Pain-induced opioid receptor trafficking and pain inhibition
Peripheral sensory neurons in the DRG are nociceptors that receive nociceptive stimuli and deliver the nociceptive information to the modulatory circuits in the spinal dorsal horn. As mentioned above, inflammatory mediators, such as bradykinin [
17], substance P [
34] and ATP [
33], can induce DOR membrane trafficking in cultured sensory neurons
in vitro. Sustained inflammation induced by complete Freud's adjuvant (CFA) also significantly increases DOR membrane trafficking in small- and medium-sized DRG neurons in intact animals [
11]. Local administration of capsaicin, an activator of vanilloid/transient receptor potential vanilloid 1 (TRPV1) selectively located in C-fibers, induces an increase in DOR membrane trafficking in small-sized DRG neurons [
11], suggesting that the enhanced membrane recruitment of DOR is tightly adapted to the modality of pain, and may account for the enhanced antinociceptive efficacy of DOR agonists under that condition. Additionally, there is a bilateral upregulation in DOR expression in the DRG neurons of small and large diameters from rats after chronic constriction of the sciatic nerve, resulting in DOR-mediated inhibition of tactile allodynia following nerve injury [
8].
The dorsal horn, especially lamina II, of the spinal cord is a critical site for the relay and processing of dynamic sensory information. While spinal administration of DOR agonists induces antinociception in naïve animals [
15], DOR-mediated analgesic effects that reverse hyperalgesia and tactile allodynia are dramatically augmented in animals with chronic inflammatory or neuropathic pain [
15,
37]. Likely, this results from the increased membrane recruitment of DOR in the dorsal horn neurons following the chronic pain. Actually, sustained inflammation induced by CFA is also reported to significantly increase the expression and membrane targeting of DOR in the spinal dorsal horn where the analgesic effect of DOR agonists is largely enhanced [
9,
10]. This adaptation of DOR during chronic inflammation may require the integrity of MOR as this adaptation is diminished in MOR knockout mice [
9]. Also, increased membrane trafficking of functional DOR has been reported in laminas III-VI neurons from rhizotomized rats [
14].
Several brain regions including the PAG and NRM are critical sites for supraspinal pain modulation. Pharmacological and electrophysiological evidence has established that the brainstem NRM, receiving major inputs from the PAG, functions as an integral relay in descending modulation of nociception. In these brain regions, DOR is located predominantly in presynaptic axon terminals, rather than on plasma membrane of presynaptic boutons, and immunolabeling for DOR is often associated with intracellular LDCVs [
38‐
40]. In general, the analgesic effect of DOR agonists applied in these two regions is weak in normal animals. Although local microfusion of DOR agonists into the NRM region shows an inhibition of the tail flick-related increase in ON-cell activity and shortens the tail flick-related pause in OFF-cell activity in intact animals [
41], microinjection of DOR agonists into either the PAG or NRM has only little or a weak effect on the thermal nociception in normal rats [
4,
6]. However, persistent inflammation induced by CFA markedly increases the anti-hyperalgesic potency of DOR agonists applied in the NRM, as indicated by a prolonged effect duration and a leftward shift of the dose-response curve with a reduced ED
50 value, an effect appearing two weeks after inflammatory injury [
12]. Also, microinjection of the DOR antagonist naltriben into the NRM enhances the hyperalgesia in the ipsilateral hindpaw, which is at least partially mediated by the increased release of endogenous opioid peptides with preferential affinity for DOR [
42]. In addition, microinjection of DOR agonists into the ventral PAG significantly inhibits mechanical allodynia in rats with neuropathic pain [
43]. Nevertheless, there is no data currently available regarding the mechanisms for the adaptation and membrane trafficking of DOR induced by chronic pain in the supraspinal sites critically involved in pain modulation.
Opioid-induced DOR trafficking and opioid analgesia
Peripheral sensory neurons in the DRG are among the critical targets of opioid analgesics acting on opioid receptors, including MOR and DOR, abundantly expressed in the cell body and terminals of DRG neurons [
33]. It has been described recently that DOR agonists can rapidly induce the membrane trafficking of intracellular DOR via Ca
2+-dependent signaling pathways in cultured sensory neurons [
33]. Prolonged exposure to morphine (48 hours) also significantly increases DOR membrane trafficking in cultured DRG neurons [
11] and cortical neurons [
3]. Similarly, sustained systemic treatment with morphine significantly induces the membrane translocation of intracellular DOR in sensory neurons in intact mice [
11]. It is believed that the DOR membrane recruitment accounts, at least in part, for the enhanced antinociceptive efficacy of DOR agonists following sustained morphine treatment, and may provide a more effective action site for peripheral analgesics [
3].
Spinal dorsal horn, as the primary processing center for nociceptive information, also contains abundant opioid receptors, therefore serving as another critical site for opioid analgesia. DOR in the spinal neurons is mostly, although not exclusively, associated with the intracellular compartments in control conditions [
3,
44]. Repeated treatment with morphine or other selective MOR agonists induces MOR-dependent membrane insertion of DOR, and increases the bioavailability of DOR in the cultured [
3] and
in vivo [
3,
14,
44] spinal neurons. The increase in functional DOR on surface membrane is thought responsible for the enhanced, DOR agonist-mediated antinociception after chronic opioid treatment [
3,
44,
45].
Through their descending pathways for pain modulation, the brainstem NRM and the midbrain PAG serve as the critical supraspinal sites for opioid analgesia. Despite the abundant expression of DOR in these areas [
19,
38,
39,
46], little DOR-mediated cellular actions have been observed under normal conditions, likely due to the intracellular location of these receptors in these two brain regions in naïve animals [
4,
47]. However, others have reported a DOR-induced weak potassium current in a small population of NRM and PAG neurons [
48,
49], but DOR agonists have no significant effect on the presynaptic GABA release in these NRM or PAG neurons from normal animals [
4,
13,
49]. Intriguingly, several recent studies have revealed that the intracellular DOR can translocate to the surface membrane and become functional in these neurons from rats chronically treated with morphine [
4,
13] (Figure
1). In these neurons, DOR agonists elicit a significant inhibition of presynaptic GABA release through activation of the newly inserted, functional DOR, which is absent in normal animals [
4,
13]. The behavioral significance of this membrane trafficking of DOR in the NRM has been functionally demonstrated by the observations that microinjection of DOR agonists into the NRM, ineffective in opioid naïve animals, produces significant antinociception in chronic morphine-treated animals, and relieves analgesic tolerance to chronic morphine [
4]. These observations further support the notion that DOR agonists may be more effective and therefore could serve as better alternative analgesics for pain control following chronic exposure to MOR agonists.