The effects of microinjection of the selective blocker of muscarinic M1 receptors pirenzepine into the neostriatum on the motor behavior of rats.

A discrimination conditioned active avoidance reflex (CAAR) model in a T maze was used in 18 rats to study the effects of bilateral microinjections of the selective muscarinic M1 receptor blocker pirenzepine into the neostriatum on the acquisition of the CAAR and behavior in an open field test. There was sharp degradation of learning of the CAAR and a significant improvement in motor activity both in the open field test and in the maze itself in rats given bilateral microinjections (pirenzepine, 0.004 mg) into the neostriatum as compared with intact controls. This suggests that changes in motor behavior (a sharp increase in locomotor activity) may be among the reasons for difficulty in learning the CAAR in rats after pirenzepine microinjections. Another reason for difficulty in learning the CAAR in these animals may be impairment of the perception of the conditioned signals (a flashing light) and poor differentiation. This is particularly indicated by the delay in the start chamber (double that seen in intact animals) on presentation of conditioned signals despite the high level of motor activity. These results and published data provide evidence for the complex nature of changes induced by blockade of muscarinic M1 receptors in the neostriatum.