Circuit-selective properties of glutamatergic inputs to the rat prelimbic cortex and their alterations in neuropathic pain

TAF15 downregulation contributes to the benefits of physical training on dendritic spines and working memory in aged mice

Moderate physical training has been shown to hinder age-related memory decline. While the benefits of physical training on hippocampal memory function are well-documented, little is known about its impact on working memory, which is linked to the prelimbic cortex (PrL), one major subdivision of the prefrontal cortex. Here, we examined the effects of physical training on spatial working memory in a well-established animal model of physical training, starting at 16 months of age and continuing for 5 months (running wheel 1 h/day and 5 days/week). This training strategy improved spatial working memory in aged mice (22-month-old), which was accompanied by an increased spine density and a lower TAF15 expression in the PrL. Specifically, physical training affected both thin and mushroom-type spines on PrL pyramidal cells, and prevented age-related loss of spines on selective segments of apical dendritic branches. Correlation analysis revealed that increased TAF15-expression was detrimental to the dendritic spines. However, physical training downregulated TAF15 expression in the PrL, preserving the dendritic spines on PrL pyramidal cells and improving working memory in trained aged mice. When TAF15 was overexpressed in the PrL via a viral approach, the benefits of physical training on the dendritic spines and working memory were abolished. These data suggest that physical training at a moderate pace might downregulate TAF15 expression in the PrL, which favors the dendritic spines on PrL pyramidal cells, thereby improving spatial working memory.

Chronic Pain-Related Cognitive Deficits: Preclinical Insights into Molecular, Cellular, and Circuit Mechanisms

Cognitive impairment is a common comorbidity of chronic pain, significantly disrupting patients' quality of life. Despite this comorbidity being clinically recognized, the underlying neuropathological mechanisms remain unclear. Recent preclinical studies have focused on the fundamental mechanisms underlying the coexistence of chronic pain and cognitive decline. Pain chronification is accompanied by structural and functional changes in the neural substrate of cognition. Based on the developments in electrophysiology and optogenetics/chemogenetics, we summarized the relevant neural circuits involved in pain-induced cognitive impairment, as well as changes in connectivity and function in brain regions. We then present the cellular and molecular alternations related to pain-induced cognitive impairment in preclinical studies, mainly including modifications in neuronal excitability and structure, synaptic plasticity, glial cells and cytokines, neurotransmitters and other neurochemicals, and the gut-brain axis. Finally, we also discussed the potential treatment strategies and future research directions.

A mesocortical glutamatergic pathway modulates neuropathic pain independent of dopamine co-release

Dysfunction in the mesocortical pathway, connecting the ventral tegmental area (VTA) to the prefrontal cortex, has been implicated in chronic pain. While extensive research has focused on the role of dopamine, the contribution of glutamatergic signaling in pain modulation remains unknown. Using in vivo calcium imaging, we observe diminished VTA glutamatergic activity targeting the prelimbic cortex (PL) in a mouse model of neuropathic pain. Optogenetic activation of VTA glutamatergic terminals in the PL alleviates neuropathic pain, whereas inhibiting these terminals in naïve mice induces pain-like responses. Importantly, this pain-modulating effect is independent of dopamine co-release, as demonstrated by CRISPR/Cas9-mediated gene deletion. Furthermore, we show that VTA neurons primarily project to excitatory neurons in the PL, and their activation restores PL outputs to the anterior cingulate cortex, a key region involved in pain processing. These findings reveal a distinct mesocortical glutamatergic pathway that critically modulates neuropathic pain independent of dopamine signaling.

Corticotropin-releasing hormone neurons control trigeminal neuralgia-induced anxiodepression via a hippocampus-to-prefrontal circuit

Anxiety and depression are frequently observed in patients suffering from trigeminal neuralgia (TN), but neural circuits and mechanisms underlying this association are poorly understood. Here, we identified a dedicated neural circuit from the ventral hippocampus (vHPC) to the medial prefrontal cortex (mPFC) that mediates TN-related anxiodepression. We found that TN caused an increase in excitatory synaptic transmission from vHPCCaMK2A neurons to mPFC inhibitory neurons marked by the expression of corticotropin-releasing hormone (CRH). Activation of CRH+ neurons subsequently led to feed-forward inhibition of layer V pyramidal neurons in the mPFC via activation of the CRH receptor 1 (CRHR1). Inhibition of the vHPCCaMK2A-mPFCCRH circuit ameliorated TN-induced anxiodepression, whereas activating this pathway sufficiently produced anxiodepressive-like behaviors. Thus, our studies identified a neural pathway driving pain-related anxiodepression and a molecular target for treating pain-related psychiatric disorders.

The role of pain modulation pathway and related brain regions in pain

Pain is a multifaceted process that encompasses unpleasant sensory and emotional experiences. The essence of the pain process is aversion, or perceived negative emotion. Central sensitization plays a significant role in initiating and perpetuating of chronic pain. Melzack proposed the concept of the "pain matrix", in which brain regions associated with pain form an interconnected network, rather than being controlled by a singular brain region. This review aims to investigate distinct brain regions involved in pain and their interconnections. In addition, it also sheds light on the reciprocal connectivity between the ascending and descending pathways that participate in pain modulation. We review the involvement of various brain areas during pain and focus on understanding the connections among them, which can contribute to a better understanding of pain mechanisms and provide opportunities for further research on therapies for improved pain management.

Loss of spines in the prelimbic cortex is detrimental to working memory in mice with early-life adversity

Adverse experiences in early life can shape neuronal structures and synaptic function in multiple brain regions, leading to deficits of distinct cognitive functions later in life. Focusing on the pyramidal cells of the prelimbic cortex (PrL), a main subregion of the medial prefrontal cortex, the impact of early-life adversity (ELA) was investigated in a well-established animal model generated by changing the rearing environment during postnatal days 2 to 9 (P2-P9), a sensitive developmental period. ELA has enduring detrimental impacts on the dendritic spines of PrL pyramidal cells, which is most apparent in a spatially circumscribed region. Specifically, ELA affects both thin and mushroom-type spines, and ELA-provoked loss of spines is observed on selective dendritic segments of PrL pyramidal cells in layers II-III and V-VI. Reduced postsynaptic puncta represented by postsynaptic density protein-95 (PSD-95), but not synaptophysin-labelled presynaptic puncta, in ELA mice supports the selective loss of spines in the PrL. Correlation analysis indicates that loss of spines and postsynaptic puncta in the PrL contributes to the poor spatial working memory of ELA mice, and thin spines may play a major role in working memory performance. To further understand whether loss of spines affects glutamatergic transmission, AMPA- and NMDA-receptor-mediated synaptic currents (EPSCs) were recorded in a group of Thy1-expressing PrL pyramidal cells. ELA mice exhibited a depressed glutamatergic transmission, which is accompanied with a decreased expression of GluR1 and NR1 subunits in the PrL. Finally, upregulating the activation of Thy1-expressing PrL pyramidal cells via excitatory DREADDs can efficiently improve the working memory performance of ELA mice in a T-maze-based task, indicating the potential of a chemogenetic approach in restoring ELA-provoked memory deficits.

Pain-related cortico-limbic plasticity and opioid signaling

Neuroplasticity in cortico-limbic circuits has been implicated in pain persistence and pain modulation in clinical and preclinical studies. The amygdala has emerged as a key player in the emotional-affective dimension of pain and pain modulation. Reciprocal interactions with medial prefrontal cortical regions undergo changes in pain conditions. Other limbic and paralimbic regions have been implicated in pain modulation as well. The cortico-limbic system is rich in opioids and opioid receptors. Preclinical evidence for their pain modulatory effects in different regions of this highly interactive system, potentially opposing functions of different opioid receptors, and knowledge gaps will be described here. There is little information about cell type- and circuit-specific functions of opioid receptor subtypes related to pain processing and pain-related plasticity in the cortico-limbic system. The important role of anterior cingulate cortex (ACC) and amygdala in MOR-dependent analgesia is most well-established, and MOR actions in the mesolimbic system appear to be similar but remain to be determined in mPFC regions other than ACC. Evidence also suggests that KOR signaling generally serves opposing functions whereas DOR signaling in the ACC has similar, if not synergistic effects, to MOR. A unifying picture of pain-related neuronal mechanisms of opioid signaling in different elements of the cortico-limbic circuitry has yet to emerge. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Impaired muscarinic modulation of the rat prelimbic cortex in neuropathic pain is sexually dimorphic and associated with cold allodynia

Cholinergic modulation of the brain cortex is critical for cognitive processes, and altered cholinergic modulation of the prefrontal cortex is emerging as an important mechanism of neuropathic pain. Sex differences in pain prevalence and perception are well known, yet the precise nature of the mechanisms responsible for sexual dimorphism in chronic neuropathic pain are poorly understood. Here we investigated potential sex differences in cholinergic modulation of layer five commissural pyramidal neurons of the rat prelimbic cortex in control conditions and in the SNI model of neuropathic pain. We discovered that cholinergic modulation is stronger in cells from male compared with female rats, and that in neuropathic pain rats, cholinergic excitation of pyramidal neurons was more severely impaired in males than in females. Finally, we found that selective pharmacological blockade of the muscarinic M1 subunit in the prefrontal cortex induces cold sensitivity (but not mechanical allodynia) in naïve animals of both sexes.

Activation of VIP interneurons in the prefrontal cortex ameliorates neuropathic pain aversiveness

While dysfunction of the medial prefrontal cortex (mPFC) has been implicated in chronic pain, the underlying neural circuits and the contribution of specific cellular populations remain unclear. Using in vivo Ca²⁺ imaging, we report that in both male and female mice, peripheral nerve injury-induced neuropathic pain causes a marked reduction of vasoactive intestinal polypeptide (VIP)-expressing interneuron activity in the prelimbic area of the mPFC, which contributes to decreased prefrontal cortical outputs. Moreover, prelimbic glutamatergic projections to GABAergic interneurons in the anterior cingulate cortex (ACC) are diminished, leading to loss of cortical-cortical inhibition and increased pyramidal neuron activity in the ACC. Chemogenetic activation of prelimbic VIP interneurons restores neuronal responses in the mPFC-ACC pathway and attenuates pain-like behaviors in mice. Furthermore, restoration of prelimbic outputs to the ACC reverses nerve injury-induced ACC hyperactivation. These findings reveal mPFC circuit changes associated with neuropathic pain and highlight VIP interneurons as potential therapeutic targets for pain treatment.

Altered prefrontal-striatal theta-band oscillatory dynamics underlie working memory deficits in neuropathic pain rats

BACKGROUND

Prelimbic medial prefrontal cortex (PL-mPFC) and nucleus accumbens core region (NAcc) play an important role in supporting several executive cognitive mechanisms, such as spatial working-memory (WM). Recently, this circuit has been also associated with both sensory and affective components of pain. However, it is still unclear whether this circuit is endogenously engaged in neuropathic pain-related cognitive dysfunctions.

METHODS

To answer this question, we induced the expression of halorhodopsin in local PL-mPFC neurons projecting to NAcc, and then selectively inhibited the terminals of these neurons in the NAcc while recording neural activity during the performance of a delayed non-match to sample (DNMS) spatial WM task. Within-subject behavioral performance and PL-mPFC to NAcc circuit neural activity was assessed after the onset of a persistent rodent neuropathic pain model - spared nerve injury (SNI).

RESULTS

Our results revealed that the induction of the neuropathy reduced WM performance, and altered the interplay between PL-mPFC and NAcc neurons namely in increasing the functional connectivity from NAcc to PL-mPFC, particularly in the theta-band spontaneous oscillations; in addition, these behavioral and functional perturbations were partially reversed by selective optogenetic inhibition of PL-mPFC neuron terminals into the NAcc during the DNMS task delay-period, without significant antinociceptive effects.

CONCLUSIONS

Altogether, these results strongly suggest that the PL-mPFC excitatory output into the NAcc plays an important role in the deregulation of WM under pain conditions.

SIGNIFICANCE

Selective optogenetic inhibition of prefrontal-striatal microcircuit reverses pain-related working memory deficits, but has no significant impact on pain responses. Neuropathic pain underlies an increase of functional connectivity between the nucleus accumbens core area and the prelimbic medial prefrontal cortex mediated by theta-band activity.

Differential Rearrangement of Excitatory Inputs to the Medial Prefrontal Cortex in Chronic Pain Models

Chronic pain patients suffer a disrupted quality of life not only from the experience of pain itself, but also from comorbid symptoms such as depression, anxiety, cognitive impairment, and sleep disturbances. The heterogeneity of these symptoms support the idea of a major involvement of the cerebral cortex in the chronic pain condition. Accordingly, abundant evidence shows that in chronic pain the activity of the medial prefrontal cortex (mPFC), a brain region that is critical for executive function and working memory, is severely impaired. Excitability of the mPFC depends on the integrated effects of intrinsic excitability and excitatory and inhibitory inputs. The main extracortical sources of excitatory input to the mPFC originate in the thalamus, hippocampus, and amygdala, which allow the mPFC to integrate multiple information streams necessary for cognitive control of pain including sensory information, context, and emotional salience. Recent techniques, such as optogenetic methods of circuit dissection, have made it possible to tease apart the contributions of individual circuit components. Here we review the synaptic properties of these main glutamatergic inputs to the rodent mPFC, how each is altered in animal models of chronic pain, and how these alterations contribute to pain-associated mPFC deactivation. By understanding the contributions of these individual circuit components, we strive to understand the broad spectrum of chronic pain and comorbid pathologies, how they are generated, and how they might be alleviated.

Sciatic Nerve Ligation Downregulates Mitochondrial Clusterin in the Rat Prefrontal Cortex

The concentration of the multifunctional protein clusterin is reduced in the plasma of subjects with degenerative scoliosis (DS) and carpal tunnel syndrome (CTS) but elevated in the cerebrospinal fluid of neuropathic pain patients successfully treated with spinal cord stimulation. The present work tries to increase the knowledge of pain-associated changes of plasma and brain clusterin by using an animal model of neuropathy. We studied the effects of sciatic nerve ligation on mechanical allodynia (von Frey test), anxiety (elevated plus maze test), plasma clusterin (enzyme-linked immunosorbent assay) and clusterin expression in the nucleus accumbens (NAC) and prefrontal cortex (PFC) of adult male Wistar rats (western blot). The possible modulatory role of high fat (HF) dieting was also studied, bearing in mind that obesity has been also reported to influence nociception, clusterin levels and prefrontal cortex activation. Animals with nerve ligation showed mechanical allodynia, anxiety and a marked downregulation of clusterin in the mitochondrial fraction of the prefrontal cortex. Animals fed on HF also exhibited a slight increase of the sensitivity to mechanical stimuli and anxiety; however, the diet did not potentiate the effects of nerve ligation. The results did not confirm a parallelism between neuropathy, obesity and alterations of plasma levels of clusterin, but strongly suggest that the protein could be involved in the functional reorganization of the prefrontal cortex which has been recently reported in chronic pain conditions.

The Medial Prefrontal Cortex as a Central Hub for Mental Comorbidities Associated with Chronic Pain

Chronic pain patients frequently develop and suffer from mental comorbidities such as depressive mood, impaired cognition, and other significant constraints of daily life, which can only insufficiently be overcome by medication. The emotional and cognitive components of pain are processed by the medial prefrontal cortex, which comprises the anterior cingulate cortex, the prelimbic, and the infralimbic cortex. All three subregions are significantly affected by chronic pain: magnetic resonance imaging has revealed gray matter loss in all these areas in chronic pain conditions. While the anterior cingulate cortex appears hyperactive, prelimbic, and infralimbic regions show reduced activity. The medial prefrontal cortex receives ascending, nociceptive input, but also exerts important top-down control of pain sensation: its projections are the main cortical input of the periaqueductal gray, which is part of the descending inhibitory pain control system at the spinal level. A multitude of neurotransmitter systems contributes to the fine-tuning of the local circuitry, of which cholinergic and GABAergic signaling are particularly emerging as relevant components of affective pain processing within the prefrontal cortex. Accordingly, factors such as distraction, positive mood, and anticipation of pain relief such as placebo can ameliorate pain by affecting mPFC function, making this cortical area a promising target region for medical as well as psychosocial interventions for pain therapy.

Amygdala physiology in pain

The amygdala has emerged as an important brain area for the emotional-affective dimension of pain and pain modulation. The amygdala receives nociceptive information through direct and indirect routes. These excitatory inputs converge on the amygdala output region (central nucleus) and can be modulated by inhibitory elements that are the target of (prefrontal) cortical modulation. For example, inhibitory neurons in the intercalated cell mass in the amygdala project to the central nucleus to serve gating functions, and so do inhibitory (PKCdelta) interneurons within the central nucleus. In pain conditions, synaptic plasticity develops in output neurons because of an excitation-inhibition imbalance and drives pain-like behaviors and pain persistence. Mechanisms of pain related neuroplasticity in the amygdala include classical transmitters, neuropeptides, biogenic amines, and various signaling pathways. An emerging concept is that differences in amygdala activity are associated with phenotypic differences in pain vulnerability and resilience and may be predetermining factors of the complexity and persistence of pain.

Bidirectional optogenetic modulation of prefrontal-hippocampal connectivity in pain-related working memory deficits

Dysfunction of the prefrontal-hippocampal circuit has been identified as a leading cause to pain-related working-memory (WM) deficits. However, the underlying mechanisms remain poorly determined. To address this issue, we implanted multichannel arrays of electrodes in the prelimbic cortex (PL-mPFC), and in the dorsal hippocampal CA1 field (dCA1) to record the neural activity during the performance of a delayed non-match to sample (DNMS) task. The prefrontal-hippocampal connectivity was selectively modulated by bidirectional optogenetic inhibition or stimulation of local PL-mPFC glutamatergic calcium/calmodulin-dependent protein kinase-II alpha (CaMKIIα) expressing neurons during the DNMS task delay-period. The within-subject behavioral performance was assessed using a persistent neuropathic pain model – spared nerve injury (SNI). Our results showed that the induction of the neuropathic pain condition affects the interplay between PL-mPFC and dCA1 regions in a frequency-dependent manner, and that occurs particularly across theta oscillations while rats performed the task. In SNI-treated rats, this disruption was reversed by the selective optogenetic inhibition of PL-mPFC CaMKIIα-expressing neurons during the last portion of the delay-period, but without any significant effect on pain responses. Finally, we found that prefrontal-hippocampal theta connectivity is strictly associated with higher performance levels. Together, our findings suggest that PL-mPFC CaMKIIα-expressing neurons could be modulated by painful conditions and their activity may be critical for prefrontal-hippocampal connectivity during WM processing.

Layer- and subregion-specific electrophysiological and morphological changes of the medial prefrontal cortex in a mouse model of neuropathic pain

Chronic neuropathic pain constitutes a serious public health problem, but the disease mechanisms are only partially understood. The involvement of different brain regions like the medial prefrontal cortex has already been established, but the comparison of the role of different subregions and layers is still inconclusive. In the current study, we performed patch-clamp recordings followed by anatomical reconstruction of pyramidal cells from different layers of the prelimbic and infralimbic subregions of the medial prefrontal cortex in neuropathic (spared nerve injury, SNI) and control mice. We found that in the prelimbic cortex, layer 2/3 pyramidal cells from SNI mice exhibited increased excitability compared to sham controls, whereas prelimbic layer 5 pyramidal neurons showed reduced excitability. Pyramidal cells in both layer 2/3 and layer 5 of the infralimbic subregion did not change their excitability, but layer 2/3 pyramidal cells displayed increased dendritic length and branching. Our findings support the view that chronic pain is associated with subregion- and layer-specific changes in the medial prefrontal cortex. They therefore provide new insights into the mechanisms underlying the chronification of pain.