Effects of pretreatment with naloxone on behaviours induced by a small dose of apomorphine

The opiate antagonist, naloxone, was used to determine whether endogenous opioids modulate behavioural effects induced by a low dose of apomorphine. Before administering apomorphine (0.075 mg/kg) or saline, rats were pretreated with naloxone (1 mg/kg) or saline. Each subject received all 4 possible treatments (saline-saline, saline-apomorphine, naloxone-saline, and naloxone-apomorphine) in random order. Naloxone reduced the frequency and altered the timing of apomorphine-induced yawning, reduced the frequency of apomorphine-induced stretching, potentiated the effect of apomorphine on delaying grooming of the body, and did not affect the hypoactivity induced by apomorphine. Moreover, like apomorphine, naloxone itself reduced activity. Furthermore, naloxone and apomorphine injected together increased the latency to groom the face. These results suggest that in some circuits, endogenous opioids interact with dopaminergic autoregulatory mechanisms.