VIP and PACAP in the CNS: Regulators of glial energy metabolism and modulators of glutamatergic signaling

VIP neurons are a homogeneous population of intracortical bipolar cells. They receive excitatory synapses from afferent circuits to the cortex and exert effects on neurons, astrocytes, and capillaries. Effects on the two latter cell types imply that VIP neurons can translate incoming neuronal signals into local metabolic actions. Indeed, VIP tightly regulates glycogen metabolism in astrocytes. In this fell type VIP regulates the expression of a number of genes related to energy metabolism, such as glycogen synthase. These effects of VIP involve the transcription factor family C/EBP and result in the Induction of at least seven new proteins by astrocytes, The actions of VIP on neurons appear to be of a modulatory nature: thus VIP enhances glutamate-mediated neurotransmission by potentiating the effects of glutamate on arachidonic acid formation and on the induction of c-fos and on BDNF expression. These effects Indicate that VIP can actually increase the strength of glutamate-mediated neurotransmission.