Contents lists available at ScienceDirect Experimental Neurology journal homepage: www.elsevier.com/locate/yexnr Review Article Piriform cortex ictogenicity in vitro Marco de Curtis a, ⁎ , Laura Uva a , Maxime Lévesque b , Gerardo Biella c , Massimo Avoli b,d a Fondazione IRCCS Istituto Neurologico Carlo Besta, Milano, Italy b Montreal Neurological Institute, McGill University, Montréal, Qc, Canada c Università di Pavia, Department of Biology, Biotechnology Lazzaro Spallanzani, Italy d Facoltà di Medicina e Odontoiatria, Sapienza Università di Roma, Roma, Italy ABSTRACT The piriform cortex is recognized to play critical roles in focal ictogenesis, both in animal models and in humans. We review here the contribution of in vitro studies performed on rodent brain tissue that were aimed at un- derstanding the ictogenic properties of the piriform cortex and the contiguous olfactory areas. During in vitro experiments, epileptiform events can be easily generated in the piriform area by diverse pro-convulsive drugs (4- aminopyridine, bicuculline, picrotoxin) or by electrical stimulation. Simultaneous intracellular and field po- tential recordings performed on in vitro preparations, which include brain slices of rats and mice and the isolated brains of guinea pigs, demonstrated that both the piriform cortex proper and the endopiriform nucleus (also considered part of the piriform area) generate interictal spikes, high-frequency oscillations and seizure-like activities that mimic focal discharges. These findings were confirmed both by optical recordings of intrinsic signals coupled with brain activity and by fast imaging of optical signals generated by voltage-sensitive dyes. Overall, these studies demonstrated that epileptiform discharges effectively propagate from the piriform structures to the limbic regions, supporting the conditions for secondarily generalized ictogenesis. 1. Introduction The piriform cortex (PC) is a three-layered paleocortex implicated in the olfactory sensory system in mammals. It is considered a primary olfactory sensory region and retains a diffuse and broad associative connectivity with its main afferent input region, the olfactory bulb (OB). The PC is broadly connected with several other olfactory and limbic areas, such as the olfactory tubercle, the anterior olfactory nu- cleus and the lateral entorhinal cortex as well as with subcortical areas (for review see Loescher and Ebert, 1996; Neville and Haberly, 2004; Vismer et al., 2015). In relation to such extended associative con- nectivity, a recent study (Meissner-Bernard et al., 2019) has confirmed that the PC is not just involved in odor perception, but represent a critical component for olfactory memory traces (for review see Wilson and Sullivan, 2011). It is also worth to emphasize that the PC conveys direct sensory information to the limbic-hippocampal system without any interposed synaptic station in subcortical and thalamic nuclei, as it is observed for all other sensory inputs. The PC is subdivided in a rostral and a caudal portion that mainly differ in the proportion of afferent/associative fiber content in the superficial plexiform layer I. The anterior piriform cortex (APC) extends between the OBs and the surface point where the lateral olfactory tract (LOT, which is formed by OB mitral cell axons) disappears as a defined structure at the ventral cortical surface (arrowhead in Fig. 1A). LOT fibers fan out in both the APC and the more caudal posterior piriform cortex (PPC), the latter being characterized by a thicker intra-PC as- sociative fiber layer (see below). In the most anterior portion of the PC a deeper region termed the endopiriform nucleus has been identified (EPN; Fig. 1B). Because of its relatively simple arrangement, at least when com- pared to the six-layered neocortex and in virtue of its extensive rostral- to-caudal homogeneous and highly laminar organization, the PC was one of the most studied cortical regions in early in vitro brain slice studies (Yamamoto and McIlwain, 1966; Harvey et al., 1974; Scholfield, 1978). As reviewed in the initial part of our review, these experiments have set the basis for defining the functional connectivity of the PC. In addition, we will address here the ability of PC networks to generate epileptiform patterns under different experimental conditions that in- clude the 4-aminopyridine (4AP) model of epileptiform synchroniza- tion. https://doi.org/10.1016/j.expneurol.2019.113014 Received 1 February 2019; Received in revised form 7 May 2019; Accepted 15 July 2019 Abbreviations: 4AP, 4-aminopyridine; APC, Anterior Piriform Cortex; EPN, Endopiriform Nucleus; EC, Entorhinal Cortex; HFOs, High-Frequency Oscillations; LOT, Lateral Olfactory Tract; OB, Olfactory Bulbs; PC, Piriform Cortex; PPC, Posterior Piriform Cortex; SLEs, Seizure-Like Events ⁎ Corresponding author at: Epilepsy Unit, Fondazione Istituto Neurologico Carlo Besta, via Celoria 11, Milano, Italy. E-mail address: marco.decurtis@istituto-besta.it (M. de Curtis). Experimental Neurology 321 (2019) 113014 Available online 16 July 2019 0014-4886/ © 2019 Elsevier Inc. All rights reserved. T