Elevation of aversive threshold in rats by intra-amygdaloid injection of morphine sulphate

Bilateral micro-injection of morphine sulphate (10 microgram, 20 microgram) into the cortico-medial amygdala produced a dose-dependent increase in aversive threshold. Similar injections into the basolateral amygdala or caudate-putamen failed to have any consistent effect on aversive thresholds. Whilst overall activity levels remained unaffected by morphine injection into either amygdaloid site, caudate animals exhibited a significant decrement in total activity in response to both morphine and control injections. Results are discussed with reference to a possible role for limbic mechanisms in morphine analgesia.