Elsevier

Progress in Neurobiology

Volume 74, Issue 2, October 2004, Pages 101-110
Progress in Neurobiology

The rhinal cortices: a wall of inhibition between the neocortex and the hippocampus

https://doi.org/10.1016/j.pneurobio.2004.08.005Get rights and content

Abstract

Anatomical data indicate that the parahippocampal region, comprised of the perirhinal (PRC), postrhinal (POR) and entorhinal (ERC) cortices, is an essential link between neocortex and hippocampus. Lesion studies demonstrated that memory functions previously ascribed to the hippocampus depend on the integrity of the rhinal cortices. This review will consider recent data suggesting that the ERC and PRC, far from being passive relay stations, actively gate impulse traffic between neocortex and hippocampus, because they are endowed with a powerful intrinsic inhibitory system. It is proposed that the cross-talk between PRC and ERC is not organized to unrestrictedly transfer information, but to select relevant inputs. The implication of these new evidences for the propagation of epileptiform activity will be considered.

Section snippets

Extrinsic and intrinsic connections of rhinal cortices

The defining feature of the parahippocampal region resides in its reciprocal connections with uni- and poly-modal association neocortical areas and with the hippocampal formation. Even though strict reciprocity between medial temporal lobe and the neocortex has been questioned in primates (Lavenex et al., 2002) and is not unequivocal in other species, tract tracing studies indicate that information transfer from the neocortex to the hippocampus depends on impulse propagation through a sequence

Physiology of perirhinal–entorhinal interactions

Although little physiological work has been performed on the rhinal cortices, it is typically assumed that they faithfully transmit inputs from the neocortex to the hippocampus and vice versa. In fact, many theories of episodic memory consolidation rely on the existence of a fast transfer of information via highly synchronized discharges of large numbers of neurons in parahippocampal cortices (Pennartz et al., 2002, Buzsáki, 1989). For instance, one model posits that during waking, information

Intrinsic inhibitory mechanisms that control propagation across rhinal cortices

Considering the evidence reported above, it should be concluded that propagation of activity between PRC and ERC, occurs with an extremely low probability. As a result, the rhinal cortices may be considered as a filter or a gate that controls bi-directional transfer of information between neocortex and hippocampus. In this section, we consider the network mechanisms that control impulse traffic through rhinal cortices.

Direct neocortical stimulation in slices of the perirhinal region kept in

Associative longitudinal propagation of excitation within PRC and ERC

The results reviewed above strongly suggest that a wall of inhibition, represented by network interactions within rhinal cortices, controls and regulates the reciprocal transfer of information between neocortex and hippocampus. In contrast with the low probability transfer seen in the transverse direction, activity propagates efficiently in the longitudinal axis along connections that span the entire rostro-caudal extent of longitudinal bands of rhinal cortex, running parallel to the rhinal

Implication for the genesis and propagation of epileptiform activity

Failure of the inhibitory control that usually dominates interactions between PRC, ERC and hippocampus may lead to permanent excitability changes that will promote limbic epileptogenesis. For instance, it has been demonstrated that propagation of olfactory-driven activity from the ERC to the PRC is facilitated in conditions of hyperexcitability, such as after kindling (Kelly and McIntyre, 1996, McIntyre and Plant, 1993) and during applications of GABA-receptor antagonists in the ERC, close to

Conclusions

The studies reviewed above delineate a new functional organization of the rhinal cortices, according to which transverse propagation of activity from the neocortex to the hippocampus via PRC and ERC is hindered by a powerful inhibitory control, whereas longitudinal propagation within both PRC and ERC is not. In keeping with this view, information transfer from the neocortex to the hippocampus (and vice-versa) may occur through the integration of coincident events within each band of rhinal

Acknowledgements

We would like to thank our collaborators Gerardo Biella, Laura Uva and Vadym Gnatkowsky for their contribution to the experiments. The study was supported by the European Community Grant VSAMUEL (IST 1999-10073), by the Italian Health Ministry to MdC and by NIH Grant R01-MH066856-01 and NSF Grant to DP.

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