Austin R. Duke

Optical Pacing of the Embryonic Heart

Light has been used to noninvasively alter the excitability of both neural and cardiac tissue 1–10. Recently, pulsed laser light has been shown to be capable of eliciting action potentials in peripheral nerves and in cultured cardiomyocytes 7–10. Here, we demonstrate for the first time optical pacing (OP) of an intact heart in vivo. Pulsed 1.875 μm infrared laser light was employed to lock the heart rate to the pulse frequency of the laser. A laser Doppler velocimetry (LDV) signal was used to verify the pacing. At low radiant exposures, embryonic quail hearts were reliably paced in vivo without detectable damage to the tissue, indicating that OP has great potential as a tool to study embryonic cardiac dynamics and development. In particular, OP can be utilized to control the heart rate, and thereby alter stresses and mechanically transduced signaling.

Transient and selective suppression of neural activity with infrared light

Analysis and control of neural circuitry requires the ability to selectively activate or inhibit neurons. Previous work showed that infrared laser light selectively excited neural activity in endogenous unmyelinated and myelinated axons. However, inhibition of neuronal firing with infrared light was only observed in limited cases, is not well understood and was not precisely controlled. Using an experimentally tractable unmyelinated preparation for detailed investigation and a myelinated preparation for validation, we report that it is possible to selectively and transiently inhibit electrically-initiated axonal activation, as well as to both block or enhance the propagation of action potentials of specific motor neurons. Thus, in addition to previously shown excitation, we demonstrate an optical method of suppressing components of the nervous system with functional spatiotemporal precision. We believe this technique is well-suited for non-invasive investigations of diverse excitable tissues and may ultimately be applied for treating neurological disorders.

Combined optical and electrical stimulation of neural tissue in vivo

Low-intensity, pulsed infrared light provides a novel nerve stimulation modality that avoids the limitations of traditional electrical methods such as necessity of contact, presence of a stimulation artifact, and relatively poor spatial precision. Infrared neural stimulation (INS) is, however, limited by a 2:1 ratio of threshold radiant exposures for damage to that for stimulation. We have shown that this ratio is increased to nearly 6:1 by combining the infrared pulse with a subthreshold electrical stimulus. Our results indicate a nonlinear relationship between the subthreshold depolarizing electrical stimulus and additional optical energy required to reach stimulation threshold. The change in optical threshold decreases linearly as the delay between the electrical and optical pulses is increased. We have shown that the high spatial precision of INS is maintained for this combined stimulation modality. Results of this study will facilitate the development of applications for infrared neural stimulation, as well as target the efforts to uncover the mechanism by which infrared light activates neural tissue.

Spatial and temporal variability in response to hybrid electro-optical stimulation

Hybrid electro-optical neural stimulation is a novel paradigm combining the advantages of optical and electrical stimulation techniques while reducing their respective limitations. However, in order to fulfill its promise, this technique requires reduced variability and improved reproducibility. Here we used a comparative physiological approach to aid the further development of this technique by identifying the spatial and temporal factors characteristic of hybrid stimulation that may contribute to experimental variability and/or a lack of reproducibility. Using transient pulses of infrared light delivered simultaneously with a bipolar electrical stimulus in either the marine mollusk Aplysia californica buccal nerve or the rat sciatic nerve, we determined the existence of a finite region of excitability with size altered by the strength of the optical stimulus and recruitment dictated by the polarity of the electrical stimulus. Hybrid stimulation radiant exposures yielding 50% probability of firing (RE₅₀) were shown to be negatively correlated with the underlying changes in electrical stimulation threshold over time. In Aplysia, but not in the rat sciatic nerve, increasing optical radiant exposures (J cm⁻²) beyond the RE₅₀ ultimately resulted in inhibition of evoked potentials. Accounting for the sources of variability identified in this study increased the reproducibility of stimulation from 35% to 93% in Aplysia and 23% to 76% in the rat with reduced variability.

Nerve fiber recruitment in the context of hybrid neural stimulation

Recently, hybrid neural stimulation combining electrical and optical techniques was demonstrated. By applying a subthreshold electrical stimulus with infrared neural stimulation (INS), hybrid stimulation was shown to reduce INS thresholds as much as 3-fold while maintaining spatial selectivity, thus overcoming the risk of thermally-induced tissue damage associated with INS and the fundamental lack of spatial specificity associated with electrical stimulation. While the potential of hybrid stimulation is evident, a better fundamental understanding of the interaction between tissue, light, thermal gradients and current is necessary before this new stimulation paradigm can be further refined and optimized for clinical implementation. A key element of this understanding is the spatial superposition of the electrical and optical stimuli. A successful hybrid stimulation paradigm requires accurate recruitment of the same neurons by each modality. If the same neurons are not targeted by both electrical and optical stimulation, then hybrid stimulation will suffer from lack of repeatability and consistency. Here we present evidence as to how light and current interact spatially within neural tissue. There exists a finite spatial region that is excitable by hybrid stimulation. This region is shown to change in size and location by altering the optical and electrical components of the hybrid stimulus. By taking advantage of these results, we are now able to achieve greater control of hybrid stimulation and can better apply this promising technology.

6.6: Presentation session: Neuroanatomy, neurogeneration, and modeling: “Combining electrical and optical techniques to develop a novel modality for neural activation”

Infrared neural stimulation (INS) provides an artifact-free, contact-free and spatially selective alternative to electrical methods of neural activation. However, INS is limited by an approximately 2:1 ratio of radiant exposures (J/cm2) inducing damage to those required for stimulation. Current laser power requirements for stimulation also provide challenges to implanting INS-based technology. While characteristics of INS are desirable in both the clinical and research arenas, these challenges must be overcome for INS to see widespread application. This study describes a novel nerve stimulation modality combining optical and electrical techniques of neural stimulation. Radiant exposures required for stimulation are reduced by as much as 3-fold when delivered concomitantly with a sub-threshold electrical stimulus, effectively increasing the aforementioned ratio from 2:1 to 6:1. Spatial selectivity attributed to INS is maintained with hybrid stimulation. These results will directly facilitate the applications and technology associated with INS.

Combining electrical and optical techniques to develop a novel modality for neural activation

Infrared neural stimulation (INS) provides an artifact-free, contact-free and spatially selective alternative to electrical methods of neural activation. However, INS is limited by an approximately 2:1 ratio of radiant exposures (J/cm2) inducing damage to those required for stimulation. Current laser power requirements for stimulation also provide challenges to implanting INS-based technology. While characteristics of INS are desirable in both the clinical and research arenas, these challenges must be overcome for INS to see widespread application. Here we describe a novel nerve stimulation modality combining optical and electrical techniques of neural stimulation. Radiant exposures required for stimulation are reduced by as much as 3-fold when delivered concomitantly with a sub-threshold electrical stimulus, effectively increasing the aforementioned ratio from 2:1 to 6:1. Spatial selectivity attributed to INS is maintained with hybrid stimulation. These results will directly facilitate the applications and technology associated with INS.