Gabapentin reverses central pain sensitization following a collagenase-induced intrathalamic hemorrhage in rats

Treatment Response to Gabapentin in Neuropathic Ocular Pain Associated with Dry Eye

PURPOSE

To investigate the response to gabapentin treatment in patients with dry eye (DE) accompanied by features of neuropathic ocular pain (NOP), and to analyze the differences between clinical manifestations of the groups according to treatment response.

METHODS

We retrospectively reviewed the records of 35 patients with DE accompanied by NOP features and obtained information on their medical history and previous ocular history. The patients underwent clinical examinations of the tear film, ocular surface, and meibomian gland and completed the Ocular Pain Assessment Survey (OPAS). One month after treatment with topical eye drops, add-on of gabapentin treatment was determined according to the Wong-Baker FACES Pain Rating Scale (WBFPS). A reduction of 2 points or more on the WBFPS was considered a positive treatment response. Enrolled patients were divided into three groups according to the treatment response: topical treatment response group (group 1, n = 11); gabapentin response group (group 2, n = 13); and gabapentin non-response group (group 3, n = 11). The medical history, clinical parameters, and OPAS scores were compared between groups.

RESULTS

The incidence of systemic comorbidities was higher in group 2 than in other groups. The corneal staining scores were lower in groups 2 and 3 than in group 1. Among the treatment response groups, group 2 showed improvements in OPAS scores of ocular pain severity, pain other than eyes, and quality of life, while group 1 showed improved OPAS scores of ocular pain severity and ocular associated factors. Group 2 exhibited lower scores of pains aggravated by mechanical and chemical stimuli than group 3.

CONCLUSIONS

Gabapentin could be effective in patients who have systemic comorbidity and less pain evoked by mechanical and chemical stimuli for the treatment of DE patients with NOP, which is refractory to topical treatment.

Use of Flavored Tablets of Gabapentin and Carprofen to Attenuate Postoperative Hypersensitivity in an Incisional Pain Model in Rats (Rattus norvegicus)

Providing postoperative analgesia to rats by oral administration, compared with injections, reduces stress from frequent handling and is technically easier for investigators. The purpose of this study was to investigate whether bacon-flavored tablets containing gabapentin, carprofen or a combination of both drugs effectively attenuates postoperative mechanical and thermal hypersensitivity in a rat model of incisional pain. Forty-eight Sprague-Dawley rats were randomly assigned to 1 of 5 treatment groups: placebo tablet; a single, subcutaneous injection of buprenorphine sustained release at 1.2 mg/kg; gabapentin 90 mg/tablet; carprofen 5 mg/tablet; gabapentin 90 mg and carprofen 5 mg/tablet (gabapentin/carprofen). Tablets were given to rats on days -3, -2, -1, 0 (surgery), 1, and 2. Rats were anesthetized using isoflurane. A 1 cm skin incision was made aseptically on the plantar surface of the left hindpaw and closed by using suture. Mechanical (von Frey monofilament) and thermal (Hargreaves method) hypersensitivity were tested daily, and analyzed on days -1, 1, 2, and 3. The amount of tablet consumed was recorded daily; postoperatively rats consumed 101 to 133 mg/kg of gabapentin, 5.5 to 5.8 mg/kg of carprofen, and 86-137/1.9-3 mg/kg of gabapentin/carprofen, respectively. Both the gabapentin and carprofen groups displayed attenuated mechanical hypersensitivity on all 3 postsurgical days and decreased thermal hypersensitivity on Day 3. The gabapentin/ carprofen group showed attenuated mechanical hypersensitivity on Day 2 and 3, but no significant reduction of thermal hypersensitivity. These data suggest that both gabapentin and carprofen, given orally by flavored tablet, effectively attenuate postoperative mechanical hypersensitivity for 3 d after surgery in a rat model of incisional pain.

Effects of intraplantar administration of Complete Freund's Adjuvant (CFA) on rotarod performance in mice

BACKGROUND AND AIMS

Preclinical animal models are crucial to study pain mechanisms and assess antinociceptive effects of medications. One major problem with current animal behavioral models is their lack of face validity with human nociception and the vulnerability for false-positive results. Here, we evaluated the usefulness of rotarod as a new way to assess inflammatory nociception in rodents.

METHODS

Adult male mice were injected with saline or Complete Freund's Adjuvant (CFA) in the left hindpaws. Mechanical allodynia and rotarod performance were evaluated before and after the administration of CFA. Mechanical allodynia was measured using von Frey filaments. Long-term effect of CFA on rotarod performance was also assessed for 2 weeks.

RESULTS

Our results showed that CFA administration decreased pain threshold and increased sensitivity to von Frey filaments compared to control group. In rotarod experiments, the starting speed of the rod rotation started at four RPM, and accelerated until it reached 40 RPM in 5 min. Rotarod performance was enhanced from day to day in the control group. However, rotarod performance in CFA group was attenuated after CFA administration, which was significant after 24 h compared to vehicle. This attenuation was blocked by ibuprofen. Haloperidol administration (positive control) produced similar results to CFA administration. CFA did not produce significant attenuation of rotarod performance after 1 week post-injection.

CONCLUSIONS

Collectively, our findings could encourage the use of rotarod assay to measure acute (but not chronic) inflammatory nociception as a useful tool in rodents.

Late-onset hypersensitivity after a lesion in the ventral posterolateral nucleus of the thalamus: A macaque model of central post-stroke pain

Central post-stroke pain (CPSP) can occur as a result of a cerebrovascular accident in the ventral posterolateral nucleus (VPL) of the thalamus. Developing therapeutic interventions for CPSP is difficult because its pathophysiology is unclear. Here we developed and characterized a macaque model of CPSP. The location of the VPL was determined by magnetic resonance imaging (MRI) and extracellular recording of neuronal activity during tactile stimulation, after which a hemorrhagic lesion was induced by injecting collagenase type IV. Histological analysis revealed that most of the lesion was localized within the VPL. Several weeks after the injection, the macaques displayed behavioral changes that were interpreted as reflecting the development of both mechanical allodynia and thermal hyperalgesia. Immunohistochemistry revealed that microglial and astrocytic activation in the perilesional areas lasted at least 3 months after injection. The present model reproduced the symptoms of patients suffering from CPSP, in which both mechanical allodynia and thermal hyperalgesia often develop several weeks after cerebrovascular accident. Further, the long-lasting glial activation revealed here may be characteristic of primate brains following injury. The present model will be useful not only for examining the neurological changes underlying CPSP, but also for testing therapeutic interventions for CPSP.

Effects of fentanyl on pain and motor behaviors following a collagenase-induced intracerebral hemorrhage in rats

Purpose

Intracerebral hemorrhage (IH) and cephalalgia are common consequences of traumatic brain injury. One of the primary obstacles for patient recovery is the paucity of treatments to support an appropriate analgesic protocol. The present study aimed to assess pain and motor behaviors following different doses of fentanyl on a rat model of IH.

Methods

Twenty-one male Sprague Dawley rats underwent a stereotaxic surgery to produce a collagenase-induced IH in the right caudoputamen nucleus. The control group (n=6) received saline subcutaneously (SC), and experimental groups received either 5 (n=6), 10 (n=6), or 20 (n=3) µg/kg of fentanyl SC, 2 hours following surgery and on 2 subsequent days. Only 3 animals received 20 µg/kg because this dose caused catalepsy for 15–20 minutes following the injection. The rat grimace scale, a neurological examination, balance beam test, and rotarod test were performed for 5 consecutive days postoperatively to evaluate pain and motor performance. At the end of the experimentation, the brains were evaluated to determine hematoma volume, and the number of reactive astrocytes and necrotic neurons.

Results

When compared to controls, the grimace scale showed that 5 µg/kg fentanyl significantly alleviated pain on day 2 only (P<0.01) and that 10 µg/kg alleviated pain on days 1 (P<0.01), 2 (P<0.001), and 3 (P<0.01). For the rotarod test, only the 10 µg/kg group showed significant decreases in performance on days 5 (P<0.05) and 6 (P<0.02). The neurological examination was not significantly different between the groups, but only the hopping test showed poor recuperation for the 5 and 10 µg/kg fentanyl group when compared to saline (P<0.01). No differences were found between the groups for the balance beam test, the histopathological results.

Conclusion

Fentanyl, at a dose of 10 µg/kg SC, provides substantial analgesia following a collagenase-induced IH in rats; however, it can alter motor performance following analgesic treatments.

Gabapentin prevents oxaliplatin-induced central sensitization in the dorsal horn neurons in rats

OBJECTIVES

The present study aims to study the alteration of glutamatergic transmission in the dorsal horn neurons and the effect of gabapentin on oxaliplatin-induced neuropathic pain in rats.

MATERIALS AND METHODS

Oxaliplatin (5 mg/kg) or saline was administered to adult male Sprague-Dawley rats. Gabapentin (60 mg/kg, IP) or vehicle was injected daily. Mechanical allodynia was assessed using a series of von Frey filaments. The expression of glutamate receptor subunits (NR2B and GluR1) and brain-derived neurotrophic factor (BDNF) was measured in the dorsal horn. The glutamatergic strength was recorded in the spinal cord slices.

RESULTS

Administration of oxaliplatin induced significant hyperreactivity to mechanical stimuli in rats, which was attenuated by gabapentin. Significant increase in the expression of BDNF was found in the dorsal horn in rats receiving oxaliplatin, which was prevented by gabapentin. Further studies also observed a significant increase in the expression of GluR1 and NR2B, as well as enhanced glutamatergic transmission in the dorsal horn neurons in rats treated with oxaliplatin. The upregulation of glutamatergic transmission was significantly reversed by gabapentin.

CONCLUSION

These results illustrated an increased expression of BDNF and enhanced glutamatergic transmission in rats with oxaliplatin-induced neuropathic pain, which was markedly attenuated by gabapentin.

Post-stroke pain hypersensitivity induced by experimental thalamic hemorrhage in rats is region-specific and demonstrates limited efficacy of gabapentin

Intractable central post-stroke pain (CPSP) is one of the most common sequelae of stroke, but has been inadequately studied to date. In this study, we first determined the relationship between the lesion site and changes in mechanical or thermal pain sensitivity in a rat CPSP model with experimental thalamic hemorrhage produced by unilateral intra-thalamic collagenase IV (ITC) injection. Then, we evaluated the efficacy of gabapentin (GBP), an anticonvulsant that binds the voltage-gated Ca(2+) channel α2δ and a commonly used anti-neuropathic pain medication. Histological case-by-case analysis showed that only lesions confined to the medial lemniscus and the ventroposterior lateral/medial nuclei of the thalamus and/or the posterior thalamic nucleus resulted in bilateral mechanical pain hypersensitivity. All of the animals displaying CPSP also had impaired motor coordination, while control rats with intra-thalamic saline developed no central pain or motor deficits. GBP had a dose-related anti-allodynic effect after a single administration (1, 10, or 100 mg/kg) on day 7 post-ITC, with significant effects lasting at least 5 h for the higher doses. However, repeated treatment, once a day for two weeks, resulted in complete loss of effectiveness (drug tolerance) at 10 mg/kg, while effectiveness remained at 100 mg/kg, although the time period of efficacious analgesia was reduced. In addition, GBP did not change the basal pain sensitivity and the motor impairment caused by the ITC lesion, suggesting selective action of GBP on the somatosensory system.