On the Coupling Mechanism Occurring at the Neuron-Nanoparticle Interface

The injection of organic photo-responsive nanoparticles (NPs) of poly(3-hexyltiophene) (P3HT) into rat models of retinitis pigmentosa was proposed as a “<i>liquid retina device</i>” to treat degenerative blindness, inducing a light evoked retinal neuron response (Maya-Vetencourt et al., 2020; Francia et al., 2022). Although the efficiency of organic polymer-based retinal devices in vivo has been proven, the interpretation of the working mechanisms that grant photostimulation at the polymer/neuron interface is still a matter of debate.<br/><br/>In our work, we focus on understanding the mechanisms which may play a role at the bio-hybrid interface, decoupling them into: (i) a photochemical effect, which in our case consists in the photocathodic behavior of P3HT in watery oxygenated environment; (ii) an electrostatic effect, due to light induced capacitive charging of the NP, known as Photo-Dember effect. With mathematical modeling and electrochemical studies, we are able to identify the relative importance of each mechanism as a function of light intensity impinging onto the substrate, cleft size and cleft resistive properties.<br/><br/>The photo-chemical mechanism, connected to the production of O₂^- in the cleft, appears to be relevant solely when light intensity increases above the physiological ranges and under conditions of strict proximity to the neuron membrane (&lt;50 nm). The capacitive effect induced by the electrostatic charging of the nanoparticle is instead appreciable only in the presence of a highly resistive medium, where ionic screening can be assumed as negligible (Debye Length&gt;&gt;cleft thickness): if this condition is verified by the system, even physiological light intensities (0.2 W/m²) are able to induce appreciable depolarization of the neurons. The highly resistive medium in the model is accounted for by the presence of adhesion proteins in the cleft. Therefore, the formation of <i>“proteic islands”</i> is suggested by the model as a fundamental ingredient to ensure the coupling among neuron and nanoparticle at reduced light intensity.<br/><br/>A preliminary confirmation of our simulation results comes from ex vivo experiments on retinal explants that received P3HT-NPs in the subretinal space either acutely after the explantation or that were taken from rats which had been subretinally injected in vivo with P3HT-NPs one month before. The recordings of retinal ganglion cell firing on acutely injected explants displayed a significantly reduced response with respect to the retinas which experiences a prolonged in vivo contact with the NPs before explantation.<br/>The combined use of modeling and physiological experiments suggests that, in vivo, NPs are engulfed by the neuronal membrane with a highly resistive medium which ensures an efficient electrostatic coupling. While at the high light intensities used in in vitro experiments both phenomena may take place, at the lower light intensities used in vivo the sole electrostatic effect is responsible for the photostimulation. The results also suggest that in vitro experiments do not faithfully reproduce the in vivo condition and that a prolonged in vivo contact between NPs and the neurons, occurring in vivo, is necessary to fully elicit the physiological effects.