Central Connections of the Ovine Olfactory Bulb Formation Identified Using Wheat Germ Agglutinin-Conjugated Horseradish Peroxidase

Abstract

Pheromonal stimuli elicit rapid behavioral and reproductive endocrine changes in the ewe. The neural pathways responsible for these effects in sheep are unknown, in part, because the olfactory bulb projections have not been examined in this species. Using the anterograde and retrograde neuronal tracer, wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP), we describe the afferent and efferent olfactory bulb connections of the Suffolk ewe. Injections of WGA-HRP limited to the main olfactory bulb resulted in retrograde labeling of cells in numerous telencephalic, diencephalic, and metencephalic regions. Terminal labeling was limited to layer Ia of ipsilateral cortical structures extending rostrally from the anterior olfactory nucleus (AON), piriform cortex, anterior-, and posterolateral-cortical amygdaloid nuclei to lateral entorhinal cortex caudally. Injections involving the accessory olfactory bulb and AON produced additional labeling of cells within the bed nucleus of the stria terminalis (BNST), medial nucleus of the amygdala, and a few cells in the posteromedial cortical nucleus of the amygdala. Terminal labeling included a small dorsomedial quadrant of BNST and also extended to the far lateral portions of the supraoptic nucleus. A clearly defined accessory olfactory tract and nucleus was not evident, perhaps due to limitations in the sensitivity of the method. With this possible exception, the afferent and efferent olfactory connections in the sheep appear similar to those reported for other species.