Multifunctional devices for optogenetic stimulation and neural recording may offer benefits to the basic study of the nervous system. Since optogenetic experiments rely on viral delivery of opsin genes and require visible light, an invasive two-step surgery is inevitable. As the majority of optogenetic studies still rely on commercially available silica fibers outfitted with metallic or semiconducting electrodes, the modulus mismatch between these devices and the brain tissue lead to profound foreign-body response posing a barrier to long-term opto-electrophysiology. Consequently, there remains a need for flexible and multifunctional neural probes that seamlessly combine viral delivery with optogenetic stimulation and electrophysiological recording in freely moving rodents.
Here we introduce an all-polymer probe that integrates an optical waveguide, 6 electrodes, and 2 microfluidic channels. This device produced via a thermal drawing process has a cross sectional diameter of 200 μm (smaller than typical silica fibers used for optical control of rodent behavior), and connectorized with optical, electrical and microfluidic interfaces weighs <0.5g. The choice of materials enabled low-loss optical transmission, and the development of a custom conductive polyethylene (CPE) composite with graphite (5% by weight) yielded electrodes with linear dimensions of 20-30 µm and impedance comparable to that of metallic microwires (100s kΩ) enabling electrophysiological recordings of isolated action potentials with high signal to noise ratio (SNR) during recording process. The probes maintained the optical, electrical, and microfluidic properties even under mechanical deformation.
The utility of our devices was confirmed by recording the optically-induced neural activity 2 weeks following the delivery of the adeno-associated virus carrying a gene for channelrhodopsin 2 into medial prefrontal cortex (mPFC) of wild type mice. Optical stimulation in the premotor area resulted in increase of locomotor activity consistent with ChR2-facilitated excitation. Multiple implantations were also performed to allow optogenetic studies of projections from the basolateral amygdala to the mPFC or the ventral hippocampus (vHPC). These circuits exhibited distinctly different latencies of optically evoked signals, and furthermore the activity was correlated to the behavioral response. Consistent with prior studies, stimulation of the BLA inputs into the vHPC resulted in a decreased time spent in the center during a standard open field test.
Finally, the flexibility of our probes was manifested in their enhanced biocompatibility as corroborated by reduced glial response and blood-brain barrier breach following up to 90 days of implantation. As our device allowed for minimally-invasive optogenetics in freely moving mice with a one-step surgery, we anticipate its future applications in systems neuroscience studies.