Opioids: The Neuroscience and Molecular Biology

Opioids are a common pharmaceutical drug used for its analgesic agents. It exerts pharmacological and physiological effects through binding to endogenous G-protein Coupled Receptors (GPCR), classified into four subtypes: μ, δ, κ and nociceptin receptors, each binding similar, yet different endogenous opioid ligands (Manglik, 2020). Opioid peptides can either act promiscuously by binding to several of the receptor subtypes or selectively binding at one subtype. This subsequently activates heterotrimeric G-proteins when bound to the GPCR (Manglik, 2020). Binding of an opioid agonist to the GPCR allows the ɑ subunit of the G-protein to exchange its bound guanosine diphosphate (GDP) molecule for guanosine triphosphate (GTP), allowing for the ɑ-GTP complex to dissociate from the β-γ complex (Pathan & Williams, 2012). Both complexes can now interact with target proteins and ion channels to activate or inhibit its effects. Figure 1 shows the ɑ-GTP and  β-γ complexes roles in the activation of potassium channel conductance and inhibition of calcium channel conductance (Pathan & Williams, 2012). Classical opioid agonist binding, however, results in the inhibition of adenylyl cyclase (Figure 1), which in turn inhibits the production of cyclic adenosine monophosphate (cAMP), a secondary messenger used to transduce and amplify the signaling pathway to activate other molecules such as proteins (Pathan & Williams, 2012). Reduced intracellular cAMP levels generally lead to inhibition of neurotransmitter release, with an exception of dopamine, amongst others.

Figure 1: Intracellular changes resulting from the binding of an opioid agonist on the GPCR. (Pathan & Williams, 2012)

Alterations to the mesocorticolimbic dopamine system mediates the long-term vulnerability to addiction (Browne et al., 2020). This system is comprised of dopamine neurons projecting from the ventral tegmental area (VTA), which terminates in the nucleus accumbens (NAc), as well as regions of the amygdala, prefrontal cortex and hippocampus (Browne et al., 2020). Opioids produce rewarding and reinforcing effects by activating μ-opioid receptors in the VTA, which causes the amplification of dopamine neurons firing, and consequently elevated dopamine neurotransmission in the NAc (Browne et al., 2020). Distinct neuronal responses can also be elicited throughout the brain via signaling from κ and δ-opioid receptors as well. The diverse actions of opioids can alter various intracellular signaling cascades, which may produce adaptations promoting behavioral and psychological abnormalities due to addiction. 

One persistent adaptation that results from long-term usage of opioids is epigenetic modifications to DNA, particularly transcription. Opioid-resulting modifications to DNA include amplified histone acetylation and reduced histone methylation (Browne et al., 2020). Histones are formed from four proteins (H2A, H2B, H3, H4), and the N-terminal “tails” undergo covalent modifications to adjust the histone’s grip on the DNA (Browne et al., 2020). Acetylation of lysine residues on histone tails reduce the electrostatic tension between the histone and the DNA, causing the chromatin to become more “unwound” to better facilitate gene transcription (Browne et al., 2020). Preclinical work has found that both repeated experimenter-administered and self-administered opioids increases global H3 acetylation in the mesolimbic dopamine system (Browne et al., 2020), thus resulting in the chromatin to become more accessible by transcription factors and RNA Polymerase II (as shown in Figure 2). 

Figure 2: Changes to histone acetylation as a result of repeated opioid exposure. (Browne et al., 2020)

On the contrary, histone methylation works to reduce the rate of transcription by inhibiting the binding of transcription factors and RNA Polymerase II onto the DNA. Although far less is known about the effects of opioids on histone methylation, the few studies examining this topic have only identified changes to the state of a specific histone tail residue, H3K9 (Browne et al., 2020). A study done by Sun et. al showed that repeated morphine treatment reduces NAc H3K9 di-methylation (H3K9me2)- the process of removing two hydrogen atoms and replacing them with methyl groups, but not mono- or tri-methylation (Sun et al., 2012). A similar reduction in H3K9me2 has been observed within the central nucleus of the amygdala, following opioid treatment, appearing to promote transcriptional activity (Sun et al., 2012). Furthermore, Sun et.al identified that there was a reduction of H3K9me2 throughout the FosB gene - a critical transcription factor that promotes drug addiction, thus reducing the suppression of this gene by reducing the effects of H3K9me2 (Sun et al., 2012). While there has been ground-breaking research regarding opioid epigenetics in the past two decades, further research is needed to fill in the gaps of opioid activity and its mechanisms. 

References

Manglik, A. (2020, January 1). Molecular basis of opioid action: From structures to new leads. National Library of Medicine. Retrieved February 19, 2023, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6898784/  

Browne, C. J., Godino, A., Salery, M., & Nestler, E. J. (2020, January 1). Epigenetic mechanisms of opioid addiction. Biological psychiatry. Retrieved February 19, 2023, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6898774/  

Sun, H., Maze, I., Dietz, D. M., Scobie, K. N., Kennedy, P. J., Damez-Werno, D., Neve, R. L., Zachariou, V., Shen, L., & Nestler, E. J. (2012, November 28). Morphine epigenomically regulates behavior through alterations in histone H3 lysine 9 dimethylation in the nucleus accumbens. The Journal of neuroscience : the official journal of the Society for Neuroscience. Retrieved February 19, 2023, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3516048/  

Pathan, H., & Williams, J. (2012, February). Basic Opioid Pharmacology: An update. British journal of pain. Retrieved February 19, 2023, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4590096/