Serotonin upregulates low- and high-threshold tetrodotoxin-resistant sodium channels in the same subpopulation of rat nociceptors

Intrathecal dopamine and serotonin enhance motor and nociceptive blockades of lidocaine in rats

The study examined the effect of intrathecal injection of dopamine (serotonin) and/or lidocaine. Intrathecal injections of dopamine (serotonin or epinephrine), lidocaine, or their combination were carried out in male Sprague Dawley rats. Neurobehavioral examinations (motor and nociceptive reactions) were performed before and after spinal injection. Intrathecal serotonin (1.5 μmol), dopamine (2.5 μmol), epinephrine (1:40000), and lidocaine (0.75 μmol) produced 29%, 33%, 29%, and 54% nociceptive blockade, whereas serotonin (1.5 μmol), dopamine (2.5 μmol), or epinephrine (1:40000) produced a longer duration of nociceptive blockade than lidocaine (0.75 μmol) (P < 0.05). Serotonin (1.5 μmol), dopamine (1.25 and 2.5 μmol), or epinephrine (1:40000 and 1:80000) prolonged the duration and increased the potency of spinal motor and nociceptive blockades of lidocaine (50% effective dose, ED50) (P < 0.05). The motor and nociceptive blockades caused by lidocaine (ED50) plus dopamine (2.5 μmol) or lidocaine (ED50) plus epinephrine (1:40000) were more outstanding than lidocaine (ED50) plus serotonin (0.75 μmol) (P < 0.05). Our study provides evidence that intrathecal dopamine or serotonin produces spinal nociceptive blockade dose-dependently. Dopamine and serotonin are less potent than lidocaine in inducing spinal nociceptive blockade. When mixed with lidocaine solution, dopamine or serotonin improves spinal motor and nociceptive blockades. The motor and nociceptive blockade caused by lidocaine (ED50) plus dopamine (2.5 μmol) is similar to that caused by lidocaine (ED50) plus epinephrine (1:40000).

Nociceptor Signalling through ion Channel Regulation via GPCRs

The prime task of nociceptors is the transformation of noxious stimuli into action potentials that are propagated along the neurites of nociceptive neurons from the periphery to the spinal cord. This function of nociceptors relies on the coordinated operation of a variety of ion channels. In this review, we summarize how members of nine different families of ion channels expressed in sensory neurons contribute to nociception. Furthermore, data on 35 different types of G protein coupled receptors are presented, activation of which controls the gating of the aforementioned ion channels. These receptors are not only targeted by more than 20 separate endogenous modulators, but can also be affected by pharmacotherapeutic agents. Thereby, this review provides information on how ion channel modulation via G protein coupled receptors in nociceptors can be exploited to provide improved analgesic therapy.

Synergistic Effects of Serotonin or Dopamine Combined With Lidocaine at Producing Nociceptive Block in Rats

BACKGROUND

The purpose of this experiment was to investigate the interactions of the local anesthetic lidocaine combined with an agent (serotonin or dopamine) as infiltrative anesthetics.

METHODS

Cutaneous analgesia was characterized by the blockade of the cutaneous trunci muscle reflex following 6 skin pinpricks on the rat back. Serotonin or dopamine at producing cutaneous analgesia in a dosage-dependent fashion was compared with lidocaine. Drug-drug interactions were calculated using the isobolographic analysis.

RESULTS

We revealed that serotonin, dopamine, and lidocaine provoked dose-related cutaneous analgesic effects. On the 50% effective dose basis, the rank of drugs' potency was found to be serotonin (1.70 [1.56-1.85] μmol) > lidocaine (5.18 [4.67-5.75] μmol) > dopamine (43.0 [40.9-45.2] μmol) (P < 0.01). At doses equivalent to their 25%, 50%, and 75% effective doses, serotonin or dopamine elicited a longer duration of action than lidocaine (P < 0.01) on producing cutaneous analgesia. Coadministration of serotonin or dopamine with lidocaine produced a synergistic effect.

CONCLUSIONS

The preclinical data showed that serotonin and dopamine produce dose-related cutaneous analgesic effects as an infiltrative anesthetic. Serotonin has a better potency with a much longer duration of action compared with lidocaine at provoking cutaneous analgesia. Serotonin or dopamine as an adjuvant increases the quality of lidocaine in cutaneous analgesia.

The role of slow and persistent TTX-resistant sodium currents in acute tumor necrosis factor-α-mediated increase in nociceptors excitability

Tetrodotoxin-resistant (TTX-r) sodium channels are key players in determining the input-output properties of peripheral nociceptive neurons. Changes in gating kinetics or in expression levels of these channels by proinflammatory mediators are likely to cause the hyperexcitability of nociceptive neurons and pain hypersensitivity observed during inflammation. Proinflammatory mediator, tumor necrosis factor-α (TNF-α), is secreted during inflammation and is associated with the early onset, as well as long-lasting, inflammation-mediated increase in excitability of peripheral nociceptive neurons. Here we studied the underlying mechanisms of the rapid component of TNF-α-mediated nociceptive hyperexcitability and acute pain hypersensitivity. We showed that TNF-α leads to rapid onset, cyclooxygenase-independent pain hypersensitivity in adult rats. Furthermore, TNF-α rapidly and substantially increases nociceptive excitability in vitro, by decreasing action potential threshold, increasing neuronal gain and decreasing accommodation. We extended on previous studies entailing p38 MAPK-dependent increase in TTX-r sodium currents by showing that TNF-α via p38 MAPK leads to increased availability of TTX-r sodium channels by partial relief of voltage dependence of their slow inactivation, thereby contributing to increase in neuronal gain. Moreover, we showed that TNF-α also in a p38 MAPK-dependent manner increases persistent TTX-r current by shifting the voltage dependence of activation to a hyperpolarized direction, thus producing an increase in inward current at functionally critical subthreshold voltages. Our results suggest that rapid modulation of the gating of TTX-r sodium channels plays a major role in the mediated nociceptive hyperexcitability of TNF-α during acute inflammation and may lead to development of effective treatments for inflammatory pain, without modulating the inflammation-induced healing processes.

Gain-of-function mutations in SCN11A cause familial episodic pain

Many ion channel genes have been associated with human genetic pain disorders. Here we report two large Chinese families with autosomal-dominant episodic pain. We performed a genome-wide linkage scan with microsatellite markers after excluding mutations in three known genes (SCN9A, SCN10A, and TRPA1) that cause similar pain syndrome to our findings, and we mapped the genetic locus to a 7.81 Mb region on chromosome 3p22.3-p21.32. By using whole-exome sequencing followed by conventional Sanger sequencing, we identified two missense mutations in the gene encoding voltage-gated sodium channel Nav1.9 (SCN11A): c.673C>T (p.Arg225Cys) and c.2423C>G (p.Ala808Gly) (one in each family). Each mutation showed a perfect cosegregation with the pain phenotype in the corresponding family, and neither of them was detected in 1,021 normal individuals. Both missense mutations were predicted to change a highly conserved amino acid residue of the human Nav1.9 channel. We expressed the two SCN11A mutants in mouse dorsal root ganglion (DRG) neurons and showed that both mutations enhanced the channel's electrical activities and induced hyperexcitablity of DRG neurons. Taken together, our results suggest that gain-of-function mutations in SCN11A can be causative of an autosomal-dominant episodic pain disorder.

The distribution of low-threshold TTX-resistant Na⁺ currents in rat trigeminal ganglion cells

The distribution of low-threshold tetrodotoxin-resistant (TTX-r) Na(+) current and its co-expression with high-threshold TTX-r Na(+) current were studied in randomly selected acutely dissociated rat trigeminal ganglion (non-identified TG cells) and TG cells serving the temporomandibular joint (TMJ-TG cells). Conditions previously shown to enhance Na(V)1.9 channel-mediated currents (holding potential (HP) -80 mV, 130-mM fluoride internally) were employed to amplify the low-threshold Na(+) current. Under these conditions, detectable low-threshold Na(+) current was exhibited by 16 out of 21 non-identified TG cells (average, 1810 ± 358 pA), and by nine of 14 TMJ-TG cells (average, 959 ± 525 pA). The low-threshold Na(+) current began to activate around -55 mV and was inactivated by holding TG cells at -60 mV and delivering 40-ms test potentials (TPs) to 0 mV. The inactivation was long lasting, recovering only 8 ± 3% over a 5-min period after the HP was returned to -80 mV. Following low-threshold Na(+) current inactivation, high-threshold TTX-r Na(+) current, evoked from HP -60 mV, was observed. High-threshold Na(+) current amplitude averaged 16,592 ± 3913 pA for TPs to 0 mV, was first detectable at an average TP of -34 ± 1.3 mV, and was ½ activated at -7.1 ± 2.3 mV. In TG cells expressing prominent low-threshold Na(+) currents, changing the external solution to one containing 0 mM Na(+) reduced the amount of current required to hold the cells at -80 mV through -50 mV, the peak effect being observed at HP -60 mV. TG cells recorded from with a more physiological pipette solution containing chloride instead of fluoride exhibited small low-threshold Na(+) currents, which were greatly increased upon superfusion of the TG cells with the adenylyl cyclase (AC) activator forskolin. These data suggest two hypotheses: (1) low- and high-threshold Na(V)1.9 and Na(V)1.8 channels, respectively, are frequently co-expressed in TG neurons serving the TMJ and other structures, and (2), Na(V)1.9 channel-mediated currents are small under physiological conditions, but may be enhanced by inflammatory mediators that increase AC activity, and may mediate an inward leak that depolarizes TG neurons, increasing their excitability.

Secondary mechanical allodynia and hyperalgesia depend on descending facilitation mediated by spinal 5-HT₄, 5-HT₆ and 5-HT₇ receptors

In the present study we determined the role of spinal 5-hydroxytriptamine (5-HT) and 5-HT(4/6/7) receptors in the long-term secondary mechanical allodynia and hyperalgesia induced by formalin in the rat. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia in both paws. In addition, formalin increased the tissue content of 5-HT in the ipsilateral, but not contralateral, dorsal part of the spinal cord compared to control animals. Intrathecal (i.t.) administration of 5,7-dihydroxytriptamine (5,7-DHT), a serotonergic neurotoxin, diminished tissue 5-HT content in the ipsilateral and contralateral dorsal parts of the spinal cord. Accordingly, i.t. 5,7-DHT prevented formalin-induced secondary allodynia and hyperalgesia in both paws. I.t. pre-treatment (-10 min) with ML-10302 (5-HT(4) agonist), EMD-386088 (5-HT(6) agonist) and LP-12 (5-HT(7) agonist) significantly increased secondary mechanical allodynia and hyperalgesia in both paws. In contrast, i.t. pre-treatment (-20 min) with GR-125487 (5-HT(4) antagonist), SB-258585 (5-HT(6) antagonist) and SB-269970 (5-HT(7) antagonist) significantly prevented formalin-induced long-term effects in both paws. In addition, these antagonists prevented the pro-nociceptive effect of ML-10302, EMD-386088 and LP-12, respectively. The i.t. post-treatment (6 days after formalin injection) with GR-125487, SB-258585 and SB-269970 reversed formalin-induced secondary allodynia and hyperalgesia in both paws. These results suggest that spinal 5-HT, released from the serotonergic projections in response to formalin injection, activates pre- or post-synaptic 5-HT(4/6/7) receptors at the dorsal root ganglion/spinal cord promoting the development and maintenance of secondary allodynia and hyperalgesia.

Characteristics of sodium currents in rat geniculate ganglion neurons

Geniculate ganglion (GG) cell bodies of chorda tympani (CT), greater superficial petrosal (GSP), and posterior auricular (PA) nerves transmit orofacial sensory information to the rostral nucleus of the solitary tract. We have used whole cell recording to investigate the characteristics of the Na(+) channels in isolated Fluorogold-labeled GG neurons that innervate different peripheral receptive fields. GG neurons expressed two classes of Na(+) channels, TTX sensitive (TTX-S) and TTX resistant (TTX-R). The majority of GG neurons expressed TTX-R currents of different amplitudes. TTX-R currents were relatively small in 60% of the neurons but were large in 12% of the sampled population. In a further 28% of the neurons, TTX completely abolished all Na(+) currents. Application of TTX completely inhibited action potential generation in all CT and PA neurons but had little effect on the generation of action potentials in 40% of GSP neurons. Most CT, GSP, and PA neurons stained positively with IB(4), and 27% of the GSP neurons were capsaicin sensitive. The majority of IB(4)-positive GSP neurons with large TTX-R Na(+) currents responded to capsaicin, whereas IB(4)-positive GSP neurons with small TTX-R Na(+) currents were capsaicin insensitive. These data demonstrate the heterogeneity of GG neurons and indicate the existence of a subset of GSP neurons sensitive to capsaicin, usually associated with nociceptors. Since there are no reports of nociceptors in the GSP receptive field, the role of these capsaicin-sensitive neurons is not clear.

Up-regulation of low-threshold tetrodotoxin-resistant Na+ current via activation of a cyclic AMP/protein kinase A pathway in nociceptor-like rat dorsal root ganglion cells

The effects of forskolin on low-threshold tetrodotoxin-resistant (TTX-r) Na(+) currents was studied in small diameter (average ≈ 25 μm) dorsal root ganglion (DRG) cells. All DRG cells included in the study were categorized as type-2 or non-type-2 based on the expression of a low-threshold A-current. In all type-2 and some non-type-2 DRG cells held at -80 mV, the adenylyl cyclase (AC) activator forskolin (10 μM) up-regulated TTX-r Na(+) currents evoked with steps to -55 mV through -35 mV (low-threshold current). Up-regulation of low-threshold current by forskolin was mimicked by the protein kinase A (PKA) agonist Sp-cAMPs and the inflammatory mediator serotonin, and blocked by the PKA antagonist Rp-cAMPs. Forskolin-induced up-regulation of low-threshold current evoked from a holding potential of -60 mV was blocked by 40 ms steps to 0 mV, which presumably induced a long lasting inactivation of the low-threshold channels. Reducing to 3 ms the duration of steps to 0 mV, significantly increased the number of DRG cells where low-threshold current was up-regulated by forskolin, presumably by reducing the long-lasting inactivation of the low-threshold channels. In the same cells, high-threshold current, evoked by 40 ms or 3 ms steps to 0 mV, was consistently up-regulated by forskolin. The selective Na(V)1.8 channel blocker A-803467 markedly blocked high-threshold current but not low-threshold current. The different voltage protocols observed to activate and inactivate the low- and high-threshold currents, and the observation that A-803467 blocked high- but not low-threshold current suggests that the two currents were mediated by different channels, possibly Na(V)1.8 and Na(V)1.9, respectively. Inflammatory mediators may simultaneously up-regulate Na(V)1.8 and Na(V)1.9 channels in the same nociceptor via a AC/PKA signaling pathway, increasing nociceptor signaling strength, and lowering nociceptor threshold, respectively.