Pain-induced adaptations in the claustro-cingulate pathway.

Persistent pain is a prevalent medical concern correlating with a hyperexcitable anterior cingulate cortex (ACC). Its activity is modulated by inputs from several brain regions, but the maladjustments that these afferent circuits undergo during the transition from acute to chronic pain still require clarification. We focus on ACC-projecting claustrum (CLAACC) neurons and their responses to sensory and aversive stimuli in a mouse model of inflammatory pain. Using chemogenetics, in vivo calcium imaging, and ex vivo electrophysiological approaches, we reveal that suppression of CLAACC activity acutely attenuates allodynia and that the claustrum preferentially transmits aversive information to the ACC. With prolonged pain, a claustro-cingulate functional impairment develops, which is mediated by a weakened excitatory drive onto ACC pyramidal neurons, resulting in a diminished claustral influence on the ACC. These findings support an instrumental role of the claustrum in the processing of nociceptive information and its susceptibility to persistent pain states

Environmental enrichment promotes resilience to neuropathic pain-induced depression and correlates with decreased excitability of the anterior cingulate cortex.

Depression is a common comorbidity of chronic pain with many patients being affected. However, efficient pharmacological treatment strategies are still lacking. Therefore, it is desirable to find additional alternative approaches. Environmental enrichment has been suggested as a method to alleviate pain-induced depression. However, the neuronal mechanisms of its beneficial effects are still elusive. The anterior cingulate cortex (ACC) plays a central role in processing pain-related negative affect and chronic pain-induced plasticity in this region correlates with depressive symptoms. We studied the consequences of different durations of environmental enrichment on pain sensitivity and chronic pain-induced depression-like behaviors in a mouse model of neuropathic pain. Furthermore, we correlated the behavioral outcomes to the activity levels of pyramidal neurons in the ACC by analyzing their electrophysiological properties ex vivo. We found that early exposure to an enriched environment alone was not sufficient to cause resilience against pain-induced depression-like symptoms. However, extending the enrichment after the injury prevented the development of depression and reduced mechanical hypersensitivity. On the cellular level, increased neuronal excitability was associated with the depressive phenotype that was reversed by the enrichment. Therefore, neuronal excitability in the ACC was inversely correlated to the extended enrichment-induced resilience to depression. These results suggest that the improvement of environmental factors enhanced the resilience to developing chronic pain-related depression. Additionally, we confirmed the association between increased neuronal excitability in the ACC and depression-like states. Therefore, this non-pharmacological intervention could serve as a potential treatment strategy for comorbid symptoms of chronic pain

B-type natriuretic peptide-induced delayed modulation of TRPV1 and P2X3 receptors of mouse trigeminal sensory neurons

Important pain transducers of noxious stimuli are small- and medium-diameter sensory neurons that express transient receptor vanilloid-1 (TRPV1) channels and/or adenosine triphosphate (ATP)-gated P2X3 receptors whose activity is upregulated by endogenous neuropeptides in acute and chronic pain models. Little is known about the role of endogenous modulators in restraining the expression and function of TRPV1 and P2X3 receptors. In dorsal root ganglia, evidence supports the involvement of the natriuretic peptide system in the modulation of nociceptive transmission especially via the B-type natriuretic peptide (BNP) that activates the natriuretic peptide receptor-A (NPR-A) to downregulate sensory neuron excitability. Since the role of BNP in trigeminal ganglia (TG) is unclear, we investigated the expression of BNP in mouse TG in situ or in primary cultures and its effect on P2X3 and TRPV1 receptors of patch-clamped cultured neurons. Against scant expression of BNP, almost all neurons expressed NPRA at membrane level. While BNP rapidly increased cGMP production and Akt kinase phosphorylation, there was no early change in passive neuronal properties or responses to capsaicin, \u3b1,\u3b2-meATP or GABA. Nonetheless, 24 h application of BNP depressed TRPV1 mediated currents (an effect blocked by the NPR-A antagonist anantin) without changing responses to \u3b1,\u3b2-meATP or GABA. Anantin alone decreased basal cGMP production and enhanced control \u3b1,\u3b2-meATP-evoked responses, implying constitutive regulation of P2X3 receptors by ambient BNP. These data suggest a slow modulatory action by BNP on TRPV1 and P2X3 receptors outlining the role of this peptide as a negative regulator of trigeminal sensory neuron excitability to nociceptive stimuli. \ua9 2013 Vilotti et al

VTA GABA neurons modulate specific learning behaviors through the control of dopamine and cholinergic systems

The mesolimbic reward system is primarily comprised of the ventral tegmental area (VTA) and the nucleus accumbens (NAc) as well as their afferent and efferent connections. This circuitry is essential for learning about stimuli associated with motivationally-relevant outcomes. Moreover, addictive drugs affect and remodel this system, which may underlie their addictive properties. In addition to dopamine (DA) neurons, the VTA also contains approximately 30% γ-aminobutyric acid (GABA) neurons. The task of signaling both rewarding and aversive events from the VTA to the NAc has mostly been ascribed to DA neurons and the role of GABA neurons has been largely neglected until recently. GABA neurons provide local inhibition of DA neurons and also long-range inhibition of projection regions, including the NAc. Here we review studies using a combination of in vivo and ex vivo electrophysiology, pharmacogenetic and optogenetic manipulations that have characterized the functional neuroanatomy of inhibitory circuits in the mesolimbic system, and describe how GABA neurons of the VTA regulate reward and aversion-related learning. We also discuss pharmacogenetic manipulation of this system with benzodiazepines (BDZs), a class of addictive drugs, which act directly on GABAA receptors located on GABA neurons of the VTA. The results gathered with each of these approaches suggest that VTA GABA neurons bi-directionally modulate activity of local DA neurons, underlying reward or aversion at the behavioral level. Conversely, long-range GABA projections from the VTA to the NAc selectively target cholinergic interneurons (CINs) to pause their firing and temporarily reduce cholinergic tone in the NAc, which modulates associative learning. Further characterization of inhibitory circuit function within and beyond the VTA is needed in order to fully understand the function of the mesolimbic system under normal and pathological conditions

An amygdala-to-cingulate cortex circuit for conflicting choices in chronic pain

Summary: Chronic pain is a complex experience with multifaceted behavioral manifestations, often leading to pain avoidance at the expense of reward approach. How pain facilitates avoidance in situations with mixed outcomes is unknown. The anterior cingulate cortex (ACC) plays a key role in pain processing and in value-based decision-making. Distinct ACC inputs inform about the sensory and emotional quality of pain. However, whether specific ACC circuits underlie pathological conflict assessment in pain remains underexplored. Here, we demonstrate that mice with chronic pain favor cold avoidance rather than reward approach in a conflicting task. This occurs along with selective strengthening of basolateral amygdala inputs onto ACC layer 2/3 pyramidal neurons. The amygdala-cingulate projection is necessary and sufficient for the conflicting cold avoidance. Further, low-frequency stimulation of this pathway restores AMPA receptor function and reduces avoidance in pain mice. Our findings provide insights into the circuits and mechanisms underlying cognitive aspects of pain and offer potential targets for treatment

PAG afferents equally target VTA DA and GABA neurons.

<p><b>A,</b> Left, schematic of the injection protocol for patch clamp experiments. Right, example of image of ChR2-EYFP infection in the PAG. Scale bar, 500 μm. <b>B,</b> High magnification confocal images showing the colocalization or exclusion of mCherry and TH in DAT-Cre or GAD65-Cre mice, respectively. Scale bars, 50 μm. <b>C,</b> Mean amplitude of the light-evoked postsynaptic currents in VTA DA (<i>n</i> = 47) and GABA (<i>n</i> = 62) neurons plotted against the percentage of connected neurons (Mann Whitney <i>U</i> test: no difference in amplitudes, <i>p</i> > 0.05; Fisher’s exact test: no difference in connectivity, p > 0.05). Scale bars, 20 ms, 20 pA. <b>D,</b> Left, schematic of the patch clamp experiments: whole-cell recordings were performed from mCherry-expressing VTA neurons while PAG afferents inputs were light-stimulated (left). Right, excitatory currents were blocked with kynurenic acid (kyn), while kyn-resistant inhibitory currents were blocked with picrotoxin (PTX). Scale bars, 20 ms, 20 pA. <b>E,</b> Proportion of kyn-sensitive glutamate inputs and PTX-sensitive GABA inputs in VTA DA (<i>n</i> = 17) and GABA (<i>n</i> = 18) neurons (Fisher’s exact test: no difference between cell types, <i>p</i> > 0.05). <b>F,</b> Mean light-evoked current amplitude plotted against <i>I</i>_h amplitude (Spearman’s rank correlation: no correlation between the variables, <i>r</i> = 0.2702, <i>p</i> > 0.05) and comparison of <i>I</i>_h between connected and non-connected VTA DA (left) or GABA neurons (right) (Mann Whitney <i>U</i> test: * <i>p</i> < 0.05).</p

Spatial distribution of VTA-projecting PAG inputs.

<p><b>A,</b> Schematic of the unilateral rabies injection protocol. <b>B-C,</b> Confocal images at low (B) and high (C) magnification showing the expression of TVA-mCherry (magenta) and RVΔG-EGFP (green) in the VTA seven days after the last injection. Scale bars, 500 μm (B) and 50 μm (C). <b>D,</b> Representative confocal images showing confined starter cell infection (i.e. TVA-mCherry and RVΔG-EGFP co-expression) in coronal sections rostral (top) or caudal (bottom) to the VTA of a DAT-Cre mouse. Antero-posterior coordinates (in mm) from Bregma are shown on the top right corner. Scale bars, 500 μm. ml, medial lemniscus; SNc, substantia nigra pars compacta; SNr, substantia nigra pars reticulata. <b>E,</b> RVΔG-EGFP alone was not able to infect neurons without TVA-mCherry expression. Antero-posterior coordinates (in mm) from Bregma are shown on the top right corner. Scale bars, 500 μm. IPN, interpeduncular nucleus. <b>F,</b> RVΔG-EGFP-expressing retrogradely labeled input neurons across the rostral, central and caudal PAG segments. Scale bars, 200 μm. <b>G,</b> Average number of inputs to VTA DA (DAT, <i>n</i> = 5) and GABA (GAD, <i>n</i> = 5) neurons along the rostro-caudal axis of the PAG (two-way ANOVA: no interaction between the cell type factor and AP coordinate factor, <i>F</i>_11,120 = 0.7027, <i>p</i> > 0.05; main effect of cell type, <i>F</i>_1,120 = 18.2, <i>p</i> < 0.0001; main effect of AP coordinate, <i>F</i>_11,120 = 6.31, <i>p</i> < 0.0001; Bonferroni post-hoc test, * <i>p</i> < 0.05). Dashed lines denote the boundaries between rostral, central and caudal PAG. Inset shows the average number starter cells per ROI in DAT-Cre and GAD65-Cre mice (two-tailed <i>t</i> test: no difference between genotypes, <i>p</i> > 0.05). <b>H,</b> Relative contribution of different PAG subregions to the total inputs to VTA DA and GABA neurons (two-way ANOVA: no interaction between the cell type factor and subregion factor, <i>F</i>_5,48 = 0.5013, <i>p</i> > 0.05; no main effect of cell type, <i>F</i>_1,48 < 0.0001, <i>p</i> > 0.05; main effect of subregion, <i>F</i>_5,48 = 42.85, <i>p</i> < 0.0001; Bonferroni post-hoc test, * <i>p</i> < 0.05, **** <i>p</i> < 0.0001). Inset shows the degree of lateralization of the PAG inputs (two-tailed <i>t</i> test: no difference between genotypes, <i>p</i> > 0.05). <b>I,</b> Color-coded representation of the relative input contribution of ipsilateral and contralateral PAG subregions.</p

Long-Lasting, Pathway-Specific Impairment of a Novel Form of Spike-Timing-Dependent Long-Term Depression by Neuropathic Pain in the Anterior Cingulate Cortex

Malfunctioning synaptic plasticity is one of the major mechanisms contributing to the development of chronic pain. We studied spike-timing dependent depression (tLTD) in the anterior cingulate cortex (ACC) of male mice, a brain region involved in processing emotional aspects of pain. tLTD onto layer 5 pyramidal neurons depended on postsynaptic calcium-influx through GluN2B-containing NMDARs and retrograde signaling via nitric oxide to reduce presynaptic release probability. After chronic constriction injury of the sciatic nerve, a model for neuropathic pain, tLTD was rapidly impaired; and this phenotype persisted even beyond the time of recovery from mechanical sensitization. Exclusion of GluN2B-containing NMDARs from the postsynaptic site specifically at projections from the anterior thalamus to the ACC caused the tLTD phenotype, whereas signaling downstream of nitric oxide synthesis remained intact. Thus, transient neuropathic pain can leave a permanent trace manifested in the disturbance of synaptic plasticity in a specific afferent pathway to the cortex. SIGNIFICANCE STATEMENT Synaptic plasticity is one of the main mechanisms that contributes to the development of chronic pain. Most studies have focused on potentiation of excitatory synaptic transmission, but very little is known about the reduction in synaptic strength. We have focused on the ACC, a brain region associated with the processing of emotional and affective components of pain. We studied spike-timing dependent LTD, which is a biologically plausible form of synaptic plasticity, that depends on the relative timing of presynaptic and postsynaptic activity. We found a long-lasting and pathway-specific suppression of the induction mechanism for spike-timing dependent LTD from the anterior thalamus to the ACC, suggesting that this pathology might be involved in altered emotional processing in pain