Activation of the VIP/VPAC2 system induces reactive astrocytosis associated with increased expression of glutamate transporters

Abstract

Vasoactive intestinal peptide (VIP) is a pleiotropic neuropeptide that acts as a neuromodulator in the CNS. Recently, secretion of several functional molecules has been identified in VIP-stimulated astrocytes in vitro. However, the relationship between VIP and its specific receptors in neurological disorders remains unknown. To investigate the role of the VIP system under pathological conditions, we performed a cold injury on the right cerebrum of adult C57BL/6 mice and observed expression patterns for VIP and its receptor, VPAC2. Immunohistochemical studies revealed VPAC2 expression in reactive astrocytes around the core lesion by post-injury day 7, which then returned to contralateral levels at post-injury day 14. By contrast, VIP immunoreactivity was detected in activated microglial cells, suggesting that microglia–astrocyte interactions in the VIP/VPAC2 system are important for the tissue repair process. In primary cultured astrocytes stimulated with N6,2′-O-dibutyryladenosine 3′,5′-cyclic monophosphate sodium salt (dbcAMP) to mimic reactive astrocytosis, VPAC2 mRNA expression was highly up-regulated compared to that of the other VIP receptors, PAC1 and VPAC1. VPAC2 activation by the selective VPAC2 agonist, Ro25-1553, induced reactive morphological and biochemical changes from a polygonal shape to a stellate shape in cultured astrocytes. Further, Ro25-1553 increased cell surface expression of the glutamate transporters GLAST and GLT-1, which can limit excitotoxic neuronal cell death. In summary, the transient expression of VPAC2 in reactive astrocytes and the up-regulation of functional glutamate transporters suggest that the VIP/VPAC2 system induces reactive astrocytosis and plays a key role in neuroprotection against excitotoxicity in neurological disorders.