Noninvasive transcranial stimulation of rat abducens nerve by focused ultrasound

Abstract

Nonpharmacologic and nonsurgical transcranial modulation of the nerve function may provide new opportunities in evaluation and treatment of cranial nerve diseases. This study investigates the possibility of using low-intensity transcranial focused ultrasound (FUS) to selectively stimulate the rat abducens nerve located above the base of the skull. FUS (frequencies of 350 kHz and 650 kHz) operating in a pulsed mode was applied to the abducens nerve of Sprague-Dawley rats under stereotactic guidance. The abductive eyeball movement ipsilateral to the side of sonication was observed at 350 kHz, using the 0.36-msec tone burst duration (TBD), 1.5-kHz pulse repetition frequency (PRF), and the overall sonication duration of 200 msec. Histologic and behavioral monitoring showed no signs of disruption in the blood brain barrier (BBB), as well as no damage to the nerves and adjacent brain tissue resulting from the sonication. As a novel functional neuro-modulatory modality, the pulsed application of FUS has potential for diagnostic and therapeutic applications in diseases of the peripheral nervous system.

Figure 1

(A) An experimental set-up. The FUS transducer (1) coupled to the rat’s head via a degassed water bag (2) was powered by two function generators and a linear power amplifier (not shown). The rodent was placed on the MRI-compatible fixation head-frame (3). (B) Estimated sonication path (triangle) on 3-D reconstruction of the rat atlas (Watson 2004); abducens nerve (red), trochlear nerve (green), oculomotor nerve (blue); The cranial nerve paths were traced in solid line while overlaying onto the rat anatomy (Greene 1968), and the rest of the approximated nerve paths were depicted in dashed lines.

Figure 2

Ipsilateral abductive eye movement induced by sonication: (A) left eye or (B) right eye in movement indicated by the arrows at the same side of sonication. (C) Quantitative estimation of eyeball movement based on video analysis: each marker indicates mean value, each vertical error bar shows standard deviation (n=7). The link to the related video is also submitted.

Figure 3

An example of histological analysis of the sonicated rat cranial nerve (H&E Staining): (A) longitudinal, (B) transversal, and (C) magnified longitudinal section, along with (D) an extracted tissue sample (brain and cranial nerves) for histology analysis. Arrow indicates the abducens nerve (noted as ‘VI’, unilateral side) and the trigeminal nerve (noted as ‘V’, its ophthalmic and maxillary branches are denoted as ‘V_o’ and ‘V_m’ respectively) are also shown.