Behaviorally adequate neuronal firing patterns are critically dependent on the specific types of ion channel expressed and on their subcellular localization. This study combines electrophysiology with genetic and pharmacological intervention in larval of both sexes to address localization and function of L-type like calcium channels in motoneurons. We demonstrate that Dmca1D (Ca1 homolog) L-type like calcium channels localize to both the somatodendritic and the axonal compartment of larval crawling motoneurons. patch-clamp recordings in genetic mosaics reveal that Dmca1D channels increase burst duration and maximum intraburst firing frequencies during crawling-like motor patterns in semi-intact animals. Genetic and acute pharmacological manipulations suggest that prolonged burst durations are caused by dendritically localized Dmca1D channels, which activate upon cholinergic synaptic input and amplify EPSPs, thus indicating a conserved function of dendritic L-type channels from to vertebrates. By contrast, maximum intraburst firing rates require axonal calcium influx through Dmca1D channels, likely to enhance sodium channel de-inactivation via a fast afterhyperpolarization through BK channel activation. Therefore, in unmyelinated motoneurons different functions of axonal and dendritic L-type like calcium channels likely operate synergistically to maximize firing output during locomotion. Nervous system function depends on the specific excitabilities of different types of neurons. Excitability is largely shaped by different combinations of voltage-dependent ion channels. Despite a high degree of conservation, the huge diversity of ion channel types and their differential localization pose challenges in assigning distinct functions to specific channels across species. We find a conserved role, from fruit flies to mammals, for L-type calcium channels in augmenting motoneuron excitability. As in spinal cord, dendritic L-type channels amplify excitatory synaptic input. In contrast to spinal motoneurons, axonal L-type channels enhance firing rates in unmyelinated motoraxons. Therefore, enhancing motoneuron excitability by L-type channels seems an old strategy, but localization and interactions with other channels are tuned to species-specific requirements.

Key pointsr We combine in situ electrophysiology with genetic manipulation in Drosophila larvae aiming to investigate the role of fast calcium-activated potassium currents for motoneurone firing patterns during locomotion.r We first demonstrate that slowpoke channels underlie fast calcium-activated potassium currents in these motoneurones.r By conducting recordings in semi-intact animals that produce crawling-like movements, we show that slowpoke channels are required specifically in motoneurones for maximum firing rates during locomotion.r Such enhancement of maximum firing rates occurs because slowpoke channels prevent depolarization block by limiting the amplitude of motoneurone depolarization in response to synaptic drive. In addition, slowpoke channels mediate a fast afterhyperpolarization that ensures the efficient recovery of sodium channels from inactivation during high frequency firing.r The results of the present study provide new insights into the mechanisms by which outward conductances facilitate neuronal excitability and also provide direct confirmation of the functional relevance of precisely regulated slowpoke channel properties in motor control.Abstract A large number of voltage-gated ion channels, their interactions with accessory subunits, and their post-transcriptional modifications generate an immense functional diversity of neurones. Therefore, a key challenge is to understand the genetic basis and precise function of specific ionic conductances for neuronal firing properties in the context of behaviour. The present study identifies slowpoke (slo) as exclusively mediating fast activating, fast inactivating BK current (I CF ) in larval Drosophila crawling motoneurones. Combining in vivo patch clamp recordings during larval crawling with pharmacology and targeted genetic manipulations reveals that I CF acts specifically in motoneurones to sculpt their firing patterns in response to a given input from the central pattern generating (CPG) networks. First, I CF curtails motoneurone postsynaptic depolarizations during rhythmical CPG drive. Second, I CF is activated during the rising phase of the action potential and mediates a fast afterhyperpolarization. Consequently, I CF is required for maximal intraburst firing rates during locomotion, probably by allowing recovery from inactivation of fast sodium channels and decreased potassium channel activation. This contrasts the common view that outward conductances oppose excitability but is in accordance with reports on transient BK and Kv3 channel function in multiple types of vertebrate neurones. Therefore, our finding that I CF enhances firing rates specifically during bursting patterns relevant to behaviour is probably of relevance to all brains.

Dendrites are highly complex 3D structures that define neuronal morphology and connectivity and are the predominant sites for synaptic input. Defects in dendritic structure are highly consistent correlates of brain diseases. However, the precise consequences of dendritic structure defects for neuronal function and behavioral performance remain unknown. Here we probe dendritic function by using genetic tools to selectively abolish dendrites in identified Drosophila wing motoneurons without affecting other neuronal properties. We find that these motoneuron dendrites are unexpectedly dispensable for synaptic targeting, qualitatively normal neuronal activity patterns during behavior, and basic behavioral performance. However, significant performance deficits in sophisticated motor behaviors, such as flight altitude control and switching between discrete courtship song elements, scale with the degree of dendritic defect. To our knowledge, our observations provide the first direct evidence that complex dendrite architecture is critically required for fine-tuning and adaptability within robust, evolutionarily constrained behavioral programs that are vital for mating success and survival. We speculate that the observed scaling of performance deficits with the degree of structural defect is consistent with gradual increases in intellectual disability during continuously advancing structural deficiencies in progressive neurological disorders.dendrite | synapse | motor behavior | courtship | Drosophila

Development of neural circuitry relies on precise matching between correct synaptic partners and appropriate synaptic strength tuning. Adaptive developmental adjustments may emerge from activity and calcium-dependent mechanisms. Calcium/calmodulin-dependent protein kinase II (CaMKII) has been associated with developmental synaptic plasticity, but its varied roles in different synapses and developmental stages make mechanistic generalizations difficult. In contrast, we focused on synaptic development roles of CaMKII in a defined sensory-motor circuit. Thus, different forms of CaMKII were expressed with UAS-Gal4 in distinct components of the giant fiber system, the escape circuit of Drosophila, consisting of photoreceptors, interneurons, motoneurons, and muscles.The results demonstrate that the constitutively active CaMKII-T287D impairs development of cholinergic synapses in giant fiber dendrites and thoracic motoneurons, preventing light-induced escape behavior. The locus of the defects is postsynaptic as demonstrated by selective expression of transgenes in distinct components of the circuit. Furthermore, defects among these cholinergic synapses varied in severity, while the glutamatergic neuromuscular junctions appeared unaffected, demonstrating differential effects of CaMKII misregulation on distinct synapses of the same circuit. Limiting transgene expression to adult circuits had no effects, supporting the role of misregulated kinase activity in the development of the system rather than in acutely mediating escape responses. Overexpression of wild-type transgenes did not affect circuit development and function, suggesting but not proving that the CaMKII-T287D effects are not due to ectopic expression. Therefore, regulated CaMKII autophosphorylation appears essential in central synapse development, and particular cholinergic synapses are affected differentially, although they operate via the same nicotinic receptor.

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