L-dopa and apomorphine disrupt long- but not short-behavioural chains

Apomorphine (0.01-0.5 mg/kg) disrupted fighting between two male ferrets. The number of bites and neck nuzzling were decreased dose-dependently. At 0.05 and 0.1 mg/kg scent-marking and collateral behaviour were increased, at 0.5 mg/kg stereotyped sniffing was induced. L-dopa (120 mg/kg + 30 mg/kg benserazide) exerted similar effects to apomorphine 0.05 mg/kg. Thus, the dopamine (DA) agonists increased short chain responses but disrupted long ones. Haloperidol (0.2 mg/kg) facilitated inter-male fighting. The number of bites increased, but scent-marking and collateral behaviour were selectively suppressed. Thus, the persistency of the long behavioural chain was enhanced. Since, at lower doses, DA-agonists expand and DA-antagonists selectively narrow the range of exhibited behavioural responses, it is suggested that the degree of stimulation of central DA-receptors determines the animal's ability to suppress responses that compete with ongoing behaviour.