SINGLE OPIOID ADMINISTRATION MODIFIES GONADAL STEROIDS IN BOTH THE CNS AND PLASMA OF MALE RATS I. CECCARELLI, A. M. DE PADOVA, P. FIORENZANI, C. MASSAFRA AND A. M. ALOISI* Pain and Stress Neurophysiology Laboratory, Neuroscience and Ap- plied Physiology Section, Department of Physiology, University of Siena, Via Aldo Moro, 2, 53100 Siena, Italy Abstract—While morphine remains one of the most widely used opioids for the treatment of painful conditions, other opioids are also commonly employed. Because of the inter- actions between opioids and gonadal hormones, in particular opioid-induced hypogonadism, this study investigated the effects of widely used opioids on plasma testosterone and estradiol levels and brain testosterone levels in male rats. Animals were s.c. injected with two concentrations of mor- phine (5 or 10 mg/kg), fentanyl (0.05 or 0.1 mg/kg), tramadol (10 or 40 mg/kg), buprenorphine (0.05 or 0.1 mg/kg) or saline (0.7 ml/kg). Four or 24 h after treatment, the rats were deeply anesthetized to collect blood samples from the abdominal aorta and to perfuse the brains with saline. Plasma and brain hormone levels were measured by radioimmunoassay. In rats studied 4 h after treatment, all the opioids except tramadol 10 mg/kg decreased plasma testosterone in comparison with saline administration. At the same time, plasma estradiol levels were lower than control in the groups treated with the low doses of morphine, tramadol and buprenorphine, while estradiol remained at control levels in the other groups. Twenty- four hours after treatment, plasma testosterone levels were different (higher) than control in the animals treated with the low doses of morphine, fentanyl and buprenorphine. Estradiol was lower than control in the low dose groups, while the high doses did not produce any changes with respect to control. Four hours after treatment, brain testosterone was drastically de- creased in all groups except buprenorphine, in which it re- mained at control levels. All groups returned to control levels at 24 h after treatment. In conclusion, opioids exert important effects on plasma and CNS sex hormone levels. The differ- ent magnitude and time-course of the effects of the differ- ent opiates on testosterone and estradiol levels are likely due to their different mechanism of action. © 2006 Pub- lished by Elsevier Ltd on behalf of IBRO. Key words: sex hormones, testosterone, estradiol, opioids, CNS. Pain, especially chronic pain, is one of the most common problems in medicine. Despite the recent introduction of many new pain-killers, opioid analgesic drugs remain among the most effective therapies for the treatment of moderate to severe pain (i.e. Rasor and Harris, 2005). At present, opioids are commonly used to treat post-surgical and cancer pain and they have recently been employed in the treatment of chronic pain of non-malignant origin. Al- though various compounds have been synthesized from the original opium, the problems of unwanted side effects persist, including hypogonadism. In men, hypogonadism is defined as a decrease below a certain level of plasma testosterone (2–3 ng/ml) (Seftel, 2005). Several reports suggest that chronic opioid therapy with morphine may result in clinically significant hypogonadism and sexual dysfunction (Aloisi et al., 2005; Paice et al., 1994; Rajago- pal and Bruera, 2003). These clinical and experimental results must be taken into account, particularly since the chronic consumption of opioids can lead to sexual dysfunction, decreased libido, increased fatigue and generally poor functioning (Rajago- pal and Bruera, 2003). Moreover, post-pubertal testoster- one deficiency is also associated with physical symptoms such as osteoporosis, decreased axillary and pubic hair, infertility, lower testicular volume, decreased strength and muscle mass (leading to muscle atrophy) and decreased bone marrow activity (leading to anemia). These effects are probably due to opioid inhibition of gonadotropin se- cretion via inhibition of hypothalamic gonadotropin releas- ing hormone (GnRH) release and to direct inhibitory ac- tions at the pituitary level via specific binding sites on the gonadotrophs (Pimpinelli et al., 2006; Fabbri et al., 1989). It is known that luteinizing hormone (LH) is regulated by numerous neurotransmitters, including endogenous opioid peptides (Delitala et al., 1983; Gore, 2001). Moreover, opioid binding sites have been found in Sertoli cells (Fabbri et al., 1989), suggesting an additional peripheral site of action able to block hormone synthesis and/or increase hormone degradation. Indeed, it was shown that morphine (10 mg/kg) injection suppresses testosterone secretion and testicular interstitial fluid formation even after pre- treatment with human chorionic gonadotropin which re- verses morphine’s suppression of LH but not the periph- eral action (Adams et al., 1993). Sex hormones may also modulate pain (Aloisi, 2003; Craft et al., 2004) and androgens in particular were found to have a protective function against chronic pain. Indeed, men have higher endogenous levels of testosterone and a lower incidence of many chronic pain syndromes than women (Becker et al., 2005). Testosterone decreases for- malin-induced behavioral responses in rats (Ceccarelli et al., 2003; Aloisi et al., 2004) while gonadectomy in male rats can increase the sensitivity to thermal stimuli applied to the tail or paws (Gaumond et al., 2002, 2005; Frye and Seliga, 2001). In addition, androgens may be lower than normal in men suffering from some chronic pain conditions (e.g. cluster headache) (Polleri, 1990). *Corresponding author. Tel: +39-0577-234103; fax: +39-0577-234037. E-mail address: aloisi@unisi.it (A. M. Aloisi). Abbreviations: DHT, 5-dihydrotestosterone; E/T, estradiol/testoster- one; LH, luteinizing hormone; RIA, radioimmunoassay. Neuroscience 140 (2006) 929 –937 0306-4522/06$30.00+0.00 © 2006 Published by Elsevier Ltd on behalf of IBRO. doi:10.1016/j.neuroscience.2006.02.044 929