Effect of oestradiol replacement on swim-induced antinociception in ovariectomized mice

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Dose- and time-dependent estradiol modulation of morphine antinociception in adult female rats

To clarify the activational role of ovarian hormones on pain and analgesia, the present study determined whether estradiol (E2) modulation of nociception and morphine antinociception in adult female rats depends on (1) the dose of E2 and (2) the interval between E2 treatment and nociceptive testing. Female rats were ovariectomized (OvX) and either oil vehicle (0), or E2 (0.25, 2.5 or 25 microg/0.1 ml vehicle) was injected s.c. two consecutive days of every four days for five cycles before testing. Either 4, 24, 48 or 96 h after the last injection, nociception was evaluated on the 50 degrees C hotplate and warm water tail withdrawal tests before and after escalating doses of s.c. morphine. Lordosis behavior and uterine weight were assessed in other rats at the same E2 doses and time points. E2 significantly lengthened latency to respond on the hotplate test at 24 h after the last injection, but had no significant effect on tail withdrawal latencies. The lower doses of E2 significantly increased morphine antinociceptive potency at 4-24 h on one or both tests, but the intermediate E2 dose significantly decreased morphine potency at 48 h on the hotplate test. Thus, E2 modulation of morphine antinociception in the adult female rat is bidirectional, and occurs at E2 doses producing cyclic changes in sexual behavior, uterine weight and vaginal cytology that are similar to those observed in gonadally intact, cycling females.

Sex differences in pain and analgesia: the role of gonadal hormones

There is now strong evidence for sex differences in pain and analgesia. These differences imply that gonadal steroid hormones such as estradiol and testosterone modulate sensitivity to pain and analgesia. The goal of this review is to present an overview of gonadal steroid modulation of pain and analgesia in animals and humans, and to describe mechanisms by which males' and females' biology may differentially predispose them to pain and to analgesic effects of drugs and stress. Evidence is presented to demonstrate that sex differences in pain and analgesia may be both quantitative and qualitative in nature. Current research suggests that sex-specific management of clinical pain will be a reality in the not-so-distant future.

Effects of a histamine H2 receptor agonist and antagonist on restraint-induced antinociception in female mice

Restraint for 1 h induced significant antinociceptive activity, as assessed by the hot plate test, in female mice. The antinociceptive activity was significant throughout the 1 h period of observation starting immediately after restraint. Prior administration of the histamine H2 receptor agonist dimaprit (1.5-6.0 mg/kg s.c.) 15 min before restraint further enhanced the restraint-induced antinociceptive activity. Furthermore, the induction of antinociceptive activity by restraint was antagonised by prior administration of histamine H2 receptor antagonists, cimetidine (2.5-10.0 mg/kg s.c.) or zolantidine (2.5-10.0 mg/kg s.c.). However, when these drugs were administered immediately after restraint for 1 h, the antinociceptive activity observed was similar to those restrained animals receiving saline injection. The histamine receptor agonist and antagonists, at the doses used in the present study, did not affect the response of unrestrained animals to the hot plate test. These results demonstrate that the effect of a histamine H2 receptor agonist and antagonists on restraint-induced antinociception is dependent upon their time of administration and may act by altering the intensity of stress, thus affecting the antinociceptive activity induced.

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.

Gender effects and central opioid analgesia

Central morphine analgesia is significantly greater in male than in female rats. Since mu and delta opioid receptor subtypes have been implicated in supraspinal analgesia, the present study evaluated whether gender or adult gonadectomy altered (a) analgesia on the tail-flick and jump tests following central administration of the mu-selective agonist, [D-Ala2, Me-Phe4, Gly(ol)5] enkephalin (DAMGO) and the delta-selective agonist, [D-Ser2,Leu5] enkephalin-Thr6 (DSLET) and (b) mu1, mu2 and delta opioid receptor binding. Sham-operated male rats displayed significantly greater magnitudes of peak and total analgesia than sham-operated females on the tail-flick test following DAMGO, but not DSLET. Gender differences were not observed for DAMGO and DSLET analgesia on the jump test. Gonadectomy failed to consistently affect either DAMGO or DSLET analgesia. Regression analyses failed to reflect significant shifts in the dose-response functions for either agonist on either measure. Gender differences were not observed for mu1, mu2, or delta binding in hypothalamus or cortex. These data are compared with analgesic responses sensitive to gender differences.