Reciprocal connections of the hippocampal area CA1, the lateral nucleus of the amygdala and cortical areas in a combined horizontal slice preparation

Abstract

The entorhinal and perirhinal cortices, the hippocampus and the amygdala are heavily interconnected limbic structures that are implicated in memory, and under pathological conditions, in seizure generation and propagation of temporal lobe epilepsy. In-vitro coronal preparations have been limited by the anatomical disposition of these structures. Here we describe a modified horizontal slice preparation that includes all these structures in the same plane. To evaluate whether axonal connectivities are preserved, fluorescent tracers were used. Most of the connections known from in-vivo studies within and between the entorhinal and perirhinal cortices, the amygdala (basolateral nucleus, lateral nucleus, and amygdalopiriform transition area) and the hippocampus were preserved in the 400 μm-thick horizontal slices employed.

Introduction

The temporal lobe is composed of the hippocampus, the amygdala and surrounding cortical areas, such as the entorhinal and perirhinal cortex. These structures are heavily interconnected by reciprocal pathways and the existence of functional interactions have been evidenced (Van Hoesen and Pandya, 1975, Steward, 1976, De Olmos et al., 1985, van Groen and Wyss, 1990, Pikkarainen et al., 1999, Shi and Cassell, 1999, Stoop and Pralong, 2000).

Furthermore, these structures are involved in learning and memory (Izquierdo and Medina, 1993, Miller et al., 1998, Yaniv and Richter, 2000) and, under pathological conditions, they are also involved in epilepsy (Gean and Shinnick Gallagher, 1987, Heinemann et al., 1993, Jones, 1993, Kelly and McIntyre, 1996, Bertram et al., 1998, Gloveli et al., 1998, Nissinen et al., 2000).

Use of coronal slices enables most electrophysiological studies in the limbic system, however, it is not possible to obtain coronal combined slices, which include the amygdala, the hippocampus, the perirhinal and the entorhinal cortex.

Horizontal slice preparations have been alternatively used for electrophysiological studies and preserved neuronal circuits in horizontal in-vitro preparations have been demonstrated in combined horizontal entorhinal-hippocampal slices (Boulton et al., 1992) and in combined slices of the amygdala (von Bohlen und Halbach and Albrecht, 1998). A combined slice preparation (including the hippocampus, the amygdala, the entorhinal and perirhinal cortex) would offer the opportunity to study effects in a more complex network however. Therefore, a preparation plane was chosen, in which all these structures could be obtained. To investigate which connectivities are preserved in this preparation in comparison to the results obtained in in-vivo studies, fluorescent tracers were applied in different structures.