Modulation of SK channels regulates locomotor alternating bursting activity in the functionally-mature spinal cord

Abstract

The spinal cord contains specialized groups of cells called pattern generators, which are capable of orchestrating rhythmic firing activity in an isolated preparation. Different patterns of activity could be generated in vitro including right-left alternating bursting and bursting in which both sides are synchronized. The cellular and network mechanisms that enable these behaviors are not fully understood. We have recently shown that Ca²⁺-activated K⁺ channels (SK channels) control the initiation and amplitude of synchronized bursting in the spinal cord. It is unclear, however, whether SK channels play a similar role in the alternating rhythmic pattern. In the current study, we used a spinal cord preparation from functionally mature mice capable of weight bearing and walking. The present results extend our previous work and show that SK channel inhibition initiates and modulates the amplitude of alternating bursting. We also show that addition of methoxamine, an α₁-adrenergic agonist, to a cocktail of serotonin, dopamine, and NMDA evokes robust and consistent alternating bursting throughout the cord.

Keywords: SK channels; fictive locomotion; methoxamine; neuronal bursting; spinal motoneurons.

Figure 1.

Induction of alternating bursting activity in the spinal cord of functionally-mature mouse. A: The activity of the ventral roots at the lower lumbar (L6) and mid-sacral (S2) segments on the left (L) and right (R) sides of a spinal cord obtained from a mouse at P11. The application of serotonin (5-HT), dopamine (DA), and NMDA induces low-amplitude, left-right alternating, rhythmic bursting at the sacral, but not the lumbar, segments. B: Recordings from a P12 mouse showing the effect of the same concentrations of serotonin, dopamine, and NMDA. Only the left side ventral root at L6 is showing low amplitude bursts (arrows), while the right side and sacral roots are silent.

Figure 2.

Effect of methoxamine on alternating activity in the lumbosacral cord. A: Left: The addition of the α₁-adrenergic agonist (methoxamine) to the 5-HT/DA/NMDA cocktail, in the same experiment as Fig. 1B, produces a seemingly synchronized high-amplitude activity in all ventral roots. Middle: Each one of these episodes of synchronous activity is a cluster of short bursts. Right: This clustered activity is composed of short alternating bursts between the left and right sides; with the lumbar bursts in phase with sacral bursts on the same side of the cord (notice the different time scales). B: An episode of fast, alternating activity produced by the same chemical cocktail in a spinal cord of a P15 mouse. Note that burst duration and cycle are similar to the experiment in (A). C: The burst parameters after methoxamine administration, plotted as a percentage of the value before adding the drug. The bars represent the mean ± standard deviation. * Statistically significant, p value < 0.05.

Figure 3.

Locomotor bursts are inhibited by SK activators and restored by SK blockers. The application of the SK activator (CyPPA) inhibits the episodic alternating bursting activity in a dose-dependent manner. After the bursting has completely ceased, the addition of the SK blocker, apamin, restores the rhythmic alternating activity. The insets show, on a different time scale, representative alternating bursts from two different episodes of activity.