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LOCAL SATELLITE SYMPOSIUM 2009

Functional Neuroanatomy of the Insect Nervous System

 

ABSTRACTS


Synaptic circuits in the Drosophila visual system: Progress towards a wiring diagram of the fly's brain

IAN A. MEINERTZHAGEN
Departments of Psychology and Neuroscience, Life Sciences Centre, Dalhousie University, Halifax

Simple invertebrate nervous systems such as the brain of Drosophila have always offered the prospect to understand the substrate of animal behaviour as a series of synaptic circuits. With few exceptions, this objective has remained elusive for both structurally defined circuits and their functional validation by electrophysiology. In recent years, the genetic reagents available in Drosophila have been coupled with new imaging technologies to reawaken the goal of defining brain function as a wiring diagram of synaptic circuits. Circuits are being reconstructed from different brain regions by means of serial-section EM. EM is required to resolve synaptic contacts having a typical volumetric density of one synapse per µm3, formed between identified neurons with processes down to <100 nm in diameter. Synaptic sites have multiple postsynaptic elements, most frequently triads, and contribute an enormous richness of microcircuits. These daunting complexities have previously been mastered only in the lamina of the fly's visual system, but new developments open similar approaches in the much larger and more complex second optic neuropile, the medulla, as well as in the mushroom body calyx of the olfactory system. Progress on these will be illustrated, and ideas on how they can contribute to knowledge of brain function discussed.

 


Ultrastructure and synaptic characteristics of neurons of the mushroom body calyx in Drosophila melanogaster

NANCY J. BUTCHER and IAN A. MEINERTZHAGEN
Departments of Psychology and Neuroscience, Dalhousie University, Halifax

Synaptic circuits form the functional basis of behaviour. To understand behaviour, we must first investigate the wiring and properties of neurons in these circuits. The fruit fly Drosophila provides an excellent genomic species to study neural circuits because it has a relatively simple nervous system and generates readily quantifiable behaviours. The mushroom body (MB) is a high-order integrative centre of the insect brain, which in Drosophila is required for olfactory learning and memory. The input neuropil, the MB calyx comprises three types of neurons: projection neurons (PNs) from the antennal lobe; Kenyon cells (KCs) intrinsic to the MB; and extrinsic neurons (ENs) from various sources. Using serial-section electron microscopy, we have characterized the morphology of these elements from computer 3-D reconstructions, and their synaptic organization. We find that each PN terminal bouton forms about seven output synapses per KC, and confirm that KCs have claw-shaped dendritic endings which encircle the PN bouton. The KC dendrites and PNs also receive inputs from EN elements, and ENs also receive occasional inputs from PN terminals. Thus PN and EN elements connect reciprocally. We also find evidence for extensive connections between ENs. The implications of these findings will be discussed.

 


Neuroanatomical organisation of the circadian system in the brain of larval Rhodnius prolixus

COLIN G. H. STEEL
Department of Biology, York University, Toronto

Rhodnius prolixus has played a formative role in the evolution of insect physiology for 80 years. While lacking the genetic tractability of Drosophila, its larger size and precise developmental timetable, dependent on a blood meal, enables studies of identified neurons and quantification of hormone levels. We have reported circadian rhythms in circulating levels of many brain neurohormones, leading us to examine the neuroanatomy of the clock system in the brain. In larvae, there are two groups of circadian clock cells in each hemisphere, which are axonally coupled both with each other and with those in the contralateral hemisphere, forming a single integrated timing system. Single axons could be traced through the brain because they contain pigment dispersing factor, visualised by fluorescence immunohistochemistry and confocal microscopy. A massive area of clock axon arborisations occurs in the anterior dorsal protocerebrum, from which radiate myriad axon branches, ramifying throughout the neuropiles or terminating among large groups of neuron somata. Such an extensive network of clock cell axons has not been seen before in any insect. This network could potentially drive rhythmicity both in behaviours and in release of neurohormones. The circadian system in larval Drosophila is rudimentary in comparison.
Supported by NSERC Discovery Grant 6669.

 


Neuroarchitecture of the clock system in the brain of Rhodnius prolixus adults and its association with neurosecretory cells

XANTHE VAFOPOULOU
Biology Department, York University, Toronto

We have shown that release of various hormonal neuropeptides from the brain of the insect Rhodnius prolixus is under circadian control, indicating their control by a clock in the brain. Previously, only behavioral rhythms were known to be controlled by insect brain clocks. Here, we describe the neuroarchitecture of the circadian system in the brain of adult Rhodnius and its association with neuroendocrine neurons. We used fluorescence immunohistochemistry for the neuropeptide pigment dispersing factor (PDF), a clock neuron marker, and confocal laser scanning microscopy to trace the axons of clock neurons through the brain. Compared with the larval stage, there are increases in the number of lateral clock neurons (LNs) and many new PDF-positive neurons appear which make contacts with these LNs, creating an extensive and elaborate network of PDF-positive axons potentially conveying rhythmicity throughout the entire dorsal protocerebrum. Crucially, LN axons make intimate contacts with the axons of various neuroendocrine cells, notably those of prothoracicotropic hormone in the lateral protocerebrum and bombyxin and LTE (Lymantrea Testis Ecdysiotropin) neurons in the medial protocerebrum, all of which are rhythmically released. We conclude the clock system controls rhythmic hormone release in a manner comparable to the mammalian suprachiasmatic nuclei.

 


Modulation of anoxic coma in locusts by the nitric oxide signaling pathway: A mechanism for regulating recovery from flash floods?

GARY A.B. ARMSTRONG and R. MELDRUM ROBERTSON
Department of Biology, Queen's University, Kingston ON

pending

 


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