Evolutionarily conserved anatomical and physiological properties of olfactory pathway till fourth order neurons in a species of grasshopper(Hieroglyphus banian)

Abstract

AcknowledgementsWe would like to thank the Center for Nanotechnology (Confocal Laser Scanning Microscope facility), Central instrumentation Laboratory (CIL, Confocal Laser Scanning Microscope facility) and School of Physics (FE-SEM facility), all at the University of Hyderabad for providing access to their infrastructure. We are grateful to Ms Nalini Manthapuram, Mr. M Prasad and Ms Sunitha for their expert technical assistance in using the CLSM and FE-SEM facilities respectively. AbstractOlfactory systems of different species show variations in structure and physiology despite some conserved characteristics. We characterized the olfactory circuit of the grasshopper Hieroglyphus banian of family Acrididae (subfamily: Hemiacridinae) and compared it to a well-studied species of locust, Schistocerca americana (subfamily: Cyrtacanthacridinae), also belonging to family Acrididae. We used in vivo electrophysiological, immunohistochemical and anatomical (bulk tract tracing) methods to elucidate the olfactory pathway from the second order neurons in antennal lobe to the fourth order neurons in β -lobe of H. banian.We observe highly conserved anatomical and physiological characteristics till the fourth order neurons in the olfactory circuit of H. banian and S. americana, though they are evolutionarily divergent (~57 million years ago). However, we found one major difference between the two species-there are four antennal lobe tracts in H. banian while only one is reported in S.americana. Besides, we are reporting for the first time, a new class of bilateral neurons which respond weakly to olfactory stimuli even though they innervate densely downstream of Kenyon cells. Abbreviations ACAAccessory calyx AL Antennal lobe bLN β -lobe neuron CT Curved tract FE-SEM Field emission scanning electron microscopy 3 GGN Giant GABAergic neuron IG Inhibitor of GGN KC Kenyon cell lALT Lateral antennal lobe tract LH Lateral horn LHN Lateral horn neuron LOG Lobus glomerulus LFP Local field potential LN Local neuron mALT Medial antennal lobe tract mlALT Medio-lateral antennal lobe tract mPN Multiglomerular projection neuron MB Mushroom body MBEN Mushroom body extrinsic neuron OR Olfactory receptor ORN Olfactory receptor neuron PSTH Peri-stimulus time histogram PN Projection neuron tALT Transverse antennal lobe tract TT Tritocerebral tract uPN Uniglomerular projection neuron Our results show that the anatomy and physiology of the neurons in the olfactory pathway in the two species are highly conserved till the fourth order. However, we found a major difference in the number of antennal lobe tracts (ALTs) to MB in H. banian, when compared to that reported in S. americana. There is only one reported ALT, the medial ALT in S. americana (Laurent and Naraghi 1994), while our tract-tracing results show that in H. banian, there are 4 ALTs-medial ALT, transverse ALT, lateral ALT and mediolateral ALT. We have also discovered a new class of bilateral mushroom body extrinsic neurons (MBEN) in the circuit. These neurons have dense innervations in the ol...