Multiphoton microscopy provides intravital imaging of neuronal structure and function in vivo in brain regions that lie within 1 mm of the surface of the brain. Imaging deeper structures is outside the range of standard multiphoton imaging. However, the use of a gradient refractive index microendoscope (GRIN lens), coupled with multiphoton microscopy, allows imaging of deeper structures in the brain.
In this paper the authors describe the use of a GRIN lens to perform calcium imaging of neurons in the geniculate ganglion, a structure lying 4 mm below the brain surface. Their interest in the geniculate ganglion was that as the initial neural station in the gustatory system, it receives inputs from all 5 types of dedicated taste receptor cells.
They were able to monitor neural activity of ensembles of geniculate ganglion neurons in response to individual as well as mixed taste stimuli. Their findings support a model of a direct match between taste receptor cells and ganglion neurons. They also demonstrate a robust imaging preparation for further work exploring molecular markers defining taste and how their stimulus quality is transmitted to the brain.