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InDrosophila, high-frequency electrical stimulation across the brain triggers a highly stereotypic repertoire of spasms known as electroconvulsive seizures (ECS). The distinctive ECS spiking discharges manifest across the nervous system and can be stably assessed throughout the seizure repertoire in the large indirect flight muscles (DLMs). ECS discharges in DLMs have been extensively used to monitor seizure activities, notably in stress (‘bang’)-sensitive mutants. However, the relationships between ECS-spike patterns and native motor programs, including flight and grooming, are not known and their similarities and distinctions remain to be characterized. We employed quantitative spike pattern analyses including: 1) overall firing frequency, 2) spike timing between contralateral fibers, and 3) short-term variability in spike interval regularity (CV2) and instantaneous firing frequency (ISI−1) to reveal distinctions amongst ECS discharges, flight and grooming motor patterns. We then examined DLM firing phenotypes in well-established mutants in excitatory cholinergic (Cha), inhibitory GABAergic (Rdl) and electrical (ShakB) synaptic transmission. The results provide an initial glimpse on the vulnerability of individual motor patterns to perturbations of respective synaptic transmission systems. We found marked alterations of ECS discharge spike patterns in terms of either seizure threshold, spike frequency or spiking regularity. In contrast, no gross alterations during grooming and only a minor reduction of firing frequency during Rdl mutant flight were observed, suggesting a role for GABAergic modulation of flight motor programs. Picrotoxin (PTX), a known pro-convulsant that inhibits GABAAreceptors, induced DLM seizure patterns that displayed some features, e.g. left-right coordination and ISI−1range, that could be found in flight or grooming, but distinct from ECS discharges. Our results indicate that these quantitative techniques may be employed to reveal overlooked relationships among aberrant motor patterns and native DLM motor programs in genetic and pharmacological analyses of underlying cellular and neural circuit function.

A seizure-paralysis repertoire characteristic of Drosophila “bang-sensitive” mutants can be evoked electroconvulsively in tethered flies, in which behavioral episodes are associated with synchronized spike discharges in different body parts. Flight muscle DLMs (dorsal longitudinal muscles) display a stereotypic sequence of initial and delayed bouts of discharges (ID and DD), interposed with giant fiber (GF) pathway failure and followed by a refractory period. We examined how seizure susceptibility and discharge patterns are modified in various K+and Na+channel mutants. Decreased numbers of Na+channels in naptsflies drastically reduced susceptibility to seizure induction, eliminated ID, and depressed DD spike generation. Mutations of different K+channels led to differential modifications of the various components in the repertoire. Altered transient K+currents in Sh133and Hk mutants promoted ID induction. However, only Sh133but not Hk mutations increased DD seizure and GF pathway failure durations. Surprisingly, modifications in sustained K+currents in eag and Shab mutants increased thresholds for DD induction and GF pathway failure. Nevertheless, both eag and Shab, like Sh133, increased DD spike generation and recovery time from GF pathway failure. Interactions between channel mutations with the bang-sensitive mutation bss demonstrated the role of membrane excitability in stress-induced seizure-paralysis behavior. Seizure induction and discharges were suppressed by naptsin bss nap double mutants, whereas Sh heightened seizure susceptibility in bss Sh133and bss ShMdouble mutants. Our results suggest that individual seizure repertoire components reflect different neural network activities that could be differentially altered by mutations of specific ion channel subunits.

Drosophila bang-sensitive mutants display a remarkable stereotyped behavioral sequence during mechanical disturbances. This seizure repertoire consists of initial and delayed bouts of spasm interposed with paralysis and followed by recovery of activity and a period of refractoriness to further stimulation. Electroconvulsive stimuli across the brain induced a similar seizure behavior in tethered flies, in which corresponding electrophysiological events could be readily recorded in indirect flight muscles [dorsal longitudinal muscles (DLMs)] of the giant fiber (GF) pathway. The DLM physiological repertoire consisted of initial and delayed discharges (IDs and DDs), a response failure and recovery, followed by a refractory period. Interestingly, wild-type flies also displayed the same electroconvulsive repertoire, albeit inducible only at higher stimulus intensities and with briefer expression. The DLM repertoire presumably originated from activities of distinct neural circuits subserving normal function and reflected the general sequence of excitation and depression of the nervous system as a whole, as shown by simultaneous recordings along the different body axes. The well characterized GF pathway facilitated localization of circuits responsible for response failure and ID and DD motor patterns by surgical manipulations, recording-stimulating site analysis, and genetic mosaic studies. A flight pattern generator is most likely the major contributor to shaping the DD pattern, with modifications by active integration of individual motor neurons and associated interneurons. The robust electroconvulsive repertoire of DLMs provides a convenient window for further genetic analysis of the interacting neural mechanisms underlying a stereotyped action pattern in Drosophila, which shows striking parallels with aspects of seizure in mammalian species.

The role of the conserved Focal Adhesion Kinase (FAK) family of protein tyrosine kinases (PTKs) in the development and physiological functions of the CNS has long been an area of interest among neuroscientists. In this report, we observe that Drosophila mutants lacking Fak56 exhibit a decreased life span, accompanied by a bang-sensitive phenotype, which is characterised by sensitivity to mechanical and high-frequency electrical stimulation. Fak56 mutant animals display lower thresholds and higher rates of seizures in response to electroconvulsive stimuli, and direct measurements of action potential conduction in larval segmental nerves demonstrate a slowed propagation speed and failure during high-frequency nerve stimulation. In addition, neuromuscular junctions in Fak56 mutant animals display transmission blockade during high-frequency activity as a result of action potential failure. Endogenous Fak56 protein is abundant in glial cells ensheathing the axon bundles, and structural alterations of segmental nerve bundles can be observed in mutants. Manipulation of Fak56 function specifically in glial cells also disrupts action potential conduction and neurotransmission, suggesting a glial component in the Fak56 bang-sensitive phenotype. Furthermore, we show that increased intracellular calcium levels result in the dephosphorylation of endogenous Fak56 protein in Drosophila cell lines, in parallel with our observations of highly variable synaptic potentials at a higher Ca2+ level in Fak56 mutant larvae. Together these findings suggest that modulation of Fak56 function is important for action potential propagation and Ca2+-regulated neuromuscular transmission in vivo.