Intrathecal Knockdown of Adenylyl Cyclase 1 Attenuates Morphine Tolerance and Withdrawal in Mice

Abstract

Opioid tolerance and opioid induced hyperalgesia during repeated opioid administration and chronic pain are associated with upregulation of adenylyl cyclase activity. Therefore, we investigated if expression of components of the cyclic-AMP pathway were increased within in the nervous system tissues of morphine tolerant mice, and if inhibiting any of these changes could alleviate pain and/or tolerance in mouse models. The objective of this study was to test the hypothesis that a reduction in adenylyl cyclase 1 (AC1) activity would attenuate morphine tolerance and hypersensitivity, and inflammatory pain using murine models. Short-hairpin RNA (shRNA) gene knockdown of Adcy1 in the spinal cord and dorsal root ganglia was accomplished using a lumbar injection of an associated adenovirus viral vector (AAV9-GFP-U6-m-Adcy1-shRNA) and negative controls (AAV9-GFP-U6-m-scrambl-shRNA). Behavioral testing such as open field testing, rotarod testing, burrowing, thermal and mechanical paw withdrawal latencies were tested after injection. Morphine tolerance (15mg/kg, sc, 5 days and 10-40mg/kg escalation over 4 days) and opioid-induced hypersenstivity were also assessed after inoculation. Lumbar intrathecal administration of a vector incorporating adeno-associated virus and short-hairpin RNA against Adcy1 did not affect baseline parameters such as open field testing, rotarod testing, and burrowing testing. Morphine tolerance and withdrawal were attenuated in Adcy1 shRNA mice compared to control vector mice. Chronic exposure to morphine leads to changes in mRNA expression of proteins involved in the cAMP signaling pathway in different areas of the nervous system. Gene knockdown of Adcy1 decreases morphine tolerance and opioid-induced hypersensitivity, which could form the basis for novel therapeutics in the future. This work was supported through K01 DA042902 to AHK and the Purdue University College of Pharmacy to VJW.