DARK Classics in Chemical Neuroscience: Morphine

As the major psychoactive agent in opium and direct precursor for heroin, morphine is a historically critical molecule in chemical neuroscience. A structurally complex phenanthrene alkaloid produced by Papaver somniferum, morphine has fascinated chemists seeking to disentangle pharmacologically beneficial analgesic effects from addiction, tolerance, and dependence liabilities. In this review, we will detail the history of morphine, from the first extraction and isolation by Sertürner in 1804 to the illicit use of morphine and proliferation of opioid use and abuse disorders currently ravaging the United States. Morphine is a molecule of great cultural relevance, as the agent that single-handedly transformed our understanding of pharmacognosy, receptor dynamics, and substance abuse and dependence disorders.

Methoxyflavones from Stachys glutinosa with binding affinity to opioid receptors: in silico, in vitro, and in vivo studies

Fractionation of the bioactive dichloromethane extract from the aerial parts of Stachys glutinosa led to the isolation of four flavones, xanthomicrol (1), sideritoflavone (2), 8-methoxycirsilineol (3), and eupatilin (4), along with two neo-clerodane diterpenes, roseostachenone (8) and a new compound, 3α,4α-epoxyroseostachenol (7). In order to study structure-activity relationships, two methoxyflavones [5-demethyltangeretin (5) and tangeretin (6)] were synthesized by the methoxylation of xanthomicrol. The isolated compounds (1-4, 7, and 8) as well as the xanthomicrol semisynthetic derivatives (5 and 6) were evaluated for their binding affinity to the μ and δ opioid receptors. Xanthomicrol was the most potent binder to both μ and δ receptors, with a Ki value of 0.83 and 3.6 μM, respectively. Xanthomicrol administered intraperitoneally in mice at a dose of 80 mg/kg significantly reduced morphine-induced antinociception in the tail flick test. Our results suggested that xanthomicrol is a μ opioid receptor antagonist. Docking experiments were carried out to acquire a deeper understanding about important structural aspects of binding of xanthomicrol. In summary, these data suggest that xanthomicrol is a valuable structure for further development into a potential μ opioid receptor antagonist.

Stilbenes as κ-selective, non-nitrogenous opioid receptor antagonists

The natural stilbene pawhuskin A has been shown to function as an opioid receptor antagonist, with preferential binding to the κ receptor. This finding encouraged assembly of a set of analogues to probe the importance of key structural features. Assays on these compounds determined that one (compound 29) shows potent opioid receptor binding activity and significantly improved selectivity for the κ receptor. These studies begin to illuminate the structural features of these non-nitrogenous opioid receptor antagonists that are required for activity.