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.