Testosterone modulation of reproductive indices vs. morphine antinociception in male rats

Sex differences in opioid receptor mediated effects: Role of androgens

An abundance of data indicates there are sex differences in endogenous opioid peptides and opioid receptors, leading to functional differences in sensitivity to opioid receptor mediated behaviors between males and females. Many of these sex differences are mediated by the effects of gonadal hormones on the endogenous opioid system. Whereas much research has examined the role of ovarian hormones on opioid receptor mediated endpoints, comparatively less research has examined the role of androgens. This review describes what is currently known regarding the influence of androgens on opioid receptor mediated endpoints and how androgens may contribute to sex differences in these effects. The review also addresses the clinical implications of androgenic modulation of opioid receptor mediated behaviors and suggests future lines of research for preclinical and clinical investigators. We conclude that further investigation into androgenic modulation of opioid receptor mediated effects may lead to new options for addressing conditions such as chronic pain and substance use disorders.

Anabolic-androgenic steroid effects on nociception and morphine antinociception in male rats

The purpose of this study was to investigate the effects of acute and chronic administration of anabolic-androgenic steroids (AAS) on nociception and morphine antinociception in acute pain models, as well as on chronic inflammatory nociception. In Experiment 1, adult, gonadally intact male rats were injected s.c. for 28 days with either 5 mg/kg testosterone (T), dihydrotestosterone (DHT), stanozolol (STAN), or safflower oil vehicle (N=12-25/group). On day 28, rats in each group were tested on acute thermal and mechanical nociceptive assays, before and after morphine treatment. In Experiment 2, rats in each group (N=8-10/group) were injected with mineral oil or complete Freund's adjuvant (CFA) into one hindpaw after 28 days of AAS treatment, and then tested for thermal hyperalgesia, mechanical allodynia, inflammation and locomotor suppression intermittently for 28 days. Experiment 3 replicated nociceptive measurements in Experiments 1 and 2, but with a single AAS or vehicle injection occurring 3h prior to testing (N=10-12/group). While chronic AAS administration tended to decrease body weight gain and alter reproductive organ weights in the expected manner, it did not significantly alter acute nociception nor attenuate the development of various chronic pain indices after CFA administration. Morphine antinociceptive potency was significantly decreased by chronic DHT on the hot plate test only. Acute AAS administration also did not significantly alter acute or chronic nociception, or morphine antinociceptive potency. Comparisons between acute and chronic AAS administration suggest that steroid tolerance did not occur in rats treated with AAS chronically. Taken together, these data do not support the hypothesis that AAS exposure alters nociception or morphine antinociception in gonadally intact males.

Sexual differentiation of rat reproductive versus opioid antinociceptive systems

BACKGROUND

It has been suggested that sexual differentiation of opioid analgesic sensitivity may parallel sexual differentiation in reproductive systems.

OBJECTIVE

The present study compared organizational and activational roles of testosterone in sexual differentiation in reproductive versus opioid antinociceptive systems in the rat, to assess whether both systems were similarly testosterone dependent.

METHODS

Male rat pups (Sprague-Dawley and Fisher 344 [F344]) were either handled or castrated on postnatal day (PND) 1, and female pups were injected with testosterone propionate (100 or 1000 microg) on PND 2. In adulthood, all rats were gonadectomized (or simply anesthetized) and implanted with either testosterone filled or blank capsules (one 10-mm capsule/100 g of body weight).

RESULTS

Two hundred one Sprague-Dawley rats and 178 F344 rats were used. In gonadally intact adults of both rat strains, the antinociceptive potency of subcutaneously injected morphine was significantly greater in males than in females (P < or = 0.05). These sex differences were eliminated by neonatal castration in males or by neonatal androgenization in females. However, adult testosterone treatment reversed the effects of neonatal castration in males. Masculinization and defeminization of sexual behavior, ovary weight, and body weight generally met conventional expectations. Compared with male controls, neonatally castrated males gained less body weight, and displayed more lordosis behavior and compromised male sexual behaviors. Compared with female controls, neonatally androgenized females gained more body weight, developed smaller ovaries, and presented less lordosis behavior and more male sexual behaviors. Overall, neonatal testosterone manipulations sufficient to masculinize or defeminize rats in terms of reproductive behavior and physiology also masculinized or defeminized morphine antinociceptive sensitivity. The effects of neonatal castration were reversed by adult testosterone treatment, indicating that sexual differentiation of opioid antinociceptive systems begins before PND 1.

CONCLUSIONS

Sensitivity to opioid antinociception begins to diverge between males and females early in life. The relationship between gonadal hormone-mediated sexual differentiation of the reproductive and the opioid antinociceptive systems suggests that the 2 systems may be functionally linked. This finding has implications for the treatment of pain and analgesia in women and men.

Study of morphine-induced dependence in gonadectomized male and female mice

In this study we evaluated the effects of sex difference and also sex hormones on the naloxone-precipitated morphine withdrawal in both orchidectomized (ORC) male and ovariectomized (OVX) female mice. Morphine (50, 50 and 75 mg/kg/day for 4 days, s.c.) was administered to animals and at 5th day naloxone (4 mg/kg, i.p.)-precipitated morphine withdrawal signs, jumpings and the percentage of weight loss, were measured. There was no significant alteration in withdrawal jumpings between male and female mice, though weight loss was significantly higher in male ones. Jumpings was significantly lower in both OVX and ORC mice and percentage of weight loss was significantly higher in OVX mice than corresponding non-operated or sham animals. In OVX mice, E(2)V (10 mg/kg, s.c.) increased number of jumpings and decreased percentage of weight loss. Progesterone (25 mg/kg, s.c.) had no effect on jumpings, whereas it decreased weight loss in OVX mice. Testosterone (2.5 mg/kg, s.c.) increased jumpings in ORC mice while it had no effect on percentage of weight loss. Our results demonstrated that sex hormones could play a role in the morphine withdrawal syndrome in both ORC male and OVX female mice.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).