The effects of morphine on the CNS

The 1973 paper by Jacquet and Lajtha, was a pivotal report in understanding the effects of opioids on nociception, as it investigated the effects of morphine on different brain structures. To do this, the authors used a microinjection technique that allowed them to specifically target subcortical sites. The animals used in this study were rats, and evaluation of the nociceptive threshold was based on the behavioural reaction to electrical shock. Two reactions were evaluated: flinch or jump responses. The main result was that, depending on the dose and the site of injection, morphine produced either an increase in nociceptive threshold (analgesia), or a decrease in the nociceptive threshold (hyperalgesia). The authors also reported severe effects characterized by hyper-reactivity and violent uncontrolled jumping in stereotyped circular leaps when morphine was administered in the periaqueductal grey matter.