A brainstem substrate for analgesia elicited by intraoral sucrose

Previous studies demonstrated that nursing or intraoral infusion of certain components of mother's milk (e.g. sugars and fats) produces calming and opiate receptor-dependent analgesia in newborn rats and humans. However, the neural circuitry underlying such analgesia is unknown. The aim of the present study was to specify the central pathways by which taste stimuli engage neural antinociceptive mechanisms. For this purpose, midcollicular transactions were used to investigate the role of the forebrain in analgesia elicited by intraoral infusion of 0.2 M sucrose in neonatal rats. Sucrose-induced analgesia persisted, and was enhanced, following midcollicular transection, indicating that it did not require neural circuits confined to the forebrain. Fos immunohistochemistry was used to identify brainstem neurons activated by a brief (90 s) intraoral infusion of a small volume (90 microl, 0.2M) of sucrose or a salt solution (0.1 M ammonium chloride) in 10-day-old rat pups. Compared with control groups (intact, cannula, distilled water), both sucrose and ammonium chloride induced Fos expression in the rostral nucleus tractus solitarius, the first relay in the ascending gustatory pathway. Sucrose also elicited Fos expression in several brainstem areas associated with centrally mediated analgesia, including the periaqueductal gray and the nucleus raphe magnus. Taken together, these findings demonstrate that analgesia elicited by intraoral sucrose does not require involvement of the forebrain. Intraoral sucrose activates neurons in the periaqueductal gray and nucleus raphe magnus, two key brainstem sites critically involved in descending pain modulation.