Catecholamine innervation of the basal forebrain III. Olfactory bulb, anterior olfactory nuclei, olfactory tubercle and piriform cortex

Abstract

The catecholamine innervation of the olfactory bulb, anterior olfactory nuclei, olfactory tubercle and piriform cortex was studied in the rat using biochemical analysis and fluorescence histochemistry. Biochemical studies demonstrate a moderate norepinephrine (NE) content in all olfactory structures, a high dopamine (DA) content in the olfactory tubercle and a low DA content in the olfactory bulb, anterior olfactory nucleus and piriform cortex. Following locus coeruleus lesions NE content decreases 71% in the olfactory bulb, 82% in the anterior olfactory nucleus, 62% in olfactory tubercle and 77% in piriform cortex. Brainstem transection caudal to locus coeruleus results in a decrease of NE in the olfactory tubercle of 35% without significant changes in other olfactory structures. Following lesions of the medial substantia nigra (SN) extending into the ventral tegmental area (VTA), there is a decrease of DA content (92%) in the olfactory tubercle. VTA lesions produce smaller decreases in olfactory tubercle DA content (79%). The area included in the SN and VTA lesions overlaps, suggesting that both the VTA and SN provide DA in nervation to the olfactory tubercle. VTA lesions also produce decreases in anterior olfactory nucleus DA content (73%) whereas medial SN lesions produce decreases in piriform cortex DA content (44%). Fluorescence histochemical studies using the glyoxylic acid‐Vibratome technique demonstrate the presence of several catecholamine (CA) fiber types in the olfactory system, including large NE fibers in the olfactory tubercle which probably originate in the caudal brainstem, fine NE fibers innervating all four olfactory structures arising from the locus coeruleus, and fine DA fibers from SN‐VTA. These innervate the olfactory tubercle, the anterior olfactory nucleus, piriform cortex and olfactory bulb. The DA innervation is very dense in all layers of the olfactory tubercle, with a variable innervation of the islands of Calleja. The DA fibers enter the olfactory tubercle as fascicles from the medial forebrain bundle. Both NE and DA fibers enter the anterior olfactory nuclei to form a sparse to moderately dense innervation with some continuing in the medial olfactory tract into the olfactory bulb. A moderately dense NE innervation is evident in the inner plexiform and granule cell layer of the accessory olfactory bulb and the granule cell layer of the olfactory bulb. Additional NE fibers are distributed sparsely throughout the remaining components of the olfactory bulb, some entering the glomeruli. There is a very sparse extrinsic DA innervation to the plexiform layer of the olfactory bulb and an intrinsic DA innervation from juxtaglomerular cells.