Olfaction in Desert Locusts- Anatomy, Function and Plasticity of the Central Olfactory System

Sammanfattning: The anatomy, function and plasticity of the primary olfactory centre, the antennal lobe (AL), of the desert locust Schistocerca gregaria was investigated using a number of neuroanatomical and neurophysiological methods. Anatomical characterisation of the AL in a number of orthopteran species made it possible to deduce a model concerning the evolution of microglomeruli. These neuropil structures, constituting the first relay centres in the olfactory information pathway of the desert locust, seem to have evolved through fragmentation of multicompartmented unique glomeruli elicited by changes in the architecture of olfactory receptor- and projection neurons. The function and plasticity of the AL was investigated by characterising the response characteristics of olfactory projection neurons in 1st, 3rd, 5th and adult solitary and gregarious locusts by means of intracellular recording and staining methods while stimulating with behaviourally relevant odours. This characterisation revealed stage- and phase-dependent plasticity in the olfactory pathway, including changes and differences in response spectra and sensitivity of olfactory projection neurons. These changes were linked with morphological changes within the AL. Plasticity of olfactory information processing and aggregation behaviour in adult gregarious locusts was also studied in relation to age and juvenile hormone (JH) level. Locusts containing a high level of JH, i. e. old locusts and locusts injected with JH, displayed an impairment of the central olfactory system that was reflected in a diminished behavioural response to the main adult aggregation pheromone component, phenylacetonitrile. Locusts containing a low level of JH, on the other hand, displayed a fully functional olfactory system and a positive response to the tested pheromone component. The morphology and ramification pattern of immunocytochemically identified aminergic and neuropeptidergic neurons were investigated as a first step toward unravelling their potential role in olfactory information processing. This study revealed a number of novel interneurons innervating the AL showing immunoreactivity to serotonin, histamine, locustatachykinin, leucokinin and FMRFamide.