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The development, morphology and genetics of sensory neurons have been extensively studied in Drosophila. Sensory neurons in the body wall of larval Drosophila in particular have been the subject of numerous anatomical studies, however, little is known about the intrinsic electrical properties of larval sensory cells.
We performed whole cell patch recordings from an identified peripheral sensory cell, the dorsal bipolar sensory neuron (dbd) and measured voltage-gated ionic currents in 1st instar larvae. Voltage clamp analysis revealed that dbds have a TEA sensitive, non-inactivating IK type potassium current as well as a 4-AP sensitive, inactivating IA type potassium current. dbds also show a voltage-gated calcium current (ICa) and a voltage-gated sodium current (INa).
This work provides a first characterization of voltage-activated ionic currents in an identified body-wall sensory neuron in larval Drosophila. Overall, we establish baseline physiology data for future studies aimed at understanding the ionic and genetic basis of sensory neuron function in fruit flies and other model organisms.
Muscles, central neurons, and peripheral sensory cells all play important roles in coordinating locomotion. To understand how a locomotor system functions, it is crucial to understand the anatomy of each component. But just knowing the anatomy is not enough. Since each component is composed of functionally linked, interdependent excitable cells, it is also essential to understand the underlying electrical properties of cells in all three network components
In Drosophila, a number of studies, performed in culture, have identified the different categories of voltage gated ion channels underlying intrinsic properties in neurons [1-5]. However, since these studies were done in culture, the identity of individual neurons could never be determined. Moreover, the characteristics of cultured neurons may not be representative of neurons in vivo [4-6]. Several studies addressed this problem in adult flies and developed techniques for recording from identified groups of neurons in acutely dissociated [7,8] and semi-intact preparations [9,10].
A separate line of work has focused on the embryonic and larval development of transmitter responses and intrinsic membrane currents in both muscles [11,12] and motor neurons [6,13,14]. In contrast to adult flies, to date, very little work has been done to characterize intrinsic properties of cells in the larval peripheral nervous system (PNS). The genetics and morphology of these cells have been studied in detail [15-17], yet researchers have lacked even the most basic information about the complement of ionic currents in larval PNS cells.
The dorsal bipolar dendrite sensory neurons (dbds) provide attractive targets for the study of sensory neuron physiology in larval Drosophila. Their anatomy has been extensively characterized [17,18]; furthermore, dbd cell bodies are easily identifiable at all developmental stages and accessible to electrophysiological recording. For these reasons, we targeted larval dbds for voltage-clamp analysis. The objective of the present study was to characterize the complement of voltage-activated currents of dbd neurons. Overall, we find that dbd neurons express multiple voltage-activated currents similar to those observed in the central nervous system (CNS) of Drosophila larvae.
Oregon-R flies were used for all electrophysiology experiments. To image dbd neuron morphology, we used larvae expressing green fluoresecent protein (GFP) in motor neurons and sensory neurons (genotype: C380-GAL4; UAS-mcd8GFP;+;+). Adults were reared at 25°C on standard media. Animals were kept on a roughly 12:12 light dark cycle.
We dissected 1st instar larvae in physiological saline on Sylgard (Dow Corning, USA) coated cover slips as published previously . Briefly, we positioned each larva dorsal side up, then used cyanoacrylate glue (Histoacryl, Braun, Melsungen, Germany) to fix the head and tail to the surface. Electrolytically sharpened tungsten needles were used to make an incision along the animal's dorsal surface. Gut and fat bodies were removed with gentle suction from a mouth pipette, then the cuticle was glued flat to the substrate. Care was taken not to disturb the CNS, visible nerves and body wall musculature.
Dissected preparations were mounted on the stage of a BX50WI compound microscope (Olympus, Center Valley, PA) in a custom-made plexiglass recording chamber. dbd neuron cell bodies were identified visually under a 63× water immersion lens. Muscles and neural sheath tissue covering dbd cell bodies were dissolved by local application of 0.2% protease (type XIV, Sigma-Aldrich, Dorset, UK) with a suction pipette as described previously . For maneuvering desheathing pipettes, we used a Narishige MHW-3 hydraulic micro-manipulator (Narishige International, London, UK).
For voltage clamp measurements of total whole-cell current, external solution contained (in mM): 135 NaCl, 5 KCl, 4 MgCl2, 2 CaCl2, 5 TES, and 36 sucrose, pH 7.1-7.2. CaCl2 was omitted and 1 μM TTX was added to the above solution to measure K+ currents. External solution for Ca2+ currents consisted of (in mM) 50 NaCl, 6 KCl, 50 BaCl2, 10 MgCl2, 10 glucose, 50 TEA-Cl, 10 HEPES, 10 4-AP, and pH 7.1-7.2. For Na+ current measurements, external saline was (in mM): 100 NaCl, 6 KCl, 2 MgCl2, 2 CaCl2, 0.2 CdCl2, 10 sucrose, 50 TEA-Cl, and10 4-AP, pH 7.1-7.2. For measurements of total whole-cell current, internal solution consisted of (in mM): 140 KCH3SO3, 2 MgCl2, 2 EGTA, 5 KCl, 20 HEPES. Internal solution for ICa and INa measurements was the same as above, but with 5 mM CsCl2 substituted for KCl.
5-10% rhodamine dextran was added to patch pipette tips in initial experiments to confirm the identities of patch-clamped dbds. We also examined dbd morphology using larvae expressing GFP in dbds. Dye filled and GFP labeled dbds were imaged with a AxioCam MRm digital camera (Carl Zeiss Ltd., Hertfordshire, UK) mounted on an Axiophot compound microscope (Carl Zeiss). Images were acquired using AxioVision 4 software (Carl Zeiss).
Whole cell voltage and current clamp recordings were performed with an Axopatch 1D amplifier (Molecular Devices, Sunnyvale, CA). Pipette resistances were 12.5-25 MΩ. Only cells with input resistances > 1GΩ were used for analysis. Mean ± SEM input resistance was 10 ± 1GΩ. For voltage clamp analysis, leak currents were subtracted before current measurements. All traces were sampled at 20 KHz and were digitized, stored and analyzed using pClamp 8.0.2 software running on a Dell desktop PC. Data were plotted using standard features in Excel (Microsoft, Redmond, WA). Final figures were made in Canvas 9 (Deneba, Victoria, CA).
To provide a baseline for future studies of sensory neuron function and development in Drosophila, we have developed a method for performing whole-cell patch clamp recordings from sensory neurons in the body wall of Drosophila larvae. As a first step, we have measured ionic currents in one type of larval sensory cell, the dbd neuron. Figure Figure1A1A shows a schematic of the location of various types of peripheral sensory neuron in the larval body wall. dbd neurons are shown in green; their dendrites span each hemi-segment in the dorsal muscle field. dbd cells send axonal projections through the intersegmental nerve to the dorsal and ventral neuropil regions of the ventral nerve cord [15,19]. For clarity, the dbd projection path is the only sensory projection path shown in Figure Figure1A.1A. Figure Figure1B1B shows a 1st instar animal expressing GFP in motor neurons and body wall sensory neurons. dbd cell bodies and distinctive bipolar dendrites are visible in multiple body segments (asterisks). Figure Figure1C1C shows a high magnification view of the center-most dbd in (B). dbd biopolar dendrites are visible (arrows); dbd morphology is distinct from that of other sensory cells (e.g. chordotonal organs, arrowhead). Figure Figure1D1D shows a 1st instar dbd (left) in the process of being filled with rhodamine dextran contained in a patch pipette (right). Because of their distinctive dendritic morphology, individual dbd neurons were readily distinguishable from other cells under DIC illumination and easily targeted for recording.
Figure Figure1E1E shows a whole cell current clamp recording from a larval dbd neuron. In response to depolarizing current injection, the cell fires action potentials. Figure Figure1F1F shows current traces in response to a series of voltage clamp steps in a larval dbd neuron. Depolarizing steps from -90 mV to 60 mV in 20 mV increments evoke transient outward currents and sustained slow outward currents typical of IA and IK type K+ channels, respectively. These large outward currents dominate the cellular response in these conditions; as a result, slow inward Ca2+ currents are obscured. However, before outward currents predominate, fast inward currents typical of voltage activated Na+ channels are visible (arrow). At some voltage steps, unclamped action currents are visible (Figure (Figure1F,1F, asterisks). The presence of these events suggests that voltage control in dendritic and/or axonal compartments of the neurons is incomplete. As in other neuron types with complex cellular geometries, measurement of voltage-activated currents in the dbd soma may be affected by unclamped currents in distant cellular compartments. All whole cell currents were abolished in the presence of 1 μM TTX (Na+ channel blocker), 10 mM 4-AP (IA blocker), 50 mM TEA (Ik blocker), and 0.2 mM external cadmium (Ca2+ channel blocker) (Figure (Figure1G).1G). Figure Figure1H1H shows the voltage clamp step protocol used in Figures 1F, G.
Our initial experiments revealed that dbd neurons display prominent transient and persistent outward currents upon depolarization. To examine the outward K+ currents underlying these responses, we performed recordings in saline containing 1 μM TTX and 0 mM Ca2+ to block Na+ channels and Ca2+ channels, respectively. In order to separate individual K+ currents, we exploited the differential voltage dependence of inactivation between Ik and IA. To isolate Ik, we held dbd resting potential at -20 mV, then measured evoked currents through a range of voltages from -80 to +55 mV in 15 mV steps. Previous work has shown that IA is inactivated at -20 mV in embryonic and larval motor neurons [6,13]. Figure Figure2A2A shows an example of typical Ik currents in a larval dbd; the responses show slow activation and little or no signs of inactivation. Previous work has shown that IA in embryonic motor neurons is released from inactivation at -90 . To measure IA, we held dbds at -90 mV, then stepped to the same voltages used to measure Ik (Figure (Figure2B).2B). Subtracting Ik at each step revealed current due to IA channels. (Figure (Figure2C).2C). To evaluate the fraction of Ik inactivation at -20 mV, we compared steady-state current levels at the end of IA subtraction traces to measured Ik currents. We found that the fraction of Ik inactivation was under 1% in all preparations. Figure 2D, E shows current voltage (I-V) relationships for Ik and IA, respectively. Currents are normalized to maximal current (I/Imax). I-V plots show that both Ik and IA begin activating between -35 and -20 mV (n = 7).
Under normal recording conditions, outward K+ currents overwhelm inward Ca2+ currents during voltage clamp experiments. Therefore, to measure ICa in dbds, we performed voltage clamp experiments with pharmacological blockers for Ik (50 mM TEA) and IA (10 mM 4-AP) and Na+ (1 μM TTX) channels in the external solution. We also substituted cesium for K+ in our internal solution to internally block K+ channels. Finally, we substituted Ba2+ for Ca2+ in our bath solution to prevent activation of Ca2+ dependent K+ channels. Ba2+ also prevents Ca2+ channel inactivation. It is important to note that because of the presence of Ba2+ in our external solution, the currents we measure in these experiments are not Ca2+ currents per se; previous work has used ICa(Ba) currents provide estimates of voltage-gated Ca2+ channel activation parameters [6,20]. Figure Figure3A3A shows typical ICa(Ba) currents in response to depolarizing steps from -90 to +60 mV (Figure (Figure3B).3B). At some voltages, unclamped inward currents are visible; this is probably caused by incomplete voltage control of distally located Ca2+ channels. Figure Figure3C3C shows the I-V relationship for ICa(Ba) in larval dbds. Currents are normalized to maximal current (I/Imax). The current begins to activate at -50 to -40 mV, and reaches peak amplitude at 5-15 mV (n = 8).
We isolated INa by blocking K+ channels with 10 mM 4-AP and 50 mM TEA as well as recording in Ca2+ free saline. Cesium was used instead of K+ in our patch solution to internally block K+ channels. INa currents sometimes escaped voltage control, indicating that (as in central neurons) spike initiation zones are probably located outside the cell body. However, using online leak subtraction protocols, and careful monitoring of input resistance and voltage clamp parameters, we were able to accurately measure INa in multiple larval dbd neurons. Figure Figure4A4A shows typical Na+ currents evoked by a series of depolarizing voltage steps (Figure (Figure4B).4B). Currents rapidly activated and inactivated within 10 ms. Figure Figure4C4C shows the normalized I-V relationship for INa in larval dbds (n = 8). These data show that in dbds, INa begins to activate at -50 to -40 mV and reaches peak amplitude at -30 to -20 mV.
In this study, we have presented measurements of voltage-gated ionic currents in dbd, an identified Drosophila larval sensory neuron. Larval dbd neurons generate action potentials and express a range of voltage activated channels, including transient and non-inactivating K+ channels, Ca2+ channels, and Na+ channels. Our study represents a technical advance in recording techniques and adds to the growing body of work aimed at understanding the biophysical properties of Drosophila neurons.
dbd neurons and Drosophila larval motor neurons both contain a similar complement of voltage gated currents. dbds and motor neurons do not show major differences in activation thresholds for voltage-gated K+, Ca+ and Na+ ion channels [6,13,20]. However, detailed quantitative comparison of activation and inactivation parameters in the two cell types is complicated by the fact that neither cell type is electrotonically compact. Distal areas of both cells are difficult to fully control during voltage clamp experiments; this inevitably leads to errors in current parameter measurements. Ionic current parameters aside, dbds do differ from motor neurons in one important respect: unlike motor neurons, dbds do not show any endogenous tonic spiking and/or rhythmic activity (data not shown).
The dbds are one of many peripheral sensory neuron subtypes that provide proprioceptive feedback into the larval ventral nerve cord in Drosophila during locomotion. This feedback is crucial for generating appropriate locomotor rhythms. Embryos lacking sensory neurons develop, but fail to hatch . When transmitter release is inhibited in the embryonic peripheral (via expression of tetanus toxin), embryos hatch and coordinated locomotor patterns are present, albeit significantly slowed . When feedback from sensory neurons is acutely inhibited in larval life, animals show severe locomotor defects . Conversely, if larval sensory neurons are acutely hyperexcited, locomotion is also inhibited . These and other studies have provided insight into the overall role of PNS neurons, but to date, the function of dbd neurons in Drosophila is not clear.
Two lines of evidence suggest that dbds act as stretch receptors in the larval body wall. First, dbd dendrites span the length of each hemi-segment, and are well positioned anatomically to provide information about hemi-segment tension. Second, neurons homologous to dbds are known to be mechanoreceptors in other insects. For example, the stretch receptor organ (SRO) in Manduca sexta is composed of segmentally repeating neurons with bipolar dendrites similar to those seen in dbds. SROs fire action potentials in response to mechanical stretching of the caterpillar body wall [24,25]. They appear to provide feedback on the overall tension of each segment during caterpillar locomotion . Whether dbds serve an identical function in Drosophila remains an open question.
Numerous studies have examined how genes influence the development of cellular morphology in the larval peripheral nervous system. But to date, very little work has been done to characterize how these genes affect sensory cell physiology through development. The present study provides a foundation for future work aimed at understanding how gene function regulates both the morphology and cellular physiology of neurons in the peripheral nervous system.
The authors declare that they have no competing interests.
AN and MB designed experiments. AN performed whole-cell patch experiments and analyzed data. SRP performed anatomy experiments, analyzed data, prepared figures, and wrote the manuscript. All authors read and approved the final manuscript.
We would like to thank Richard Baines for technical assistance with whole-cell patch technique and Astrid Prinz for critical reading of the manuscript. This work was supported by grants from the Cambridge Commonwealth Trust (Nehru Cambridge Scholarship and Overseas Research Studentship to AN), the Wellcome Trust (Programme Grant 075934 to MB) and the Royal Society (Newton International Fellowship to SRP). MB is a Royal Society Research Professor.