Chronic Morphine Induces Adaptations in Opioid Receptor Signaling in a Thalamostriatal Circuit That Are Location Dependent, Sex Specific, and Regulated by µ-Opioid Receptor Phosphorylation.

Chronic opioid exposure induces tolerance to the pain-relieving effects of opioids but sensitization to some other effects. While the occurrence of these adaptations is well understood, the underlying cellular mechanisms are less clear. This study aimed to determine how chronic treatment with morphine, a prototypical opioid agonist, induced adaptations to subsequent morphine signaling in different subcellular contexts. Opioids acutely inhibit glutamatergic transmission from medial thalamic (MThal) inputs to the dorsomedial striatum (DMS) via activity at µ-opioid receptors (MORs). MORs are present in somatic and presynaptic compartments of MThal neurons terminating in the DMS. We investigated the effects of chronic morphine treatment on subsequent morphine signaling at MThal-DMS synapses and MThal cell bodies in male and female mice. Surprisingly, chronic morphine treatment increased subsequent morphine inhibition of MThal-DMS synaptic transmission (morphine facilitation) in male, but not female, mice. At MThal cell bodies, chronic morphine treatment decreased subsequent morphine activation of potassium conductance (morphine tolerance) in both male and female mice. In knock-in mice expressing phosphorylation-deficient MORs, chronic morphine treatment resulted in tolerance to, rather than facilitation of, subsequent morphine signaling at MThal-DMS terminals, suggesting phosphorylation deficiency unmasks adaptations that counter the facilitation observed at presynaptic terminals in wild-type mice. The results of this study suggest that the effects of chronic morphine exposure are not ubiquitous; rather adaptations in MOR function may be determined by multiple factors such as subcellular receptor distribution, influence of local circuitry, and sex.

Chronic morphine induces adaptations in opioid receptor signaling in a thalamo-cortico-striatal circuit that are projection-dependent, sex-specific and regulated by mu opioid receptor phosphorylation.

Chronic opioid exposure induces tolerance to the pain-relieving effects of opioids but sensitization to some other effects. While the occurrence of these adaptations is well-understood, the underlying cellular mechanisms are less clear. This study aimed to determine how chronic treatment with morphine, a prototypical opioid agonist, induced adaptations to subsequent morphine signaling in different subcellular contexts. Opioids acutely inhibit glutamatergic transmission from medial thalamic (MThal) inputs to the dorsomedial striatum (DMS) and anterior cingulate cortex (ACC) via activity at µ-opioid receptors (MORs). MORs are present in somatic and presynaptic compartments of MThal neurons terminating in both the DMS and ACC. We investigated the effects of chronic morphine treatment on subsequent morphine signaling at MThal-DMS synapses, MThal-ACC synapses, and MThal cell bodies in male and female mice. Surprisingly, chronic morphine treatment increased subsequent morphine inhibition of MThal-DMS synaptic transmission (morphine facilitation), but decreased subsequent morphine inhibition of transmission at MThal-ACC synapses (morphine tolerance) in a sex-specific manner; these adaptations were present in male but not female mice. Additionally, these adaptations were not observed in knockin mice expressing phosphorylation-deficient MORs, suggesting a role of MOR phosphorylation in mediating both facilitation and tolerance to morphine within this circuit. The results of this study suggest that the effects of chronic morphine exposure are not ubiquitous; rather adaptations in MOR function may be determined by multiple factors such as subcellular receptor distribution, influence of local circuitry and sex.

Mu Opioid Receptors Acutely Regulate Adenosine Signaling in Striatal Glutamate Afferents.

Endogenous adenosine plays a crucial role in maintaining energy homeostasis, and adenosine levels are tightly regulated across neural circuits. In the dorsal medial striatum (DMS), adenosine inhibits neurotransmitter release, but the source and mechanism underlying its accumulation are largely unknown. Opioids also inhibit neurotransmitter release in the DMS and influence adenosine accumulation after prolonged exposure. However, how these two neurotransmitter systems interact acutely is also largely unknown. This study demonstrates that activation of µ opioid receptors, but not δ opioid receptors or κ opioid receptors, inhibits tonic activation of adenosine A1Rs via a cAMP-dependent mechanism in both male and female mice. Further, selectively knocking out µ opioid receptors from thalamic presynaptic terminals and postsynaptic medium spiny neurons (MSNs) revealed that activation of µ opioid receptors on D1R-positive MSNs, but not D2R-positive MSNs, is necessary to inhibit tonic adenosine signaling on presynaptic terminals. Given the role of D1R-positive MSNs in movement and motivated behaviors, these findings reveal a novel mechanism by which these neurons regulate their own synaptic inputs.SIGNIFICANCE STATEMENT Understanding interactions between neuromodulatory systems within brain circuits is a fundamental question in neuroscience. The present work uncovers a novel role of opioids in acutely inhibiting adenosine accumulation and subsequent adenosine receptor signaling in the striatum by inhibiting the production of cAMP. Adenosine receptor signaling regulates striatal neurotransmitters, including glutamate, GABA, dopamine, and acetylcholine. Furthermore, interactions between adenosine2A receptors and numerous other GPCRs, including D2 dopamine and CB1 cannabinoid receptors, suggest that endogenous adenosine broadly modulates striatal GPCR signaling. Additionally, this work discovered that the source of resting endogenous extracellular adenosine is likely D1, but not D2 receptor-positive medium spiny neurons, suggesting that opioid signaling and manipulation of D1R-expressing medium spiny neuron cAMP activity can broadly affect striatal function and behavior.

Electrophysiological signatures of acute systemic lipopolysaccharide-induced inflammation: potential implications for delirium science.

BACKGROUND: Novel preventive therapies are needed for postoperative delirium, which especially affects older patients. A mouse model is presented that captures inflammation-associated cortical slow wave activity (SWA) observed in patients, allowing exploration of the mechanistic role of prostaglandin-adenosine signalling. METHODS: EEG and cortical cytokine measurements (interleukin 6, monocyte chemoattractant protein-1) were obtained from adult and aged mice. Behaviour, SWA, and functional connectivity were assayed before and after systemic administration of lipopolysaccharide (LPS)+piroxicam (cyclooxygenase inhibitor) or LPS+caffeine (adenosine receptor antagonist). To avoid the confounder of inflammation-driven changes in movement which alter SWA and connectivity, electrophysiological recordings were classified as occurring during quiescence or movement, and propensity score matching was used to match distributions of movement magnitude between baseline and post-LPS administration. RESULTS: LPS produces increases in cortical cytokines and behavioural quiescence. In movement-matched data, LPS produces increases in SWA (likelihood-ratio test: χ2(4)=21.51, P<0.001), but not connectivity (χ2(4)=6.39, P=0.17). Increases in SWA associate with interleukin 6 (P<0.001) and monocyte chemoattractant protein-1 (P=0.001) and are suppressed by piroxicam (P<0.001) and caffeine (P=0.046). Aged animals compared with adult animals show similar LPS-induced SWA during movement, but exaggerated cytokine response and increased SWA during quiescence. CONCLUSIONS: Cytokine-SWA correlations during wakefulness are consistent with observations in patients with delirium. Absence of connectivity effects after accounting for movement changes suggests decreased connectivity in patients is a biomarker of hypoactivity. Exaggerated effects in quiescent aged animals are consistent with increased hypoactive delirium in older patients. Prostaglandin-adenosine signalling may link inflammation to neural changes and hence delirium.

The Roles of Rods, Cones, and Melanopsin in Photoresponses of M4 Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs) and Optokinetic Visual Behavior.

Intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate not only image-forming vision like other ganglion cells, but also non-image-forming physiological responses to light such as pupil constriction and circadian photoentrainment. All ipRGCs respond to light through their endogenous photopigment melanopsin as well as rod/cone-driven synaptic inputs. A major knowledge gap is how melanopsin, rods, and cones differentially drive ipRGC photoresponses and image-forming vision. We whole-cell-recorded from M4-type ipRGCs lacking melanopsin, rod input, or cone input to dissect the roles of each component in ipRGCs' responses to steady and temporally modulated (≥0.3 Hz) lights. We also used a behavioral assay to determine how the elimination of melanopsin, rod, or cone function impacts the optokinetic visual behavior of mice. Results showed that the initial, transient peak in an M4 cell's responses to 10-s light steps arises from rod and cone inputs. Both the sustainability and poststimulus persistence of these light-step responses depend only on rod and/or cone inputs, which is unexpected because these ipRGC photoresponse properties have often been attributed primarily to melanopsin. For temporally varying stimuli, the enhancement of response sustainedness involves melanopsin, whereas stimulus tracking is mediated by rod and cone inputs. Finally, the behavioral assay showed that while all three photoreceptive systems are nearly equally important for contrast sensitivity, only cones and rods contribute to spatial acuity.

Mechanisms creating transient and sustained photoresponses in mammalian retinal ganglion cells.

Retinal neurons use sustained and transient light responses to encode visual stimuli of different frequency ranges, but the underlying mechanisms remain poorly understood. In particular, although earlier studies in retinal ganglion cells (RGCs) proposed seven potential mechanisms, all seven have since been disputed, and it remains unknown whether different RGC types use different mechanisms or how many mechanisms are used by each type. Here, we conduct a comprehensive survey in mice and rats of 12 candidate mechanisms that could conceivably produce tonic rod/cone-driven ON responses in intrinsically photosensitive RGCs (ipRGCs) and transient ON responses in three types of direction-selective RGCs (TRHR+, Hoxd10+ ON, and Hoxd10+ ON-OFF cells). We find that the tonic kinetics of ipRGCs arises from their substantially above-threshold resting potentials, input from sustained ON bipolar cells, absence of amacrine cell inhibition of presynaptic ON bipolar cells, and mGluR7-mediated maintenance of light-evoked glutamatergic input. All three types of direction-selective RGCs receive input from transient ON bipolar cells, and each type uses additional strategies to promote photoresponse transience: presynaptic inhibition and dopaminergic modulation for TRHR+ cells, center/surround antagonism and relatively negative resting potentials for Hoxd10+ ON cells, and presynaptic inhibition for Hoxd10+ ON-OFF cells. We find that the sustained nature of ipRGCs' rod/cone-driven responses depends neither on melanopsin nor on N-methyl-d-aspartate (NMDA) receptors, whereas the transience of the direction-selective cells' responses is influenced neither by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor desensitization nor by glutamate uptake. For all cells, we further rule out spike frequency adaptation and intracellular Ca2+ as determinants of photoresponse kinetics. In conclusion, different RGC types use diverse mechanisms to produce sustained or transient light responses. Parenthetically, we find evidence in both mice and rats that the kinetics of light-induced mGluR6 deactivation determines whether an ON bipolar cell responds tonically or transiently to light.

All spiking, sustained ON displaced amacrine cells receive gap-junction input from melanopsin ganglion cells.

Retinal neurons exhibit sustained versus transient light responses, which are thought to encode low- and high-frequency stimuli, respectively. This dichotomy has been recognized since the earliest intracellular recordings from the 1960s, but the underlying mechanisms are not yet fully understood. We report that in the ganglion cell layer of rat retinas, all spiking amacrine interneurons with sustained ON photoresponses receive gap-junction input from intrinsically photosensitive retinal ganglion cells (ipRGCs), recently discovered photoreceptors that specialize in prolonged irradiance detection. This input presumably allows ipRGCs to regulate the secretion of neuromodulators from these interneurons. We have identified three morphological varieties of such ipRGC-driven displaced amacrine cells: (1) monostratified cells with dendrites terminating exclusively in sublamina S5 of the inner plexiform layer, (2) bistratified cells with dendrites in both S1 and S5, and (3) polyaxonal cells with dendrites and axons stratifying in S5. Most of these amacrine cells are wide field, although some are medium field. The three classes respond to light differently, suggesting that they probably perform diverse functions. These results demonstrate that ipRGCs are a major source of tonic visual information within the retina and exert widespread intraretinal influence. They also add to recent evidence that ganglion cells signal not only to the brain.