Loss of muscarinic antinociception by antisense inhibition of M 1 receptors

The effect on cholinergic analgesia of inactivation of the M 1 gene by an antisense oligodeoxyribonucleotide (aODN) was investigated in the mouse hot plate test. Mice received a single intracerebroventricular (i.c.v.) injection of anti‐M 1 aODN (0.3, 1.0 or 2.0 nmol per injection), degenerate ODN (dODN) or vehicle on days 1, 4 and 7. A dose‐dependent inhibition of the antinociception induced by the muscarinic agonists oxotremorine (0.1 mg kg −1 s.c.) and McN‐A‐343 (30 μg per mouse i.c.v.) and the cholinesterase inhibitor physostigmine (0.2 mg kg −1 s.c.) was observed 24 h after the last i.c.v. injection of aODN. Time‐course experiments revealed that, after the end of the aODN treatment, sensitivity to analgesic drugs progressively appeared reaching the normal range at 96 h. The anti‐M 1 aODN was selective against muscarinic antinociception since the enhancement of pain threshold produced by morphine and baclofen were not affected by the above‐mentioned treatment. dODN, used as control, did not affect muscarinic antinociception. Binding studies evidenced a selective reduction of M 1 receptor levels in the hippocampus of aODN‐treated mice. Neither aODN, dODN nor vehicle produced any behavioural impairment of mice as revealed by the rota‐rod and Animex experiments. These results indicate that activation of M 1 muscarinic receptor subtype is fundamental to induce central cholinergic analgesia in mice.British Journal of Pharmacology (2000) 129 , 1633–1640; doi: 10.1038/sj.bjp.0703268