Investigating intrathecal adeno-associated virus vectors for the study of analgesic interactions at the spinal level

Abstract

Pain signals from the periphery enter the central nervous system (CNS) via axons of primary afferent sensory neurons residing in dorsal root ganglia (DRG). Inhibition of pain signals directly at the spinal level provides an opportunity to reduce pain at the most basic level. Several G protein-coupled receptors, including opioid and adrenergic receptors, are known to be potent regulators of pain signaling at the spinal level. Co-activation of different subtypes of these receptors can lead to synergistic inhibition of pain signaling. Understanding how these receptors interact at the cellular level may provide insight into how to exploit such mechanisms for improved pain management strategies. This thesis has taken two approaches to advance the study of interactions involving opioid and adrenergic receptors at the spinal level: develop the use of adeno-associated virus (AAV) vectors for gene-transfer to sensory neurons where the expression of involved proteins can be modulated, and to further characterize these interactions by understanding of the cellular processes underlying their outcomes. The first part of this thesis describes the transduction pattern in DRG and CNS resulting from intrathecal delivery of AAV serotypes 5, 8, and 9. The results obtained demonstrate differential tropism of AAV vectors for subpopulations of primary afferent neurons and some brain areas. The second part of this thesis characterizes interactions between spinally delivered combinations of an opioid and an adrenergic agonist, or two opioid agonists, based on the requirement of protein kinase C-epsilon (PKC-epsilon) for analgesic synergy. The results obtained suggest that PKC-epsilon is not involved in analgesic activity of agonists delivered singly, but is necessary for synergistic interactions resulting from specific agonist combinations. The last part of this thesis integrates the primary results from the first two parts, and suggests ways that differences in tropism of AAV vectors can be exploited to study various aspects of signaling interactions between G protein-coupled receptors at the spinal level.