Widely distributed in the central nervous system, dynorphin A binds to three opioid subtypes with different affinities, especially to κ-opioid receptors (Fallon and Leslie, 1986; Schwarzer, 2009). κ-Opioid receptors are also widely distributed in the central nervous system (Yuferov et al., 2004; Shippenberg et al., 2007b; Bruchas et al., 2010), and the dynorphin/κ-opioid receptor system plays an important role in pain/analgesia, temperature, emotions and neuroendocrine functions (Pfeiffer et al., 1986; Bodnar, 2010). The dynorphin/κ-opioid receptor pathway is also a major anti-reward system and participates the development of drug addiction. There is growing evidence that administration of dynorphin A and other related opioid peptides alleviates withdrawal symptoms of morphine physical dependence (Takemori et al., 1993; Hooke et al., 1995). κ-Opioid agonists, do not produce reinforcing effects but reduce drug abuse under certain conditions. It was reported that the κ-opioid receptor agonists ethylketocyclazocine and U50,488 attenuated cocaine behavioral sensitization, conditioned place preference (CPP) acquisition and self-administration in rhesus monkeys by repressing the release of dopamine (Maisonneuve et al., 1994). It was also reported that the dynorphin/κ-opioid receptor system antagonized rewarding effects in drug abuse and inhibited brain reward function by suppressing dopamine release from the mesolimbic reward pathway (Chartoff et al., 2008; Mysels and Sullivan, 2009). On the contrary, it was reported that κ-opioid receptor antagonists nor-BNI and arodyn blocked stress-induced reinstatement of cocaine-induced self-administration or CPP acquisition (Beardsley et al., 2005; Carey et al., 2007). These reports suggest a complex role of the dynorphin/κ-opioid receptor system in the drug abuse development.
Bulleyaconitine A (BAA), isolated from the rhizomes of Aconitum bulleyanum, is a C19-diterpenoid alkaloid without activities of binding to opioid receptors (Wang et al., 2007). As it is a nonnarcotic analgesic and has lower toxicity and wider treatment window than aconitine, BAA has been widely prescribed in China to treat various forms of chronic pain over four decades (Bello-Ramirez and Nava-Ocampo, 2004; Xie et al., 2018). Accumulated evidence demonstrated that BAA and its analogs aconitine (C19-diterpenoid), bullatine A (C20-diterpenoid) and lappaconitine (C18-diterpenoid) produced antinociception without induction of antinociceptive tolerance in various rodent models of pain hypersensitivity, including neuropathic pain, bone cancer pain, inflammatory pain, diabetic pain, and visceral pain (Li et al., 2016a; Li et al., 2016b; Huang et al., 2017a; Huang et al., 2020a; Huang et al., 2020b). Our recent studies further uncovered that BAA, aconitines, bullatine A and lappaconitine alleviated pain directly through stimulating spinal microglial dynorphin A expression and subsequent activating κ-opioid receptors in a variety of rodent models of pain hypersensitivity (Huang et al., 2017a; Li et al., 2017; Sun et al., 2018). In addition, BAA and bullatine A injection blocks chronic morphine-induced antinociceptive tolerance in rats and mice (Li et al., 2016a; Huang et al., 2017a). These reports lead to the hypothesis that aconitines including BAA may have potential therapeutic properties in the treatment of morphine physical dependence and psychological dependence.
In this study we aimed to assess the anti-addictive effects of BAA in morphine-induced physical and psychological dependence. We first tested whether subcutaneous injection of BAA attenuated morphine-induced physical dependence in a mouse model, in which application of naloxone induced withdrawal signs including shakes, jumps, genital licks, fecal excretion and body weight loss (Lenard and Roerig, 2005). We further assessed whether subcutaneous BAA inhibited morphine-induced psychological dependence in the CPP test, a widely used model to assess the reinforcing effect of drugs of abuse in laboratories (Wu et al., 2016). As like other addictive drugs, morphine CPP acquisition is considered to constitute a part of the addiction process associated with the opioid reinforcing properties. Finally, we explored the mechanisms underlying BAA-induced anti-addictive effects, particularly the involvement of microglial dynorphin A expression and subsequent κ-opioid receptor activation in brain. Our results, for the first time, uncover that BAA inhibits morphine-induced physical and psychological dependence through brain microglial expression of dynorphin A, and suggest that microglial expression of dynorphin A is a potential target for the treatment of opioid addiction and abuse.