THE LOCOMOTOR EFFECTS OF A PUTATIVE DOPAMINE D-3 RECEPTOR AGONIST IN DEVELOPING RATS

Dopamine receptors have been categorized into subfamilies D-1 and D-2, each with separate roles in dopamine-mediated behaviors. Of the D-2 s ubfamily, the dopamine D-3 receptor has been cloned, but the behaviora l effects of selectively stimulating the D-3 receptor an largely unkno wn. The purpose of this study was to quantify the locomotor responses of developing rats to the putative dopamine D-3 receptor agonist, 7-hy droxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT). One of three doses of 7-OH-DPAT (0.01, 0.10, 1.00 mg/kg) or saline was injected subcutane ously into rats at the age of 10, 20, 30, or 60 days. Five minutes aft er the injection, rats were placed in automated activity monitors whic h recorded locomotor behavior at 5 min intervals for 2 h. The high dos e of 7-OH-DPAT increased locomotor activity in rats of all ages. The m edium and low doses increased activity in 10- and 20-day-old rats but not in 30- or 60-day-old rats. The level of drug-induced activation pe aked at 20 days of age. In 30- and 60-day-old rats, but not 10- and 20 -day-old rats, a period of locomotor suppression preceded the activati on in response to the high dose of 7-OH-DPAT. In rats aged 20 days and older, the middle and low doses decreased locomotion early in the tes t session, but activation did not ensue. This dose-response pattern ac ross ontogeny closely resembles that induced by quinpirole, an agonist at the dopamine D-2 receptor subfamily.