et al, 2013), and that while the hippocampus is engaged in the episodic memory of different sensory modalities (Plailly et al, 2007; Lehn et al, 2013), it has a privileged role for the first olfactory associations (Yeshurun et al, 2009).…”

“…The contribution of each of the medial temporal lobe components to the memory process and their connectivity with the neocortex has been widely investigated (Suzuki and Amaral, 1994; Burwell and Amaral, 1998; Witter et al, 2000; Squire et al, 2004; Davachi, 2006; Diana et al, 2007; Eichenbaum et al, 2007). In summary, the cortical projections encompass two parallel pathways.…”

RNA interference‐mediated phosphodiesterase 4D splice variants knock‐down in the prefrontal cortex produces antidepressant‐like and cognition‐enhancing effects

“…In addition, long‐form PDE4D knock‐down in the PFC enhanced spatial memory in the MWM task, although the MWM was initially used for testing hippocampus‐dependent spatial memory (Morris et al ., ). Given that specific down‐regulation of long‐form PDE4D in the PFC did not cause any changes in the hippocampus including PDE4D expression, cAMP signalling and neurogenesis (data not shown), the PFC should be the main region responsible for the cognition‐enhancing effects, although the crosstalk of cortico‐parahippocampal‐hippocampal connectivity may not be excluded (Witter et al ., ).…”

The mechanisms for pattern completion and pattern separation in the hippocampus

“…The CA1 neurons would then activate, via their termination in the deep layers of the entorhinal cortex, at least the pyramidal cells in the deep layers of the entorhinal cortex (see Figure 1). These entorhinal cortex layer 5 neurons would then, by virtue of their backprojections (Lavenex and Amaral, 2000; Witter et al, 2000a) to the parts of cerebral cortex that originally provided the inputs to the hippocampus, terminate in the superficial layers (including layer 1) of those neocortical areas, where synapses would be made onto the distal parts of the dendrites of the (superficial and deep) cortical pyramidal cells (Rolls, 1989a,b,c; Markov et al, 2013). The areas of cerebral neocortex in which this recall would be produced could include multimodal cortical areas [e.g., the cortex in the superior temporal sulcus which receives inputs from temporal, parietal and occipital cortical areas, and from which it is thought that cortical areas such as 39 and 40 related to language developed; and the orbitofrontal and anterior cingulate cortex to retrieve the reward / affective aspects of an episodic memory (Rolls, 2014a,b)], and also areas of unimodal association cortex (e.g., inferior temporal visual cortex).…”