Pulsed infrared radiation excites cultured neonatal spiral and vestibular ganglion neurons by modulating mitochondrial calcium cycling

Abstract

Cochlear implants are currently the most effective solution for profound sensorineural hearing loss, and vestibular prostheses are under development to treat bilateral vestibulopathies. Electrical current spread in these neuroprostheses limits channel independence and, in some cases, may impair their performance. In comparison, optical stimuli that are spatially confined may result in a significant functional improvement. Pulsed infrared radiation (IR) has previously been shown to elicit responses in neurons. This study analyzes the response of neonatal rat spiral and vestibular ganglion neurons in vitro to IR (wavelength = 1,863 nm) using Ca(2+) imaging. Both types of neurons responded consistently with robust intracellular Ca(2+) ([Ca(2+)]i) transients that matched the low-frequency IR pulses applied (4 ms, 0.25-1 pps). Radiant exposures of ∼637 mJ/cm(2) resulted in continual neuronal activation. Temperature or [Ca(2+)] variations in the media did not alter the IR-evoked transients, ruling out extracellular Ca(2+) involvement or primary mediation by thermal effects on the plasma membrane. While blockage of Na(+), K(+), and Ca(2+) plasma membrane channels did not alter the IR-evoked response, blocking of mitochondrial Ca(2+) cycling with CGP-37157 or ruthenium red reversibly inhibited the IR-evoked [Ca(2+)]i transients. Additionally, the magnitude of the IR-evoked transients was dependent on ryanodine and cyclopiazonic acid-dependent Ca(2+) release. These results suggest that IR modulation of intracellular calcium cycling contributes to stimulation of spiral and vestibular ganglion neurons. As a whole, the results suggest selective excitation of neurons in the IR beam path and the potential of IR stimulation in future auditory and vestibular prostheses.

Target structures for cochlear infrared neural stimulation

“…7,8,28 For the next steps, several possibilities have been discussed and some may occur in sequence. Steps include the generation of a depolarizing capacitive current, 9,29-31 the activation of temperature-sensitive ion channels such as the TRP channels, 11,32-34 increase of intracellular calcium concentration, [13][14][15] and the generation of a stress relaxation wave that mechanically stimulate hair cells.…”

“…The increase in free calcium caused by infrared irradiation was found in the vestibular system 13,15 in cardiomyocytes 14 and the cortex. 16,17 While it has been well documented that the intracellular calcium concentration increases upon irradiation, it is not clear how the increased calcium results in an action potential.…”

“…11,12 Other experiments demonstrated that IR increases the intracellular calcium concentration. [13][14][15][16][17][18] Recently, it has been demonstrated that the increase in intracellular calcium concentration results from a direct interaction of the radiation with intracellular calcium stores. 15 Pulse lengths for INS are typically 100 μs for cochlear stimulation and about 1 ms for peripheral nerve or cortical stimulation.…”

Millisecond infrared laser pulses depolarize and elicit action potentials on in-vitro dorsal root ganglion neurons

“…Infrared laser pulses have been shown to induce intracellular calcium transients implicating mitochondria in neonatal cardiomyocytes [10] and in neonatal spiral and vestibular ganglion neurons [11] and to depolarize membranes in HEK293 cells [12], dorsal root ganglion neurons [13], oocytes, HEK cells and artificial layers [14], retinal and vestibular primary neurons [15,16], hippocampal neurons [17], spiral ganglion neurons [18], brain slices [19] and in vestibular hair cells and afferent neurons [20]. What remains unclear is whether an universal photothermal mechanism exists and how the transient heating induced by the IR laser pulse elicits membrane depolarization of neurons and action potentials or modulates intracellular signalling.…”

Regulation of Cellular Molecular Signaling by Light (Invited Paper)

“…Photothermal effect, as another type of photodamage, can also generate slow Ca 2+ increase. For example, the heating effect of infrared laser at 1863 nm could stimulate mitochondria to induce Ca 2+ increase [88,89].…”