A new subdivision of anterior piriform cortex and associated deep nucleus with novel features of interest for olfaction and epilepsy

The anterior part of the piriform cortex (the APC) has been the focus of co rtical-level studies of olfactory coding and associative processes and has attracted considerable attention as a result of a unique capacity to initia te generalized tonic-clonic seizures. Based on analysis of cytoarchitecture , connections, and immunocytochemical markers, a new subdivision of the APC and an associated deep nucleus are distinguished in the rat. As a result o f its ventrorostral location in the APC, the new subdivision is termed the APC(VR). The deep nucleus is termed the pre-endopiriform nucleus (pEn) base d on location and certain parallels to the endopiriform nucleus. The APC, h as unique features of interest for normal function: immunostaining suggests that it receives input fi om tufted cells in the olfactory bulb in additio n to mitral cells, and it provides a heavy, rather selective projection fro m the piriform cortex to the ventrolateral orbital cortex (VLO), a prefront al area where chemosensory, visual, and spatial information converges. The APC, also has di- and tri-synaptic projections to the VLO via the pEn and t he submedial thalamic nucleus. The pEn is of particular interest from a pat hological standpoint because it corresponds in location to the physiologica lly defined "deep piriform cortex" ("area tempestas") from which convulsant s initiate temporal lobe seizures, and blockade reduces ischemic damage to the hippocampus. Immunostaining revealed novel features of the pEn and APC( VR) that could alter excitability, including a near-absence of gamma -amino butyric acid (GABA)ergic "cartridge'' endings on axon initial segments, few cholecystokinin (CCK)-positive basket cells, and very low gamma -aminobuty ric acid transporter-1 (GAT1)-like immunoreactivity. Normal functions of th e APC(VR)-pEn may require a shaping of neuronal activity by inhibitory proc esses in a fashion that renders this region susceptible to pathological beh avior. (C) 2001 Wiley-Liss, Inc.