Pharmacological analysis of excitatory and inhibitory synaptic transmission in horizontal brainstem slices preserving three subnuclei of spinal trigeminal nucleus

Mechanisms of Peripheral and Central Pain Sensitization: Focus on Ocular Pain

Persistent ocular pain caused by corneal inflammation and/or nerve injury is accompanied by significant alterations along the pain axis. Both primary sensory neurons in the trigeminal nerves and secondary neurons in the spinal trigeminal nucleus are subjected to profound morphological and functional changes, leading to peripheral and central pain sensitization. Several studies using animal models of inflammatory and neuropathic ocular pain have provided insight about the mechanisms involved in these maladaptive changes. Recently, the advent of new techniques such as optogenetics or genetic neuronal labelling has allowed the investigation of identified circuits involved in nociception, both at the spinal and trigeminal level. In this review, we will describe some of the mechanisms that contribute to the perception of ocular pain at the periphery and at the spinal trigeminal nucleus. Recent advances in the discovery of molecular and cellular mechanisms contributing to peripheral and central pain sensitization of the trigeminal pathways will be also presented.

Modulation of excitatory synaptic transmissions by TRPV1 in the spinal trigeminal subnucleus caudalis neurons of neuropathic pain rats

The present study examined contribution of the transient receptor potential vanilloid 1 channel (TRPV1) to the chronic orofacial pain. Bilateral partial nerve ligation (PNL) of the mental nerve, a branch of trigeminal nerve, was performed to induce neuropathic pain. The withdrawal threshold in response to mechanical stimulation of the lower lip skin was substantially reduced after the surgery in the PNL rats while it remained unchanged in the sham rats. This reduction in the PNL rats was alleviated by pregabalin injected intraperitoneally (10 mg/kg) and intracisternally (10, 30, 100 μg). Furthermore, an intracisternal injection of AMG9810, an antagonist of TRPV1, (1.5, 5.0 μg) attenuated the reduction of withdrawal threshold. Spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) were recorded from the spinal trigeminal subnucleus caudalis (Vc) neurons in the brainstem slice, which receive the orofacial nociceptive signals. In the PNL rats, superfusion of capsaicin (0.03, 0.1 μM) enhanced their frequency without effect on the amplitude and the highest concentration (0.3 μM) increased both the frequency and amplitude. In the sham rats, only 0.3 μM capsaicin increased their frequency. Thus, capsaicin-induced facilitation of sEPSCs and mEPSCs in the PNL rats was significantly stronger than that in the sham rats. AMG9810 (0.1 μM) attenuated the capsaicin's effect. Capsaicin was ineffective on the trigeminal tract-evoked EPSCs in the PNL and sham rats. These results suggest that the chronic orofacial pain in the PNL model results from facilitation of the spontaneous excitatory synaptic transmission in the Vc region through TRPV1 at least partly.

Olfactory stimulation Inhibits Nociceptive Signal Processing at the Input Stage of the Central Trigeminal System

The spinal trigeminal nucleus caudalis (SpVc) in the mammalian brainstem serves a pivotal function in pain processing. As the main relay center for nociceptive signals, SpVc conducts pain-related signals from various regions of the head toward higher levels of central processing such as the thalamus. SpVc also receives modulatory signals from other brain areas, which can alleviate the perception of headache. We studied the impact of olfactory co-stimulation on pain-related behavior and SpVc neural activity in mice. Using the TRPA1 agonist allyl isothiocyanate (AITC) as noxious stimulus, we quantified the aversive response and the perceived pain intensity by evaluating explorative running and the mouse grimace scale, respectively. We found that the floral odorants phenylethyl alcohol (PEA) and lavender oil mitigated the aversive response to AITC. Consistent with this finding, a newly developed, automated quantification of c-Fos expression in SpVc revealed that co-stimulation with PEA or lavender profoundly reduced network activity in the presence of AITC. These results demonstrated a substantial analgesic potential of odor stimulation in the trigeminal system and provide an explanation for the palliative effect of odors in the treatment of headache.

Voltage-dependent calcium channel β subunit-derived peptides reduce excitatory neurotransmission and arterial blood pressure

AIMS

Voltage-dependent calcium channels (VDCCs) play an important role in various physiological functions in the nervous system and the cardiovascular system. In L-, N-, P/Q-, and R-type VDCCs, β subunit assists the channels for membrane targeting and modulates channel properties. In this study, we investigated whether an inhibition of the β subunit binding to α subunit, the pore-forming main subunit of VDCCs, have any effect on channel activation and physiological functions.

MAIN METHODS

Peptides derived from the specific regions of β subunit that bind to the α-interaction domain in I-II linker of α subunit were manufactured, presuming that the peptides interrupt α-β subunit interaction in the channel complex. Then, they were tested on voltage-activated Ca²⁺ currents recorded in acutely isolated trigeminal ganglion (TG) neurons, excitatory postsynaptic currents (EPSCs) in the spinal dorsal horn neurons, and arterial blood pressure (BP) recorded from the rat femoral artery.

KEY FINDINGS

When applied internally through patch pipettes, the peptides decreased the peak amplitudes of the voltage-activated Ca²⁺ currents. After fusing with HIV transactivator of transcription (TAT) sequence to penetrate cell membrane, the peptides significantly decreased the peak amplitudes of Ca²⁺ currents and the peak amplitudes of EPSCs upon the external application through bath solution. Furthermore, the TAT-fused peptides dose dependently reduced the rat BP when administered intravenously.

SIGNIFICANCE

These data suggest that an interruption of α-β subunit association in VDCC complex inhibits channel activation, thereby reducing VDCC-mediated physiological functions such as excitatory neurotransmission and arterial BP.

P2X7 receptors exert a permissive effect on the activation of presynaptic AMPA receptors in rat trigeminal caudal nucleus glutamatergic nerve terminals

Background

Purine receptors play roles in peripheral and central sensitization and are associated with migraine headache. We investigated the possibility that ATP plays a permissive role in the activation of AMPA receptors thus inducing Glu release from nerve terminals isolated from the rat trigeminal caudal nucleus (TCN).

Methods

Nerve endings isolated from the rat TCN were loaded with [³H]D-aspartic acid ([³H]D-ASP), layered into thermostated superfusion chambers, and perfused continuously with physiological medium, alone or with various test drugs. Radioactivity was measured to assess [³H]D-ASP release under different experimental conditions.

Results

Synaptosomal [³H]D-ASP spontaneous release was stimulated by ATP and to an even greater extent by the ATP analogue benzoylbenzoylATP (BzATP). The stimulation of [³H]D-ASP basal release by the purinergic agonists was prevented by the selective P2X7 receptor antagonist A438079. AMPA had no effect on basal [³H]D-ASP release, but the release observed when synaptosomes were exposed to AMPA plus a purinoceptor agonist exceeded that observed with ATP or BzATP alone. The selective AMPA receptor antagonist NBQX blocked this “excess” release. Co-exposure to AMPA and BzATP, each at a concentration with no release-stimulating effects, evoked a significant increase in [³H]D-ASP basal release, which was prevented by exposure to a selective AMPA antagonist.

Conclusions

P2X7 receptors expressed on glutamatergic nerve terminals in the rat TCN can mediate Glu release directly and indirectly by facilitating the activation of presynaptic AMPA receptors. The high level of glial ATP that occurs during chronic pain states can promote widespread release of Glu as well as can increase the function of AMPA receptors. In this manner, ATP contributes to the AMPA receptor activation involved in the onset and maintenance of the central sensitization associated with chronic pain.

Involvement of presynaptic TRPV1 channels in prostaglandin E2-induced facilitation of spontaneous synaptic transmission in the rat spinal trigeminal subnucleus caudalis

Prostaglandin E₂ (PGE₂) synthesized in the central nervous system influences various physiological functions including nociception. Recently, we have demonstrated that PGE₂ facilitates spontaneous synaptic transmission through presynaptic EP1 receptors in the spinal trigeminal subnucleus caudalis (Vc) neurons that receive nociceptive signals from the orofacial area. Increasing evidence suggests that the action of PGE₂ is related to activation of transient receptor potential vanilloid 1 (TRPV1) channels. The present study investigated whether TRPV1 channels contribute to the facilitatory effect of PGE₂ on synaptic transmission in the Vc neurons. Spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) were recorded from Vc neurons in the rat brainstem slice by whole-cell patch-clamp mode. Superfusion of capsaicin (0.3, 1.0 μM) concentration-dependently increased the frequency of both sEPSCs and sIPSCs without any significant effect on their amplitude. The effect of capsaicin was completely abolished by a TRPV1 channel blocker AMG9810 (0.1 μM). PGE₂ (5.0 μM) increased the frequency of sEPSCs and sIPSCs. This facilitatory effect of PGE₂ was attenuated by AMG9810 and in neurons desensitized by repeated application of capsaicin. While a low concentration of either PGE₂ (1.0 μM) or capsaicin (0.1 μM) had an insignificant effect on the sEPSCs and sIPSCs, co-application of these drugs increased their frequency. The present study demonstrated involvement of the presynaptic TRPV1 channels in PGE₂-induced facilitation of spontaneous synaptic transmissions and suggests interaction of PGE₂ with TRPV1 channels in modification of nociceptive signals from the orofacial area to the Vc neurons.

Trigeminal long-term potentiation as a cellular substrate for migraine

Most previous studies suggest that the subnucleus caudalis (Vc) of spinal trigeminal nucleus (Vsp) plays a key role in the generation and maintenance of migraine, a type of primary headache, by participating in the trigeminovascular system. Furthermore, the excitability of the Vc with the stimulation of the peripheral nociceptive fibers innervating the intracranial vessels or dura matter is regarded as a main cellular substrate for migraine. Here, a revised hypothesis is introduced, reinforcing the previous hypothesis and complementing it. This hypothesis suggests that, besides the Vc, much broader areas of the trigeminal sensory nuclei (Vsn), i.e., the principal sensory nucleus (Vp), the oralis nucleus (Vo), and the interpolaris nucleus (Vi), contribute to process and integrate pain signals generated in the head. In addition, the plasticity of synaptic transmission between nuclei or subnuclei in the Vsn, in particular, the Vsp, can be a cellular model for migraine, in the same way as the hippocampal synaptic plasticity is a model for learning and memory. This hypothesis will contribute to the discovery of new therapeutic tools for patients with migraine.

Novel trigeminal slice preparation method for studying mechanisms of nociception transmission

BACKGROUND

The trigeminal subnucleus caudalis (Vc) plays a critical role in transmission and modulation of nociceptive afferent inputs, and exhibits a similar layer construction to the spinal dorsal horn. However, afferent inputs enter the brainstem and project to a separately located nucleus. It has previously been difficult to record responses of the Vc to afferent fiber activation in a brainstem slice preparation. The aim of the present study was to establish a novel brainstem slice preparation method to study trigeminal nociceptive transmission mechanisms.

NEW METHOD

Thirty adult 6-7-week-old C57/BL6J male mice were included in the study. Obliquely sliced brainstem sections at a thickness of 600μm, which included the Vc and the root entry zone to the brainstem, were prepared. The Vc response to electrical stimulation of afferent fibers was observed as a change in intracellular calcium concentration by fluorescence intensity response.

RESULTS

Electrical stimulation of afferent inputs to the trigeminal nerve increased fluorescent intensity in the Vc, which was completely diminished by tetrodotoxin and significantly suppressed by the AMPA/kainate antagonist CNQX (paired t-test, P<0.001), although the non-competitive NMDA antagonist (+)-MK801 maleate resulted in no changes. These results suggested a glutamate receptor-mediated response.

COMPARISON WITH EXISTING METHODS/CONCLUSION

This brainstem slice preparation will be useful for investigating nociceptive transmission mechanisms of the trigeminal nerve.

Trigeminal brainstem modulation of persistent orbicularis oculi muscle activity in a rat model of dry eye

Altered corneal reflex activity is a common feature of dry eye disease (DE). Trigeminal sensory nerves supply the ocular surface and terminate at the trigeminal interpolaris/caudalis (ViVc) transition and spinomedullary (VcC1) regions. Although both regions contribute to corneal reflexes, their role under dry eye conditions is not well defined. This study assessed the influence of local inhibitory and excitatory amino acid neurotransmission at the ViVc transition and VcC1 regions on hypertonic saline (HS) evoked orbicularis oculi muscle activity (OOemg) in urethane-anesthetized male rats after exorbital gland removal (DE). HS increased the magnitude of long-duration OOemg activity (OOemgL, >200ms) in DE compared to sham rats, while short-duration OOemg activity (OOemgS, <200ms) was similar for both groups. Inhibition of the ViVc transition by muscimol, a GABAA receptor agonist, greatly reduced HS-evoked OOemgL activity in DE rats, whereas injections at the VcC1 region had only minor effects in both groups. Blockade of GABAA receptors by bicuculline methiodide at the ViVc transition or VcC1 region increased HS-evoked OOemgL activity in DE rats. Blockade of N-methyl-D-aspartate (NMDA) receptors at either region reduced HS-evoked OOemgL activity in DE and sham rats. GABAαβ3 receptor density was reduced at the ViVc transition, while NMDA receptor density was increased at both regions in DE rats. Loss of GABAergic inhibition at the ViVc transition coupled with enhanced NMDA excitatory amino acid neurotransmission at the ViVc transition and the VcC1 region likely contribute to altered corneal reflexes under dry eye conditions.

Effects of prostaglandin E2 on synaptic transmission in the rat spinal trigeminal subnucleus caudalis

The spinal trigeminal subnucleus caudalis (Vc) receives preferentially nociceptive afferent signals from the orofacial area. Nociceptive stimuli to the orofacial area induce cyclooxygenase both peripherally and centrally, which can synthesize a major prostanoid prostaglandin E2 (PGE2) that implicates in diverse physiological functions. To clarify the roles of centrally-synthesized PGE2 in nociception, effects of exogenous PGE2 on synaptic transmission in the Vc neurons were investigated in the rat brainstem slice. Spontaneously occurring excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) were recorded, respectively, under pharmacological blockade of inhibitory and excitatory transmission by whole-cell patch-clamp mode. Perfusion of PGE2 (1-5 μM) increased the frequency of sIPSCs in a concentration-dependent manner but had no significant effect on the amplitude. Similarly to the effects on sIPSCs, PGE2 increased the sEPSC frequency without any effect on the amplitude. These facilitatory effects of PGE2 on spontaneous synaptic transmissions were blocked by an EP1 antagonist SC19220 but not by an EP4 antagonist AH23848. Electrical stimulation of the trigeminal tract evoked short latency EPSCs (eEPSCs) in the Vc neurons. PGE2 (5 μM) was ineffective on the eEPSCs. The present study demonstrated that PGE2 facilitated spontaneous synaptic transmissions in the Vc neurons through activating the presynaptic EP1 receptors but had no effect on the trigeminal tract-mediated excitatory transmission. These results suggest that centrally-synthesized PGE2 modifies the synaptic transmission in the Vc region, thereby contributing to the processing of nociceptive signals originated from the orofacial area.

Nicotinic receptors modulate the function of presynaptic AMPA receptors on glutamatergic nerve terminals in the trigeminal caudal nucleus

In this study, we demonstrate the existence on trigeminal caudal nucleus (TCN) glutamatergic terminals of α4β2 nicotinic receptors (nAChRs) capable of enhancing the terminals' spontaneous release of [(3)H]d-aspartate ([(3)H]D-Asp). In rat TCN synaptosomes, spontaneous [(3)H]D-Asp release was increased by nicotine and the nicotinic receptor agonists (±)epibatidine and RJR2403. The increase was potentiated by the positive allosteric modulator of nAChRs LY2087101, inhibited by the nicotinic antagonists mecamylamine (MEC) and dihydro-β-erythroidine hydrobromide (DHβE), and unaffected by α-bungarotoxin (α-BgTx) and methyllycaconitine (MLA). Evidence of functional interaction was observed between the α4β2 nAChRs and cyclothiazide-sensitive, alfa-amino-3-hydroxy-5-methyl-4-isoxazolone propionate (AMPA) receptors co-localized on the TCN synaptosomes. Brief pre-exposure of synaptosomes to 30 μM nicotine or 10 μM RJR2403 abolished the AMPA (100 μM) -induced potentiation of [K(+)]e-evoked [(3)H]D-Asp release, an effect that seems to be caused by nicotine-induced increases in the internalization of AMPA receptors. Indeed, the effects of nicotine-pretreatment were not seen in synaptosomes containing pre-entrapped pep2-SVKI, a peptide known to compete for the binding of GluA2 subunit to scaffolding proteins involved in AMPA endocytosis, while entrapment of pep2-SVKE, an inactive peptide used as negative control, was inefficacious. These findings show that nicotine can negatively modulate the function of AMPA receptors present on glutamatergic nerve terminals in the rat TCN. Dynamic control of AMPA receptors by the nicotinic cholinergic system has been observed under other experimental conditions, and it can contribute to the control of synaptic plasticity such as long-term depression and potentiation. Nicotine's ability to reduce the functionality of presynaptic AMPA receptors could contribute to its analgesic effects by diminishing glutamatergic transmission from the primary afferent terminals that convey nociceptive input to TCN.

Differential roles of signal transduction mechanisms in long-term potentiation of excitatory synaptic transmission induced by activation of group I mGluRs in the spinal trigeminal subnucleus oralis

Group I metabotropic glutamate receptors (mGluR1 and 5) have been implicated in long-term potentiation (LTP), a persistent increase of synaptic efficiency, in the central nervous system including the spinal trigeminal nucleus (Vsp). In the ascending pathway from the caudalis (Vc) to the oralis (Vo) subnuclus in Vsp, it has been shown that the activation of group I mGluRs (mGluR1 and 5) with their agonist (S)-3,5-dihydroxyphenylglycine (DHPG) produces a delayed type of LTP of excitatory synaptic transmission and this LTP was mediated by mGluR1. Further, this study attempts to pharmacologically characterize essential signaling components for the expression of DHPG-induced LTP. As a result, it is found that the group I mGluRs essentially use G protein-mediated activation of the phospholipase C (PLC) pathway to express the LTP. However, recruited signaling molecules following the activation of PLC are differentially involved in the expression of LTP: i.e. IP3 receptor, intracellular Ca(2+) rise, CaMKII and ERK function as positive regulators, whereas PKC as a negative regulator. Furthermore, both L-type voltage-dependent Ca(2+) channel and canonical transient receptor potential channel positively contribute to the expression of LTP. Taken together, these results suggest that signaling molecules recruited by the activation of group I mGluRs collaboratively or oppositely control the optimal expression of synaptic plasticity at excitatory synapses in the Vo.

Large conductance calcium-activated potassium channels: their expression and modulation of glutamate release from nerve terminals isolated from rat trigeminal caudal nucleus and cerebral cortex

Large conductance, calcium-activated potassium channels [big potassium (BK) channel] consist of a tetramer of pore-forming α-subunit and distinct accessory β-subunits (β1-4) that modify the channel's properties. In this study, we analyzed the effects of BK channel activators and blockers on glutamate and γ-aminobutyric acid (GABA) release from synaptosomes isolated from the cerebral cortices or trigeminal caudal nuclei (TCN) of rats. Real-time polymerase chain reaction was used to characterize BK channel α and β(1-4) subunit expression in the cortex and in the trigeminal ganglia (TG), whose neurons project primary terminal afferents into the TCN. Immunocytochemistry was used to localize these subunits on cortical and TCN synaptosomes. The BK channels regulating [(3)H]D-aspartate release from primary afferent nerve terminals projecting into the TCN displayed limited sensitivity to iberiotoxin, whereas those expressed on cortical synaptosomes were highly sensitive to this toxin. BK channels did not appear to be present on GABAergic nerve terminals from the TCN since [(3)H]-γ-aminobutyric acid release in this model was unaffected by BK channel activators or blockers. Gene expression studies revealed expression levels of the α subunit in the TG that were only 31.2 ± 2.1% of those found in cortical tissues. The β4 subunit was the accessory subunit expressed most abundantly in both the cortex and TG. Levels of β1 and β2 were low in both these areas although β2 expression in the TG was higher than that found in the cortex. Immunocytochemistry experiments showed that co-localization of α and β4 subunits (the accessory subunit most abundantly expressed in both brain areas) was more common in TCN synaptosomes than in cortical synaptosomes. On the basis of these findings, it is reasonable to hypothesize that BK channels expressed on glutamatergic terminals in the TCN and cortex have distinct pharmacological profiles, which probably reflect different α and β subunit combinations. Channels in the cortex seem to be composed mainly of α subunits and to a lesser degree by α and β4 subunits, whereas in the TG the α + β4 combination seems to prevail (although α and/or α + β2 channels cannot be excluded). In light of the BK channels' selective control of excitatory transmission and their pharmacological diversity, their effects on primary glutamatergic afferents projecting to TCN represent a potential target for drug therapy of migraines and other types of orofacial pain.

Physiological temperatures drive glutamate release onto trigeminal superficial dorsal horn neurons

Trigeminal sensory afferent fibers terminating in nucleus caudalis (Vc) relay sensory information from craniofacial regions to the brain and are known to express transient receptor potential (TRP) ion channels. TRP channels are activated by H(+), thermal, and chemical stimuli. The present study investigated the relationships among the spontaneous release of glutamate, temperature, and TRPV1 localization at synapses in the Vc. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded from Vc neurons (n = 151) in horizontal brain-stem slices obtained from Sprague-Dawley rats. Neurons had basal sEPSC rates that fell into two distinct frequency categories: High (≥10 Hz) or Low (<10 Hz) at 35°C. Of all recorded neurons, those with High basal release rates (67%) at near-physiological temperatures greatly reduced their sEPSC rate when cooled to 30°C without amplitude changes. Such responses persisted during blockade of action potentials indicating that the High rate of glutamate release arises from presynaptic thermal mechanisms. Neurons with Low basal frequencies (33%) showed minor thermal changes in sEPSC rate that were abolished after addition of TTX, suggesting these responses were indirect and required local circuits. Activation of TRPV1 with capsaicin (100 nM) increased miniature EPSC (mEPSC) frequency in 70% of neurons, but half of these neurons had Low basal mEPSC rates and no temperature sensitivity. Our evidence indicates that normal temperatures (35-37°C) drive spontaneous excitatory synaptic activity within superficial Vc by a mechanism independent of presynaptic action potentials. Thus thermally sensitive inputs on superficial Vc neurons may tonically activate these neurons without afferent stimulation.

Morphology and connections of intratrigeminal cells and axons in the macaque monkey

Trigeminal primary afferent fibers have small receptive fields and discrete submodalities, but second order trigeminal neurons often display larger receptive fields with complex, multimodal responses. Moreover, while most large caliber afferents terminate exclusively in the principal trigeminal nucleus, and pars caudalis (sVc) of the spinal trigeminal nucleus receives almost exclusively small caliber afferents, the characteristics of second order neurons do not always reflect this dichotomy. These surprising characteristics may be due to a network of intratrigeminal connections modifying primary afferent contributions. This study characterizes the distribution and morphology of intratrigeminal cells and axons in a macaque monkeys. Tracer injections centered in the principal nucleus (pV) and adjacent pars oralis retrogradely labeled neurons bilaterally in pars interpolaris (sVi), but only ipsilaterally, in sVc. Labeled axons terminated contralaterally within sVi and caudalis. Features of the intratrigeminal cells in ipsilateral sVc suggest that both nociceptive and non-nociceptive neurons project to principalis. A commissural projection to contralateral principalis was also revealed. Injections into sVc labeled cells and terminals in pV and pars oralis on both sides, indicating the presence of bilateral reciprocal connections. Labeled terminals and cells were also present bilaterally in sVi and in contralateral sVc. Interpolaris injections produced labeling patterns similar to those of sVc. Thus, the rostral and caudal poles of the macaque trigeminal complex are richly interconnected by ipsilateral ascending and descending connections providing an anatomical substrate for complex analysis of oro-facial stimuli. Sparser reciprocal crossed intratrigeminal connections may be important for conjugate reflex movements, such as the corneal blink reflex.

5-HT(1B) receptors inhibit glutamate release from primary afferent terminals in rat medullary dorsal horn neurons

BACKGROUND AND PURPOSE

Although 5-HT(1B) receptors are expressed in trigeminal sensory neurons, it is still not known whether these receptors can modulate nociceptive transmission from primary afferents onto medullary dorsal horn neurons.

EXPERIMENTAL APPROACH

Primary afferent-evoked EPSCs were recorded from medullary dorsal horn neurons of rat horizontal brain stem slices using a conventional whole-cell patch clamp technique under a voltage-clamp condition.

KEY RESULTS

CP93129, a selective 5-HT(1B) receptor agonist, reversibly and concentration-dependently decreased the amplitude of glutamatergic EPSCs and increased the paired-pulse ratio. In addition, CP93129 reduced the frequency of spontaneous miniature EPSCs without affecting the current amplitude. The CP93129-induced inhibition of EPSCs was significantly occluded by GR55562, a 5-HT(1B/1D) receptor antagonist, but not LY310762, a 5-HT(1D) receptor antagonist. Sumatriptan, an anti-migraine drug, also decreased EPSC amplitude, and this effect was partially blocked by either GR55562 or LY310762. On the other hand, primary afferent-evoked EPSCs were mediated by the Ca(2+) influx passing through both presynaptic N-type and P/Q-type Ca(2+) channels. The CP93129-induced inhibition of EPSCs was significantly occluded by ω-conotoxin GVIA, an N-type Ca(2+) channel blocker.

CONCLUSIONS AND IMPLICATIONS

The present results suggest that the activation of presynaptic 5-HT(1B) receptors reduces glutamate release from primary afferent terminals onto medullary dorsal horn neurons, and that 5-HT(1B) receptors could be, at the very least, a potential target for the treatment of pain from orofacial tissues.

NMDA receptor, PKC and ERK prevent fos expression induced by the activation of group I metabotropic glutamate receptors in the spinal trigeminal subnucleus oralis

Fos, a protein product of immediate early gene c-fos, has been used as a marker for activation of nociceptive neurons in central nervous system including spinal trigeminal nucleus (Vsp). By noxious stimulation applied to orofacial area, the expression of Fos occurred in the Vsp pars oralis (Vo), the subnucleus receiving inputs from trigeminal primary afferents that predominantly innervate intraoral receptive fields. The present study demonstrates that the in vitro activation of group I metabotropic glutamate receptors (mGluRs; mGluR1 and 5) by bath-application of their well-known agonist (S)-3,5-dihydroxyphenylglycine (DHPG) increased the number of Fos-expressing neurons in the Vo area. In addition, bath application of DHPG caused inward currents, a parameter of neuronal excitation, in the Vo neurons held at -70 mV in voltage-clamp mode of whole-cell recordings. In further experiments characterizing two phenomena, the increased Fos expression in the Vo was mediated by an additive activation of both mGluR1 and mGluR5, which required the activation of N-methyl-D-aspartate (NMDA) receptors, protein kinase C (PKC) and extracellular signal-regulated kinase (ERK). In contrast, the inward currents were mediated only by mGluR1, but not by others. The data resulting from this in vitro study indicate that the DHPG-induced membrane depolarisation or neuronal excitation may be upstream to, or skip, the NMDA receptor, PKC and ERK pathways for the DHPG-induced Fos expression.

N-methyl-D-aspartate receptor-mediated modulations of the anti-allodynic effects of 5-HT1B/1D receptor stimulation in a rat model of trigeminal neuropathic pain

Previous studies showed that triptans and other 5-HT(1B/1D)-receptor agonists attenuate hyper-responsiveness to mechanical stimulation of the face in a rat model of trigeminal neuropathic pain, probably by activating 5-HT(1B/1D)-receptors on primary afferent nociceptive fibers. We now tested whether blockade of post-synaptic receptors for the excitatory amino acid glutamate released by these fibers would increase this action. We thus evaluated whether (±)1-hydroxy-3-aminopyrrolidine-2-one (HA-966), an antagonist at the glycine/D-serine site of N-methyl-D-aspartate (NMDA)-receptors, would potentiate the anti-allodynic action of dihydroergotamine and zolmitriptan in rats with chronic constriction injury to the infraorbital nerve (CCI-ION). Complementary studies were performed with other NMDA-receptor ligands and in rats with chronic constriction injury to the sciatic nerve (CCI-SN) for comparison. Injury was produced by loose ligatures of the nerves. Responsiveness to mechanical stimulation (vibrissae or hindpaw territories) with von Frey filaments was used to evaluate allodynia 2 weeks after nerve ligature. Rats received NMDA-receptor ligands or saline 20 min before dihydroergotamine (25-100 μg/kg, i.v.) or zolmitriptan (25-100 μg/kg, s.c.). HA-966 (2.5mg/kg, s.c.), inactive on its own, enhanced the anti-allodynic effects of dihydroergotamine (eightfold increase) and zolmitriptan (threefold increase) in CCI-ION rats, but these drugs exerted no effects in allodynic CCI-SN rats. NMDA-receptor blockade by memantine (5mg/kg, i.p.) also enhanced, whereas activation at glycine/NMDA site by D-cycloserine (3mg/kg, i.p.) reduced the anti-allodynic properties of zolmitriptan in CCI-ION rats. Combined administration of NMDA-receptor antagonist and 5-HT(1B/1D)-receptor agonist may be a promising approach for alleviating trigeminal neuropathic pain.

Signal transduction mechanisms underlying group I mGluR-mediated increase in frequency and amplitude of spontaneous EPSCs in the spinal trigeminal subnucleus oralis of the rat

Group I mGluRs (mGluR1 and 5) pre- and/or postsynaptically regulate synaptic transmission at glutamatergic synapses. By recording spontaneous EPSCs (sEPSCs) in the spinal trigeminal subnucleus oralis (Vo), we here investigated the regulation of glutamatergic transmission through the activation of group I mGluRs. Bath-applied DHPG (10 μM/5 min), activating the group I mGluRs, increased sEPSCs both in frequency and amplitude; particularly, the increased amplitude was long-lasting. The DHPG-induced increases of sEPSC frequency and amplitude were not NMDA receptor-dependent. The DHPG-induced increase in the frequency of sEPSCs, the presynaptic effect being further confirmed by the DHPG effect on paired-pulse ratio of trigeminal tract-evoked EPSCs, an index of presynaptic modulation, was significantly but partially reduced by blockades of voltage-dependent sodium channel, mGluR1 or mGluR5. Interestingly, PKC inhibition markedly enhanced the DHPG-induced increase of sEPSC frequency, which was mainly accomplished through mGluR1, indicating an inhibitory role of PKC. In contrast, the DHPG-induced increase of sEPSC amplitude was not affected by mGluR1 or mGluR5 antagonists although the long-lasting property of the increase was disappeared; however, the increase was completely inhibited by blocking both mGluR1 and mGluR5. Further study of signal transduction mechanisms revealed that PLC and CaMKII mediated the increases of sEPSC in both frequency and amplitude by DHPG, while IP₃ receptor, NO and ERK only that of amplitude during DHPG application. Altogether, these results indicate that the activation of group I mGluRs and their signal transduction pathways differentially regulate glutamate release and synaptic responses in Vo, thereby contributing to the processing of somatosensory signals from orofacial region.

N-methyl-D-aspartate-dependent long-term potentiation of excitatory transmission in trigeminal subnucleus oralis

This study for the first time demonstrates early developmental changes of passive/active membrane properties, and long-term potentiation (LTP) of excitatory synaptic transmission at spinal trigeminal subnucleus caudalis (Vc)-to-oralis (Vo) synapses. During postnatal development, the probability of Vo neurons with monosynaptic excitatory postsynaptic currents (EPSCs) upon Vc stimulation significantly increased, whereas the input resistances of Vo neurons and the latencies of monosynaptic EPSCs significantly decreased. Application of a 'pairing' protocol that comprises 2 Hz-conditioning stimulation of Vc with postsynaptic depolarization of Vo neuron to +30 mV generated LTP of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor-mediated monosynaptic EPSC amplitude in more than 70% of Vo neurons. The induction of LTP required the activation of N-methyl-D-aspartate receptor, but its magnitudes had correlation neither with postnatal ages nor with baseline EPSC amplitudes.

GABAA receptor-mediated tonic currents in substantia gelatinosa neurons of rat spinal trigeminal nucleus pars caudalis

In the present study, we describe GABAA receptor-mediated tonic inhibitory currents in the substantia gelatinosa (SG) region of rat spinal trigeminal nucleus pars caudalis (Vc). The GABA(A) receptor-mediated tonic currents were identified by bath-application of the GABAA receptor antagonists, picrotoxin (1mM), SR95531 (100microM) and bicuculline (100microM). All three antagonists completely blocked outward spontaneous (phasic) inhibitory postsynaptic currents, but only picrotoxin and bicuculline induced a significant (>5pA) inward shift of holding currents at a holding potential (Vh) of 0mV in 60-70% of SG neurons, revealing the existence of tonic outward currents. The tonic currents were resistant to further the blockades of glycine receptors or those in addition to glutamate receptors and voltage-dependent sodium channels. An acute bath-application of THDOC (0.1microM), the stress-related neurosteroid, did enhance tonic currents, but only in a small population of SG neurons. In addition, slices incubated with THDOC for 30min increased the probability of neurons with significant tonic currents. The GABAergic tonic inhibition demonstrated in this study may play a significant role in the sensory processing system of the Vc.