Control of dopamine communication by opioids: glutamate enters the discussion

Opioid use disorder (OUD) has become an increasingly widespread public health concern within the United States, affecting nearly 2.7 million individuals over the age of 12 [1]. This, coupled with a rise in opioid-related overdose deaths, has reinvigorated efforts to understand the intricacies of opioid interaction with the brain’s reward circuitry. While heroin, morphine, and related opioids have long been known to act through mu opioid receptors (MORs) to mediate the rewarding effects of these drugs, defining the cellular substrates at which MORs influence reward-seeking behavior remains an ongoing process.

In this issue, McGovern et al. investigate the roles that MORs located on ventral tegmental area (VTA) glutamate neurons play in opioid reward, and how they might mediate behavioral effects of the opioid oxycodone in male and female mice [2]. A decades old classic model of opioid function in the VTA and other brain areas (e.g., the hippocampus) suggests that principal output neurons like VTA dopamine (DA) neurons, or pyramidal neurons in the hippocampus, are excited through the process of ‘disinhibition’, whereby the activation of MORs by opioids inhibits GABA release onto the principal neurons [3]. This model of opioid action in VTA has since been incorporated into models seeking to define how opioids can produce DA-mediated drug reward.

In the present study, the authors used RNAscope^TM in situ hybridization to identify MORs in a subpopulation of neurons defined as glutamatergic because they express the type-2 vesicular glutamate transporter (VGlut2). These MOR-expressing glutamate neurons are found primarily in the anterior VTA, in contrast to the MOR-expressing GABAergic neurons (expressing the vesicular GABA transporter VGaT) that are predominately found in posterior VTA. The authors also report that co-expression of MORs, together with both VGlut2 and VGaT markers is relatively rare (<10%), indicating separate populations of GABAergic and glutamatergic VTA neurons expressing MORs. The local circuit function of MOR-VGlut2⁺ neurons was assessed by intracellular recordings from VTA DA neurons while optogenetically activating VGlut2⁺ neurons. These experiments revealed that MOR-VGlut2⁺ neurons mediate short latency, monosynaptic excitatory postsynaptic currents (EPSCs) in VTA DA neurons, and that these EPSCs are inhibited by selective activation of MORs. Thus, MOR-VGlut2⁺ neurons can synaptically excite VTA DA neurons, and this excitation can be diminished by activation of presynaptic MORs.

Since excitation of VTA DA neurons is known to increase DA release in the nucleus accumbens core (NAcc), the authors also evaluated the contribution of VTA VGlut2⁺ neurons to this DA release. They used optogenetic stimulation of VGlut2⁺ VTA neurons together with fiber photometry in NAcc to measure fluorescencemediated by the genetically expressed DA sensor, GRAB_DA. The authors measured DA in NAcc, because, unlike the nucleus accumbens shell, it does not receive input from VTA neurons that co-release DA and glutamate (i.e., TH⁺/VGlut2⁺ neurons). This permitted the use of VGlut2-Cre mice to examine the isolated effect of TH^-/VGlut2⁺ neurons on DA release. Using this approach, the authors found that optogenetic activation of VTA VGlut2 neurons increased DA release in NAcc and this was inhibited by systemic oxycodone administration, supporting a mechanism whereby MORs presynaptically inhibit glutamate release onto DA neurons to decrease DA release. Although these findings are consistent with the proposed glutamate-DA neuron circuit mechanism, the authors acknowledge that other sites might also be involved, such as MORs on NAcc cholinergic interneurons [4]. Nevertheless, the idea that NAcc DA levels are determined in part by opioid effects on VTA MOR-glutamate neurons implies that a revision of the classic disinhibition hypothesis is necessary, and hints at finer control of mesolimbic DA release by MORs.

The authors also assessed the activity of VTA VGlut2⁺ neurons during opioid-seeking behavior using fiber photometry measurement of intracellular calcium using GCaMP6m. In male and female mice trained to self-administer oral oxycodone, Ca²⁺ signals increased during oral oxycodone seeking (nose poke) and taking (drug port head entry). Moreover, following extinction of the operant response, re-exposure to a cue previously paired with oral oxycodone self-administration increased drug seeking and augmented VGlut2⁺ neuron calcium signals. Thus, VGlut2⁺ neurons were activated during opioid seeking and taking and were recruited during cue-induced oxycodone seeking. This is consistent with studies showing reward-related activation of VTA VGlut2⁺ neurons [5].

Lastly, to determine whether inhibition of VTA VGlut2⁺ neurons influences oxycodone-seeking behavior, the authors chemogenetically inhibited them during oxycodone self-administration or cue-induced reinstatement. Inhibition of VTA VGlut2⁺ neurons did not alter oxycodone self-administration in males or females but reduced cue-induced reinstatement of oxycodone seeking in males only. Thus, despite the fact that VTA VGlut2⁺ neurons showed similar increases in Ca2⁺ signal during cue-induced reinstatement in males and females, inhibition of these neurons resulted in sex-specific inhibition of this reinstatement behavior. Although the reasons for this discrepancy were not further explored, the implication of this finding for sex-dependent differences in opioid relapse remains intriguing.

Whereas the contribution of VGlut2⁺ VTA neurons to encoding both reward and aversion has become more widely appreciated in recent years, the present study refines this view to further demonstrate their involvement in opioid-driven behaviors. Thus, as shown in earlier studies, although the excitation of VTA DA neurons through MOR-mediated disinhibition is critical to some forms of opioid seeking in rodent models, the identification of MOR-sensitive glutamatergic VTA neurons provides an additional control point for opioids to regulate the contribution of the VTA to opioid reward and potentially aversion. Thus, this study suggests that MOR control of glutamate release onto VTA DA neurons should be incorporated into models of opioid action in the VTA and its projections to limbic and cortical regions.

Author contributions

AFH wrote the initial draft. CRL reviewed, edited, and prepared the final submission. Both authors reviewed, edited, and approved the final version.

Funding

CRL and AFH are supported by the Intramural Research Program of NIH, NIDA.

Competing interests

The authors declare no competing interests.