Inhibitors of catechol-O-methyltransferase sensitize mice to pain

Abstract

Background and purpose: Catechol-O-methyltransferase (COMT) inhibitors are used in Parkinson's disease in which pain is an important symptom. COMT polymorphisms modulate pain and opioid analgesia in humans. In rats, COMT inhibitors have been shown to be pro-nociceptive in acute pain models, but also to attenuate allodynia and hyperalgesia in a model of diabetic neuropathy. Here, we have assessed the effects of acute and repeated administrations of COMT inhibitors on mechanical, thermal and carrageenan-induced nociception in male mice.

Experimental approach: We used single and repeated administration of a peripherally restricted, short-acting (nitecapone) and also a centrally acting (3,5-dinitrocatechol, OR-486) COMT inhibitor. We also tested CGP 28014, an indirect inhibitor of COMT enzyme. Effects of OR-486 on thermal nociception were also studied in COMT deficient mice. Effects on spinal pathways were assessed in rats given intrathecal nitecapone.

Key results: After single administration, both nitecapone and OR-486 reduced mechanical nociceptive thresholds and thermal nociceptive latencies (hot plate test) at 2 and 3 h, regardless of their brain penetration. These effects were still present after chronic treatment with COMT inhibitors for 5 days. Intraplantar injection of carrageenan reduced nociceptive latencies and both COMT inhibitors potentiated this reduction without modifying inflammation. CGP 28014 shortened paw flick latencies. OR-486 did not modify hot plate times in Comt gene deficient mice. Intrathecal nitecapone modified neither thermal nor mechanical nociception.

Conclusions and implications: Pro-nociceptive effects of COMT inhibitors were confirmed. The pro-nociceptive effects were primarily mediated via mechanisms acting outside the brain and spinal cord. COMT protein was required for these actions.

Figure 1

(A) Striatal catechol-O-methyltransferase (COMT) activity in male C57/BL6J mice 1 and 3 h after i.p. administration of 3,5-dinitrocatechol (OR-486) (30 mg·kg⁻¹; 1 h, n = 7; 3 h, n = 13) or nitecapone (30 mg·kg⁻¹; 1 h, n = 8; 3 h, n = 16). Baseline (0 h) represents COMT activity of vehicle (carboxymethylcellulose, CMC; n = 21) treated animals. **P < 0.01; ***P < 0.001 versus vehicle group; one-way anova followed by a Bonferroni test. COMT activity of the OR-486 group was also lower than that of the nitecapone group 1 and 3 h after drug administration (1 h, P < 0.001; 3 h, P < 0.01). (B) Hepatic COMT activity after the same i.p. treatment as above (OR-486: 1 h, n = 7; 3 h, n = 6; nitecapone: 1 h, n = 8; 3 h, n = 5; vehicle: n = 12). **P < 0.01 versus vehicle group. COMT activity of the OR-486 group was also lower than that of the nitecapone group 1 h after drug administration (P < 0.001).

Figure 2

(A) Mechanical nociceptive thresholds after acute i.p. administration of 3,5-dinitrocatechol (OR-486) (30 mg·kg⁻¹; n = 16), nitecapone (30 mg·kg⁻¹; n = 16) or saline (n = 21). Results are expressed in grams. *P < 0.05; **P < 0.01 versus vehicle group; two-way repeated anova followed by a Bonferroni test. Thresholds of the OR-486 group were also significantly lower than of the nitecapone group at 2 and 3 h after treatment (at 2 h, P < 0.01; at 3 h, P < 0.05). (B) Change in nociceptive latencies in hot plate test after acute i.p. administration of OR-486 (30 mg·kg⁻¹; n = 16), nitecapone (30 mg·kg⁻¹; n = 16) or saline (n = 21). Results are expressed as percentages of the maximum possible effect (MPE%). Statistics: **P < 0.01; ***P < 0.001 versus vehicle group.

Figure 3

(A) Mechanical nociceptive thresholds after chronic (repeated) i.p. administration of 3,5-dinitrocatechol (OR-486) (30 mg·kg⁻¹; n = 16), nitecapone (30 mg·kg⁻¹; n = 16) or saline (n = 21). Results are expressed in grams. ***P < 0.001 versus vehicle group. (B) Thermal nociceptive latencies in hot plate test after repeated i.p. administration of OR-486 (30 mg·kg⁻¹; n = 16), nitecapone (30 mg·kg⁻¹; n = 16) or saline (n = 21). Results are expressed as percentage of the maximum possible effect (MPE%). Statistics: **P < 0.01; ***P < 0.001 versus vehicle group.

Figure 4

(A) Effect of repeated i.p. administration of 3,5-dinitrocatechol (OR-486) (30 mg·kg⁻¹; n = 16), nitecapone (30 mg·kg⁻¹; n = 16) and saline (n = 21) on paw flick responses 3 h after carrageenan injection in inflamed and non-inflamed paws. Results are expressed in seconds. *P < 0.05; **P < 0.01; ***P < 0.001 versus corresponding paw of the vehicle group. (B) Effect of repeated i.p. administration of OR-486 (30 mg·kg⁻¹; n = 16), nitecapone (30 mg·kg⁻¹; n = 16) or saline (n = 21) on carrageenan-induced nociception. Nociceptive latencies were measured with hot plate test 3 h after induction of inflammation and are expressed in seconds. *P < 0.05; ***P < 0.001 versus vehicle group.

Figure 5

(A) Effect of acute i.p. administration of CGP 28014 (30 mg·kg⁻¹; n = 15), or vehicle (carboxymethylcellulose, CMC; n = 15) on paw flick responses 2 and 3 h after drug administration. *P < 0.05 versus vehicle group. (B) Effect of acute i.p. administration of CGP 28014 (30 mg·kg⁻¹; n = 15), or vehicle (CMC; n = 15) on paw flick responses 3, 4 and 5 h after carrageenan injection in inflamed paws. Results are expressed as percentage of the maximum possible effect (MPE%).

Figure 6

(A) Effect of intrathecal administration of 600 µM (n = 11) or 200 µM (n = 11) of nitecapone or vehicle (n = 14) on mechanical nociceptive thresholds in male Wistar rats. Thresholds are expressed in grams. There was a time effect (P < 0.05) but no treatment effect. (B) Effect of intrathecal administration of 600 µM (n = 11) or 200 µM (n = 11) of nitecapone or vehicle (n = 14) on hot plate latencies in male Wistar rats. Latencies are expressed as percentage of the maximal possible response (MPE%). There was a significant effect of time (P < 0.05) but no effect of treatment.