Brainstem neurons expressing c-Fos immunoreactivity following irritant chemical stimulation of the rat's tongue

Separation of Oral Cooling and Warming Requires TRPM8

Cooling sensations arise inside the mouth during ingestive and homeostasis behaviors. Oral presence of cooling temperature engages the cold and menthol receptor TRPM8 (transient receptor potential melastatin 8) on trigeminal afferents. Yet, how TRPM8 influences brain and behavioral responses to oral temperature is undefined. Here we used in vivo neurophysiology to record action potentials stimulated by cooling and warming of oral tissues from trigeminal nucleus caudalis neurons in female and male wild-type and TRPM8 gene deficient mice. Using these lines, we also measured orobehavioral licking responses to cool and warm water in a novel, temperature-controlled fluid choice test. Capture of antidromic electrophysiological responses to thalamic stimulation identified that wild-type central trigeminal neurons showed diverse responses to oral cooling. Some neurons displayed relatively strong excitation to cold <10°C (COLD neurons) while others responded to only a segment of mild cool temperatures below 30°C (COOL neurons). Notably, TRPM8 deficient mice retained COLD-type but lacked COOL cells. This deficit impaired population responses to mild cooling temperatures below 30°C and allowed warmth-like (≥35°C) neural activity to pervade the normally innocuous cool temperature range, predicting TRPM8 deficient mice would show anomalously similar orobehavioral responses to warm and cool temperatures. Accordingly, TRPM8 deficient mice avoided both warm (35°C) and mild cool (≤30°C) water and sought colder temperatures in fluid licking tests, whereas control mice avoided warm but were indifferent to mild cool and colder water. Results imply TRPM8 input separates cool from warm temperature sensing and suggest other thermoreceptors also participate in oral cooling sensation.

Sensory Effects of Nicotine and Tobacco

INTRODUCTION

Ingestion of nicotine by smoking, vaping, or other means elicits various effects including reward, antinociception, and aversion due to irritation, bitter taste, and unpleasant side effects such as nausea and dizziness.

AIMS AND METHODS

Here we review the sensory effects of nicotine and the underlying neurobiological processes.

RESULTS AND CONCLUSIONS

Nicotine elicits oral irritation and pain via the activation of neuronal nicotinic acetylcholine receptors (nAChRs) expressed by trigeminal nociceptors. These nociceptors excite neurons in the trigeminal subnucleus caudalis (Vc) and other brainstem regions in a manner that is significantly reduced by the nAChR antagonist mecamylamine. Vc neurons are excited by lingual application of nicotine and exhibit a progressive decline in firing to subsequent applications, consistent with desensitization of peripheral sensory neurons and progressively declining ratings of oral irritation in human psychophysical experiments. Nicotine also elicits a nAChR-mediated bitter taste via excitation of gustatory afferents. Nicotine solutions are avoided even when sweeteners are added. Studies employing oral self-administration have yielded mixed results: Some studies show avoidance of nicotine while others report increased nicotine intake over time, particularly in adolescents and females. Nicotine is consistently reported to increase human pain threshold and tolerance levels. In animal studies, nicotine is antinociceptive when delivered by inhalation of tobacco smoke or systemic infusion, intrathecally, and by intracranial microinjection in the pedunculopontine tegmentum, ventrolateral periaqueductal gray, and rostral ventromedial medulla. The antinociception is thought to be mediated by descending inhibition of spinal nociceptive transmission. Menthol cross-desensitizes nicotine-evoked oral irritation, reducing harshness that may account for its popularity as a flavor additive to tobacco products.

IMPLICATIONS

Nicotine activates brain systems underlying reward and antinociception, but at the same time elicits aversive sensory effects including oral irritation and pain, bitter taste, and other unpleasant side effects mediated largely by nicotinic acetylcholine receptors (nAChRs). This review discusses the competing aversive and antinociceptive effects of nicotine and exposure to tobacco smoke, and the underlying neurobiology. An improved understanding of the interacting effects of nicotine will hopefully inform novel approaches to mitigate nicotine and tobacco use.

Enhanced Ocular Surface and Intraoral Nociception via a Transient Receptor Potential Vanilloid 1 Mechanism in a Rat Model of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA), characterized by low arterial oxygen saturation during sleep, is associated with an increased risk of orofacial pain. In this study, we simulated chronic intermittent hypoxia (CIH) during the sleep/rest phase (light phase) to determine the role of transient receptor potential vanilloid 1 (TRPV1) in mediating enhanced orofacial nocifensive behavior and trigeminal spinal subnucleus caudalis (Vc) neuronal responses to capsaicin (a TRPV1 agonist) stimulation in a rat model of OSA. Rats were subjected to CIH (nadir O₂, 5%) during the light phase for 8 or 16 consecutive days. CIH yielded enhanced behavioral responses to capsaicin after application to the ocular surface and intraoral mucosa, which was reversed under normoxic conditions. The percentage of TRPV1-immunoreactive trigeminal ganglion neurons was greater in CIH rats than in normoxic rats and recovered under normoxic conditions after CIH. The ratio of large-sized TRPV1-immunoreactive trigeminal ganglion neurons increased in CIH rats. The density of TRPV1 positive primary afferent terminals in the superficial laminae of Vc was higher in CIH rats. Phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive cells intermingled with the central terminal of TRPV1-positive afferents in the Vc. The number of pERK-immunoreactive cells following low-dose capsaicin (0.33 µM) application to the tongue was significantly greater in the middle portion of the Vc of CIH rats than of normoxic rats and recovered under normoxic conditions after CIH. These data suggest that CIH during the sleep (light) phase is sufficient to transiently enhance pain on the ocular surface and intraoral mucosa via TRPV1-dependent mechanisms.

Reflex regulation of breathing by the paratrigeminal nucleus via multiple bulbar circuits

Sensory neurons of the jugular vagal ganglia innervate the respiratory tract and project to the poorly studied medullary paratrigeminal nucleus. In the present study, we used neuroanatomical tracing, pharmacology and physiology in guinea pig to investigate the paratrigeminal neural circuits mediating jugular ganglia-evoked respiratory reflexes. Retrogradely traced laryngeal jugular ganglia neurons were largely (> 60%) unmyelinated and expressed the neuropeptide substance P and calcitonin gene-related peptide, although a population (~ 30%) of larger diameter myelinated jugular neurons was defined by the expression of vGlut1. Within the brainstem, vagal afferent terminals were confined to the caudal two-thirds of the paratrigeminal nucleus. Electrical stimulation of the laryngeal mucosa evoked a vagally mediated respiratory slowing that was mimicked by laryngeal capsaicin application. These laryngeal reflexes were modestly reduced by neuropeptide receptor antagonist microinjections into the paratrigeminal nucleus, but abolished by ionotropic glutamate receptor antagonists. D,L-Homocysteic acid microinjections into the paratrigeminal nucleus mimicked the laryngeal-evoked respiratory slowing, whereas capsaicin microinjections evoked a persistent tachypnoea that was insensitive to glutamatergic inhibition but abolished by neuropeptide receptor antagonists. Extensive projections from paratrigeminal neurons were anterogradely traced throughout the pontomedullary respiratory column. Dual retrograde tracing from pontine and ventrolateral medullary termination sites, as well as immunohistochemical staining for calbindin and neurokinin 1 receptors, supported the existence of different subpopulations of paratrigeminal neurons. Collectively, these data provide anatomical and functional evidence for at least two types of post-synaptic paratrigeminal neurons involved in respiratory reflexes, highlighting an unrecognised complexity in sensory processing in this region of the brainstem.

VGLUT1 or VGLUT2 mRNA-positive neurons in spinal trigeminal nucleus provide collateral projections to both the thalamus and the parabrachial nucleus in rats

The trigemino-thalamic (T-T) and trigemino-parabrachial (T-P) pathways are strongly implicated in the sensory-discriminative and affective/emotional aspects of orofacial pain, respectively. These T-T and T-P projection fibers originate from the spinal trigeminal nucleus (Vsp). We previously determined that many vesicular glutamate transporter (VGLUT1 and/or VGLUT2) mRNA-positive neurons were distributed in the Vsp of the adult rat, and most of these neurons sent their axons to the thalamus or cerebellum. However, whether VGLUT1 or VGLUT2 mRNA-positive projection neurons exist that send their axons to both the thalamus and the parabrachial nucleus (PBN) has not been reported. Thus, in the present study, dual retrograde tract tracing was used in combination with fluorescence in situ hybridization (FISH) for VGLUT1 or VGLUT2 mRNA to identify the existence of VGLUT1 or VGLUT2 mRNA neurons that send collateral projections to both the thalamus and the PBN. Neurons in the Vsp that send collateral projections to both the thalamus and the PBN were mainly VGLUT2 mRNA-positive, with a proportion of 90.3%, 93.0% and 85.4% in the oral (Vo), interpolar (Vi) and caudal (Vc) subnucleus of the Vsp, respectively. Moreover, approximately 34.0% of the collateral projection neurons in the Vc showed Fos immunopositivity after injection of formalin into the lip, and parts of calcitonin gene-related peptide (CGRP)-immunopositive axonal varicosities were in direct contact with the Vc collateral projection neurons. These results indicate that most collateral projection neurons in the Vsp, particularly in the Vc, which express mainly VGLUT2, may relay orofacial nociceptive information directly to the thalamus and PBN via axon collaterals.

Somatotopy in the Medullary Dorsal Horn As a Basis for Orofacial Reflex Behavior

The somatotopy of the trigeminocervical complex of the rat was defined as a basis for describing circuitry for reflex behaviors directed through the facial motor nucleus. Thus, transganglionic transport of horseradish peroxidase conjugates applied to individual nerves/peripheral receptive fields showed that nerves innervating oropharyngeal structures projected most rostrally, followed by nerves innervating snout, periocular, and then periauricular receptive fields most caudally. Nerves innervating mucosae or glabrous receptive fields terminated densely in laminae I, II, and V of the trigeminocervical complex, while those innervating hairy skin terminated in laminae I-V. Projections to lamina II exhibited the most focused somatotopy when individual cases were compared. Retrograde transport of FluoroGold (FG) deposited into the facial motor nucleus resulted in labeled neurons almost solely in lamina V of the trigeminocervical complex. The distribution of these labeled neurons paralleled the somatotopy of primary afferent fibers, e.g., those labeled after FG injections into a functional group of motoneurons innervating lip musculature were found most rostrally while those labeled after injections into motoneurons innervating snout, periocular and preauricular muscles, respectively, were found at progressively more caudal levels. Anterograde transport of injections of biotinylated dextran amine into lamina V at different rostrocaudal levels of the trigeminocervical complex confirmed the notion that the somatotopy of orofacial sensory fields parallels the musculotopy of facial motor neurons. These data suggest that neurons in lamina V are important interneurons in a simple orofacial reflex circuit consisting of a sensory neuron, interneuron and motor neuron. Moreover, the somatotopy of primary afferent fibers from the head and neck confirms the "onion skin hypothesis" and suggests rostral cervical dermatomes blend seamlessly with "cranial dermatomes." The transition area between subnucleus interpolaris and subnucleus caudalis is addressed while the paratrigeminal nucleus is discussed as an interface between the somatic and visceral nervous systems.

Anatomical evidence of pruriceptive trigeminothalamic and trigeminoparabrachial projection neurons in mice

Itch is relayed to higher centers by projection neurons in the spinal and medullary dorsal horn. We employed a double-label method to map the ascending projections of pruriceptive and nociceptive trigeminal and spinal neurons. The retrograde tracer fluorogold (FG) was stereotaxically injected into the right thalamus or lateral parabrachial area (LPb) in mice. Seven days later, mice received intradermal (id) microinjection of histamine, chloroquine, capsaicin, or vehicle into the left cheek. Histamine, chloroquine, and capsaicin intradermally elicited similar distributions of Fos-positive neurons in the medial aspect of the superficial medullary and spinal dorsal horn from the trigeminal subnucleus caudalis to C2. Among neurons retrogradely labeled from the thalamus, 43%, 8%, and 22% were Fos-positive following id histamine, chloroquine, or capsaicin. Among the Fos-positive neurons following pruritic or capsaicin stimuli, ∼1-2% were retrogradely labeled with FG. Trigeminoparabrachial projection neurons exhibited a higher incidence of double labeling in the superficial dorsal horn. Among the neurons retrogradely labeled from LPb, 36%, 29%, and 33% were Fos positive following id injection of histamine, chloroquine, and capsaicin, respectively. Among Fos-positive neurons elicited by id histamine, chloroquine, and capsaicin, respectively, 3.7%, 4.3%, and 4.1% were retrogradely labeled from LPb. The present results indicate that, overall, relatively small subpopulations of pruriceptive and/or nociceptive neurons innervating the cheek project to thalamus or LPb. These results imply that the vast majority of pruritogen- and algogen-responsive spinal neurons are likely to function as interneurons relaying information to projection neurons and/or participating in segmental nocifensive circuits.

Projections from the dorsal peduncular cortex to the trigeminal subnucleus caudalis (medullary dorsal horn) and other lower brainstem areas in rats

This study has revealed direct projections from the dorsal peduncular cortex (DP) in the medial prefrontal cortex (mPfC) to the trigeminal brainstem sensory nuclear complex and other lower brainstem areas in rats. We first examined the distribution of mPfC neurons projecting directly to the medullary dorsal horn (trigeminal subnucleus caudalis [Vc]) and trigeminal subnucleus oralis (Vo) which are known to receive direct projections from the lateral prefrontal cortex (insular cortex). After injections of the retrograde tracer Fluorogold (FG) into the rostro-dorsomedial part of laminae I/II of Vc (rdm-I/II-Vc), many neurons were labeled bilaterally (with an ipsilateral predominance) in the rostrocaudal middle level of DP (mid-DP) and not in other mPfC areas. After FG injections into the lateral and caudal parts of laminae I/II of Vc, or the Vo, no neurons were labeled in the mPfC. We then examined projections from the mid-DP by using the anterograde tracer biotinylated dextranamine (BDA). After BDA injections into the mid-DP, many axons and terminals were labeled bilaterally (with an ipsilateral predominance) in the rdm-I/II-Vc, periaqueductal gray and solitary tract nucleus, and ipsilaterally in the parabrachial nucleus and trigeminal mesencephalic nucleus. In addition, the connections of the mid-DP with the insular cortex were examined. Many BDA-labeled axons and terminals from the mid-DP were also found ipsilaterally in the caudalmost level of the granular and dysgranular insular cortex (GI/DI). After BDA injections into the caudalmost GI/DI, many axons and terminals were labeled ipsilaterally in the mid-DP. The projections from the mid-DP to the rdm-I/II-Vc and other brainstem nuclei suggest that mid-DP neurons may regulate intraoral and perioral sensory processing (including nociceptive processing) of rdm-I/II-Vc neurons directly or indirectly through the brainstem nuclei. The reciprocal connections between the mid-DP and caudalmost GI/DI suggest that this regulation may involve mid-DP interactions with the caudalmost GI/DI neurons.

Effects of gustatory nerve transection and/or ovariectomy on oral capsaicin avoidance in rats

The incidence of chronic oral pain such as burning mouth syndrome is greater in peri-menopausal females, and was postulated to be associated with gustatory nerve damage. We investigated whether bilateral transection of the chorda tympani, with or without accompanying ovariectomy, affected oral capsaicin avoidance in rats. Female rats had restricted access to 2 bottles, 1 bottle containing capsaicin (concentration range: 0.33-33 μM/L) and the other vehicle. Percent volume of capsaicin consumption and lick counts were measured. The concentration series was tested before and 0.5, 3, 6, 9, and 12 months after the following surgical procedures: (a) bilateral transection of the chorda tympani (CTx); (b) ovariectomy (OVx); (3) CTx plus OVx; or (4) sham CT surgery. Before surgery there was a concentration-dependent decrease in licks and volume of capsaicin consumed, with a threshold between 0.1 and 0.3 ppm. The majority of drink licks occurred during the first 9 minutes of access. Over the 12-month test period, the CTx group did not exhibit reduced capsaicin consumption, and consumed significantly more capsaicin at 6 and 9 months postsurgery. Rats in the OVx group consistently consumed significantly less capsaicin and exhibited significantly higher counts of capsaicin-evoked Fos-like immunoreactivity in the dorsomedial trigeminal subnucleus caudalis (Vc) compared to all other treatment groups. That CTx, with or without OVx, did not enhance capsaicin avoidance indicates that damage to the gustatory system does not disinhibit trigeminal nociceptive transmission.

Pain fiber anesthetic reduces brainstem Fos after tooth extraction

We recently demonstrated that pain-sensing neurons in the trigeminal system can be selectively anesthetized by co-application of QX-314 with the TRPV1 receptor agonist, capsaicin (QX cocktail). Here we examined whether this new anesthetic strategy can block the neuronal changes in the brainstem following molar tooth extraction in the rat. Adult male Sprague-Dawley rats received infiltration injection of anesthetic 10 min prior to lower molar tooth extraction. Neuronal activation was determined by immunohistochemistry for the proto-oncogene protein c-Fos in transverse sections of the trigeminal subnucleus caudalis (Sp5C). After tooth extraction, c-Fos-like immunoreactivity (Fos-LI) detected in the dorsomedial region of bilateral Sp5C was highest at 2 hrs (p < .01 vs. naïve ipsilateral) and declined to pre-injury levels by 8 hrs. Pre-administration of the QX cocktail significantly reduced to sham levels Fos-LI examined 2 hrs after tooth extraction; reduced Fos-LI was also observed with the conventional local anesthetic lidocaine. Pulpal anesthesia by infiltration injection was confirmed by inhibition of the jaw-opening reflex in response to electrical tooth pulp stimulation. Our results suggest that the QX cocktail anesthetic is effective in reducing neuronal activation following tooth extraction. Thus, a selective pain fiber 'nociceptive anesthetic' strategy may provide an effective local anesthetic option for dental patients in the clinic.

Modulation of central gustatory coding by temperature

Changes in oral temperature can influence taste perception, indicating overlap among mechanisms for taste and oral somesthesis. Medullary gustatory neurons can show cosensitivity to temperature, albeit how these cells process combined taste and thermal input is poorly understood. Here, we electrophysiologically recorded orosensory responses (spikes) from 39 taste-sensitive neurons in the nucleus tractus solitarii of anesthetized mice during oral delivery of tastants adjusted to innocuous cool (16 and 18°C), room (22°C, baseline), and warm (30 and 37°C) oral temperatures. Stimuli included (in mM) 100 sucrose, 30 NaCl, 3 HCl, 3 quinine, an umami mixture, and water. Although cooled water excited few cells, water warmed to 30 and 37°C significantly excited 33% and 64% of neurons, respectively. Warmth induced responses of comparable magnitude to room temperature tastants. Furthermore, warming taste solutions influenced the distribution of gustatory responses among neurons and increased (P < 0.05) neuronal breadth of tuning across taste qualities. The influence of warmth on response magnitude was stimulus specific. Across neurons, warming facilitated responses to sucrose and umami in a superadditive manner, as these responses exceeded (P < 0.05) the arithmetic sum of activity to warming alone and the taste stimulus tested at room temperature. Superadditive increases (P < 0.05) in responding were also noted in some cells for warmed HCl. Yet warming induced only simple additive or subtractive effects on responses to quinine and NaCl. Data show temperature is a parameter of gustatory processing, like taste quality and concentration, in medullary circuits for taste.

Capsaicin avoidance as a measure of chemical hyperalgesia in orofacial nerve injury models

Many patients suffer from trigeminal neuralgia and other types of orofacial pain that are poorly treated, necessitating preclininal animal models for development of mechanisms-based therapies. The present study assessed capsaicin avoidance and other nocifensive behavioral responses in three models of orofacial nerve injury in rats: chronic constriction injury (CCI) of the mental nerves, partial tight ligation of mental nerves, and CCI of lingual nerves. We additionally investigated if nerve injury resulted in enhanced capsaicin-evoked activation of neurons in trigeminal caudalis (Vc) or nucleus of the solitary tract (NTS) based on expression of Fos-like immunoreactivity (FLI). Mental nerve CCI resulted in an enhancement of capsaicin avoidance in a two-bottle preference paradigm, while neither mental nerve injury produced thermal hyperalgesia or mechanical allodynia. CCI of lingual nerves did not affect capsaicin avoidance. Counts of FLI in Vc were significantly higher in the lingual sham and mental nerve CCI groups compared to mental shams; FLI counts in NTS did not differ among groups. Mental nerve CCI may have induced central sensitization of chemical nociception since increased capsaicin avoidance was accompanied by greater activation of Vc neurons in response to oral capsaicin.

Metabotropic glutamate receptor 5 contributes to inflammatory tongue pain via extracellular signal-regulated kinase signaling in the trigeminal spinal subnucleus caudalis and upper cervical spinal cord

Background

In the orofacial region, limited information is available concerning pathological tongue pain, such as inflammatory pain or neuropathic pain occurring in the tongue. Here, we tried for the first time to establish a novel animal model of inflammatory tongue pain in rats and to investigate the roles of metabotropic glutamate receptor 5 (mGluR5)-extracellular signal-regulated kinase (ERK) signaling in this process.

Methods

Complete Freund’s adjuvant (CFA) was submucosally injected into the tongue to induce the inflammatory pain phenotype that was confirmed by behavioral testing. Expression of phosphorylated ERK (pERK) and mGluR5 in the trigeminal subnucleus caudalis (Vc) and upper cervical spinal cord (C1-C2) were detected with immunohistochemical staining and Western blotting. pERK inhibitor, a selective mGluR5 antagonist or agonist was continuously administered for 7 days via an intrathecal (i.t.) route. Local inflammatory responses were verified by tongue histology.

Results

Submucosal injection of CFA into the tongue produced a long-lasting mechanical allodynia and heat hyperalgesia at the inflamed site, concomitant with an increase in the pERK immunoreactivity in the Vc and C1-C2. The distribution of pERK-IR cells was laminar specific, ipsilaterally dominant, somatotopically relevant, and rostrocaudally restricted. Western blot analysis also showed an enhanced activation of ERK in the Vc and C1-C2 following CFA injection. Continuous i.t. administration of the pERK inhibitor and a selective mGluR5 antagonist significantly depressed the mechanical allodynia and heat hyperalgesia in the CFA-injected tongue. In addition, the number of pERK-IR cells in ipsilateral Vc and C1-C2 was also decreased by both drugs. Moreover, continuous i.t. administration of a selective mGluR5 agonist induced mechanical allodynia in naive rats.

Conclusions

The present study constructed a new animal model of inflammatory tongue pain in rodents, and demonstrated pivotal roles of the mGluR5-pERK signaling in the development of mechanical and heat hypersensitivity that evolved in the inflamed tongue. This tongue-inflamed model might be useful for future studies to further elucidate molecular and cellular mechanisms of pathological tongue pain such as burning mouth syndrome.

The role of trigeminal interpolaris-caudalis transition zone in persistent orofacial pain

Previous studies have established the role of the medullary dorsal horn or the subnucleus caudalis of the spinal trigeminal complex, a homolog of the dorsal horn of the spinal cord, in trigeminal pain processing. In addition to the medullary dorsal horn, recent studies have pointed out increased excitability and sensitization of trigeminal interpolaris and caudalis transition zone (Vi/Vc) following deep orofacial injury, involving neuron-glia-cytokine interactions. The Vi/Vc transition zone accesses rostral brain regions that are important for descending pain modulation, and somatovisceral and somatoautonomic processing and plays a unique role in coordinating trigeminal nocifensive responses.

Expression of TRPV4 in the stimulated rat oral mucous membrane--nociceptive mechanisms of lingual conical papillae

The study was supported by 2006-2007 Aid Program for Overseas Training of the Promotion and Mutual Aid Corporation for Private School of Japan and International Exchange Grant, Osaka Dental University. We studied the function of TRPV4 expression and its neuronal activation in response to noxious stimulation of oral mucosa. The intermolar region of dorsal lingual eminence (IDLE) of rats was stimulated with 10 microl of either normal saline or 5% formalin. Immunohistological studies of the TRPV4, pERK and serotonin (5HT) expression in designated regions of tongues and brainstems were performed for studying the descending pain modulatory system in response to nociception. Specimens of the experimental IDLE demonstrated a significant increase of TRPV4 activity in particular in stratum basale of conical papillae (p < 0.01). pERK-IR positive neurons were significantly increased in the RMg (p < 0.05), Sp5C (p < 0.05) and Md (p < 0.01); TRPV4-IR neurons were found to show a similar distribution with pERK-IR cells in the peripheral Sp5C (p < 0.05). A significant increase of 5HT expression was observed in the RMg (p < 0.01), RPa (p < 0.01) and ROb (p < 0.05). The results suggest that TRPV4 in the oral mucosa is nociceptor of peripheral hyperalgesia, and pERK expression in the Sp5C is closely related with central hyperalgesia of the nociception. Furthermore, pERK-IR cells of the central 5HT nervous system are activated to accelerate 5HT release for neuronal modulation of the descending pain modulatory system in response to nociception.

Effects of systemic bicuculline or morphine on formalin-evoked pain-related behaviour and c-Fos expression in trigeminal nuclei after formalin injection into the lip or tongue in rats

This study examined differences in nociceptive responses between lip and tongue. Formalin-induced pain-related behaviour and c-Fos expression in the trigeminal caudal nucleus (Vc) with/without systemic preadministration of a gamma-aminobutyric acid (GABA) type A receptor antagonist, bicuculline (2 mg/kg, i.p., 10 min before formalin injection) or a micro-opioid receptor agonist, morphine (3 mg/kg, i.p., 10 min before formalin injection) have been studied. Formalin injection into the upper lip induced an immediate pain-related behaviour, mostly face-rubbing behaviour, for 15 min (phase 1, mean +/- SEM/5 min, 81.2 +/- 30.1), followed by a more increased activity for 15 min (phase 2, 205.4 +/- 43.6) and a decline to baseline for next 15 min (phase 3, 63.9 +/- 28.0). Formalin injection into the tongue induced similar amount of pain-related behaviour at phase 1 (67.9 +/- 16.7), followed by similar activity at phase 2 (48.6 +/- 6.2), and lesser behaviour at phase 3 (20.4 +/- 7.6). The behaviour at phase 2 decreased following preadministration of bicuculline or morphine when formalin was injected into the lip (b, 62.5 +/- 14.5; m, 95.8 +/- 10.0) but not into the tongue (b, 31.0 +/- 9.2; m, 77.4 +/- 27.0). A considerable numbers of c-Fos-immunoreactive (IR) cells were induced in the caudal and inter-medio-lateral center of superficial layers of the Vc (VcI/II; mean +/- SEM/section = 225.8 +/- 12.9) and magnocellular zone of the Vc (VcIII/IV; 67.1 +/- 4.7) 2 h after formalin injection into the lip. Much smaller numbers of c-Fos-IR cells were induced in the rostral and dorso-medial one-fourth of the VcI/II (72.6 +/- 3.7) and VcIII/IV (55.6 +/- 6.6) after formalin injection into the tongue. Following preadministration with systemic bicuculline or morphine, the formalin-induced c-Fos-IR cells were decreased more in the VcI/II when formalin was injected into the lip (VcI/II, 102.4 +/- 8.0; VcIII/IV, 32.8 +/- 1.4) than into the tongue (VcI/II, 49.5 +/- 8.1; VcIII/IV, 31.7 +/- 5.3). These results show that the lip is more sensitive to formalin-induced noxious stimulation and regulated more through GABA(A) and micro-opioid receptors than the tongue.

Modulation of paratrigeminal nociceptive neurons following temporomandibular joint inflammation in rats

To evaluate the involvement of paratrigeminal nucleus (Pa5) nociceptive neurons in temporomandibular joint (TMJ) inflammation-induced pain and its autonomic correlates, we conducted behavioral, single unit recording and Fos immunohistochemical studies in anesthetized rats. Nocifensive behaviors to mechanical, heat or cold stimulation of the lateral face over the TMJ region were significantly enhanced in the TMJ-inflamed rats for 10-14 days after injection of complete Freund's adjuvant (CFA) into the TMJ and gradually decreased at the end of the 14-day observation period. Lowering of the nocifensive threshold in TMJ-inflamed rats lasted longer in vagus nerve-transected rats than vagus nerve-intact rats. A large number of Fos-like immunoreactive (LI) cells were observed in the Pa5, and half of them were retrogradely labeled with Fluorogold (FG) injected into the parabrachial nucleus. Background activity of Pa5 wide dynamic range and nociceptive specific neurons was significantly higher in the TMJ-inflamed rats when compared with controls. Responses to mechanical stimuli were significantly higher in NS neurons in the TMJ-inflamed rats. All thermal responsive Pa5 neurons were exclusively sensitive to cold and the response to cold was significantly higher in the TMJ-inflamed rats compared with control rats. Vagus nerve stimulation significantly decreased responses to mechanical and cold stimuli as well as the background activity in TMJ-treated rats but not in TMJ-untreated rats. The present findings suggest that populations of Pa5 neurons are nociceptive and involved in TMJ inflammation-induced pain as well as in autonomic processes related to TMJ pain.

Trigeminal transition zone/rostral ventromedial medulla connections and facilitation of orofacial hyperalgesia after masseter inflammation in rats

Recent studies have implicated a role for the trigeminal interpolaris/caudalis (Vi/Vc) transition zone in response to orofacial injury. Using combined neuronal tracing and Fos protein immunocytochemistry, we investigated functional connections between the Vi/Vc transition zone and rostral ventromedial medulla (RVM), a key structure in descending pain modulation. Rats were injected with a retrograde tracer, FluoroGold, into the RVM 7 days before injection of an inflammatory agent, complete Freund's adjuvant, into the masseter muscle and perfused at 2 hours postinflammation. A population of neurons in the ventral Vi/Vc overlapping with caudal ventrolateral medulla, and lamina V of the trigeminal subnucleus caudalis (Vc), exhibited FluoroGold/Fos double staining, suggesting the activation of the trigeminal-RVM pathway after inflammation. No double-labeled neurons were found in the dorsal Vi/Vc and laminae I-IV of Vc. Injection of an anterograde tracer, Phaseolus vulgaris leucoagglutinin, into the RVM resulted in labeling profiles overlapped with the region that showed FluoroGold/Fos double labeling, suggesting reciprocal connections between RVM and Vi/Vc. Lesions of Vc with a soma-selective neurotoxin, ibotenic acid, significantly reduced inflammation-induced Fos expression as well as the number of FluoroGold/Fos double-labeled neurons in the ventral Vi/Vc (P<0.05). Compared with control rats, lesions of the RVM (n=6) or Vi/Vc (n=6) with ibotenic acid led to the elimination or attenuation of masseter hyperalgesia/allodynia developed after masseter inflammation (P<0.05-0.01). The present study demonstrates reciprocal connections between the ventral Vi/Vc transition zone and RVM. The Vi/Vc-RVM pathway is activated after orofacial deep tissue injury and plays a critical role in facilitating orofacial hyperalgesia.

Mechanisms of orofacial pain control in the central nervous system

Recent advances in the study of pain have revealed somatotopic- and modality-dependent processing and the integration of nociceptive signals in the brain and spinal cord. This review summarizes the uniqueness of the trigeminal sensory nucleus (TSN) in structure and function as it relates to orofacial pain control. The oral nociceptive signal is primarily processed in the rostral TSN above the obex, the nucleus principalis (Vp), and the subnuclei oralis (SpVo) and interpolaris (SpVi), while secondarily processed in the subnucleus caudalis (SpVc). In contrast, the facial nociceptive signal is primarily processed in the SpVc. The neurons projecting to the thalamus are localized mostly in the Vp, moderately in the SpVi, and modestly in the ventrolateral SpVo and the SpVc. Orofacial sensory inputs are modulated in many different ways: by interneurons in the TSN proper, through reciprocal connection between the TSN and rostral ventromedial medulla, and by the cerebral cortex. A wide variety of neuroactive substances, including substance P, gamma-aminobutyric acid, serotonin and nitric oxide (NO) could be involved in the modulatory functions of these curcuits. The earliest expression of NO synthase (NOS) in the developing rat brain is observed in a discrete neuronal population in the SpVo at embryonic day 15. NOS expression in the SpVc is late at postnatal day 10. The neurons receiving intraoral signals are intimately related with the sensorimotor reflexive function through the SpVo. In summary, a better understanding of the trigeminal sensory system--which differs from the spinal system--will help to find potential therapeutic targets and lend to developing new analgesics for orofacial-specific pain with high efficacy and fewer side effects.

Effects of postnatal nicotine exposure on apoptotic markers in the developing piglet brain

Exposure to cigarette smoke is a risk factor for the sudden infant death syndrome (SIDS), but the ability to distinguish between the neuropathological effects of pre- versus postnatal exposure is limited in the clinical setting. To test whether postnatal nicotine exposure could contribute to the increased neuronal expression of apoptotic markers that we have previously observed in SIDS infants, as well as including study of gender influences, we developed a piglet model to mimic passive smoking in the early postnatal period. Piglets were exposed to nicotine (2 mg/kg/day infused via an implanted osmotic minipump) within 48 h of birth until the age of 13-14 days, when the brain was collected for study. Four piglet groups included: control females (n=7), control males (n=7), nicotine females (n=7), and nicotine males (n=7). Apoptotic markers included immunohistochemistry for activated caspase-3, and for DNA fragmentation or terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling (TUNEL) in seven nuclei of the brainstem caudal medulla and two subregions of the hippocampus (CA4 and dentate gyrus). Among control females compared with males, there was less active caspase-3 and less TUNEL in the dorsal motor nucleus of vagus (DMNV), and there was less TUNEL in the nucleus of the spinal trigeminal tract (NSTT). Compared with controls, nicotine-exposed male piglets had increased TUNEL staining in the cuneate nucleus (P=0.05), and increased active caspase-3 in the hypoglossal, gracile and dentate gyrus (P<0.05 for each). Nicotine-exposed females showed no change in TUNEL staining in any of the nuclei studied, but increased active caspase-3 in the hypoglossal, DMNV and NSTT (P<0.05 for each). These results show for the first time that postnatal nicotine exposure can lead to an increase in apoptotic markers in the brain. In piglets, these effects showed regional and gender-specific differences, suggesting that passive, postnatal nicotine exposure may be responsible for some neuropathological changes observed in infants dying from SIDS.

Differential rostral projections of caudal brainstem neurons receiving trigeminal input after masseter inflammation

To understand the functional significance of orofacial injury-induced neuronal activation, this study examined the rostral projection of caudal brainstem neurons that were activated by masseteric inflammation. Rats were injected with a retrograde tracer, Fluorogold, into the nucleus submedius of the thalamus (Sm), parabrachial nucleus (PB), lateral hypothalamus (LH), or medial ventroposterior thalamic nucleus (VPM) 7 days before injection of an inflammatory agent, complete Freund's adjuvant (CFA), into the masseter muscle. Rats were perfused at 2 hours after inflammation, and brainstem tissues were processed for Fos-Fluorogold double immunocytochemistry. Although there was no difference in Fos expression among the four groups (n=4 per site), the rostral projection of Fos-positive neurons showed dramatic differences. In the ventral portion of the trigeminal subnuclei interpolaris/caudalis (Vi/Vc) transition zone, the percentage of Fos-positive neurons projecting to the Sm (39.7%) was significantly higher than that projecting to the LH (5.4%) or VPM (5.6%; P<.001). The anesthesia alone also induced Fos expression in ventral Vi/Vc neurons, but these neurons did not project to Sm. In the caudal laminated Vc and dorsal Vi/Vc, the PB was the major site of rostral projection of Fos-positive neurons. In the caudal ventrolateral medulla and nucleus tractus solitarius, Fos-positive neurons projected to the Sm, PB, and LH. Most VPM-projecting neurons examined did not show Fos-like immunoreactivity after masseter inflammation. These findings emphasize the importance of the trigeminal Vi/Vc transition zone in response to orofacial deep tissue injury. Furthermore, the results differentiate the ventral and dorsal portions of the Vi/Vc transition zone, in that the Sm received projection mainly from activated neurons in the ventral Vi/Vc. The activation of Vi/Vc neurons and associated ascending pathways may facilitate somatoautonomic and somatovisceral integration and descending pain modulation after orofacial deep tissue injury.

Activation of neurons in trigeminal caudalis by noxious oral acidic or salt stimuli is not reduced by amiloride

We investigated the possible role of amiloride-sensitive ion channels of the ENaC/DEGenerin superfamily in the activation of trigeminal nociceptive neurons elicited by noxious chemical stimulation of the oral mucosa using two methodologies, single-unit recording and c-fos immunohistochemistry. In pentobarbital-anesthetized rats, single-unit recordings were made from neurons in superficial laminae of dorsomedial trigeminal subnucleus caudalis (Vc) that responded to noxious thermal and chemical stimuli applied to the dorsal tongue. Successive application of each of three chemicals (250 mM pentanoic acid, n=6 units; 250 mM citric acid, n=8; 5 M NaCl, n=6) evoked responses that were not affected following topical application of amiloride (1 mM). In separate experiments, pentobarbital-anesthetized rats received one of the following stimuli delivered to the dorsal tongue: 250 mM pentanoic acid (n=6); 1 mM amiloride followed by 250 mM pentanoic (N=6); 5 M NaCl (n=5); or 1 mM amiloride followed by 5 M NaCl (n=5). Two hours later they were perfused with 4% paraformaldehyde and the brain stems processed for c-fos immunoreactivity. Both pentanoic acid and 5 M NaCl evoked similar numbers and patterns of fos-like immunoreactivity (FLI) in dorsomedial Vc and other brain stem regions, with no significant difference in counts of FLI in animals pretreated with amiloride. These results suggest that amiloride-sensitive Na(+) channels are not essential in mediating the activation of intraoral trigeminal nociceptors.

Reduced aversion to oral capsaicin following neurotoxic destruction of superficial medullary neurons expressing NK-1 receptors

Aversion to capsaicin (0.1-10 ppm) was assessed using a two-bottle paired preference paradigm, before and after intracisternal injection of substance P conjugated to saporin (SP-SAP) to ablate neurons in superficial medullary and cervical dorsal horn that express NK-1 receptors. Before SP-SAP, there was a concentration-dependent decrease in consumption of capsaicin with a threshold of 0.1-0.3 ppm. Following SP-SAP, significantly more capsaicin solution was consumed at 1- and 10-ppm concentrations. These results support a role for substance P in the mediation of high, but not low, levels of capsaicin-induced oral irritation.

Quinine and citric acid elicit distinctive Fos-like immunoreactivity in the rat nucleus of the solitary tract

The present experiment investigated Fos-like immunoreactivity (FLI) in the nucleus of the solitary tract (NST) after intraoral infusions of 0.1 M citric acid, 0.3 M NaCl, and 0.3-30 mM quinine monohydrochloride (QHCl) in awake, behaving rats. Increases in QHCl concentration produced increases in the numbers of FLI-labeled neurons in the rostral part of the intermediate (i(r)) and rostral (r) NST, but the topographic distribution of FLI was consistent across QHCl concentrations and distinctive compared with effects of citric acid. Quinine elicited FLI concentrated in the medial third of the nucleus; acid elicited more broadly distributed FLI concentrated farther laterally. Surprisingly, in contrast to QHCl and citric acid, NaCl produced FLI that was indistinguishable from that produced by water. Although the functional significance of these patterns is unknown, citric acid and QHCl are nonpreferred stimuli but produced different oromotor behaviors. QHCl (30 mM) elicited approximately 3.2 times as many gapes as citric acid (0.1 M), and acid elicited more ingestive responses. Parallel differences in FLI expression suggest that different NST regions may have distinctive roles in triggering oromotor behaviors.

Mapping of brain stem neuronal circuitry active during swallowing

A poorly understood neural circuit in the brain stem controls swallowing. This experiment studied the swallowing circuit in the rat brain stem by means of fos immunocytochemistry. The fos protein is a marker of activated neurons, and under experimental conditions, repetition of a behavior causes the fos protein to be produced in the neurons involved in that behavior. The fos technique has been successfully used to delineate neural circuits involved in reflex glottic closure, cough, and vocalization; however, the technique has not been used to map the swallowing circuit. Nine rats were used in this study. Swallows were evoked in anesthetized rats for 1 hour, then, after a 4-hour delay to allow maximum fos production, the rats were painlessly sacrificed by perfusion. The brain stems were removed and sectioned in the frontal plane, and every fourth section was immunoreacted for fos protein. All sections were examined by light microscopy, and cells positive for fos were marked on drawings of brain stem structures for different levels throughout the brain stem. Control animals underwent sham experiments. After subtraction of the areas of fos labeling seen in controls, all experimental rats showed fos-labeled neurons in very discrete and localized areas, including practically all regions implicated by prior neurophysiology studies of swallowing. The distribution of labeled neurons was more dispersed through the brain stem than current theories of swallowing would suggest. Specifically, recent studies of swallowing control have focused on the nucleus of the solitary tract (NST) and the region surrounding the nucleus ambiguus (periambigual area) just rostral to the obex. These areas contained fos-labeled neurons, but unexpectedly, heavy labeling was found in the same areas caudal to the obex. Areas containing the heaviest labeling were specific subnuclei of the NST and surrounding reticular formation; the periambigual area; and the intermediate reticular zone in the pons and caudal medulla. Interestingly, none of these anatomic structures had uniform fos labeling; this finding suggests that the unlabeled areas are involved in other oromotor behaviors, or that the specific protocol did not activate the full population of swallowing-related neurons. A notable finding of this study is a candidate for the central pattern generator (CPG) of swallowing. Careful lesioning studies in cats strongly suggest that a region in the rostral-medial medulla contains the CPG for swallowing, although the exact location of the CPG was never pinpointed. In the homologous region of the rat brain stem, fos labeling was only found in a small group of neurons within the gigantocellular reticular formation that may be a candidate for the CPG. In summary, correlation with prior physiology experiments suggests that this experiment appears to have delineated many, if not all, of the components of the swallowing circuit for the first time in any mammal. In addition, other areas were found that might also be swallowing-related. One notable example is a small group of fos-labeled cells that may be the CPG for swallowing. Further studies are required to clarify the specific roles of the fos-labeled neurons seen in this study.

Neuroplasticity Induced by Tooth Pulp Stimulation in Trigeminal Subnucleus Oralis Involves NMDA Receptor Mechanisms

We have recently demonstrated that application of the mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant increases in jaw muscle electromyographic (EMG) activity and neuroplastic changes in trigeminal (V) subnucleus caudalis. Since subnucleus oralis (Vo) as well as caudalis receives projections from molar pulp afferents and is also an integral brain stem relay of afferent input from orofacial structures, we tested whether MO application to the exposed pulp induces neuroplastic changes in oralis neurons and whether microinjection of MK-801, a noncompetitive NMDA antagonist, into the Vo influences the pulp/MO-induced neuroplastic changes in chloralose/urethan-anesthetized rats. Single neuronal activity was recorded in Vo, and neurons classified as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), nociceptive-specific (NS), deep (D), or skin/mucosa and deep (S + D). The spontaneous activity, mechanoreceptive field (RF) size, mechanical threshold, and response to suprathreshold mechanical stimuli applied to the neuronal RF were assessed prior to and throughout a 40- to 60-min period after MO application to the maxillary molar pulp. In animals pretreated with saline microinjection (0.3 μl) into the Vo, MO application to the pulp produced a significant increase in spontaneous activity, expansion of the pinch or deep RF, decrease in the mechanical threshold, and increase in response to suprathreshold mechanical stimuli of the nociceptive (WDR, NS, and S + D) neurons except for those nociceptive neurons having their RF only in the intraoral region. The pulpal application of MO did not produce any significant neuroplastic changes in LTM neurons. Furthermore, in animals pretreated with MK-801 microinjection (3 μg/0.3 μl) into the Vo, MO application to the pulp did not produce any significant changes in the RF and response properties of nociceptive neurons. In other animals pretreated with saline (0.3 μl) or MK-801 (3 μg/0.3 μl) microinjected into the Vo, mineral oil application to the pulp did not produce any significant changes in RF and response properties of nociceptive neurons. These findings indicate that the application of MO to the tooth pulp can induce significant neuroplastic changes in oralis nociceptive neurons and that central NMDA receptor mechanisms may be involved in these neuroplastic changes.

Nicotine as an Addictive Substance: A Critical Examination of the Basic Concepts and Empirical Evidence

The present review is a critical analysis of the concepts behind and the empirical data supporting the view that tobacco use represents an addiction to nicotine. It deals with general aspects of the notion of addiction, while concentrating on specific problems associated with incorporating nicotine into current frameworks. The notion of addiction suffers from unprecedented definitional difficulties. The definitions offered by various authorities are very different, even contradictory. Definitions that reasonably include nicotine are so broad and vague that they allow many trivial things, such as salt, sugar, and watching television, to be considered addictive. Definitions that exclude the trivia also exclude nicotine. The addiction hypothesis, in general, is strongly shaped by views that certain drugs bring about a molecular level subversion of rationality. The main human evidence for this is verbal reports of smokers who say that they can't quit. On the other hand, the existence of many millions of successful quitters suggests that most people can quit. Some smokers don't quit, but whether they can't is another matter. The addiction hypothesis would be greatly strengthened by the demonstration that any drug of abuse produces special changes in the brain. It has yet to be shown that any drug produces changes in the brain different from those produced by many innocuous substances and events. The effects of nicotine on the brain are similar to those of sugar, salt, exercise, and other harmless substances and events. Apart from numerous conceptual and definitional inadequacies with the addiction concept in general, the notion that nicotine is addictive lacks reasonable empirical support. Nicotine does not have the properties of reference drugs of abuse. There are so many findings that conflict so starkly with the view that nicotine is addictive that it increasingly appears that adhering to the nicotine addiction thesis is only defensible on extra-scientific grounds.

Sensitization, desensitization and stimulus-induced recovery of trigeminal neuronal responses to oral capsaicin and nicotine

Repeated application of capsaicin at a 1-min interstimulus interval (ISI) to the tongue induces a progressively increasing irritant sensation (sensitization), followed after a rest period by reduced sensitivity to further capsaicin (desensitization). Sequential reapplication of capsaicin induces irritation that eventually increases to initial levels: stimulus-induced recovery (SIR). In contrast, repeated application of nicotine elicits a declining irritant sensation across trials. To investigate possible neural correlates of these phenomena, we recorded from single units in superficial laminae of the dorsomedial trigeminal subnucleus caudalis (Vc) that responded to noxious thermal (54 degrees C) and chemical (1 M pentanoic acid) stimulation of the tongue of anesthetized rats. We then recorded responses to either capsaicin (330 microM) or nicotine (0.6 M), delivered either once, repeatedly at 1-min ISI, or continually by constant flow. After the initial capsaicin application and a rest period, the capsaicin was reapplied in the identical manner to test for SIR. The mean response of 14 Vc units to sequential application of pentanoic acid did not vary significantly across trials, indicating lack of tachyphylaxis or sensitization. The averaged response of 11 Vc units to repeated capsaicin increased significantly across the first eight trials and then plateaued. Following the rest period, spontaneous firing had returned to the precapsaicin level. With capsaicin reapplication, the averaged response increased again after a significant delay (due to desensitization), but did not reattain the peak firing rate achieved in the initial series (partial SIR). Constant-flow application of capsaicin induced an identical sensitization followed by nearly complete SIR. A single application of capsaicin induced a significant rise in firing in eight other units, but the rate of rise and maximal firing rate were both much lower compared with repetitive or constant-flow capsaicin. When capsaicin was reapplied once after the rest period, there was no change in firing rate indicating absence of SIR. These results indicate that maintenance of the capsaicin concentration induces a progressive increase in neuronal response that parallels sensitization. With recurrent capsaicin application, desensitization can be overcome to result in a delayed recovery of Vc responses similar to SIR. In contrast, the averaged response of 17 Vc units to repeated or constant-flow application of nicotine increased only over the first 3 min, and then decreased to spontaneous levels even as nicotine was still being applied. These results are consistent with the decrease in the perceived irritation elicited by sequential application of nicotine in humans.

Trigeminohypothalamic and reticulohypothalamic tract neurons in the upper cervical spinal cord and caudal medulla of the rat

Sensory information that arises in orofacial organs facilitates exploratory, ingestive, and defensive behaviors that are essential to overall fitness and survival. Because the hypothalamus plays an important role in the execution of these behaviors, sensory signals conveyed by the trigeminal nerve must be available to this brain structure. Recent anatomical studies have shown that a large number of neurons in the upper cervical spinal cord and caudal medulla project directly to the hypothalamus. The goal of the present study was to identify the types of information that these neurons carry to the hypothalamus and to map the route of their ascending axonal projections. Single-unit recording and antidromic microstimulation techniques were used to identify 81 hypothalamic-projecting neurons in the caudal medulla and upper cervical (C(1)) spinal cord that exhibited trigeminal receptive fields. Of the 72 neurons whose locations were identified, 54 were in laminae I-V of the dorsal horn at the level of C(1) (n = 22) or nucleus caudalis (Vc, n = 32) and were considered trigeminohypothalamic tract (THT) neurons because these regions are within the main projection territory of trigeminal primary afferent fibers. The remaining 18 neurons were in the adjacent lateral reticular formation (LRF) and were considered reticulohypothalamic tract (RHT) neurons. The receptive fields of THT neurons were restricted to the innervation territory of the trigeminal nerve and included the tongue and lips, cornea, intracranial dura, and vibrissae. Based on their responses to mechanical stimulation of cutaneous or intraoral receptive fields, the majority of THT neurons were classified as nociceptive (38% high-threshold, HT, 42% wide-dynamic-range, WDR), but in comparison to the spinohypothalamic tract (SHT), a relatively high percentage of low-threshold (LT) neurons were also found (20%). Responses to thermal stimuli were found more commonly in WDR than in HT neurons: 75% of HT and 93% of WDR neurons responded to heat, while 16% of HT and 54% of WDR neurons responded to cold. These neurons responded primarily to noxious intensities of thermal stimulation. In contrast, all LT neurons responded to innocuous and noxious intensities of both heat and cold stimuli, a phenomenon that has not been described for other populations of mechanoreceptive LT neurons at spinal or trigeminal levels. In contrast to THT neurons, RHT neurons exhibited large and complex receptive fields, which extended over both orofacial ("trigeminal") and extracephalic ("non-trigeminal") skin areas. Their responses to stimulation of trigeminal receptive fields were greater than their responses to stimulation of non-trigeminal receptive fields, and their responses to innocuous stimuli were induced only when applied to trigeminal receptive fields. As described for SHT axons, the axons of THT and RHT neurons ascended through the contralateral brain stem to the supraoptic decussation (SOD) in the lateral hypothalamus; 57% of them then crossed the midline to reach the ipsilateral hypothalamus. Collateral projections were found in the superior colliculus, substantia nigra, red nucleus, anterior pretectal nucleus, and in the lateral, perifornical, dorsomedial, suprachiasmatic, and supraoptic hypothalamic nuclei. Additional projections (which have not been described previously for SHT neurons) were found rostral to the hypothalamus in the caudate-putamen, globus pallidus, and substantia innominata. The findings that nonnociceptive signals reach the hypothalamus primarily through the direct THT route, whereas nociceptive signals reach the hypothalamus through both the direct THT and the indirect RHT routes suggest that highly prioritized painful signals are transferred in parallel channels to ensure that this critical information reaches the hypothalamus, a brain area that regulates homeostasis and other humoral responses required for the survival of the organism.

Spinal sources of noxious visceral and noxious deep somatic afferent drive onto the ventrolateral periaqueductal gray of the rat

Studies utilizing the expression of Fos protein as a marker of neuronal activation have revealed that pain of deep somatic or visceral origin selectively activates the ventrolateral periaqueductal gray (vlPAG). Previous anatomical tracing studies revealed that spinal afferents to the vlPAG arose from the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus at all spinal segmental levels, with approximately 50% of vlPAG-projecting spinal neurons found within the upper cervical spinal cord. This study utilized detection of Fos protein to determine the specific populations of vlPAG-projecting spinal neurons activated by noxious deep somatic or noxious visceral stimulation. Pain of cardiac or peritoneal (i.e., visceral) origin activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the thoracic cord, whereas pain of hindlimb (i.e., deep somatic) origin activated neurons in the same laminar regions but in the lumbosacral cord. Each of these deep noxious manipulations also activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the upper cervical spinal cord. In a second set of experiments, the combination of retrograde tracing and Fos immunohistochemistry revealed that vlPAG-projecting spinal neurons activated by deep somatic pain were located in both the upper cervical and lumbosacral cord, whereas those activated by visceral pain were restricted to the thoracic spinal cord. Thus pain arising from visceral versus deep somatic body regions influences neural activity within the vlPAG via distinct spinal pathways. The findings also highlight the potential significance of the upper cervical cord in integrating pain arising from deep structures throughout the body.

GABA(A) receptor-mediated effects on expression of c-Fos in rat trigeminal nucleus following high- and low-intensity afferent stimulation

We examined the effects of systemic administration of a GABA(A) receptor agonist, muscimol, or antagonist, bicuculline, on the expression of c-Fos protein induced 3h after electrical stimulation of the trigeminal ganglion at low (0.1 mA) or high intensities (1. 0 mA) in the urethane-anesthetized rat. In saline-treated rats, 10 min stimulation of the trigeminal ganglion induced c-Fos-immunopositive neurons throughout the full extent of the ipsilateral superficial layers of the trigeminal nucleus caudalis, and dorsal or dorsomedial part of the nuclei rostral to obex (trigeminal nucleus principalis, dorsomedial nucleus of trigeminal nucleus oralis, dorsomedial nucleus of trigeminal nucleus interpolaris). Animals stimulated at 1. 0 mA induced a significantly higher number of labeled neurons in all trigeminal sensory nucleus than animals stimulated at 0.1 mA. In rats treated with 1mg/kg i.p. muscimol and stimulated at 0.1 mA, the numbers of Fos-positive neurons in trigeminal nucleus caudalis, dorsomedial nucleus of trigeminal nucleus interpolaris, and dorsomedial nucleus of trigeminal nucleus oralis were significantly decreased. However, after stimulation at 1.0 mA, the numbers of Fos-positive neurons in the superficial layers of trigeminal nucleus caudalis was increased and no changes occurred in the numbers of Fos-positive neurons in the magnocellular zone of trigeminal nucleus caudalis, the dorsomedial nucleus of trigeminal nucleus interpolaris, or dorsomedial nucleus of trigeminal nucleus oralis compared to saline-treated controls. In rats treated with 2mg/kg i.p. bicuculline and stimulated at 0.1 mA, the number of Fos-positive neurons increased in the superficial layers of trigeminal nucleus caudalis and trigeminal nucleus principalis. However, after stimulation at 1.0 mA, the number of Fos-positive neurons was unchanged in superficial layers of trigeminal nucleus caudalis, but decreased in the magnocellular zone of trigeminal nucleus caudalis, dorsomedial nucleus of trigeminal nucleus interpolaris and dorsomedial nucleus of trigeminal nucleus oralis. There was a specific loss of Fos-positive neurons in the maxillary and ophthalmic divisions (ventrolateral half) of trigeminal nucleus caudalis. These results indicate that the expression of c-Fos in the trigeminal nucleus is differentially regulated through GABA(A) receptors in a manner that is dependent on the nucleus and the type of primary afferents that are activated by different stimulus intensities.

Psychophysical and neurobiological evidence that the oral sensation elicited by carbonated water is of chemogenic origin

The sensation produced by carbonated beverages has been attributed to chemical excitation of nociceptors in the oral cavity via the conversion of CO(2) to carbonic acid in a reaction catalyzed by carbonic anhydrase. In separate studies, we tested if the carbonic anyhdrase blocker, acetazolamide, reduced either the intensity of sensation in humans or c-fos expression by trigeminal neurons in rats, evoked by application of carbonated water to the tongue. In the psychophysical experiment, one-half of the dorsal tongue was pretreated with acetazolamide (1 or 2%), after which the tongue was exposed bilaterally to carbonated water. In a two-alternative forced-choice paradigm, subjects chose which side of the tongue yielded a stronger sensation and additionally rated the magnitude of sensation on each side. Pretreatment with acetazolamide reduced the magnitude of sensation elicited by carbonated water in a concentration-dependent manner, since a significant majority of subjects chose the untreated side of the tongue as having a stronger sensation and assigned significantly higher intensity ratings to that side. Acetazolamide did not affect the irritant sensation from citric acid, while capsaicin pretreatment reduced both the sensation elicited by carbonated water and the irritation induced by citric acid application. In a separate experiment using rats, delivery of carbonated water to the tongue significantly increased the number of cells expressing c-fos-like immunoreactivity in the dorsomedial trigeminal nucleus caudalis (versus saline controls); this was significantly reduced by pretreatment with acetazolamide. Our results support the hypothesis that carbonated water activates lingual nociceptors via conversion of CO(2) to carbonic acid; the nociceptors in turn excite trigeminal neurons involved in signaling oral irritation.

Chronic tooth pulp inflammation causes transient and persistent expression of Fos in dynorphin-rich regions of rat brainstem

We have analyzed central Fos immunoreactivity (Fos-IR) brainstems of adult rats after three clinically relevant dental injuries: filled dentin (DF) cavities that cause mild pulp injury and heal within 1-2 weeks; open pulp exposures (PX) that cause gradual pulp loss and subsequent periodontal lesions; and filled pulp exposures (PXF). By 1 week after DF cavities, no Fos-IR remained except for sites such as lateral-ventral periolivary nucleus (LVPO) that had Fos-IR in all rats including controls. PX injury induced (1) a delayed transient expression of Fos at 1-2 weeks at three loci (ipsilateral neurons in dorsomedial nucleus oralis, paratrigeminal nucleus, and trigeminal tract), (2) persistent ipsilateral Fos for at least 4 weeks after injury in dynorphin (Dyn)-rich regions (rostral lateral solitary nucleus, periobex dorsal nucleus caudalis), and (3) late Fos-IR at 2-4 weeks (bilateral superficial cervical dorsal horn, contralateral dorsal nucleus caudalis, contralateral rostral lateral solitary nucleus). Rats with PXF injury were examined at 2 weeks, and they had greater numbers and more extensive rostro-caudal distribution of Fos neurons than the PX group. One week after PX injury, Fos-IR neurons were found in regions with strong Dyn-IR central fibers. Co-expression of Dyn and Fos was found in some unusually large neurons of the ipsilateral rostral lateral solitary nucleus, trigeminal tract, and dorsal nucleus caudalis. Immunocytochemistry for the p75 low affinity neurotrophin receptor (p75NTR) or for calcitonin gene-related peptide (CGRP) showed no consistent change in trigeminal central endings in any Fos-reactive brainstem areas, despite the extensive structural and cytochemical reorganization of the peripheral endings of the dental neurons. The Fos responses of central neurons to tooth injury have some unusual temporal and spatial patterns in adult rats compared to other trigeminal injury models.

Acute and chronic craniofacial pain: brainstem mechanisms of nociceptive transmission and neuroplasticity, and their clinical correlates

This paper reviews the recent advances in knowledge of brainstem mechanisms related to craniofacial pain. It also draws attention to their clinical implications, and concludes with a brief overview and suggestions for future research directions. It first describes the general organizational features of the trigeminal brainstem sensory nuclear complex (VBSNC), including its input and output properties and intrinsic characteristics that are commensurate with its strategic role as the major brainstem relay of many types of somatosensory information derived from the face and mouth. The VBSNC plays a crucial role in craniofacial nociceptive transmission, as evidenced by clinical, behavioral, morphological, and electrophysiological data that have been especially derived from studies of the relay of cutaneous nociceptive afferent inputs through the subnucleus caudalis of the VBSNC. The recent literature, however, indicates that some fundamental differences exist in the processing of cutaneous vs. other craniofacial nociceptive inputs to the VBSNC, and that rostral components of the VBSNC may also play important roles in some of these processes. Modulatory mechanisms are also highlighted, including the neurochemical substrate by which nociceptive transmission in the VBSNC can be modulated. In addition, the long-term consequences of peripheral injury and inflammation and, in particular, the neuroplastic changes that can be induced in the VBSNC are emphasized in view of the likely role that central sensitization, as well as peripheral sensitization, can play in acute and chronic pain. The recent findings also provide new insights into craniofacial pain behavior and are particularly relevant to many approaches currently in use for the management of pain and to the development of new diagnostic and therapeutic procedures aimed at manipulating peripheral inputs and central processes underlying nociceptive transmission and its control within the VBSNC.

Activation of spinal wide dynamic range neurons by intracutaneous microinjection of nicotine

Nicotine evokes pain in the skin and oral mucosa and excites a subpopulation of cutaneous nociceptors, but little is known about the central transmission of chemogenic pain. We have investigated the responses of lumbar spinal wide dynamic range (WDR)-type dorsal horn neurons to intracutaneous (ic) microinjection of nicotine in pentobarbital-anesthetized rats. Nearly all (97%) units responded to nicotine microinjected ic (1 microl) into the low-threshold region of the hind-paw mechanosensitive receptive field in a concentration-related manner (0.01-10%). Responses to repeated injections of 10% nicotine exhibited tachyphylaxis at 5-, 10-, and 15-min interstimulus intervals. Significant tachyphylaxis was not seen with 1% nicotine. All nicotine-responsive units tested (n = 30) also responded to ic histamine (1 microl, 3%) and did not exhibit tachyphylaxis to repeated histamine. However, there was significant cross-tachyphylaxis of nicotine to histamine. Thus 5 min after ic nicotine, histamine-evoked responses were attenuated significantly compared with the initial histamine-evoked response prior to nicotine, with partial recovery over the ensuing 15 min. Neuronal excitation by ic nicotine was not mediated by histamine H1 receptors because ic injection of the H1 receptor antagonist, cetirizine, had no effect on ic nicotine-evoked responses, whereas it significantly attenuated ic histamine-evoked responses in the same neurons. The lowest-threshold portion of cutaneous receptive fields showed a significant expansion in area at 20 min after ic nicotine 10%, indicative of sensitization. Responses to 1% nicotine were significantly reduced after ic injection of the nicotinic antagonist, mecamylamine (0.1% ic), with no recovery over the ensuing 40-60 min. These data indicate that nicotine ic excites spinal WDR neurons, partly via neuronal nicotinic acetylcholine receptors that are presumably expressed in cutaneous nociceptor terminals. Repeated injections of high concentrations of nicotine led to tachyphylaxis and cross-tachyphylaxis with histamine, possibly relevant to peripheral analgesic effects of nicotine.

Masseteric inflammation-induced Fos protein expression in the trigeminal interpolaris/caudalis transition zone: contribution of somatosensory-vagal-adrenal integration

The effects of vagotomy and adrenalectomy on the expression of Fos protein in brainstem neurons following the inflammation of masseter muscle were examined in order to differentiate the Fos activation related to nociceptive processing in contrast to that due to somatoautonomic processing. The inflammation was induced by a unilateral injection of complete Freund's adjuvant (CFA) into the masseter muscle under methohexital anesthesia after a small skin-cut (S-cut). After the CFA injection, Fos positive neurons were identified in bilateral spinal trigeminal nucleus (VSP), nucleus tractus solitarius (NTS), ventrolateral medulla (VLM) and inferior medial olivary nucleus (IOM). At the level of the trigeminal subnucleus interpolaris/caudalis (Vi/Vc) transition zone, there was a selective induction of Fos-like immunoreactivity (LI) in the VSP and NTS, when compared to control rats (anesthesia with or without S-cut). A major portion of the Fos-LI in the VSP at the level of the caudal Vc was apparently activated by S-cut. Bilateral adrenalectomy or a unilateral vagotomy resulted in a selective reduction of inflammation-induced Fos-LI in the VSP at the Vi/Vc transition zone (P<0.05) and NTS (P<0.05), but had less effect on Fos-LI in the caudal Vc. These results suggest that the inflammation of the masseter muscle, an injury of orofacial deep tissue, results in a widespread change in neuronal activity in the VSP and NTS that depends in part on the integrity of the adrenal cortex and vagus. Thus, in addition to somatotopically organized nociceptive responses, orofacial deep tissue injury also is coupled to somatovisceral and somatoautonomic processing that contribute to central neural activation.

Neurobiological and psychophysical mechanisms underlying the oral sensation produced by carbonated water

Carbonated drinks elicit a sensation that is highly sought after, yet the underlying neural mechanisms are ill-defined. We hypothesize that CO(2) is converted via carbonic anhydrase into carbonic acid, which excites lingual nociceptors that project to the trigeminal nuclei. We investigated this hypothesis using three methodological approaches. Electrophysiological methods were used to record responses of single units located in superficial laminae of the dorsomedial aspect of trigeminal subnucleus caudalis (Vc) evoked by lingual application of carbonated water in anesthetized rats. After pretreatment of the tongue with the carbonic anhydrase inhibitor dorzolamide, neuronal responses to carbonated water were significantly attenuated, followed by recovery. Using c-Fos immunohistochemistry, we investigated the distribution of brainstem neurons activated by intraoral carbonated water. Fos-like immunoreactivity (FLI) was significantly higher in the superficial laminae of dorsomedial and ventrolateral Vc in animals treated with carbonated water versus controls. Dorzolamide pretreatment significantly reduced FLI in dorsomedial Vc. We also examined the sensation elicited by carbonated water in human psychophysical studies. When one side of the tongue was pretreated with dorzolamide, followed by bilateral application of carbonated water, a significant majority of subjects chose the untreated side as having a stronger sensation and assigned significantly higher intensity ratings to that side. Dorzolamide did not reduce irritation elicited by pentanoic acid. The present data support the hypothesis that carbonated water excites lingual nociceptors via a carbonic anhydrase-dependent process, in turn exciting neurons in Vc that are presumably involved in signaling oral irritant sensations.

Persistent Fos protein expression after orofacial deep or cutaneous tissue inflammation in rats: implications for persistent orofacial pain

This study was designed to systematically examine the effects of persistent orofacial tissue injury on prolonged neuronal activation in the trigeminal nociceptive pathways by directly comparing the effects of orofacial deep vs. cutaneous tissue inflammation on brainstem Fos protein expression, a marker of neuronal activation. Complete Freund's adjuvant (CFA) was injected unilaterally into the rat temporomandibular joint (TMJ) or perioral (PO) skin to produce inflammation in deep or cutaneous tissues, respectively. Rats were perfused 2 hours, 24 hours, 3 days, or 10 days following CFA injection. The TMJ and PO inflammation-induced Fos expression paralleled the intensity and course of inflammation over the 10-day observation period, suggesting that the increase in intensities and persistence of Fos protein expression may be associated with a maintained increase in peripheral input. Compared to PO CFA injection, the injection of CFA into the TMJ produced a significantly stronger inflammation associated with a greater Fos expression. In TMJ- but not in PO-inflamed rats, Fos-like immunoreactivity (LI) spread from superficial to deep upper cervical dorsal horn as the inflammation persisted and there was a dominant ipsilateral Fos-labeling in the paratrigeminal nucleus. Common to TMJ and PO inflammation, Fos-LI was induced in the trigeminal subnuclei interpolaris and caudalis, C1-2 dorsal horn, and other medullary nuclei. Substantial bilateral Fos-LI was found in the interpolaris-caudalis trigeminal transition zone. Further analysis revealed that Fos-LI in the ventral transition zone was equivalent bilaterally, whereas Fos-LI in the dorsal transition zone was predominantly ipsilateral to the inflammation. The differential induction of Fos expression suggests that an increase in TMJ C-fiber input after inflammation and robust central neuronal hyperexcitability contribute to persistent pain associated with temporomandibular disorders.

Fos-like immunoreactivity in the brain stem following oral quinine stimulation in decerebrate rats

The present study compared the distribution of Fos-like immunoreactivity (FLI) following intraoral stimulation with quinine monohydrochloride (QHCl) in awake intact rats to the pattern obtained in chronic supracollicular decerebrate (CD) rats. Because the behavioral rejection response to QHCl is evident in the CD rat, it was hypothesized that the pattern of FLI in the lower brain stem should be similar in both groups. Overall, the distribution of FLI in the brain stem was quite similar in both intact and CD groups, and QHCl stimulation increased FLI in the rostral (gustatory) nucleus of the solitary tract, the parabrachial nucleus (PBN), and the lateral reticular formation (RF) compared with an unstimulated control group. The CD group differed from the intact group, however, with a trend toward less FLI in the RF and a shift in the pattern of label away from the external subdivision of the PBN. CD rats also had increased FLI in the caudal nucleus of the solitary tract, with or without intraoral infusions. The distribution of QHCl-induced FLI in the brain stem of intact rats thus indicates both local sensorimotor processing as well as the influence of forebrain structures.

The effect of fentanyl on c-fos expression in the trigeminal brainstem complex produced by pulpal heat stimulation in the ferret

We have previously shown that Fos-like immunoreactivity (Fos-LI) is evoked in the brainstem of ferrets following stimulation of pulpal A delta and C fibers originating from the maxillary canine. This study evaluated the effects of the mu-opioid receptor agonist fentanyl on Fos expression evoked by noxious thermal stimulation of the right maxillary and mandibular canines in pentobarbital/chloral hydrate anesthetized adult male ferrets. Pulpal heating evoked Fos expression in two distinct regions of the spinal trigeminal nuclear complex: the transitional region between subnucleus interpolaris and caudalis (Vi/Vc) and within the subnucleus caudalis (Vc). More Fos positive cells were expressed in both regions ipsilateral to the site of stimulation compared with the contralateral side (P < 0.05, ANOVA). Pretreatment with fentanyl significantly and dose-dependently suppressed the number of Fos positive cells in both the Vi/Vc transitional region and Vc (P < 0.05, ANOVA). The suppressive effect of fentanyl on Fos expression was blocked by the intravenous administration of naloxone, an opioid antagonist, indicating a specific opioid receptor effect. In addition, opioid receptor antagonism with naloxone alone enhanced Fos expression in Vi/Vc and Vc in response to heat stimulation. The administration of naloxone without heat stimulation failed to evoke Fos expression in Vi/ Vc and Vc. These findings suggest that the activation of trigeminal Vi/Vc and Vc neurons by noxious dental heat stimulation is controlled by a naloxone sensitive endogenous opioid system as indicated by Fos expression. Collectively, these results suggest that neuronal populations in Vi/Vc and Vc regions may contribute to pain responses to noxious dental stimulation and these responses can be modulated by both endogenous and exogenous opioids.

The uroprotection of mesna on cyclophosphamide cystitis in rats. Its consequences on behaviour and brain activities

We studied the uroprotective effect of mesna, at doses of 40-300 mg/kg/i.p., in single or fractioned injections, on the development of cyclophosphamide (CP, 100 mg/kg/i.p.) cystitis in rats. The study concerns the histological, behavioural and nervous aspects of the disease. The specific effects of mesna, when injected alone, have also been considered. The mesna itself does not have specific deleterious effects, except at a dose of 300 mg/kg which provokes a moderate vesical inflammation although without consequence on the animal's behaviour. Mesna offers good protection against CP cystitis for only certain posologies. The uroprotective effects of mesna reach maxima at doses of 40-100 mg/kg and for fractioned injections given over the entire time frame of the urinary toxic release. The uroprotective effects of other posologies are only partial. The nervous activities were studied through the expression of Fos protein. The repetitive intraperitoneal injection of mesna induced a spinal activity and a preferential contralateral activity of the trigemino/reticular areas of the brainstem spinal cord junction--an effect which was reduced in the presence of CP. The prevention of cystitis by mesna was accompanied only by a reduction in spinal Fos activity, the supraspinal activities remaining high and in strict relationship with the vagal afferent activity. In conclusion, the uroprotective effect of mesna, which requires appropriate posologies, has led to the confirmation of the spinal actions of the CP cystitis, probably via the pelvic nerve, but did not allow a clear distinction between the consequences of the systemic (vagal) and local (spinal, pelvic) actions of CP at supraspinal level.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Cells of origin of the trigeminohypothalamic tract in the rat

Recent studies have demonstrated that a large number of spinal cord neurons convey somatosensory and visceral nociceptive information directly from cervical, lumbar, and sacral spinal cord segments to the hypothalamus. Because sensory information from head and orofacial structures is processed by all subnuclei of the trigeminal brainstem nuclear complex (TBNC) we hypothesized that all of them contain neurons that project directly to the hypothalamus. In the present study, we used the retrograde tracer Fluoro-Gold to examine this hypothesis. Fluoro-Gold injections that filled most of the hypothalamus on one side labeled approximately 1,000 neurons (best case = 1,048, mean = 718 +/- 240) bilaterally (70% contralateral) within all trigeminal subnuclei and C1-2. Of these neurons, 86% were distributed caudal to the obex (22% in C2, 22% in C1, 23% in subnucleus caudalis, and 18% in the transition zone between subnuclei caudalis and interpolaris), and 14% rostral to the obex (6% in subnucleus interpolaris, 4% in subnucleus oralis, and 4% in subnucleus principalis). Caudal to the obex, most labeled neurons were found in laminae I-II and V and the paratrigeminal nucleus, and fewer neurons in laminae III-IV and X. The distribution of retrogradely labeled neurons in TBNC gray matter areas that receive monosynaptic input from trigeminal primary afferent fibers innervating extracranial orofacial structures (such as the cornea, nose, tongue, teeth, lips, vibrissae, and skin) and intracranial structures (such as the meninges and cerebral blood vessels) suggests that sensory and nociceptive information originating in these tissues could be transferred to the hypothalamus directly by this pathway.

Activation of neurons in rat trigeminal subnucleus caudalis by different irritant chemicals applied to oral or ocular mucosa

To investigate the role of trigeminal subnucleus caudalis in neural mechanisms of irritation, we recorded single-unit responses to application of a variety of irritant chemicals to the tongue or ocular mucosa in thiopental-anesthetized rats. Recordings were made from wide dynamic range (WDR) and nociceptive-specific units in superficial layers of the dorsomedial caudalis (0-3 mm caudal to obex) responsive to mechanical stimulation and noxious heating of the ipsilateral tongue ("tongue" units) and from WDR units in ventrolateral caudalis (0-2 caudal to obex) responsive to mechanical and noxious thermal stimulation of cornea-conjunctiva and frequently also surrounding skin ("cornea-conjunctival" units). The following chemicals were delivered topically (0.1 ml) onto the dorsal anterior tongue or instilled into the ipsilateral eye: capsaicin (0.001-1% = 3.3 x 10(-2) to 3.3 x 10(-5) M), ethanol (15-80%), histamine (0.01-10% = 9 x 10(-1) to 9 x 10(-4) M), mustard oil (allyl-isothiocyanate, 4-100% = 4 x 10(-1) to 10 M), NaCl (0.5-5 M), nicotine (0.01-10% = 6 x 10(-1) to 6 x 10(-4) M), acidified phosphate buffer (pH 1-6), piperine (0.01-1% = 3.5 x 10(-2) to 3.5 x 10(-4) M), serotonin (5-HT; 0.3-3% = 1.4 x 10(-1) to 1.4 x 10(-2) M), and carbonated water. The dose-response relationship and possible tachyphylaxis were tested for each chemical. Of 32 tongue units, 31 responded to one or more, and frequently all, chemicals tested. The population responded to 75.3% of the various chemicals tested (</=10 per unit). The incidence of responses was independent of the order of chemicals tested, except for capsaicin, which reduced subsequent responses. Responses to histamine, nicotine, 5-HT, and ethanol had a more rapid onset and shorter duration compared with capsaicin, acid, and mustard oil. Responses to all chemicals increased in a dose-related manner. Successive responses to repeated application decreased significantly for nicotine, 5-HT, capsaicin, and piperine. Spontaneous firing increased significantly 5-10 min after initial application of capsaicin. Of 31 corneal-conjunctival units, 29 responded to one or more chemicals, and the population responded to 65% of all chemicals tested. Responses increased in a dose-related manner for all chemicals, and successive responses decreased significantly for histamine, nicotine, ethanol, acid, and capsaicin. Responses of tongue units to histamine and nicotine were reduced significantly by ceterizine (H1 antagonist) and mecamylamine, respectively. Mecamylamine also significantly reduced responses of corneal-conjunctival units to nicotine. Different classes of irritant chemicals contacting the oral or ocular mucosa can activate individual sensory neurons in caudalis, presumably via independent peripheral transduction mechanisms. Multireceptive units with input from the tongue or cornea-conjunctiva exhibited a similar spectrum of excitability to different irritant chemicals. Such neurons would not be capable of discriminating among different chemically evoked irritant sensations but could contribute to a common chemical sense.

The NMDA receptor antagonist MK-801 reduces capsaicin-induced c-fos expression within rat trigeminal nucleus caudalis

The effect of the N-methyl-D-aspartate (NMDA) receptor antagonist (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-i mine hydrogen maleate (MK-801) was examined on c-fos-like immunoreactivity (c-fos-LI) in urethane-anesthetized Sprague-Dawley rats using a polyclonal antibody. C-fos, an indicator of neuronal activation, was assessed within the trigeminal nucleus caudalis (TNC), area postrema. lateral reticular and solitary tract nuclei 2 h after intracisternal injection of capsaicin. C-fos-positive cells were counted at three representative levels corresponding to obex, -2.05 mm and -6.45 mm in 18 tissue sections (50 microm). A weighted average was obtained reflecting total brainstem expression within lamina I, II of TNC using a recently validated method. Capsaicin (0.1, 1, 5, 10 and 15 nmol) caused a dose-dependent labeling of cells in lamina I, II at obex similar to that previously reported after intracisternal blood or carrageenin administration in rats and guinea pigs. MK-801 (0.3, 1 and 3 mg/kg) administered i.p. 30 min before capsaicin (5 nmol in 100 microl artificial CSF) reduced significantly and dose-dependently (12%, 36% and 47%, respectively) the c-fos-LI cells in TNC at each level from rostral to caudal but not in solitary tract, area postrema and lateral reticular nuclei, and for unexplained reasons, increased c-fos-LI within the inferior olive. These results suggest that NMDA receptors provide a potential therapeutic target for cephalic pain (e.g. migraine) due to trigeminovascular activation from meningeal afferents.