Low-Intensity Pulsed Ultrasound Neuromodulation of a Rodent's Spinal Cord Suppresses Motor Evoked Potentials

Abstract

Objective: Here we investigate the ability of low-intensity ultrasound (LIUS) applied to the spinal cord to modulate the transmission of motor signals.

Methods: Male adult Sprague-Dawley rats (n = 10, 250-300 g, 15 weeks old) were used in this study. Anesthesia was initially induced with 2% isoflurane carried by oxygen at 4 L/min via a nose cone. Cranial, upper extremity, and lower extremity electrodes were placed. A thoracic laminectomy was performed to expose the spinal cord at the T11 and T12 vertebral levels. A LIUS transducer was coupled to the exposed spinal cord, and motor evoked potentials (MEPs) were acquired each minute for either 5- or 10-minutes of sonication. Following the sonication period, the ultrasound was turned off and post-sonication MEPs were acquired for an additional 5 minutes.

Results: Hindlimb MEP amplitude significantly decreased during sonication in both the 5- (p < 0.001) and 10-min (p = 0.004) cohorts with a corresponding gradual recovery to baseline. Forelimb MEP amplitude did not demonstrate any statistically significant changes during sonication in either the 5- (p = 0.46) or 10-min (p = 0.80) trials.

Conclusion: LIUS applied to the spinal cord suppresses MEP signals caudal to the site of sonication, with recovery of MEPs to baseline after sonication.

Significance: LIUS can suppress motor signals in the spinal cord and may be useful in treating movement disorders driven by excessive excitation of spinal neurons.

Figure 1.

Illustration of the animal model. (A) Cranial electrodes placed 2 mm posterior and lateral from the bregma. (B) Upper extremity electrodes placed in brachioradialis muscle. (C) Low-intensity ultrasound placed in direct contact with the dorsum of the spinal cord. (D) Lower extremity electrode placed in the soleus muscle.

Figure 2.

Depiction of the low-intensity pulsed ultrasound parameters.

Figure 3.

Experimental set-up. Each experiment trial was separated into multiple 1-minute-windows and each window was named according to the period and its time order.

Figure 4.

Results of the 5-min sonication trial. (A) Representative motor evoked potential; (B) Normalized amplitude across before, during, and post sonication periods; (C) Normalized latency across the 3 stages; (D) Boxplot of normalized forelimb amplitude; (E) Boxplot of normalized hindlimb amplitude. The amplitude was measured from the average MEPs for each 1-min time segment. **, p < 0.01, ***, p < 0.001. BL: Baseline, DS; During Sonication, PS: Post Sonication. Time segment number corresponds to the minute of sonication or post sonication.

Figure 5.

Results for the 10-min sonication trial. (A) Representative motor evoked potential; (B) Normalized amplitudes before (baseline), during and post sonication; (C) Normalized latency for the 3 periods; (D) Boxplot of normalized forelimb amplitude; (E) Boxplot of normalized hindlimb amplitude. The amplitude was measured from the average MEPs for each 1-min time segment. **, p < 0.01, ***, p < 0.001. BL: Baseline, DS; During Sonication, PS: Post Sonication. Time segment number corresponds to the minute of sonication or post sonication.

Figure 6.

Depiction of the average MEP waveform during each minute of the trial. (A) Waveforms from the 5-min trial. (B) Waveforms from the 10-min trial. BL: Baseline, DS; During Sonication, PS: Post Sonication.

Figure 7.

LIUS does not produce damage to spinal cord as demonstrated with tissue staining and qPCR data. (A) Spinal cord samples were collected and stained with H&E and Masson’s trichome. These stains show that sonication did not lead to gross tissue deficits, such as hemorrhaging or necrosis. (B) qPCR data demonstrates that there is no significant difference in immune/inflammation and fibrosis genes between the control and stimulated groups. ns, not significant. *, p < 0.05 (vs. non-stim (−)).

Figure 8.

Temperature change during ten minutes of sonication in two rat ex vivo spinal cord samples. (A) Depiction of experimental set-up; (B) Temperature during pre-, during-, and post-sonication. Created with BioRender.com.