Maintenance of beta-endorphin analgesia across age cohorts

Qualitative sex differences in kappa-opioid analgesia in mice are dependent on age

The effects of aging on sex differences in analgesia from the kappa-opioid agonist, U50,488H (U50), were examined in C57BL/6J mice. U50 analgesia can be blocked by the N-methyl-d-aspartate receptor antagonist, MK-801 (MK), in male rodents and gonadectomized females, but not hormonally intact or estrogen-replaced females, suggesting the existence of alternate neurochemical mediation in females. We now report that MK antagonism of U50 analgesia is age-dependent in females. That is, reproductively senescent females display MK-sensitive U50 analgesia qualitatively similar to that displayed by males or hormonally deprived young females. Age-related reductions in U50 analgesic magnitude were also observed in females. Thus, age and gender are likely to alter the clinical efficacy of analgesic drugs active at kappa-opioid receptors.

Age differences in nociception and pain behaviours in the rat

Much remains to be learned about the effects of ageing on pain. Studies of life-span changes in nociception and pain behaviours in the rat are equivocal making it difficult to draw firm conclusions. This paper reviews the available data and finds that age differences in nociception may be dependent on the pain test employed. Specifically, reflexive responses to nociceptive stimuli do not change with age while there may be no change or a linear decrease with age on more highly organized tests of nociception. Interestingly, age differences in pain behaviours on models of tissue injury and inflammation may not be linear. It is shown that important changes that begin at mid-life in neuroanatomy, neurochemistry and endogenous pain inhibition may be associated with alterations in pain sensitivity. Several testable hypotheses which might encourage future research in this domain are developed throughout this paper.

Age differences in the response to the formalin test in rats

We report the results of a study designed to assess age differences in the response to the formalin test, a model of tissue injury and inflammation, while controlling for differences in weight and motoric abilities in three groups of adult male Long-Evans rats: young (3 months old), middle-aged (18 months old), and old (24 months old). The first part of the study assessed initial differences in responsivity and found that the middle-aged group showed the greatest response, whereas the young and old groups did not differ from each other. In the second part of the study, the young and middle-aged animals were followed for a 4-month period. The formalin test was repeated at 2-month intervals. These results indicate that there may be an age-associated change in the sensitivity to tonic pain and that this sensitivity may peak at mid-life.

Age-related changes in the spinal antinociceptive effects of DAGO, DPDPE and beta-endorphin in the rat

These studies were designed to investigate how the aging process alters the spinal antinociceptive efficacy of mu (mu), delta (delta) and epsilon (epsilon) opioid receptor agonists administered intrathecally (i.t.) in rats. Various doses of the mu agonist DAGO, the delta agonist DPDPE or the putative epsilon beta-endorphin were injected i.t. in young (5-6-month-old), mature (15-16-month-old) and aged (25-26-month-old) Fischer 344 rats. Antinociception was measured using the rat tail-flick analgesiometric assay. The data demonstrated a decline in spinal opioid-induced antinociception as a function of age. For instance, the i.t. dose of DPDPE or beta-endorphin needed to produce antinociception in the 25-26-month-old rats was higher than that needed to elevate tail-flick latency in the young and mature animals. We also noted that the i.t. doses of the opioid agonists needed to produce 'antinociception' in the aged cohort were within a range of spinal doses that produced motor impairment. Apparently, the aging process alters the ability of opioid receptors to mediate antinociception. Perhaps an age-related decrease in the number and/or affinity of opioid receptor sites in the rat spinal cord accounts for these observations.

Effects of aging on spinal opioid-induced antinociception

Initial experiments were conducted to determine whether or not the aging process alters the ability of young, mature, or aged male Fischer 344 rats (5- to 6-, 15- to 16-, and 25- to 26-months-old, respectively) to respond to thermal nociceptive stimuli. Using the tail-flick analgesiometric assay, 25- to 26-month-old rats responded significantly faster to the heat source than 15- to 16-month-old animals, but no significant differences were noted between the 5- to 6-month-old and aged rats. Another series of investigations compared the effects of aging on the spinal antinociceptive properties of the mu opioid agonist [D-Ala2,N-methyl-Phe4,Gly5-ol] enkephalin (DAMPGO) and the delta agonist [D-Pen2,D-Pen5] enkephalin (DPDPE). In these studies, young, mature, and aged rats were injected intrathecally (IT) with different doses of DAMPGO or DPDPE, and opioid-induced antinociception was tested on the tail-flick test. All three age groups responded to IT DAMPGO in a dose-dependent manner but, for the most part, higher spinal doses were required to produce significant elevations in tail-flick latency in the aged cohort of rats. The spinal analgesic effects of DPDPE also declined with advanced age. The aging process apparently alters the pain-inhibitory function of mu and delta opioid receptors in the rat spinal cord.

Interactions among aging, gender, and gonadectomy effects upon naloxone hypophagia in rats

The present study examined the dose-dependent (0.25-5 mg/kg) effects of systemic naloxone upon deprivation-induced intake and high-fat intake as functions of age (4, 8, 14, and 20 months), gender, and gonadectomy in rats. Significant increases in body weight were observed as functions of age and gonadectomy. Whereas aging significantly reduced basal deprivation-induced intake, it generally failed to alter basal high-fat intake. Whereas age, gender, and gonadectomy failed to alter the decreases in deprivation-induced intake following low (0.25-2.5 mg/kg) naloxone doses, sham males displayed significantly greater age-related and gender-related inhibition following the 5 mg/kg dose of naloxone. Young gonadectomized rats displayed significant increases in naloxone's inhibition of deprivation-induced intake as well. More dramatic changes occurred in naloxone's inhibition of high-fat intake. Naloxone's potency increased in sham female rats as a function of age, and decreased in sham males and ovariectomized females as a function of age. Whereas sham males and ovariectomized females were most sensitive to naloxone's inhibition of high-fat intake at young ages, sham females were most sensitive at older ages. These data indicate that effects of age, gender, and gonadectomy upon naloxone-induced hypophagia dissociate as a function of the type of intake. Because selective opioid antagonist studies demonstrate that deprivation-induced intake is mediated by the mu1 receptor and high-fat intake is mediated by kappa and mu2 receptors, it is postulated that the differential effects of aging, gender, and gonadectomy variables upon opioid mediation of the two forms of intake may reflect their interaction with different opioid receptor subtypes.

Interactions among aging, gender, and gonadectomy effects upon morphine antinociception in rats

In addition to age-related deficits in morphine antinociception in female rats, gender and gonadectomy differences have also been observed, with male rats displaying greater magnitudes of effects than females and castrated males. Since there are little data indicating how aging, gender, and gonadectomy interact in modulating morphine antinociception, the present study evaluated alterations in this response as functions of age (6, 12, 18, and 24 months), gender, and gonadal status (intact, gonadectomized) across a dose range (1-10 mg/kg) and time course (0.5-2 h) on the tail-flick test. The maximal percentage effect (MPE) of morphine (1 mg/kg) was significantly increased in castrated males (18 months), sham females (18 and 24 months), and ovariectomized females (18 months) relative to 6-month-old groups. Increases in the MPE of morphine (1 mg/kg) occurred in sham females (24 months) relative to corresponding sham males and ovariectomized females. The MPE of morphine (2.5 mg/kg) was significantly increased in sham males (18 months) and decreased in sham females (12 months). Decreases in the MPE of morphine (2.5 mg/kg) occurred in castrated males (18 and 24 months) as well as sham (18 months) and ovariectomized (18 and 24 months) females relative to sham males. Whereas the MPE of morphine (5 mg/kg) was unchanged by these variables, the MPE of morphine (10 mg/kg) was significantly decreased in sham females (18 and 24 months) relative to females aged 6 months, as well as males and ovariectomized females aged 24 months.(ABSTRACT TRUNCATED AT 250 WORDS)

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.