An Anterior Cingulate Cortex-to-Midbrain Projection Controls Chronic Itch in Mice

An excitatory neural circuit for descending inhibition of itch processing

Itch serves as a self-protection mechanism against harmful external agents, whereas uncontrolled and persistent itch severely influences the quality of life of patients and aggravates their diseases. Unfortunately, the existing treatments are largely ineffective. The current difficulty in treatment may be closely related to the fact that the central neural mechanisms underlying itch processing, especially descending inhibition of itch, are poorly understood. Here, we demonstrate that an excitatory descending neural circuit from rostral anterior cingulate cortex pyramidal (rACCPy) neurons to periaqueductal gray GABAergic (PAGGABA) neurons plays a key role in the inhibition of itch. The activity of itch-tagged rACCPy neurons decreases during the itch-evoked scratching period. Artificial activation or inhibition of the neural circuits significantly impairs or enhances itch processing, respectively. Thus, an excitatory neural circuit is identified as playing a crucial inhibitory role in descending regulation of itch, suggesting that it could be a potential target for treating itch.

Differential regulation of pruritic sensation and emotion by cannabinoid type 1 receptors on mPFC glutamatergic and GABAergic neurons

Itch causes a strong urge to scratch and induces negative emotions, such as aversion and anxiety. Antihistamine medications are key in the clinical management of pruritus, but their therapeutic efficacy in controlling moderate and severe itching remains limited. The neural circuits in the brain that process itching and itch-induced aversion and anxiety remain unclear so far. Human brain imaging suggests that the medial prefrontal cortex (mPFC) is involved in processing the emotional and motivational components of itching. In this study, we investigated the mechanisms by which glutamatergic and GABAergic neurons in mPFC differentially regulated pruritic sensation and emotion through cannabinoid type 1 receptors (CB1Rs). Chloroquinoline (CQ)-induced acute and calcipotriol (MC903)-induced chronic itch models were established. Fiberoptic calcium imaging was used to detect the activity of the two types of neurons in response to itching. The CB1R antagonist AM251 (0.5 mg in 200 nL) was microinjected into the mPFC through the implanted cannula. We showed that chemogenetic activation of glutamatergic neurons and inhibition of GABAergic neurons in the mPFC reduced scratching and chronic itch-induced anxiety. GABAergic, but not glutamatergic, neurons were involved in acute itch-induced aversion. CB1Rs on glutamatergic and GABAergic neurons modulated chronic itch-induced scratching and anxiety in divergent manners. However, CB1Rs did not affect acute itch-induced scratching. CB1Rs on GABAergic, but not glutamatergic, neurons regulated acute itch-induced aversion. These results may guide the development of therapeutic strategies targeting CB1Rs to treat itch-induced sensory and emotional responses.

Anterior Cingulate Cortex Contributes to the Hyperlocomotion under Nitrogen Narcosis

Nitrogen narcosis is a neurological syndrome that manifests when humans or animals encounter hyperbaric nitrogen, resulting in a range of motor, emotional, and cognitive abnormalities. The anterior cingulate cortex (ACC) is known for its significant involvement in regulating motivation, cognition, and action. However, its specific contribution to nitrogen narcosis-induced hyperlocomotion and the underlying mechanisms remain poorly understood. Here we report that exposure to hyperbaric nitrogen notably increased the locomotor activity of mice in a pressure-dependent manner. Concurrently, this exposure induced heightened activation among neurons in both the ACC and dorsal medial striatum (DMS). Notably, chemogenetic inhibition of ACC neurons effectively suppressed hyperlocomotion. Conversely, chemogenetic excitation lowered the hyperbaric pressure threshold required to induce hyperlocomotion. Moreover, both chemogenetic inhibition and genetic ablation of activity-dependent neurons within the ACC reduced the hyperlocomotion. Further investigation revealed that ACC neurons project to the DMS, and chemogenetic inhibition of ACC-DMS projections resulted in a reduction in hyperlocomotion. Finally, nitrogen narcosis led to an increase in local field potentials in the theta frequency band and a decrease in the alpha frequency band in both the ACC and DMS. These results collectively suggest that excitatory neurons within the ACC, along with their projections to the DMS, play a pivotal role in regulating the hyperlocomotion induced by exposure to hyperbaric nitrogen.

A thalamic nucleus reuniens-lateral septum-lateral hypothalamus circuit for comorbid anxiety-like behaviors in chronic itch

Chronic itch often clinically coexists with anxiety symptoms, creating a vicious cycle of itch-anxiety comorbidities that are difficult to treat. However, the neuronal circuit mechanisms underlying the comorbidity of anxiety in chronic itch remain elusive. Here, we report anxiety-like behaviors in mouse models of chronic itch and identify γ-aminobutyric acid-releasing (GABAergic) neurons in the lateral septum (LS) as the key player in chronic itch-induced anxiety. In addition, chronic itch is accompanied with enhanced activity and synaptic plasticity of excitatory projections from the thalamic nucleus reuniens (Re) onto LS GABAergic neurons. Selective chemogenetic inhibition of the Re → LS circuit notably alleviated chronic itch-induced anxiety, with no impact on anxiety induced by restraint stress. Last, GABAergic neurons in lateral hypothalamus (LH) receive monosynaptic inhibition from LS GABAergic neurons to mediate chronic itch-induced anxiety. These findings underscore the potential significance of the Re → LS → LH pathway in regulating anxiety-like comorbid symptoms associated with chronic itch.

CCL2 Potentiates Inflammation Pain and Related Anxiety-Like Behavior Through NMDA Signaling in Anterior Cingulate Cortex

Previous studies have shown that the C-C motif chemokine ligand 2 (CCL2) is widely expressed in the nervous system and involved in regulating the development of chronic pain and related anxiety-like behaviors, but its precise mechanism is still unclear. This paper provides an in-depth examination of the involvement of CCL2-CCR2 signaling in the anterior cingulate cortex (ACC) in intraplantar injection of complete Freund's adjuvant (CFA) leading to inflammatory pain and its concomitant anxiety-like behaviors by modulation of glutamatergic N-methyl-D-aspartate receptor (NMDAR). Our findings suggest that local bilateral injection of CCR2 antagonist in the ACC inhibits CFA-induced inflammatory pain and anxiety-like behavior. Meanwhile, the expression of CCR2 and CCL2 was significantly increased in ACC after 14 days of intraplantar injection of CFA, and CCR2 was mainly expressed in excitatory neurons. Whole-cell patch-clamp recordings showed that the CCR2 inhibitor RS504393 reduced the frequency of miniature excitatory postsynaptic currents (mEPSC) in ACC, and CCL2 was involved in the regulation of NMDAR-induced current in ACC neurons in the pathological state. In addition, local injection of the NR2B inhibitor of NMDAR subunits, Ro 25-6981, attenuated the effects of CCL2-induced hyperalgesia and anxiety-like behavior in the ACC. In summary, CCL2 acts on CCR2 in ACC excitatory neurons and participates in the regulation of CFA-induced pain and related anxiety-like behaviors through upregulation of NR2B. CCR2 in the ACC neuron may be a potential target for the treatment of chronic inflammatory pain and pain-related anxiety.

Prelimbic cortex-nucleus accumbens core projection positively regulates itch and itch-related aversion

Itch is one of the most common clinical symptoms in patients with diseases of the skin, liver, or kidney, and it strongly triggers aversive emotion and scratching behavior. Previous studies have confirmed the role of the prelimbic cortex (Prl) and the nucleus accumbens core (NAcC), which are reward and motivation regulatory centers, in the regulation of itch. However, it is currently unclear whether the Prl-NAcC projection, an important pathway connecting these two brain regions, is involved in the regulation of itch and its associated negative emotions. In this study, rat models of acute neck and cheek itch were established by subcutaneous injection of 5-HT, compound 48/80, or chloroquine. Immunofluorescence experiments determined that the number of c-Fos-immunopositive neurons in the Prl increased during acute itch. Chemogenetic inhibition of Prl glutamatergic neurons or Prl-NAcC glutamatergic projections can inhibit both histaminergic and nonhistaminergic itch-scratching behaviors and rectify the itch-related conditioned place aversion (CPA) behavior associated with nonhistaminergic itch. The Prl-NAcC projection may play an important role in the positive regulation of itch-scratching behavior by mediating the negative emotions related to itch.

Orexin neurons play contrasting roles in itch and pain neural processing via projecting to the periaqueductal gray

Pain and itch are recognized as antagonistically regulated sensations; pain suppresses itch, whilst pain inhibition enhances itch. The neural mechanisms at the central nervous system (CNS) underlying these pain-itch interactions still need to be explored. Here, we revealed the contrasting role of orexin-producing neurons (ORX neurons) in the lateral hypothalamus (LH), which suppresses pain while enhancing itch neural processing, by applying optogenetics to the acute pruritus and pain model. We also revealed that the circuit of ORX neurons from LH to periaqueductal gray regions served in the contrasting modulation of itch and pain processing using optogenetic terminal inhibition techniques. Additionally, by using an atopic dermatitis model, we confirmed the involvement of ORX neurons in regulating chronic itch processing, which could lead to a novel therapeutic target for persistent pruritus in clinical settings. Our findings provide new insight into the mechanism of antagonistic regulation between pain and itch in the CNS.

The sexually divergent cFos activation map of fear extinction

Objective

Post-traumatic stress disorder (PTSD) is a neuropsychiatric disorder that can develop after experiencing or witnessing a traumatic event. Exposure therapy is a common treatment for PTSD, but it has varying levels of efficacy depending on sex. In this study, we aimed to compare the sexual dimorphism in brain activation during the extinction of fear conditioning in male and female rats by detecting the c-fos levels in the whole brain.

Methods

Thirty-two rats (Male: n = 16; Female: n = 16) were randomly separated into the extinction group as well as the non-extinction group, and fear conditioning was followed by extinction and non-extinction, respectively. Subsequently, brain sections from the sacrificed animal were performed immunofluorescence and the collected data were analyzed by repeated two-way ANOVAs as well as Pearson Correlation Coefficient.

Results

Our findings showed that most brain areas activated during extinction were similar in both male and female rats, except for the reuniens thalamic nucleus and ventral hippocampi. Furthermore, we found differences in the correlation between c-fos activation levels and freezing behavior during extinction between male and female rats. Specifically, in male rats, c-fos activation in the anterior cingulate cortex was negatively correlated with the freezing level, while c-fos activation in the retrosplenial granular cortex was positively correlated with the freezing level; but in female rats did not exhibit any correlation between c-fos activation and freezing level. Finally, the functional connectivity analysis revealed differences in the neural networks involved in extinction learning between male and female rats. In male rats, the infralimbic cortex and insular cortex, anterior cingulate cortex and retrosplenial granular cortex, and dorsal dentate gyrus and dCA3 were strongly correlated after extinction. In female rats, prelimbic cortex and basolateral amygdala, insular cortex and dCA3, and anterior cingulate cortex and dCA1 were significantly correlated.

Conclusion

These results suggest divergent neural networks involved in extinction learning in male and female rats and provide a clue for improving the clinical treatment of exposure therapy based on the sexual difference.

Loosely synchronized activation of anterior cingulate cortical neurons for scratching response during histamine-induced itch

Itch is a distinctive sensation that causes a specific affection and scratching reaction. The anterior cingulate cortex (ACC) has been linked to itch sensation in numerous studies; however, its precise function in processing pruritic inputs remains unknown. Distinguishing the precise role of the ACC in itch sensation can be challenging because of its capacity to conduct heterologous neurophysiological activities. Here, we used in vivo calcium imaging to examine how ACC neurons in free-moving mice react to pruritogenic histamine. In particular, we focused on how the activity of the ACC neurons varied before and after the scratching response. We discovered that although the change in neuronal activity was not synchronized with the scratching reaction, the overall activity of itch-responsive neurons promptly decreased after the scratching response. These findings suggest that the ACC does not directly elicit the feeling of itchiness.

Excitatory Projections from the Prefrontal Cortex to Nucleus Accumbens Core D1-MSNs and κ Opioid Receptor Modulate Itch-Related Scratching Behaviors

Itch is an uncomfortable and complex sensation that elicits the desire to scratch. The nucleus accumbens (NAc) activity is important in driving sensation, motivation, and emotion. Excitatory afferents from the medial prefrontal cortex (mPFC), amygdala, and hippocampus are crucial in tuning the activity of dopamine receptor D1-expressing and D2-expressing medium spiny neurons (Drd1-MSN and Drd2-MSN) in the NAc. However, a cell-type and neural circuity-based mechanism of the NAc underlying acute itch remains unclear. We found that acute itch induced by compound 48/80 (C48/80) decreased the intrinsic membrane excitability in Drd1-MSNs, but not in Drd2-MSNs, in the NAc core of male mice. Chemogenetic activation of Drd1-MSNs alleviated C48/80-induced scratching behaviors but not itch-related anxiety-like behaviors. In addition, C48/80 enhanced the frequency of spontaneous EPSCs (sEPSCs) and reduced the paired-pulse ratio (PPR) of electrical stimulation-evoked EPSCs in Drd1-MSNs. Furthermore, C48/80 increased excitatory synaptic afferents to Drd1-MSNs from the mPFC, not from the basolateral amygdala (BLA) or ventral hippocampus (vHipp). Consistently, the intrinsic excitability of mPFC-NAc projecting pyramidal neurons was increased after C48/80 treatment. Chemogenetic inhibition of mPFC-NAc excitatory synaptic afferents relieved the scratching behaviors. Moreover, pharmacological activation of κ opioid receptor (KOR) in the NAc core suppressed C48/80-induced scratching behaviors, and the modulation of KOR activity in the NAc resulted in the changes of presynaptic excitatory inputs to Drd1-MSNs in C48/80-treated mice. Together, these results reveal the neural plasticity in synapses of NAc Drd1-MSNs from the mPFC underlying acute itch and indicate the modulatory role of the KOR in itch-related scratching behaviors.SIGNIFICANCE STATEMENT Itch stimuli cause strongly scratching desire and anxiety in patients. However, the related neural mechanisms remain largely unclear. In the present study, we demonstrated that the pruritogen compound 48/80 (C48/80) shapes the excitability of dopamine receptor D1-expressing medium spiny neurons (Drd1-MSNs) in the nucleus accumbens (NAc) core and the glutamatergic synaptic afferents from medial prefrontal cortex (mPFC) to these neurons. Chemogenetic activation of Drd1-MSNs or inhibition of mPFC-NAc excitatory synaptic afferents relieves the scratching behaviors. In addition, pharmacological activation of κ opioid receptor (KOR) in the NAc core alleviates C48/80-induced itch. Thus, targeting mPFC-NAc Drd1-MSNs or KOR may provide effective treatments for itch.