Transient BK outward current enhances motoneurone firing rates during Drosophila larval locomotion

Abstract

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.

Altered synaptic plasticity and central pattern generator dysfunction in aDrosophilamodel of PNKD3 paroxysmal dyskinesia

“…3C-F). Interestingly, these data are mirrored by prior work showing that reducing SLO expression enhances the frequency of CPG-induced motoneuron bursts 29 . Thus, loss and gain of BK channel function results in bidirectional alterations in the endogenous motoneuron firing in Drosophila larvae.…”

“…The function of pre-motor circuits and motoneurons driving larval locomotion can be interrogated via sharp electrode recordings and ex vivo calcium imaging [29][30][31] . Furthermore, the neuromuscular junction (NMJ) of Drosophila larvae represents a widely studied model glutamatergic synapse in which alterations in neurotransmitter release and/or synaptic plasticity can be readily assessed 32 .…”

Tyramine action on motoneuron excitability and adaptable tyramine/octopamine ratios adjust Drosophila locomotion to nutritional state

“…The firing responses of larval RP2 MNs to somatic current injections were recorded in current clamp mode. RP2 responded to square-pulse current injection with a characteristic biphasic depolarization and a marked delay to the first action potential, both of which were previously described (23)(24)(25). A representative example trace for current injection just above firing threshold is shown before any pharmacological manipulation ( Fig.…”

“…Ca 2+ currents were recorded in a voltage clamp as previously described (30). Transient and sustained K + outward currents were separated as in Kadas et al (25). Ca 2+ imaging was conducted as in Ryglewski et al (43).…”

BK channels and a cGMP-dependent protein kinase (PKG) function through independent mechanisms to regulate the tolerance of synaptic transmission to acute oxidative stress at theDrosophilalarval neuromuscular junction

“…Slo 4 mutants on a w 1118 genetic background were used as BK channel loss-of-function mutants (Atkinson, Robertson, & Ganetzky, 1991). Slo 4 is a gamma ray-induced chromosomal inversion on the third chromosome between 96A17 and 96F5-8 (Atkinson et al, 1991;Kadas, Ryglewski, & Duch, 2015). The Slo 4 mutants were gifted by Dr. Gregory Macleod, but were originally obtained from Dr. Nigel Atkinson.…”