P2X2 Receptor Terminal Field Demarcates a "Transition Zone" for Gustatory and Mechanosensory Processing in the Mouse Nucleus Tractus Solitarius.

Peripheral gustatory neurons express P2X2 purinergic receptors and terminate in the rostral portion of the nucleus tractus solitarius (rNTS), but a relationship between the P2X2 terminal field and taste evoked activity has not been established. Additionally, a portion of somatosensory neurons from the trigeminal nerve, which are devoid of P2X2 expression, also terminate in the lateral rNTS. We hypothesized that P2X2 receptor expression on afferent nerve endings could be used as an anatomical tool for segregating gustatory from mechanosensory responsive regions in the mouse rNTS. C57BL/6 mice were used to record extracellular activity from neurons within the rNTS and the laterally adjacent reticular formation and trigeminal nucleus. Histological reconstruction of electrolytic lesions indicated that gustatory activity coincided with electrode tracks that traversed through P2X2 terminal fields. Gustatory recordings made more rostral in the rNTS had receptive fields located in the anterior oral cavity (AO), whereas gustatory recordings made more caudal in the rNTS had receptive fields located in the posterior oral cavity (PO). Mechanosensory neurons with AO receptive fields were recorded near the lateral border of the P2X2 terminal field and became numerous on electrode tracks made lateral to the P2X2 terminal field. In contrast, mechanosensory responses with PO receptive fields were recorded within the P2X2 terminal field along with gustatory activity and transitioned to mechanosensory only outside the P2X2 terminal field. Collectively, our results indicate that the lateral border of the P2X2 terminal field, demarcates a faithful "transition zone," where AO responses transition from gustatory to mechanosensory.