Title : A PI 3 K - mediated negative feedback regulates Drosophila motor neuron excitability

Negative feedback processes, which can enable maintenance of neuronal homeostasis, are widely observed in neuronal systems 1-3. For example, neuronal silencing via tetrodotoxin application both in vivo and in vitro increases excitability 4-6. This effect occurs in vitro via both increased sodium currents and decreased potassium currents. However, the signalling pathways responsible for these excitability changes remain unclear. The mammalian type II metabotropic glutamate receptors, which are G-protein coupled receptors activated by glutamate, are well positioned to mediate negative feedback. When localized presynaptically, these receptors can act as autoinhibitors of glutamate release 7-10. Because these receptors are located outside of the active zone 11 , activation is thought to occur only during conditions of elevated glutamate release and might serve to prevent glutamate-mediated neurotoxicity. Agonists for these receptors are proposed for treatment of schizophrenia, anxiety and epilepsy, among others 12, 13 , but although many of the acute effects of type II mGluR activation on neuronal physiology have been elucidated 14, 15 , possible long term effects on neuronal function, such as through changes in ion channel gene expression, remain essentially unexplored. The Drosophila mGluRA affects the rate of onset of long-term facilitation (LTF), a reporter for motor neuron excitability: In Drosophila, the single mGluRA 2 gene encodes a protein most similar to the mammalian type II mGluR 16. mGluRA is located presynaptically at the neuromuscular junction (nmj), which suggests that mGluRA might regulate transmitter release from motor neurons. Elimination of mGluRA by the null mutation mGluRA 112b affects a form of synaptic plasticity termed long-term facilitation (LTF) 16, 17 , which is induced when a motor neuron is subjected to repetitive nerve stimulation at low bath [Ca 2+ ]. At a certain point in the stimulus train, an abrupt increase in transmitter release and hence muscle depolarization (termed excitatory junctional potential, or ejp) is observed (Figure 1A). This abrupt increase is caused by an abrupt increase in the duration of nerve terminal depolarization and hence Ca 2+ influx, and reflects a progressive increase in motor neuron excitability induced by the repetitive nerve stimulation: when an excitability threshold is reached, LTF occurs 17-19. In Drosophila, many genotypes that increase motor neuron excitability by decreasing K + currents or increasing Na + currents increase the rate of onset of LTF. For example, altered activities of frequenin and Hyperkinetic, which act via K + channels, or paralytic and pumilio, which act via …