Effect of naloxone on serum testosterone in adult male rabbits

From Pharmacology to Physiology: Endocrine Functions of μ-Opioid Receptor Networks

The steady rise in opioid users and abusers has uncovered multiple detrimental health consequences of perturbed opioid receptor signaling, thereby creating the need to better understand the biology of these systems. Among endogenous opioid networks, μ-receptors have received special attention due to their unprecedented biological complexity and broad implications in homeostatic functions. Here, we review the origin, molecular biology, and physiology of endogenous opioids with a special focus on μ-opioid receptor networks within the endocrine system. Moreover, we summarize the current evidence supporting an involvement of the latter in regulating distinct endocrine functions. Finally, we combine these insights to present an integrated perspective on μ-opioid receptor biology and provide an outlook on future studies and unresolved questions in this field.

Effect of opioid antagonists on sex hormone secretion

BACKGROUND

Endogenous opioids have roles in various functions in different parts of the body, including intestinal motility, suppression of pain, reinforcement of behavior, and regulation of the hypothalamic-pituitary-gonadal axis. The endogenous opioid system is also recognized to be involved in the negative-feedback regulation of the release of LH and testosterone.

AIM

The reviewed articles herein show the development of the current model of this regulation, the evidence supporting it, and also the observed effects of opioid antagonist (naloxone, naltrexone, and nalmefene) on the system.

MATERIALS AND METHODS

Review of the studies published during the years 1979-1996 (no significant studies made after that). Search from databases Pubmed, SciFinder, and Medline with search words opioid antagonists, hormones, LH, testosterone, and GnRH, in different combinations.

RESULTS/CONCLUSIONS

Opioid antagonists seem to increase the secretion of GnRH in the hypothalamus which then causes a pulsatile release of LH in the pituitary and secretion of testosterone. According to the experiments, the frequency of pulses and concentration of LH and testosterone in plasma seem to increase. These effects are seen in both men and women (at early follicular phase). More research is needed to investigate the consequences of these effects in general.

Effect of small doses of naloxone on sexual exhaustion in White New Zealand male rabbits

The objective of the present study was to determine the effect of small doses of naloxone on sexual exhaustion in White New Zealand male rabbits. Twelve young and 12 adult male rabbits 6-12 months old and 14-20 months of age, respectively, were selected from a commercial farm. Each male rabbit was housed individually in galvanized cages (90 cm x 60 cm x 40 cm). The rabbits were housed in an open shed exposed to natural photoperiod (12 L 12 D, 19 degrees N). Daily temperature fluctuated through the year from 28 to 16 degrees C. Humidity was 45+/-5%. Water and food (rabbit chow PMI) was supplied ad libitum. After sexual behaviour for each studied group was established, the males were given a 6-day rest, and 3 days before next trial, six males of each group (treated) received a subcutaneous implant of 8 mg of naloxone in a crystalline nitrocellulose pellet formulated to be completely absorbed in 15 days. The remaining six males were sham-treated (control). At the end of the resting period as previously described, the sexual behavior of each group was studied and compared using a Mann-Whitney statistical U-test. The effect of naloxone on sexual behavior was analyzed with a Wilcoxon test for correlated samples. With regard to sexual activity between young and adult rabbits, it was observed that there was a significant difference between groups (P=0.00275, Z=2.8823, adjusted Z=2.99.43) showing that younger rabbits mounted/ejaculated from 9 to 10 females compared with 6 to 8 mounted/ejaculated by older rabbits. When naloxone was administered to both groups, there was a significant difference when comparing sexual behavior before and after administration of naloxone (table first and second trial). Young rabbits treated with naloxone mounted/ejaculated 11-12 females while older rabbits mounted nine females before reaching sexual exhaustion. A significant difference was observed when comparing the number of estrous females that were mounted/ejaculated between groups. Environmental photoperiod and temperature changes were not considered. It was concluded that endogenous opioids are important modulators of behavioral and hormonal interactions related to sexual behavior.

The effect of small doses of naloxone on the initiation and duration of the first estrus after weaning in sows

The present research was conducted with the objective of studying the pharmacological effect of small doses of naloxone on the initiation and duration of the first estrus after weaning in the sow. For this purpose, 32 multiparous sows were used. Sows were divided at random into two groups. Group 1 (n=16) was treated by i.m. injection with 2mg naloxone at 12h intervals from 3 days before until 3 days after weaning. Group 2 (n=16) served as the control group and received saline solution at the same times as treatments for group 1. First estrus after weaning occurred at 85+/-5.2 and 108.3+/-5h (P<0.05) in naloxone- and saline-treated sows. Duration of estrus was 89.6+/-3.9 and 49.6+/-3.9h (P<0.05) in naloxone-treated and control animals, respectively. It was concluded that naloxone treatment advanced the time of appearance and duration of the first estrus after weaning in sows giving further support that endogenous opioids (EOP) are modulators of sexual behavior in female pigs.

The effect of low doses of naloxone on the preovulatory surge of LH and on the onset and duration of oestrus in the ewe with induced oestrus during the non-breeding season

The objective of this work was to study the effect of the endogenous opiate peptide (EOP) antagonist, naloxone, on the preovulatory LH surge and on the time of onset and duration of oestrus in the ewe with induced oestrus during the non-breeding season. Forty Suffolk X Hampshire ewes 2-3-years-old and 50+/- 4kg were studied, ewes were divided at random in two groups of 20, housed in open paddocks under natural photoperiod (19 degrees latitude N); were fed with hay and commercial pellets, and provided water ad libitum. Group one received an intravaginal sponge with 45mg of medroxiprogesterone acetate for 14 days, and upon sponge withdrawal 250IU of eCG was administered i.m. Group two received the same treatment as group 1 but in addition they received two i.m. injections of 0.5mg of naloxone, one given on sponge withdrawal and the second 24h later (total dose 1.0mg). Oestrus in naloxone-treated ewes was present 32+/- 2h and in control ewes in 35+/- 3h after sponge withdrawal. Duration of oestrus in control ewes was shorter (27+/- 2.5h), than naloxone-treated ewes (39+/- 6h); (P<0.0001). The LH surge in naloxone-treated ewes was initiated 5h after onset of oestrus, and 8h after onset of oestrus in control ewes, and the difference was significative (P<0.0006). It was concluded that EOP are important modulators of reproductive function in the ewe.

Effect of naloxone on serum prolactin levels in adult male rabbits

In order to clarify the possible physiological role of endogenous opioid peptides (EOP), we studied the effect of low doses of naloxone (specific opiate antagonist) on plasma prolactin levels in male rabbits. Five groups of five male rabbits each was injected daily between 8-9 a.m., with naloxone 12.5, 25, 50, 100 and 200 microg/kg/day. An additional group of ten animals was injected with saline solution and considered the control group. Blood samples were taken at baseline before naloxone administration and later at 90 min and 1, 2, 4, 7, 10 days after its administration. Samples were also taken 4 days after stopping naloxone administration (day 14). Plasma prolactin levels were measured by RIA. A significant constant decrease in PRL levels was seen with the 12.5 microg at 90 minutes, while with the remaining doses a progressive decrease was recorded throughout the study.