Beyond the primary olfactory cortex: olfactory-related areas in the neocortex, thalamus and hypothalamus

The extrinsic projections from the primary olfactory cortex to other areas of the forebrain in the rat are described, on the basis of experiments using anterograde and retrograde axonal tracers, as well as electrophysiological recording of unit activity. The areas shown to receive olfactory inputs are (i) in the neocortex: the lateral and ventrolateral orbital areas, and the ventral agranular insular area, all of which are in the dorsal bank of the rhinal sulcus; (ii) in the thalamus: the central segment of the mediodorsal nucleus and the ventral part of the submedical nucleus; (iii) in the hypothalamus: the lateral hypothalamic area, especially its caudal half, and probably the nucleus gemiru, with some input also to more medial hypothalamic structures; (iv) the hippocampus and dentate gyrus; and (v) the deep nuclei of the amygdala. Anterograde and retrograde axonal tracing experiments have shown that all of these areas receive fibers from cells in or closely related to the primary olfactory cortex, and cells in all of them except the nucleus gemini and the deep nuclei of the amygdala have been shown to be driven by electrical stimulation of the olfactory bulb.