Intrinsic and efferent connections of the endopiriform nucleus in rat

The endopiriform nucleus is a large group of multipolar cells located deep to the piriform cortex. The function of this nucleus is unknown, but studie s with animal models suggest that it plays an important role in temporal lo be epileptogenesis. To address questions concerning mechanisms of epileptog enesis and to gain insights into its normal function, efferent axons from t he endopiriform nucleus were labeled by anterograde transport from small ex tracellular injections of Phaseolus vulgaris leucoagglutinin. Several principles of organization were derived: (1) heavy local and long i ntrinsic connections are present throughout the endopiriform nucleus; (2) e ndopiriform efferents target cortical rather than nuclear structures; (3) e xtensive projections from the endopiriform nucleus extend to most basal for ebrain ay eas including the piriform cortex, entorhinal cortex, insular cor tex, orbital cortex, and all cortical amygdaloid areas. The perirhinal cort ex, olfactory tubercle, and most subdivisions of the hippocampal formation receive light projections; (4) projections are highly distributed spatially within all target; areas; (5) efferent axone from the endopiriform nucleus are unmyelinated and give rise to boutons along their entire course rather than arborizing locally; and (6) the endopiriform nucleus and piriform cor tex share target areas, but efferents from the endopiriform nucleus lack th e precise laminar order of those from the piriform cortex, and provide a he avy caudal to rostral pathway that is lacking in the cortex. The significance of these findings for the triggering of generalized seizur es from the deep piriform region are discussed. An hypothesis for a role of the endopiriform nucleus in memory storage is presented. (C) 1999 Wiley-Li ss, Inc.