For a detailed account of the construction of implantable optical fibers and optical patch cables, see Sparta et al. (2011). Briefly, for chronic fibers 200 μm core, 0.37 NA standard multimode

hard cladding fiber (Thor Labs) was stripped of the cladding, threaded through a Nintedanib clinical trial 230 μm multimode zirconia ferrule, polished, and cut to a length of 6 mm. They were then tested for light output and sterilized using 70% ethanol before implantation into the brain. Optical patch cables connecting the fiber optic rotary joint to the chronic fiber consisted of a 60 μm core multimode fiber (0.22 NA) were threaded through furcation tubing connected to a 127 μm ID bore ceramic zirconia ferrule and a multimode FC multimode ferrule assembly (Precision Fiber products). Optical patch cables connecting the fiber optic rotary interfaced to the laser housing consisted of FC multimode ferrule assemblies on both ends. Light transmission was measured for all implantable

optical fibers before BMN-673 implantation and after the animal was sacrificed. Only optical fibers with > 80% light transmission prior to implantation were used. Two to three weeks after AAV-Ef1a-DIO-ChR2-eYFP virus injection and fiber implantation into the VTA, male VGAT-ires-CRE mice were food restricted to 85–90% of their free-feeding bodyweight. Mice were food restricted for 3 days while also tethered to custom-made optical patch cables for 1 hr/day before they were trained

for habituation purposes. Mice were then trained in sound-attenuated mouse chambers (Med Associates) equipped with a white noise and Resminostat tone generator, cue lights, and a receptacle for sucrose delivery that could detect head entries and individual licks. Mice were trained for one session per day that lasted ∼60 min. Each session consisted of 40 trials wherein a randomized 60–120 s intertrial interval was followed by a 5 s tone/light cue presentation that terminated with delivery of 20 μl of a 10% sucrose solution. During each training session, the chronic optical fiber was connected to a patch cable that interfaced with a FC/PC fiber optic rotary joint (Doric Lenses), which then interfaced with a 473 nm solid-state laser outside the chamber. Mice were trained for 20–25 days until stable responding was observed to both cue and reward presentation. After training, mice underwent behavioral sessions where they received a 5 s laser stimulation (12 mW into the brain) starting either at the onset of the cue or the reward delivery in a counterbalanced fashion. Laser stimulation sessions were always flanked by nonstimulation sessions on the previous and following days, where the laser light was blocked from reaching the brain by a piece of material that prevented light transmission inside the connecter sleeve between the optical patch cable to the chronic fiber.

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