Background

Optogenetics has transformed neuroscience by enabling the targeted manipulation of genetically defined neural cell types (Deisseroth, 2011 and 2015). This typically involves expressing a light-activated ion channel using a Cre driver mouse and implanting an optical fiber in the brain (Park et al., 2015b, 2015a and 2016). In this way, it is possible to activate or inactivate specific cell types with light and thereby test their causal role in behavior (Kim et al., 2013; Montgomery et al., 2015; Shin et al., 2017).

Optogenetics also has the potential to transform the study of the PNS, but the complex anatomy of the PNS poses unique challenges for the focused delivery of light to specific tissues (Towne et al., 2013; Montgomery et al., 2016; Maimon et al., 2018). The PNS consists of sensory and motor neurons, with cell bodies located in widely distributed peripheral ganglia. The major sensory ganglia include dorsal root ganglia, which consist of clusters of cell bodies of spinal sensory neurons that are organized bilaterally along the length of the spine, and nodose ganglia, which contain the cell bodies of vagal sensory neurons (Berthoud and Neuhuber, 2000).

These two sensory systems broadly innervate a range of peripheral organs that differ in their size, shape, location, and accessibility, necessitating individualized strategies for light delivery to different tissues (Berthoud, 2008; de Lartigue, 2016; Williams et al., 2016). Moreover, some of these organs are highly mobile during normal physiology or behavior (e.g., movement of the intestine during digestion), which further complicates the positioning and anchoring of optical fibers. Thus, unlike in the brain, it is a considerable challenge to develop robust strategies for the delivery of light selectively to peripheral tissues in freely behaving animals.

In principle, it is possible to target peripheral tissues with light using a wired fiber optic that stimulates the animal via the back or rectum (Caggiano et al., 2016; Hibberd et al., 2018), analogous to the intracranial fiber optics commonly used in the brain. While this approach has the advantage that it can be readily coupled with off-the-shelf components (e.g., lasers and LEDs), the abdominal viscera differs from the brain, in that it lacks a stable interface for securing large fiber optics and preventing motion. Moreover, the inflexible nature of most optical fibers can cause the shearing of tissues and nerves, or of the fiber itself, during normal animal movement. For this reason, wired fiber optics have been used less frequently for peripheral optogenetics in behaving animals (Maimon et al., 2017 and 2018; Towne et al., 2013).

An alternative approach is to implant a wireless probe that internalizes the light source, thereby bypassing physical constraints associated with fiber optic cables. Strategies for such wireless, radio-frequency(RF)-powered devices have recently been described (Montgomery et al., 2015 and 2016; Park et al., 2015a and 2015b), but it remains a major challenge to achieve long-term, organ-restricted illumination, without impeding normal behavior or physiology. For example, a wirelessly powered micro-light emitting diode (µLED) has been secured to the rat bladder using a circumferential elastomer sleeve (Mickle et al., 2019), but this device impedes normal organ expansion. A wireless flexible device that sutures a µLED onto the heart surface has been reported (Gutruf et al., 2019), but it has not been demonstrated to function for more than eight days, precluding most behavioral studies. More generally, affixing any µLED to a target organ surface results in light back-scatter and non-specific optogenetic illumination of nearby tissues.

Recently, we developed and published a new class of implantable wireless devices, demonstrating their applicability to study the vagus nerve (Kim et al., 2021). This new wireless optogenetic implant delivers light to nerve endings from inside the stomach, via a pre-curved tether. This intragastric light delivery enables illumination of stomach innervating nerves, with much greater specificity than µLEDs affixed to the organ surface, and could in principle be extended to study other organs, such as the intestine. Here, we describe the technical procedures for the fabrication of these devices and their in vivo applications.