Antisense knockdowns of M1 receptors induces transient anterograde amnesia in mice

The effect on memory processes of inactivation of the M1 gene by an antisense oligodeoxyribonucleotide (aODN) was investigated in the mouse passive avoidance test. Mice received a single intracerebroventricular (i.c.v.) injection of M1 aODN (0.3, 1.0 or 2.0 nmol per injection), degenerated ODN (dODN) or vehicle on days 1, 4 and 7. An amnesic effect, comparable to that produced by antimuscarinic drugs, was observed 12, 24, 48 and 72 h after the last i.c.v. aODN injection, whereas dODN and vehicle, used as controls, did not produce any effect. Reduction in the entrance latency to the dark compartment induced by aODN disappeared 7 days after the end of aODN treatment, which indicates the absence of any irreversible damage or toxicity caused by aODN. Quantitative reverse transcription-polymerase chain reaction analysis demonstrated that a decrease in M1 mRNA levels occurred only in the aODN-treated group, being absent in all control groups. Furthermore, a reduction in M1 receptors was observed in the hippocampus of aODN-treated mice. Neither aODN, dODN nor vehicle produced any behavioral impairment of mice. These results indicate that the integrity and functionality of M1 receptors are fundamental in the modulation of memory processes.