Pacemaker and plateau potentials shape output of a developing locomotor network

Abstract

Background: During development, spinal networks undergo an intense period of maturation in which immature forms of motor behavior are observed. Such behaviors are transient, giving way to more mature activity as development proceeds. The processes governing age-specific transitions in motor behavior are not fully understood.

Results: Using in vivo patch clamp electrophysiology, we have characterized ionic conductances and firing patterns of developing zebrafish spinal neurons. We find that a kernel of spinal interneurons, the ipsilateral caudal (IC) cells, generate inherent bursting activity that depends upon a persistent sodium current (I(NaP)). We further show that developmental transitions in motor behavior are accompanied by changes in IC cell bursting: during early life, these cells generate low frequency membrane oscillations that likely drive "coiling," an immature form of motor output. As fish mature to swimming stages, IC cells switch to a sustained mode of bursting that permits generation of high-frequency oscillations during locomotion. Finally, we find that perturbation of IC cell bursting disrupts motor output at both coiling and swimming stages.

Conclusions: Our results suggest that neurons with unique bursting characteristics are a fundamental component of developing motor networks. During development, these may shape network output and promote stage-specific reconfigurations in motor behavior.

Figure 1

Spinal Neuron Characteristics during the Coiling to Burst Swimming Developmental Period (A) Timeline depicting developmental period encompassing coiling (17–29 hpf) to burst swimming (∼30-48 hpf). Lower panels: consecutive frames of a single coil (left) and three cycles of burst swimming (right). Time (in ms) is shown in bottom right of each frame. A single coil lasts ∼1 s, whereas a single swim cycles lasts ∼30 msec. The scale bar represents 0.5 mm. (B) Schematic (left) and micrographs (right) of primary neuron classes that participate in SNA. Ipsilateral caudal (IC) somata are found in the caudal hindbrain/rostral spinal cord and extend axons ventrolaterally. Ventrolateral descending (VeLD) somata have axons that course ventrally before turning to descend laterally. Commissural primary ascending (CoPA) interneurons have dorsal somata and axons that cross the commisure to ascend contralaterally (hatched line). Motoneurons (Mns) have ventral somata and axons innervating the muscle. Arrows and arrowheads denote position of soma and axons respectively. A-P, anterior-posterior orientation. (C) Network activity during the coiling period. (a) Early coiling stage neurons generate periodic depolarizations (PDs; arrows). (b) Mid-coiling stages neurons generate PDs (arrows) that intersperse with synaptic bursts (SBs; asterisks). (c) At late coiling stages (upper traces), PDs (arrows) are infrequent. Lower traces show activity in a simultaneously recorded muscle cell, revealing that PDs drive motor output. Right-hand panels: expanded sweeps of the same recording showing a single PD in register with neuromuscular activity. (D) At burst swimming stages, neurons (upper trace) generate synaptic drive for burst swimming. Lower traces show activity in a simultaneously recorded muscle cell, revealing that locomotor drive evokes rhythmic neuromuscular activity. Arrow marks stimulus artifact. Right-hand panels: expanded sweep of the same record showing locomotor related EPSPs during swimming. Traces in (Ca–Cc) were obtained from separate IC cell recordings of 17 hpf, 23 hpf, and 26 hpf fish. Trace in (D) was obtained from a Mn at 42 hpf.

Figure 2

Effects of Ion Channel Blockers on SNA (A) Upper panel: voltage recording depicting effects of TTX (0.5 μM) on SNA. Lower panel: expanded traces of above recording during control (a) and after 1 min (b) and 2.5 min (c) TTX perfusion. PDs (arrows) and SBs (asterisks) were completely abolished by this treatment (c). (B) After 7 min cadmium (100 μM) perfusion, SBs (asterisks) were abolished, whereas PDs (arrows) persisted. Note the presence of low-amplitude oscillations after cadmium treatment, which represent filtered PDs originating from electrically coupled contralateral neurons [5]. (C) Nifedipine (100 μM) reduced SBs but failed to inhibit PDs. (D) ZD7288 (40 μM) blocked neither PDs (arrows) nor SBs (asterisks). (E) Bar charts depicting effects of cadmium, nifedipine and ZD7288 on mean PD amplitude (left-hand panel), width (middle panel), and frequency (right-hand panel). Note that PD frequency was significantly reduced by cadmium (control = 0.18 ± 0.03 Hz, cadmium = 0.06 ± 0.01 Hz; ^∗p = 1.6 × 10⁻³) and nifedipine (control = 0.43 ± 0.09 Hz, nifedipine = 0.28 ± 0.06 Hz; ^∗p = 0.02). Cadmium also significantly increased PD widths (control = 1.21 ± 0.74 s, cadmium = 2.07 ± 0.16 s; ^∗p = 4.7 × 10⁻⁴). Data are represented as mean ± SEM. See also Figure S1.

Figure 3

Spinal Neurons Express I_NaP during the SNA Period (A) In the presence of cadmium and caesium (see the Supplemental Experimental Procedures) 55 mV/s voltage ramps (inset) generated a persistent inward current in IC (purple), VeLD (blue), CoPA (green), and Mn (black) cells at 24 hpf. All traces are leak subtracted. (B and C) Persistent inward currents (black) were blocked by 1 μM TTX and 5 μM riluzole (gray). Subtraction currents reveal isolated sodium current (red). (D) IV plot of riluzole-sensitive current (mean of 14 neurons). (E) Boltzmann fit of riluzole sensitive conductance. V_1/2, half activation voltage; K, Boltzmann constant. (F) Effects of riluzole (5 μM) on SNA recorded from an IC cell at 23 hpf. The lower panel shows expanded regions of activity in control (a) and after 8 min (b) and 20 min (c) riluzole. Arrows and asterisks show PD and SB, respectively. (G) Overlaid frames (100 ms intervals) of behavior in control and after 5 μM riluzole treatment. (H) Bar chart of average coiling frequency. Riluzole (5 μM) abolished coiling behavior (control n = 10, riluzole = 15; ^∗p = 3.1 × 10⁻⁷), whereas 100 μM cadmium reduced coiling frequency (control = 12, cadmium n = 11; ^∗p = 0.005) and 200 μM ZD7288 (control n = 14, ZD7288 n = 16; p = 0.2) had no effect. Data are represented as mean ± SEM. See also Figure S2.

Figure 4

IC Cells Have Unique Cellular Characteristics during Coiling Stages (A and B) Raw traces (A) and mean current responses (B) to 55mV/s voltage ramps in the absence of calcium and potassium channel blockers (see the Supplemental Experimental Procedures). IC cells (purple) exhibited inward rectification, whereas VeLD (blue), CoPA (green), and Mn (black) cells did not. (C) Mean PD amplitude (amp) did not differ between IC and VeLD neurons (IC = 36.41 ± 2.77 mV, n = 31; VeLD = 32.86 ± 2.30, n = 22, p = 0.43) but was significantly smaller in CoPA and Mn cells (CoPA = 17.86 ± 4.00, n = 13, p = 0.002; Mn = 14.98 ± 1.12, n = 26, p = 1.87 × 10⁻⁹). (D) Mean baseline membrane potential of IC cells was not significantly different from I_NaP threshold (−66.21 ± 1.04 mV, hatched gray line) at 20–22 hpf (mean = −68.13 ± 1.60 mV, n = 7, p = 0.31) and 23–25 hpf (mean = −67.37 ± 2.33 mV, n = 9, p = 0.61) but was highly significantly different at 26–28 hpf (mean = −78.06 ± 2.10, n = 19, p = 1 × 10⁻⁴). Baseline membrane potential of VeLD (20–22 hpf, n = 6; 23–25 hpf, n = 7; 26–28 hpf, n = 18), CoPA (20–22 hpf, n = 4; 23–25 hpf, n = 6; 26–28 hpf, n = 7), and Mn (20–22 hpf, n = 16; 23–25 hpf, n = 6; 26–28 hpf, n = 15) cells was significantly different from I_NaP threshold at all developmental stages (p < 0.05). (E) Left-hand panel: representative voltage traces showing IC cell membrane oscillations evoked by current injection. Right-hand panel: expanded sweep of the same record showing onset of oscillatory response. (F) On dialysis with QX-314 (4 mM), IC cells were unable to generate membrane oscillations or action potentials when depolarized. (G) IC cell responses in control and after 4 min bath perfusion with riluzole (5 μM). Riluzole abolished oscillations, but cells still fired single action potentials (asterisk) in response to depolarization. (H) Membrane responses of VeLD, CoPA, and Mns to a family of current pulses. These cells generated single or tonic action potential discharges at suprathreshold potentials. Traces in (E–H) are derived from separate current clamp recordings spanning 26–28 hpf. Data in (C) and (D) represented as mean ± SEM. Current commands are depicted by gray traces. See also Figure S3.

Figure 5

IC Cells Generate I_NaP-Dependent Sustained Bursts at the Early Swimming Stage (A) Left-hand panel: simultaneous voltage recordings of an IC and red muscle (M) cell during fictive swimming in a 35 hpf embryo. Right-hand panel: expanded region of the same record showing rhythmic bursting activity occurring in register with neuromuscular drive. (B) Left-hand panel: voltage recording of IC cell response to current injection. Current injection evoked a sustained burst (in this case lasting 2.8 s). Right-hand panel: expanded region of the same record showing step-like depolarization at burst onset. (C) Five minute perfusion with riluzole (5 μM) abolished sustained bursting. Note that cells were able to fire single action potentials (asterisk) in the presence of riluzole. (D) Voltage responses of non-bursting primary neurons to a family of current pulses. Upward deflections are tonic (Mn) or single (VeLD, CoPA) action potential discharges. Traces in (A–D) are derived from separate current clamp recordings spanning 32–46 hpf. Current commands are depicted by gray traces. See also Figure S4.

Figure 6

Locomotor-Related Activity in IC cells (A) Left-hand panels: paired voltage recording of locomotor activity recorded from an IC cell and Mn at 38 hpf. Right-hand panel: expanded sweep taken from same episode showing rhythmic IC cell bursting occurring in register with rhythmic Mn EPSPs. Note transient spikes (asterisks) often occurred at peak of Mn EPSPs. Arrows mark stimulus artifacts. (B) Left-hand panels: representative IC cell locomotor drive recorded with control intracellular solution (upper trace) and intracellular solution containing 4 mM QX-314 (lower trace). Right-hand panels: expanded excerpts of the same locomotor episodes. Note that large amplitude rhythmic bursts are not observed in QX-314 dialyzed cells. Recordings were obtained from embryos at 35 hpf (control) and 37 hpf (QX-314), respectively. (C) Representative voltage clamp recordings of locomotor-related drive in 33–35 hpf IC cells. At the estimated chloride ion reversal potential (−43 mV), the cell received rhythmic inward currents superimposed upon a small sustained inward current (a). At the estimated cation reversal potential (7 mV), the same cell received outward currents (b). At an intermediate holding potential (−23 mV), rhythmic inward currents interspersed with volleys of outward current were observed (c). (D) Simultaneous voltage recordings of electrical coupling between an IC cell (upper trace) and a Mn (lower trace). Depolarization of IC cells causes tonic depolarization of the coupled Mn (a). Similarly, injection of IC cells (upper trace) with hyperpolarizing current steps evokes hyperpolarizing responses in the coupled Mn (lower trace) (b).Traces in (a) and (b) were obtained from embryos at 30 and 34 hpf, respectively. Current commands are depicted by gray traces. See also Figure S5.

Figure 7

Possible Roles of IC Cells during Swimming (A) Locomotor-related activity recorded from a 37 hpf primary Mn in control saline and after 10 min incubation in riluzole (5 μM). Arrows mark stimulus artifacts. (B) Effects of dopamine (DA) on IC cell firing at 30–48 hpf. Sustained bursts (left-hand trace) were abolished by 5 min perfusion with 10 μM DA (middle trace) but recovered after 5 min wash (right-hand trace). Current commands are depicted by gray traces. (C) Dopamine (DA) did not affect action potential firing in other neurons. Current commands are depicted by gray traces. (D) Example of locomotor-related drive recorded from Mns exposed to control saline, after 5 min dopamine (DA) perfusion and after 10 min wash. Arrows mark stimulus artifacts. (E) Plot of example swim trajectories of control and dopamine-treated fish at 42 hpf. (F) Bar charts depicting effects of riluzole and dopamine (DA) on the duration of Mn fictive episode duration (left-hand and middle graphs) and swim episode length (right-hand graph). Riluzole reduced the duration of locomotor drive from 4.84 ± 1.17 s to 1.29 ± 0.42 (^∗p = 1.8 × 10⁻³). Similarly dopamine reduced locomotor drive duration from 16.14 ± 0.76 s to 2.17 ± 0.39 s (^∗p = 9 × 10⁻³) and swim duration from 2.14 ± 0.33 s to 1.17 ± 0.20 s, ^∗p = 0.01). Data are represented as mean ± SEM. See also Figure S6.