Antisense oligonucleotide knockdown of mGlu5 receptor attenuates the antinociceptive tolerance and up-regulated expression of spinal protein kinase C associated with chronic morphine treatment

Spinal metabotropic glutamate receptor 5 (mGlu5 receptor) is known to influence the development of intrathecal morphine antinociceptive tolerance. However, the signaling mechanisms remain unknown. We carried out intrathecal administration of an antisense oligodeoxynucleotide (ODN), which results in reduced expression of spinal mGlu5 receptor, to determine its effects on morphine tolerance and spinal protein kinase C (PKC) expression. Rats were treated intrathecally with saline, morphine, mGlu5 receptor antisense ODN or mGlu5 receptor mismatched ODN. Behavioral tests were used to test the thermal and mechanical pain thresholds. Eight days later, rats were sacrificed and spinal cords were harvested to assess the expression of spinal PKC (α, γ and ε) by Western blotting and real-time polymerase chain reaction (PCR). Compared to control, intrathecal mGlu5 receptor antisense ODN resulted in a ~ 53.9% reduction of spinal mGlu5 receptor after 8 days treatment. The mGlu5 receptor antisense ODN prevented the development of morphine tolerance. Expression of spinal PKC (α, γ and ε) was up-regulated at the mRNA and protein levels during the development of tolerance. Meanwhile, antisense ODN but not mismatched ODN reduced the spinal dorsal horn levels of PKC (α, γ and ε) which had been up-regulated after morphine exposure. We conclude that mGlu5 receptor participates in the development of morphine tolerance. Expression of spinal PKC (α, γ and ε) at the mRNA and protein levels increased during morphine tolerance. Antisense ODN of mGlu5 receptor prevented the tolerance and inhibited the altered expression of spinal PKC (α, γ and ε) during the development of tolerance.