Question

a.Name the class of drug and the subjective effects it has on a user. In addition to the risk of death associated with respiratory failure due to drug overdose, the addictive quality of the drug can destroy a user’s life. One drug used to reduce the impact of the addiction is Methadone. Describe the action of both Heroin and Methadone at the receptor level.

b.Nutt et al. (2015) present the addictive qualities of drugs like heroin as evidence against current models of addiction that focus entirely on dopamine. Summarise the key arguments and conclusions that are based on the impact of drugs like heroine on the brain and a person’s experience associated with drug taking.

c.Explain why methadone would be beneficial for people addicted to heroin. In your answer outline why this difference in drug action of methadone (compared to heroin) makes sense in terms of our understanding of receptor function and also on a more general level, how reward and value are coded by the brain.

(Around 100 words each)

  

Answer 1

First Question (a)

Describe the action of both Heroin and Methadone at the receptor level.

Heroin:

Heroin is synthetically derived from the morphine alkaloid in opium and is approximately twice as potent as morphine. Heroin acts agonistically on central nervous system (CNS) opioid receptors mu, kappa, and delta.

Mu receptor effects account for both the analgesic effects (Mu1) and the respiratory depression and euphoria (Mu2). Activation of Mu2 receptors also causes miosis, reduced gastrointestinal (GI) motility, and physiologic dependence. Kappa receptor activation causes some degree of analgesia as well. Delta receptors are more involved in spinal analgesia phenomena.

Heroin metabolizes in the CNS to monoacetylmorphine, which is a more potent mu-receptor agonist than morphine. Peripheral tissues metabolize heroin to 6-monoacetylmorphine and then morphine.

When taken orally, heroin undergoes first-pass metabolism to morphine via deacetylation. Therefore, unlike intravenous administration, oral ingestion does not cause a rapid onset of effects and is less desirable to users. 6-monoacetylmorphine and morphine activate opioid receptors.

Methadone:

Methadone primarily acts as an agonist at the μ-opioid receptor, but, unlike most other opioids, it also antagonizes the N-methyl, d-aspartate (NMDA) receptor and blocks serotonin and norepinephrine transporters. Methadone also blocks the cardiac potassium channel, hERG, which can cause a prolonged QT interval on the ECG. Typical μ-opioid agonist effects of methadone include miosis, analgesia, sedation, possible euphoria, decrease in gut motility, and respiratory depression; like other opioids, tolerance may develop to some of these effects.