Neural pathways connecting the deutocerebrum and lateral protocerebrum in the brains of decapod crustaceans

The olfactory and accessory lobes of eureptantian decapod crustaceans are b ilateral brain neuropil regions located within the deutocerebrum. Although the olfactory lobe seems to receive only primary olfactory inputs, the acce ssory lobe receives higher-order multimodal (including olfactory) inputs. T he output pathways from both the olfactory and accessory lobes are provided by the axons of a large population of projection neurons, whose somata lie adjacent to the lobes. The axons of these neurons form a large tract that projects bilaterally to the medulla terminalis and hemiellipsoid body in th e lateral protocerebrum. To gain insights into the ways in which olfactory information is processed on leaving the deutocerebrum, we examined the neur oanatomy of the projection neuron pathways of three species of eureptantian decapod crustaceans: the freshwater crayfish, Procambarus clarkii and Orco nectes rusticus, and the clawed lobster, Homarus americanus. Projection neu rons were labeled by focal injections of the lipophilic tracers DiI and DiA into the olfactory and accessory lobes. In all three species, projection n eurons innervating the accessory lobe were found to exclusively innervate t he neuropils of the hemiellipsoid body. In contrast, projection neurons inn ervating the olfactory lobes primarily target neuropil regions of the medul la terminalis. The results of this study indicate, therefore, that the proj ection neuron pathways from the olfactory and accessory lobes project to se parate, largely nonoverlapping regions of the lateral protocerebrum. The im plications of these findings for our understanding of the processing of olf actory information in the brains of decapod crustaceans are discussed. J. C omp. Neurol. 441:9-22 2001. (C) 2001 Wiley-Liss, Inc.