The spinal antinociceptive effects of cholinergic drugs in rats: receptor subtype specificity in different nociceptive tests

Abstract

Background: Several studies have shown that muscarinic cholinergic agonists cause antinociception in humans and animals when given by both spinal and non-spinal parenteral routes. It is uncertain which subtype of muscarinic receptor is involved in spinally mediated antinociceptive effects caused by these drugs. The cholinergic receptor agonists McN-A-343 (M1 selective; 3.89 to 389 nmol) and carbachol (non-selective; 0.029 to 29 nmol) were used in a rat acute pain model to investigate the involvement of M1 and non-M1 subtypes in spinally mediated antinociception. The drugs were injected intrathecally and results from experiments in which drug actions were carefully confined to the spinal cord were used to construct agonist dose response curves.

Results: McN-A-343 frequently diffused rostrally to the brain, away from the lumbosacral site of injection. Thus, in spite of its receptor subtype selectivity, McN-A-343 is a poor probe to use in attempting to identify receptor subtypes involved in spinal cord antinociceptive systems. However, in some experiments McN-A-343 caused spinally mediated antinociception assessed by the electrical current threshold test. Antinociception assessed by the tail flick latency test with intrathecal McN-A-343 was observed and found to involve supraspinal mechanisms. Carbachol caused spinally mediated antinociception assessed by both electrical current threshold and tail flick latency.

Conclusions: The results suggest that M1 receptors are involved in spinally mediated antinociception revealed by electrical current threshold; other cholinergic receptors (non-M1) are involved in thermal antinociception at the spinal cord. This contrasts with previous work on spinally mediated cholinergic antinociception. These differences are believed to be due to difficulties in restricting the action of these drugs to the spinal cord.

Figure 2

Dose response curves for intrathecal carbachol, all data All ECT (neck and tail) and TFL values recorded in the rats that received intrathecal doses of carbachol that subsequently had positive lignocaine tests. Values for each testing site and modality have been combined at each dose of carbachol and shown as means ± SEM; n = 8–12.

Figure 3

Dose response curves for the spinally mediated antinociceptive effects of intrathecal McN-A-343 The values for ECT (neck, panel A; tail, panel B) and TFL (panel C) are shown on a scatter plot against dose of intrathecal McN-A-343. Only the results from testing times when there was no significant rise in ECT (n) values are shown. These values have been subjected to a logistic regression shown by the continuous line bounded by ± 95% CI shown as broken lines.

Figure 4

Dose response curves for intrathecal McN-A-343, all data All ECT (neck and tail) and TFL values recorded in the rats that received intrathecal doses of McN-A-343 that subsequently had positive lignocaine tests. Values for each testing site and modality have been combined at each dose of McN-A-343 and shown as means ± SEM; n = 20–25.

Figure 5

Dose response curves for the spinally mediated antinociceptive effects of intrathecal carbachol The values for ECT (neck, panel A; tail, panel B) and TFL (panel C) are shown on a scatter plot against dose of intrathecal carbachol. Only the results from testing times when there was no significant rise in ECT (n) values are shown. These values have been subjected to a logistic regression shown by the continuous line bounded by ± 95% CI shown as broken lines.

Figure 1

Experimental protocol for nociceptive testing This figure shows the experimental protocol used in all experiments. X values represent ECT (tail and neck) and TFL measurements made before intrathecal (IT) drug administration and Y values represents measurements made after drug administration.