A novel hot-plate test sensitive to hyperalgesic stimuli and non-opioid analgesics

It is widely accepted that the classical constant-temperature hot-plate test is insensitive to cyclooxygenase inhibitors. In the current study, we developed a variant of the hot-plate test procedure (modified hot-plate (MHP) test) to measure inflammatory nociception in freely moving rats and mice. Following left and right hind paw stimulation with a phlogogen and vehicle, respectively, the animals were placed individually on a hot-plate surface at 51ºC and the withdrawal latency for each paw was determined simultaneously in measurements performed at 15, 60, 180, and 360 min post-challenge. Plantar stimulation of rats (250 and 500 µg/paw) and mice (125-500 µg/paw) with carrageenan led to a rapid hyperalgesic response of the ipsilateral paw that reached a plateau from 15 to 360 min after challenge. Pretreatment with indomethacin (4 mg/kg, ip) inhibited the phenomenon at all the times analyzed. Similarly, plantar stimulation of rats and mice with prostaglandin E2 (0.5 and 1 µg/paw) also resulted in rapid hyperalgesia which was first detected 15 min post-challenge. Finally, we observed that the MHP test was more sensitive than the classical Hargreaves' test, being able to detect about 4- and 10-fold lower doses of prostaglandin E2 and carrageenan, respectively. In conclusion, the MHP test is a simple and sensitive method for detecting peripheral hyperalgesia and analgesia in rats and mice. This test represents a low-cost alternative for the study of inflammatory pain in freely moving animals.

The inactivation of the basolateral nucleus of the rat amygdala has an anxiolytic effect in the elevated T-maze and light/dark transition tests

Pharmacological evidence indicates that the basolateral nucleus of the amygdala (BLA) is involved in the mediation of inhibitory avoidance but not of escape behavior in the elevated T-maze test. These defensive responses have been associated with generalized anxiety disorder (GAD) and panic disorder, respectively. In the present study, we determined whether the BLA plays a differential role in the control of inhibitory avoidance and escape responses in the elevated T-maze. Male Wistar rats (250-280 g, N = 9-10 in each treatment group) were pre-exposed to one of the open arms of the maze for 30 min and 24 h later tested in the model after inactivation of the BLA by a local injection of the GABA A receptor agonist muscimol (8 nmol in 0.2 µL). It has been shown that a prior forced exposure to one of the open arms of the maze, by shortening latencies to withdrawal from the open arm during the test, improves the escape task as a behavioral index of panic. The effects of muscimol in the elevated T-maze were compared to those caused by this GABA agonist in the avoidance reaction generated in the light/dark transition test. This defensive behavior has also been associated with GAD. In the elevated T-maze, intra-BLA injection of muscimol impaired inhibitory avoidance (control: 187.70 ± 14.90 s, muscimol: 37.10 ± 2.63 s), indicating an anxiolytic effect, without interfering with escape performance. The drug also showed an anxiolytic effect in the light/dark transition test as indicated by the increase in the time spent in the lighted compartment (control: 23.50 ± 2.45 s, muscimol: 47.30 ± 4.48 s). The present findings point to involvement of the BLA in the modulation of defensive responses that have been associated with GAD.

A pyrazolyl-thiazole derivative causes antinociception in mice

The present study investigates the antinociceptive effect of the pyrazolyl-thiazole derivative 2-(5-trichloromethyl-5-hydroxy-3-phenyl-4,5-dihydro-1 H-pyrazol-1-yl)-4-(4-bromophenyl)-5-methylthiazole (B50) in mice. Male albino Swiss mice (30-40 g) were used in the acetic acid-induced abdominal writhes and tail-immersion tests. B50 caused dose-dependent antinociception (8, 23 and 80 µmol/kg, sc) in the acetic acid writhing assay (number of writhes: vehicle: 27.69 ± 6.15; B50 (8 µmol/kg): 16.92 ± 3.84; B50 (23 µmol/kg): 13.85 ± 3.84; B50 (80 µmol/kg): 9.54 ± 3.08; data are reported as means ± SEM for 9 animals per group). On the other hand, B50 did not cause antinociception in the tail immersion assay. Naloxone (2.75 µmol/kg, sc) prevented B50-induced antinociception (number of writhes: vehicle-saline: 31.11 ± 3.15; vehicle-naloxone: 27.41 ± 3.70; B50 (80 µmol/kg)-saline: 8.70 ± 3.33; B50 (80 µmol/kg)-naloxone: 31.84 ± 4.26; morphine-saline: 2.04 ± 3.52; morphine-naloxone: 21.11 ± 4.26; 8-9 animals per group). The removal of the methyl group of the thiazole ring of B50 or substitution of the bromo substituent with the methyl at position 4 of the phenyl group, which is attached to the thiazole ring of B50, resulted in loss of activity, suggesting that these substituents are important for antinociceptive activity. B50 had no effect on spontaneous locomotion or rotarod performance, indicating that the antinociceptive effect of B50 is not related to nonspecific motor effects. The antinociceptive profile of B50 seems to be closer to nonsteroidal anti-inflammatory drugs than to classic opioid agents, since it had no analgesic effect in a thermally motivated test.

Cortisol and 17-a-hydroxy-progesterone levels in infants with refractory hypotension born at 30 weeks of gestation or less

Refractory hypotension is frequent in very low-birth weight infants, whose hypothalamic-pituitary-adrenal axis has been suggested to be immature. The objective of the present study was to evaluate basal cortisol and 17-a-OH-progesterone in the first 36 h of life in preterm infants with and without refractory hypotension (mean arterial blood pressure below the lower limit for gestational age throughout the study despite aggressive volume expansion and use of vasopressors). Thirty-five infants with £30 weeks of gestation and a birth weight £1250 g, with no postnatal use of corticosteroid or death in the first 48 h were studied. Mean arterial pressure was measured every 4 h during the first 48 h. Cortisol and 17-a-OH-progesterone were determined at 12 and 36 h and patients were divided into refractory hypotensive (N = 15) and control (N = 20) groups. The groups were not different regarding type of delivery, use of prenatal corticosteroid, requirement of mechanical ventilation, use of vasopressor drugs, morphine, fentanyl, prophylactic indomethacin, and mean sample timing. Although refractory hypotensive newborns were more immature, were smaller, suffered more deaths after 48 h of life and had a higher SNAPPE-2 score, their cortisol and 17-a-OH-progesterone levels were not different from controls at 12 h and at 36 h. The increase of cortisol in newborns with refractory hypotension 36 h after birth was significantly higher than in controls. Despite the fact that refractory hypotensive very low-birth weight neonates were submitted to a very stressful condition, their cortisol and 17-a-OH-progesterone levels were similar to controls.

Antinociceptive properties in mice of a lectin isolated from the marine alga Amansia multifida Lamouroux

The antinociceptive effects of a lectin (LEC) isolated from the marine alga Amansia multifida were determined in Swiss mice. The LEC (1, 5, and 10 mg/kg) inhibited acetic acid-induced abdominal writhings in a dose-dependent manner after intraperitoneal or oral administration. A partial but significant inhibition of writhings was observed after the combination of LEC (10 mg/kg) with avidin (1 mg/kg), a potent inhibitor of the hemmaglutinant activity of the lectin. However, total writhing inhibition was demonstrable in the group of mice treated with LEC plus mannose (1 mg/kg), as compared to LEC alone or to control groups. Furthermore, avidin and mainly mannose also play a role in antinociception, somehow facilitating the interaction of LEC with its active cell sites. In the formalin test, although both phases of the response were significantly inhibited, the effect of LEC was predominant during phase 2, causing inhibition of licking time that ranged from 48 to 88% after oral (5 and 10 mg/kg) and intraperitoneal (1 to 5 mg/kg) administration. As is the case with morphine, the effect of LEC (2 mg/kg) was reversed by naloxone (2 mg/kg), indicating the involvement of the opioid system. LEC was also effective in the hot-plate test, producing inhibitory responses to the thermal stimulus, and its effects were blocked by naloxone. In the pentobarbital-induced sleeping time, although LEC did not alter the onset of sleep significantly, it increased the time of sleep within the same dose range compared to control. These results show that LEC presents antinociceptive effects of both central and peripheral origin, possibly involving the participation of the opioid system.

Role of the cardiac nerve in the adaptive changes of heart rate in response to an aversive stimulus in Megalobulimus mogianensis

The effect of an aversive stimulus represented by contact with a hot plate on the heart rate of Megalobulimus mogianensis was evaluated with electrocardiogram recording in intact snails (N = 8). All stimulated animals showed an increase in heart rate, with mean values ranging from 35.6 ± 1.2 (basal heart rate) to 43.8 ± 0.9 bpm (post-stimulation heart rate). The cardioacceleration was followed by gradual recovery of the basal heart rate, with mean recovery times varying from 4.3 ± 0.3 to 5.8 ± 0.6 min. Repetition of the stimulus did not affect the magnitude of variation nor did it influence the basal heart rate recovery time. To investigate the role of the cardiac nerve in mediating the heart rate alterations induced by the aversive stimulus, denervated (N = 8) and sham-operated (N = 8) animals were also tested. Although the aversive stimulus caused the heart rate to increase significantly in both experimental groups, the mean increase in heart rate in denervated animals (4.4 ± 0.4 bpm) was 57% of the value obtained in sham-operated animals (7.7 ± 1.3 bpm), indicating that the cardiac nerve is responsible for 43% of the cardioacceleration induced by the aversive stimulus. The cardioacceleration observed in denervated snails may be due to an increase in venous return promoted by the intense muscular activity associated with the withdrawal response. Humoral factors may also be involved. A probable delaying inhibitory effect of the cardiac nerve on the recuperation of the basal heart rate is suggested.

Training-related changes in the R-R interval at the onset of passive movements in humans

The aim of the present study was to determine whether training-related alterations in muscle mechanoreflex activation affect cardiac vagal withdrawal at the onset of exercise. Eighteen male volunteers divided into 9 controls (26 ± 1.9 years) and 9 racket players (25 ± 1.9 years) performed 10 s of voluntary and passive movement characterized by the wrist flexion of their dominant and non-dominant limbs. The respiratory cycle was divided into four phases and the phase 4 R-R interval was measured before and immediately following the initiation of either voluntary or passive movement. At the onset of voluntary exercise, the decrease in R-R interval was similar between dominant and non-dominant forearms in both controls (166 ± 20 vs 180 ± 34 ms, respectively; P > 0.05) and racket players (202 ± 29 vs 201 ± 31 ms, respectively; P > 0.05). Following passive movement, the non-dominant forearm of racket players elicited greater changes than the dominant forearm (129 ± 30 vs 77 ± 17 ms; P < 0.05), as well as both the dominant (54 ± 20 ms; P < 0.05) and non-dominant (59 ± 14 ms; P < 0.05) forearms of control subjects. In contrast, changes in R-R interval elicited by the racket players' dominant forearm were similar to that observed in the control group, indicating that changes in R-R interval at the onset of passive exercise were not attenuated in the dominant forearm of racket players. In summary, cardiac vagal withdrawal induced by muscle mechanoreflex stimulation is well-maintained, despite long-term exposure to training.

Streptozotocin-induced mechanical hypernociception is not dependent on hyperglycemia

Since streptozotocin (STZ)-induced diabetes is a widely used model of painful diabetic neuropathy, the aim of the present study was to design a rational protocol to investigate whether the development of mechanical hypernociception induced by STZ depends exclusively on hyperglycemia. Male Wistar rats (180-200 g; N = 6-7 per group) received a single intravenous injection of STZ at three different doses (10, 20, or 40 mg/kg). Only the higher dose (40 mg/kg) induced a significant increase in blood glucose levels, glucose tolerance and deficiency in weight gain. However, all STZ-treated rats (hyperglycemic or not) developed persistent (for at least 20 days) and indistinguishable bilateral mechanical hypernociception that was not prevented by daily insulin treatment (2 IU twice a day, sc). Systemic morphine (2 mg/kg) but not local (intraplantar) morphine treatment (8 µg/paw) significantly inhibited the mechanical hypernociception induced by STZ (10 or 40 mg/kg). In addition, intraplantar injection of STZ at doses that did not cause hyperglycemia (30, 100 or 300 µg/paw) induced ipsilateral mechanical hypernociception for at least 8 h that was inhibited by local and systemic morphine treatment (8 µg/paw or 2 mg/kg, respectively), but not by dexamethasone (1 mg/kg, sc). The results of this study demonstrate that systemic administration of STZ induces mechanical hypernociception that does not depend on hyperglycemia and intraplantar STZ induces mechanical sensitization of primary sensory neurons responsive to local morphine treatment.

Autonomic control of cardiorespiratory interactions in fish, amphibians and reptiles

Control of the heart rate and cardiorespiratory interactions (CRI) is predominantly parasympathetic in all jawed vertebrates, with the sympathetic nervous system having some influence in tetrapods. Respiratory sinus arrhythmia (RSA) has been described as a solely mammalian phenomenon but respiration-related beat-to-beat control of the heart has been described in fish and reptiles. Though they are both important, the relative roles of feed-forward central control and peripheral reflexes in generating CRI vary between groups of fishes and probably between other vertebrates. CRI may relate to two locations for the vagal preganglionic neurons (VPN) and in particular cardiac VPN in the brainstem. This has been described in representatives from all vertebrate groups, though the proportion in each location is variable. Air-breathing fishes, amphibians and reptiles breathe discontinuously and the onset of a bout of breathing is characteristically accompanied by an immediate increase in heart rate plus, in the latter two groups, a left-right shunting of blood through the pulmonary circuit. Both the increase in heart rate and opening of a sphincter on the pulmonary artery are due to withdrawal of vagal tone. An increase in heart rate following a meal in snakes is related to withdrawal of vagal tone plus a non-adrenergic-non-cholinergic effect that may be due to humoral factors released by the gut. Histamine is one candidate for this role.

Evaluation of piroxicam-β-cyclodextrin as a preemptive analgesic in functional endoscopic sinus surgery

The preemptive analgesic efficacy and adverse effects of preoperatively administered piroxicam-β-cyclodextrin for post-endoscopic sinus surgery pain was determined in a prospective, double-blind, randomized, clinical study. Seventy-five American Society of Anesthesiologists status I-II patients, aged 18-65 years, were divided into three groups with similar demographic characteristics: group 1 received 20 mg piroxicam-β-cyclodextrin, group 2 received 40 mg piroxicam-β-cyclodextrin and group 3 received placebo orally before induction of general anesthesia. A blinded observer recorded the incidence and severity of pain at admission to the post-anesthesia care unit (PACU), at 15, 30, and 45 min in the PACU, and 1, 2, 4, 6, and 24 h postoperatively. All patients received patient-controlled morphine analgesia during the postoperative period and consumption was recorded for 24 h. During the PACU period, mean visual analogue scale values were significantly lower in groups 1 and 2 compared to group 3 (P < 0.05). During the postoperative period, morphine consumption was 3.03 ± 2.54, 2.7 ± 2.8, and 5.56 ± 3.12 mg for each group, respectively (P < 0.05). As a side effect, bleeding was observed in groups 1 and 3, nausea and vomiting in all groups, and edema only in group 3. However, no significant differences were detected in any of the parameters analyzed, which also included epigastric pain, constipation/diarrhea and headache. Similar hematological test results were obtained for all groups. Preemptive administration of piroxicam-β-cyclodextrin effectively reduced analgesic consumption, and 40 mg of the drug was more effective than 20 mg piroxicam-β-cyclodextrin without side effects during the postoperative period.

Participation of endogenous opioids in the antinociception induced by resistance exercise in rats

Exercise is a low-cost intervention that promotes health and contributes to the maintenance of the quality of life. The present study was designed to investigate the influence of different resistance exercise protocols on the nociceptive threshold of rats. Female Wistar rats were used to perform exercises in a weight-lifting exercise model. The following groups were examined (N = 6 per group): untrained rats (control group); an acute protocol group consisting of rats submitted to 15 sets of 15 repetitions of resistance exercise (acute group); rats exercised with 3 sets of 10 repetitions, three times per week for 12 weeks (trained group), and a group consisting of trained rats that were further submitted to the acute protocol (trained-acute group). The nociceptive threshold was measured by the paw-withdrawal test, in which the withdrawal threshold (escape reaction) was measured by an apparatus applying force to the plantar surface of the animal paw. The opioid antagonist naloxone (2 mg/kg) was administered subcutaneously 10 min before the exercise protocols. The trained group demonstrated antinociception only up to day 45 of the 12-week training period. A significant increase (37%, P < 0.05) in the nociceptive threshold was produced immediately after exercise, decreasing to 15% after 15 min, when the acute exercise protocol was used. Naloxone reversed this effect. These data show that the acute resistance exercise protocol was effective in producing antinociception for 15 min. This antinociceptive effect is mediated by the activation of opioid receptors.

Follow-up of patients treated with retinoic acid for the control of radioiodine non-responsive advanced thyroid carcinoma

During thyroid tumor progression, cellular de-differentiation may occur and it is commonly accompanied by metastatic spread and loss of iodine uptake. Retinoic acid (RA) administration might increase iodine uptake in about 40% of patients, suggesting that RA could be a promising therapeutic option for radioiodine non-responsive thyroid carcinoma, although a prospective study with a long-term follow-up has not been reported. This was a clinical prospective study assessing the value of 13-cis-RA in patients with advanced thyroid carcinoma and its impact on major outcomes such as tumor regression and cancer-related death with a long-term follow-up of patients submitted to radioiodine (131I) therapy after RA administration. Sixteen patients with inoperable disease and no significant radioiodine uptake on post-therapy scan were selected. Patients were treated orally with 13-cis-RA at a dose of 1.0 to 1.5 mg·kg-1·day-1 for 5 weeks and then submitted to radioiodine therapy (150 mCi) after thyroxine withdrawal. A whole body scan was obtained 5 to 7 days after the radioactive iodine therapy. RECIST criteria were used to evaluate the response. An objective partial response rate was observed in 18.8%, a stable disease rate in 25% and a progression disease rate in 56.2%. Five patients died (62.5%) in the group classified as progression of disease. Progression-free survival rate (PFS) ranged from 72 to 12 months, with a median PFS of 26.5 months. RA may be an option for advanced de-differentiated thyroid cancer, due to the low rate of side effects.

Percutaneous sciatic nerve block with tramadol induces analgesia and motor blockade in two animal pain models

Local anesthetic efficacy of tramadol has been reported following intradermal application. Our aim was to investigate the effect of perineural tramadol as the sole analgesic in two pain models. Male Wistar rats (280-380 g; N = 5/group) were used in these experiments. A neurostimulation-guided sciatic nerve block was performed and 2% lidocaine or tramadol (1.25 and 5 mg) was perineurally injected in two different animal pain models. In the flinching behavior test, the number of flinches was evaluated and in the plantar incision model, mechanical and heat thresholds were measured. Motor effects of lidocaine and tramadol were quantified and a motor block score elaborated. Tramadol, 1.25 mg, completely blocked the first and reduced the second phase of the flinching behavior test. In the plantar incision model, tramadol (1.25 mg) increased both paw withdrawal latency in response to radiant heat (8.3 ± 1.1, 12.7 ± 1.8, 8.4 ± 0.8, and 11.1 ± 3.3 s) and mechanical threshold in response to von Frey filaments (459 ± 82.8, 447.5 ± 91.7, 320.1 ± 120, 126.43 ± 92.8 mN) at 5, 15, 30, and 60 min, respectively. Sham block or contralateral sciatic nerve block did not differ from perineural saline injection throughout the study in either model. The effect of tramadol was not antagonized by intraperitoneal naloxone. High dose tramadol (5 mg) blocked motor function as well as 2% lidocaine. In conclusion, tramadol blocks nociception and motor function in vivo similar to local anesthetics.

Estrous cycle and stress: influence of progesterone on the female brain

The female brain operates in a constantly changing chemical milieu caused by cyclical changes in gonadal hormones during the estrous cycle (menstrual cycle in women). Such hormones are highly lipophilic and pass readily from the plasma to the brain where they can influence neuronal function. It is becoming clear that the rapid reduction in peripheral circulating progesterone, which occurs during the late diestrous phase of the cycle, can trigger a withdrawal-like response, in which changes in GABA A receptor expression render hyper-responsive certain brain areas involved in processing responses to stressful stimuli. The periaqueductal gray matter (PAG) is recognised as an important region for integrating anxiety/defence responses. Withdrawal from progesterone, via actions of its neuroactive metabolite allopregnanolone, triggers up-regulation of extrasynaptic GABA A receptors on GABAergic neurons in the PAG. As a consequence, ongoing GABAergic tone on the output cells decreases, leading to an increase in functional excitability of the circuitry and enhanced responsiveness to stressful stimuli during the late diestrous phase. These changes during late diestrus could be prevented by short-term neurosteroid administration, timed to produce a more gradual fall in the peripheral concentration of allopregnanolone than the rapid decrease that occurs naturally, thus removing the trigger for the central withdrawal response.

High-frequency electroacupuncture versus carprofen in an incisional pain model in rats

The objective of the present study was to compare the effect of electroacupuncture (EA) and carprofen (CP) on postoperative incisional pain using the plantar incision (PI) model in rats. A 1-cm longitudinal incision was made through skin, fascia and muscles of a hind paw of male Wistar rats and the development of mechanical and thermal hypersensitivity was determined over 4 days using the von Frey and Hargreaves methods, respectively. Based on the experimental treatments received on the third postoperative day, the animals were divided into the following groups: PI+CP (CP, 2 mg/kg, po); PI+EAST36 (100-Hz EA applied bilaterally at the Zusanli point (ST36)); PI+EANP (EA applied to a non-acupoint region); PI+IMMO (immobilization only); PI (vehicle). In the von Frey test, the PI+EAST36 group had higher withdrawal force thresholds in response to mechanical stimuli than the PI, PI+IMMO and PI+EANP groups at several times studied. Furthermore, the PI+EAST36 group showed paw withdrawal thresholds in response to mechanical stimuli that were similar to those of the PI+CP group. In the Hargreaves test, all groups had latencies higher than those observed with PI. The PI+EAST36 group was similar to the PI+IMMO, PI+EANP and PI+CP groups. We conclude that 100-Hz EA at the ST36 point, but not at non-acupoints, can reduce mechanical nociception in the rat model of incisional pain, and its effectiveness is comparable to that of carprofen.