Applying X-ray micro-tomography to learning and memory

Metabolic processes and neural pathways such as those for glucose metabolism and olfaction in insects are similar to, and in some cases identical to, those found in vertebrates. In particular, the peripheral architecture of the insect and mammalian olfactory system is surprisingly similar. Thus one insect, the western honeybee (Apis mellifera), has become a model organism for the investigation of olfactory signal processing in the brain. A. Mellifera has superior abilities to identify, classify, learn and remember odoriferous environmental chemicals. The use of x-ray MicroCT imaging for the non-invasive study of insects (termed diagnostic radioentomology 'DR') is increasing. This experiment was conducted as a 'proof of principle' study to identify whether DR scanning live insects (honeybees) can be used to non-invasively examine learning and memory with respect to brain architecture and function. A bench-top MicroCT scanner was used to scan live bees. To enhance tissue differentiation, radiographic contrast was injected directly into the haemolymph. Brain volume was measured using BeeView software. Gross brain architecture was visualised in 2D and 3D projections. Scanning of live bees enabled minimally-invasive imaging of physiological processes for the first time such as passage of contrast from gut to haemolymph as well as preliminary brain perfusion studies. In particular, the learning and memory neuropils in the antennal lobes were assessed by measuring changes in antennal lobe volumes. Future experiments will correlate neuropil volume changes with classic PER learning and memory studies. One advantage of using an insect model is that multiple experiments can be conducted without the need for lengthy ethics approvals that apply to vertebrate experiments. The advantage of using live insects is that longitudinal studies can be performed on individuals therefore reducing, to a great extent, the inherent inter-cohort errors which occur. Once proof of principles is established in the insect model, experiments such as these can easily be extended to vertebrates.