Exploratory behaviour and aversive thresholds following intra-amygdaloid application of opiates in rats

Rats were bilaterally implanted with guide cannulae aimed at the central or medial nucleus of the amygdala. Microinjections of morphine (10 microgram) at both sites significantly elevated the threshold of response in the flinch-jump test; but only at medial sites did naloxone (1 microgram) antagonise this effect. However, in the hole-board test, an opposite pattern of results emerged. Morphine injections into the central nucleus produced naloxone-reversible reductions in both exploration and activity whilst, in the medial nucleus, the morphine-induced decrease in exploration was not reversed by naloxone. It is concluded that (1) the presence or absence of naloxone-sensitive opiate receptors cannot always be deduced on the basis of a single behavioural test and (2) within the amygdaloid complex, two distinct naloxone-sensitive opiate systems appear to be involved in the modulation of behavioural responses to different forms of stimulation.