Dissociable effects of histamine H1 antagonists on reaction-time performance in rats

The most pronounced side effect of antiallergic histaminergic drugs (H1 antagonists) is sedation. These effects have been linked with the effects of histaminergic drugs on central H1 receptors. In the present study, we investigated the dose-response relationship of different antihistamines on the performance in a reaction-time task that has been developed for rats. The dose-response relationship of diphenhydramine, cetirizine and terfenadine were examined for the various behavioural measures in this task (i.e., reaction time, motor time, premature responses and number of trials completed). In addition, the effects of scopolamine were assessed to evaluate the cholinergic profile in this task. Diphenhydramine did not reliably affect the reaction time, but increased the motor time and the proportion of premature responses, and decreased the number of trials completed in a session. A low dose of cetirizine decreased the reaction time, whereas an increase in reaction time was found for the high dose. The motor time was increased after both doses of cetirizine. Terfenadine did not affect the responding of rats in the reaction-time task at the doses tested. The effects of scopolamine were very similar to those of diphenhydramine. The reaction-time task used in this study was able to dissociate different types of antihistamines on aspects of psychomotor function, which were likely to be related to central muscarinic or H1 antagonism. These findings suggest that the reaction-time task may be a sensitive tool for assessing effects of drugs on psychomotor function.