Serotonergic nerve fibers in the primary olfactory pathway of the larval sea lamprey,Petromyzon marinus

In this study, serotonin (5-hydroxytryptamine; 5HT)-immunoreactive (5HT-IR) neuronal fibers were identified in the primary olfactory pathway of the sea lamprey. These neurons are likely part of a nonolfactory neural system that innervates the olfactory sac. Cell bodies with 5HT immunoreactivity predominated in the lamina propria of the rostral portion of the nasal cavity and were less prevalent adjacent to the olfactory epithelium. The 5HT-IR fibers were parallel to axons of the olfactory receptor neurons in the lamina propria of the olfactory mucosa and in the olfactory nerve. Serotonergic fibers crossed from the olfactory nerve into the olfactory bulb or branched in the caudal portion of the olfactory nerve and terminated at the junction of the olfactory nerve with the olfactory bulb. In the dorsal olfactory bulb, 5HT-IR fibers coursed along the layer of olfactory fibers. Throughout the layer with glomeruli and mitral cells, 5HT-IR fibers were seen along the border of glomerular units. Experimental lesion of the olfactory nerve was used to determine the origin of 5HT-IR fibers rostral to the olfactory bulb. The loss of these fibers and their reappearance during outgrowth of olfactory receptor neurons inferred that they emanate from the cell bodies in the olfactory sac. The results from this study suggest that axons of olfactory receptor neurons in larval lampreys receive modulation by 5HT from these neuronal fibers.