Effects of Chronic Morphine Treatment on Pre- and Postsynaptic Thalamo-Cortico-Striatal Mu-Opioid Receptor Signaling

Abstract

Opioids are regarded as the most effective therapy for pain. However, their clinical use is limited by their dangerous side effects, high abuse liability, and the rapid development of tolerance to their analgesic properties, whereby with prolonged use, increasing doses are required to achieve the same degree of pain relief. Despite widespread opioid use, the cellular and circuit adaptations that drive tolerance and addiction are not fully understood. One challenge in establishing the adaptations relevant to these processes is the widespread expression of the mu-opioid receptor (MOR), the receptor through which effects of clinical opioids are primarily mediated, throughout the nervous system. The goal of this dissertation is to characterize how chronic exposure to morphine, a prototypical opioid, alters MOR signaling within the somatic and presynaptic compartments of a physiologically relevant thalamo-cortico-striatal circuit, and determine the mechanisms underlying the observed effects. Glutamatergic neurons originating in the medial thalamus (MThal) send projections to the dorsomedial striatum (DMS) and anterior cingulate cortex (ACC). These interconnected brain regions are involved in mediating pain perception and reward behaviors, but how chronic opioid exposure alters synaptic transmission within this circuitry is not well-studied. In chapter 2, I compare chronic morphine effects on subsequent MOR signaling at MThal cell bodies and MThal-DMS presynaptic terminals in male and female mice. I demonstrate that chronic morphine treatment induces cellular tolerance at MThal cell bodies, where subsequent morphine responses are diminished, but facilitation at MThal-DMS terminals, where morphine responses are enhanced. Moreover, presynaptic facilitation is sex-specific, occurring only in male mice, while tolerance at cell bodies occurs in both sexes. Using MOR phosphorylation deficient mice, I demonstrate that MThal-DMS facilitation appears to be driven by MOR phosphorylation, a critical regulatory process. In chapter 3, I examine chronic morphine effects at MThal-ACC terminals innervating both excitatory and inhibitory pathways. In contrast to our findings in chapter 2, chronic morphine treatment at these terminals induces presynaptic tolerance, rather than facilitation. Again, these effects are sex-specific, where tolerance within the inhibitory pathway is only seen in male mice but tolerance within the excitatory pathway is seen in both sexes, and mediated by receptor phosphorylation. At MThal cell bodies, a fraction of cells did not respond to morphine. This observation motivated me in chapter 4 to explore whether functional MThal projections to the DMS arise from both MOR-expressing and MOR-lacking subpopulations. After confirming this to be the case, I examined how chronic morphine treatment differentially alters MOR signaling within these subpopulations separately to determine whether the adaptations driving the facilitation observed in chapter 2 were cell autonomous or circuit-level. I report that at MOR-expressing terminals, chronic morphine induces tolerance, rather than facilitation, with no apparent changes in signaling at MOR-lacking terminals. Taken together with my previous findings, this suggests the presence of multiple opposing adaptations in MOR signaling at MThal-DMS synapses; chronic morphine induces tolerance at the level of the cellular- or receptor-level while simultaneously inducing adaptations elsewhere in the circuit that mediate facilitation. Overall, the work presented here provides insight into chronic morphine-induced adaptations at different subcellular compartments within a physiologically relevant thalamo-cortico-striatal circuit. Moreover, it highlights the complexity with which chronic opioid exposure alters physiology to produce behavioral outcomes. These effects are not ubiquitous, but rather depend on multiple factors such as sex, brain region, and the microenvironment of the circuit.