Stressor controllability and stress-induced analgesia

Cats produce analgesia in rats on the tail-flick test: naltrexone sensitivity is determined by the nociceptive test stimulus

Previous research has demonstrated that exposure to a cat produces a naltrexone-reversible antinociception as assessed in the formalin test in rats. Because different neurochemical mechanisms inhibit different forms of nociception, the present study examined whether presentation of a cat would also produce a naltrexone-reversible antinociception in the tail-flick response to radiant heat and electric shock. Exposure to the cat produced antinociception in both tail-flick paradigms. Naltrexone blocked the inhibition of the thermally evoked tail-flick response, but had no effect in the electric shock tail-flick paradigm. These results indicate that opioid mediation of stress-induced analgesia is determined, in part, by the nociceptive test employed.

Antagonism of morphine analgesia by nonopioid cold-water swim analgesia: direct evidence for collateral inhibition

The demonstrated existence of opioid and nonopioid forms of pain control has raised questions as to how they interact. Previous indirect evidence suggests that activation of one system inhibited the activation of the other. The present study assessed this directly using morphine as an opiate form of analgesia and continuous cold-water swims (CCWS, 4 degrees C, 2 min) as the nonopioid form. A significant reduction in morphine (8 mg/kg, SC) analgesia on the tail-flick test was observed if rats were acutely exposed to CCWS immediately prior to morphine administration. The inability of naloxone (10 mg/kg, SC) to reduce CCWS analgesia verified its nonopioid nature. The antagonism of morphine (3 mg/kg, SC) analgesia was greater following preexposure to 2 min of CCWS than 1 min of CCWS. CCWS was also more effective in antagonizing analgesia induced by the 3 mg/kg than the 8 mg/kg dose of morphine. The antagonism of morphine analgesia by CCWS was dependent upon the temporal patterning of stimulus presentation: exposure to CCWS 20 or 60 min prior to morphine failed to alter subsequent morphine analgesia. A significant reduction in analgesia induced by intraperitoneal administration of morphine (10 mg/kg) was also observed when CCWS was presented immediately prior to injection, suggesting that pharmacokinetic factors such as altered drug absorbance by CCWS-induced vasoconstriction do not appear to explain these effects. These data provide direct support for the existence of collateral inhibitory mechanisms activated by CCWS and morphine, and suggests that these opioid and nonopioid forms of analgesia do not function synergistically, but instead involve some form of hierarchical order.

Analgesia and impact induced by anticipation stress: involvement of the endogenous opioid peptide system

Anticipation of an unavoidable painful stimulation, i.e. anticipation stress, induces analgesia. This analgesia may result from activation of the endogenous opioid peptide system as it is blocked by naloxone. The present paper characterizes the anticipation stress further by following the kinetics of the analgesia rise, the impact of the stress on the stress-sensitive organs and the involvement of the endogenous opioid peptide system in the anticipation stress effects. Adult male rats exposed repeatedly to placing into a conditioning box followed by a painful stimulation develop analgesia almost immediately after a transfer to the box. Anticipation acts as a specific trigger, no other side stressor is effective. Anticipation stress has a significant impact on stress-sensitive organs: weights of the adrenals increase while those of the thymus and spleen decrease. These changes are associated with a significant increase of plasma corticosterone. Blockade of the endogenous opioid system by naloxone before the exposure to anticipation stress potentiates these stress-induced impacts, especially the decrease of weight of the thymus. Plasma corticosterone levels are not affected by naloxone. During the anticipation stress, the amount of opioid receptors, i.e. of [3H]naloxone binding sites, in the hypothalamus, but not in the striatum, decreases. The possible biological role of stress-induced activation of endogenous opioid system, namely the maintenance of the intensity of stress reaction within certain limits and thus the prevention of the self-destructive effects of stress reactions, is discussed.

Changes in mu opiate receptors following inescapable shock

Rats exposed to inescapable shock exhibit profound hypoalgesia. Pharmacological evidence has suggested that changes in endogenous opiate activity may be responsible for the hypoalgesic response. We measured the binding of [3H]DAGO, a selective mu-opiate receptor agonist, in brains of rats exposed to no shock, inescapable shock, or escapable shock. Binding of [3H]DAGO in the midbrains of rats in the inescapable shock group was decreased relative to the other two groups. The decrease in binding appeared to result from a decrease in number of mu-receptors and not a change in affinity. These results support the hypothesis that inescapable shock produces long-term changes in endogenous opiate systems.

Organismic variables and pain inhibition: roles of gender and aging

Multiple pain-inhibitory systems dependent upon both opioid and nonopioid mechanisms of action have been identified, particularly in the rodent. The experimental subject has typically been the young, adult male rat, and generalizations concerning these systems have been made from this subject pool. This review focuses upon the roles of two organismic factors, aging and gender, in the modulation of analgesic processes. Using an array of age cohorts (4, 9, 14, 19, 24 months), these data illustrate that aging produces differential decrements in the analgesic responses following morphine, different parameters of footshock, continuous cold-water swims (CCWS: a nonopioid stressor), intermittent cold-water swims (ICWS: an opioid stressor) and 2-deoxy-D-glucose (a mixed opioid/nonopioid stressor). In contrast, neither beta-endorphin nor food deprivation analgesia is affected by aging. This review identifies that CCWS and ICWS analgesia are sensitive to gender differences, gonadectomy differences and steroid replacement differences such that females display less analgesia than males, gonadectomy reduces both analgesic responses, and that testosterone is most effective in reinstating gonadectomy-induced analgesic deficits. These data are considered in terms of therapeutic implications for the organismic variables under study as well as for the conceptual and methodological modifications that must be made in studying intrinsic pain inhibition.

Effects of naltrexone on learning and performance of conditional fear-induced freezing and opioid analgesia

In the presence of Pavlovian conditional aversive stimuli rats engage in defensive freezing and exhibit a decreased sensitivity to painful stimulation. Administration of the opioid antagonist naltrexone (7 mg/ml/kg) prior to training in a context fear conditioning procedure using footshock caused an increase in freezing but did not affect suppression of formalin-induced recuperative behaviors relative to controls during a test session 24 hr later. The same dose of naltrexone given during the test session reversed conditional analgesia without altering the level of freezing. These results suggest that full expression of endogenous opioid analgesia during training is not necessary for conditional responding.