GluA1 in Central Amygdala Promotes Opioid Use and Reverses Inhibitory Effect of Pain.

Increasing evidence suggests that long-term opioids and pain induce similar adaptive changes in the brain's reward circuits, however, how pain alters the addictive properties of opioids remains poorly understood. In this study using a rat model of morphine self-administration (MSA), we found that short-term pain, induced by an intraplantar injection of complete Freund's adjuvant (CFA), acutely decreased voluntary morphine intake, but not food intake, only at a morphine dose that did not affect pain itself. Pre-treatment with indomethacin, a non-opioid inhibitor of pain, before the pain induction blocked the decrease in morphine intake. In rats with steady MSA, the protein level of GluA1 subunits of glutamate AMPA receptors (AMPARs) was significantly increased, but that of GluA2 was decreased, resulting in an increased GluA1/GluA2 ratio in central nucleus of the amygdala (CeA). In contrast, pain decreased the GluA1/GluA2 ratio in the CeA of rats with MSA. Microinjection of NASPM, a selective inhibitor of homomeric GluA1-AMPARs, into CeA inhibited morphine intake. Furthermore, viral overexpression of GluA1 protein in CeA maintained morphine intake at a higher level than controls and reversed the pain-induced reduction in morphine intake. These findings suggest that CeA GluA1 promotes opioid use and its upregulation is sufficient to increase opioid consumption, which counteracts the acute inhibitory effect of pain on opioid intake. These results demonstrate that the CeA GluA1 is a shared target of opioid and pain in regulation of opioid use, which may aid in future development of therapeutic applications in opioid abuse.

Persistent pain facilitates response to morphine reward by downregulation of central amygdala GABAergic function.

Opioid-based analgesics are widely used for treating chronic pain, but opioids are highly addictive when repeatedly used because of their strong rewarding effects. In recent years, abuse of prescription opioids has dramatically increased, including incidences of misuse of opioid drugs prescribed for pain control. Despite this issue in current clinical pain management, it remains unknown how pain influences the abuse liability of prescription opioids. Pain as aversive experience may affect opioid reward of positive emotion through common brain sites involved in emotion processing. In this study, on a rat model of chronic pain, we determined how persistent pain altered behavioral responses to morphine reward measured by the paradigm of unbiased conditioned place preference (CPP), focusing on GABAergic synaptic activity in neurons of the central nucleus of the amygdala (CeA), an important brain region for emotional processing of both pain and reward. We found that pain reduced the minimum number of morphine-conditioning sessions required for inducing CPP behavior. Both pain and morphine conditioning that elicited CPP inhibited GABA synaptic transmission in CeA neurons. Pharmacological activation of CeA GABAA receptors reduced the pain and inhibited CPP induced both by an effective dose of morphine and by a sub-threshold dose of morphine under pain condition. Furthermore, inhibition of CeA GABAA receptors mimicked the pain effect, rendering the sub-threshold dose of morphine effective in CPP induction. These findings suggest that pain facilitates behavioral responses to morphine reward by predisposing the inhibitory GABA function in the CeA circuitry involved in the behavior of opioid reward.

Brain-derived neurotrophic factor-mediated downregulation of brainstem K+-Cl- cotransporter and cell-type-specific GABA impairment for activation of descending pain facilitation.

Chronic pain is thought to be partly caused by a loss of GABAergic inhibition and resultant neuronal hyperactivation in the central pain-modulating system, but the underlying mechanisms for pain-modulating neurons in the brain are unclear. In this study, we investigated the cellular mechanisms for activation of brainstem descending pain facilitation in rats under persistent pain conditions. In the nucleus raphe magnus (NRM), a critical relay in the brain's descending pain-modulating system, persistent inflammatory pain induced by complete Freund's adjuvant decreased the protein level of K(+)-Cl(-) cotransporter (KCC2) in both total and synaptosomal preparations. Persistent pain also shifted the equilibrium potential of GABAergic inhibitory postsynaptic current (EIPSC) to a more positive level and increased the firing of evoked action potentials selectively in µ-opioid receptor (MOR)-expressing NRM neurons, but not in MOR-lacking NRM neurons. Microinjection of brain-derived neurotrophic factor (BDNF) into the NRM inhibited the KCC2 protein level in the NRM, and both BDNF administration and KCC2 inhibition by furosemide mimicked the pain-induced effects on EIPSC and excitability in MOR-expressing neurons. Furthermore, inhibiting BDNF signaling by NRM infusion of tyrosine receptor kinase B-IgG or blocking KCC2 with furosemide prevented these pain effects in MOR-expressing neurons. These findings demonstrate a cellular mechanism by which the hyperactivity of NRM MOR-expressing neurons, presumably responsible for descending pain facilitation, contributes to pain sensitization through the signaling cascade of BDNF-KCC2-GABA impairment in the development of chronic pain.