Acquisition of brightness discrimination in the rat is impaired by opiates with psychotomimetic properties.

The acquisition of shock avoidance behavior by rats was studied in an automated Y-maze which incorporates a simultaneous brightness discrimination paradigm. When administered prior to each of 5 consecutive daily sessions, two opiate derivatives with psychotomimetic properties, cyclazocine and N-allylnormetazocine, impaired the acquisition of brightness discrimination at doses which also increased movements between trials. These effects were similar to those produced by phencyclidine, ketamine, and a high dose of d-amphetamine. Pretreatment with morphine, pentazocine and scopolamine at motor stimulant doses before each training session did not affect acquisition of brightness discrimination. Nalorphine, naltrexone, and chlorpromazine also had no effect on brightness discrimination, even at motor depressant doses. Whereas the motor stimulation produced by morphine or pentazocine was blocked by naltrexone, the motor stimulation and discrimination disruption produced by cyclazocine or N-allylnormetazocine were only incompletely antagonized by naltrexone. The results demonstrate similarities between psychotomimetic opiates and phencyclidine-like compounds and may reflect the sensory or cognitive disturbance produced by these drugs in man.