Polydatin protects cardiomyocytes against myocardial infarction injury by activating Sirt3.

Myocardial infarction (MI), which is characterized by chamber dilation and left ventricular (LV) dysfunction, represents a major cause of morbidity and mortality worldwide. Polydatin (PD), a monocrystalline and polyphenolic drug isolated from a traditional Chinese herb (Polygonum cuspidatum), alleviates mitochondrial dysfunction. We investigated the effects and underlying mechanisms of PD in post-MI cardiac dysfunction. We constructed an MI model by left anterior descending (LAD) coronary artery ligation using wild-type (WT) and Sirt3 knockout (Sirt3-/-) mice. Cardiac function, cardiomyocytes autophagy levels, apoptosis and mitochondria biogenesis in mice that underwent cardiac MI injury were compared between groups. PD significantly improved cardiac function, increased autophagy levels and decreased cardiomyocytes apoptosis after MI. Furthermore, PD improved mitochondrial biogenesis, which is evidenced by increased ATP content, citrate synthase (CS) activity and complexes I/II/III/IV/V activities in the cardiomyocytes subjected to MI injury. Interestingly, Sirt3 knockout abolished the protective effects of PD administration. PD inhibited apoptosis in cultured neonatal mouse ventricular myocytes subjected to hypoxia for 6h to simulate MI injury. PD increased GFP-LC3 puncta, and reduced the accumulation of protein aggresomes and p62 in cardiomyocytes after hypoxia. Interestingly, the knock-down of Sirt3 nullified the PD-induced beneficial effects. Thus, the protective effects of PD are associated with the up-regulation of autophagy and improvement of mitochondrial biogenesis through Sirt3 activity.

Α-Dendrotoxin-sensitive Kv1 channels contribute to conduction failure of polymodal nociceptive C-fibers from rat coccygeal nerve.

It is known that some patients with diabetic neuropathy are usually accompanied by abnormal painful sensations. Evidence has accumulated that diabetic neuropathic pain is associated with the hyperexcitability of peripheral nociceptors. Previously, we demonstrated that reduced conduction failure of polymodal nociceptive C-fibers and enhanced voltage-dependent sodium currents of small dorsal root ganglion (DRG) neurons contribute to diabetic hyperalgesia. To further investigate whether and how potassium channels are involved in the conduction failure, α-dendrotoxin (α-DTX), a selective blocker of the low-threshold sustained Kv1 channel, was chosen to examine its functional capability in modulating the conduction properties of polymodal nociceptive C-fibers and the excitability of sensory neurons. We found that α-DTX reduced the conduction failure of C-fibers from coccygeal nerve in vivo accompanied by an increased initial conduction velocity but a decreased activity-dependent slowing of conduction velocity. In addition, the number of APs evoked by step currents was significantly enhanced after the treatment with α-DTX in small-diameter sensory neurons. Further study of the mechanism indicates α-DTX-sensitive K(+) current significantly reduced and the activation of this current in peak and steady state shifted to depolarization for diabetic neurons. Expression of Kv channel subunits Kv1.2 and Kv1.6 was downregulated in both small dorsal root ganglion neurons and peripheral C-fibers. Taken together, these results suggest that α-DTX-sensitive Kv1 channels might play an important role in regulating the conduction properties of polymodal nociceptive C-fibers and firing properties of sensory neurons.

High-entropy superparaelectrics with locally diverse ferroic distortion for high-capacitive energy storage.

Superparaelectrics are considered promising candidate materials for achieving superior energy storage capabilities. However, due to the complicated local structural design, simultaneously achieving high recoverable energy density (Wrec) and energy storage efficiency (η) under high electric fields remains a challenge in bulk superparaelectrics. Here, we propose utilizing entropy engineering to disrupt long-range ferroic orders into local polymorphic distortion disorder with multiple BO6 tilt types and diverse heterogeneous polarization configurations. This strategy reduces the switching barriers, thereby facilitating the emergence of superparaelectric behaviors with ideal polarization forms. Furthermore, it enables high polarization response, negligible remnant polarization, delayed polarization saturation, and enhanced breakdown electric fields (Eb) in high-entropy superparaelectrics. Consequently, an extraordinary Wrec of 15.48 J cm-3 and an ultrahigh η of 90.02% are achieved at a high Eb of 710 kV cm-1, surpassing the comprehensive energy storage performance of previously reported bulk superparaelectrics. This work demonstrates that entropy engineering is a viable strategy for designing high-performance superparaelectrics.

Modulation of action potential trains in rabbit saphenous nerve unmyelinated fibers.

Usually, the main axon is assumed to faithfully conduct action potentials (APs). Recent data have indicated that neural processing can occur along the axonal path. However, the patterns and mechanisms of temporal coding are not clear. In the present study, single fiber recording was used to analyze activity-dependent modulation of AP trains in the main axons of C fibers in the rabbit saphenous nerve. Trains of 5 superthreshold electrical pulses at interstimulus intervals of 20 or 50 ms were applied to the nerve trunk for 200 s. The interspike intervals (ISIs) for these trains were compared to the input interstimulus intervals. Three basic types of C fibers were observed in response to repeated stimuli: first, the ISI between the first and second AP (ISI1-2) of type 1 was longer than the interstimulus interval; second, the ISI1-2 of type 2 showed wavelike fluctuations around the interstimulus interval, and third, the ISI1-2 of type 3 exhibited shorter intervals for a long period. Furthermore, both 4-aminopyridine-sensitive potassium and hyperpolarization-activated cation currents were involved in the modulation of ISI1-2 of train pulses. These data provide new evidence that multiple modes of neural conduction can occur along the main axons of C fibers.

Reduced conduction failure of the main axon of polymodal nociceptive C-fibres contributes to painful diabetic neuropathy in rats.

Painful diabetic neuropathy is a common complication of diabetes mellitus and can affect many aspects of life and severely limit patients' daily functions. Signals of painful diabetic neuropathy are believed to originate in the peripheral nervous system. However, its peripheral mechanism of hyperalgesia has remained elusive. Numerous studies have accumulated that polymodal nociceptive C-fibres play a crucial role in the generation and conduction of pain signals and sensitization of which following injury or inflammation leads to marked hyperalgesia. Traditionally, the number of nociceptive primary afferent firings is believed to be determined at the free nerve endings, while the extended main axon of unmyelinated C-fibres only involves the reliable and faithful propagation of firing series to the central terminals. We challenged this classic view by showing that conduction of action potential can fail to occur in response to repetitive activity when they travel down the main axon of polymodal nociceptive C-fibres. Quantitative analysis of conduction failure revealed that the degree of conduction failure displays a frequency-dependent manner. Local administration of low threshold, rapidly activating potassium current blocker, α-dendrotoxin (0.5 nM) and persistent sodium current blocker, low doses of tetrodotoxin (<100 nM) on the main axon of C-fibres can reciprocally regulate the degree of conduction failure, confirming that conduction failure did occur along the main axon of polymodal nociceptive C-fibres. Following streptozotocin-induced diabetes, a subset of polymodal nociceptive C-fibres exhibited high-firing-frequency to suprathreshold mechanical stimulation, which account for about one-third of the whole population of polymodal nociceptive C-fibres tested. These high-firing-frequency polymodal nociceptive C-fibres in rats with diabetes displayed a marked reduction of conduction failure. Delivery of low concentrations of tetrodotoxin and Nav1.8 selective blocker, A-803467 on the main axon of C-fibres was found to markedly enhance the conduction failure in a dose-dependent manner in diabetic rats. Upregulated expression of sodium channel subunits Nav1.7 and Nav1.8 in both small dorsal root ganglion neurons and peripheral C-fibres as well as enhanced transient and persistent sodium current and increased excitability in small dorsal root ganglion neurons from diabetic rats might underlie the reduced conduction failure in the diabetic high-firing-frequency polymodal nociceptive C-fibres. This study shed new light on the functional capability in the pain signals processing for the main axon of polymodal nociceptive C-fibres and revealed a novel mechanism underlying diabetic hyperalgesia.

Achieving Ultrahigh Energy Storage Performance for NaNbO3-Based Lead-Free Antiferroelectric Ceramics via the Coupling of the Stable Antiferroelectric R Phase and Nanodomain Engineering.

NaNbO3(NN)-based lead-free eco-friendly antiferroelectric (AFE) ceramics with an extremely high maximum polarization (Pm) are believed to be a promising alternative to traditional lead-based ceramics. Nevertheless, the high energy dissipation resulting from the large polarization hysteresis, which arises from the AFE-ferroelectric (FE) phase transition, poses a great challenge to the application of this promising ceramic. Herein, an excellent recoverable energy storage density (Wrec) was attained by intentionally designing a (0.86 - x) NaNbO3-0.14CaTiO3-xBiMg2/3Nb1/3O3 (NN-CT-xBMN) relaxor antiferroelectric ceramic, attributed to the synergistic effect of the stable AFE R phase and nanodomain engineering to overcome the bottleneck. The obtained results illustrate that the inclusion of BMN causes the transition from AFE microdomains to nanodomains and stabilizes the relaxor AFE orthorhombic R phase, which generates a highly stable polarization field response with low hysteresis and delays the AFE-FE phase transition, thus improving energy storage density. As a consequence, a high Wrec of 5.41 J cm-3 with an excellent conversion efficiency η of 86.7% was obtained in the NN-CT-0.08BMN ceramic. Moreover, the NN-CT-0.08BMN ceramic exhibits superior stability in temperature (25-150 °C), frequency (1-600 Hz), and fatigue behavior (10°-104 cycles) together with a large current density (CD = 810 A cm-2), ultrahigh power density (PD = 118 MW cm-3), and ultrafast discharge rate (t0.9 < 0.7 µs). This superior energy storage density, coupled with outstanding stability, suggests that the NN-CT-0.08BMN ceramic has the potential to be a promising candidate for pulsed power applications and power electronics.

Reduced expression of SynGAP, a neuronal GTPase-activating protein, enhances capsaicin-induced peripheral sensitization.

Synaptic GTPase-activating protein (SynGAP) is a neuronal-specific Ras/Rap-GAP that increases the hydrolysis rate of GTP to GDP, converting Ras/Rap from the active into the inactive form. The Ras protein family modulates a wide range of cellular pathways including those involved in sensitization of sensory neurons. Since GAPs regulate Ras activity, SynGAP might be an important regulator of peripheral sensitization and pain. Therefore, we evaluated excitability, stimulus-evoked release of the neuropeptide calcitonin gene-related peptide (CGRP), and nociception from wild-type (WT) mice and those with a heterozygous mutation of the SynGAP gene (SynGAP(+/-)). Our results demonstrate that SynGAP is expressed in primary afferent sensory neurons and that the capsaicin-stimulated CGRP release from spinal cord slices was two-fold higher from SynGAP(+/-) mice than that observed from WT mouse tissue, consistent with an increase in expression of the capsaicin receptor, transient receptor potential cation channel subfamily V member 1 (TRPV1), in SynGAP(+/-) dorsal root ganglia. However, there was no difference between the two genotypes in potassium-stimulated release of CGRP, the number of action potentials generated by a ramp of depolarizing current, or mechanical hypernociception elicited by intraplantar injection of capsaicin. In contrast, capsaicin-induced thermal hypernociception occurred at lower doses of capsaicin and had a longer duration in SynGAP(+/-) mice than WT mice. These results provide the first evidence that SynGAP is an important regulator of neuropeptide release from primary sensory neurons and can modulate capsaicin-induced hypernociception, demonstrating the importance of GAP regulation in signaling pathways that play a role in peripheral sensitization.

Noise enhances subthreshold oscillations in injured primary sensory neurons.

Noise can play a constructive role in the detection of weak signals in various kinds of peripheral receptors and neurons. What the mechanism underlying the effect of noise is remains unclear. Here, the perforated patch-clamp technique was used on isolated cells from chronic compression of the dorsal root ganglion (DRG) model. Our data provided new insight indicating that, under conditions without external signals, noise can enhance subthreshold oscillations, which was observed in a certain type of neurons with high-frequency (20-100 Hz) intrinsic resonance from injured DRG neurons. The occurrence of subthreshold oscillation considerably decreased the threshold potential for generating repetitive firing. The above effects of noise can be abolished by blocking the persistent sodium current (I(Na, P)). Utilizing a mathematical neuron model we further simulated the effect of noise on subthreshold oscillation and firing, and also found that noise can enhance the electrical activity through autonomous stochastic resonance. Accordingly, we propose a new concept of the effects of noise on neural intrinsic activity, which suggests that noise may be an important factor for modulating the excitability of neurons and generation of chronic pain signals.

Conduction failures in rabbit saphenous nerve unmyelinated fibers.

Recent experimental and theoretical data indicate that the functional capabilities of axons with specialized structures are much more diverse than traditionally thought. However, few observations were concerned with the main axons without arborization. In the present study, electrical stimulation of the saphenous nerve at different frequencies (2, 5, 10, 20 Hz) was used to test the role of activity-dependent effects on the pattern of action potentials that propagate along individual unmyelinated fibers (C fibers) within the trunk of the saphenous nerve in rabbits. Three basic types of C fiber responses to repetitive stimulation were observed: type-1 fibers showed an entrained response without conduction failure; type-2 fibers discharged with intermittent conduction failures; while only sporadic conduction failures happened in type 3. The failure modality in type-2 and type-3 fibers is closely related to the conductive distance as well as the frequency and duration of stimuli which lead to a critical level of conduction velocity slowing. A novel fluctuation in interspike intervals was always observed immediately before the occurrence of the failures, implying that the fluctuation of conduction velocity is correlated with imminent failures. Both the 4-aminopyridine-sensitive potassium current and hyperpolarization-activated cation current were recognized to be involved in the regulation of conduction failure patterns. The results confirmed, at least in part, the existence of conduction failures in the main axon of C fibers, suggesting that axonal operations may also be determinants for adaptation phenomenon and information processing in peripheral nervous system.

Responsiveness of a neural pacemaker near the bifurcation point.

We compared the responsiveness of a neural firing pacemaker in different dynamic states during the process of period-adding bifurcation to excitatory and inhibitory electrical field stimulus. In the region far from the bifurcation point, with the increase of the intensity of excitatory stimulus, the firing rate increased in an approximately linear manner and no firing pattern transition was observed. While in the region near the bifurcation point, the firing rate increased markedly higher accompanied with the transition of firing pattern when the intensity of excitatory stimulus remained the same. The stimulus-response of the region near the bifurcation point shifted upward significantly compared to that of the region far from the bifurcation point. Inhibitory stimulus with the same intensity, however, decreased the firing rate slightly without the transition of firing pattern in the region near the bifurcation point. These results suggest that the responsiveness in the region near the bifurcation point is more sensitive than that in the region far from the bifurcation point, which we named "critical sensitivity", and this has directional selectivity.

A novel intrinsic analgesic mechanism: the enhancement of the conduction failure along polymodal nociceptive C-fibers.

Although conduction failure has been observed in nociceptive C-fibers, little is known regarding its significance or therapeutic potential. In a previous study, we demonstrated that C-fiber conduction failure, which is regarded as an intrinsic self-inhibition mechanism, was reduced in circumstances of painful diabetic neuropathy. In this study, we extend this finding in the complete Freund's adjuvant model of inflammatory pain and validate that the degree of conduction failure decreased and led to a greater amount of pain signals conveyed to the central nervous system. In complete Freund's adjuvant-injected animals, conduction failure occurred in a C-fiber-selective, activity-dependent manner and was associated with an increase in the rising slope of the C-fiber after-hyperpolarization potential. To target conduction failure in a therapeutic modality, we used ZD7288, an antagonist of hyperpolarization-activated, cyclic nucleotide-modulated channels which are activated by hyperpolarization and play a pivotal role in both inflammatory and neuropathic pain. ZD7288 promoted conduction failure by suppressing Ih as a mechanism to reduce the rising slope of the after-hyperpolarization potential. Moreover, perineuronal injection of ZD7288 inhibited abnormal mechanical allodynia and thermal hyperalgesia without affecting motor function or heart rate. Our data highlight the analgesic potential of local ZD7288 application and identify conduction failure as a novel target for analgesic therapeutic development.

Effects of gabapentin on spontaneous discharges and subthreshold membrane potential oscillation of type A neurons in injured DRG.

Ectopic spontaneous discharges play a critical role for both initiation and maintenance of the neuropathic pain state. Gabapentin (GBP) has been shown to be effective in animal models of neuropathic pain as well as in chronic pain patients. To investigate the peripheral mechanisms of GBP, the effects of GBP on spontaneous discharges and subthreshold membrane potential oscillation (SMPO) of chronically compressed dorsal root ganglion (DRG) were examined electrophysiolocally in vitro. The rate of spontaneous discharges was transitorily enhanced when GBP was applied to the DRG. When the concentration was under 5microM, only enhanced effect was observed, while spontaneous discharges were completely suppressed when the concentration of GBP was beyond 5microM. The similar doses of GBP blocking the spontaneous discharges failed to block the propagation of impulses by electrical nerve stimulation. Furthermore, we found that the SMPO of injured DRG cells can be selectively abolished by GBP without interrupting spike propagation. The results suggest that the inhibitory effect of GBP on SMPO might be one of the membrane mechanisms of action of GBP. This may partially explain the antinociceptive action of GBP by directly suppression nociceptive afferent input to the spinal cord.

Different firing patterns induced by veratridine and aconitine in injured dorsal root ganglion neurons.

Ectopic spontaneous activity originated from the injured dorsal root ganglion (DRG) neurons in rats was recorded through single dorsal root fiber. The firing patterns induced by veratridine and aconitine, inhibitors of inactivation gate of sodium channel operating on different binding sites, were compared. In the same neuron, veratridine (1.5 approximately 5.0 micromol/L) caused slow wave oscillations of interspike intervals (ISIs), while aconitine (10 approximately 200 micromol/L) caused tonic firing. Moreover, even if the background firing patterns were various and the reagent concentrations used were different, veratridine and aconitine still induced slow wave oscillations and tonic firing patterns, respectively. The results suggest that veratridine and aconitine induce different firing patterns in injured DRG neurons, which may relate to their inhibitory effects on different binding sites of the sodium channel.

Effects of Systemic Magnesium on Post-operative Analgesia: Is the Current Evidence Strong Enough?

BACKGROUND: Clinical studies have been previously carried out on the efficacy of systemic magnesium to minimize postoperative pain, however, with controversial results. A quantitative meta-analysis was performed to evaluate the analgesic efficacy and safety of systemic magnesium on post-operative pain. STUDY DESIGN: Comprehensive systematic review of all relevant, publsished randomized controlled trials. METHODS: A search was conducted of published literature in MEDLINE, PsycINFO, Scopus, EMBASE, and the Cochrane Central Register of Controlled Trials (CENTRAL) databases from inception to Sep-Oct 2014. Randomized controlled trials (RCTs) that compared magnesium with placebo were identified. Effects were summarized using standardized mean differences (SMDs), weighed mean differences (WMD), or odds ratio (OR) with suitable effect model. RESULTS: Twenty-seven RCTs involving 1,504 patients were included. In total, peri-operative magnesium significantly reduced the pain score at rest (SMD, -1.43, 95% CI, -2.74 to -0.12, < 0.01). Magnesium significantly reduced analgesic consumption (SMD, -1.72, 95% CI, -3.21 to -0.23) in patients undergoing urogenital, orthopaedic, and cardiovascular surgeries, but was inconclusive for patients receiving gastrointestinal surgeries. The obvious analgesia of systemic magnesium was observed on reducing the pain score during movement at 24 hours after operation (SMD, -0.05, 95% CI, -0.43 to 0.32). Moreover, magnesium administration showed a beneficial effect with regard to intra-operative hemodynamics and reduced extubation time in the cardiovascular surgery patients (WMD, -29.34 min, 95% CI, -35.74 to -22.94, P < 0.01). LIMITATIONS: Focused only on the quality of analgesia on postoperative pain with regards to surgery type. CONCLUSIONS: Our study suggests that systemic magnesium during general anesthesia significantly decreases post-operative pain scores without increasing adverse events. It should be noted that since there are 18 ongoing RCTs without published data, it is still premature to draw conclusions on the long-term analgesic effects of magnesium as well as potential gender or age difference.

Subthreshold membrane potential oscillation mediates the excitatory effect of norepinephrine in chronically compressed dorsal root ganglion neurons in the rat.

Injured dorsal root ganglion (DRG) neurons often develop adrenergic sensitivity. To investigate the mechanisms of this phenomenon, the effects of norepinephrine (NE) on membrane potential of large- and medium-sized A-type neurons from chronically compressed DRG were recorded electrophysiologically in vitro. NE induced a depolarization in both control (26/36) and injured (56/62) neurons, whereas the incidence and amplitude of NE-induced depolarization in the injured neurons were significantly higher than that in controls. Following NE-induced depolarization, a subthreshold membrane potential oscillation (SMPO) was triggered or enhanced that initiated or increased repetitive firing in a fraction of injured neurons (15/56). After the SMPO was selectively abolished by application of tetrodotoxin (TTX), NE-induced depolarization failed to produce repetitive firing, even with a greater depolarization. Application of Rp-cAMPS (500 microM), a selective inhibitor of protein kinase A (PKA), decreased both SMPO and repetitive firing evoked by NE application or by intracellular current injection. Conversely, Sp-cAMPS (500 microM), a PKA activator, had a facilitating effect on both the SMPO and the repetitive firing. These results strongly suggest that a PKA mediated triggering and enhancement of SMPO may be responsible for the excitatory effects of NE on sensory neurons in neuropathic rats.

[Three types of induced bursting rhythm in rat injured nerves].

Firing patterns of injured nerve fibers were recorded using the single-fiber firing recording technique. Under the same background firing pattern, three types of bursting were induced separately by EGTA, veratridine or high [Ca(2+)](o) in the same type of nerve fibers. The results suggest that different firing patterns are related to different stimuli, which means that each firing pattern carries corresponding neural information.

[Triggered oscillations in type A dorsal root ganglion neurons induced by veratridine].

Veratridine, a blocker of inactive gate of sodium channel, was used to perfuse L5 dorsal root ganglion (DRG) topically. Afferent activities of type A single fiber from these DRGs were recorded. It was found that after a 10-min bath of veratridine (1.8-3 micromol/L), some of the primary silent DRG neurons were triggered by touch or pressure on the receptive fields or by electrical stimulation of the sciatic nerve to produce high-frequency firing, which was termed triggered oscillation presenting a U-type of interspike intervals (ISI) or other types of oscillations. The longer the intervals between stimulating pulses, the more stimulating pulses were needed to trigger the oscillation. The oscillation, triggered by electric stimuli with different duration or patterns, had no significant difference in their patterns. The duration of the inhibitory period after a triggered oscillation was generally 30-90 s. It was also observed that this kind of triggered oscillation was induced by afferent pulses of the same neurons. These results suggest that triggered oscillation, which may contribute to the fit of triggered pain, can be produced in primary sensory neurons after application of veratridine.

Gastrodin inhibits allodynia and hyperalgesia in painful diabetic neuropathy rats by decreasing excitability of nociceptive primary sensory neurons.

Painful diabetic neuropathy (PDN) is a common complication of diabetes mellitus and adversely affects the patients' quality of life. Evidence has accumulated that PDN is associated with hyperexcitability of peripheral nociceptive primary sensory neurons. However, the precise cellular mechanism underlying PDN remains elusive. This may result in the lacking of effective therapies for the treatment of PDN. The phenolic glucoside, gastrodin, which is a main constituent of the Chinese herbal medicine Gastrodia elata Blume, has been widely used as an anticonvulsant, sedative, and analgesic since ancient times. However, the cellular mechanisms underlying its analgesic actions are not well understood. By utilizing a combination of behavioral surveys and electrophysiological recordings, the present study investigated the role of gastrodin in an experimental rat model of STZ-induced PDN and to further explore the underlying cellular mechanisms. Intraperitoneal administration of gastrodin effectively attenuated both the mechanical allodynia and thermal hyperalgesia induced by STZ injection. Whole-cell patch clamp recordings were obtained from nociceptive, capsaicin-sensitive small diameter neurons of the intact dorsal root ganglion (DRG). Recordings from diabetic rats revealed that the abnormal hyperexcitability of neurons was greatly abolished by application of GAS. To determine which currents were involved in the antinociceptive action of gastrodin, we examined the effects of gastrodin on transient sodium currents (I(NaT)) and potassium currents in diabetic small DRG neurons. Diabetes caused a prominent enhancement of I(NaT) and a decrease of potassium currents, especially slowly inactivating potassium currents (I(AS)); these effects were completely reversed by GAS in a dose-dependent manner. Furthermore, changes in activation and inactivation kinetics of I(NaT) and total potassium current as well as I(AS) currents induced by STZ were normalized by GAS. This study provides a clear cellular basis for the peripheral analgesic action of gastrodin for the treatment of chronic pain, including PDN.

5-HT induces enhanced phrenic nerve activity via 5-HT(2A) receptor/PKC mechanism in anesthetized rats.

Respiratory behavior expresses diverse forms of plasticity by altering breathing patterns. Failure of respiratory neuroplasticity often leads to malfunctions. Long-term facilitation (LTF), the most frequently studied model induced by episodic hypoxia to produce long-lasting enhancement of phrenic motor output, is thought to be serotonin 2A (5-HT(2A)) receptor-dependent. Previous studies have described 5-HT-induced prompt apnea in intact animals. However, the role of exogenous 5-HT in mediating respiratory neuroplasticity is less attended in vivo study. We hypothesized that an in vivo 5-HT challenge contributes to respiratory neuroplasticity. Here, we found that systemic bolus administration of 5-HT exerted an initial transient inhibition followed by marked facilitation, forming a biphasic pattern of phrenic nerve activity in artificially ventilated, midcervically vagotomized, and anesthetized adult rats. The facilitatory phase corresponded to the enhanced phrenic nerve activity that lasted for at least one hour after drug exposure, characterized as phrenic LTF (pLTF). The 5-HT-induced biphasic pattern and pLTF were 5-HT(2A) receptor-dependent and coupled to protein kinase C (PKC) activation. The initial inhibition of phrenic nerve activity was found to be nodose ganglion-associated, whereas the subsequent facilitation was carotid body-associated, establishing a peripheral inhibitory-facilitatory afferent balance. Immunoreactive expressions of 5-HT/5-HT(2A) receptors and phospho-PKC isoforms/PKC substrate provide morphological evidence of existence of a 5-HT/5-HT(2A) receptor/PKC mechanism in the nodose ganglion and the carotid body. We speculate that 5-HT challenge in vivo may contribute to respiratory neuroplasticity, to yield pLTF or augmented pLTF in animals with reduced or absent peripheral inhibitory inputs.

Direct effects of HIV-1 Tat on excitability and survival of primary dorsal root ganglion neurons: possible contribution to HIV-1-associated pain.

The vast majority of people living with human immunodeficiency virus type 1 (HIV-1) have pain syndrome, which has a significant impact on their quality of life. The underlying causes of HIV-1-associated pain are not likely attributable to direct viral infection of the nervous system due to the lack of evidence of neuronal infection by HIV-1. However, HIV-1 proteins are possibly involved as they have been implicated in neuronal damage and death. The current study assesses the direct effects of HIV-1 Tat, one of potent neurotoxic viral proteins released from HIV-1-infected cells, on the excitability and survival of rat primary dorsal root ganglion (DRG) neurons. We demonstrated that HIV-1 Tat triggered rapid and sustained enhancement of the excitability of small-diameter rat primary DRG neurons, which was accompanied by marked reductions in the rheobase and resting membrane potential (RMP), and an increase in the resistance at threshold (R(Th)). Such Tat-induced DRG hyperexcitability may be a consequence of the inhibition of cyclin-dependent kinase 5 (Cdk5) activity. Tat rapidly inhibited Cdk5 kinase activity and mRNA production, and roscovitine, a well-known Cdk5 inhibitor, induced a very similar pattern of DRG hyperexcitability. Indeed, pre-application of Tat prevented roscovitine from having additional effects on the RMP and action potentials (APs) of DRGs. However, Tat-mediated actions on the rheobase and R(Th) were accelerated by roscovitine. These results suggest that Tat-mediated changes in DRG excitability are partly facilitated by Cdk5 inhibition. In addition, Cdk5 is most abundant in DRG neurons and participates in the regulation of pain signaling. We also demonstrated that HIV-1 Tat markedly induced apoptosis of primary DRG neurons after exposure for longer than 48 h. Together, this work indicates that HIV-1 proteins are capable of producing pain signaling through direct actions on excitability and survival of sensory neurons.