Opposite effects of a D1 and a D2 agonist on oral movements in rats

Oral movements in rats administered one of three doses of either a D1 agonist (SK&F 38393) or a D2 agonist (LY171555) were observed via closed-circuit television and simultaneously recorded using a computerized video analysis system which measured the distance between two fluorescent dots painted above and below the rat's mouth. The D1 agonist SK&F 38393 induced a dose-dependent increase in tremorous oral movements, tongue protrusions, and, at the highest dose, increased repetitive chewing movements. Conversely, the D2 agonist LY171555 produced an inhibition or oral activity at all dose levels. At the lowest dose tested this appeared to reflect a non-specific decrease in activity, for there was an inhibition of all categories of behavior measured, as well as of all amplitudes of computer-scored movements and slow, sluggish movements were recorded. But higher doses of LY171555 induced hyperactivity and stereotyped, repetitive head movements whereas chewing movements, tremorous oral movements, and tongue protrusions were still decreased. D1 and D2 dopamine receptors appear to have opposite effects on oral movements.