Opioid research in the time of the opioid crisis

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Pain, negative affective states and opioid-based analgesics: Safer pain therapies to dampen addiction

Across centuries and civilizations opioids have been used to relieve pain. In our modern societies, opioid-based analgesics remain one of the most efficient treatments for acute pain. However, the long-term use of opioids can lead to the development of analgesic tolerance, opioid-induced hyperalgesia, opioid use disorders, and overdose, which can ultimately produce respiratory depressant effects with fatal consequences. In addition to the nociceptive sensory component of pain, negative affective states arising from persistent pain represent a risk factor for developing an opioid use disorder. Several studies have indicated that the increase in prescribed opioid analgesics since the 1990s represents the root of our current opioid epidemic. In this review, we will present our current knowledge on the endogenous opioid system within the pain neuroaxis and the plastic changes occurring in this system that may underlie the occurrence of pain-induced negative affect leading to misuse and abuse of opioid medications. Dissecting the allostatic neuronal changes occurring during pain is the most promising avenue to uncover novel targets for the development of safer pain medications. We will discuss this along with current and potential approaches to treat pain-induced negative affective states that lead to drug misuse. Moreover, this chapter will provide a discussion on potential avenues to reduce the abuse potential of new analgesic drugs and highlight a basis for future research and drug development based on recent advances in this field.

Akt Is Essential for Insulin Modulation of Amphetamine-Induced Human Dopamine Transporter Cell-Surface Redistribution

Uptake by the dopamine transporter (DAT) is the primary pathway for the clearance of extracellular dopamine (DA) and consequently for regulating the magnitude and duration of dopaminergic signaling. Amphetamine (AMPH) has been shown to decrease simultaneously DAT cell-surface expression and [(3)H]DA uptake. We have shown that insulin and its subsequent signaling through the phosphatidylinositol 3-kinase (PI3K)-dependent pathway oppose this effect of AMPH by promoting increased cell-surface expression. Here, we used human embryonic kidney 293 cells stably expressing the human DAT (hDAT cells) to investigate the downstream cellular components important for this effect of insulin. Akt is a protein kinase effector immediately downstream of PI3K. Both overexpression of a dominant-negative mutant of Akt (K179R) and the addition of 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine HCl (ML9), a pharmacological inhibitor of Akt, decreased cell-surface expression of DAT, suggesting a role of basal Akt signaling in the homoeostasis of DAT. Moreover, expression of a constitutively active Akt mutant reduced the ability of AMPH to decrease hDAT cell-surface expression as well as [(3)H]DA uptake. In contrast, overexpression of K179R blocked the ability of insulin to oppose AMPH-induced reduction of hDAT cell-surface expression and [(3)H]DA uptake, as did ML9. Our data demonstrate that hDAT cell-surface expression is regulated by the insulin signaling pathway and that Akt plays a key role in the hormonal modulation of AMPH-induced hDAT trafficking and in the regulation of basal hDAT cell-surface expression.