A pharmacological study of the modulation of neuronal and behavioural nociceptive responses in the rat trigeminal region

Locus coeruleus-noradrenergic modulation of trigeminal pain: Implications for trigeminal neuralgia and psychiatric comorbidities

Trigeminal neuralgia is the most common neuropathic pain involving the craniofacial region. Due to the complex pathophysiology, it is therapeutically difficult to manage. Noradrenaline plays an essential role in the modulation of arousal, attention, cognitive function, stress, and pain. The locus coeruleus, the largest source of noradrenaline in the brain, is involved in the sensory and emotional processing of pain. This review summarizes the knowledge about the involvement of noradrenaline in acute and chronic trigeminal pain conditions and how the activity of the locus coeruleus noradrenergic neurons changes in response to acute and chronic pain conditions and how these changes might be involved in pain-related comorbidities including anxiety, depression, and sleep disturbance.

Clonidine Reduces Nociceptive Responses in Mouse Orofacial Formalin Model: Potentiation by Sigma-1 Receptor Antagonist BD1047 without Impaired Motor Coordination

Although the administration of clonidine, an alpha-2 adrenoceptor agonist, significantly attenuates nociception and hyperalgesia in several pain models, clinical trials of clonidine are limited by its side effects such as drowsiness, hypotension and sedation. Recently, we determined that the sigma-1 receptor antagonist BD1047 dose-dependently reduced nociceptive responses in a mouse orofacial formalin model. Here we examined whether intraperitoneal injection of clonidine suppressed the nociceptive responses in the orofacial formalin test, and whether co-administration with BD1047 enhances lower-dose clonidine-induced anti-nociceptive effects without the disruption of motor coordination and blood pressure. Formalin (5%, 10 µL) was subcutaneously injected into the right upper lip, and the rubbing responses with the ipsilateral fore- or hind-paw were counted for 45 min. Clonidine (10, 30 or 100 µg/kg) was intraperitoneally administered 30 min before formalin injection. Clonidine alone dose-dependently reduced nociceptive responses in both the first and second phases. Co-localization for alpha-2A adrenoceptors and sigma-1 receptors was determined in trigeminal ganglion cells. Interestingly, the sub-effective dose of BD1047 (3 mg/kg) significantly potentiated the anti-nociceptive effect of lower-dose clonidine (10 or 30 µg/kg) in the second phase. In particular, the middle dose of clonidine (30 µg/kg) in combination with BD1047 produced an anti-nociceptive effect similar to that of the high-dose clonidine, but without a significant motor dysfunction or hypotension. In contrast, mice treated with the high dose of clonidine developed severe impairment in motor coordination and blood pressure. These data suggest that a combination of low-dose clonidine with BD1047 may be a novel and safe therapeutic strategy for orofacial pain management.

Functional neuroanatomy of the central noradrenergic system

The central noradrenergic neurone, like the peripheral sympathetic neurone, is characterized by a diffusely arborizing terminal axonal network. The central neurones aggregate in distinct brainstem nuclei, of which the locus coeruleus (LC) is the most prominent. LC neurones project widely to most areas of the neuraxis, where they mediate dual effects: neuronal excitation by α₁-adrenoceptors and inhibition by α₂-adrenoceptors. The LC plays an important role in physiological regulatory networks. In the sleep/arousal network the LC promotes wakefulness, via excitatory projections to the cerebral cortex and other wakefulness-promoting nuclei, and inhibitory projections to sleep-promoting nuclei. The LC, together with other pontine noradrenergic nuclei, modulates autonomic functions by excitatory projections to preganglionic sympathetic, and inhibitory projections to preganglionic parasympathetic neurones. The LC also modulates the acute effects of light on physiological functions ('photomodulation'): stimulation of arousal and sympathetic activity by light via the LC opposes the inhibitory effects of light mediated by the ventrolateral preoptic nucleus on arousal and by the paraventricular nucleus on sympathetic activity. Photostimulation of arousal by light via the LC may enable diurnal animals to function during daytime. LC neurones degenerate early and progressively in Parkinson's disease and Alzheimer's disease, leading to cognitive impairment, depression and sleep disturbance.

Functional neuroanatomy of the noradrenergic locus coeruleus: its roles in the regulation of arousal and autonomic function part I: principles of functional organisation

The locus coeruleus (LC) is the major noradrenergic nucleus of the brain, giving rise to fibres innervating extensive areas throughout the neuraxis. Recent advances in neuroscience have resulted in the unravelling of the neuronal circuits controlling a number of physiological functions in which the LC plays a central role. Two such functions are the regulation of arousal and autonomic activity, which are inseparably linked largely via the involvement of the LC. The LC is a major wakefulness-promoting nucleus, resulting from dense excitatory projections to the majority of the cerebral cortex, cholinergic neurones of the basal forebrain, cortically-projecting neurones of the thalamus, serotoninergic neurones of the dorsal raphe and cholinergic neurones of the pedunculopontine and laterodorsal tegmental nucleus, and substantial inhibitory projections to sleep-promoting GABAergic neurones of the basal forebrain and ventrolateral preoptic area. Activation of the LC thus results in the enhancement of alertness through the innervation of these varied nuclei. The importance of the LC in controlling autonomic function results from both direct projections to the spinal cord and projections to autonomic nuclei including the dorsal motor nucleus of the vagus, the nucleus ambiguus, the rostroventrolateral medulla, the Edinger-Westphal nucleus, the caudal raphe, the salivatory nuclei, the paraventricular nucleus, and the amygdala. LC activation produces an increase in sympathetic activity and a decrease in parasympathetic activity via these projections. Alterations in LC activity therefore result in complex patterns of neuronal activity throughout the brain, observed as changes in measures of arousal and autonomic function.

Nitric oxide/cGMP pathway mediates orofacial antinociception induced by electroacupuncture at the St36 acupoint

The aim of the present study was to test the hypothesis that electroacupuncture (EA) at acupoint St36 induces antinociception by activation of the L-arg/NO/cGMP pathway. Nociception was produced by thermal stimuli applied to the face of Wistar rats and latency was measured by face withdrawal. Electric stimulation of acupoint St36 for 20 min induced antinociception in this model, which was maintained for 150 min. For comparison, a so-called dry needle group (DN) was used, which received needling at the same point without stimulation. The antinociception obtained by stimulation of acupoint St36 was only achieved when high frequency (100 Hz) was used, whereas low frequencies (5 and 30 Hz) were not capable of achieving this effect. Subcutaneous administration of both inhibitors of NO synthase (N-nitro-L-arginine) and guanylyl cyclase (ODQ) and intraperitoneal administration of specific antagonists of neuronal NO synthase (L-NNA) and inductible NO synthase (aminoguanidine) antagonized the antinociception induced by St36 stimulation. The results of this paper suggest that stimulation of acupoint St36 at high frequency induces antinociception, which seems to be related to L-arg/NO/cGMP pathway activation.

Successful treatment of trigeminal neuralgia with milnacipran

Paroxysmal pain in a 64-year-old woman diagnosed with trigeminal neuralgia disappeared with the administration of carbamazepine, but carbamazepine had to be discontinued because of intolerable lassitude and liver dysfunction. Afterward, the paroxysmal pain reoccurred, and depressive symptoms appeared. Milnacipran was then administered at a dosage of 50 mg/d for 2 months, and the paroxysmal pain and depression disappeared completely. Carbamazepine is the drug of first choice for trigeminal neuralgia, but the present results suggest that milnacipran is worth investigating for patients who do not respond to carbamazepine, who cannot stay on carbamazepine because of side effects, and who exhibit depressive symptoms.

Activation of alpha2-adrenoceptors in the trigeminal region produces sex-specific modulation of nociception in the rat

Sex-related differences in the sensitivity to pain and in the response to analgesics have been reported including higher perceptual responses to experimentally induced pain and the higher prevalence of many pain syndromes in women compared with men. This study examines whether alpha2-adrenoceptor-mediated antinociceptive effects are reduced by estrogen which could account for the sex-related differences in pain perception and modulation. Clonidine, an alpha2-adrenoceptor agonist, has been shown to inhibit noxious stimulus-evoked nociceptive behavior as well as the responses of nociceptive neurons in the medullary dorsal horn. Intracisternal microinjection of clonidine (7 microg/5 microl) through the implanted PE-10 cannulae dorsal to the trigeminal region in male, ovariectomized (OVX), and diestrous (DiE) Sprague-Dawley rats produced a strong antinociceptive effect on N-methyl-D-aspartic acid (NMDA)-induced nociceptive scratching behavior and heat-induced face withdrawal nociceptive tests. However, it failed to produce any inhibition in the estradiol-treated ovariectomized (OVX+E) group regardless of the dose of estradiol (1, 10 or 100 microg/100 microl sesame oil) or in the proestrous (ProE) group. Further, clonidine produced dose-dependent effects in male and OVX groups but not in the OVX+E group on the NMDA-induced nociceptive behavior. Finally, the effect of clonidine was reversed by yohimbine, an alpha2-adrenoceptor antagonist, in male and OVX groups on thermal nociceptive test. These results lead us to conclude that activation of alpha2-adrenoceptors produces sex-specific, estrogen dependent modulation of nociception in the trigeminal region of the rat. A decreased alpha2-adrenoceptor-mediated inhibition could be one of the factors responsible for the higher prevalence of pain syndromes in females.

Neuronal activity regulates viral replication of herpes simplex virus type 1 in the nervous system

Herpes simplex virus types 1 and 2 (HSV-1, -2) infect and also establish latency in neurons. In the present study, the authors investigated the influence of neuronal activity on the replication of HSV-1. The results showed that the sodium channel blocker tetrodotoxin (TTX) and the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) could significantly increase viral replication in primary neuronal cultures, by two- to fourfold. In contrast, KCl reduced viral production by at least 80% in the same cultures. Inhibitors of GABA(A) receptors completely abolished the effects of GABA. Intravitreously injected TTX in a mouse corneal scarification model enhanced the viral titers > 10-fold in both the trigeminal ganglia and the brain. At 2 h post infection, both TTX and GABA significantly up-regulated the levels of transcription for the viral immediate early (IE) genes ICP0, ICP4, and ICP27, as revealed by real time PCR. These results indicate that the neuronal excitation status may dictate the efficiency of HSV-1 viral replication, probably by regulating the levels of viral IE gene expression. These are the first findings connecting neuronal activity to the molecular mechanisms of HSV replication in the nervous system, which may significantly influence our view of herpesvirus infection and latency.

Trigeminal somatosensory innervation of the head of a teleost fish with particular reference to nociception

Trigeminal somatosensory receptors have not been characterised in teleost fish and studies in elasmobranchs have failed to identify nociceptors. The present study examined the trigeminal nerve of a teleost fish, the rainbow trout (Oncorhynchus mykiss) to determine what types of somatosensory receptors were present on the head of the trout specifically searching for nociceptors. Single unit recordings were made from receptive fields on the head of the fish innervated by the trigeminal nerve. Each receptive field was tested for sensitivity to mechanical, thermal and chemical stimulation. Five different receptor types were found: fast adapting receptors responding to mechanical stimulation; slowly adapting receptors responding to mechanical stimuli; polymodal nociceptors responding to mechanical, noxious thermal and chemical stimulation; mechanothermal nociceptors responding to mechanical stimulation and noxious heat; and mechanochemical receptors responsive to mechanical and chemical stimulation. Mechanical thresholds, receptive field diameter, conduction velocities and thermal thresholds of the receptors were determined and there was no significant difference between the receptor types in terms of these properties. Three shapes of action potential (AP) were recorded from these receptors: type 1 with no inflexion; type 2 with an inflexion on depolarisation; and type 3 with an inflexion on repolarisation. Conduction velocity, amplitude and duration of the APs, afterhypolarisation amplitude and duration, as well as the maximum rate of depolarisation were measured for each action potential type. No major differences were found when making comparisons within receptor type and between receptor types. The fish nociceptors had similar physiological properties to nociceptors found in higher vertebrates.

Effect of antisense knock-down of alpha(2a)- and alpha(2c)-adrenoceptors on the antinociceptive action of clonidine on trigeminal nociception in the rat

Although activation of alpha(2)-adrenoceptors is known to play an important role in mediating antinociception, the contribution of various alpha(2)-adrenoceptor subtypes in modulating trigeminal nociception remains unknown since subtype specific agonists and antagonists are not available. The present study investigated the functional role of alpha(2)-adrenoceptor subtypes in modulating the N-methyl-D-aspartate-induced nociceptive behavior in the medullary dorsal horn by using antisense oligodeoxynucleotides to selectively knock-down the receptor subtypes. Microinjection of N-methyl-D-aspartate (2 nmol in 10 microl) through a cannula implanted dorsal to the medullary dorsal horn produced a total of 164.9+/-8.8 scratches in the facial region (n=14), and the scratching behavior lasted for 77.8+/-5.2s (n=14). Microinjection of clonidine, an alpha(2)-agonist (7 microg in 5 microl), 15 min prior to administration of N-methyl-D-aspartate, produced a reduction of 71.6% (n=12) in the number of scratches and a reduction of 57.5% (n=12) in the duration. The inhibitory effect of clonidine was blocked by idazoxan (n=4) and yohimbine (n=4), alpha(2) antagonists. In rats pretreated with the antisense probe to the alpha(2A) adrenoceptor, clonidine only produced a reduction of 7.3% in the number of scratches (n=12) and a reduction of 9% in the duration (n=12). The antinociceptive effect of clonidine recovered completely 4 days after termination of the alpha(2A) antisense oligodeoxynucleotide treatment. In contrast to the alpha(2A) antisense-treated animals, clonidine reduced the number of scratches and the duration by 85.5% (n=9) and 82.1% (n=9), respectively, in rats pretreated with the sense probe to the alpha(2A) adrenoceptor. The effect of clonidine was not altered in rats pretreated with the antisense or the sense probes to the alpha(2C) adrenoceptor. In the alpha(2C) antisense pretreated rats, clonidine reduced the number of scratches and the duration by 60.8% (n=11) and 44.5 % (n=11), respectively. In the sense-pretreated rats, clonidine produced a reduction of 69.1% in the number of scratches (n=9) and a reduction of 55.1% in the duration (n=9). In order to assess the effectiveness of the antisense treatment, the receptor expression was examined by immunohistochemistry. Antisense treatment reduced alpha(2A) and alpha(2C) receptor immunoreactivity in the medullary dorsal horn compared to the sense and the vehicle-treated animals. Quantitative image analysis revealed a significant decrease in pixel intensity following the antisense treatment. These results indicate that activation of alpha(2A) adrenoceptor plays an important role in mediating the antinociceptive effect of clonidine in the medullary dorsal horn in the rat.

Intrathecal administration of 5-HT(3) receptor agonist modulates jaw muscle activity evoked by injection of mustard oil into the temporomandibular joint in the rat

The effect of intrathecal administration of the 5-HT(3) receptor agonist 2-methyl-5-hydroxytryptamine (2m-5HT) on jaw muscle activity evoked by mustard oil (MO) injection into the temporomandibular joint of anesthetized rats was examined. One microgram or 100 microg of 2m-5HT significantly enhanced or suppressed jaw muscle responses, respectively. Pre-administration of tropisetron, a 5-HT(3) receptor antagonist, attenuated the effect of 2m-5HT. These results indicate that activation of 5-HT(3) receptors can modulate trigeminal nociceptive responses.

Interaction of nitrous oxide and propofol to reduce hypertensive response to stimulation

PURPOSE

To evaluate the interaction between nitrous oxide and propofol for the suppression of hypertension following electrical stimulation of the mental nerve in the rabbit.

METHODS

Male Japan White rabbits were tracheostomized, cannulated and mechanically ventilated under isoflurane anesthesia. Square wave pulses (5 V, 0.5 msec, 50 Hz for 5 sec) were delivered to the left mental nerve. Animals received nitrous oxide 20, 40, 60 and 80% (Group 1); propofol 200, 400, 600 and 800 microg x kg(-1) min(-1) (Group 2); or combinations of nitrous oxide and propofol at 10 + 100, 20 + 200, 30 + 300 and 40 % + 400 microg x kg(-1) x min(-1) (Group 3). Systolic blood pressure was recorded from immediately before to maximal increase following nerve stimulation. Probit analysis was used to obtain ED(50) values for 50% suppression of blood pressure elevation. Isobolographic analysis was used to evaluate the interaction between nitrous oxide and propofol.

RESULTS

ED(50) values are 52.9% for nitrous oxide (Group 1), 464.1 microg x kg(-1) min(-1) for propofol (Group 2), 21.7 % + 217.1 microg x kg(-1) min(-1) for nitrous oxide and propofol combination (Group 3) and 24. 7 % + 247.1 microg x kg(-1) x min(-1) for the theoretically additive combination of nitrous oxide and propofol, respectively.

CONCLUSION

The interaction between nitrous oxide and propofol for the suppression of blood pressure elevation following electrical stimulation of the mental nerve is additive.

Dopamine reuptake inhibition in the rostral agranular insular cortex produces antinociception

We provide evidence for an antinociceptive effect of dopamine in the rat cerebral cortex that is mediated through descending nociceptive inhibition of spinal neurons. Injection of the dopamine reuptake inhibitor GBR-12935 in the rostral agranular insular cortex (RAIC), a cortical area that receives a dense dopaminergic projection and is involved in descending antinociception (Burkey et al.,1996), resulted in dose-dependent inhibition of formalin-induced nociceptive behavior, without any alteration of motor function. Injection of the dopamine reuptake inhibitor in the surrounding cortical areas had no effect on nociceptive behaviors. GBR-12935 also produced a reduction in noxious stimulus-induced c-fos expression in nociceptive areas of the spinal dorsal horn, suggesting that dopamine in the RAIC acts in part through descending antinociception. Electrophysiological recording from single wide dynamic range-type spinal dorsal horn neurons confirmed the descending nociceptive inhibitory effect. GBR-12935 in the RAIC significantly reduced neuronal responses evoked by noxious thermal stimulation of the skin, an effect that was reversed by local administration of the selective D1 receptor antagonist SCH-23390. Finally, administration of SCH-23390 alone in the RAIC decreased paw withdrawal latencies from noxious heat, suggesting that dopamine acts tonically in the cortex to inhibit nociception.

Paradoxical inhibition of nociceptive neurons in the dorsal horn of the rat spinal cord during a nociceptive hindlimb reflex

Nociceptive-specific and multireceptive neurons in the lumbar dorsal horn are excited by noxious stimuli applied to the hindpaw and inhibited by noxious stimuli applied to distant body regions. Given that at least a subset of these neurons are part of the circuit for nociceptive reflexes, inhibition of nociceptive-specific and multireceptive neurons should inhibit nociceptive reflexes. Unfortunately, previous attempts to test this hypothesis have been inconclusive because of methodological differences between electrophysiological and behavioral experiments. The present study overcame this problem by recording neural and reflex activity simultaneously. Rats were anesthetized with halothane and surgically prepared for single-unit recording from the lumbar dorsal horn. Hindpaw heat caused a burst of activity that reliably preceded hindpaw withdrawal in 10 nociceptive-specific and 17 multireceptive neurons. A distant noxious stimulus (tail in 50 degrees C water or ear pinch) inhibited the evoked activity of both nociceptive-specific and multireceptive neurons and simultaneously changed the topography of the hindpaw reflex from flexion to extension without altering reflex latency. The present data are consistent with previous reports of inhibition of nociceptive-specific and multireceptive neurons during application of a distant noxious stimulus. However, inhibition of nociceptive-specific and multireceptive neurons concomitant with a shift in the hindlimb reflex from flexion to extension suggests that these neurons are part of the circuit for flexor reflexes specifically. Presumably, lateral inhibition from the flexor to extensor circuit allows for the release of hindlimb extension when neurons in the flexion circuit are inhibited by a distant noxious stimulus. Such a system reduces the chance of injury by allowing for withdrawal reflexes to a single noxious stimulus and escape reactions, such as running and jumping, to multiple noxious stimuli.

Midbrain periaqueductal gray (PAG) inhibits nociceptive inputs to sacral dorsal horn nociceptive neurons through alpha2-adrenergic receptors

Modulation of sacral spinal dorsal horn neurons by the ventrolateral PAG was studied by extracellular recording combined with microiontophoretic applications of alpha-adrenergic agonists or antagonists. Bicuculline (BIC, 15 ng) microinjected into the ventrolateral PAG produced a consistent inhibition of the responses of nociceptive dorsal horn neurons. After PAG-BIC applications, the total number of spikes per heat stimulation period was significantly decreased to a mean of 37 +/- 19% (n = 8) of the pre-BIC control. Local iontophoresis of the selective alpha2-adrenoceptor antagonists idazoxan or yohimbine but not the selective alpha1 antagonist benoxathian significantly reversed PAG-BIC-evoked inhibition. At low ejection currents, clonidine, an alpha2-adrenoceptor agonist, markedly reduced noxious heat-evoked responses but had no consistent action on the responses to iontophoresed excitatory amino acids [EAA; N-methyl--aspartate (NMDA) or kainic acid]. At ejection currents higher than required to block descending inhibition, idazoxan potentiated responses to both heat and EAA iontophoresis. At higher ejection currents, EAA responses were inhibited by clonidine. This indicates that both presynaptic and postsynaptic alpha2 receptors are capable of inhibiting the recorded neurons. Activation of the alpha1 adrenoceptors by iontophoresis of methoxamine often led to a marked increase in the responses to kainic acid and, to a lesser extent, to NMDA iontophoresis or noxious heat. Together with previously reported work, the current experiments demonstrate that PAG neurons inhibit nociceptive dorsal horn neurons primarily through an indirect alpha2 adrenoceptor mechanism. In this same population of dorsal horn neurons, norepinephrine has a direct alpha1-mediated excitatory effect.

alpha2-adrenoceptors modulate NMDA-evoked responses of neurons in superficial and deeper dorsal horn of the medulla

Extracellular single unit recordings were made from neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in 21 male rats anesthetized with urethan. NMDA produced an antagonist-reversible excitation of 46 nociceptive as well as nonnociceptive neurons. Microiontophoretic application of a preferential alpha2-adrenoceptor (alpha2AR) agonist, (2-[2, 6-dichloroaniline]-2-imidazoline) hydrochloride (clonidine), reduced the NMDA-evoked responses of 86% (6/7) of nociceptive-specific (NS) neurons, 82% (9/11) of wide dynamic range (WDR) neurons, and 67% (4/6) of low-threshold (LT) neurons in the superficial dorsal horn. In the deeper dorsal horn, clonidine inhibited the NMDA-evoked responses of 94% (16/17) of NS and WDR neurons and 60% (3/5) of LT neurons. Clonidine facilitated the NMDA-evoked responses in 14% (1/17) of NS, 9% (1/11) of WDR, and 33% (2/6) of LT neurons in the superficial dorsal horn. Idazoxan, an alpha2AR antagonist, reversed the inhibitory effect of clonidine in 90% (9/10) of neurons, whereas prazosin, an alpha1-adrenoceptor antagonist with affinity for alpha2BAR, and alpha2CAR, were ineffective. We suggest that activation of alpha2ARs produces a predominantly inhibitory modulation of the NMDA-evoked responses of nociceptive neurons in the medullary dorsal horn.

Direct comparison of heat-evoked activity of nociceptive neurons in the dorsal horn with the hindpaw withdrawal reflex in the rat

Although the sensory coding of nociceptive neurons in the dorsal horn has been studied extensively, surprisingly little is known about how these neurons contribute to nociceptive reflexes. The objective of the present study was to examine the characteristics of dorsal horn neurons capable of initiating hindpaw withdrawal. To this end, neural and reflex activity were measured simultaneously in response to noxious radiant heat applied to the hindpaw in lightly anesthetized rats. Subsets of both multireceptive (MR; 52/95) and nociceptive-specific (NS; 19/46) neurons showed a consistent burst of activity that preceded the reflex. However, when compared with NS neurons, MR neurons as a group were: more likely to be active before the reflex (55 vs. 41%); more active before the reflex (31 vs. 23 Hz); and active earlier (2.8 vs. 2.3 s before the reflex). Subsets of MR neurons were active before the reflex regardless of receptive field size or location in the dorsal horn. In contrast, NS neurons with small receptive fields or those located outside of superficial laminae were rarely active before the reflex and thus unlikely to be part of the reflex circuit. These results suggest that current classification schemes, in particular MR and NS categories, cannot be used as the sole criterion to predict involvement in nociceptive reflexes. However, simultaneous measurement of neural and reflex activity provides an opportunity to determine the characteristics of nociceptive neurons involved in withdrawal reflexes.