Opioid-based drugs are mainstays for pain management despite their significant side effects and addictive liability. Abuse of opioid drugs such as heroin is linked to many serious health problems, including fatal overdose, spontaneous abortion, infectious disease such as hepatitis and HIV/AIDS, and cardiovascular and pulmonary problems. Given both their clinical significance and adverse impact on public health, it is imperative that we understand mechanisms underlying the physiological and behavioral effects of opioids. The work proposed herein centers on a key neural substrate of opioid reward - the ventral tegmental area (VTA) - and challenges conventional wisdom concerning the signaling pathway mediating the opioid-induced disinhibition of dopaminergic (DA) neurons, a key mechanism of opioid reward. The opioid-induced disinhibition of DA neurons in the VTA involves the direct hyperpolarization of VTA GABA neurons. G protein-gated inwardly- rectifying K+ (GIRK/KIR3) channels are widely-considered to mediate the opioid-induced hyperpolarization of GABA neurons, due largely to their documented roles in metabotropic postsynaptic inhibition in many neuron populations. Our recent attempt to validate this paradigm failed, however, revealing that GIRK channels do not mediate the inhibition of VTA GABA neurons, the disinhibition of VTA DA neurons, or reward-related behavioral effects of opioids. Instead, our findings suggest that the acute inhibitory actions of opioids on VTA GABA neurons are mediated by the inhibition of adenylyl cyclase and consequent activation of an ion channel exhibiting the unique regulatory and biophysical signature of the Trek subfamily of 2-pore (K2P) K+ channels. The goal of this study is to test the hypothesis that opioids indirectly stimulate VTA dopamine neurons, and evoke reward-relevant behaviors, by activating Trek channels in VTA GABA neurons. At present, there are scant data concerning Trek expression in the VTA and no reports of Trek channel involvement in opioid signaling. As such, we will begin in AIM 1 by determining whether it is Trek1 or Trek2 that carries the MOR- activated K+ current in VTA GABA neurons. Well-characterized function-blocking antibodies directed against Trek1 and Trek2, as well as single-cell RT-PCR, will be applied to electrophysiological studies involving VTA GABA neurons in slices.
In AIM 2, we will use available Trek knockout mice to measure the impact of Trek ablation on opioid signaling in the VTA, and on the motor-stimulatory and reinforcing effects of morphine.
In AIM 3, we will seek a better understanding of the novel observation that the MOR-activated K+ current in VTA GABA neurons is significantly enhanced in Girk knockout mice. Electrophysiological and behavioral approaches will be used to probe the relationship between the MOR-activated K+ current in VTA GABA neurons and the complex adaptations linked to chronic drug administration. The proposed work will reframe our understanding of signaling downstream from opioid receptors and as such, may have significant implications for diagnostic or therapeutic strategies relevant to pain management and addiction.
Opioid-based drugs target neural circuitry important for pain processing and reward, actions that explain both their beneficial (analgesic) and untoward (addictive) effects. This proposal challenges conventional wisdom concerning the molecular details of opioid signaling in a key neuron population involved in reward. A clear understanding of the molecular mechanisms of opioid reward is crucial to our understanding of addiction and to the design of more selective and effective therapeutic approaches to pain management.
