Functional role of the neurotransmitter glutamate in CNS of Apis mellifera

LOCATELLI F; BUNDROCK G; MUELLER U

Goettingen, Alemania.

Congreso; XI Meeting of the German Neuroscience Society & XXX Göttingen Neurobiology Conference; 2005

German Neuroscience Society

Functional role of the neurotransmitter glutamate in CNS of Apis mellifera Fernando Locatelli1, Gesine Bundrock1 & Uli Müller1,2 1 Institut für Neurobiologie, Freie Universität, Berlin 2 Zoologie-Physiologie, Universität des Saarlandes, Saarbrücken Previous works aimed at studying the role of the neurotransmitter glutamate in learning and memory in Apis mellifera were based on interfering glutamate recycling. These works lead to controversial interpretations of glutamate function, since different drugs that are expected to block glutamate recycling demonstrated somehow opposite effects on memory, either amnesic or facilitatory. In order to find an explanation for this controversy, we tested the effectiveness of the drugs to modulate glutamate levels. Given also the quite opposite effects of some of these drugs in memory and the tight relation between glutamate and GABA metabolisms, effect of the drugs on both neurotransmitters were tested. Glutamate and GABA levels were measured in CNS after injection of drugs reported to induce the amnesic and the facilitatory effects: a) inhibitors of plasma-membrane glutamate transporters, which are directly involved in removing glutamate from the synaptic cleft; b) inhibitors of the enzyme glutamine-synthetase, which converts glutamate into glutamine in glial cells and constitutes a critical step in glutamate re-uptake. Results demonstrate that while drugs reduce glutamate levels in the insect brain, both kinds of drugs induce opposite changes in GABA levels, probably explaining the reported differential modulation of learning processes. In order to gain specificity, temporal and spatial resolution, we designed an alternative pharmacological approach based in the photo-release of glutamate at specific brain sites. The role of glutamate was evaluated in a non associative form of learning, the habituation of proboscis extension reflex by repetitive sucrose-stimulation of the antennae. Animals were injected systemically either vehicle or caged-Glutamate and neurotransmitter release was induced by light-flashes directed either to the mushroom bodies, or antennal lobes (AL). Glutamate release was assessed at different times respect to the stimulation protocol in order to evaluate a role of the neurotransmitter in habituation and dishabituation processes. Results showed that glutamate release into the contra-lateral AL respect to the stimulated antennae enhances the dishabituating effect of sucrose stimulation of the contra-lateral antennae. This result constitutes the first direct evidence of a functional role of glutamate in CNS in Apis mellifera and suggests a role of this neurotransmitter in the sucrose signaling pathway. The observation that the effects were observed only after stimulation of specific brain sites and in precise temporal relation with the stimuli, argues in favor of the photo-release of glutamate as an effective and precise tool to further study the functional role of glutamate in CNS of insects. Supported by DAAD, Fundacion Antorchas and SFB-515(DFG). Keywords: Glutamate, GABA, honeybee, learning, photo-release