Identifying local and descending inputs for primary sensory neurons

5' Transgenes drive leaky expression of 3' transgenes in Cre-inducible bi-cistronic vectors

Molecular cloning techniques enabling contemporaneous expression of two or more protein-coding sequences provide an invaluable tool for understanding the molecular regulation of cellular functions. The Cre-lox system is used for inducing the expression of recombinant proteins encoded within a bi-/poly-cistronic cassette. However, leak expression of transgenes is often observed in the absence of Cre recombinase activity, compromising the utility of this approach. To investigate the mechanism of leak expression, we generated Cre-inducible bi-cistronic vectors to monitor the expression of transgenes positioned either 5' or 3' of a 2A peptide or internal ribosomal entry site (IRES) sequence. Cells transfected with these bi-cistronic vectors exhibited Cre-independent leak expression specifically of transgenes positioned 3' of the 2A peptide or IRES sequence. Similarly, AAV-FLEX vectors encoding bi-cistronic cassettes or fusion proteins revealed the selective Cre-independent leak expression of transgenes positioned at the 3' end of the open reading frame. Our data demonstrate that 5' transgenes confer promoter-like activity that drives the expression of 3' transgenes. An additional lox-STOP-lox cassette between the 2A sequence and 3' transgene dramatically decreased Cre-independent transgene expression. Our findings highlight the need for appropriate experimental controls when using Cre-inducible bi-/poly-cistronic constructs and inform improved design of vectors for more tightly regulated inducible transgene expression.

Spatially organized striatum-wide acetylcholine dynamics for the learning and extinction of Pavlovian cues and actions

Striatal acetylcholine (ACh) has been linked to behavioral flexibility. A key component of flexibility is down-regulating responding as valued cues and actions become decoupled from positive outcomes. We used array fiber photometry in mice to investigate how ACh release across the striatum evolves during learning and extinction of Pavlovian associations. Changes in multi-phasic release to cues and consummatory actions were bi-directional and region-specific. Following extinction, increases in cue-evoked ACh release emerged in the anterior dorsal striatum (aDS) which preceded a down-regulation of anticipatory behavior. Silencing ACh release from cholinergic interneurons in the aDS blocked behavioral extinction. Dopamine release dipped below baseline for down-shifted cues, but glutamate input onto cholinergic interneurons did not change, suggesting an intrastriatal mechanism for the emergence of ACh increases. Our large-scale mapping of striatal ACh dynamics during learning pinpoints region-specific elevations in ACh release positioned to down-regulate behavior during extinction, a central feature of flexible behavior.

Spinal projecting neurons in rostral ventromedial medulla co-regulate motor and sympathetic tone

Many behaviors require the coordinated actions of somatic and autonomic functions. However, the underlying mechanisms remain elusive. By opto-stimulating different populations of descending spinal projecting neurons (SPNs) in anesthetized mice, we show that stimulation of excitatory SPNs in the rostral ventromedial medulla (rVMM) resulted in a simultaneous increase in somatomotor and sympathetic activities. Conversely, opto-stimulation of rVMM inhibitory SPNs decreased both activities. Anatomically, these SPNs innervate both sympathetic preganglionic neurons and motor-related regions in the spinal cord. Fiber-photometry recording indicated that the activities of rVMM SPNs correlate with different levels of muscle and sympathetic tone during distinct arousal states. Inhibiting rVMM excitatory SPNs reduced basal muscle and sympathetic tone, impairing locomotion initiation and high-speed performance. In contrast, silencing the inhibitory population abolished muscle atonia and sympathetic hypoactivity during rapid eye movement (REM) sleep. Together, these results identify rVMM SPNs as descending spinal projecting pathways controlling the tone of both the somatomotor and sympathetic systems.

Mixed representations of choice and outcome by GABA/glutamate cotransmitting neurons in the entopeduncular nucleus

The basal ganglia (BG) are an evolutionarily conserved and phylogenetically old set of sub-cortical nuclei that guide action selection, evaluation, and reinforcement. The entopeduncular nucleus (EP) is a major BG output nucleus that contains a population of GABA/glutamate cotransmitting neurons (EP Sst+ ) that specifically target the lateral habenula (LHb) and whose function in behavior remains mysterious. Here we use a probabilistic switching task that requires an animal to maintain flexible relationships between action selection and evaluation to examine when and how GABA/glutamate cotransmitting neurons contribute to behavior. We find that EP Sst+ neurons are strongly engaged during this task and show bidirectional changes in activity during the choice and outcome periods of a trial. We then tested the effects of either permanently blocking cotransmission or modifying the GABA/glutamate ratio on behavior in well-trained animals. Neither manipulation produced detectable changes in behavior despite significant changes in synaptic transmission in the LHb, demonstrating that the outputs of these neurons are not required for on-going action-outcome updating in a probabilistic switching task.

The 'in's and out's' of descending pain modulation from the rostral ventromedial medulla

The descending-pain modulating circuit controls the experience of pain by modulating transmission of sensory signals through the dorsal horn. This circuit's key output node, the rostral ventromedial medulla (RVM), integrates 'top-down' and 'bottom-up' inputs that regulate functionally defined RVM cell types, 'OFF-cells' and 'ON-cells', which respectively suppress or facilitate pain-related sensory processing. While recent advances have sought molecular definition of RVM cell types, conflicting behavioral findings highlight challenges involved in aligning functional and molecularly defined types. This review summarizes current understanding, derived primarily from rodent studies but with corroborating evidence from human imaging, of the role of RVM populations in pain modulation and persistent pain states and explores recent advances outlining inputs to, and outputs from, RVM pain-modulating neurons.

A flowchart for adequate controls in virus-based monosynaptic tracing experiments identified Cre-independent leakage of the TVA receptor in RΦGT mice

BACKGROUND

A pseudotyped modified rabies virus lacking the rabies glycoprotein (G-protein), which is crucial for transsynaptic spread, can be used for monosynaptic retrograde tracing. By coupling the pseudotyped virus with transgene expression of the G-protein and the avian leukosis and sarcoma virus subgroup A receptor (TVA), which is necessary for cell entry of the virus, researchers can investigate specific neuronal populations. Responder mouse lines, like the RΦGT mouse line, carry the genes encoding the G-protein and TVA under Cre-dependent expression. These mouse lines are valuable tools because they reduce the number of viral injections needed compared to when using helper viruses. Since RΦGT mice do not express Cre themselves, introducing the pseudotyped rabies virus into their brain should not result in viral cell entry or spread.

RESULTS

We present a straightforward flowchart for adequate controls in tracing experiments, which we employed to demonstrate Cre-independent expression of TVA in RΦGT mice.

CONCLUSIONS

Our observations revealed TVA leakage, indicating that RΦGT mice should be used with caution for transgene expression of TVA. Inaccurate tracing outcomes may occur if TVA is expressed in the absence of Cre since background leakage leads to nonspecific cell entry. Moreover, conducting appropriate control experiments can identify the source of potential caveats in virus-based neuronal tracing experiments.

How do opioids control pain circuits in the brainstem during opioid-induced disorders and in chronic pain? Implications for the treatment of chronic pain

ABSTRACT

Brainstem areas involved in descending pain modulation are crucial for the analgesic actions of opioids. However, the role of opioids in these areas during tolerance, opioid-induced hyperalgesia (OIH), and in chronic pain settings remains underappreciated. We conducted a revision of the recent studies performed in the main brainstem areas devoted to descending pain modulation with a special focus on the medullary dorsal reticular nucleus (DRt), as a distinctive pain facilitatory area and a key player in the diffuse noxious inhibitory control paradigm. We show that maladaptive processes within the signaling of the µ-opioid receptor (MOR), which entail desensitization and a switch to excitatory signaling, occur in the brainstem, contributing to tolerance and OIH. In the context of chronic pain, the alterations found are complex and depend on the area and model of chronic pain. For example, the downregulation of MOR and δ-opioid receptor (DOR) in some areas, including the DRt, during neuropathic pain likely contributes to the inefficacy of opioids. However, the upregulation of MOR and DOR, at the rostral ventromedial medulla, in inflammatory pain models, suggests therapeutic avenues to explore. Mechanistically, the rationale for the diversity and complexity of alterations in the brainstem is likely provided by the alternative splicing of opioid receptors and the heteromerization of MOR. In conclusion, this review emphasizes how important it is to consider the effects of opioids at these circuits when using opioids for the treatment of chronic pain and for the development of safer and effective opioids.

Dissecting neural circuits from rostral ventromedial medulla to spinal trigeminal nucleus bidirectionally modulating craniofacial mechanical sensitivity

Chronic craniofacial pain is intractable and its mechanisms remain unclarified. The rostral ventromedial medulla (RVM) plays a crucial role in descending pain facilitation and inhibition. It is unclear how the descending circuits from the RVM to spinal trigeminal nucleus (Sp5) are organized to bidirectionally modulate craniofacial nociception. We used viral tracing, in vivo optogenetics, calcium signaling recording, and chemogenetic manipulations to investigate the structure and function of RVM-Sp5 circuits. We found that most RVM neurons projecting to Sp5 were GABAergic or glutamatergic and facilitated or inhibited craniofacial nociception, respectively. Both GABAergic interneurons and glutamatergic projection neurons in Sp5 received RVM inputs: the former were antinociceptive, whereas the latter were pronociceptive. Furthermore, we demonstrated activation of both GABAergic and glutamatergic Sp5 neurons receiving RVM inputs in inflammation- or dysfunction-induced masseter hyperalgesia. Activating GABAergic Sp5 neurons or inhibiting glutamatergic Sp5 neurons that receive RVM projections reversed masseter hyperalgesia. Our study identifies specific cell types and projections of RVM-Sp5 circuits involved in facilitating or inhibiting craniofacial nociception respectively. Selective manipulation of RVM-Sp5 circuits can be used as potential treatment strategy to relieve chronic craniofacial muscle pain.

Elevated Serotonin in Mouse Spinal Dorsal Horn Is Pronociceptive

Serotonergic neurons in the rostral ventral medulla (RVM) contribute to bidirectional control of pain through modulation of spinal and trigeminal nociceptive networks. Deficits in this pathway are believed to contribute to pathologic pain states, but whether changes in serotonergic mechanisms are pro- or antinociceptive is debated. We used a combination of optogenetics and fiber photometry to examine these mechanisms more closely. We find that optogenetic activation of RVM serotonergic afferents in the spinal cord of naive mice produces mechanical hypersensitivity and conditioned place aversion (CPA). Neuropathic pain, produced by chronic constriction injury of the infraorbital nerve (CCI-ION), evoked a tonic increase in serotonin (5HT) concentrations within the spinal trigeminal nucleus caudalis (SpVc), measured with liquid chromatography-tandem mass spectroscopy (LC-MS/MS). By contract, CCI-ION had no effect on the phasic serotonin transients in SpVc, evoked by noxious pinch, and measured with fiber photometry of a serotonin sensor. These findings suggest that serotonin release in the spinal cord is pronociceptive and that an increase in sustained serotonin signaling, rather than phasic or event driven increases, potentiate nociception in models of chronic pain.

Sensory spinal interoceptive pathways and energy balance regulation

Interoception plays an important role in homeostatic regulation of energy intake and metabolism. Major interoceptive pathways include gut-to-brain and adipose tissue-to brain signaling via vagal sensory nerves and hormones, such as leptin. However, signaling via spinal sensory neurons is rapidly emerging as an additional important signaling pathway. Here we provide an in-depth review of the known anatomy and functions of spinal sensory pathways and discuss potential mechanisms relevant for energy balance homeostasis in health and disease. Because sensory innervation by dorsal root ganglia (DRG) neurons goes far beyond vagally innervated viscera and includes adipose tissue, skeletal muscle, and skin, it is in a position to provide much more complete metabolic information to the brain. Molecular and anatomical identification of function specific DRG neurons will be important steps in designing pharmacological and neuromodulation approaches to affect energy balance regulation in disease states such as obesity, diabetes, and cancer.

Cell-type specific molecular architecture for mu opioid receptor function in pain and addiction circuits

Opioids are potent analgesics broadly used for pain management; however, they can produce dangerous side effects including addiction and respiratory depression. These harmful effects have led to an epidemic of opioid abuse and overdose deaths, creating an urgent need for the development of both safer pain medications and treatments for opioid use disorders. Both the analgesic and addictive properties of opioids are mediated by the mu opioid receptor (MOR), making resolution of the cell types and neural circuits responsible for each of the effects of opioids a critical research goal. Single-cell RNA sequencing (scRNA-seq) technology is enabling the identification of MOR-expressing cell types throughout the nervous system, creating new opportunities for mapping distinct opioid effects onto newly discovered cell types. Here, we describe molecularly defined MOR-expressing neuronal cell types throughout the peripheral and central nervous systems and their potential contributions to opioid analgesia and addiction.

Descending GABAergic pathway links brain sugar-sensing to peripheral nociceptive gating in Drosophila

Although painful stimuli elicit defensive responses including escape behavior for survival, starved animals often prioritize feeding over escape even in a noxious environment. This behavioral priority is typically mediated by suppression of noxious inputs through descending control in the brain, yet underlying molecular and cellular mechanisms are incompletely understood. Here we identify a cluster of GABAergic neurons in Drosophila larval brain, designated as SEZ-localized Descending GABAergic neurons (SDGs), that project descending axons onto the axon terminals of the peripheral nociceptive neurons and prevent presynaptic activity through GABAB receptors. Remarkably, glucose feeding to starved larvae causes sustained activation of SDGs through glucose-sensing neurons and subsequent insulin signaling in SDGs, which attenuates nociception and thereby suppresses escape behavior in response to multiple noxious stimuli. These findings illustrate a neural mechanism by which sugar sensing neurons in the brain engages descending GABAergic neurons in nociceptive gating to achieve hierarchical interaction between feeding and escape behavior.

Elevated serotonin in mouse spinal dorsal horn is pronociceptive

Serotonergic neurons in the rostral ventral medulla (RVM) contribute to bidirectional control of pain through modulation of spinal and trigeminal nociceptive networks. Deficits in this pathway are believed to contribute to pathological pain states, but whether changes in serotonergic mechanisms are pro or anti-nociceptive are debated. We used a combination of optogenetics and fiber photometry to examine these mechanisms more closely. We find that optogenetic activation of RVM serotonergic afferents in the spinal cord of naïve mice produces mechanical hypersensitivity and conditioned place aversion. Neuropathic pain, produced by chronic constriction injury of the infraorbital nerve (CCI-ION), evoked a tonic increase in serotonin concentrations within the spinal trigeminal nucleus caudalis (SpVc), measured with liquid chromatography-tandem mass spectroscopy (LC-MS/MS). By contract, CCI-ION had no effect on the phasic serotonin transients in SpVc, evoked by noxious pinch, and measured with fiber photometry of a serotonin sensor. These findings suggest that serotonin release in the spinal cord is pronociceptive and that an increase is sustained serotonin signaling, rather than phasic or event driven increases, potentiate nociception in models of chronic pain.

Principal neurons in the olfactory cortex mediate bidirectional modulation of seizures

Although the piriform cortex (PC) has been previously implicated as a critical node for seizure generation and propagation, the underlying neural mechanism has remained unclear. Here, we found increased excitability in PC neurons during amygdala kindling acquisition. Optogenetic or chemogenetic activation of PC pyramidal neurons promoted kindling progression, whereas inhibition of these neurons retarded seizure activities induced by electrical kindling in the amygdala. Furthermore, chemogenetic inhibition of PC pyramidal neurons alleviated the severity of kainic acid-induced acute seizures. These results demonstrate that PC pyramidal neurons bidirectionally modulate seizures in temporal lobe epilepsy, providing evidence for the efficacy of PC pyramidal neurons as a potential therapeutic target for epileptogenesis. KEY POINTS: While the piriform cortex (PC) is an important olfactory centre critically involved in olfactory processing and plays a crucial role in epilepsy due to its close connection with the limbic system, how the PC regulates epileptogenesis is largely unknown. In this study, we evaluated the neuronal activity and the role of pyramidal neurons in the PC in the mouse amygdala kindling model of epilepsy. PC pyramidal neurons are hyperexcited during epileptogenesis. Optogenetic and chemogenetic activation of PC pyramidal neurons significantly promoted seizures in the amygdala kindling model, whereas selective inhibition of these neurons produced an anti-epileptic effect for both electrical kindling and kainic acid-induced acute seizures. The results of the present study indicate that PC pyramidal neurons bidirectionally modulate seizure activity.

Mitofusin-2 in nucleus accumbens D2-MSNs regulates social dominance and neuronal function

The nucleus accumbens (NAc) is a brain hub regulating motivated behaviors, including social competitiveness. Mitochondrial function in the NAc links anxiety with social competitiveness, and the mitochondrial fusion protein mitofusin 2 (Mfn2) in NAc neurons regulates anxiety-related behaviors. However, it remains unexplored whether accumbal Mfn2 levels also affect social behavior and whether Mfn2 actions in the emotional and social domain are driven by distinct cell types. Here, we found that subordinate-prone highly anxious rats show decreased accumbal Mfn2 levels and that Mfn2 overexpression promotes dominant behavior. In mice, selective Mfn2 downregulation in NAc dopamine D2 receptor-expressing medium spiny neurons (D2-MSNs) induced social subordination, accompanied by decreased accumbal mitochondrial functions and decreased excitability in D2-MSNs. Instead, D1-MSN-targeted Mfn2 downregulation affected competitive ability only transiently and likely because of an increase in anxiety-like behaviors. Our results assign dissociable cell-type specific roles to Mfn2 in the NAc in modulating social dominance and anxiety.

Cell type-specific dissection of sensory pathways involved in descending modulation

Decades of research have suggested that stimulation of supraspinal structures, such as the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM), inhibits nocifensive responses to noxious stimulation through a process known as descending modulation. Electrical stimulation and pharmacologic manipulations of the PAG and RVM identified transmitters and neuronal firing patterns that represented distinct cell types. Advances in mouse genetics, in vivo imaging, and circuit tracing methods, in addition to chemogenetic and optogenetic approaches, allowed the characterization of the cells and circuits involved in descending modulation in further detail. Recent work has revealed the importance of PAG and RVM neuronal cell types in the descending modulation of pruriceptive as well as nociceptive behaviors, underscoring their roles in coordinating complex behavioral responses to sensory input. This review summarizes how new technical advances that enable cell type-specific manipulation and recording of neuronal activity have supported, as well as expanded, long-standing views on descending modulation.

Somatostatin Neurons from Periaqueductal Gray to Medulla Facilitate Neuropathic Pain in Male Mice

Projections from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM) are known to engage in descending pain modulation, but how the neural substrates of the PAG-RVM projections contribute to neuropathic pain remains largely unknown. In this study, we showed somatostatin-expressing glutamatergic neurons in the lateral/ventrolateral PAG that facilitate mechanical and thermal hypersensitivity in a mouse model of chemotherapy-induced neuropathic pain. We found that these neurons form direct excitatory connections with neurons in the RVM region. Inhibition of this PAG-RVM projection alleviates mechanical and thermal hypersensitivity associated with neuropathy, whereas its activation enhances hypersensitivity in the mice. Thus, our findings revealed that somatostatin neurons within the PAG-RVM axial are crucial for descending pain facilitation and can potentially be exploited as a useful therapeutic target for neuropathic pain. PERSPECTIVE: We report the profound contribution of somatostatin neurons within the PAG-RVM projections to descending pain facilitation underlying neuropathic pain. These results may help to develop central therapeutic strategies for neuropathic pain.

Identification of brain-to-spinal circuits controlling the laterality and duration of mechanical allodynia in mice

Mechanical allodynia (MA) represents one prevalent symptom of chronic pain. Previously we and others have identified spinal and brain circuits that transmit or modulate the initial establishment of MA. However, brain-derived descending pathways that control the laterality and duration of MA are still poorly understood. Here we report that the contralateral brain-to-spinal circuits, from Oprm1 neurons in the lateral parabrachial nucleus (lPBNOprm1), via Pdyn neurons in the dorsal medial regions of hypothalamus (dmHPdyn), to the spinal dorsal horn (SDH), act to prevent nerve injury from inducing contralateral MA and reduce the duration of bilateral MA induced by capsaicin. Ablating/silencing dmH-projecting lPBNOprm1 neurons or SDH-projecting dmHPdyn neurons, deleting Dyn peptide from dmH, or blocking spinal κ-opioid receptors all led to long-lasting bilateral MA. Conversely, activation of dmHPdyn neurons or their axonal terminals in SDH can suppress sustained bilateral MA induced by lPBN lesion.

Neurons in the caudal ventrolateral medulla mediate descending pain control

Supraspinal brain regions modify nociceptive signals in response to various stressors including stimuli that elevate pain thresholds. The medulla oblongata has previously been implicated in this type of pain control, but the neurons and molecular circuits involved have remained elusive. Here we identify catecholaminergic neurons in the caudal ventrolateral medulla that are activated by noxious stimuli in mice. Upon activation, these neurons produce bilateral feed-forward inhibition that attenuates nociceptive responses through a pathway involving the locus coeruleus and norepinephrine in the spinal cord. This pathway is sufficient to attenuate injury-induced heat allodynia and is required for counter-stimulus induced analgesia to noxious heat. Our findings define a component of the pain modulatory system that regulates nociceptive responses.

Molecular identification of bulbospinal ON neurons by GPER, which drives pain and morphine tolerance

The rostral ventromedial medulla (RVM) exerts bidirectional descending modulation of pain attributable to the activity of electrophysiologically identified pronociceptive ON and antinociceptive OFF neurons. Here, we report that GABAergic ON neurons specifically express G protein-coupled estrogen receptor (GPER). GPER+ neurons exhibited characteristic ON-like responses upon peripheral nociceptive stimulation. Optogenetic activation of GPER+ neurons facilitated, but their ablation abrogated, pain. Furthermore, activation of GPER caused depolarization of ON cells, potentiated pain, and ameliorated morphine analgesia through desensitizing μ-type opioid receptor-mediated (MOR-mediated) activation of potassium currents. In contrast, genetic ablation or pharmacological blockade of GPER attenuated pain, enhanced morphine analgesia, and delayed the development of morphine tolerance in diverse preclinical pain models. Our data strongly indicate that GPER is a marker for GABAergic ON cells and illuminate the mechanisms underlying hormonal regulation of pain and analgesia, thus highlighting GPER as a promising target for the treatment of pain and opioid tolerance.

Targeted anatomical and functional identification of antinociceptive and pronociceptive serotonergic neurons that project to the spinal dorsal horn

Spinally projecting serotonergic neurons play a key role in controlling pain sensitivity and can either increase or decrease nociception depending on physiological context. It is currently unknown how serotonergic neurons mediate these opposing effects. Utilizing virus-based strategies and Tph2-Cre transgenic mice, we identified two anatomically separated populations of serotonergic hindbrain neurons located in the lateral paragigantocellularis (LPGi) and the medial hindbrain, which respectively innervate the superficial and deep spinal dorsal horn and have contrasting effects on sensory perception. Our tracing experiments revealed that serotonergic neurons of the LPGi were much more susceptible to transduction with spinally injected AAV2retro vectors than medial hindbrain serotonergic neurons. Taking advantage of this difference, we employed intersectional chemogenetic approaches to demonstrate that activation of the LPGi serotonergic projections decreases thermal sensitivity, whereas activation of medial serotonergic neurons increases sensitivity to mechanical von Frey stimulation. Together these results suggest that there are functionally distinct classes of serotonergic hindbrain neurons that differ in their anatomical location in the hindbrain, their postsynaptic targets in the spinal cord, and their impact on nociceptive sensitivity. The LPGi neurons that give rise to rather global and bilateral projections throughout the rostrocaudal extent of the spinal cord appear to be ideally poised to contribute to widespread systemic pain control.

The anatomy of head pain

Pain-sensitive structures in the head and neck, including the scalp, periosteum, meninges, and blood vessels, are innervated predominantly by the trigeminal and upper cervical nerves. The trigeminal nerve supplies most of the sensation to the head and face, with the ophthalmic division (V1) providing innervation to much of the supratentorial dura mater and vessels. This creates referral patterns for pain that may be misleading to clinicians and patients, as described by studies involving awake craniotomies and stimulation with electrical and mechanical stimuli. Most brain parenchyma and supratentorial vessels refer pain to the ipsilateral V1 territory, and less commonly the V2 or V3 region. The upper cervical nerves provide innervation to the posterior scalp, while the periauricular region and posterior fossa are territories with shared innervation. Afferent fibers that innervate the head and neck send nociceptive input to the trigeminocervical complex, which then projects to additional pain processing areas in the brainstem, thalamus, hypothalamus, and cortex. This chapter discusses the pain-sensitive structures in the head and neck, including pain referral patterns for many of these structures. It also provides an overview of peripheral and central nervous system structures responsible for transmitting and interpreting these nociceptive signals.

Layer-specific pain relief pathways originating from primary motor cortex

The primary motor cortex (M1) is involved in the control of voluntary movements and is extensively mapped in this capacity. Although the M1 is implicated in modulation of pain, the underlying circuitry and causal underpinnings remain elusive. We unexpectedly unraveled a connection from the M1 to the nucleus accumbens reward circuitry through a M1 layer 6-mediodorsal thalamus pathway, which specifically suppresses negative emotional valence and associated coping behaviors in neuropathic pain. By contrast, layer 5 M1 neurons connect with specific cell populations in zona incerta and periaqueductal gray to suppress sensory hypersensitivity without altering pain affect. Thus, the M1 employs distinct, layer-specific pathways to attune sensory and aversive-emotional components of neuropathic pain, which can be exploited for purposes of pain relief.

Segmental and descending control of primary afferent input to the spinal lamina X

ABSTRACT

Despite being involved in a number of functions, such as nociception and locomotion, spinal lamina X remains one of the least studied central nervous system regions. Here, we show that Aδ- and C-afferent inputs to lamina X neurons are presynaptically inhibited by homo- and heterosegmental afferents as well as by descending fibers from the corticospinal tract, dorsolateral funiculus, and anterior funiculus. Activation of descending tracts suppresses primary afferent-evoked action potentials and also elicits excitatory (mono- and polysynaptic) and inhibitory postsynaptic responses in lamina X neurons. Thus, primary afferent input to lamina X is subject to both spinal and supraspinal control being regulated by at least 5 distinct pathways.

The whisking oscillator circuit

Central oscillators are primordial neural circuits that generate and control rhythmic movements1,2. Mechanistic understanding of these circuits requires genetic identification of the oscillator neurons and their synaptic connections to enable targeted electrophysiological recording and causal manipulation during behaviours. However, such targeting remains a challenge with mammalian systems. Here we delimit the oscillator circuit that drives rhythmic whisking-a motor action that is central to foraging and active sensing in rodents3,4. We found that the whisking oscillator consists of parvalbumin-expressing inhibitory neurons located in the vibrissa intermediate reticular nucleus (vIRtPV) in the brainstem. vIRtPV neurons receive descending excitatory inputs and form recurrent inhibitory connections among themselves. Silencing vIRtPV neurons eliminated rhythmic whisking and resulted in sustained vibrissae protraction. In vivo recording of opto-tagged vIRtPV neurons in awake mice showed that these cells spike tonically when animals are at rest, and transition to rhythmic bursting at the onset of whisking, suggesting that rhythm generation is probably the result of network dynamics, as opposed to intrinsic cellular properties. Notably, ablating inhibitory synaptic inputs to vIRtPV neurons quenched their rhythmic bursting, impaired the tonic-to-bursting transition and abolished regular whisking. Thus, the whisking oscillator is an all-inhibitory network and recurrent synaptic inhibition has a key role in its rhythmogenesis.

Kappa opioids inhibit the GABA/glycine terminals of rostral ventromedial medulla projections in the superficial dorsal horn of the spinal cord

Descending projections from neurons in the rostral ventromedial medulla (RVM) make synapses within the superficial dorsal horn (SDH) of the spinal cord that are involved in the modulation of nociception, the development of chronic pain and itch, and an important analgesic target for opioids. This projection is primarily inhibitory, but the relative contribution of GABAergic and glycinergic transmission is unknown and there is limited knowledge about the SDH neurons targeted. Additionally, the details of how spinal opioids mediate analgesia remain unclear, and no study has investigated the opioid modulation of this synapse. We address this using ex vivo optogenetic stimulation of RVM fibres in conjunction with whole-cell patch-clamp recordings from the SDH in spinal cord slices. We demonstrate that both GABAergic and glycinergic neurotransmission is employed and show that SDH target neurons have diverse morphological and electrical properties, consistent with both inhibitory and excitatory interneurons. Then, we describe a subtype of SDH neurons that have a glycine-dominant input, indicating that the quality of descending inhibition across cells is not uniform. Finally, we discovered that the kappa-opioid receptor agonist U69593 presynaptically suppressed most RVM-SDH synapses. By contrast, the mu-opioid receptor agonist DAMGO acted both pre- and post-synaptically at a subset of synapses, and the delta-opioid receptor agonist deltorphin II had little effect. These data provide important mechanistic information about a descending control pathway that regulates spinal circuits. This information is necessary to understand how sensory inputs are shaped and develop more reliable and effective alternatives to current opioid analgesics. Abstract figure legend We combined ex vivo optogenetic stimulation of RVM fibres with whole cell electrophysiology of SDH neurons to investigate the final synapse in a key descending pain modulatory pathway. We demonstrate that both glycine and GABA mediate signalling at the RVM-SDH synapse, that the SDH targets of RVM projections have diverse electrical and morphological characteristics, and that presynaptic inhibition is directly and consistently achieved by kappa opioid agonists. Opioid receptors shown are sized relative to the proportion of neurons that responded to its specific agonists (81 and 84percent of DF and non-DF neurons responded to kappa opioid receptor agonists, respectively. Responses that occurred in <255 percentage of neurons are not indicated here). This article is protected by copyright. All rights reserved.

Medullary kappa-opioid receptor neurons inhibit pain and itch through a descending circuit

In perilous and stressful situations, the ability to suppress pain can be critical for survival. The rostral ventromedial medulla contains neurons that robustly inhibit nocioception at the level of the spinal cord through a top-down modulatory pathway. Although much is known about the role of the rostral ventromedial medulla in the inhibition of pain, the precise ability to directly manipulate pain-inhibitory neurons in the rostral ventromedial medulla has never been achieved. We now expose a cellular circuit that inhibits nocioception and itch in mice. Through a combination of molecular, tracing and behavioural approaches, we found that rostral ventromedial medulla neurons containing the kappa-opioid receptor inhibit itch and nocioception. With chemogenetic inhibition, we uncovered that these neurons are required for stress-induced analgesia. Using intersectional chemogenetic and pharmacological approaches, we determined that rostral ventromedial medulla kappa-opioid receptor neurons inhibit nocioception and itch through a descending circuit. Lastly, we identified a dynorphinergic pathway arising from the periaqueductal grey that modulates nociception within the rostral ventromedial medulla. These discoveries highlight a distinct population of rostral ventromedial medulla neurons capable of broadly and robustly inhibiting itch and nocioception.

Shifting the Balance: How Top-Down and Bottom-Up Input Modulate Pain via the Rostral Ventromedial Medulla

The sensory experience of pain depends not only on the transmission of noxious information (nociception), but on the state of the body in a biological, psychological, and social milieu. A brainstem pain-modulating system with its output node in the rostral ventromedial medulla (RVM) can regulate the threshold and gain for nociceptive transmission. This review considers the current understanding of how RVM pain-modulating neurons, namely ON-cells and OFF-cells, are engaged by “top-down” cognitive and emotional factors, as well as by “bottom-up” sensory inputs, to enhance or suppress pain.

Dissecting the Neural Circuitry for Pain Modulation and Chronic Pain: Insights from Optogenetics

Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage. The processing of pain involves complicated modulation at the levels of the periphery, spinal cord, and brain. The pathogenesis of chronic pain is still not fully understood, which makes the clinical treatment challenging. Optogenetics, which combines optical and genetic technologies, can precisely intervene in the activity of specific groups of neurons and elements of the related circuits. Taking advantage of optogenetics, researchers have achieved a body of new findings that shed light on the cellular and circuit mechanisms of pain transmission, pain modulation, and chronic pain both in the periphery and the central nervous system. In this review, we summarize recent findings in pain research using optogenetic approaches and discuss their significance in understanding the pathogenesis of chronic pain.

Testing the Role of Glutamate NMDA Receptors in Peripheral Trigeminal Nociception Implicated in Migraine Pain

The pro-nociceptive role of glutamate in the CNS in migraine pathophysiology is well established. Glutamate, released from trigeminal afferents, activates second order nociceptive neurons in the brainstem. However, the function of peripheral glutamate receptors in the trigeminovascular system suggested as the origin site for migraine pain, is less known. In the current project, we used calcium imaging and patch clamp recordings from trigeminal ganglion (TG) neurons, immunolabelling, CGRP assay and direct electrophysiological recordings from rat meningeal afferents to investigate the role of glutamate in trigeminal nociception. Glutamate, aspartate, and, to a lesser extent, NMDA under free-magnesium conditions, evoked calcium transients in a fraction of isolated TG neurons, indicating functional expression of NMDA receptors. The fraction of NMDA sensitive neurons was increased by the migraine mediator CGRP. NMDA also activated slowly desensitizing currents in 37% of TG neurons. However, neither glutamate nor NMDA changed the level of extracellular CGRP. TG neurons expressed both GluN2A and GluN2B subunits of NMDA receptors. In addition, after removal of magnesium, NMDA activated persistent spiking activity in a fraction of trigeminal nerve fibers in meninges. Thus, glutamate activates NMDA receptors in somas of TG neurons and their meningeal nerve terminals in magnesium-dependent manner. These findings suggest that peripherally released glutamate can promote excitation of meningeal afferents implicated in generation of migraine pain in conditions of inherited or acquired reduced magnesium blockage of NMDA channels and support the usage of magnesium supplements in migraine.

Simultaneous brain, brainstem and spinal cord pharmacological-fMRI reveals involvement of an endogenous opioid network in attentional analgesia

Pain perception is decreased by shifting attentional focus away from a threatening event. This attentional analgesia engages parallel descending control pathways from anterior cingulate (ACC) to locus coeruleus, and ACC to periaqueductal grey (PAG) – rostral ventromedial medulla (RVM), indicating possible roles for noradrenergic or opioidergic neuromodulators. To determine which pathway modulates nociceptive activity in humans, we used simultaneous whole brain-spinal cord pharmacological-fMRI (N = 39) across three sessions. Noxious thermal forearm stimulation generated somatotopic-activation of dorsal horn (DH) whose activity correlated with pain report and mirrored attentional pain modulation. Activity in an adjacent cluster reported the interaction between task and noxious stimulus. Effective connectivity analysis revealed that ACC interacts with PAG and RVM to modulate spinal cord activity. Blocking endogenous opioids with Naltrexone impairs attentional analgesia and disrupts RVM-spinal and ACC-PAG connectivity. Noradrenergic augmentation with Reboxetine did not alter attentional analgesia. Cognitive pain modulation involves opioidergic ACC-PAG-RVM descending control which suppresses spinal nociceptive activity.

The role of endogenous opioid neuropeptides in neurostimulation-driven analgesia

Due to the prevalence of chronic pain worldwide, there is an urgent need to improve pain management strategies. While opioid drugs have long been used to treat chronic pain, their use is severely limited by adverse effects and abuse liability. Neurostimulation techniques have emerged as a promising option for chronic pain that is refractory to other treatments. While different neurostimulation strategies have been applied to many neural structures implicated in pain processing, there is variability in efficacy between patients, underscoring the need to optimize neurostimulation techniques for use in pain management. This optimization requires a deeper understanding of the mechanisms underlying neurostimulation-induced pain relief. Here, we discuss the most commonly used neurostimulation techniques for treating chronic pain. We present evidence that neurostimulation-induced analgesia is in part driven by the release of endogenous opioids and that this endogenous opioid release is a common endpoint between different methods of neurostimulation. Finally, we introduce technological and clinical innovations that are being explored to optimize neurostimulation techniques for the treatment of pain, including multidisciplinary efforts between neuroscience research and clinical treatment that may refine the efficacy of neurostimulation based on its underlying mechanisms.

Engaging endogenous opioid circuits in pain affective processes

The pervasive use of opioid compounds for pain relief is rooted in their utility as one of the most effective therapeutic strategies for providing analgesia. While the detrimental side effects of these compounds have significantly contributed to the current opioid epidemic, opioids still provide millions of patients with reprieve from the relentless and agonizing experience of pain. The human experience of pain has long recognized the perceived unpleasantness entangled with a unique sensation that is immediate and identifiable from the first-person subjective vantage point as "painful." From this phenomenological perspective, how is it that opioids interfere with pain perception? Evidence from human lesion, neuroimaging, and preclinical functional neuroanatomy approaches is sculpting the view that opioids predominately alleviate the affective or inferential appraisal of nociceptive neural information. Thus, opioids weaken pain-associated unpleasantness rather than modulate perceived sensory qualities. Here, we discuss the historical theories of pain to demonstrate how modern neuroscience is revisiting these ideas to deconstruct the brain mechanisms driving the emergence of aversive pain perceptions. We further detail how targeting opioidergic signaling within affective or emotional brain circuits remains a strong avenue for developing targeted pharmacological and gene-therapy analgesic treatments that might reduce the dependence on current clinical opioid options.

Brain circuits for pain and its treatment

[Figure: see text].

Identification and characterization of rostral ventromedial medulla neurons synaptically connected to the urinary bladder afferents in female rats with or without neonatal cystitis

The neurons in the rostral ventromedial medulla (RVM) play a major role in pain modulation. We have previously shown that early-life noxious bladder stimuli in rats resulted in an overall spinal GABAergic disinhibition and a long-lasting bladder/colon sensitization when tested in adulthood. However, the neuromolecular alterations within RVM neurons in the pathophysiology of early life bladder inflammation have not been elucidated. In this study, we have identified and characterized RVM neurons that are synaptically linked to the bladder and colon and examined the effect of neonatal bladder inflammation on molecular expressions of these neurons. A transient bladder inflammation was induced by intravesicular instillation of protamine sulfate and zymosan during postnatal days 14 through 16 (P14-16) followed by pseudorabies virus PRV-152 and PRV-614 injections into the bladder and colon, respectively, on postnatal day P60. Tissues were examined 96 h postinoculation for serotonergic, GABAergic, and enkephalinergic expressions using in situ hybridization and/or immunohistochemistry techniques. The results revealed that > 50% of RVM neurons that are synaptically connected to the bladder (i.e., PRV-152+) were GABAergic, 40% enkephalinergic, and about 14% expressing serotonergic marker tryptophan hydroxylase 2 (TpH2). Neonatal cystitis resulted in a significant increase in converging neurons in RVM receiving dual synaptic inputs from the bladder and colon. In addition, neonatal cystitis significantly downregulated vesicular GABA transporter (VGAT) with a concomitant increase in TpH2 expression in bladder-linked RVM neurons, suggesting an alteration in supraspinal signaling. These alterations of synaptic connectivity and GABAergic/serotonergic expressions in RVM neurons may contribute to bladder pain modulation and cross-organ visceral sensitivity.

G-Protein-Coupled Estrogen Receptor (GPER) in the Rostral Ventromedial Medulla Is Essential for Mobilizing Descending Inhibition of Itch

Chronic itch is a troublesome condition and often difficult to cure. Emerging evidence suggests that the periaqueductal gray (PAG)-rostral ventromedial medulla (RVM) pathway may play an important role in the regulation of itch, but the cellular organization and molecular mechanisms remain incompletely understood. Here, we report that a group of RVM neurons distinctively express the G-protein-coupled estrogen receptor (GPER), which mediates descending inhibition of itch. We found that GPER⁺ neurons in the RVM were activated in chronic itch conditions in rats and mice. Selective ablation or chemogenetic suppression of RVM GPER⁺ neurons resulted in mechanical alloknesis and increased scratching in response to pruritogens, whereas chemogenetic activation of GPER⁺ neurons abrogated itch responses, indicating that GPER⁺ neurons are antipruritic. Moreover, GPER-deficient mice and rats of either sex exhibited hypersensitivity to mechanical and chemical itch, a phenotype reversible by the µ type opioid receptor (MOR) antagonism. Additionally, significant MOR phosphorylation in the RVM was detected in chronic itch models in wild-type but not in GPER^-/- rats. Therefore, GPER not only identifies a population of medullary antipruritic neurons but may also determine the descending antipruritic tone through regulating µ opioid signaling.SIGNIFICANCE STATEMENT Therapeutic options for itch are limited because of an as yet incomplete understanding of the mechanisms of itch processing. Our data have provided novel insights into the cellular organization and molecular mechanisms of descending regulation of itch in normal and pathologic conditions. GPER⁺ neurons (largely GABAergic) in the RVM are antipruritic neurons under tonic opioidergic inhibition, activation of GPER promotes phosphorylation of MOR and disinhibition of the antipruritic GPER⁺ neurons from inhibitory opioidergic inputs, and failure to mobilize GPER⁺ neurons may result in the exacerbation of itch. Our data also illuminate on some of the outstanding questions in the field, such as the mechanisms underlying sex bias in itch, pain, and opioid analgesia and the paradoxical effects of morphine on pain and itch.

G protein-coupled estrogen receptor in the rostral ventromedial medulla contributes to the chronification of postoperative pain

Aims

Chronification of postoperative pain is a common clinical phenomenon following surgical operation, and it perplexes a great number of patients. Estrogen and its membrane receptor (G protein‐coupled estrogen receptor, GPER) play a crucial role in pain regulation. Here, we explored the role of GPER in the rostral ventromedial medulla (RVM) during chronic postoperative pain and search for the possible mechanism.

Methods and Results

Postoperative pain was induced in mice or rats via a plantar incision surgery. Behavioral tests were conducted to detect both thermal and mechanical pain, showing a small part (16.2%) of mice developed into pain persisting state with consistent low pain threshold on 14 days after incision surgery compared with the pain recovery mice. Immunofluorescent staining assay revealed that the GPER‐positive neurons in the RVM were significantly activated in pain persisting rats. In addition, RT‐PCR and immunoblot analyses showed that the levels of GPER and phosphorylated μ‐type opioid receptor (p‐MOR) in the RVM of pain persisting mice were apparently increased on 14 days after incision surgery. Furthermore, chemogenetic activation of GPER‐positive neurons in the RVM of Gper‐Cre mice could reverse the pain threshold of pain recovery mice. Conversely, chemogenetic inhibition of GPER‐positive neurons in the RVM could prevent mice from being in the pain persistent state.

Conclusion

Our findings demonstrated that the GPER in the RVM was responsible for the chronification of postoperative pain and the downstream pathway might be involved in MOR phosphorylation.

Monoaminergic and Opioidergic Modulation of Brainstem Circuits: New Insights Into the Clinical Challenges of Pain Treatment?

The treatment of neuropathic pain remains a clinical challenge. Analgesic drugs and antidepressants are frequently ineffective, and opioids may induce side effects, including hyperalgesia. Recent results on brainstem pain modulatory circuits may explain those clinical challenges. The dual action of noradrenergic (NA) modulation was demonstrated in animal models of neuropathic pain. Besides the well-established antinociception due to spinal effects, the NA system may induce pronociception by directly acting on brainstem pain modulatory circuits, namely, at the locus coeruleus (LC) and medullary dorsal reticular nucleus (DRt). The serotoninergic system also has a dual action depending on the targeted spinal receptor, with an exacerbated activity of the excitatory 5-hydroxytryptamine 3 (5-HT3) receptors in neuropathic pain models. Opioids are involved in the modulation of descending modulatory circuits. During neuropathic pain, the opioidergic modulation of brainstem pain control areas is altered, with the release of enhanced local opioids along with reduced expression and desensitization of μ-opioid receptors (MOR). In the DRt, the installation of neuropathic pain increases the levels of enkephalins (ENKs) and induces desensitization of MOR, which may enhance descending facilitation (DF) from the DRt and impact the efficacy of exogenous opioids. On the whole, the data discussed in this review indicate the high plasticity of brainstem pain control circuits involving monoaminergic and opioidergic control. The data from studies of these neurochemical systems in neuropathic models indicate the importance of designing drugs that target multiple neurochemical systems, namely, maximizing the antinociceptive effects of antidepressants that inhibit the reuptake of serotonin and noradrenaline and preventing desensitization and tolerance of MOR at the brainstem.

IFI16 directly senses viral RNA and enhances RIG-I transcription and activation to restrict influenza virus infection

The retinoic acid-inducible gene I (RIG-I) receptor senses cytoplasmic viral RNA and activates type I interferons (IFN-I) and downstream antiviral immune responses. How RIG-I binds to viral RNA and how its activation is regulated remains unclear. Here, using IFI16 knockout cells and p204-deficient mice, we demonstrate that the DNA sensor IFI16 enhances IFN-I production to inhibit influenza A virus (IAV) replication. IFI16 positively upregulates RIG-I transcription through direct binding to and recruitment of RNA polymerase II to the RIG-I promoter. IFI16 also binds to influenza viral RNA via its HINa domain and to RIG-I protein with its PYRIN domain, thus promoting IAV-induced K63-linked polyubiquitination and RIG-I activation. Our work demonstrates that IFI16 is a positive regulator of RIG-I signalling during influenza virus infection, highlighting its role in the RIG-I-like-receptor-mediated innate immune response to IAV and other RNA viruses, and suggesting its possible exploitation to modulate the antiviral response.

Posterior subthalamic nucleus (PSTh) mediates innate fear-associated hypothermia in mice

The neural mechanisms of fear-associated thermoregulation remain unclear. Innate fear odor 2-methyl-2-thiazoline (2MT) elicits rapid hypothermia and elevated tail temperature, indicative of vasodilation-induced heat dissipation, in wild-type mice, but not in mice lacking Trpa1-the chemosensor for 2MT. Here we report that Trpa1-/- mice show diminished 2MT-evoked c-fos expression in the posterior subthalamic nucleus (PSTh), external lateral parabrachial subnucleus (PBel) and nucleus of the solitary tract (NTS). Whereas tetanus toxin light chain-mediated inactivation of NTS-projecting PSTh neurons suppress, optogenetic activation of direct PSTh-rostral NTS pathway induces hypothermia and tail vasodilation. Furthermore, selective opto-stimulation of 2MT-activated, PSTh-projecting PBel neurons by capturing activated neuronal ensembles (CANE) causes hypothermia. Conversely, chemogenetic suppression of vGlut2+ neurons in PBel or PSTh, or PSTh-projecting PBel neurons attenuates 2MT-evoked hypothermia and tail vasodilation. These studies identify PSTh as a major thermoregulatory hub that connects PBel to NTS to mediate 2MT-evoked innate fear-associated hypothermia and tail vasodilation.

The cellular mechanism by which the rostral ventromedial medulla acts on the spinal cord during chronic pain

Clinical therapies for chronic pain are limited. While targeted drugs are promising therapies for chronic pain, they exhibit insufficient efficacy and poor targeting. The occurrence of chronic pain partly results from central changes caused by alterations in neurons in the rostral ventromedial medulla (RVM) in the brainstem regulatory pathway. The RVM, which plays a key role in the descending pain control pathway, greatly contributes to the development and maintenance of pain. However, the exact roles of the RVM in chronic pain remain unclear, making it difficult to develop new drugs targeting the RVM and related pathways. Here, we first discuss the roles of the RVM and related circuits in chronic pain. Then, we analyze synaptic transmission between RVM neurons and spinal cord neurons, specifically focusing on the release of neurotransmitters, to explore the cellular mechanisms by which the RVM regulates chronic pain. Finally, we propose some ideas for the development of drugs targeting the RVM.

Presynaptic Inhibition of Pain and Touch in the Spinal Cord: From Receptors to Circuits

Sensory primary afferent fibers, conveying touch, pain, itch, and proprioception, synapse onto spinal cord dorsal horn neurons. Primary afferent central terminals express a wide variety of receptors that modulate glutamate and peptide release. Regulation of the amount and timing of neurotransmitter release critically affects the integration of postsynaptic responses and the coding of sensory information. The role of GABA (γ-aminobutyric acid) receptors expressed on afferent central terminals is particularly important in sensory processing, both in physiological conditions and in sensitized states induced by chronic pain. During the last decade, techniques of opto- and chemogenetic stimulation and neuronal selective labeling have provided interesting insights on this topic. This review focused on the recent advances about the modulatory effects of presynaptic GABAergic receptors in spinal cord dorsal horn and the neural circuits involved in these mechanisms.

Supraspinal Opioid Circuits Differentially Modulate Spinal Neuronal Responses in Neuropathic Rats

BACKGROUND

The anterior cingulate cortex and central nucleus of the amygdala connect widely with brainstem nuclei involved in descending modulation, including the rostral ventromedial medulla. Endogenous opioids in these circuits participate in pain modulation. The hypothesis was that a differential opioidergic role for the brain nuclei listed in regulation of spinal neuronal responses because separable effects on pain behaviors in awake animals were previously observed.

METHODS

This study utilized in vivo electrophysiology to determine the effects of morphine microinjection into the anterior cingulate cortex, right or left central nucleus of the amygdala, or the rostral ventromedial medulla on spinal wide dynamic range neuronal responses in isoflurane-anesthetized, male Sprague-Dawley rats. Ongoing activity in the ventrobasal thalamus was also measured. In total, 33 spinal nerve ligated and 26 control age- and weight-matched control rats were used.

RESULTS

Brainstem morphine reduced neuronal firing to 60-g von Frey stimulation in control rats (to 65 ± 12% of control response (means ± 95% CI), P < 0.001) with a greater inhibition in neuropathic rats (to 53 ± 17% of control response, P < 0.001). Contrasting anterior cingulate cortex morphine had only marginal modulatory effects on spinal neuronal responses with limited variance in effect between control and neuropathic rats. The inhibitory effects of morphine in the central nucleus of the amygdala were dependent on pain state and laterality; only right-side morphine reduced neuronal firing to 60-g stimulation in neuropathic rats (to 65 ± 14% of control response, P = 0.001). In addition, in neuropathic rats elevated ongoing neuronal activity in the ventral posterolateral thalamus was not inhibited by anterior cingulate cortex morphine, in contrast to evoked responses.

CONCLUSIONS

Cumulatively the data support opioid modulation of evoked responses predominately through a lateralized output from the right amygdala, as well as from the brainstem that is enhanced in injured conditions. Minimal modulation of dorsal horn responses was observed after anterior cingulate cortex opioid administration regardless of injury state.

Central Nervous System Targets: Supraspinal Mechanisms of Analgesia

While the acute sensation of pain is protective, signaling the presence of actual or potential bodily harm, its persistence is unpleasant. When pain becomes chronic, it has limited evolutionarily advantage. Despite the differing nature of acute and chronic pain, a common theme is that sufferers seek pain relief. The possibility to medicate pain types as varied as a toothache or postsurgical pain reflects the diverse range of mechanism(s) by which pain-relieving "analgesic" therapies may reduce, eliminate, or prevent pain. Systemic application of an analgesic able to cross the blood-brain barrier can result in pain modulation via interaction with targets at different sites in the central nervous system. A so-called supraspinal mechanism of action indicates manipulation of a brain-defined circuitry. Pre-clinical studies demonstrate that, according to the brain circuitry targeted, varying therapeutic pain-relieving effects may be observed that relate to an impact on, for example, sensory and/or affective qualities of pain. In many cases, this translates to the clinic. Regardless of the brain circuitry manipulated, modulation of brain processing often directly impacts multiple aspects of nociceptive transmission, including spinal neuronal signaling. Consideration of supraspinal mechanisms of analgesia and ensuing pain relief must take into account nonbrain-mediated effects; therefore, in this review, the supraspinally mediated analgesic actions of opioidergic, anti-convulsant, and anti-depressant drugs are discussed. The persistence of poor treatment outcomes and/or side effect profiles of currently used analgesics highlight the need for the development of novel therapeutics or more precise use of available agents. Fully uncovering the complex biology of nociception, as well as currently used analgesic mechanism(s) and site(s) of action, will expedite this process.

Neurons that regulate mouse torpor

The advent of endothermy, which is achieved through the continuous homeostatic regulation of body temperature and metabolism^1,2, is a defining feature of mammalian and avian evolution. However, when challenged by food deprivation or harsh environmental conditions, many mammalian species initiate adaptive energy-conserving survival strategies-including torpor and hibernation-during which their body temperature decreases far below its homeostatic set-point^3-5. How homeothermic mammals initiate and regulate these hypothermic states remains largely unknown. Here we show that entry into mouse torpor, a fasting-induced state with a greatly decreased metabolic rate and a body temperature as low as 20 °C⁶, is regulated by neurons in the medial and lateral preoptic area of the hypothalamus. We show that restimulation of neurons that were activated during a previous bout of torpor is sufficient to initiate the key features of torpor, even in mice that are not calorically restricted. Among these neurons we identify a population of glutamatergic Adcyap1-positive cells, the activity of which accurately determines when mice naturally initiate and exit torpor, and the inhibition of which disrupts the natural process of torpor entry, maintenance and arousal. Taken together, our results reveal a specific neuronal population in the mouse hypothalamus that serves as a core regulator of torpor. This work forms a basis for the future exploration of mechanisms and circuitry that regulate extreme hypothermic and hypometabolic states, and enables genetic access to monitor, initiate, manipulate and study these ancient adaptations of homeotherm biology.

Stimulus predictability moderates the withdrawal strategy in response to repetitive noxious stimulation in humans

Innate defensive behaviors such as reflexes are found across all species, constituting preprogrammed responses to external threats that are not anticipated. Previous studies indicated that the excitability of the reflex arcs like spinal nociceptive withdrawal reflex (NWR) pathways in humans are modulated by several cognitive factors. This study assesses how the predictability of a threat affects the biomechanical pattern of the withdrawal response, showing that distal and proximal muscles are differentially modulated by descending control.

General anesthetics activate a potent central pain-suppression circuit in the amygdala

General anesthesia (GA) can produce analgesia (loss of pain) independent of inducing loss of consciousness, but the underlying mechanisms remain unclear. We hypothesized that GA suppresses pain in part by activating supraspinal analgesic circuits. We discovered a distinct population of GABAergic neurons activated by GA in the mouse central amygdala (CeA_GA neurons). In vivo calcium imaging revealed that different GA drugs activate a shared ensemble of CeA_GA neurons. CeA_GA neurons also possess basal activity that mostly reflects animals' internal state rather than external stimuli. Optogenetic activation of CeA_GA potently suppressed both pain-elicited reflexive and self-recuperating behaviors across sensory modalities and abolished neuropathic pain-induced mechanical (hyper-)sensitivity. Conversely, inhibition of CeA_GA activity exacerbated pain, produced strong aversion and canceled the analgesic effect of low-dose ketamine. CeA_GA neurons have widespread inhibitory projections to many affective pain-processing centers. Our study points to CeA_GA as a potential powerful therapeutic target for alleviating chronic pain.