Carrageenan induced phosphorylation of Akt is dependent on neurokinin-1 expressing neurons in the superficial dorsal horn

Rats pretreated with spinal saporin conjugated to a stabilized form of substance P had substantial loss of neurons with neurokinin 1 receptors throughout their superficial, but not deep dorsal horns. Animals pre-treated with substance P-saporin exhibited no change in locomotor ability and a small, but significant decrease in carrageenan-induced mechanical allodynia when compared to animals pre-treated with spinal saporin alone. Importantly, carrageenan-induced phosphorylation of Akt was blocked, in the substance P-saporin treated group, throughout the spinal cord grey matter. In marked contrast, carrageenan induced-trafficking of the GluA1 receptor subunit increased equivalently in both treatment groups.We infer from these data that 1) phosphorylation of Akt in the deep dorsal horn is dependent on prior activation of NK1 receptor bearing cells in superficial dorsal horn, and 2) there are parallel spinal intracellular cascades initiated by the carrageenan injection downstream of PI-3K activation, including one containing Akt and another involving GluA1 trafficking into neuronal plasma membranes that separately lead to enhanced pain behavior. These results imply that the two pathways downstream of PI-3K can be activated separately and therefore should be able to be inhibited independently.Akt is a serine/threonine kinase that plays a pivotal role in many essential cellular processes including cell survival and apoptosis. Spinal phosphorylation of neuronal Akt at both the ser 473 and the thr 308 sites occurs following peripheral tissue injury; this can be observed throughout the superficial dorsal horn [1-4], but also is elicited prominently in lateral lamina V and in α-motor neurons [3]. This is a topic of interest, as spinal blockade of Akt phosphorylation (P-Akt) or phosphatidylinositol 3-kinase (PI-3K), its upstream activator, results in amelioration of injury-induced pain behavior [1,2,4,5]. More generally, phosphorylation of Akt is held to be an indicator of neur