Response threshold to aversive stimuli in stimulated early protein-malnourished rats

Two animal models of pain were used to study the effects of short-term protein malnutrition and environmental stimulation on the response threshold to aversive stimuli. Eighty male Wistar rats were used. Half of the pups were submitted to malnutrition by feeding their mothers a 6% protein diet from 0 to 21 days of age while the mothers of the other half (controls) were well nourished, receiving 16% protein. From 22 to 70 days all rats were fed commercial lab chow. Half of the animals in the malnourished and control groups were maintained under stimulating conditions, including a 3-min daily handling from 0 to 70 days and an enriched living cage after weaning. The other half was reared in a standard living cage. At 70 days, independent groups of rats were exposed to the shock threshold or to the tail-flick test. The results showed lower body and brain weights in malnourished rats when compared with controls at weaning and testing. In the shock threshold test the malnourished animals were more sensitive to electric shock and environmental stimulation increased the shock threshold. No differences due to diet or environmental stimulation were found in the tail-flick procedure. These results demonstrate that protein malnutrition imposed only during the lactation period is efficient in inducing hyperreactivity to electric shock and that environmental stimulation attenuates the differences in shock threshold produced by protein malnutrition

Effects of naltrexone and cross-tolerance to morphine in a learned helplessness paradigm

Opiates have been implicated in learned helplessness (LH), a phenomenon known to be related to opiate stress-induced analgesia (SIA). In the present study, we investigated the role of opiates in the induction of LH and SIA under different conditions. Adult female Wistar rats were trained either by receiving 60 inescapable 1-mA footshocks (IS group, N = 114) or by confinement in the shock box (control or NS group, N = 92). The pain threshold of some of the animals was immediately evaluated in a tail-flick test while the rest were used 24 h later in a shuttle box experiment to examine their escape performance. The opiate antagonist naltrexone (0 or 8 mg/kg, ip) and the previous induction of cross-tolerance to morphine by the chronic administration of morphine (0 or 10 mg/kg, sc, for 13 days) were used to identify opiate involvement. Analysis of variance revealed that only animals in the IS group demonstrated antinociception and an escape deficit, both of which were resistant to the procedures applied before the training session. However, the escape deficit could be reversed if the treatments were given before the test session. We conclude that, under our conditions, induction of the LH deficit in escape performance is not opiate-mediated although its expression is opiate-modulated

Sucrose ingestion causes opioid analgesia

The intake of saccharin solutions for relatively long periods of time causes analgesia in rats, as measured in the hot-plate test, an experimental procedure involving supraspinal components. In order to investigate the effects of sweet substance intake on pain modulation using a different model, male albino Wistar rats weighing 180-200 g received either tap water or sucrose solutions (250 g/l) for 1 day or 14 days as their only source of liquid. Each rat consumed an average of 15.6 g sucrose/day. Their tail withdrawal latencies in the tail-flick test (probably a spinal reflex) were measured immediately before and after this treatment. An analgesia index was calculated from the withdrawal latencies before and after treatment. The indexes (mean ± SEM, N = 12) for the groups receiving tap water for 1 day or 14 days, and sucrose solution for 1 day or 14 days were 0.09 ± 0.04, 0.10 ± 0.05, 0.15 ± 0.08 and 0.49 ± 0.07, respectively. One-way ANOVA indicated a significant difference (F(3,47) = 9.521, P<0.001) and the Tukey multiple comparison test (P<0.05) showed that the analgesia index of the 14-day sucrose-treated animals differed from all other groups. Naloxone-treated rats (N = 7) receiving sucrose exhibited an analgesia index of 0.20 ± 0.10 while rats receiving only sucrose (N = 7) had an index of 0.68 ± 0.11 (t = 0.254, 10 degrees of freedom, P<0.03). This result indicates that the analgesic effect of sucrose depends on the time during which the solution is consumed and extends the analgesic effects of sweet substance intake, such as saccharin, to a model other than the hot-plate test, with similar results. Endogenous opioids may be involved in the central regulation of the sweet substance-produced analgesia.

Antinociceptive effect of intrathecal neostigmine evaluated in rats by two different pain models

The analgesic efficacy of cholinergic agonists and anticholinesterase agents has been widely recognized. The analgesic effect obtained by activating cholinergic mechanisms, however, seems to depend on the experimental pain model utilized for its evaluation. The antinociceptive effect of intraspinal neostigmine was examined in rats submitted concurrently to the tail flick and formalin tests. Neostigmine (8.25 and 16.5 nmol) produced a dose-dependent antinociceptive effect in the tail flick test (a model of phasic pain) and reduced the first phase (phasic pain) of the animal response to formalin also in a dose-dependent manner. The second phase (tonic pain) of the response to formalin, however, was slightly reduced after a longer period of time only by the higher dose of the anticholinesterase. The effect of neostigmine was not significantly different when the drug was injected into rats submitted exclusively to the tail flick test. The second phase of the animal response to formalin was slightly reduced by neostigmine (8.25 nmol) and strongly inhibited by the higher dose of the anticholinesterase when injection was made after the first phase. We conclude that phasic and tonic pain can both be controlled by high doses of neostigmine. In addition, we show that inhibition by a lower dose of neostigmine of the formalin-induced phasic pain did not prevent the subsequent occurrence of tonic pain produced by the irritant

Antinociception induced by stimulating amygdaloid nuclei in rats: changes produced by systemically administered antagonists

The antinociceptive effects of stimulating the medial (ME) and central (CE) nuclei of the amygdala in rats were evaluated by the changes in the latency for the tail withdrawal reflex to noxious heating of the skin. A 30-s period of sine-wave stimulation of the ME or CE produced a significant and short increase in the duration of tail flick latency. A 15-s period of stimulation was ineffective. Repeated stimulation of these nuclei at 48-h intervals produced progressively smaller effects. The antinociception evoked from the ME was significantly reduced by the previous systemic administration of naloxone, methysergide, atropine, phenoxybenzamine, and propranolol, but not by mecamylamine, all given at the dose of 1.0 mg/kg. Previous systemic administration of naloxone, atropine, and propranolol, but not methysergide, phenoxybenzamine, or mecamylamine, was effective against the effects of stimulating the CE. We conclude that the antinociceptive effects of stimulating the ME involve at least opioid, serotonergic, adrenergic, and muscarinic cholinergic descending mechanisms. The effects of stimulating the CE involve at least opioid, ß-adrenergic, and muscarinic cholinergic descending mechanisms.

Acute phenobarbital administration induces hyperalgesia: pharmacological evidence for the involvement of supraspinal GABA-A receptors

The aim of the present study was to determine if phenobarbital affects the nociception threshold. Systemic (1-20 mg/kg) phenobarbital administration dose dependently induced hyperalgesia in the tail-flick, hot-plate and formalin tests in rats and in the abdominal constriction test in mice. Formalin and abdominal constriction tests were the most sensitive procedures for the detection of hyperalgesia in response to phenobarbital compared with the tail-flick and hot-plate tests. The hyperalgesia induced by systemic phenobarbital was blocked by previous administration of 1 mg/kg ip picrotoxin or either 1-2 mg/kg sc or 10 ng icv bicuculline. Intracerebroventricular phenobarbital administration (5 µg) induced hyperalgesia in the tail-flick test. In contrast, intrathecal phenobarbital administration (5 µg) induced antinociception and blocked systemic-induced hyperalgesia in this test. We suggest that phenobarbital may mediate hyperalgesia through GABA-A receptors at supraspinal levels and antinociception through the same kind of receptors at spinal levels.

Antinociceptive potency of aminoglycoside antibiotics and magnesium chloride: a comparative study on models of phasic and incisional pain in rats

A close relationship exists between calcium concentration in the central nervous system and nociceptive processing. Aminoglycoside antibiotics and magnesium interact with N- and P/Q-type voltage-operated calcium channels. In the present study we compare the antinociceptive potency of intrathecal administration of aminoglycoside antibiotics and magnesium chloride in the tail-flick test and on incisional pain in rats, taken as models of phasic and persistent post-surgical pain, respectively. The order of potency in the tail-flick test was gentamicin (ED50 = 3.34 µg; confidence limits 2.65 and 4.2) > streptomycin (5.68 µg; 3.76 and 8.57) = neomycin (9.22 µg; 6.98 and 12.17) > magnesium (19.49 µg; 11.46 and 33.13). The order of potency to reduce incisional pain was gentamicin (ED50 = 2.06 µg; confidence limits 1.46 and 2.9) > streptomycin (47.86 µg; 26.3 and 87.1) = neomycin (83.17 µg; 51.6 and 133.9). The dose-response curves for each test did not deviate significantly from parallelism. We conclude that neomycin and streptomycin are more potent against phasic pain than against persistent pain, whereas gentamicin is equipotent against both types of pain. Magnesium was less potent than the antibiotics and effective in the tail-flick test only.

Acute stress-induced antinociception is cGMP-dependent but heme oxygenase-independent

Endogenous carbon monoxide (CO), which is produced by the enzyme heme oxygenase (HO), participates as a neuromodulator in physiological processes such as thermoregulation and nociception by stimulating the formation of 3′,5′-cyclic guanosine monophosphate (cGMP). In particular, the acute physical restraint-induced fever of rats can be blocked by inhibiting the enzyme HO. A previous study reported that the HO-CO-cGMP pathway plays a key phasic antinociceptive role in modulating noninflammatory acute pain. Thus, this study evaluated the involvement of the HO-CO-cGMP pathway in antinociception induced by acute stress in male Wistar rats (250-300 g; n=8/group) using the analgesia index (AI) in the tail flick test. The results showed that antinociception induced by acute stress was not dependent on the HO-CO-cGMP pathway, as neither treatment with the HO inhibitor ZnDBPG nor heme-lysinate altered the AI. However, antinociception was dependent on cGMP activity because pretreatment with the guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-a] quinoxaline-1-one (ODQ) blocked the increase in the AI induced by acute stress.

Involvement of the midbrain periaqueductal gray 5-HT1A receptors in social conflict induced analgesia in mice

Recent results from our laboratory have shown that 30-bites social conflict in mice produces a high-intensity, short-term analgesia which is attenuated by systemically injected 5-HT1A receptor agonists, such as BAY R 1531 (6-methoxy-4-(di-n-propylamino)-1,3,4,5-tetrahydrobenz(c,d)indole hydrochloride) and gepirone. The present study investigated the effects of these drugs, as well as the 5-HT1A receptor antagonist WAY 100135 (N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide) injected into the midbrain periaqueductal gray matter of mice on 30-bites analgesia. Four to five days after guide-cannula implantation, each mouse received microinjection of gepirone (30 nmol/0.2 mu l), BAY R 1531 (10 nmol/0.2 mu l), WAY 100135 (10 nmol/0.2 mu l), saline (0.9% NaCl) or vehicle (saline + 4% Tween 80) 5 min before either an aggressive (30 bites) or a non-aggressive interaction. Nociception was assessed by the tail-flick test made before as well as 1, 5, 10 and 20 min after social interaction. The full 5-HT1A receptor agonist BAY R 1531 blocked, whereas, WAY 100135 and gepirone intensified 30-bites analgesia, Neither non-aggressive interaction, per se, nor the three compounds given after this type of social interaction significantly changed nociception. These results indicate that 5-HT1A receptors in the periaqueductal gray inhibit analgesia induced by social conflict in mice. (C) 1998 Elsevier B.V. B.V.

Effect of a nanostructured dendrimer-naloxonazine complex on endogenous opioid peptides mu(1) receptor-mediated post-ictal antinociception

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

ANTINOCICEPTIVE EFFECT OF AMITRAZ IN MICE AND RATS

Amitraz, a formamidine insecticide and acaricide used in veterinary practice, presents side effects related to its pharmacological activity on az-adrenergic receptors. The present study was undertaken to investigate the antinociceptive effect of amitraz in rats and mice. The tail-flick test was used to determine the duration of the antinociceptive effect of the intraperitoneal tip) administration of amitraz (1 and 2 mg/kg, 10 animals per group) in male Wistar rats weighing 180-220 g. The writhing test (using 10 ml/kg of a 0.6% acetic acid solution as a painful stimulus). was performed in 140 male Swiss mice weighing 20-30 g, divided into 14 groups that received ip injections of saline, amitraz (0.5, 1.0, 1.5, 2.0 and 4.0 mg/kg), xylazine or detomidine (1.0, 1.5, 2.0 and 4.0 mg/ kg), in order to compare the effect of amitraz to that caused by xylazine and detomidine, and to investigate the participation of alpha(2)-adrenergic receptors which were blocked by idazoxan (1 mg/kg). Amitraz induced a significant antinociceptive effect in both rats and mice. This effect is blocked in mice by idazoxan.

Open elevated plus maze-induced antinociception in rats: A non-opioid type of pain inhibition?

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Sufentanil-2-hydroxypropyl-beta-cyclodextrin inclusion complex for pain treatment: Physicochemical, cytotoxicity, and pharmacological evaluation

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Estudo das propriedades analgésicas da Achillea millefolium L. (Asteraceae)

This study was designed to separately evaluate the analgesic capacity of 15% tea from Achillea millefolium leaves and florid ends. Three different tests were performed. The animals were separated in 3 groups: group I (control) that received water, group II that received florid end tea, and group III that received leaf tea. The first test was the writhing test induced by intraperitoneal injection of acetic acid in male Swiss mice. The second test consisted of the measurement of tail flick reflex latency (TFRL) in male Wistar rats after receiving a painful thermal stimulus. The groups received the teas or water per os, prior to observation. In the third test, the Wistar rats were submitted to an approximately 1 cm long incision in their tails to test the effect of the teas on induced inflammatory pain. The results showed that the effect of the 15% leaf tea on the mice writhing and TFRL test were not statistically significant. The florid ends tea showed a significantly increase in number of writhes in mice and TFRL in rats after administration. With regard to the effect of the teas on inflammatory pain, ingestion of both preparations caused an increase in the force threshold necessary for the avoidance reflex, indicating antihyperalgesic action of both the leaf and florid ends 15% teas of Achillea millefolium.

Anti-ulcerogenic and analgesic activities of the leaves of Wilbrandia ebracteata in mice

Wilbrandia ebracteata (Cogn.) Cogn. is a medicinal plant belonging to the Cucurbitaceae family used popularly as an antiulcer and analgesic medicine. The hydromethanol extract of leaves was investigated to determine its anti-ulcerogenic (ethanol and indomethacin induced gastric damage) and analgesic (writhing and tail-flick tests) activities in mice (efficacy), its acute toxicity (safety), and its phytochemistry (quality control). Oral administration of leaf extract at a dose of 1000 mg/kg body wt. significantly reduced 73.3% of the total area of lesion in ethanol-induced gastric damage, but was inactive in an indomethacin-induced gastric damage test. The hydromethanol extract was also inactive in both analgesic tests. Oral administration of the leaf extract did not produce mortality in mice, while the LD50 value of the roots was 22.10 mg/kg body wt. in female mice and 58.31 mg/kg body wt. in male mice. Leaves of W. ebracteata reacted positively for steroids, flavonols, flavanones, saponins, tannins and xanthones and negative for other compounds, including cucurbitacins. Leaf extract of W. ebracteata was active as an anti-ulcerogenic, probably through increasing gastric defensive factors, and flavonoids might be the main constituent responsible for this activity.