Firing characteristics of deep dorsal horn neurons after acute spinal transection during administration of agonists for 5-HT1B/1D and NMDA receptors

Abstract

Spinal cord injury (SCI) results in a loss of serotonin (5-HT) to the spinal cord and a loss of inhibition to deep dorsal horn (DDH) neurons, which produces an exaggerated excitatory drive to motoneurons. The mechanism of this excitatory drive could involve the DDH neurons triggering long excitatory postsynaptic potentials in motoneurons, which may ultimately drive muscle spasms. Modifying the activity of DDH neurons with drugs such as NMDA or the 5-HT_1B/1D receptor agonist zolmitriptan could have a large effect on motoneuron activity and, therefore, on muscle spasms. In this study, we characterize the firing properties of DDH neurons after acute spinal transection in adult mice during administration of zolmitriptan and NMDA, using the in vitro sacral cord preparation and extracellular electrophysiology. DDH neurons can be categorized into three major types with distinct evoked and spontaneous firing characteristics: burst (bursting), simple (single spiking), and tonic (spontaneously tonic firing) neurons. The burst neurons likely contribute to muscle spasm mechanisms because of their bursting behavior. Only the burst neurons show significant changes in their firing characteristics during zolmitriptan and NMDA administration. Zolmitriptan suppresses the burst neurons by reducing their evoked spikes, burst duration, and spontaneous firing rate. Conversely, NMDA facilitates them by enhancing their burst duration and spontaneous firing rate. These results suggest that zolmitriptan may exert its antispastic effect on the burst neurons via activation of 5-HT_1B/1D receptors, whereas activation of NMDA receptors may facilitate the burst neurons in contributing to muscle spasm mechanisms following SCI.

Keywords: N-methyl-d-aspartate; deep dorsal horn neurons; serotonin; spinal cord injury.

Fig. 1.

Three firing response types of DDH neurons and their distribution in the dorsal horn: representative recording traces showing 3 distinct firing response types of DDH neurons upon 0.2-ms dorsal root stimulation (arrows, stimulus artifacts) in a single trial: burst neuron (A), simple neuron (B), and tonic neuron (C) (top) with its evoked spike (bottom). C, bottom: an expanded view of the second arrow (top) for a better view of the evoked spike. An evoked spike was also found in an expanded view of the first arrow (not shown). D: distribution of DDH neuron types as incidence (%) across various depths measured from the dorsal surface (n = 190).

Fig. 2.

Firing properties of 3 distinct neuron types after acute spinal transection: group data comparing firing properties (means ± SE) of burst, simple, and tonic neurons in response to increasing stimulus intensity in control. A: evoked spike count for burst (n = 96, except n = 95 at 10×), simple (n = 72), and tonic (n = 22) neurons. B: field potential for burst (n = 96, except n = 95 at 10×), simple (n = 72), and tonic (n = 22) neurons. C: first-spike latency (only nonzero values) for burst (n = 64, 1×; n = 92, 2×; n = 96, 5×; n = 94, 10×), simple (n = 39, 1×; n = 64, 2×; n = 69, 5×; n = 69, 10×), and tonic (n = 16, 1×; n = 18, 2×; n = 20, 5×; n = 21, 10×) neurons. D: spontaneous firing rate for burst (n = 96, except n = 95 at 10×), simple (n = 72), and tonic (n = 22) neurons. Significant difference from simple (*P < 0.017), burst (†P < 0.017), and tonic (‡P < 0.017) neurons shown by post hoc pairwise comparison (t-test) of group means with the Bonferroni correction.

Fig. 3.

Effects of NMDA on the firing properties of burst neurons after acute spinal transection. A: representative recording traces showing the effect of NMDA (bottom) on a burst neuron, compared with that in control (top), upon dorsal root stimulation (0.2 ms; arrow). B–F: group data showing the effects of NMDA on the firing properties (means ± SE) of burst neurons in response to increasing stimulus intensity compared with those during control and washout conditions. B: evoked spike count during control (n = 34 for all intensities), NMDA (n = 26, 1× and 2×; n = 28, 5×; n = 32, 10×), and washout (same as NMDA) conditions. C: field potential during control (n = 34 for all intensities), NMDA (n = 26, 1× and 2×; n = 28, 5×; n = 32, 10×), and washout (same as NMDA) conditions. D: burst duration during control (n = 34 for all intensities), NMDA (n = 26, 1× and 2×; n = 28, 5×; n = 32, 10×), and washout (same as NMDA) conditions. E: first-spike latency (only nonzero values) during control (n = 18, 1×; n = 31, 2×; n = 34, 5× and 10×), NMDA (n = 15, 1×; n = 23, 2×; n = 26, 5×; n = 30, 10×), and washout (n = 14, 1×; n = 22, 2×; n = 26, 5×; n = 29, 10×) conditions. F: spontaneous firing rate during control (n = 34 for all intensities), NMDA (n = 26, 1× and 2×; n = 28, 5×; n = 32, 10×), and washout (same as NMDA) conditions. Significant difference from control (*P < 0.017) and washout (†P < 0.017) conditions shown by post hoc pairwise comparison (t-test) of group means with the Bonferroni correction.

Fig. 4.

Effects of zolmitriptan on the firing properties of burst neurons after acute spinal transection. A: representative recording traces showing the effect of zolmitriptan (bottom) on a burst neuron, compared with that in control (top), upon dorsal root stimulation (0.2 ms; arrow). B–F: group data showing the effects of zolmitriptan on the firing properties (means ± SE) of burst neurons in response to increasing stimulus intensity compared with those during control and washout conditions. B: evoked spike count during control (n = 25 for all intensities), zolmitriptan (n = 24, 1×, 2×, and 5×; n = 25, 10×), and washout (same as zolmitriptan) conditions. C: field potential during control (n = 25 for all intensities), zolmitriptan (n = 24, 1×, 2×, and 5×; n = 25, 10×), and washout (same as zolmitriptan) conditions. D: burst duration during control (n = 25 for all intensities), zolmitriptan (n = 24, 1×, 2×, and 5×; n = 25, 10×), and washout (same as zolmitriptan) conditions. E: first-spike latency (only nonzero values) during control (n = 18, 1×; n = 24, 2×, 5×, and 10×), zolmitriptan (n = 16, 1×; n = 21, 2×; n = 22, 5×; n = 21, 10×), and washout (n = 13, 1×; n = 21, 2×, 5×, and 10×) conditions. F: spontaneous firing rate during control (n = 25 for all intensities), zolmitriptan (n = 24, 1×, 2×, and 5×; n = 25, 10×), and washout (same as zolmitriptan) conditions. Significant difference from zolmitriptan (*P < 0.017) and washout (†P < 0.017) conditions shown by post hoc pairwise comparison (t-test) of group means with the Bonferroni correction.

Fig. 5.

Effects of NMDA on the firing properties of simple neurons after acute spinal transection. A: representative recording traces showing the effect of NMDA (bottom) on a simple neuron, compared with that in control (top), upon dorsal root stimulation (0.2 ms; arrow). B–E: group data showing the effects of NMDA on the firing properties (means ± SE) of simple neurons in response to increasing stimulus intensity compared with those during control and washout conditions. B: evoked spike count during control (n = 23 for all intensities), NMDA (n = 17, 1× and 2×; n = 21, 5×; n = 19, 10×), and washout (same as NMDA) conditions. C: field potential during control (n = 23 for all intensities), NMDA (n = 17, 1× and 2×; n = 21, 5×; n = 19, 10×), and washout (same as NMDA) conditions. D: first-spike latency (only nonzero values) during control (n = 10, 1×; n = 21, 2×; n = 23, 5× and 10×), NMDA (n = 8, 1×; n = 13, 2×; n = 17, 5×; n = 15, 10×), and washout (n = 7, 1×; n = 13, 2×; n = 17, 5×; n = 16, 10×) conditions. E: spontaneous firing rate during control (n = 23 for all intensities), NMDA (n = 17, 1× and 2×; n = 21, 5×; n = 19, 10×), and washout (same as NMDA) conditions.

Fig. 6.

Effects of zolmitriptan on the firing properties of simple neurons after acute spinal transection. A: representative recording traces showing the effect of zolmitriptan (bottom) on a simple neuron, compared with that in control (top), upon dorsal root stimulation (0.2 ms; arrow). B–E: group data showing the effects of zolmitriptan on the firing properties (means ± SE) of simple neurons in response to increasing stimulus intensity compared with those during control and washout conditions. B: evoked spike count during control (n = 23 for all intensities), zolmitriptan (n = 20, 1×, 2×, and 5×; n = 22, 10×), and washout (n = 21, 1×, 2×, and 5×; n = 22, 10×) conditions. C: field potential during control (n = 23 for all intensities), zolmitriptan (n = 21, 1×, 2×, and 5×; n = 23, 10×), and washout (n = 21, 1×, 2×, and 5×; n = 22, 10×) conditions. D: first-spike latency (only nonzero values) during control (n = 15, 1×; n = 19, 2×; n = 22, 5× and 10×), zolmitriptan (n = 13, 1×; n = 17, 2×; n = 19, 5×; n = 22, 10×), and washout (n = 13, 1×; n = 18, 2× and 5×; n = 20, 10×) conditions. E: spontaneous firing rate during control (n = 23 for all intensities), zolmitriptan (n = 20, 1×, 2×, and 5×; n = 22, 10×), and washout (n = 21, 1×, 2×, and 5×; n = 22, 10×) conditions.