The role of 5-hydroxytryptamine3 receptors in the vagal afferent activation-induced inhibition of the first cervical dorsal horn spinal neurons projected from tooth pulp in the rat

Transcutaneous auricular VNS applied to experimental pain: A paired behavioral and EEG study using thermonociceptive CO2 laser

Background

Transcutaneous auricular Vagal Nerve Stimulation (taVNS) is a non-invasive neurostimulation technique with potential analgesic effects. Several studies based on subjective behavioral responses suggest that taVNS modulates nociception differently with either pro-nociceptive or anti-nociceptive effects.

Objective

This study aimed to characterize how taVNS alters pain perception, by investigating its effects on event-related potentials (ERPs) elicited by different types of spinothalamic and lemniscal somatosensory stimuli, combined with quantitative sensory testing (detection threshold and intensity ratings).

Methods

We performed 3 experiments designed to study the time-dependent effects of taVNS and compare with standard cervical VNS (cVNS). In Experiment 1, we assessed the effects of taVNS after 3 hours of stimulation. In Experiment 2, we focused on the immediate effects of the duty cycle (OFF vs. ON phases). Experiments 1 and 2 included 22 and 15 healthy participants respectively. Both experiments consisted of a 2-day cross-over protocol, in which subjects received taVNS and sham stimulation sequentially. In addition, subjects received a set of nociceptive (thermonociceptive CO₂ laser, mechanical pinprick) and non-nociceptive (vibrotactile, cool) stimuli, for which we recorded detection thresholds, intensity of perception and ERPs. Finally, in Experiment 3, we tested 13 epileptic patients with an implanted cVNS by comparing OFF vs. ON cycles, using a similar experimental procedure.

Results

Neither taVNS nor cVNS appeared to modulate the cerebral and behavioral aspects of somatosensory perception.

Conclusion

The potential effect of taVNS on nociception requires a cautious interpretation, as we found no objective change in behavioral and cerebral responses to spinothalamic and lemniscal somatosensory stimulations.

Noninvasive vagus nerve stimulation and morphine transiently inhibit trigeminal pain signaling in a chronic headache model

Noninvasive vagus nerve stimulation suppressed persistent trigeminal nociception in a chronic headache model similarly to morphine and may provide a safe, nonaddictive abortive therapy for chronic headache.

Introduction:

Chronic headache conditions are characterized by persistent sensitization of the trigeminal system, which involves dysfunction of descending pain modulation. We previously reported that noninvasive vagus nerve stimulation (nVNS) inhibits trigeminal nociception in models of episodic migraine through a mechanism involving enhanced serotonergic and GABAergic descending pain signaling.

Objectives:

The analgesic effectiveness of nVNS and morphine were investigated in an animal model of chronic headache mediated by the combination of the 3 migraine risk factors of neck muscle tension, paradoxical sleep deprivation, and pungent odors.

Methods:

Sprague–Dawley rats were injected with complete Freund's adjuvant in the trapezius and sleep deprived for 1 night to promote trigeminal sensitization. After 7 days, animals were exposed to a pungent odor, and mechanical nocifensive head withdrawal responses were determined using von Frey filaments. Beginning on day 3 after odor exposure, animals were treated daily with either nVNS or morphine for 7 days.

Results:

Exposure of animals sensitized by neck inflammation and sleep deprivation to a pungent odor resulted in a prolonged state of trigeminal nociception. Daily administration of nVNS or morphine significantly repressed the nocifensive response; however, cessation resulted in a return to heightened pretreatment nocifensive levels.

Conclusions:

The combination of reported migraine risk factors promotes a state of sustained trigeminal hypersensitivity characteristic of chronic headache. Daily nVNS was similarly effective as morphine in inhibiting nociception and may represent a safer, opioid-sparing therapeutic option for other chronic pain disorders involving sensitization of the trigeminal system by promoting descending pain modulation.

Inhibition of Trigeminal Nociception by Non-invasive Vagus Nerve Stimulation: Investigating the Role of GABAergic and Serotonergic Pathways in a Model of Episodic Migraine

Migraine is a prevalent neurological disease that is characterized by unpredictable episodic attacks of intense head pain. The underlying pathology involves sensitization and activation of the trigeminal system. Although non-invasive vagus nerve stimulation (nVNS) is recommended for the treatment of migraine, the abortive mechanism of action is not well-understood. The goal of this study was to compare the ability of nVNS and sumatriptan to inhibit trigeminal activation in two animal models of episodic migraine and to investigate the receptor mechanism of action of nVNS. Nocifensive head withdrawal response was investigated in adult male Sprague Dawley rats using von Frey filaments. To induce trigeminal nociceptor sensitization, complete Freund's adjuvant was injected in the trapezius muscle and trigeminal neurons were activated by exposure to a pungent odor or injection of the nitric oxide donor sodium nitroprusside. Some animals received nVNS or sumatriptan as treatment. Some animals were injected intracisternally with antagonists of GABA_A, 5-HT3 or 5-HT7 receptors prior to nVNS since these receptors are implicated in descending modulation. While unsensitized animals exposed to the pungent odor or nitric oxide alone did not exhibit enhanced mechanical nociception, sensitized animals with neck muscle inflammation displayed increased trigeminal nocifensive responses. The enhanced nociceptive response to both stimuli was attenuated by nVNS and sumatriptan. Administration of antagonists of GABA_A, 5-HT3, and 5-HT7 receptors in the upper spinal cord suppressed the anti-nocifensive effect of nVNS. Our findings suggest that nVNS inhibits trigeminal activation to a similar degree as sumatriptan in episodic migraine models via involvement of GABAergic and serotonergic signaling to enhance central descending pain modulation.

Resveratrol suppresses nociceptive jaw-opening reflex via 5HT3 receptor-mediated GABAergic inhibition

Systemic administration of the dietary constituent, resveratrol, was previously shown to inhibit the nociceptive jaw-opening reflex (JOR) via the endogenous opioid system. The present study investigated whether resveratrol could similarly affect the JOR under in vivo conditions via 5HT₃ receptor-mediated GABAergic inhibition. We used electrical stimulation of the tongue in pentobarbital-anesthetized rats to evoke the JOR, which was recorded as the anterior belly of the digastric muscle electromyograms (dEMG). Intravenous administration of resveratrol (2 mg/kg) reduced the dEMG amplitude in response to three times the determined threshold electrical stimulation, with maximum inhibition reached within approximately 10 min. These inhibitory effects on the JOR were reversible to control levels after approximately 20 min. Pretreatment of rats with either 5HT₃ receptor antagonist, ondansetron (0.25-1 mg/kg, i.p.), or GABA_A receptor antagonist, bicuculline (0.5-1 mg/kg, i.p.), significantly and dose-dependently attenuated the inhibitory effects of resveratrol on dEMG amplitude compared with untreated controls. These findings suggest that resveratrol also attenuates the nociceptive JOR via 5HT₃ receptor-mediated GABAergic inhibition. The present study therefore provides new insight into a possible mechanism underlying resveratrol-induced trigeminal antinociception via the descending pain control system and highlights a potential therapeutic agent for complementary alternative medicine.

The dietary constituent resveratrol suppresses nociceptive neurotransmission via the NMDA receptor

Background

Although we have previously reported that intravenous resveratrol administration inhibits the nociceptive neuronal activity of spinal trigeminal nucleus caudalis neurons, the site of the central effect remains unclear. The aim of the present study was to examine whether acute intravenous resveratrol administration in the rat attenuates central glutamatergic transmission of spinal trigeminal nucleus caudalis neurons responding to nociceptive mechanical stimulation in vivo, using extracellular single-unit recordings and microiontophoretic techniques.

Results

Extracellular single-unit recordings using multibarrel electrodes were made from the spinal trigeminal nucleus caudalis wide dynamic range neurons responding to orofacial mechanical stimulation in pentobarbital anesthetized rats. These neurons also responded to iontophoretic application of glutamate, and the evoked neuronal discharge frequency was significantly increased in a current-dependent and reversible manner. The mean firing frequency evoked by the iontophoretic application of glutamate (30, 50, and 70 nA) was mimicked by the application of 10 g, 60 g, and noxious pinch mechanical stimulation, respectively. The mean firing frequency of spinal trigeminal nucleus caudalis wide dynamic range neurons responding to iontophoretic application of glutamate and N-methyl-D-aspartate were also significantly inhibited by intravenous administration of resveratrol (2 mg/kg) and the maximal inhibition of discharge frequency was observed within 10 min. These inhibitory effects lasted approximately 20 min. The relative magnitude of inhibition by resveratrol of the glutamate-evoked spinal trigeminal nucleus caudalis wide dynamic range neuronal discharge frequency was similar to that for N-methyl-D-aspartate iontophoretic application.

Conclusion

These results suggest that resveratrol suppresses glutamatergic neurotransmission of the spinal trigeminal nucleus caudalis neurons responding to nociceptive mechanical stimulation via the N-methyl-D-aspartate receptor in vivo, and resveratrol may be useful as a complementary or alternative therapeutic agent for the treatment of trigeminal nociceptive pain.

Acute intravenous administration of dietary constituent theanine suppresses noxious neuronal transmission of trigeminal spinal nucleus caudalis in rats

Theanine is a non-dietary amino acid linked to the modulation of synaptic transmission in the central nervous system, although the acute effects of theanine in vivo, particularly on nociceptive transmission in the trigeminal system, remain to be determined. The present study investigated whether acute intravenous theanine administration to rats attenuates the excitability of wide dynamic range (WDR) spinal trigeminal nucleus caudalis (SpVc) neurons in response to nociceptive and non-nociceptive mechanical stimulation in vivo. Extracellular single unit recordings were made from 15 SpVc neurons in response to orofacial mechanical stimulation of pentobarbital-anesthetized rats, and responses to non-noxious and noxious mechanical stimuli were analyzed. The mean firing frequency of SpVc WDR neurons in response to all mechanical stimuli was dose-dependently inhibited by theanine (10, 50, and 100mM, i.v.) with the maximum inhibition of discharge frequency reached within 5min. These inhibitory effects were reversed after approximately 10min. The relative magnitude of theanine's inhibition of SpVc WDR neuronal discharge frequency was significantly greater for noxious than non-noxious stimulation. Iontophoretic application of l-glutamate induced the mean firing frequency of SpVc WDR neuron responding to noxious mechanical stimulation was also inhibited by intravenous administration of 100mM theanine. These results suggest that acute intravenous theanine administration suppresses glutaminergic noxious synaptic transmission in the SpVc, implicating theanine as a potential complementary and alternative therapeutic agent for the treatment of trigeminal nociceptive pain.

Systemic administration of the dietary constituent resveratrol inhibits the nociceptive jaw-opening reflex in rats via the endogenous opioid system

The aim of the present study was to investigate whether, under in vivo conditions, systemic administration of resveratrol could attenuate the rat nociceptive jaw-opening reflex (JOR) via the endogenous opioid system. The JOR evoked by electrical stimulation of the tongue was recorded as digastric muscle electromyograms (dEMG) in pentobarbital-anesthetized rats. The amplitude of the dEMG increased significantly in proportion to the intensity of electrical stimulation (from 1× to 5 × threshold for the JOR). dEMG amplitude in response to 3× threshold electrical stimulation of the tongue was dose-dependently inhibited by intravenous administration of resveratrol (0.5-2mg/kg). Maximum inhibition of dEMG amplitude was seen within approximately 10min. These inhibitory effects were reversible, with dEMG responses returning to control levels after approximately 20min. Pretreatment of rats with naloxone resulted in significant, dose-dependent attenuation of the inhibitory effects of resveratrol on dEMG amplitude compared with control. These findings suggest that resveratrol inhibits the nociceptive JOR via the endogenous opioid system. Further, the findings of the present study strongly support the idea that resveratrol, which is not known to have any toxic side effects, combined with an opioid could be a potential therapeutic agent for the prevention of acute trigeminal nociception.

Modulatory Mechanism of Nociceptive Neuronal Activity by Dietary Constituent Resveratrol

Changes to somatic sensory pathways caused by peripheral tissue, inflammation or injury can result in behavioral hypersensitivity and pathological pain, such as hyperalgesia. Resveratrol, a plant polyphenol found in red wine and various food products, is known to have several beneficial biological actions. Recent reports indicate that resveratrol can modulate neuronal excitability, including nociceptive sensory transmission. As such, it is possible that this dietary constituent could be a complementary alternative medicine (CAM) candidate, specifically a therapeutic agent. The focus of this review is on the mechanisms underlying the modulatory effects of resveratrol on nociceptive neuronal activity associated with pain relief. In addition, we discuss the contribution of resveratrol to the relief of nociceptive and/or pathological pain and its potential role as a functional food and a CAM.

Facilitatory actions of serotonin type 3 receptors on GABAergic inhibitory synaptic transmission in the spinal superficial dorsal horn

Analgesic effects of serotonin (5-hydroxytryptamine [5-HT]) type 3 (5-HT3) receptors may involve the release of gamma-aminobutyric acid (GABA) in the spinal dorsal horn. However, the precise synaptic mechanisms for 5-HT3 receptor-mediated spinal analgesia are not clear. In this study, we investigated whether GABAergic neurons in the superficial dorsal horn (SDH) express functional 5-HT3 receptors and how these 5-HT3 receptors affect GABAergic inhibitory synaptic transmission in the SDH, by using slice preparations from adult glutamate decarboxylase 67-green fluorescent protein (GAD67-GFP) knock-in mice. Tight-seal whole cell recordings from GFP-positive and -negative neurons showed that 5-HT3 receptor-specific agonist 2-methyl-serotonin (2-Me-5-HT) induced inward currents in a substantial population of both GFP-positive and -negative neurons. Additionally, we confirmed expression of 5-HT3 receptors in both types of neurons by single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis. Further, GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs)-both those evoked by electrical stimulation and those occurring spontaneously in tetrodotoxin (i.e., miniature IPSCs [mIPSCs])-were recorded from GFP-negative neurons. 2-Me-5-HT increased the amplitude of the evoked IPSCs and the frequency of mIPSCs. The amplitude of mIPSCs was not affected by 2-Me-5-HT, suggesting that 5-HT augments GABAergic synaptic transmission via presynaptic mechanisms. The present observations indicate that 5-HT3 receptors are expressed on both somadendritic regions and presynaptic terminals of GABAergic neurons and regulate GABAA receptor-mediated inhibitory synaptic transmission in the SDH. Taken together, these results provide clues for the underlying mechanisms of the antinociceptive actions of 5-HT3 receptors in the spinal dorsal horn.

Excitability of small-diameter trigeminal ganglion neurons by 5-HT is mediated by enhancement of the tetrodotoxin-resistant sodium current due to the activation of 5-HT(4) receptors and/or by the inhibition of the transient potassium current

The aims of the present study were to investigate whether the activation of the 5-HT receptor subtypes (5-HT(4) and 5-HT(3)) acted significantly on the modification of the tetrodotoxin-resistant sodium current (I(NaR)) in small-sized rat trigeminal ganglion (TG) neurons and whether the inhibition of the transient K(+) current (I(A)) contributed to the excitability in those neurons. 5-HT applications in at concentrations ranging from 0.01-10 microM significantly increased the peak I(NaR). One micromolar 5-HT application caused the greatest increase in the peak I(NaR) amplitude accompanied by a hyperpolarizing shift in the activation curve. A similar modification of I(NaR) properties was also obtained via the application of the 5-HT(4) receptor agonist, RS 67333, in concentrations ranging from 0.001-1 microM. The largest effects of 5-HT (1 microM) and RS 67333 (0.1 microM) on the modification of I(NaR) were abolished by pretreatment with ICS 205-930 (a 5-HT(3/4) receptor antagonist, 10 microM), which showed no significant effect on the baseline I(NaR). However, ICS 205-930 application at 30 microM caused a significant decrease in the baseline I(NaR). Phenylbiguanide (a 5-HT(3) receptor agonist) did not significantly alter I(NaR) properties when applied in concentrations ranging from 1 to 100 microM. The application of 0.1 microM RS 67333 decreased the transient K(+) current (I(A)) by approximately 31%. The threshold for action potential generation was significantly lower after the application of 0.1 microM RS 67333. Furthermore, 0.1 microM RS 67333 application increased the number of action potentials and the resting membrane potential got more positive, but it decreased the duration of depolarization phase of action potential. In addition, neither the additional application of 1 microM 5-HT in the presence of 10 microM forskolin, a stimulator of adenylyl cyclase, nor the opposite applications of 5-HT and forskolin caused the enhancement of increased I(NaR), which indicates the presence of an 'occluding effect.' These results suggest that the 5-HT-induced modification of I(NaR) is mediated by the activation of 5-HT(4) receptors, involving a cAMP-dependent signaling pathway, and that the inhibition of I(A) following the application of a 5-HT(4) receptor agonist also contributes to the increased number of action potentials.

Visceral sensitivity and expression of 5-hydroxytryptamine and c-fos in the spinal dorsal horn in a rat model with irritable bowel syndrome

AIM: To study the visceral sensitivity by rectal balloon distension and expression of 5-hydroxy-tryptamine (5-HT) and c-fos in the spinal dorsal horn of a rat model with constipation-predominant irritable bowel syndrome (C-IBS).

METHODS: The rat model was established by intragastric injection with ice-cold water (0-4℃). The perception thresholds and the number of abdominal withdrawal reflexes (AWRs) were recorded during rectal balloon distention in order to evaluate visceral sensitivity in the model group (A, n = 10) and control group (B, n = 10). Expression of 5-HT and c-fos in the spinal dorsal horn was shown by immunohistochemical staining, and analyzed semi-quantitatively by computerized color image analyzer using immunoreactive areas and optical density (OD). The statistical difference of the OD and immunoreactive areas between the two groups was examined by t test.

RESULTS: The perception threshold in group A was slightly higher than that in group B during rectal balloon distention, but there was no significant difference (0.59 ± 0.09 vs 0.57 ± 0.13, P > 0.05). The number of AWRs to the lower balloon content (1.0 mL) was much lower in group A than in group B (10.3 ± 3.3 vs 18.3 ± 5.5, P < 0.05), although no difference was found to the higher balloon content (1.5 and 2.0 mL). Immunoreactive area and OD of 5-HT- and c-fos-positive neurons and fibers in group A were significantly higher than those in group B (5-HT immunoreactive area, 146.5 ± 15.1 vs 109.3 ± 18.5; 5-HT OD, 45826 ± 2563.2 vs 29358 ± 8965.5; c-fos immunoreactive area, 125.4 ± 23.3 vs 88.7 ± 23.2; c-fos OD, 46258 ± 4642 vs 33238 ± 4587; all P < 0.05).

CONCLUSION: The model of C-IBS has a decreased visceral sensitivity to rectal balloon distention. The increased expression of 5-HT and c-fos in the spinal dorsal horn is probably involved in the process.

Activation of central 5HT2A receptors reduces the craniofacial nociception of rats

We assessed the contribution of central 5HT2A receptors to the craniofacial tissue nociception in naïve male rats. First, we tested whether activation of central 5HT2A receptors affected nociceptive neural activities recorded from superficial laminae of the trigeminal subnucleus caudalis (Vc)/upper cervical spinal cord junction (Vc/C2) region. Two types of units, such as deep-nociceptive or skin-wide dynamic range (WDR) units were identified from extracellular recordings. Topical administration of 5HT2A receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) onto the Vc/C2 region significantly reduced deep-nociceptive unit discharges evoked by formalin injection into the masseter muscle. Noxious pinch stimulation to the facial skin-evoked skin-WDR unit discharges was significantly reduced by topical administration of 0.1 mg/rat DOI onto the Vc/C2 region. Second, we tested whether i.c.v. administration of DOI affected Fos-like immunoreactivity (-LI) evoked by formalin injection into the masseter muscle. Fos-LI was significantly induced mainly at the ventrolateral (vl) area of trigeminal subnucleus interpolaris (Vi)/Vc junction (vl-Vi/Vc) region and Vc/C2 region in vehicle-treated rats. Formalin-evoked Fos-LI was significantly reduced in laminae I-II of the Vc/C2, but not vl-Vi/Vc region after i.c.v. administration of DOI. Finally, orofacial nocifensive behavioral activities evoked by formalin injection into the masseter muscle were significantly reduced by intracisternal administration of DOI. These results suggest that 5HT2A receptors in the Vc/C2 region mediate antinociceptive effects in the craniofacial nociception.

Activation of α2-adrenoreceptors suppresses the excitability of C1 spinal neurons having convergent inputs from tooth pulp and superior sagittal sinus in rats

The aim of the present study was to test the hypothesis that activation of alpha(2)-adrenoreceptors modulates the excitability of C1 neurons having convergent inputs from both the tooth pulp (TP) and the superior sagittal sinus (SSS), by using the microiontophoretic techniques of drug application and immunohistochemical approaches. Extracellular single-unit recordings were made from 38 C1 neurons responding to electrical stimulation of TP under pentobarbital-anesthetized rats. Seventy-one percent of C1 neurons (27/38) that responded to TP stimulation also responded to electrical stimulation of the SSS. In these neurons, L: -glutamate-evoked C1 neuronal discharge firings were increased in a dose-dependent manner. The mean glutamate-evoked firing rates were dose-dependently inhibited after microiontophoretic application of clonidine (alpha(2)-adrenoreceptor/imidazoline I(1) receptor agonist). The inhibition of glutamate-evoked C1 mean firings by clonidine was antagonized by the co-application of idazoxan (alpha(2)-adrenoreceptor/imidazoline I(2) receptor antagonist), yohimbine (alpha(2)-adrenoreceptor) but not the alpha(1)-adrenoreceptor antagonist, prazosin with affinity for alpha(2B)- and alpha(2C)-adrenoreceptors. The mean spontaneous discharge frequencies were significantly inhibited by the microiontophoretic application of clonidine and this inhibition was reversed by the co-application of idazoxan, yohimbine. Microiontophoresis of clonidine also resulted in a reduction of TP-/SSS-evoked activity and this effect was reversed by the co-application of yohimbine. Immunoreactivity for alpha(2A)-adrenoreceptor was found in the superficial layers of I-III in the C1 region. These results suggest that alpha(2)-adrenoreceptor agonist clonidine inhibits the excitability of C1 neurons having convergent inputs from TP and SSS afferents, and that the activation of alpha(2A)-adrenoreceptors onto C1 dorsal horn neurons may contribute as a useful therapeutic target for the alleviation of trigeminal referred pain associated with migraine and tooth pain.

Central serotonin 3 receptors play an important role in the modulation of nociceptive neural activity of trigeminal subnucleus caudalis and nocifensive orofacial behavior in rats with persistent temporomandibular joint inflammation

The role of central serotonin 3 receptors on neural activities recorded from superficial laminae of trigeminal subnucleus caudalis/upper cervical spinal cord junction region was investigated using rats with (Complete Freund's Adjuvant day 7 group) or without (non-Complete Freund's Adjuvant group) persistent temporomandibular joint inflammation evoked by Complete Freund's Adjuvant for 7 days. We identified two types of units, Deep-wide dynamic range units and Skin-wide dynamic range units from extracellular recordings. Deep-wide dynamic range units have mechanoreceptive fields in the deep craniofacial tissues including masseter muscle but do not have cutaneous mechanoreceptive fields. Deep-wide dynamic range unit discharges evoked by the formalin injection into masseter muscle were significantly enhanced in the late phase in Complete Freund's Adjuvant day 7 group. Discharges of Skin-wide dynamic range units evoked by the noxious pinch stimulation to facial skin in Complete Freund's Adjuvant day 7 group were significantly enhanced compared with those in non-Complete Freund's Adjuvant group. Topical administration of central serotonin 3 receptor antagonist, tropisetron, onto trigeminal subnucleus caudalis/upper cervical spinal cord junction region significantly reduced both formalin-evoked Deep-wide dynamic range unit and pinch-evoked Skin-wide dynamic range unit discharges in non-Complete Freund's Adjuvant and Complete Freund's Adjuvant day 7 groups significantly. The inhibitory effects of tropisetron on pinch-evoked Skin-wide dynamic range unit discharges were prolonged in Complete Freund's Adjuvant day 7 group compared with those in non-Complete Freund's Adjuvant group. The role of central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region was also tested by orofacial formalin test in Complete Freund's Adjuvant day 7 group. Intracisternal administration of tropisetron decreased the orofacial nocifensive behavior in the late phase evoked by the injection of formalin into the masseter muscle. These results suggest that central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region are involved in mediating pronociceptive effects in both superficial and deep craniofacial tissues nociception during persistent temporomandibular joint inflammation.

Modulation of human 5-hydroxytryptamine type 3AB receptors by volatile anesthetics and n-alcohols

Functional 5-hydroxytryptamine type 3 (5-HT3) receptors can be formed by 5-HT3A subunits alone or in combination with the 5-HT3B subunit, but only the 5-HT3A receptor has been previously studied with respect to the modulation by volatile anesthetics and n-alcohols. Using two-electrode voltage-clamp, we show for the first time the modulation of heteromeric human (h)5-HT3AB receptors, expressed in Xenopus oocytes, by a series of n-alcohols and halogenated volatile anesthetics. At twice their anesthetic concentration, compounds having a molecular volume of less than 110 A3 enhanced submaximal 5-HT-evoked current. Compounds larger than 110 A3 inhibited submaximal 5-HT-evoked current. In experiments examining 5-HT concentration-response relationships, chloroform and butanol caused a slight decrease in the 5-HT EC50. Sevoflurane and octanol inhibited 5-HT-evoked current at all 5-HT concentrations tested but had no effect upon the 5-HT EC50. Compared with previous data on homomeric h5-HT3A receptors, the presence of the h5-HT3B subunit reduces the enhancement of h5-HT3 receptors by smaller halogenated volatile anesthetics and n-alcohols. In summary, these results suggest that heteromeric h5-HT3AB receptors are modulated by halogenated volatile anesthetics at clinically relevant concentrations, in addition to n-alcohols, suggesting that these receptors may be another physiological target for these compounds. The modulation is dependent upon the molecular volume of the compound, further supporting the concept of an anesthetic binding pocket of limited volume common on other Cys-loop ligand-gated ion channels. Incorporation of the 5-HT3B subunit alters either the anesthetic binding site or the allosteric interactions between anesthetic binding and channel opening.