No abstract

The degree of behavioral control that an organism has over a stressor is a potent modulator of the stressor's impact; uncontrollable stressors produce numerous outcomes that do not occur if the stressor is controllable. Research on controllability has focused on brainstem nuclei such as the dorsal raphe nucleus (DRN). Here we find that the infralimbic and prelimbic regions of the ventral medial prefrontal cortex (mPFCv) in rats detect whether a stressor is under the organism's control. When a stressor is controllable, stress-induced activation of the DRN is inhibited by the mPFCv, and the behavioral sequelae of uncontrollable stress are blocked. This suggests a new function for the mPFCv and implies that the presence of control inhibits stress-induced neural activity in brainstem nuclei, in contrast to the prevalent view that such activity is induced by a lack of control.

This review will introduce the concept of toll-like receptor (TLR)–mediated glial activation as central to all of the following: neuropathic pain, compromised acute opioid analgesia, and unwanted opioid side effects (tolerance, dependence, and reward). Attenuation of glial activation has previously been demonstrated both to alleviate exaggerated pain states induced by experimental pain models and to reduce the development of opioid tolerance. Here we demonstrate that selective acute antagonism of TLR4 results in reversal of neuropathic pain as well as potentiation of opioid analgesia. Attenuating central nervous system glial activation was also found to reduce the development of opioid dependence, and opioid reward at a behavioral (conditioned place preference) and neurochemical (nucleus accumbens microdialysis of morphine-induced elevations in dopamine) level of analysis. Moreover, a novel antagonism of TLR4 by (+)- and (˗)-isomer opioid antagonists has now been characterized, and both antiallodynic and morphine analgesia potentiating activity shown. Opioid agonists were found to also possess TLR4 agonistic activity, predictive of glial activation. Targeting glial activation is a novel and as yet clinically unexploited method for treatment of neuropathic pain. Moreover, these data indicate that attenuation of glial activation, by general or selective TLR antagonistic mechanisms, may also be a clinical method for separating the beneficial (analgesia) and unwanted (tolerance, dependence, and reward) actions of opioids, thereby improving the safety and efficacy of their use.

Opioid action was thought to exert reinforcing effects solely via the initial agonism of opioid receptors. Here we present evidence for an additional novel contributor to opioid reward: the innate immune pattern-recognition receptor, Toll-like receptor 4 (TLR4), and its MyD88-dependent signaling. Blockade of TLR4/MD2 by administration of the non-opioid, unnatural isomer of naloxone, (+)-naloxone (rats), or two independent genetic knockouts of MyD88-TLR4-dependent signaling (mice), suppressed opioid-induced conditioned place preference. (+)-Naloxone also reduced opioid (remifentanil) self-administration (rats), another commonly used behavioral measure of drug reward. Moreover, pharmacological blockade of morphine-TLR4/MD2 activity potently reduced morphine-induced elevations of extracellular dopamine in rat nucleus accumbens, a region critical for opioid reinforcement. Importantly, opioid-TLR4 actions are not a unidirectional influence on opioid pharmacodynamics, since TLR4 −/− mice had reduced oxycodone-induced p38 and JNK phosphorylation, whilst displaying potentiated analgesia. Similar to our recent reports of morphine-TLR4/MD2 binding, here we provide a combination of in silico and biophysical data to support (+)-naloxone and remifentanil binding to TLR4/MD2. Collectively, these data indicate that the actions of opioids at classical opioid receptors, together with their newly identified TLR4/MD2 actions, affect the mesolimbic dopamine system which amplifies opioid-induced elevations in extracellular dopamine levels and therefore possibly explaining altered opioid reward behaviors. Thus, the discovery of TLR4/MD2 recognition of opioids as foreign xenobiotic substances adds to the existing hypothesized neuronal reinforcement mechanisms, identifies a new drug target in TLR4/MD2 for the treatment of addictions, and provides further evidence supporting a role for central proinflammatory immune signaling in drug reward.