Firing dynamics and modulatory actions of supraspinal dopaminergic neurons during zebrafish locomotor behavior

Abstract

Background: Dopamine (DA) has long been known to have modulatory effects on vertebrate motor circuits. However, the types of information encoded by supraspinal DAergic neurons and their relationship to motor behavior remain unknown.

Results: By conducting electrophysiological recordings from awake, paralyzed zebrafish larvae that can produce behaviorally relevant activity patterns, we show that supraspinal DAergic neurons generate two forms of output: tonic spiking and phasic bursting. Using paired supraspinal DA neuron and motoneuron recordings, we further show that these firing modes are associated with specific behavioral states. Tonic spiking is prevalent during periods of inactivity while bursting strongly correlates with locomotor output. Targeted laser ablation of supraspinal DA neurons reduces motor episode frequency without affecting basic parameters of motor output, strongly suggesting that these cells regulate spinal network excitability.

Conclusions: Our findings reveal how vertebrate motor circuit flexibility is temporally controlled by supraspinal DAergic pathways and provide important insights into the functional significance of this evolutionarily conserved cell population.

Figure 1

Identification of DDNs in the Diencephalon of ETvmat2:GFP Zebrafish Larvae at 4 dpf (A) Ventral (left) and lateral (right) schematic overview of DC2 (dark green) and DC4/5 (light green) neurons in the diencephalon (dashed line represents the anterior diencephalic border) of 4 dpf larvae. (B) Ventral view of GFP-labeled cells in the diencephalon. Prospective DDNs in DC2 comprise a small cluster of strongly fluorescent, large-diameter cells (dashed boxes) near the anterior aspect of the diencephalon. (C) Lateral images of the posterior tuberculum labeled with GFP (green, top) and anti-tyrosine hydroxylase antibodies (Anti-TH, yellow, middle). Merged image (bottom) reveals that all large, GFP-positive neurons in the anterior aspect of posterior tuberculum are immunoreactive for TH. (D) Lateral view of a GFP-expressing neuron in the anterior posterior tuberculum (green) labeled with neurobiotin (NB, red) and co-stained with anti-tyrosine hydroxylase antibodies (Anti-TH, yellow). Merged image (bottom right) reveals NB-labeled cells are GFP and TH positive. (E and F) Lower-magnification contrast inverted composite images of the same NB-labeled neuron in (D) depicting the soma and primary axon in the diencephalon (E; arrows) and the primary axon (F; arrows) extending from the hindbrain (HB) to the spinal cord (SC). Extensive axonal arborization is observed near the soma (arrow heads in E). (G and H) At the level of the hindbrain, the primary axon branches into the periphery, innervating the otic capsule (OC, top in G), cranial neuromasts (CNM, bottom in G), lateral line (top in H), and trunk neuromasts (TNM, bottom in H). White dashed lines in (G) and (H) mark the OC (top) and CNM and TNM (bottom). Bottom panels in (G) and (H) depict contrast-inverted image of NB labeling (left), bright field (middle), and merged NB-bright field images (right). (I and J) Ventral (I) and lateral (J) schematic overviews of DDNs and their arborization patterns. Axons project caudally into the spinal cord (solid green lines) and into the periphery (dashed green lines). In (B)–(H), anterior is left and posterior is right. In (C)–(H), dorsal is up and ventral is down. Scale bars in (B)–(H) represent 10 μm. (A), (I), and (J) are not to scale.

Figure 2

Endogenous DDN Activity Patterns (A–F) Left: representative loose patch (A–C) and perforated patch clamp (D–F) activity patterns recorded from GFP-positive DDNs of awake, paralyzed larvae at 4 dpf. Right: excerpts of activity demarcated by dashed boxes in left panels shown on an expanded timescale. Inset in (D) shows underlying slow membrane oscillations during tonic spiking. Action potentials are truncated (hatched lines). Traces in (A)–(F) are derived from different preparations. (G–J) Box-and-whisker plots of tonic action potential (AP) frequency (G), burst duration (H) within burst AP frequency (I), and the frequency of bursting activity (J) obtained during loose and perforated patch recordings in cells that exhibited primarily tonic spiking (black) or repetitive bouts of bursting (gray). In (G)–(J), filled circles depict raw data points; upper and lower hinges of the box correspond to the first and third quartiles; whiskers extend to 1.5× the interquartile range; and lines within boxes represent median. Scale bars for time in panels (A)–(F) are illustrated in (F). Scale bars for voltage in panels (D)–(F) are illustrated in (F). See also Figure S1.

Figure 3

DDNs Receive Glutamatergic and GABAergic Inputs (A) Addition of picrotoxin (PIC) during whole-cell DDN recordings of TTX-treated fish isolated a population of events (left) that were abolished by application of kynurenic acid (KYN; right). (B) Addition of KYN isolated a second population of events (left) that were abolished by addition of PIC (right). (C) Overlays of glutamatergic currents isolated with picrotoxin and GABAergic currents isolated with kynurenic acid (gray traces). Black traces represent current averages. (D–F) Box-and-whisker plots of glutamatergic and GABAergic mPSC amplitudes (D), rise times (E), and half-widths (F). In (D)–(F), filled circles depict raw data points; upper and lower hinges of the box correspond to the first and third quartiles; whiskers extend to 1.5× the interquartile range; and lines within boxes represent median. (G and H) Whole-cell voltage clamp recordings of endogenous synaptic currents recorded from a QX-314-dialyzed DDN of awake, paralyzed larvae clamped at the reversal potential for chloride (G) or cationic (H) currents. (I) Voltage recording derived from the same cell depicting tonic and burst activity. Note that action potentials were blocked in the recorded cell by addition of QX-314 to the pipette solution. Right panels in (G)–(I) are excerpts (dashed boxes) of activity shown on an expanded timescale. Scale bars for time in (G)–(I) are shown in (I).

Figure 4

Autonomous Spike Activity in DDNs (A–D) Left: extracellular loose (A and B) and perforated (C and D) patch clamp recordings of DDN activity in awake, paralyzed larvae bathed in control saline. Traces in (A)–(D) are derived from different DDNs. Right: effects of kynurenic acid (2–4 mM) and picrotoxin (50–100 μM) on DDNs that exhibit tonic and intermittent burst spiking (A and C) or repetitive burst spiking (B and D). Note that addition of these blockers unmasks low-frequency autonomous spiking activity. (E–H) Box-and-whisker plots showing the effects of synaptic blockers on interspike interval (ISI; log scaled) and coefficient of variation in loose (E and F) and perforated (G and H) patch clamp recordings. In (E)–(H), filled circles depict raw data points; upper and lower hinges of the box correspond to the first and third quartiles; whiskers extend to 1.5× the interquartile range; and lines within boxes represent median. Scale bars of (A) and (B) are shown in (B); scale bars of (C) and (D) are shown in (D). See also Figure S2.

Figure 5

Paired Recordings Reveal the Relationship between Spinal Motor Circuit State and DDN Spike Activity (A and B) Simultaneous loose patch DDN and motoneuron whole-cell recordings obtained from awake, paralyzed larvae at 4 dpf. (A) During periods of motor network inactivity, motoneurons (Mn) do not receive rhythmic locomotor drive, and DDNs spike tonically. (B) During episodes of fictive beat-glide swimming, Mn receive repetitive bouts of locomotor drive during beat periods (denoted by gray bars in lower trace) that often occur simultaneously with DDN bursts (denoted by black bars on upper trace). (C) Simultaneous loose patch DDN and whole-cell red muscle (RM) fiber recording show that DDN bursting (denoted by black bars in upper trace) coincides with rhythmic bouts of locomotor drive to the muscle (denoted by gray bars in lower trace). (D) Plot of delay in onset of DDN burst activity relative to motor episode onset (bottom) with box-and-whisker plot of same data (top). Motor episode occurs at 0 ms (dotted line). (E) Log-scaled duration of DDN bursts and beat episodes. In (D) and (E), filled circles depict raw data points; upper and lower hinges of the box correspond to the first and third quartiles; whiskers extend to 1.5× the interquartile range; and lines within boxes represent median. See also Figure S3.

Figure 6

Effects of DDN Ablation on Zebrafish Behavior (A and B) Ventral views of GFP (left), anti-tyrosine hydroxylase (anti-TH; middle), and merged GFP/anti-TH (right) labeling in the diencephalon of control (A) and ablated (B) ETvmat2:GFP larvae at 4 dpf. Note that the anterior-most neurons corresponding to DDNs in DC2 (arrowheads in A) are absent in ablated fish. (C) Swimming trajectories of 4 dpf control and laser ablated zebrafish recorded over a 10 min period. (D) Left: cumulative distance that individual control (black) and laser-ablated (red) fish travel over the 10 min period. Right: box-and-whisker plot of total distance traveled in 10 min period for control and laser-ablated 4 dpf larvae. (E and F) Raster plots of identified episodes of beat-glide swimming in non-ablated (E) and ablated (F) 4 dpf larvae over the 10 min recording (top). Bottom: corresponding regions of raster plots over an expanded timescale. Each row represents a single fish during the recording. (G and H) Box-and-whisker plots of the percent of time spent beat-glide swimming over the 10 min observation period (G) and the duration of individual beat-glide bouts (H). (I) Plots of average velocity as a function of time for bouts of beat-glide activity. At the onset of the beat period (black arrow), velocity increases sharply and subsequently declines to baseline levels during the glide period. For box-and-whisker plots in (D), (G), and (H), filled circles depict raw data points; upper and lower hinges of the box correspond to the first and third quartiles; whiskers extend to 1.5× the interquartile range; and lines within boxes represent median. Scale bars of (A) and (B) represent 10 μm; anterior is left, posterior is right. See also Figures S4 and S5.