Persistent neuropathic pain is the coordinated activation and sensitization of glial and neuronal elements both peripherally and centrally. Here, we have investigated the role of glial fibrillary acidic protein (GFAP)-positive astrocytes in the central nervous system (CNS) and non-myelinating Schwann cells in the peripheral nervous system (PNS) and their individual contributions to persistent neuropathic pain. We used three-dimensional reconstruction of ultrastructural data to establish the morphological relationship between astrocyte processes and incoming C and A-delta fiber synapses with second-order pain neurons. We found that not only do astrocytes contact 100% of the C and A-delta fiber glomerular synapses, but they also provide a high degree of ensheathment of each glomerulus. This encapsulation of the glomerular synapses puts astrocytes in a position to potentially modulate neuronal activity and synapse structure. Next, we used two glial-specific transgenic mouse models to interfere vesicular gliotransmitter release. However, neither blocking IP3-dependent Ca2+ release or SNARE-dependent vesicle release had any effect on nociception. We then used two glial-specific transgenic mouse models that interfere with the NFkB-COX2 inflammatory pathway and observed a robust yet temporary alleviation of pain behavior from one to five weeks post-nerve injury. This finding indicates that the NFkB-COX2 signaling pathway in GFAP-positive glia is critical to the maintenance of a specific phase of persistent neuropathic pain. GFAP-positive glia include peripheral non-myelinating Schwann cells, which ensheath unmyelinated nociceptive neurons, as well as central astrocytes, which ensheath neuronal synapses throughout the CNS. We used the tet-Off transgenic mouse expression system in a novel manner to tease apart the peripheral vs central roles of GFAP-positive cells. The administration of oxytetracycline, a blood-brain barrier impermeable analog of doxycycline, was capable of turning off transgene expression in the PNS without affecting transgene expression in the CNS. The blockade of peripheral transgene expression reversed the alleviation of pain behavior post-nerve injury in mice with suppressed NFkB activity. Thus, the suppression of NFkB in astrocytes is insufficient to relieve mechanical sensitization following nerve injury. The implicates non-myelinating Schwann cells in an important role in the maintenance of a specific phase of persistent neuropathic pain from one to five weeks post-injury.Doctor of Philosoph
