Reduced Anti-Allodynic Effect of the Adenosine A1-Receptor Agonist R-Phenylisopropyladenosine on Repeated Intrathecal Administration and Lack of Cross-Tolerance with Morphine in a Rat Model of Central Pain

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Abstract
We examined the development of tolerance to the anti-allodynic effect of chronic intrathecal (IT) administration of the adenosine analog R-phenylisopropyladenosine (RPIA) in a rat model of central pain after ischemic spinal cord injury. After 10 days of IT R-PIA treatment, the effect of IT morphine was also assessed to examine whether cross-tolerance between R-PIA and morphine was present. IT R-PIA completely alleviated allodynia-like behaviors to mechanical and cold stimuli in spinally injured rats. The anti-allodynic effect of R-PIA was maintained for 6-7 days with twice-daily administration and was reduced thereafter, particularly with respect to cold allodynia. IT morphine alleviated mechanical and cold allodynia in rats rendered tolerant to R-PIA to a degree comparable to that in R-PIA-naive (control) rats, which indicates that the anti-allodynic property of R-PIA is independent of the mechanisms by which morphine acts. The possibility of using agonists of adenosine receptors in treating refractory pain in patients with spinal cord injury is discussed. Implications: There is often no satisfactory treatment for chronic pain after spinal cord injury. Our study suggests such pain can be treated with a spinal injection of R-phenylisopropyladenosine in rats. Reduced effect to R-phenylisopropyladenosine was noted with repeated administrations. However, there was no cross-tolerance to morphine. (Anesth Analg 1998;87:1367-71)