Spontaneous painful disease in companion animals can facilitate the development of chronic pain therapies for humans.

OBJECTIVE: To outline the role that spontaneous osteoarthritis (OA) in companion animals can play in translational research and therapeutic pharmacological development. OUTLINE: Narrative review summarizing the opportunities and limitations of naturally occurring, spontaneous OA as models of human OA pain, with a focus on companion animal pets. The background leading to considering inserting spontaneous disease models in the translational paradigm is provided. The utility of this model is discussed in terms of outcome measures that have been validated as being related to pain, and in terms of the potential for target discovery is outlined. The limitations to using companion animal pets as models of human disease are discussed. CONCLUSIONS: Although many steps along the translational drug development pathway have been identified as needing improvement, spontaneous painful OA in companion animals offers translational potential. Such 'models' may better reflect the complex genetic, environmental, temporal and physiological influences present in humans and current data suggests the predictive validity of the models are good. The opportunity for target discovery exists but is, as yet, unproven.

GWAS Identifies New Loci for Painful Temporomandibular Disorder: Hispanic Community Health Study/Study of Latinos.

Temporomandibular disorder (TMD) is a musculoskeletal condition characterized by pain and reduced function in the temporomandibular joint and/or associated masticatory musculature. Prevalence in the United States is 5% and twice as high among women as men. We conducted a discovery genome-wide association study (GWAS) of TMD in 10,153 participants (769 cases, 9,384 controls) of the US Hispanic Community Health Study/Study of Latinos (HCHS/SOL). The most promising single-nucleotide polymorphisms (SNPs) were tested in meta-analysis of 4 independent cohorts. One replication cohort was from the United States, and the others were from Germany, Finland, and Brazil, totaling 1,911 TMD cases and 6,903 controls. A locus near the sarcoglycan alpha ( SGCA), rs4794106, was suggestive in the discovery analysis ( P = 2.6 × 106) and replicated (i.e., 1-tailed P = 0.016) in the Brazilian cohort. In the discovery cohort, sex-stratified analysis identified 2 additional genome-wide significant loci in females. One lying upstream of the relaxin/insulin-like family peptide receptor 2 ( RXP2) (chromosome 13, rs60249166, odds ratio [OR] = 0.65, P = 3.6 × 10-8) was replicated among females in the meta-analysis (1-tailed P = 0.052). The other (chromosome 17, rs1531554, OR = 0.68, P = 2.9 × 10-8) was replicated among females (1-tailed P = 0.002), as well as replicated in meta-analysis of both sexes (1-tailed P = 0.021). A novel locus at genome-wide level of significance (rs73460075, OR = 0.56, P = 3.8 × 10-8) in the intron of the dystrophin gene DMD (X chromosome), and a suggestive locus on chromosome 7 (rs73271865, P = 2.9 × 10-7) upstream of the Sp4 Transcription Factor ( SP4) gene were identified in the discovery cohort, but neither of these was replicated. The SGCA gene encodes SGCA, which is involved in the cellular structure of muscle fibers and, along with DMD, forms part of the dystrophin-glycoprotein complex. Functional annotation suggested that several of these variants reside in loci that regulate processes relevant to TMD pathobiologic processes.

Painful Temporomandibular Disorder: Decade of Discovery from OPPERA Studies.

In 2006, the OPPERA project (Orofacial Pain: Prospective Evaluation and Risk Assessment) set out to identify risk factors for development of painful temporomandibular disorder (TMD). A decade later, this review summarizes its key findings. At 4 US study sites, OPPERA recruited and examined 3,258 community-based TMD-free adults assessing genetic and phenotypic measures of biological, psychosocial, clinical, and health status characteristics. During follow-up, 4% of participants per annum developed clinically verified TMD, although that was a "symptom iceberg" when compared with the 19% annual rate of facial pain symptoms. The most influential predictors of clinical TMD were simple checklists of comorbid health conditions and nonpainful orofacial symptoms. Self-reports of jaw parafunction were markedly stronger predictors than corresponding examiner assessments. The strongest psychosocial predictor was frequency of somatic symptoms, although not somatic reactivity. Pressure pain thresholds measured at cranial sites only weakly predicted incident TMD yet were strongly associated with chronic TMD, cross-sectionally, in OPPERA's separate case-control study. The puzzle was resolved in OPPERA's nested case-control study where repeated measures of pressure pain thresholds revealed fluctuation that coincided with TMD's onset, persistence, and recovery but did not predict its incidence. The nested case-control study likewise furnished novel evidence that deteriorating sleep quality predicted TMD incidence. Three hundred genes were investigated, implicating 6 single-nucleotide polymorphisms (SNPs) as risk factors for chronic TMD, while another 6 SNPs were associated with intermediate phenotypes for TMD. One study identified a serotonergic pathway in which multiple SNPs influenced risk of chronic TMD. Two other studies investigating gene-environment interactions found that effects of stress on pain were modified by variation in the gene encoding catechol O-methyltransferase. Lessons learned from OPPERA have verified some implicated risk factors for TMD and refuted others, redirecting our thinking. Now it is time to apply those lessons to studies investigating treatment and prevention of TMD.

COMT Diplotype Amplifies Effect of Stress on Risk of Temporomandibular Pain.

When measured once, psychological stress predicts development of painful temporomandibular disorder (TMD). However, a single measurement fails to characterize the dynamic nature of stress over time. Moreover, effects of stress on pain likely vary according to biological susceptibility. We hypothesized that temporal escalation in stress exacerbates risk for TMD, and the effect is amplified by allelic variants in a gene, catechol-O-methyltransferase (COMT), regulating catechol neurotransmitter catabolism. We used data from the Orofacial Pain: Prospective Evaluation and Risk Assessment prospective cohort study of 2,707 community-dwelling adults with no lifetime history of TMD on enrollment. At baseline and quarterly periods thereafter, the Perceived Stress Scale (PSS) measured psychological stress. Genotyped DNA from blood samples determined COMT diplotypes. During follow-up of 0.25 to 5.2 y, 248 adults developed examiner-verified incident TMD. PSS scores at baseline were 20% greater (P < 0.001) in adults who developed incident TMD compared with TMD-free controls. Baseline PSS scores increased by 9% (P = 0.003) during follow-up in cases but remained stable in controls. This stress escalation was limited to incident cases with COMT diplotypes coding for low-activity COMT, signifying impaired catabolism of catecholamines. Cox regression models confirmed significant effects on TMD hazard of both baseline PSS (P < 0.001), modeled as a time-constant covariate, and change in PSS (P < 0.001), modeled as a time-varying covariate. Furthermore, a significant (P = 0.04) interaction of COMT diplotype and time-varying stress showed that a postbaseline increase of 1.0 standard deviation in PSS more than doubled risk of TMD incidence in subjects with low-activity COMT diplotypes (hazard ratio = 2.35; 95% confidence limits: 1.66, 3.32), an effect not found in subjects with high-activity COMT diplotypes (hazard ratio = 1.42; 95% confidence limits: 0.96, 2.09). Findings provide novel insights into dynamic effects of psychological stress on TMD pain, highlighting that effects are most pronounced in individuals whose genetic susceptibility increases responsiveness to catecholamine neurotransmitters.

Sleep apnea symptoms and risk of temporomandibular disorder: OPPERA cohort.

The authors tested the hypothesis that obstructive sleep apnea (OSA) signs/symptoms are associated with the occurrence of temporomandibular disorder (TMD), using the OPPERA prospective cohort study of adults aged 18 to 44 years at enrollment (n = 2,604) and the OPPERA case-control study of chronic TMD (n = 1,716). In both the OPPERA cohort and case-control studies, TMD was examiner determined according to established research diagnostic criteria. People were considered to have high likelihood of OSA if they reported a history of sleep apnea or ≥ 2 hallmarks of OSA: loud snoring, daytime sleepiness, witnessed apnea, and hypertension. Cox proportional hazards regression estimated hazard ratios (HRs) and 95% confidence limits (CL) for first-onset TMD. Logistic regression estimated odds ratios (OR) and 95% CL for chronic TMD. In the cohort, 248 individuals developed first-onset TMD during the median 2.8-year follow-up. High likelihood of OSA was associated with greater incidence of first-onset TMD (adjusted HR = 1.73; 95% CL, 1.14, 2.62). In the case-control study, high likelihood of OSA was associated with higher odds of chronic TMD (adjusted OR = 3.63; 95% CL, 2.03, 6.52). Both studies supported a significant association of OSA symptoms and TMD, with prospective cohort evidence finding that OSA symptoms preceded first-onset TMD.

Janus molecule I: dichotomous effects of COMT in neuropathic vs nociceptive pain modalities.

The enzyme catechol-O-methyltransferase (COMT) has been shown to play a critical role in pain perception by regulating levels of epinephrine (Epi) and norepinephrine (NE). Although the key contribution of catecholamines to the perception of pain has been recognized for a long time, there is a clear dichotomy of observations. More than a century of research has demonstrated that increasing adrenergic transmission in the spinal cord decreases pain sensitivity in animals. Equally abundant evidence demonstrates the opposite effect of adrenergic signaling in the peripheral nervous system, where adrenergic signaling increases pain sensitivity. Viewing pain processing within spinal and peripheral compartments and determining the directionality of adrenergic signaling helps clarify the seemingly contradictory findings of the pain modulatory properties of adrenergic receptor agonists and antagonists presented in other reviews. Available evidence suggests that adrenergic signaling contributes to pain phenotypes through α(1/2) and ß(2/3) receptors. While stimulation of α(2) adrenergic receptors seems to uniformly produce analgesia, stimulation of α(1) or ß receptors produces either analgesic or hyperalgesic effects. Establishing the directionality of adrenergic receptor modulation of pain processing, and related COMT activity in different pain models are needed to bring meaning to recent human molecular genetic findings. This will enable the translation of current findings into meaningful clinical applications such as diagnostic markers and novel therapeutic targets for complex human pain conditions.

Acupuncture points are large fields: the fuzziness of acupuncture point localization by doctors in practice.

BACKGROUND: Acupuncture textbooks, schools, practitioners and clinical researchers designing randomized controlled trials on acupuncture all assume that acupuncture points are small and must be located precisely. METHOD: Seventy-one medical doctors with ≥200 h acupuncture training and ≥2 years of clinical experience independently identified 23 commonly used acupuncture points on a male volunteer, using sticky transparent films with an X/Y grid placed asymmetrically around acupuncture points. RESULTS: For each acupuncture point, the field covering 95% (68%) of all point locations varied from 2.7 (0.7) cm(2) for PC-6 up to 41.4 (10.2) cm(2) for ST-38. Commonly-used acupuncture points showed unexpectedly large variance in location: 95% (or 68%) areas were SP-6: 12.2 cm(2) (3.0 cm(2) ), ST-36: 20.7 cm(2) (5.1 cm(2) ), LI-15: 18.7 cm(2) (4.6 cm(2) ), BL-23: 22.4 cm(2) (5.6 cm(2) ) and BL-54: 22.5 cm(2) (5.6 cm(2) ). Points close to anatomical landmarks (forearm, ankle, poplitea; BL-60, BL-40, TW-5, PC-6) were located with less variance. Precision of point location was independent of length of acupuncture experience, kind of training or medical specialty. CONCLUSIONS: In respect to the high degree of variation in the localization of acupuncture points, we suggest that the term 'acupuncture field' is more appropriate than 'acupuncture points' to describe the clinical reality; for the design of sham-controlled acupuncture trials, we recommend a minimum distance of 6 cm between verum and sham points on face, hands and feet, and up to 12 cm for all other parts of the body.

Functional properties of tooth pulp neurons responding to thermal stimulation.

The response properties of tooth pulp neurons that respond to noxious thermal stimulation of the dental pulp have been not well-studied. The present study was designed to characterize the response properties of tooth pulp neurons to noxious thermal stimulation of the dental pulp. Experiments were conducted on 25 male ferrets, and heat stimulation was applied by a computer-controlled thermode. Only 15% of tooth pulp neurons (n = 39) responded to noxious thermal stimulation of the teeth. Tooth pulp neurons were found in both the superficial and deep nuclear regions of the subnucleus caudalis (Vc) and in the interface between the nucleus caudalis and interpolaris (Vc/Vi). Thirty-seven neurons had cutaneous receptive fields and were classified as either NS (16) or WDR (21) neurons. Repeated heat stimulation of the dental pulp sensitized and increased the number of electrically evoked potentials of tooth pulp neurons. These results provide evidence that both the Vc and Vc/Vi regions contain neurons that respond to noxious thermal stimulation of the dental pulp, and that these cells may contribute to the sensitization process associated with symptomatic pulpitis.

Pituitary function in paediatric survivors of severe traumatic brain injury.

BACKGROUND: Traumatic brain injury (TBI)-mediated hypopituitarism is an increasingly recognised problem. Paediatric survivors of TBI may be vulnerable to the possible effects of pituitary deficits as pituitary hormones control normal growth and development. Research concerning pituitary dysfunction following childhood TBI is limited. AIM: To identify pituitary dysfunction in paediatric survivors of severe TBI. METHODS: Of 1020 children who sustained a TBI and were admitted to the Royal Children's Hospital, Melbourne, Australia over 10 years, 117 were identified as survivors of severe TBI. 54 patients (31 males) were enrolled and administered questionnaires regarding quality of life and possible endocrine dysfunction. Where indicated, hormone testing was performed. RESULTS: 29 of the 54 patients underwent hormonal investigations, while 21 who had satisfactory questionnaires did not (four patients had already been diagnosed with pituitary deficiencies). In those 29 patients, TBI occurred at ages ranging from 0.25 to 16.80 years (median 9.7 years). Time from TBI to study ranged from 0.9 to 8.5 years (median 4.5 years). Of the 54 patients, nine had pituitary dysfunction, of whom four had multiple pituitary hormone deficiencies. CONCLUSIONS: Our study that confirms that paediatric survivors of severe TBI may develop pituitary dysfunction. Pituitary function should therefore be determined in these patients.

Influence of psychological factors on risk of temporomandibular disorders.

Psychological characteristics potentially may be a cause or consequence of temporomandibular disorder (TMD). We hypothesized that psychological characteristics associated with pain sensitivity would influence risk of first-onset TMD, but the effect could be attributed to variation in the gene encoding catechol-O-methyltransferase (COMT). We undertook a prospective cohort study of healthy female volunteers aged 18-34 yrs. At baseline, participants were genotyped, they completed psychological questionnaires, and underwent quantitative sensory testing to determine pain sensitivity. We followed 171 participants for up to three years, and 8.8% of them were diagnosed with first-onset TMD. Depression, perceived stress, and mood were associated with pain sensitivity and were predictive of 2- to 3-fold increases in risk of TMD (P < 0.05). However, the magnitude of increased TMD risk due to psychological factors remained unchanged after adjustment for the COMT haplotype. Psychological factors linked to pain sensitivity influenced TMD risk independently of the effects of the COMT haplotype on TMD risk.

Human catechol-O-methyltransferase haplotypes modulate protein expression by altering mRNA secondary structure.

Catechol-O-methyltransferase (COMT) is a key regulator of pain perception, cognitive function, and affective mood. Three common haplotypes of the human COMT gene, divergent in two synonymous and one nonsynonymous position, code for differences in COMT enzymatic activity and are associated with pain sensitivity. Haplotypes divergent in synonymous changes exhibited the largest difference in COMT enzymatic activity, due to a reduced amount of translated protein. The major COMT haplotypes varied with respect to messenger RNA local stem-loop structures, such that the most stable structure was associated with the lowest protein levels and enzymatic activity. Site-directed mutagenesis that eliminated the stable structure restored the amount of translated protein. These data highlight the functional significance of synonymous variations and suggest the importance of haplotypes over single-nucleotide polymorphisms for analysis of genetic variations.

Enlarged parietal foramina: MR imaging features in the fetus and neonate.

Enlarged parietal foramina are believed to be benign and familial and due to a variable degree of defective intramembranous ossification of the parietal bones. We report 2 patients with this condition in whom fetal and neonatal MR imaging studies illustrate the antenatal and perinatal evolution of this condition and the associated persistence of a falcine sinus. We discuss its relationship to the spectrum of cephalocoeles.

Health related quality of life and psychological outcome in patients treated for craniopharyngioma in childhood.

UNLABELLED: Patients with craniopharyngioma are at risk for many adverse effects related to the tumour's invasive behaviour and its proximity to many vital structures. Profound psychosocial problems, memory impairment, pituitary and hypothalamic dysfunction in addition to the physical handicap of visual loss are frequently recognized sequelae of craniopharyngioma treatment. OBJECTIVES: To examine health related quality of life (QoL) and psychological outcomes of patients treated for craniopharyngioma at the Royal Children's Hospital, Melbourne, between January 1980 and September 2003. PATIENTS: Seven (17.4%) of 46 (26 male) had died. Thirty-nine remained, of whom 30 were contactable. Eighteen of 30 (8 male), mean age 21.2 +/- 6.7 years, agreed to evaluation, of whom 16/18 (88.9%) had three or more pituitary hormone deficiencies, 11/18 had visual impairment and 9/18 obesity. MEASUREMENTS: The Adult GH-Deficient Assessment (AGHDA) and Psychological General Well-Being (PGWB) questionnaires were employed to assess quality of life in patients and age- and sex-matched healthy controls. Additional psychological assessment, including intellectual and academic skills, emotional function, and adaptive behaviour, had been undertaken in 12 patients at a previous time. RESULTS: High levels of physical morbidity and psychological disability were described. The General Health score of patients was significantly worse than for controls on PGWB (p = 0.025), anxiety was higher in those who had surgery alone (p = 0.008) and subjective QoL associated with GHD using AGHDA was lower (p = 0.006). Few craniopharyngioma survivors (18/30) were available for evaluation, demonstrating difficulties in attempts to assess this complex group. The discrepancy between results of objective and subjective measures of QoL is discussed in terms of adaptation to illness, disabilities and changed perception of life fulfilment. CONCLUSIONS: Craniopharyngioma and its treatment result in significant, complex medical, social, psychological and emotional difficulties. The degree of global disability is not reflected in subjective QoL reports for this group, highlighting the need for careful selection of assessment instruments.

Increased perception of post-ischemic paresthesias in depressed subjects.

A psychophysical assessment of sensory activity linked to unmyelinated and myelinated primary afferents was conducted by estimating the intensity of thermal and tactile post-ischemic paresthesias in 11 nontreated depressed subjects (Zung's index > or =50) and 19 controls. Blood flow in the dominant forearm was arrested until ischemic pain tolerance was reached. Ischemic pain and post-ischemic paresthesias were numerically rated. The duration of blood flow occlusion to the time of ischemic pain tolerance was similar in both groups. Thermal (warm/cool) and tactile (tingling) paresthesias were 96% and 57% more intense in depressed than in control subjects, respectively. Zung's depression scores were positively correlated with the tingling and thermal paresthesias. Ischemic pain intensity correlated positively with thermal paresthesias. These findings suggest that depression is associated with enhanced sensory paresthesias that are known to be predominately linked to unmyelinated afferent activity.

Sex differences in chest pain in patients with documented coronary artery disease and exercise-induced ischemia: Results from the PIMI study.

BACKGROUND: Sex differences in the pathophysiologic course of coronary artery disease (CAD) are widely recognized, yet accurate diagnosis of coronary artery disease in women remains challenging. METHODS: To determine sex differences in the clinical manifestation of CAD, we studied chest pain reported during daily activities, exercise, and mental stress in 170 men and 26 women. All patients had documented CAD (>50% narrowing in at least 1 major coronary artery or prior myocardial infarction) and all had 1-mm ST-segment depression on treadmill exercise. We collected psychologic test results, serum samples (potassium, epinephrine, norepinephrine, cortisol, b-endorphin, and glucose), and cardiac function, sensory threshold, and autonomic function data at specified times before, during, or after exercise and mental stress tests to assess measures of depression, anxiety, and neurohormonal and thermal pain perception. RESULTS: Women reported chest pain more often than men during daily activities (P =.04) and during laboratory mental stressors (P =.01) but not during exercise. Men had lower scores than women on measures of depression, trait anxiety, harm avoidance, and reward dependence (P <.05 for all). Women had significantly lower plasma b-endorphin levels at rest (4.2 +/- 3.9 vs 5.0 +/- 2.5 pmol/L for men, P =.005) and at maximal mental stress (6.4 +/- 5.1 vs 7.4 +/- 3.5 pmol/L for men, P <.01). A higher proportion of women than men had marked pain sensitivity to graded heat stimuli applied to skin (hot pain threshold <41 degrees C, 33% vs 10%, P =.001). CONCLUSIONS: Our results reflect sex differences in the affective and discriminative aspects of pain perception and may help explain sex-related differences in clinical presentations.

Estrous cycle modulation of nociceptive behaviors elicited by electrical stimulation and formalin.

The impact of circulating ovarian hormones on nociceptive behaviors elicited by phasic and tonic stimuli was evaluated in rats using two behavioral tests: an operant escape task and the formalin test. The operant escape task was structured to separately evaluate hindlimb flexion reflexes, the latency of escape, and the amplitude of peak vocalization to a series of phasic electrocutaneous stimuli (0.05-0.8 mA), whereas the formalin test evaluated nociceptive behaviors elicited by tonic stimulation following a subcutaneous injection of dilute formalin (1%). Hindlimb reflex amplitude, escape latency, and peak vocalization varied across the estrous cycle, such that rats were most sensitive to electrical stimuli during proestrus (reflex and escape latency) and diestrus (vocalization). Furthermore, morphine-induced (3 mg/kg sc) attenuation of hindlimb reflex amplitude was sensitive to estrous cycling. During proestrus, morphine produced less attenuation of hindlimb reflex amplitude than during nonproestrus phases. However, estrous cycling did not alter nociceptive behaviors elicited by 1% formalin. These data support the notion that circulating ovarian hormones may differentially modulate behaviors associated with phasic and tonic pain.

Lack of effect of ondansetron on the pharmacokinetics and analgesic effects of morphine and metabolites after single-dose morphine administration in healthy volunteers.

AIMS: The purpose of this investigation was to study the influence of ondansetron on the single-dose pharmacokinetics and the analgesic effects elicited by morphine and the 3- and 6-glucuronide metabolites of morphine in healthy volunteers. METHODS: This was a randomized, double-blind, placebo-controlled, two-way crossover study in which six male and six female subjects were administered a single 10 mg intravenous dose of morphine sulphate, followed 30 min later by a single 16 mg intravenous dose of ondansetron hydrochloride or placebo. Serum and urine concentrations of morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) samples were quantified over 48 h using high performance liquid chromatography with detection by mass spectrometry. Analgesia was assessed in the volunteers with a contact thermode device to provide a thermal pain stimulus. Four analgesic response variables were measured including thermal pain threshold, thermal pain tolerance, temporal summation of pain and mood state. RESULTS: The two treatments appeared to be equivalent based on the 90% confidence intervals (0.6, 1.67) of the least squares means ratio. All least squares means ratio confidence intervals for each parameter, for each analyte fell within the specified range, demonstrating a lack of an interaction. CONCLUSIONS: The results of this study suggest that administration of ondansetron (16 mg i.v.) does not alter the pharmacokinetics of morphine and its 3- or 6-glucuronide metabolites to a clinically significant extent, nor does it affect the overall analgesic response to morphine as measured by the contact thermode system.

Long-lasting inflammation and long-term hyperalgesia after subcutaneous formalin injection into the rat hindpaw.

Subcutaneous formalin injection is widely used as a nociceptive stimulus in the rat. This procedure evokes overt behaviors that last about 90 minutes. However, little is known about the duration of paw inflammation and alterations in pain sensitivity to noxious stimuli after 2 hours. We studied the nociceptive responses to thermal and mechanical stimuli 2 hours to 4 weeks after formalin injection into the dorsal or plantar side of the hindpaw. Thirty-two adult male Sprague-Dawley rats were divided into 3 groups: In group I, 50 microL of 5% formalin was injected into the plantar side (n = 12); in group II, 50 microL of 5% formalin was injected into the dorsal side (n = 12); in group III, 50 microL saline was injected into the dorsal or plantar side of the hindpaw (n = 8). Nociceptive responses to thermal and mechanical stimuli applied to the dorsal or plantar surfaces of the injected and the contralateral hindpaws were recorded. The injection of formalin into the rat's hindpaw produced a hypoalgesic region around the injection site. In contrast, hyperalgesic responses to thermal and mechanical stimulation were induced on the opposite surface of the injected hindpaw as well as in the contralateral noninjected hindpaw. The hyperalgesic responses, which were observed 2 hours after formalin administration, were enhanced 1 to 3 days after injection and lasted 3 to 4 weeks. These results suggest that peripheral inflammation after subcutaneous formalin injection produces a long-lasting sensitization. Possible mechanisms for these changes in nociception are discussed.

The effects of noxious dental heating on the jaw-opening reflex and trigeminal Fos expression in the ferret.

Previous studies have established that the activation of peripheral nociceptors alters the central processing of nociceptive stimuli. In this study, we examined whether noxious heating of the dental pulp enhances the nociceptive jaw-opening reflex (JOR) and the expression of the immediate early gene c-fos in chloral hydrate/pentobarbital-anesthetized ferrets. We hypothesized that the application of noxious heat to the dental pulp, a procedure that evokes a preferential activation of pulpal C-fibers, will enhance JOR responses to electrical stimulation of the tooth pulp and that this enhanced response will be associated with the expression of Fos protein in discrete regions of the trigeminal nucleus. Consistent with our predictions, we observed that noxious heat conditioning enhanced the JOR as indicated by an increase in the magnitude of the signal averaged digastric electromyogram response evoked by electrical stimuli applied to either a heat-conditioned maxillary canine or the contralateral nonconditioned canine. The enhancement in JOR responses was independent of temporal summation of the electrical stimulus for test stimuli delivered at either 1.0 or 0.1 Hz. Sensitization of the JOR was associated with an increase in the number of immunohistochemically identified Fos-positive nuclei in trigeminal caudalis (Vc) and the transition zone between trigeminal interpolaris and caudalis (Vi/Vc) ipsilateral to the site of stimulation compared with sham stimulated animals. These findings suggest that neuronal populations in Vc and Vi/Vc play a role in the enhanced reflex responses to tooth pulp stimulation and may contribute to the pain and hyperalgesia associated with a symptomatic pulpitis.

Effects of systemic morphine on escape latency and a hindlimb reflex response in the rat.

The present study uses focal electrical stimulation of myelinated nociceptors to simultaneously assess behavioral responses that are organized at spinal and supraspinal sites in the rat. Hindlimb reflex amplitude and the latency to operant escape responses by a forelimb were recorded for each stimulus presentation to a hindlimb across a wide range of intensities. This paradigm provided a tool whereby effects of morphine on conscious escape responses could be delineated from effects on a segmental flexion reflex over a range of doses. Administration of morphine (3 mg/kg and 10 mg/kg, subcutaneously) increased the latency of escape responses and decreased the amplitude of reflex responses in a dose-dependent manner. However, morphine produced a greater suppression of reflex responses compared with the increase in effects on escape latencies. The effects of morphine on escape latency were not expressed at the highest stimulus intensities (0.6 to 0.8 mA), whereas reflex responses were attenuated at all suprathreshold stimulus intensities. Thus, electrically evoked, spinal-mediated responses of rats are not affected by morphine in the same manner as electrically evoked supraspinal-mediated nociceptive behaviors. However, both measures confirm evidence that responses elicited by activation of myelinated afferents are less powerfully affected by morphine than responses to input from unmyelinated nociceptors.