An ethogram analysis of cutaneous thermal pain sensitivity and oxycodone reward-related behaviors in rats.

Inter-relationships between pain sensitivity, drug reward, and drug misuse are of considerable interest given that many analgesics exhibit misuse potential. Here we studied rats as they underwent a series of pain- and reward-related tests: cutaneous thermal reflex pain, induction and extinction of conditioned place preference to oxycodone (0.56 mg/kg), and finally the impact of neuropathic pain on reflex pain and reinstatement of conditioned place preference. Oxycodone induced a significant conditioned place preference that extinguished throughout repeated testing. Correlations identified of particular interest included an association between reflex pain and oxycodone-induced behavioral sensitization, and between rates of behavioral sensitization and extinction of conditioned place preference. Multidimensional scaling analysis followed by k-clustering identified three clusters: (1) reflex pain, rate of behavioral sensitization and rate of extinction of conditioned place preference (2) basal locomotion, locomotor habituation, acute oxycodone-stimulated locomotion and rate of change in reflex pain during repeated testing, and (3) magnitude of conditioned place preference. Nerve constriction injury markedly enhanced reflex pain but did not reinstate conditioned place preference. These results suggest that high rates of behavioral sensitization predicts faster rates of extinction of oxycodone seeking/reward, and suggest that cutaneous thermal reflex pain may be predictive of both.

Influence of TRPV1 on Thermal Nociception in Rats with Temporomandibular Joint Persistent Inflammation Evaluated by the Operant Orofacial Pain Assessment Device (OPAD).

Background: Temporomandibular joint (TMJ)-associated inflammation contributes to the pain reported by patients with temporomandibular disorders (TMD). It is common for patients diagnosed with TMD to report pain in the masticatory muscles and temporomandibular joints, headache, and jaw movement disturbances. Although TMD can have different origins, including trauma and malocclusion disorder, anxiety/depression substantially impacts the development and maintenance of TMD. In general, rodent studies on orofacial pain mechanisms involve the use of tests originally developed for other body regions, which were adapted to the orofacial area. To overcome limitations and expand knowledge in orofacial pain, our group validated and characterized an operant assessment paradigm in rats with both hot and cold stimuli as well mechanical stimuli. Nevertheless, persistent inflammation of the TMJ has not been evaluated with this operant orofacial pain assessment device (OPAD). Methods: We characterized the thermal orofacial sensitivity for cold, neutral, and hot stimuli during the development of TMD using the OPAD behavior test. In addition, we evaluated the role of transient receptor potential vanilloid 1 (TRPV1) expressing nociceptors in rats with persistent TMJ inflammation. The experiments were performed in male and female rats with TMJ inflammation induced by carrageenan (CARR). Additionally, resiniferatoxin (RTX) was administered into the TMJs prior CARR to lesion TRPV1-expressing neurons to evaluate the role of TRPV1-expressing neurons. Results: We evidenced an increase in the number of facial contacts and changes in the number of reward licks per stimulus on neutral (37°C) and cold (21°C) temperatures. However, at the hot temperature (42°C), the inflammation did not induce changes in the OPAD test. The prior administration of RTX in the TMJ prevented the allodynia and thermal hyperalgesia induced by CARR. Conclusion: We showed that TRPV-expressing neurons are involved in the sensitivity to carrageenan-induced pain in male and female rats evaluated in the OPAD.

An ethogram analysis of cutaneous thermal pain sensitivity and oxycodone reward-related behaviors in rats.

Inter-relationships between pain sensitivity, drug reward, and drug misuse are of considerable interest given that many analgesics exhibit misuse potential. Here we studied rats as they underwent a series of pain- and reward-related tests: cutaneous thermal reflex pain, induction and extinction of conditioned place preference to oxycodone (0.56 mg/kg), and finally the impact of neuropathic pain on reflex pain and reinstatement of conditioned place preference. Oxycodone induced a significant conditioned place preference that was extinguished throughout repeated testing. Correlations identified of particular interest included an association between reflex pain and oxycodone-induced behavioral sensitization, and between rates of behavioral sensitization and extinction of conditioned place preference. Multidimensional scaling analysis followed by k-clustering identified three clusters: (1) reflex pain and the rate of change in reflex pain response throughout repeated testing, (2) basal locomotion, locomotor habituation, and acute oxycodone-stimulated locomotion, and (3) behavioral sensitization, strength of conditioned place preference, and rate of extinction. Nerve constriction injury markedly enhanced reflex pain but did not reinstate conditioned place preference. These results support the notion that behavioral sensitization relates to the acquisition and extinction of oxycodone seeking/reward, but suggest that generally cutaneous thermal reflex pain poorly predicts oxycodone reward-related behaviors except for behavioral sensitization.

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

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

Oxycodone decreases anxiety-like behavior in the elevated plus-maze test in male and female rats.

The prescription opioid oxycodone is widely used for the treatment of pain in humans. Oxycodone misuse is more common among people with an anxiety disorder than those without one. Therefore, oxycodone might be misused for its anxiolytic properties. We investigated if oxycodone affects anxiety-like behavior in adult male and female rats. The rats were treated with oxycodone (0.178, 0.32, 0.56, or 1 mg/kg), and anxiety-like behavior was investigated in the elevated plus-maze test. Immediately after the elevated plus-maze test, a small open field test was conducted to determine the effects of oxycodone on locomotor activity. In the elevated plus-maze test, oxycodone increased the percentage of time spent on the open arms, the percentage of open arm entries, time on the open arms, open arm entries, and the distance traveled. The males treated with vehicle had a lower percentage of open arm entries than the females treated with vehicle, and oxycodone treatment led to a greater increase in the percentage of open arm entries in the males than females. Furthermore, the females spent more time on the open arms, made more open arm entries, spent less time in the closed arms, and traveled a greater distance than the males. In the small open field test, treatment with oxycodone did not affect locomotor activity or rearing. Sex differences were observed; the females traveled a greater distance and displayed more rearing than the males. In conclusion, oxycodone decreases anxiety-like behavior in rats, and oxycodone has a greater anxiolytic-like effect in males than females.

Assessment of Nerve Injury-induced Mechanical Hypersensitivity in Rats using an Orofacial Operant Pain Assay.

Pain has sensory and affective components. Unlike traditional, reflex-based pain assays, operant pain assays can produce more clinically relevant results by addressing the cognitive and motivational aspects of pain in rodents. This paper presents a protocol for assessing mechanical hypersensitivity following chronic constriction injury of the infraorbital nerves (CCI-ION) in rats using an orofacial operant pain system. Before CCI-ION surgery, rats were trained in an orofacial pain assessment device (OPAD) to drink sweetened condensed milk while making facial contact with the metal spiked bars and lick-tube. In this assay, rats can choose between receiving milk as a positive reinforcer or escaping an aversive mechanical stimulus that is produced by a vertical row of small pyramid-shaped spikes on each side of the reward access hole. Following 2 weeks of training in the OPAD and before the CCI-ION surgery, baseline mechanical sensitivity data were recorded for 5 days for each rat during a 10 min testing session. During a session, the operant system automatically records the number of reward bottle activations (licks) and facial contacts, contact duration, and latency to the first lick, among other measures. Following baseline measurements, rats underwent either CCI-ION or sham surgery. In this protocol, mechanical hypersensitivity was quantified by measuring the number of licks, latency to the first lick, the number of contacts, and the ratio of licks to facial contacts (L/F). The data showed that CCI-ION resulted in a significant decrease in the number of licks and the L/F ratio and an increase in the latency to the first lick, indicating mechanical hypersensitivity. These data support the use of operant-based pain assays to assess mechanical pain sensitivity in preclinical pain research.

Effects of Oxaliplatin on Facial Sensitivity to Cool Temperatures and TRPM8 Expressing Trigeminal Ganglion Neurons in Mice.

The chemotherapeutic agent oxaliplatin is commonly used to treat colorectal cancer. Although effective as a chemotherapeutic, it frequently produces painful peripheral neuropathies. These neuropathies can be divided into an acute sensitivity to cool temperatures in the mouth and face, and chronic neuropathic pain in the limbs and possible numbness. The chronic neuropathy also includes sensitivity to cool temperatures. Neurons that detect cool temperatures are reported to utilize Transient Receptor Potential Cation Channel, Subfamily M, Member 8 (TRPM8). Therefore, we investigated the effects of oxaliplatin on facial nociception to cool temperatures (18°C) in mice and on TRPM8 expressing trigeminal ganglion (TRG) neurons. Paclitaxel, a chemotherapeutic that is used to treat breast cancer, was included for comparison because it produces neuropathies, but acute cool temperature sensitivity in the oral cavity or face is not typically reported. Behavioral testing of facial sensitivity to 18°C indicated no hypersensitivity either acutely or chronically following either chemotherapeutic agent. However, whole cell voltage clamp experiments in TRPM8 expressing TRG neurons indicated that both oxaliplatin and paclitaxel increased Hyperpolarization-Activated Cyclic Nucleotide-Gated channel (HCN), voltage gated sodium channel (Nav), and menthol evoked TRPM8 currents. Voltage gated potassium channel (Kv) currents were not altered. Histological examination of TRPM8 fibers in the skin of the whisker pads demonstrated that the TRPM8 expressing axons and possible Merkel cell-neurite complexes were damaged by oxaliplatin. These findings indicate that oxaliplatin induces a rapid degeneration of TRG neuron axons that express TRPM8, which prevents evoked activation of the sensitized neurons and likely leads to reduced sensitivity to touch and cool temperatures. The changes in HCN, Nav, and TRPM8 currents suggest that spontaneous firing of action potentials may be increased in the deafferented neurons within the ganglion, possibly producing spontaneously induced cooling or nociceptive sensations.

Pharmacological Characterization of Orofacial Nociception in Female Rats Following Nitroglycerin Administration.

Rodent models of human disease can be valuable for understanding the mechanisms of a disease and for identifying novel therapies. However, it is critical that these models be vetted prior to committing resources to developing novel therapeutics. Failure to confirm the model can lead to significant losses in time and resources. One model used for migraine headache is to administer nitroglycerin to rodents. Nitroglycerin is known to produce migraine-like pain in humans and is presumed to do the same in rodents. It is not known, however, if the mechanism for nitroglycerin headaches involves the same pathological processes as migraine. In the absence of known mechanisms, it becomes imperative that the model not only translates into successful clinical trials but also successfully reverse translates by demonstrating efficacy of current therapeutics. In this study female rats were given nitroglycerin and nociception was evaluated in OPADs. Estrous was not monitored. Based on the ED50 of nitroglycerin a dose of 10 mg/kg was used for experiments. Sumatriptan, caffeine, buprenorphine and morphine were administered to evaluate the reverse translatability of the model. We found that nitroglycerin did not produce mechanical allodynia in the face of the rats, which is reported to be a consequence of migraine in humans. Nitroglycerin reduced the animals' participation in the assay. The reduced activity was verified using an assay to measure exploratory behavior. Furthermore, the effects of nitroglycerin were not reversed or prevented by agents that are effective acute therapies for migraine. Two interesting findings from this study, however, were that morphine and nitroglycerin interact to increase the rats' tolerance of mechanical stimuli on their faces, and they work in concert to slow down the central motor pattern generator for licking on the reward bottle. These interactions suggest that nitroglycerin generated nitric oxide and mu opioid receptors interact with the same neuronal circuits in an additive manner. The interaction of nitroglycerin and morphine on sensory and motor circuits deserves additional examination. In conclusion, based on the results of this study the use of nitroglycerin at these doses in naïve female rats is not recommended as a model for migraine headaches.

Behavioral characteristics of capsaicin mediated cutaneous, myogenic, and arthrogenic orofacial nociception in rats.

OBJECTIVE: To assess changes in orofacial tactile sensitivity and gnawing related to capsaicin-mediated cutaneous, myogenic, and arthrogenic nociception in the rat. DESIGN: After recovery from anesthesia, orofacial tactile sensitivity and gnawing were assessed using operant testing methods following capsaicin application. Twenty female CD-Hairless rats were tested with bilateral capsaicin cream application to the cheek or with isoflurane anesthesia alone. Following several weeks of recovery, animals (n = 20) received either 10 µL unilateral masseter injections of vehicle, or phosphate buffered saline (PBS) to assess injection sensitization. After several weeks, masseter capsaicin (1.0%) injections (10 µL) were assessed compared to vehicle and PBS (n = 13). Weeks later capsaicin TMJ injections were evaluated. Animals (n = 11) received either 10 µL unilateral TMJ injections of capsaicin solution (1%) or vehicle. RESULTS: Capsaicin cream to the skin significantly altered gnawing activity (increased puncture time by 248 s (p = 0.0002)) and tactile sensitivity (decreased tolerated bottle distance by 0.980 cm compared to isoflurane only (p = 0.0001)). Similarly, capsaicin masseter injection increased puncture time (339.6 s, p = 0.07) and decreased tolerated bottle distance (1.04 cm, p = 0.005) compared to vehicle. However, intra-articular capsaicin in the TMJ only modified gnawing (increased puncture time by 133 s), with no changes found in tactile sensitivity compared to vehicle. CONCLUSION: Application of capsaicin to the skin and masseter had similar behavioral effects; however, intra-articular injections to the TMJ only affected gnawing. These data indicate the behavioral changes in rodent models of myogenic and cutaneous pain may be markedly different than models of arthrogenic pain originating from the TMJ.

Pain control through selective chemo-axotomy of centrally projecting TRPV1+ sensory neurons.

Agonists of the vanilloid receptor transient vanilloid potential 1 (TRPV1) are emerging as highly efficacious nonopioid analgesics in preclinical studies. These drugs selectively lesion TRPV1+ primary sensory afferents, which are responsible for the transmission of many noxious stimulus modalities. Resiniferatoxin (RTX) is a very potent and selective TRPV1 agonist and is a promising candidate for treating many types of pain. Recent work establishing intrathecal application of RTX for the treatment of pain resulting from advanced cancer has demonstrated profound analgesia in client-owned dogs with osteosarcoma. The present study uses transcriptomics and histochemistry to examine the molecular mechanism of RTX action in rats, in clinical canine subjects, and in 1 human subject with advanced cancer treated for pain using intrathecal RTX. In all 3 species, we observe a strong analgesic action, yet this was accompanied by limited transcriptional alterations at the level of the dorsal root ganglion. Functional and neuroanatomical studies demonstrated that intrathecal RTX largely spares susceptible neuronal perikarya, which remain active peripherally but unable to transmit signals to the spinal cord. The results demonstrate that central chemo-axotomy of the TRPV1+ afferents underlies RTX analgesia and refine the neurobiology underlying effective clinical use of TRPV1 agonists for pain control.

Advanced glycation endproducts (AGEs) in saliva of patients with multiple myeloma - a pilot study.

Multiple myeloma (MM) is a hematological cancer with underlying causes associated with increased oxidative stress. Through signaling of their receptor RAGE, advanced glycation endproducts (AGEs) are known to increase oxidative stress associated with malignant transformation. In the present study, we have demonstrated that the levels of these compounds are increased in the saliva of myeloma patients with bone lesions. This data may provide a potential marker for bone lesions in MM and a potential target for the treatment of myeloma by blocking the AGEs or their receptor.

Sex differences in mouse Transient Receptor Potential Cation Channel, Subfamily M, Member 8 expressing trigeminal ganglion neurons.

The detection of cool temperatures is thought to be mediated by primary afferent neurons that express the cool temperature sensing protein Transient Receptor Potential Cation Channel, Subfamily M, Member 8 (TRPM8). Using mice, this study tested the hypothesis that sex differences in sensitivity to cool temperatures were mediated by differences in neurons that express TRPM8. Ion currents from TRPM8 expressing trigeminal ganglion (TRG) neurons in females demonstrated larger hyperpolarization-activated cyclic nucleotide-gated currents (Ih) than male neurons at both 30° and 18°C. Additionally, female neurons' voltage gated potassium currents (Ik) were suppressed by cooling, whereas male Ik was not significantly affected. At the holding potential tested (-60mV) TRPM8 currents were not visibly activated in either sex by cooling. Modeling the effect of Ih and Ik on membrane potentials demonstrated that at 30° the membrane potential in both sexes is unstable. At 18°, female TRPM8 TRG neurons develop a large oscillating pattern in their membrane potential, whereas male neurons become highly stable. These findings suggest that the differences in Ih and Ik in the TRPM8 TRG neurons of male and female mice likely leads to greater sensitivity of female mice to the cool temperature. This hypothesis was confirmed in an operant reward/conflict assay. Female mice contacted an 18°C surface for approximately half the time that males contacted the cool surface. At 33° and 10°C male and female mice contacted the stimulus for similar amounts of time. These data suggest that sex differences in the functioning of Ih and Ik in TRPM8 expressing primary afferent neurons leads to differences in cool temperature sensitivity.

Trigeminal neuroplasticity underlies allodynia in a preclinical model of mild closed head traumatic brain injury (cTBI).

Post-traumatic headache (PTH) following TBI is a common and often persisting pain disability. PTH is often associated with a multimodal central pain sensitization on the skin surface described as allodynia. However, the particular neurobiology underlying cTBI-induced pain disorders are not known. These studies were performed to assess trigeminal sensory sensitization and to determine if sensitization measured behaviorally correlated with detectable changes in portions of the trigeminal sensory system (TSS), particularly trigeminal nucleus, thalamus, and sensory cortex. Thermal stimulation is particularly well suited to evaluate sensitization and was used in these studies. Recent advances in the use of reward/conflict paradigms permit use of operant measures of behavior, versus reflex-driven response behaviors, for thermal sensitization studies. Thus, to quantitate facial thermal sensitization (allodynia) in the setting of acute TBI, the current study utilized an operant orofacial pain reward/conflict testing paradigm to assess facial thermal sensitivity in uninjured control animals compared with those two weeks after cTBI in a rodent model. Significant reductions in facial contact/lick behaviors were observed in the TBI animals using either cool or warm challenge temperatures compared with behaviors in the normal animals. These facial thermal sensitizations correlated with detectable changes in multiple levels of the TSS. The immunohistochemical (IHC) studies revealed significant alterations in the expression of the serotonin (5-HT), neurokinin 1 receptor (NK1R), norepinephrine (NE), and gamma-aminobutyric acid (GABA) in the caudal trigeminal nucleus, thalamic VPL/VPM nucleus, and sensory cortex of the orofacial pain pathways. There was a strong correlation between increased expression of certain IHC markers and increased behavioral markers for facial sensitization. The authors conclude that TBI-induced changes observed in the TSS are consistent with the expression of generalized facial allodynia following cTBI. To our knowledge, this is the first report of orofacial sensitization correlated with changes in selected neuromodulators/neurotransmitters in the TSS following experimental mild TBI.

Phosphorylation of the N-methyl-d-aspartate receptor is increased in the nucleus accumbens during both acute and extended morphine withdrawal.

Opioid withdrawal causes a dysphoric state that can lead to complications in pain patients and can propagate use in drug abusers and addicts. Opioid withdrawal changes the activity of neurons in the nucleus accumbens, an area rich in both opioid-binding mu opioid receptors and glutamate-binding NMDA receptors. Because the accumbens is an area important for reward and aversion, plastic changes in this area during withdrawal could alter future behaviors in animals. We discovered an increase in phosphorylation of serine 897 in the NR1 subunit of the NMDA receptor (pNR1) during acute morphine withdrawal. This serine can be phosphorylated by protein kinase A (PKA) and dephosphorylated by calcineurin. We next demonstrated that this increased pNR1 change is associated with an increase in NR1 surface expression. NR1 surface expression and pNR1 levels during acute withdrawal were both reduced by the NMDA receptor antagonist MK-801 (dizocilpine hydrogen maleate) and the PKA inhibitor H-89(N-[2-[[3-(4-bromophenyl)-2-propenyl]amino]ethyl]-5-isoquinolinesulfonamide dihydrochloride hydrate). We also found that pNR1 levels remained high after an extended morphine withdrawal period of 2 months, correlated with reward-seeking behavior for palatable food, and were associated with a decrease in accumbal calcineurin levels. These data suggest that NR1 phosphorylation changes during the acute withdrawal phase can be long lasting and may reflect a permanent change in NMDA receptors in the accumbens. These altered NMDA receptors in the accumbens could play a role in long-lasting behaviors associated with reward and opioid use.

A novel operant-based behavioral assay of mechanical allodynia in the orofacial region of rats.

BACKGROUND: Detecting behaviors related to orofacial pain in rodent models often relies on subjective investigator grades or methods that place the animal in a stressful environment. In this study, an operant-based behavioral assay is presented for the assessment of orofacial tactile sensitivity in the rat. NEW METHODS: In the testing chamber, rats are provided access to a sweetened condensed milk bottle; however, a 360° array of stainless steel wire loops impedes access. To receive the reward, an animal must engage the wires across the orofacial region. Contact with the bottle triggers a motor, requiring the animal to accept increasing pressure on the face during the test. To evaluate this approach, tolerated bottle distance was measured for 10 hairless Sprague Dawley rats at baseline and 30 min after application of capsaicin cream (0.1%) to the face. The experiment was repeated to evaluate the ability of morphine to reverse this effect. RESULTS: The application of capsaicin cream reduced tolerated bottle distance measures relative to baseline (p<0.05). As long as morphine did not cause reduced participation due to sedation, subcutaneous morphine dosing reduced the effects of capsaicin (p<0.001). Comparison with existing method: For behavioral tests, experimenters often make subjective decisions of an animal's response. Operant methods can reduce these effects by measuring an animal's selection in a reward-conflict decision. Herein, a method to measure orofacial sensitivity is presented using an operant system. CONCLUSIONS: This operant device allows for consistent measurement of heightened tactile sensitivity in the orofacial regions of the rat.

Altered vasoactive intestinal peptides expression in irritable bowel syndrome patients and rats with trinitrobenzene sulfonic acid-induced colitis.

AIM: To investigate the vasoactive intestinal peptides (VIP) expression in irritable bowel syndrome (IBS) and trinitrobenzene sulfonic acid (TNBS) induced colitis. METHODS: The VIP gene expression and protein plasma levels were measured in adult participants (45.8% male) who met Rome III criteria for IBS for longer than 6 mo and in a rat model of colitis as induced by TNBS. Plasma and colons were collected from naïve and inflamed rats. Markers assessing inflammation (i.e., weight changes and myeloperoxidase levels) were assessed on days 2, 7, 14 and 28 and compared to controls. Visceral hypersensitivity of the rats was assessed with colo-rectal distension and mechanical threshold testing on hind paws. IBS patients (n = 12) were age, gender, race, and BMI-matched with healthy controls (n = 12). Peripheral whole blood and plasma from fasting participants was collected and VIP plasma levels were assayed using a VIP peptide-enzyme immunoassay. Human gene expression of VIP was analyzed using a custom PCR array. RESULTS: TNBS induced colitis in the rats was confirmed with weight loss (13.7 ± 3.2 g) and increased myeloperoxidase activity. Visceral hypersensitivity to colo-rectal distension was increased in TNBS treated rats up to 21 d and resolved by day 28. Somatic hypersensitivity was also increased up to 14 d post TNBS induction of colitis. The expression of an inflammatory marker myeloperoxidase was significantly elevated in the intracellular granules of neutrophils in rat models following TNBS treatment compared to naïve rats. This confirmed the induction of inflammation in rats following TNBS treatment. VIP plasma concentration was significantly increased in rats following TNBS treatment as compared to naïve animals (P < 0.05). Likewise, the VIP gene expression from peripheral whole blood was significantly upregulated by 2.91-fold in IBS patients when compared to controls (P < 0.00001; 95%CI). VIP plasma protein was not significantly different when compared with controls (P = 0.193). CONCLUSION: Alterations in VIP expression may play a role in IBS. Therefore, a better understanding of the physiology of VIP could lead to new therapeutics.

Ionic conjugates of lidocaine and sweeteners as better tasting local anesthetics for dentistry.

Lidocaine is the most widely utilized intraoral injected dental anesthetic, used for more than 500 million dental injections per year. Local anesthesia is essential for pain-free dentistry, yet intraoral injections are often considered painful and a source of anxiety for many patients. Any new anesthetics that will reduce the stress and anxiety of dental injection are expected to be beneficial. A novel chemical approach to taste modulation is proposed, in which the lidocaine cation is coupled with anionic sweeteners such as saccarinate and acesulfamate. The ionic conjugates synthesized using anion exchange techniques, were much less bitter, demonstrated a high local anesthetic potential in animal studies, and were as safe as the original hydrochloride. Based on the currently robust market for lidocaine it is expected that the resulting anesthetics will be in high demand in clinical practices worldwide.

Operant assays for assessing pain in preclinical rodent models: highlights from an orofacial assay.

Despite an immense investment of resources, pain remains at epidemic proportions. Given this, there has been an increased effort toward appraising the process by which new painkillers are developed, focusing specifically on why so few analgesics make it from the benchside to the bedside. The use of behavioral assays and animal modeling for the preclinical stages of analgesic development is being reexamined to determine whether they are truly relevant, meaningful, and predictive. Consequently, there is a strengthening consensus that the traditional reflex-based assays upon which several decades of preclinical pain research has been based are inadequate. Thus, investigators have recently turned to the development of new preclinical assays with improved face, content, and predictive validity. In this regard, operant pain assays show considerable promise, as they are more sensitive, present better validity, and, importantly, better encompass the psychological and affective dimensions of pain that trouble human pain sufferers. Here, we briefly compare and contrast reflex assays with operant assays, and we introduce a particular operant orofacial pain assay used in a variety of experiments to emphasize how operant pain assays can be applied to preclinical studies of pain.

The effects of a co-application of menthol and capsaicin on nociceptive behaviors of the rat on the operant orofacial pain assessment device.

BACKGROUND: Transient receptor potential (TRP) cation channels are involved in the perception of hot and cold pain and are targets for pain relief in humans. We hypothesized that agonists of TRPV1 and TRPM8/TRPA1, capsaicin and menthol, would alter nociceptive behaviors in the rat, but their opposite effects on temperature detection would attenuate one another if combined. METHODS: Rats were tested on the Orofacial Pain Assessment Device (OPAD, Stoelting Co.) at three temperatures within a 17 min behavioral session (33°C, 21°C, 45°C). RESULTS: The lick/face ratio (L/F: reward licking events divided by the number of stimulus contacts. Each time there is a licking event a contact is being made.) is a measure of nociception on the OPAD and this was equally reduced at 45°C and 21°C suggesting they are both nociceptive and/or aversive to rats. However, rats consumed (licks) equal amounts at 33°C and 21°C but less at 45°C suggesting that heat is more nociceptive than cold at these temperatures in the orofacial pain model. When menthol and capsaicin were applied alone they both induced nociceptive behaviors like lower L/F ratios and licks. When applied together though, the licks at 21°C were equal to those at 33°C and both were significantly higher than at 45°C. CONCLUSIONS: This suggests that the cool temperature is less nociceptive when TRPM8/TRPA1 and TRPV1 are co-activated. These results suggest that co-activation of TRP channels can reduce certain nociceptive behaviors. These data demonstrate that the motivational aspects of nociception can be influenced selectively by TRP channel modulation and that certain aspects of pain can be dissociated and therefore targeted selectively in the clinic.