Yohimbine potentiates cold-water swim analgesia: re-evaluation of a noradrenergic role

Neurogenic inflammation and arthritis

Inflammation and inflammatory diseases are sexually dimorphic, but the underlying causes for this observed sexual dimorphism are poorly understood. We discuss neural-immune mechanisms that underlie sexual dimorphism in three critical aspects of the inflammatory process-plasma extravasation, neutrophil function, and inflammatory hyperalgesia. Plasma extravasation and accumulation/activation of leukocytes into tissues are critical components in inflammation and are required for several other aspects of the inflammatory response. Pain (hyperalgesia) also markedly influences the magnitude of other components of the inflammatory response and induces a feedback control of plasma extravasation and neutrophil function. More important, this feedback control itself is powerfully modulated by vagal afferent activity and both the function of the primary afferent nociceptor and the modulation of inflammatory hyperalgesia by vagal afferent activity are highly sexually dimorphic.

Changes in anxiety levels are followed by changes in behavioral strategy in mice subjected to stress and in the extent of stress-induced analgesia

The experiments reported here demonstrated that corasol increased the extent of analgesia induced by stress and decreased the duration of immobility in mice in a forced swimming test in cold water. Administration of diazepam led to the opposite changes and counteracted the actions of the anxiogen. The effects of the anxiolytic were more apparent in NMRI than mongrel mice, while in mongrel mice the effects of the anxiogen were more marked. Changes in measures following administration of agents were reciprocal in nature. These results lead to the conclusion that that these changes are determined by the level of anxiety, and that the strain differences between mongrel and NMRI mice are also linked with this factor.

Kappa-opioids produce significantly greater analgesia in women than in men

Sex differences in human responses to nociceptive stimuli and painful pathological conditions have generally indicated that women report higher pain levels or exhibit less tolerance than men for given stimulus intensities (reviewed in ref. 1 and 2). However, studies have not evaluated sex differences in analgesic responses. We recently reported that the opioid agonist-antagonist pentazocine, which acts predominantly at kappa-receptors, produced significantly better postoperative analgesia in females than in males in patients who underwent surgery for the removal of their third molars (wisdom teeth). In the current study, we evaluated the hypothesis that this sex difference is a characteristic of kappa-opioid agonism. In order to determine whether there are sex differences associated with kappa-opioid agonism, the analgesic efficacy of two other predominantly kappa-opioid analgesics, nalbuphine and butorphanol; was compared in males and females who underwent surgery for the removal of third molar teeth. We found that both nalbuphine and butorphanol produced significantly greater analgesia in females as compared with males. Considering our earlier findings, we conclude that kappa-opioid analgesia is greater in females than in males, probably reflecting a difference in kappa-opioid-activated endogenous pain modulating circuits.

Stress-induced behavioral responses and multiple opioid systems in the brain

Various stressor produce a wide range of behavioral responses such as analgesia, catalepsy and motor suppression, which are sensitive to opioid receptor antagonists. These behavioral responses in stress are accompanied by changes in the contents of opioid peptides, the mRNAs encoding their precursors and opioid receptor binding in the brain. In the present article, experimental data concerning stress-induced analgesia and motor suppression is reviewed and discussed in relation to a possible involvement of different opioid systems in the various observed behavioral responses in stress. Pharmacological studies with subtype-selective antagonists have demonstrated that not only mu- but also delta- and/or kappa-opioid receptors are involved in opioid-mediated stress-induced analgesia. There are two types of stress-induced analgesia referred to as opioid-mediated and non-opioid mediated forms. It has been proposed that the intensity and temporal pattern of stressor may be a critical factor determining the nature of stress-induced analgesia. Accumulated evidence demonstrate that these two forms of pain inhibitory systems interact each other according to a collateral inhibition model. Recent studies show that parallel activation of multiple opioid receptors mediates non-opioid froms of stress-induced analgesia. Dynorphins, by acting at kappa-opioid receptors, may play a pivotal role in the expression of stress-induced motor suppression, whereas enkephalins may act to attenuate this response.

Effects of acute selective 5-HT1, 5-HT2, 5-HT3 receptor and alpha 2 adrenoceptor blockade on naloxone-induced antinociception

Several studies have demonstrated a paradoxical form of antinociception induced by the repeated administration of opioid antagonists accompanied by exposure to a painful stimulus. The underlying mechanism of this naloxone-induced antinociception (NIA) is still unknown, but the results of several studies suggest that it is a non-opioid response. This study was designed to investigate serotonergic and noradrenergic involvement in NIA. Rats were treated daily with systemic injections of 5 mg/kg naloxone, followed by a 45-s hot plate test of nociception (temperature = 51.5 +/- 0.5 degree C). After rats reached plateau levels of NIA, they received a test trial in which they were treated with various doses of different selective 5-HT or alpha 2 adrenoceptor antagonists in addition to naloxone before the hot plate test. Rats treated with 0.16, 0.32 and 0.63 mg/kg pirenperone or 2.5 mg/kg ritanserin showed significant reductions in paw lick latency with respect to rats treated with vehicle. In addition, high doses of yohimbine (7.5-10 mg/kg) also effectively reversed NIA. In contrast, NIA was not affected by acute blockade of 5-HT1 or 5-HT3 receptors by methiothepin or MDL 72222, respectively, or by the alpha 2 adrenoceptor blocker idazoxan. None of the 5-HT or alpha 2 adrenoceptor antagonists had any effect on the paw lick latencies of saline-treated rats. A possible role of 5-HT2 receptors in the antinociception induced by opioid receptor blockade is discussed.

Effect of naloxone on the habituation of novelty-induced hypoalgesia: the collateral inhibition hypothesis revisited

Repeated daily administration of the opiate receptor antagonist naloxone prior to hotplate tests provokes longer paw-lick latencies by attenuating the habituation of novelty-induced hypoalgesia. This hypoalgesia has been found to persist when pain tests are subsequently conducted following saline administration. The present experiments were conducted to determine whether the substrates mediating the hypoalgesia observed during naloxone and saline tests are similar or distinct. Neither the hypoalgesia observed during naloxone nor saline tests were affected by the induction of tolerance to the hypoalgesic effect of morphine, suggesting that both effects are mediated by nonopioid antinociceptive mechanisms. Previous work from our laboratory demonstrated that the hypoalgesia observed during naloxone tests is inhibited by clonidine, enhanced by yohimbine, and unaffected by prazosin and phentolamine. In the present article, we report a similar pattern of results for the hypoalgesia observed during saline tests. It is concluded that the substrates mediating both effects are similar. The results are discussed in relation to the possibility that an opioid substrate involved in habituative learning may be inhibitory on a nonopioid antinociceptive substrate.

Naloxone-induced hypoalgesia: lack of involvement of the GABA-benzodiazepine receptor complex

Previous evidence has demonstrated that repeated daily administration of the opiate receptor antagonist naloxone prior to assessment of pain sensitivity provokes the development of a nonopioid form of hypoalgesia. The present experiments assessed whether the GABA-benzodiazepine receptor complex may be involved in the mediation of this effect. Male Wistar rats were administered 10 mg/kg naloxone prior to hot-plate tests (48.5 degrees C) for pain sensitivity for 8 consecutive days. Control animals were administered saline prior to, and naloxone 2-4 h after, assessment of pain reactivity. Beginning on the fourth or fifth day of this regimen, animals tested under the influence of naloxone displayed longer paw-lick latencies than controls. Preadministration of the GABAA agonist muscimol (1.0-5.0 mg/kg) and GABAA antagonist bicuculline (0.25-1.0 mg/kg) failed to affect paw-lick latencies in naloxone-tested and control rats. The GABAB receptor agonist baclofen (1.0-5.0 mg/kg) and the benzodiazepine receptor agonist diazepam (1.0-5.0 mg/kg) both elevated paw-lick latencies to the same degree in both groups of animals. These results suggest that the GABA-benzodiazepine receptor complex is not involved in the mediation of naloxone-induced hypoalgesia.

Clonidine and yohimbine modulate the effects of naloxone on novelty-induced hypoalgesia

Previous research has shown that repeated daily pretreatment with the opiate receptor blocker naloxone retards the development of habituation to novelty-induced hypoalgesia. The present experiments were conducted in order to determine whether noradrenergic substrates mediates this effect. Animals in the NAL condition were administered 10 mg/kg naloxone prior to assessment of pain sensitivity on a 48.5 degrees C hot plate. Control animals (SAL condition) were administered saline prior to pain assessment, and naloxone 2-4 h later. Paw lick latencies declined over repeated tests in SAL animals, suggesting the habituation of novelty hypoalgesia. Naloxone pretreatment attenuated this decline. The longer paw lick latencies observed in NAL condition animals were reduced by administration of 2 microgram/kg clonidine, a specific noradrenergic alpha-2 receptor agonist, and enhanced in a dose dependent (0.5-4.0 mg/kg) fashion by the alpha-2 antagonist yohimbine. Clonidine and yohimbine either failed to alter pain reactivity in control animals, or produced less marked effects than those observed in naloxone-exposed animals. These results suggest that noradrenergic substrates mediate naloxone's effects on novelty hypoalgesia.

Roles of gender and gonadectomy in pilocarpine and clonidine analgesia in rats

Central and systemic morphine analgesia as well as both opioid and nonopioid forms of swim analgesia display gender differences with male rats showing greater magnitudes of analgesia than female rats. Since nonopioid swim analgesia is dependent upon muscarinic cholinergic and alpha 2-noradrenergic mechanisms, the present study evaluated in rats whether gender, adult gonadectomy or estrous phase altered analgesia induced by either the muscarinic cholinergic receptor agonist, pilocarpine or the alpha 2-noradrenergic receptor agonist, clonidine. Pilocarpine (1-10 mg/kg) analgesia was significantly greater in male rats. Female rats displayed 7-fold and 3-fold rightward shifts in peak analgesia on the tail-flick and jump tests respectively. Clonidine (100-500 micrograms/kg) analgesia was significantly greater on both nociceptive tests in males, but only produced a 2-fold rightward shift in peak analgesia in females on the jump test. Whereas castration failed to shift either dose-response curve, ovariectomy mitigated the gender differences in pilocarpine and clonidine analgesia. Both pilocarpine and clonidine analgesia were not altered by estrous phase changes. These data indicate that gender differences in analgesia are not specific to opioid systems.

Alpha-adrenoceptor involvement in swim stress-induced antinociception in the mouse

Three different intensities of swim stress produced stress-induced antinociception (SIA) in mice which was assessed either by the reduction in the number of abdominal constrictions produced by acetic acid or by an increase in reaction time on a hot-plate. The involvement of alpha-adrenoceptors in the three models of SIA was investigated using selective antagonists. SIA produced by the mild stress of a 30 s warm water swim was attenuated by idazoxan (0.5-1 mg kg-1), and by yohimbine at a dose (1 mg kg-1) which reduced antinociception produced by clonidine (12.5-50 micrograms kg-1). Indoramin (1-2 mg kg-1) did not affect this model of SIA, but reversed phenylephrine induced inhibition of the constrictions. A 3 min room temperature swim increased reaction times on the hot-plate and this naloxone-sensitive SIA was reduced significantly by prazosin (1-2 mg kg-1), idazoxan (0.5-1 mg kg-1) and yohimbine (0.5-1 mg kg-1) but enhanced by clonidine (0.5 mg kg-1) and noradrenaline (NA) (10 micrograms i.c.v.). Mice treated with 6-hydroxydopamine (60 + 60 micrograms i.c.v.) were hypersensitive to the hot-plate and did not develop SIA. Levels of noradrenaline in the brain (minus the cerebellum) were decreased after the room temperature swim SIA. The most severe stress of a cold water swim produced SIA on the hot-plate which was initially naloxone-insensitive.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opiates: 1988

This paper is the eleventh installment in our annual review of the research during the past year involving the endogenous opiate system. It is concerned with nonanalgesic and behavioral studies of the opiate peptides that were published during 1988. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

Reduction in opioid and non-opioid forms of swim analgesia by 5-HT2 receptor antagonists

Acute exposure to continuous (CCWS) or intermittent (ICWS) cold-water swims elicits non-opioid and opioid forms of analgesia respectively. Intrathecal administration of methysergide blocks ICWS, but not CCWS analgesia. The present study evaluated the role of serotonin (5-HT) receptor subtypes in the mediation of CCWS and ICWS analgesia on the tail-flick and jump tests following administration of methysergide, a non-specific 5-HT antagonist and pirenpirone and ketanserin, two 5-HT2 receptor subtype antagonists. Systemic methysergide was more effective in reducing CCWS analgesia (50-58%, 0.1-1.0 mg/kg) than ICWS analgesia (21%, 5 mg/kg) on both pain tests. Systemic pirenpirone (0.04-0.2 mg/kg) and ketanserin (1-5 mg/kg) were also more effective in reducing CCWS analgesia (43-57%) on both tests than ICWS analgesia (pirenpirone: 0.4 mg/kg, 34%; ketanserin: 5 mg/kg, 21%) on the tail-flick test. Indeed, both 5-HT2 receptor antagonists potentiated ICWS analgesia on the jump test. While serotonin antagonist effects upon hypothermia could not account for CCWS analgesia effects, similar potentiations in ICWS analgesia and hypothermia were observed following pirenpirone and ketanserin. Finally, both 5-HT2 receptor antagonists differentially reduced CCWS hypothermia and potentiated ICWS hypothermia. These data suggest differential serotonergic modulation of the two forms of swim analgesia with opioid-mediated ICWS analgesia acting through spinal 5-HT1 receptors and non-opioid-mediated CCWS analgesia acting through supraspinal 5-HT2 receptors.

Effect of yohimbine on nociceptive threshold in normoglycemic and streptozotocin-treated hyperglycemic mice

The effects of yohimbine (0.1, 1, 3 and 10 mg/kg SC) on nociceptive threshold were tested in mice using the tail-immersion and hot-plate tests. The tail-flick (withdrawal) latency, a monosynaptic spinal nociceptive response, was significantly lowered by yohimbine. This hyperalgesic response was at its peak 0.5 hr after yohimbine injection. The tail-flick latencies expressed as % of basal latencies were, 95 +/- 8, 100 +/- 10, 62 +/- 10, 33 +/- 7 and 28 +/- 4 in vehicle and 0.1, 1, 3 and 10 mg/kg in yohimbine-treated groups respectively. Yohimbine-induced hyperalgesia was observed when stimulus temperature was either 50 degrees C or 45 degrees C; however, the opiate antagonist naloxone (3 mg/kg SC) induced a hyperalgesic response at 50 degrees C and an analgesic response at 45 degrees C stimulus temperature. Streptozotocin-induced hyperglycemia did not influence the hyperalgesic response of yohimbine. In the hot-plate (60 degrees C) test, which involves higher centers and a polysynaptic nociceptive reflex, yohimbine did not modify the jump latency. The data provide evidence for the presence of a tonic spinal noradrenergic inhibitory control of nociceptive mechanism(s) which does not appear to be readily altered by hyperglycemia.