In Vivo Calcium Imaging of Neuronal Ensembles in Networks of Primary Sensory Neurons in Intact Trigeminal Ganglia

Calcium imaging is an important tool for monitoring signaling mechanisms that either influx Ca²⁺ into the cytosol or release Ca²⁺ from intracellular Ca²⁺ stores. The use of Pirt-GCaMP3 mice, which have the Ca²⁺sensor GCaMP3 inserted downstream of the Pirt promoter region and expressed in nearly all (>95%) primary sensory neurons, allows for simultaneous monitoring of the activity of >3,000 neurons via Ca²⁺ imaging of trigeminal ganglion (TG) primary sensory neurons. This allows investigators to study neural network responses to noxious stimuli and somatosensory and hormonal responses in vivo, which would otherwise be very difficult to detect using other methods. Specifically, the ability to monitor the TG area innervating the face and other areas of the head separately allows for monitoring primary sensory neurons activated by stimuli of the face area before later layers of neuron processing. The number of neurons generating Ca²⁺ transients and the amplitude of Ca²⁺ transients, which indicate their sensitivity to different sensory modalities, can also be observed. Additionally, investigators can measure the diameter of neurons, which is an indicator of the type of fiber activated (non-noxious mechano-fibers vs. noxious nociceptive fibers, Aβ, Aδ, and C fibers). Therefore, Pirt-GCaMP3 calcium imaging of TG would be a powerful tool to dissect specific sensory modalities and neuronal cell types to study trigeminal origin pain (orofacial and craniofacial, dental or/tooth pain, headache, migraine, temporomandibular joint pain, trigeminal neuralgia), itch and touch and any other somatosensations, as well as hormonal responses.