Dose-related response of male rats to apomorphine: snout contact in the open-field.

Apomorphine (0.01 to 5 mg/kg, SC) was administered to male rats observed singly in the open-field. The behavior of each rat was coded using a microprocessor during 3 preinjection and 9 postinjection trials of 6 min duration over a 2 hr session. The behavior categories included grooming, yawning, turning, nodding and gnawing, as well as snout contact and nonsnout contact variants of locomoting, rearing and sitting. Dose-dependent increases in the time spent in snout contact with the field surface were noted throughout the complete dose range. Both the peak and duration of the snout contact epoch increased with the dose of apomorphine. The integrated time spent in all types of snout contact proved to be the best behavioral measure for discriminating between doses of apomorphine even though the topography of snout contact response changed as a function of the dose.