The Neural connections between the primary olfactory center and higher brain regions in the male moth : morphological and physiological properties of projection neurons and centrifugal neurons

Abstract

In the insect brain, different types of neurons provide input and output to the primary olfactory center, the antennal lobe. The principal output neurons, projection neurons, convey odor information from the antennal lobe to higher brain areas by projecting in one of six parallel tracts (m-ALT, ml-ALT, l-ALT, t-ALT, d-ALT, and dm-ALT). In turn, centrifugal neurons innervating higher brain areas provide input to, and modulate, the processing in the antennal lobe. However, the functional role of the parallel tracts and centrifugal neurons in olfactory processing, is yet poorly understood. Here, we have investigated these two types of neurons in the noctuid moth Helicoverpa Armigera, by using in vivo intracellular recording and iontophoretic staining of individual neurons, in combination with confocal microscopy. The results demonstrate that morphologically diverse projection neurons shows different physiological responses to different odors. This suggests that individual tracts are associated with distinct functional roles. Neurons in the m-ALT are more narrowly tuned to odors and appeared to encode odor identity, whereas ml-ALT and l-ALT are more broadly tuned as serve other functions. In addition, the comparison of output area of pheromone-sensitive projection neurons confined to the m-ALT in the lateral protocerebrum showed non-overlapping terminal regions with the non-pheromone m-ALT projection neurons. Projection neurons exhibited several different physiological response patterns in response to odors. This suggests that individual projection neurons may encode various features of the olfactory stimuli through different response patterns. The labelling and identification of a new centrifugal neuron, the bilateral, paired centrifugal neuron, indicates that this neuron respond directly to the olfactory pathways and may serve as a feedback neuron. Contrary, the CSD neuron exhibited no response to any of the tested odors, which suggests that this neuron influence AL processing based on internal states of the animal or non-olfactory input, rather than a direct connection with olfactory pathways.