Figure 19. Model of VIP action on cortical astrocytes. VIP released from neurons interacts with receptors on glial cells. Secretion of cytokines, growth factors and protease nexin-1 occurs after stimulation of high-affinity receptors linked to intracellular calcium mobilization. Low-affinity VIP receptors are linked to adenylate cyclase and mediate glycogenolysis and other functions. A neuron-glia-neuron relationship is proposed for VIP's action in the CNS.