Question

Total Discussion Description: Board 6 1. Compare the interactions Posts: with opioid receptors, analgesic effect, andr espiratory depressant effect of (a) morphine, (b) buprenorphine, and () naltrexone. (3 Points)

Answer 1

Morphine belongs to the to a class of drugs known as Opiod Analgesics; Buprenorphine belongs to a class of drugs called mixed opioid agonist-antagonists and Naltrexone belongs to a class of drugs known as Opiate antagonists.

Now to understand the drug interactions with opiod receptors first we should know what are the common opiod receptors. The major types of opioid receptors are - delta (δ),kappa (κ), mu (μ) , Nociceptin receptor & zeta (ζ) receptors. Now the analgesic effect and respiratory depressant affect of the drugs depends on the interactions it have on the opiod receptors.

Morphine and its metabolites act as agonists of the mu and the kappa opioid receptors which derive later into analgesia. However, in studies with delta opioid receptor, it was reported a reduction in the rewarding effect of morphine suggesting that the rewarding effect of morphine is related to activity towards the delta opioid receptor in the nucleus accumbens. From the three target receptors of morphine, the mu receptor is associated with the side effects of the morphine such as modifications in the respiratory system (respiratory depression) and addiction.

Buprenorphine's analgesic effect is due to partial agonist activity at mu-opioid receptors. Buprenorphine is also a kappa-opioid receptor antagonist. The partial agonist activity means that opioid receptor antagonists only partially reverse the effects of buprenorphine. The binding to the mu and kappa receptors results in hyperpolarization and reduced neuronal excitability. Furthermore, buprenorphine slowly dissociates from its receptor. This observation would account for the longer duration of action compared to morphine, the unpredictability of its reversal by opioid antagonists, and its low level of manifest physical dependence. Though Buprenorphine has onset, it has longer duration of action than Morphine.

Naltrexone is thought to act as a competitive antagonist at mc, κ, and δ receptors in the CNS, with the highest affintiy for the μ receptor. Naltrexone competitively binds to such receptors and may block the effects of endogenous opioids. This leads to the antagonization of effects of opiates, including respiratory depression, miosis, euphoria, and drug craving.