SEPARATE AND COMBINED EFFECTS OF DOPAMINE D-1 AND D-2 RECEPTOR AGONISTS ON KEY PECKING IN THE DEVELOPING CHICK

The separate and combined effects of dopamine D-1 and D-2 agonists on key-peck responding of young chicks for heat reinforcement were determ ined. In the first experiment, 1- and I-day-old chicks (n = 96) were i njected SC with either distilled water or 5 mg/kg reserpine 18 h befor e testing. Twenty minutes before a 24-trial autoshaping session, chick s were injected IP with either distilled water or 10 mg/kg SKF 38393 a nd 2 mg/kg quinpirole. Chicks receiving both dopamine agonists had enh anced key-peck responding in both pretreatment conditions. In Experime nt 2, 1- and 4-day-old chicks (n = 192) of two strains received 5 mg/k g reserpine SC 18 h before testing. Twenty minutes before their autosh aping session these chicks were injected IP with either distilled wate r, 10 mg/kg SKF 38393, 2 mg/kg quinpirole, or 10 mg/kg SKF 38393 and 2 mg/kg quinpirole. After reserpine pretreatment, chicks of both strain s responded on more trials when given both dopamine agonists compared with controls given either distilled water or single agonist treatment . No age or strain differences in key pecking were produced by the com bined administration of these dopamine agonists. Therefore, functional coupling of the dopamine D-1 and D-2 receptors is found within 1 day after hatching in the domestic chick.