Effects of low‐dose scopolamine on locomotor activity: No dissociation between cognitive and non‐effects

Abstract

Attempts have been made to dissociate the cognitive effects of scopolamine from its non-cognitive effects. It has been suggested that low doses of scopolamine may induce memory impairment without inducing significant non-cognitive effects. We therefore tested changes in locomotor activity (as a non-cognitive effect) in rats treated with low-dose scopolamine (which is believed to induce cognitive effects only). In this study, locomotor activity (as a non-cognitive effect) induced by low doses of this drug was evaluated by using two methods and rat strains. In the first study (circular box method, an automated open-field), scopolamine hydrobromide (HBr), scopolamine methylbromide (MeBr) or saline was injected subcutaneously into male Sprague-Dawley rats. After 30 min, rats were put into an automated open-field and locomotor activity was quantified by recording interruptions of infrared beams, with print-outs every 2 min for 16 min. Locomotor activity was assessed by summing these recordings. In the second study (closed platform), scopolamine HBr or saline was injected intraperitoneally into male Long-Evans rats. Twenty minutes later, the rats were placed in the center of a square-shaped closed platform (with 3×3 divisions). Locomotor activity was defined as the sum of crossings (traversing of four paws of the rat from one area into another of nine) and rears, which were recorded every 5 min for 20 min. Results from the circular box study showed that scopolamine HBr produced a marked increase in locomotor activity whereas scopolamine MeBr produced a non-significant decrease in locomotor activity. The closed platform data also demonstrated that scopolamine HBr increased locomotor activity significantly. These data show that scopolamine can induce non-cognitive effects (such as increased locomotor activity), even at low doses. Our results also imply that the increase in locomotor activity is mediated centrally.