Dendrites are dispensable for basic motoneuron function but essential for fine tuning of behavior

Abstract

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.

Keywords: Drosophila; courtship; dendrite; motor behavior; synapse.

Fig. 1.

Selective manipulation of dendrites. (A) Schematic of Drosophila dorsal longitudinal wing depressor muscle and its innervation by motoneurons, MN1–5. (B) Representative image of MN5 dendritic structure in a control. (C) MN5 structure after targeted RNAi knockdown of Dscam1. (D) Schematic of DLM fibers with MN1–5 axon arbors. (E and F) MN1–5 axonal projections and collateral branches are similar in controls (E) and after dendrite elimination (F). (G) Schematic of the Drosophila neuronal escape circuit. Visual input is relayed to the GF interneuron that makes a mixed electrical chemical synapse onto the PSI, which in turn bypasses all dendrites and synapses onto the axons of MN1–5. (H) Postsynaptic responses in the DLM muscle after GF stimulation are identical in controls and after genetic elimination of most motoneuron dendrites, indicating normal speed of action potential propagation and synaptic transmission. (I–L) Reliability of synaptic transmission is not affected by dendritic defects. Refractory period remains unaltered (J); the pathway follows stimulation frequencies of approximately 130 Hz with 100% reliability (K) and up to 200 Hz with 50% reliability (L). (M) MN5 voltage-gated Ca²⁺ currents are qualitatively similar in controls and with 90% dendrite reduction, both with respect to low-voltage activated (LVA) (N) and high-voltage activated (HVA) currents (O). (P) Activation voltages of LVA and HVA currents are identical in controls (n = 22) and manipulated neurons (n =15), but current amplitudes as recorded from the soma are larger in neurons with significantly reduced dendrites. (Q and R) A-type (Q) and sustained (R) K⁺ currents are qualitatively similar in controls and MN5 with dendritic defects. (S) Activation voltages of transient and sustained K⁺ currents are not affected by Dscam1 RNAi. Transient K⁺ current is identical to controls (n = 31), but sustained K⁺ current displays a larger amplitude in MN5 with defective dendrites (n = 16). (T) Positive phototaxis and negative geotaxis responses (U) are normal after targeted expression of Dscam1 RNAi under the control of C380-GAL4;; Cha-GAL80.

Fig. 2.

Dendrites are dispensable for synaptic input to wing motoneurons, and manipulated animals can fly. (A) Flies with Dscam1 RNAi-induced dendritic defects in wing motoneurons readily engage in tethered flight. (B–D) Significant dendritic defects cause only mild flight behavioral deficits. (B) Total flight duration is decreased in animals with wing motoneuron dendritic defects (dark gray bar) compared with control (white and light gray bars). (C and D) Both the number of flight initiations upon wind stimuli to the head (C) and the mean duration of individual flight bouts (D) are not reduced significantly. (E) In MN5 with defective dendrites lamellipodia-like structures that decorate the primary neurite are in direct proximity to immunolabeled presynaptic active zones of other neurons. White boxes show regions for which selective enlargements of single optical sections are shown (i, ii, lamellipodia-like structures in green, active zones in magenta). Dotted white lines in i encircle active zone. Dotted white lines in ii depict outline of lamellilpodia. (F–I) Picoinjection of nicotine (10⁻⁵ M in saline, 3-ms injection duration) into MN5 lamellipodia regions reliably evokes postsynaptic potentials as recorded from the soma both in controls (F) and without any normal dendrites (G). Recordings are performed in 10⁻⁷ M TTX to exclude possible indirect effects via excitation of other neurons. (H and I) Overlays of eight sweeps (black traces) and average postsynaptic responses (red traces) display larger amplitudes in manipulated animals than in controls, because signal attenuation to the somatic recording site is reduced. (J–M) Picoinjections of GABA (10⁻⁴ M in saline, 3-ms injection duration) show that MN5 lamellipodia express GABA_A receptors in controls (J and L) and after Dscam1 RNAi-induced dendritic defects (K and M). (N and O) Longer-duration injections (2 s) of nicotine in the absence of TTX reliably evoke action potentials in controls (N) and after dendrite elimination (O), demonstrating that even in the absence of all normal dendrites strong cholinergic excitation is sufficient to drive motoneuron firing.

Fig. 3.

Dendritic defects scale with specific flight defects. (A–C) Compared with controls (A), Dscam1 RNAi reduces MN5 dendrites either by approximately 50% (B) or by 90% (C). (D–H) Independent of the dendritic defect, MN5 fires tonically during tethered flight (E, control; F, 50% dendrite reduction; G, 90% dendrite reduction). Dendritic defect severity scales with decreases in average firing frequencies (D), as do deviations from a constant interspike interval (H). (I–P) Spike frequency modulations in response to optomotor input scale down significantly with the severity of dendritic defects. (L) Presenting vertically moving stripes simulates rising (downward stripes) or decreasing (upward stripes) flight altitude, resulting in MN5 firing frequency modulations in control flies (I). These also occur with a 50% dendrite reduction (J) but are strongly reduced with 90% dendrites absent (K). (M–O) Mean firing frequency modulations (eight animals per group) during downs (gray) and ups (white); M, control; N, 50% dendrite reduction; O, 90% dendrite reduction. ΔF signifies maximum frequency modulations and Δt the durations between minimum and maximum frequencies. (P) ΔF is significantly reduced only with a reduction in dendrites by more than 90%. Δt scales significantly with the dendritic defect severity (ANOVA, Newman Keuls post hoc test, P ≤ 0.05).

Fig. 4.

Song quality and mating success scale with the severity of dendritic defects. (A–C) During courtship song the male fly orients toward the female, spreads out one wing (A), and produces two distinctly different song elements by up and down wing movements. (B) Sound patterns with low amplitude and an average frequency of 155 ± 10 Hz are named sine song, whereas high-amplitude sound pulses with an average IPI of 33 ± 3 ms are named pulse song. (C) Animals with a 50% reduction in wing depressor motoneuron dendrites produce normal sine and pulse song elements. (D) Even with 90% dendrite reduction sine and pulse song frequencies and amplitudes are similar to controls. (E) However, mating success within 20 min decreases from 96% in controls (white), over 64% with 50% dendrite reduction (light gray) to 21% with 90% dendrite reduction (dark gray). Thus, mating failure rates scale with the dendritic defect severity. (F) Similarly, the time to mating increases significantly with the dendritic defect severity. (G) However, all three groups spend the same amounts of time singing. (H and I) IPI (H) and sine song frequency (I) are statistically identical in all groups. (J) The average sine bout duration is significantly increased in animals with Dscam1 RNAi expression. (K) Summing up all sine and pulse song bouts reveals a statistically significant increase of the sine to pulse song ratio with 50% dendrite reduction compared with controls. In animals with 90% reduction of wing motoneuron dendrites the sine to pulse song ratio is significantly further increased (Kruskal Wallis ANOVA with post hoc U tests).