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

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.

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.

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.

Correction: Periaqueductal efferents to dopamine and GABA neurons of the VTA.

[This corrects the article DOI: 10.1371/journal.pone.0190297.].

Periaqueductal efferents to dopamine and GABA neurons of the VTA.

Neurons in the periaqueductal gray (PAG) modulate threat responses and nociception. Activity in the ventral tegmental area (VTA) on the other hand can cause reinforcement and aversion. While in many situations these behaviors are related, the anatomical substrate of a crosstalk between the PAG and VTA remains poorly understood. Here we describe the anatomical and electrophysiological organization of the VTA-projecting PAG neurons. Using rabies-based, cell type-specific retrograde tracing, we observed that PAG to VTA projection neurons are evenly distributed along the rostro-caudal axis of the PAG, but concentrated in its posterior and ventrolateral segments. Optogenetic projection targeting demonstrated that the PAG-to-VTA pathway is predominantly excitatory and targets similar proportions of Ih-expressing VTA DA and GABA neurons. Taken together, these results set the framework for functional analysis of the interplay between PAG and VTA in the regulation of reward and aversion.

Convergence of Reinforcing and Anhedonic Cocaine Effects in the Ventral Pallidum.

Addiction is a disorder of behavioral symptoms including enhanced incentive salience of drug-associated cues, but also a negative affective state. Cocaine-evoked synaptic plasticity in the reward system, particularly the nucleus accumbens (NAc), drives drug-adaptive behavior. However, how information is integrated downstream of the NAc remains unclear. Here, we identify the ventral pallidum (VP) as a site of convergence of medium spiny neurons expressing dopamine (DA) receptor type 1 (D1-MSNs) and type 2 (D2-MSNs) of the NAc. Repeated in vivo cocaine exposure potentiated output of D1-MSNs, but weakened output of D2-MSNs, occluding LTP and LTD at these synapses, respectively. Selectively restoring basal transmission at D1-MSN-to-VP synapses abolished locomotor sensitization, whereas restoring transmission at D2-MSN-to-VP synapses normalized motivational deficits. Our results support a model by which drug-evoked synaptic plasticity in the VP mediates opposing behavioral symptoms; targeting the VP may provide novel therapeutic strategies for addictive disorders.

VTA Projection Neurons Releasing GABA and Glutamate in the Dentate Gyrus.

Both dopamine and nondopamine neurons from the ventral tegmental area (VTA) project to a variety of brain regions. Here we examine nondopaminergic neurons in the mouse VTA that send long-range projections to the hippocampus. Using a combination of retrograde tracers, optogenetic tools, and electrophysiological recordings, we show that VTA GABAergic axons make synaptic contacts in the granule cell layer of the dentate gyrus, where we can elicit small postsynaptic currents. Surprisingly, the currents displayed a partial sensitivity to both bicuculline and NBQX, suggesting that these mesohippocampal neurons corelease both GABA and glutamate. Finally, we show that this projection is functional in vivo and its stimulation reduces granule cell-firing rates under anesthesia. Altogether, the present results describe a novel connection between GABA and glutamate coreleasing of cells of the VTA and the dentate gyrus. This connection could be relevant for a variety of functions, including reward-related memory and neurogenesis.

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.