Circadian rhythms in the fly's optic lobe
In a longstanding collaboration with the laboratory of Dr.Elzbieta Pyza, who is in the Department of Cytology at the Zoological Institute of the Jagiellonian University in Kraków, Poland, we study a novel form of neuronal plasticity in a visual system, using the first neuropile, or lamina, of the optic lobe in the fruit fly, Drosophila or the experimentally more amenable housefly Musca domestica. The lamina is modular in composition, and each module -- called a cartridges, comprises a small, fixed number of identified neurons. Two of these (L1, L2) exhibit a day/night modulation in the size of their axons. These size changes occur concurrently with synaptic changes discovered previously by Dr. Pyza, and both sets of changes, axon size and numbers of synapses, in some cases have a circadian basis. We are in the process of identifying the circuits that procure these changes, to understand, in particular, the role of various wide-field optic lobe neurons (below), which are thought to liberate neuromodulators that act simultaneously right across the entire optic lobe. One such cell (LBO5HT) is immunoreactive to 5-HT, or serotonin (green channel), while another is immunoreactive to a peptide homologue of PDH and expresses the gene pigment dispersing factor (red channel). Both of these cells probably have accomplice cells or neurotransmitters, for example those that express a member of the FMRFamide gene family of neuropeptides. Some of the PDH cells in Drosophila are also immunoreactive to the product of the period gene, which is widely implicated in the expression of circadian rhythms. Microinjections of peptide and amine neuromodulators by Harjit Seyan, or of classical transmitters such as GABA or glutamate, alter the lamina axon calibre of L1 and L2. Such changes are presumably downstream to more immediate responses, possibly as part of a wider set of unproven structural changes among these cells' dendrites, and probably involving the surrounding lamina epithelial glia.
Frontal section of the optic lobe in the housefly Musca domestica.