Brain Circuits Mediating Opposing Effects on Emotion and Pain.

The amygdala is important for processing emotion, including negative emotion such as anxiety and depression induced by chronic pain. Although remarkable progress has been achieved in recent years on amygdala regulation of both negative (fear) and positive (reward) behavioral responses, our current understanding is still limited regarding how the amygdala processes and integrates these negative and positive emotion responses within the amygdala circuits. In this study with optogenetic stimulation of specific brain circuits, we investigated how amygdala circuits regulate negative and positive emotion behaviors, using pain as an emotional assay in male rats. We report here that activation of the excitatory pathway from the parabrachial nucleus (PBN) that relays peripheral pain signals to the central nucleus of amygdala (CeA) is sufficient to cause behaviors of negative emotion including anxiety, depression, and aversion in normal rats. In strong contrast, activation of the excitatory pathway from basolateral amygdala (BLA) that conveys processed corticolimbic signals to CeA dramatically opposes these behaviors of negative emotion, reducing anxiety and depression, and induces behavior of reward. Surprisingly, activating the PBN-CeA pathway to simulate pain signals does not change pain sensitivity itself, but activating the BLA-CeA pathway inhibits basal and sensitized pain. These findings demonstrate that the pain signal conveyed through the PBN-CeA pathway is sufficient to drive negative emotion and that the corticolimbic signal via the BLA-CeA pathway counteracts the negative emotion, suggesting a top-down brain mechanism for cognitive control of negative emotion under stressful environmental conditions such as pain.SIGNIFICANCE STATEMENT It remains unclear how the amygdala circuits integrate both negative and positive emotional responses and the brain circuits that link peripheral pain to negative emotion are largely unknown. Using optogenetic stimulation, this study shows that the excitatory projection from the parabrachial nucleus to the central nucleus of amygdala (CeA) is sufficient to drive behaviors of negative emotion including anxiety, depression, and aversion in rats. Conversely, activation of the excitatory projection from basolateral amygdala to CeA counteracts each of these behaviors of negative emotion. Thus, this study identifies a brain pathway that mediates pain-driven negative emotion and a brain pathway that counteracts these emotion behaviors in a top-down mechanism for brain control of negative emotion.

Pain vulnerability and DNA methyltransferase 3a involved in the affective dimension of chronic pain.

Chronic pain with comorbid emotional disorders is a prevalent neurological disease in patients under various pathological conditions, yet patients show considerable difference in their vulnerability to developing chronic pain. Understanding the neurobiological basis underlying this pain vulnerability is essential to develop targeted therapies of higher efficiency in pain treatment of precision medicine. However, this pain vulnerability has not been addressed in preclinical pain research in animals to date. In this study, we investigated individual variance in both sensory and affective/emotional dimensions of pain behaviors in response to chronic neuropathic pain condition in a mouse model of chronic pain. We found that mice displayed considerably diverse sensitivities in the chronic pain-induced anxiety- and depression-like behaviors of affective pain. Importantly, the mouse group that was more vulnerable to developing anxiety was also more vulnerable to developing depressive behavior under the chronic pain condition. In contrast, there was relatively much less variance in individual responses in the sensory dimension of pain sensitization. Molecular analysis revealed that those mice vulnerable to developing the emotional disorders showed a significant reduction in the protein level of DNA methyltransferase 3a in the emotion-processing central nucleus of the amygdala. In addition, social stress also revealed significant individual variance in anxiety behavior in mice. These findings suggest that individual pain vulnerability may be inherent mostly in the emotional/affective component of chronic pain and remain consistent in different aspects of negative emotion, in which adaptive changes in the function of DNA methyltransferase 3a for DNA methylation in central amygdala may play an important role. This may open a new avenue of basic research into the neurobiological mechanisms underlying pain vulnerability.

Persistent pain maintains morphine-seeking behavior after morphine withdrawal through reduced MeCP2 repression of GluA1 in rat central amygdala.

As long-term opioids are increasingly used for control of chronic pain, how pain affects the rewarding effect of opioids and hence risk of prescription opioid misuse and abuse remains a healthcare concern and a challenging issue in current pain management. In this study, using a rat model of morphine self-administration, we investigated the molecular mechanisms underlying the impact of pain on operant behavior of morphine intake and morphine seeking before and after morphine withdrawal. We found that rats with persistent pain consumed a similar amount of daily morphine to that in control rats without pain, but maintained their level-pressing behavior of morphine seeking after abstinence of morphine at 0.2 mg/kg, whereas this behavior was gradually diminished in control rats. In the central nucleus of amygdala (CeA), a limbic structure critically involved in the affective dimension of pain, proteins of GluA1 subunits of glutamate AMPA receptors were upregulated during morphine withdrawal, and viral knockdown of CeA GluA1 eliminated the morphine-seeking behavior in withdrawn rats of the pain group. Chromatin immunoprecipitation analysis revealed that the methyl CpG-binding protein 2 (MeCP2) was enriched in the promoter region of Gria1 encoding GluA1 and this enrichment was significantly attenuated in withdrawn rats of the pain group. Furthermore, viral overexpression of CeA MeCP2 repressed the GluA1 level and eliminated the maintenance of morphine-seeking behavior after morphine withdrawal. These results suggest direct MeCp2 repression of GluA1 function as a likely mechanism for morphine-seeking behavior maintained by long-lasting affective pain after morphine withdrawal.

Central amygdala GluA1 facilitates associative learning of opioid reward.

GluA1 subunits of AMPA glutamate receptors are implicated in the synaptic plasticity induced by drugs of abuse for behaviors of drug addiction, but GluA1 roles in emotional learning and memories of drug reward in the development of drug addiction remain unclear. In this study of the central nucleus of the amygdala (CeA), which is critical in emotional learning of drug reward, we investigated how adaptive changes in the expression of GluA1 subunits affected the learning process of opioid-induced context-reward association (associative learning) for the acquisition of reward-related behavior. In CeA neurons, we found that CeA GluA1 expression was significantly increased 2 h after conditioning treatment with morphine, but not 24 h after the conditioning when the behavior of conditioned place reference (CPP) was fully established in rats. Adenoviral overexpression of GluA1 subunits in CeA accelerated associative learning, as shown by reduced minimum time of morphine conditioning required for CPP acquisition and by facilitated CPP extinction through extinction training with no morphine involved. Adenoviral shRNA-mediated downregulation of CeA GluA1 produced opposite effects, inhibiting the processes of both CPP acquisition and CPP extinction. Adenoviral knockdown of CeA GluA2 subunits facilitated CPP acquisition, but did not alter CPP extinction. Whole-cell recording revealed enhanced electrophysiological properties of postsynaptic GluA2-lacking AMPA receptors in adenoviral GluA1-infected CeA neurons. These results suggest that increased GluA1 expression of CeA AMPA receptors facilitates the associative learning of context-drug reward, an important process in both development and relapse of drug-seeking behaviors in drug addiction.

Optogenetic activation of brainstem serotonergic neurons induces persistent pain sensitization.

BACKGROUND: The rostral ventromedial medulla (RVM) is a key brainstem structure that conveys powerful descending influence of the central pain-modulating system on spinal pain transmission and processing. Serotonergic (5-HT) neurons are a major component in the heterogeneous populations of RVM neurons and in the descending pathways from RVM. However, the descending influence of RVM 5-HT neurons on pain behaviors remains unclear. RESULTS: In this study using optogenetic stimulation in tryptophan hydroxylase 2 (TPH2)- Channelrhodopsin 2 (ChR2) transgenic mice, we determined the behavioral effects of selective activation of RVM 5-HT neurons on mechanical and thermal pain behaviors in vivo. We found that ChR2-EYFP-positive neurons strongly co-localized with TPH2-positive (5-HT) neurons in RVM. Optogenetic stimulation significantly increased c-fos expression in 5-HT cells in the RVM of TPH2-ChR2 mice, but not in wild type mice. Behaviorally, the optogenetic stimulation decreased both mechanical and thermal pain threshold in an intensity-dependent manner, with repeated stimulation producing sensitized pain behavior for up to two weeks. CONCLUSIONS: These results suggest that selective activation of RVM 5-HT neurons exerts a predominant effect of pain facilitation under control conditions.

The projection from dorsal medial prefrontal cortex to basolateral amygdala promotes behaviors of negative emotion in rats.

Brain circuits between medial prefrontal cortex (mPFC) and amygdala have been implicated in cortical control of emotion, especially anxiety. Studies in recent years focus on differential roles of subregions of mPFC and amygdala, and reciprocal pathways between mPFC and amygdala in regulation of emotional behaviors. It has been shown that, while the projection from ventral mPFC to basomedial amygdala has an anxiolytic effect, the reciprocal projections between dorsal mPFC (dmPFC) and basolateral amygdala (BLA) are generally involved in an anxiogenic effect in various conditions with increased anxiety. However, the function of the projection from dmPFC to BLA in regulation of general emotional behaviors under normal conditions remains unclear. In this study, we used optogenetic analysis to identify how this dmPFC-BLA pathway regulates various emotional behaviors in normal rats. We found that optogenetic stimulation of the dmPFC-BLA pathway promoted a behavioral state of negative emotion, increasing anxiety-like and depressive-like behaviors and producing aversive behavior of place avoidance. Conversely, optogenetic inhibition of this pathway produced opposite effects, reducing anxiety-like and depressive-like behaviors, and inducing behaviors of place preference of reward. These findings suggest that activity of the dmPFC-BLA pathway is sufficient to drive a negative emotion state and the mPFC-amygdala circuit is tonically active in cortical regulation of emotional behaviors.

Upregulation of nuclear factor of activated T-cells by nerve injury contributes to development of neuropathic pain.

Nerve injury induces long-term changes in gene expression in the nociceptive circuitry and can lead to chronic neuropathic pain. However, the transcriptional mechanism involved in neuropathic pain is poorly understood. Nuclear factor of activated T-cells (NFATc) is a transcriptional factor regulated by the Ca(2+)-dependent protein phosphatase calcineurin. In this study, we determined nerve injury-induced changes in the expression of NFATc1-c4 in the dorsal root ganglia (DRG) and spinal cords and their role in the development of neuropathic pain. The mRNA of NFATc1-c4 was detected in the rat DRG and dorsal spinal cord. Nerve injury transiently elevated NFATc1-c3 mRNA levels and persistently increased NFATc4 and C-C chemokine receptor type 2 (CCR2) mRNA levels in the DRG. However, NFATc1-c4 mRNA levels in the spinal cord were not altered significantly by nerve injury. Nerve injury also significantly increased the protein level of dephosphorylated NFATc4 in the DRG. Intrathecal injection of the specific NFATc inhibitor 11R-VIVIT or the calcineurin inhibitor FK-506 (tacrolimus) early after nerve injury significantly attenuated the development of tactile allodynia. In addition, treatment with FK-506 or 11R-VIVIT significantly reduced the mRNA levels of NFATc4 and CCR2 but not large-conductance Ca(2+)-activated K(+) channels, in the DRG after nerve injury. Our findings suggest that peripheral nerve injury causes a time-dependent change in NFATc1-c4 expression in the DRG. Calcineurin-NFATc-mediated expression of pronociceptive cytokines contributes to the transition from acute to chronic pain after nerve injury.

Gamma aminobutyric acid transporter subtype 1 gene knockout mice: a new model for attention deficit/hyperactivity disorder.

Attention deficit/hyperactivity disorder (ADHD) is characterized by hyperactivity, impaired sustained attention, impulsivity, and is usually accompanied by varying degrees of learning difficulties and lack of motor coordination. However, the pathophysiology and etiology of ADHD remain inconclusive so far. Our previous studies have demonstrated that the gamma aminobutyric acid transporter subtype 1 (GAT1) gene knockout (ko) mouse (gat1-/-) is hyperactive and exhibited impaired memory performance in the Morris water maze. In the current study, we found that the gat1-/- mice showed low levels of attentional focusing and increased impulsivity. In addition, the gat1-/- mice displayed ataxia characterized by defects in motor coordination and balance skills. The hyperactivity in the ko mice was reduced by both methylphenidate and amphetamine. Collectively, these results suggest that GAT1 ko mouse is a new animal model for ADHD studying and GAT1 may be a new target to treat ADHD.

Enhanced learning and memory in GAT1 heterozygous mice.

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system. The termination of GABA transmission is through the action of a family of membrane proteins, called GABA transporters (GAT1-4). It is well established that GABA system is involved in the modulation of memory. Our previous study showed that homozygous GAT1(-/-) mice exhibited impaired hippocampus-dependent learning and memory. To evaluate the impact of endogenous reduced GABA reuptake on mice cognitive behaviors, the ability of learning and memory of heterozygous GAT1(+/-) mice was detected by the passive avoidance paradigm and Morris water maze. The hole board paradigm was also used to measure changes in anxiety-related behavior or exploratory behavior in such mice. As one form of synaptic plasticity, long-term potentiation was recorded in the mouse hippocampal CA1 area. We found that GAT1(+/-) mice displayed increased learning and memory, decreased anxiety-like behaviors, and highest synaptic plasticity compared with wild-type and homozygous GAT1(-/-) mice. Our results suggest that a moderate reduction in GAT1 activity causes the enhancement of learning and memory in mice.

Synaptic plasticity in two cell types of central amygdala for regulation of emotion and pain.

The amygdala is a critical brain site for regulation of emotion-associated behaviors such as pain and anxiety. Recent studies suggest that differential cell types and synaptic circuits within the amygdala complex mediate interacting and opposing effects on emotion and pain. However, the underlying cellular and circuit mechanisms are poorly understood at present. Here we used optogenetics combined with electrophysiological analysis of synaptic inputs to investigate pain-induced synaptic plasticity within the amygdala circuits in rats. We found that 50% of the cell population in the lateral division of the central nucleus of the amygdala (CeAl) received glutamate inputs from both basolateral amygdala (BLA) and from the parabrachial nucleus (PBN), and 39% of the remaining CeAl cells received glutamate inputs only from PBN. Inflammatory pain lasting 3 days, which induced anxiety, produced sensitization in synaptic activities of the BLA-CeAl-medial division of CeA (CeAm) pathway primarily through a postsynaptic mechanism. Moreover, in CeAl cells receiving only PBN inputs, pain significantly augmented the synaptic strength of the PBN inputs. In contrast, in CeAl cells receiving both BLA and PBN inputs, pain selectively increased the synaptic strength of BLA inputs, but not the PBN inputs. Electrophysiological analysis of synaptic currents showed that the increased synaptic strength in both cases involved a postsynaptic mechanism. These findings reveal two main populations of CeAl cells that have differential profiles of synaptic inputs and show distinct plasticity in their inputs in response to anxiety-associated pain, suggesting that the specific input plasticity in the two populations of CeAl cells may encode a different role in amygdala regulation of pain and emotion.

Impact of dialysis modality choice on the survival of end-stage renal disease patients with congestive heart failure in southern China: A retrospective cohort study.

Background and object: Heart failure is one of the common complications in patients with end-stage renal disease (ESRD) and a major cause of death in these patients. The choice of dialysis modality for ESRD patients with congestive heart failure (CHF) is still inconclusive. The purpose of this study was to compare the prognosis of hemodialysis (HD) and peritoneal dialysis (PD) among ESRD patients with CHF and provide a basis for clinical decision-making. Materials and methods: This was a retrospective study conducted at Guangdong Provincial Hospital of Traditional Chinese Medicine that included patients with CHF requiring long-term renal replacement therapy between January 1, 2012 and December 31, 2017. The end of follow-up was December 31, 2020. All patients were divided into HD and PD groups and sub grouped by age, and we used univariate and multifactorial Cox regression analyses to calculate the relative hazard ratios (HR) of the different dialysis types and adjusted for differences in baseline data using propensity score matching (PSM). Result: A total of 121 patients with PD and 156 patients with HD were included in this study. Among younger ESRD patients (≤65 years of age) with CHF, the prognosis of HD was worse than that of PD [HR = 1.84, 95% confidence interval (CI) = 1.01-3.34], and this disadvantage remained significant in the fully adjusted model [sex, age at dialysis initiation, Charlson comorbidities index, body mass index, prealbumin, hemoglobin, and left ventricular ejection fraction (LVEF)] and after PSM. In the older group (>65 years of age), the prognosis of HD was better than that of PD (HR = 0.46, 95% CI = 0.25-0.85), and the protective effect remained in the fully adjusted model and after PSM. The aforementioned survival differences across the cohort were maintained in patients with preserved LVEF (>55%), but could not be reproduced in patients with reduced LVEF (≤55%). Conclusion: In southern China, PD is a better choice for younger patients with ESRD, CHF and preserved LVEF, and HD is the better option for older patients.

Nerve growth factor-regulated emergence of functional delta-opioid receptors.

Sorting of intracellular G-protein-coupled receptors (GPCRs) either to lysosomes for degradation or to plasma membrane for surface insertion and functional expression is a key process regulating signaling strength of GPCRs across the plasma membrane in adult mammalian cells. However, little is known about the molecular mechanisms governing the dynamic process of receptor sorting to the plasma membrane for functional expression under normal and pathological conditions. In this study, we demonstrate that delta-opioid receptor (DOPr), a GPCR constitutively targeted to intracellular compartments, is driven to the surface membrane of central synaptic terminals and becomes functional by the neurotrophin nerve growth factor (NGF) in native brainstem neurons. The NGF-triggered DOPr translocation is predominantly mediated by the signaling pathway involving the tyrosine receptor kinase A, Ca(2+)-mobilizing phospholipase C, and Ca(2+)/calmodulin-dependent protein kinase II. Importantly, it requires interactions with the cytoplasmic sorting protein NHERF-1 (Na(+)/H(+) exchange regulatory factor-1) and N-ethyl-maleimide-sensitive factor-regulated exocytosis. In addition, this NGF-mediated mechanism is likely responsible for the emergence of functional DOPr induced by chronic opioids. Thus, NGF may function as a key molecular switch that redirects the sorting of intracellularly targeted DOPr to plasma membrane, resulting in new functional DOPr on central synapses under chronic opioid conditions.

Regulation of increased glutamatergic input to spinal dorsal horn neurons by mGluR5 in diabetic neuropathic pain.

Diabetic neuropathic pain is associated with increased glutamatergic input in the spinal dorsal horn. Group I metabotropic glutamate receptors (mGluRs) are involved in the control of neuronal excitability, but their role in the regulation of synaptic transmission in diabetic neuropathy remains poorly understood. Here we studied the role of spinal mGluR5 and mGluR1 in controlling glutamatergic input in a rat model of painful diabetic neuropathy induced by streptozotocin. Whole-cell patch-clamp recordings of lamina II neurons were performed in spinal cord slices. The amplitude of excitatory post-synaptic currents (EPSCs) evoked from the dorsal root and the frequency of spontaneous EPSCs (sEPSCs) were significantly higher in diabetic than in control rats. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) inhibited evoked EPSCs and sEPSCs more in diabetic than in control rats. Also, the percentage of neurons in which sEPSCs and evoked EPSCs were affected by MPEP or the group I mGluR agonist was significantly higher in diabetic than in control rats. However, blocking mGluR1 had no significant effect on evoked EPSCs and sEPSCs in either groups. The mGluR5 protein level in the dorsal root ganglion, but not in the dorsal spinal cord, was significantly increased in diabetic rats compared with that in control rats. Furthermore, intrathecal administration of MPEP significantly increased the nociceptive pressure threshold only in diabetic rats. These findings suggest that increased mGluR5 expression on primary afferent neurons contributes to increased glutamatergic input to spinal dorsal horn neurons and nociceptive transmission in diabetic neuropathic pain.

Reduction in voltage-gated K+ channel activity in primary sensory neurons in painful diabetic neuropathy: role of brain-derived neurotrophic factor.

Abnormal hyperexcitability of primary sensory neurons plays an important role in neuropathic pain. Voltage-gated potassium (Kv) channels regulate neuronal excitability by affecting the resting membrane potential and influencing the repolarization and frequency of the action potential. In this study, we determined changes in Kv channels in dorsal root ganglion (DRG) neurons in a rat model of diabetic neuropathic pain. The densities of total Kv, A-type (IA) and sustained delayed (IK) currents were markedly reduced in medium- and large-, but not in small-, diameter DRG neurons in diabetic rats. Quantitative RT-PCR analysis revealed that the mRNA levels of IA subunits, including Kv1.4, Kv3.4, Kv4.2, and Kv4.3, in the DRG were reduced approximately 50% in diabetic rats compared with those in control rats. However, there were no significant differences in the mRNA levels of IK subunits (Kv1.1, Kv1.2, Kv2.1, and Kv2.2) in the DRG between the two groups. Incubation with brain-derived neurotrophic factor (BDNF) caused a large reduction in Kv currents, especially IA currents, in medium and large DRG neurons from control rats. Furthermore, the reductions in Kv currents and mRNA levels of IA subunits in diabetic rats were normalized by pre-treatment with anti-BDNF antibody or K252a, a TrkB tyrosine kinase inhibitor. In addition, the number of medium and large DRG neurons with BDNF immunoreactivity was greater in diabetic than control rats. Collectively, our findings suggest that diabetes primarily reduces Kv channel activity in medium and large DRG neurons. Increased BDNF activity in these neurons likely contributes to the reduction in Kv channel function through TrkB receptor stimulation in painful diabetic neuropathy.

GluA1 in Central Amygdala Promotes Opioid Use and Reverses Inhibitory Effect of Pain.

Increasing evidence suggests that long-term opioids and pain induce similar adaptive changes in the brain's reward circuits, however, how pain alters the addictive properties of opioids remains poorly understood. In this study using a rat model of morphine self-administration (MSA), we found that short-term pain, induced by an intraplantar injection of complete Freund's adjuvant (CFA), acutely decreased voluntary morphine intake, but not food intake, only at a morphine dose that did not affect pain itself. Pre-treatment with indomethacin, a non-opioid inhibitor of pain, before the pain induction blocked the decrease in morphine intake. In rats with steady MSA, the protein level of GluA1 subunits of glutamate AMPA receptors (AMPARs) was significantly increased, but that of GluA2 was decreased, resulting in an increased GluA1/GluA2 ratio in central nucleus of the amygdala (CeA). In contrast, pain decreased the GluA1/GluA2 ratio in the CeA of rats with MSA. Microinjection of NASPM, a selective inhibitor of homomeric GluA1-AMPARs, into CeA inhibited morphine intake. Furthermore, viral overexpression of GluA1 protein in CeA maintained morphine intake at a higher level than controls and reversed the pain-induced reduction in morphine intake. These findings suggest that CeA GluA1 promotes opioid use and its upregulation is sufficient to increase opioid consumption, which counteracts the acute inhibitory effect of pain on opioid intake. These results demonstrate that the CeA GluA1 is a shared target of opioid and pain in regulation of opioid use, which may aid in future development of therapeutic applications in opioid abuse.

Plasticity and emerging role of BKCa channels in nociceptive control in neuropathic pain.

Large-conductance Ca(2+)-activated K(+) (BK(Ca), MaxiK) channels are important for the regulation of neuronal excitability. Peripheral nerve injury causes plasticity of primary afferent neurons and spinal dorsal horn neurons, leading to central sensitization and neuropathic pain. However, little is known about changes in the BK(Ca) channels in the dorsal root ganglion (DRG) and spinal dorsal horn and their role in the control of nociception in neuropathic pain. Here we show that L5 and L6 spinal nerve ligation in rats resulted in a substantial reduction in both the mRNA and protein levels of BK(Ca) channels in the DRG but not in the spinal cord. Nerve injury primarily reduced the BK(Ca) channel immunoreactivity in small- and medium-sized DRG neurons. Furthermore, although the BK(Ca) channel immunoreactivity was decreased in the lateral dorsal horn, there was an increase in the BK(Ca) channel immunoreactivity present on dorsal horn neurons near the dorsal root entry zone. Blocking the BK(Ca) channel with iberiotoxin at the spinal level significantly reduced the mechanical nociceptive withdrawal threshold in control and nerve-injured rats. Intrathecal injection of the BK(Ca) channel opener [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one] dose dependently reversed allodynia and hyperalgesia in nerve-ligated rats but it had no significant effect on nociception in control rats. Our study provides novel information that nerve injury suppresses BK(Ca) channel expression in the DRG and induces a redistribution of BK(Ca) channels in the spinal dorsal horn. BK(Ca) channels are increasingly involved in the control of sensory input in neuropathic pain and may represent a new target for neuropathic pain treatment.

A functional link between T-type calcium channels and mu-opioid receptor expression in adult primary sensory neurons.

The mu-opioid receptor agonists have a preferential effect on nociception in adults but their analgesic effect is less selective in neonates. Here we presented our finding that the mu-opioid receptor agonists had no effect on high voltage-activated Ca(2+) channels (HVACCs) in adult dorsal root ganglion (DRG) neurons that exhibited a prominent T-type Ca(2+) current. We also determined the mechanisms underlying the mu-opioid agonists' lack of effect on HVACCs in these neurons. The mu-opioid agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO), morphine, and morphine 6-beta-D-glucuronide had no effect on either T-type or HVACC currents despite the presence of a large N-type Ca(2+) current in neurons with T-type Ca(2+) currents. DAMGO still had no effect on HVACC currents when T-type Ca(2+) channels were blocked in these neurons. However, intracellular dialysis of GTP-gamma-S to activate G proteins significantly attenuated HVACC currents. DRG neurons with T-type Ca(2+) currents showed little responses to capsaicin. Single-cell RT-PCR analysis revealed that the mu-opioid receptor mRNA was present only in adult DRG neurons lacking prominent T-type Ca(2+) currents. In the neonatal DRG, DAMGO inhibited HVACC currents in 31% neurons with T-type Ca(2+) currents. The mu-opioid receptor mRNA was detected in all neurons without T-type Ca(2+) currents and also in 28.6% of neurons with T-type Ca(2+) currents in the neonatal DRG. Our study provides novel information that adult DRG neurons with prominent T-type Ca(2+) currents do not express mu-opioid receptors. Expression of T-type Ca(2+) (Ca(V)3.2) channels and mu-opioid receptors may be developmentally co-regulated as some DRG neurons differentiate toward becoming nociceptive neurons.

Role of M2, M3, and M4 muscarinic receptor subtypes in the spinal cholinergic control of nociception revealed using siRNA in rats.

Muscarinic acetylcholine receptors (mAChRs) are involved in the control of nociception in the spinal cord. The M(2), M(3), and M(4) mAChR subtypes are present in the spinal dorsal horn. However, the role of the individual subtypes in the anti-nociceptive effect produced by mAChR agonists is uncertain. Here, we determined the contribution of M(2), M(3), and M(4) subtypes to spinal muscarinic analgesia by using small-interference RNA (siRNA) targeting specific mAChR subtypes in rats. The neuronal uptake and distribution of a chitosan-siRNA conjugated fluorescent dye in the spinal cord and dorsal root ganglion were confirmed after intrathecal injection. The control and gene-specific siRNA-chitosan complexes were injected intrathecally for three consecutive days. Quantitative reverse-transcription polymerase chain reaction analysis showed that treatment with siRNA targeting M(2), M(3), or M(4) subtype produced a large reduction in the corresponding mRNA levels in the dorsal root ganglion and dorsal spinal cord. Also, the protein levels of the mAChR subtypes in the spinal cord were significantly down-regulated by siRNA treatment, as determined by the immunoprecipitation and receptor-binding assay. Treatment with the M(2)-siRNA caused a large reduction in the inhibitory effect of muscarine on the nociceptive withdrawal threshold. Furthermore, M(4) knockdown at the spinal level significantly reduced the anti-nociceptive effect of muscarine. However, the anti-nociceptive effect of muscarine was not significantly changed by the M(3)-specific siRNA. Our study suggests that chitosan nanoparticles can be used for efficient delivery of siRNA into the neuronal tissues in vivo. Our findings also provide important functional evidence that M(2) and M(4), but not M(3), contribute to nociceptive regulation by mAChRs at the spinal level.

REST overexpression in mice causes deficits in spontaneous locomotion.

Overexpression of REST has been implicated in brain tumors, ischemic insults, epilepsy, and movement disorders such as Huntington's disease. However, owing to the lack of a conditional REST overexpression animal model, the mechanism of action of REST overexpression in these disorders has not been established in vivo. We created a REST overexpression mouse model using the human REST (hREST) gene. Our results using these mice confirm that hREST expression parallels endogenous REST expression in embryonic mouse brains. Further analyses indicate that REST represses the dopamine receptor 2 (Drd2) gene, which encodes a critical nigrostriatal receptor involved in regulating movement, in vivo. Overexpression of REST using Drd2-Cre in adult mice results in increased REST and decreased DRD2 expression in the striatum, a major site of DRD2 expression, and phenocopies the spontaneous locomotion deficits seen upon global DRD2 deletion or specific DRD2 deletion from indirect-pathway medium spiny neurons. Thus, our studies using this mouse model not only reveal a new function of REST in regulating spontaneous locomotion but also suggest that REST overexpression in DRD2-expressing cells results in spontaneous locomotion deficits.