The nociceptin/orphanin FQ/NOP receptor system as a target for treatment of alcohol abuse: a review of recent work in alcohol-preferring rats

The intracerebroventricular administration of the 17 amino acid peptide nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the NOP receptor (previously referred to as ORL-1 or OP4 receptor), reduces voluntary 10% ethanol intake in genetically selected Marchigian Sardinian alcohol-preferring (msP) rats. Studies aimed at the pharmacological characterization of the receptor, which mediates the effect, have shown that the C-terminal 13 amino acid sequence is crucial for activity and that the selective NOP receptor antagonist [Nphe(1)]N/OFQ(1-13)NH(2) blocks the effect of N/OFQ on ethanol drinking. In place conditioning studies, N/OFQ abolishes the conditioned place preference (CPP) induced by ethanol in msP rats, or by morphine in nonselected Wistar rats; these findings suggest that N/OFQ is able to abolish the rewarding properties of ethanol and morphine. Moreover, N/OFQ inhibits reinstatement of alcohol-seeking behavior induced to electric footshock stress, as well as reinstatement of alcohol-seeking behavior induced by ethanol-paired cues. Together, these findings suggest that N/OFQ and its receptor may represent an interesting target for pharmacological treatment of alcohol abuse.     

Nociceptin/orphanin FQ receptors modulate glutamate extracellular levels in the substantia nigra pars reticulata. A microdialysis study in the awake freely moving rat

Intracerebral microdialysis was employed in awake freely moving rats to investigate the effects of nociceptin/orphanin FQ receptor ligands on glutamate extracellular levels in the substantia nigra pars reticulata. Nociceptin/orphanin FQ, ineffective at 0.1 microM, induced a prolonged stimulation of nigral glutamate levels at 1 and 10 microM (mean effect of 137+/-9 and 167+/-13%, respectively, of basal values). These effects were prevented by the novel nociceptin/orphanin FQ receptor antagonist [Nphe(1)]nociceptin/orphanin FQ(1-13)NH(2) (100 and 300 microM, respectively) but not by the non-selective opioid receptor antagonist naloxone (10 microM). [Nphe(1)]nociceptin/orphanin FQ(1-13)NH(2) (100 microM) inhibited by itself glutamate outflow (maximal reduction to 71+/-4%) while naloxone was ineffective. The nociceptin/orphanin FQ receptor ligand [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin/orphanin FQ(1-13)NH(2) also facilitated glutamate outflow at 10 microM (mean effect of 145+/-10%). Intranigral perfusion with tetrodotoxin (1 microM) or with the dopamine D(2) receptor antagonist raclopride (1 microM), failed to affect basal glutamate output and prevented the facilitatory effect of nociceptin/orphanin FQ (10 microM). However, perfusion with the GABA(A) receptor antagonist bicuculline (10 microM) increased local glutamate extracellular levels by itself and attenuated the effect of the peptide.Our data suggest that nociceptin/orphanin FQ increases glutamate extracellular levels in the substantia nigra pars reticulata via activation of nociceptin/orphanin FQ receptors located on non-glutamatergic, possibly dopaminergic and GABAergic, neuronal elements.     

The nociceptin receptor antagonist [Nphe1,Arg14,Lys15]nociceptin/orphanin FQ-NH2 blocks the stimulatory effects of nociceptin/orphanin FQ on the HPA axis in rats

Nociceptin/orphanin FQ (N/OFQ) is an opioid-related peptide that stimulates corticosterone release after i.c.v. administration in non-stressed rats. We employed in situ hybridization histochemistry to investigate N/OFQ-stimulated activation of the HPA axis at the hypothalamic and pituitary level. We have demonstrated that N/OFQ-induced activation of the HPA axis is mediated via the central N/OFQ peptide receptor (NOP) using the recently described selective NOP antagonist [Nphe(1),Arg(14),Lys(15)]nociceptin/orphanin FQ-NH(2) (UFP-101).We found that, at 30 min post-i.c.v. injection, N/OFQ dose-dependently increased plasma adrenocorticotrophin hormone and corticosterone compared with the vehicle-injected controls. N/OFQ (1.0 microg) significantly increased CRF mRNA but not AVP mRNA within the parvocellular hypothalamic paraventricular nucleus compared with the control group, and significantly increased pro-opiomelanocortin (POMC) mRNA in the anterior pituitary. While UFP-101 (1.0 microg) alone had no significant effect on plasma corticosterone concentration it blocked the effect of N/OFQ (1.0 microg) on plasma corticosterone levels when compared with N/OFQ administered alone. UFP-101 also blocked the N/OFQ-induced increase in CRF mRNA and POMC mRNA. These results demonstrate that centrally administered N/OFQ activates the HPA axis via up-regulation of CRF and POMC mRNA and stimulation of corticosterone release in rats. Further, we have demonstrated for the first time that the selective NOP receptor antagonist UFP-101 blocks these effects indicating that N/OFQ-induced HPA axis activation is mediated via central NOP receptors.     

Nociceptin/orphanin FQ acts as a functional antagonist of corticotropin-releasing factor to inhibit its anorectic effect

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the NOP opioid receptor (previously referred to as ORL1 or OP4 receptor), exerts a variety of behavioral effects. N/OFQ as well as the synthetic NOP receptor agonist Ro 64-6198 have been reported to possess antistress properties and to elicit a pronounced hyperphagic effect in freely feeding rats. These findings have raised our interest to investigate possible interactions in the control of ingestive behavior between N/OFQ and corticotropin-releasing factor (CRF), which is well known to be a major mediator of stress and to possess anorectic properties. These studies have shown that intracerebroventricular injections of N/OFQ or of Ro 64-6198 reverse the anorectic action evoked by intracerebroventricular administration of CRF. The anti-anorectic effect of N/OFQ or Ro 64-6198 is antagonized by the selective NOP receptor antagonist [Nphe1]N/OFQ1-13NH2, providing evidence that it is mediated by this receptor. The effect occurs at doses that are not hyperphagic per se and is clearly selective versus the anorectic action of CRF since N/OFQ or Ro 64-6198 do not influence the anorectic effect of Escherichia coli lipopolysaccharide (LPS). Neither N/OFQ nor Ro 64-6198 shows affinity for CRF receptors, suggesting that NOP receptor agonists might act as functional antagonists of CRF with regard to its anorectic action. Microinjection studies have revealed that the bed nucleus of the stria terminalis (BNST) is highly sensitive to the anorectic action of CRF, as well as to the anti-anorectic action of N/OFQ; pretreatment with 0.025-0.25 microg/site of N/OFQ into the BNST blocked the anorectic action of 0.1 microg/site of CRF given in the same area. On the other hand, intra-BNST microinjection of 0.025-0.25 microg/site of N/OFQ did not modify basal food intake. Thus, the BNST may be the site where the functional antagonism between N/OFQ and CRF takes place. These findings raise interest for the N/OFQ-NOP receptor system as a pharmacological target to block the anorectic effect of CRF. In comparison to CRF receptor antagonists, NOP receptor agonists may have the advantage of not inhibiting the hypothalamic-pituitary-adrenal (HPA) axis.     

Nociceptin/orphanin FQ decreases serotonin efflux in the rat brain but in contrast to a kappa-opioid has no antagonistic effect on mu-opioid-induced increases in serotonin efflux

Similar to kappa-opioids, nociceptin/orphanin FQ (OFQ) exerts anti-mu-opioid actions. This may involve interactions within the circuitry controlling 5-HT neurons in the dorsal raphe nucleus (DRN) that project to the nucleus accumbens (NAcc). To test this hypothesis, we compared the effects of OFQ and kappa-opioids on 5-HT efflux in the CNS of freely behaving rats. First, OFQ (30-300 microM) infused into the DRN for 120 min dose-dependently decreased 5-HT efflux in the DRN. The opioid receptor-like 1 (ORL-1) antagonist [Nphe(1)]nociceptin(1-13)NH(2) blocked this effect. Using dual-probe microdialysis we observed that OFQ (300 microM) infused into the DRN for 120 min produced parallel decreases in 5-HT efflux in the DRN and NAcc, suggesting that ORL-1 receptors in the DRN inhibit serotonergic neurons projecting to the NAcc. Also, 5-HT efflux in the NAcc was dose-dependently decreased during OFQ (30-300 microM) infusion into the NAcc. This suggests that OFQ can reduce 5-HT efflux in the NAcc both by inhibiting serotonergic neurons in the DRN and by stimulating ORL-1 receptors in the NAcc. Similar to OFQ, the kappa-opioids U-50,488 (300 microM) and dynorphin A(1-13) (300 microM) infused into the DRN for 120 min decreased 5-HT efflux in the DRN. This effect was blocked only by the kappa-opioid receptor antagonist nor-BNI. Lastly, we compared the ability of OFQ and U-50,488 to block mu-opioid-induced increases in 5-HT. The kappa-opioid U-50,488 (1000 microM) attenuated the increase in 5-HT induced by the mu-opioid agonist endomorphin-1 (300 microM) in the DRN. In contrast, OFQ (300-1000 microM) did not alter mu-opioid-induced increases in 5-HT efflux. In summary, kappa-opioids and OFQ both decreased 5-HT efflux in the CNS. However, in contrast to kappa-opioids, which reversed mu-opioid-induced increases in 5-HT efflux, the anti-mu-opioid effects of OFQ apparently do not involve changes in 5-HT transmission under our experimental conditions.     

Antagonistic effects of [Nphe1]nociceptin(1-13)NH2 on nociceptin receptor mediated inhibition of cAMP formation in Chinese hamster ovary cells stably expressing the recombinant human nociceptin receptor

Nociceptin/orphanin FQ (NC) is the endogenous ligand for the nociceptin receptor (NCR) which is negatively coupled to adenylyl cyclase to inhibit the formation of cAMP. In this study we describe the inhibitory action of the novel NC analogue, [Nphe1]nociceptin(1-13)NH2 on cAMP formation in Chinese hamster ovary cells expressing the human NCR. NC, NC(1-13)NH2, the pseudopeptides [Phe1psi(CH2-NH)Gly2]NC(1-17)NH2 and [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2, the hexapeptide, acetyl-Arg-Tyr-Tyr-Arg-Trp-Lys-NH2 and buprenorphine all produced a concentration dependent inhibition of forskolin stimulated cAMP formation. This inhibition was competitively reversed by [Nphe1]NC(1-13)NH2 with essentially identical pA2 values (6.12-6.48). [Nphe1]NC(1-13)NH2 showed per se a negligible residual agonist activity (alpha < 0.15).     

Daily GnRH and LH secretion in ewes is not modified by exogenous melatonin during seasonal anestrus

The effect of central, short-term melatonin administration on daily GnRH and LH secretion was studied in ewes during seasonal anestrus. Melatonin, in a total dose of 32 micrograms and the vehicle were perfused for 4 hours into the mediobasal hypothalamus/median eminence (MBH/ME). The mean GnRH concentration during perfusion with melatonin decreased significantly (P < 0.05), as compared to the concentration during the preceding perfusion with the vehicle only. This change resulted from high variations in GnRH concentration noted during the initial phase of perfusion rather than from an action of melatonin. Melatonin perfused into the MBH/ME did not significantly affect LH secretion. A higher dose of melatonin and vehicle were also infused intracerebroventricularly (icv.) in either intact (300 micrograms for 3 hours) or ovariectomized (OVX) ewes (400 micrograms for 4 hours, 100 micrograms/100 microliters/h). In the intact animals, melatonin did not significantly affect LH secretion. Interestingly, melatonin significantly decreased (P < 0.05) the number of LH peaks in OVX ewes. These results demonstrate that melatonin delivered for a few hours directly into the central nervous system did not affect either daily hypothalamic GnRH release or pituitary LH secretion in intact ewes during seasonal anestrus, but did modify pulsatile LH secretion in ewes deprived of the negative feedback of estradiol.     

Gonadotropin-releasing hormone mRNA in the rat: distribution and neuronal content over the estrous cycle and after castration of males.

The decapeptide gonadotropin-releasing hormone (GnRH) stimulates release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary. In the present study we used a 51-base oligonucleotide probe and in situ hybridization to study the neuronal content of GnRH mRNA at several time points in the estrous cycle and 7 days after castration of male rats. GnRH mRNA containing cells were found in the medial septum (SEPT), the vertical and horizontal limbs of the diagonal band of Broca (DBB), and throughout the preoptic area (POA) from the organum vasculosum of the lamina terminalis (OVLT) to its caudal merger with the anterior hypothalamus. The number of neurons producing detectable quantities of GnRH mRNA was not different either among females killed at 0700 h proestrus, 1000 h estrus, or 1900 h of diestrus 1 or between intact male rats and male rats killed 1 week after castration. We did, however, detect a significant difference in the number of GnRH mRNA producing neurons between males and females (P less than 0.05), where females had 20% more labeled cells. We detected no significant difference in the relative copy number of GnRH mRNA molecules (grains per labeled cell) either over the estrous cycle or between intact and castrate males. However, females overall had 24% more grains per labeled cell than males (P less than 0.05). These results suggest that gonadal steroid regulation of GnRH both over the estrous cycle and after short-term castration of males is mediated primarily by cellular processes subsequent to GnRH gene regulation. Furthermore, these results suggest that biosynthetic activity of GnRH is higher in females than in males.     

In vivo release rates of substance P in the preoptic/anterior hypothalamic area of ovariectomized and ovariectomized estrogen-primed rats: correlation with luteinizing hormone and prolactin levels

Push-pull cannulae were implanted into the preoptic/anterior hypothalamic area (POA) of ovariectomized (OVX) rats. After recovery animals were treated with estradiol (E2) or corn oil and they were perfused 3 days later. Substance P (SP) concentrations were measured in 15 min perfusate fractions, blood samples were taken at similar intervals. SP concentration in POA perfusates were readily measurable. Following estrogen priming SP release increased significantly each afternoon prior to the estrogen induced prolactin and luteinizing hormone (LH) surges. No such increase of SP release was observed in OVX rats with constant LH and prolactin levels throughout the day. Mean SP rates in OVX rats were significantly higher in comparison to OVX estrogen-primed rats. These results indicate that SP may be involved in the feedback mechanisms of estrogen on prolactin and LH release. Authenticity of SP in POA perfusates was made probable by high-performance liquid chromatography (HPLC) where synthetic SP eluted at the same retention time as the signal measured in push-pull perfusates.     

Binding and GTPgammaS autoradiographic analysis of preproorphanin precursor peptide products at the ORL1 and opioid receptors

Utilizing agonist-stimulated GTPγS autoradiography, we analyzed the ability of preproorphanin FQ (ppOFQ) peptides to stimulate [³⁵S]-GTPγS binding in adult rat brain. Orphanin FQ (OFQ) stimulated [³⁵S]-GTPγS binding in a pattern similar to that described for [¹²⁵I]-OFQ at the endogenous opioid receptor-like (ORL1) receptor. The ppOFQ peptides nocistatin and orphanin FQ2 (OFQ II_1–17) had no effect, suggesting that they do not mediate their reported analgesic effects via a G_i/o-coupled receptor (i.e. opioid or ORL1). Unlike OFQ II_1–17, high concentrations of its C-terminal extension, OFQ II_1–28, stimulated [³⁵S]-GTPγS binding in a mu (μ) opioid receptor-like distribution and the effect was blocked by naloxone. To explore these observations, we evaluated the receptor binding profile of OFQ II_1–28 at the cloned ORL1 and μ opioid receptors. OFQ II_1–28 had no specific binding at either ORL1 or μ opioid receptors at concentrations up to 50 μM. This lack of affinity was not consistent with a μ-mediated effect, as suggested by preliminary observation using functional autoradiography in rat brain sections. Although behavioral studies suggest that OFQ II_1–28 possesses analgesic activity, this effect does not appear to be mediated via direct binding at the μ opioid receptor. Taken together, these findings support the view that (1) OFQ is the only ppOFQ peptide that binds to and activates the ORL1 receptor and (2) OFQ II_1–28 does not bind or stimulate [³⁵S]-GTPγS binding in cells expressing the μ opioid receptor.     

Ethanol induced changes in cyclin-dependent kinase-5 activity and its activators, P35, P67 (Munc-18) in rat brain

kappa(3) opioid receptors have a unique binding and analgesic profile, as originally defined by naloxone benzoylhydrazone (NalBzoH). Although antisense studies demonstrated the close relationship between kappa(3) opioid and Orphan opioid receptor-like receptor (ORL1) and implied they were generated from the same gene, these studies also revealed differences in the sensitivity profiles of NalBzoH and orphanin FQ/nociceptin (OFQ/N), indicating that they were not identical. To help define the relationship between kappa(3) and ORL1 receptors, we utilized BE(2)-C human neuroblastoma cells that natively express functional ORL1 and kappa(3) opioid receptors. (125)I-[Tyr(14)]OFQ/N binds to a single population of receptors in BE(2)-C cells. Competition binding and adenylyl cyclase studies clearly illustrated marked selectivity differences between the ORL1 and the kappa(3) sites. Furthermore, antisense DNA targeting ORL1 blocked the inhibition of cAMP by OFQ/N, but not by NalBzoH. Thus, the receptor mechanisms mediating the activity of OFQ/N and NalBzoH in BE(2)-C cells are distinct.     

The role of the opioid receptor-like (ORL1) receptor in motor stimulatory and rewarding actions of buprenorphine and morphine

We have previously shown that the ability of buprenorphine to activate the opioid receptor-like (ORL1) receptor compromises its antinociceptive effect. Furthermore, morphine has been shown to alter the level of orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the ORL1 receptor, raising the possibility that the endogenous OFQ/N/ORL1 receptor system may be involved in the actions of these opioids. Thus, using mice lacking the ORL1 receptor and their wild-type littermates, the present study assessed the role of the ORL1 receptor in psychomotor stimulant and rewarding actions of buprenorphine and morphine. Morphine (5, 10 mg/kg) dose-dependently increased motor activity and induced conditioned place preference. However, the magnitude of each response was comparable for the mutant mice and their wild-type littermates. In contrast, buprenorphine (1 mg/kg) induced greater motor stimulation in ORL1 receptor knockout mice as compared with their wild-type littermates. Further, single conditioning with buprenorphine (3 mg/kg) induced place preference in mutant mice but not in their wild-type littermates. The results of binding assay showed that buprenorphine concentration-dependently (0-1000 nM) displaced specific binding of [(3)H]-OFQ/N in brain membrane of wild-type mice. Together, the present results suggest that the ability of buprenorphine to interact with the ORL1 receptor modulates its acute motor stimulatory and rewarding effects.     

Are circulating leptin and luteinizing hormone synchronized in patients with polycystic ovary syndrome?

Animal and human studies suggest that leptin modulates hypothalamic-pituitary-gonadal axis functions. Leptin may stimulate gonadotrophin-releasing hormone (GnRH) release from the hypothalamus and luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretion from the pituitary. A synchronicity of LH and leptin pulses has been described in healthy women, suggesting that leptin probably also regulates the episodic secretion of LH. In some pathological conditions, such as polycystic ovarian syndrome (PCOS), LH-leptin interactions are not known. The aim of the present investigation was to assess the episodic fluctuations of circulating LH and leptin in PCOS patients compared to regularly menstruating women. Six PCOS patients and six normal cycling (NC) women of similar age and body mass index (BMI) were studied. To assess episodic hormone secretion, blood samples were collected at 10-min intervals for 6 h. LH and leptin concentrations were measured in all samples. For pulse analysis the cluster algorithm was used. To detect an interaction between LH and leptin pulses, an analysis of copulsatility was employed. LH concentrations were significantly higher in the PCOS group in comparison to NC women, however serum leptin concentrations and leptin pulse characteristics for PCOS patients did not differ from NC women. A strong synchronicity between LH and leptin pulses was observed in NC women; 11 coincident leptin pulses were counted with a phase shift of 0 min (P = 0.027), 18 pulses with a phase shift of -1 (P = 0.025) and 24 pulses with a phase shift of -2 (P = 0.028). PCOS patients also exhibited a synchronicity between LH and leptin pulses but weaker (only 20 of 39 pulses) and with a phase shift greater than in normal women, leptin pulses preceding LH pulses by 20 min (P = 0.0163). These results demonstrate that circulating leptin and LH are synchronized in normal women and patients with PCOS. The real significance of the apparent copulsatility between LH and leptin must be elucidated, as well as the mechanisms that account for the ultradian leptin release.     

Synaptic changes in the hypothalamus of the prepuberal female rat administered estrogen

Subcutaneous administration of estradiol benzoate (EB) to prepuberal female rats can advance vaginal opening, phasic pituitary gland luteinizing hormone (LH) secretion, and ovulation, presumably through a neural mechanism. This study investigated whether these effects are associated with changes in synaptic profiles in the arcuate nucleus of the hypothalamus (ARC) and the preoptic area (POA). Twenty-five-day-old female rats were adminstered EB, EB followed by progesterone on day 27, or oil vehicle alone; or they received no treatment. Blood was collected by jugular venipuncture at 1600 hr, on day 27, and plasma was assayed for LH by radioimmunoassay. Rat brains were immediately perfused for electron microscopy, and the ARC and POA were dissected out. Tissue blocks from these areas were processed with phosphotungstic acid for selective staining of neural synapses. Serum LH was markedly elevated in the EB-treated rats compared with controls. In the treated groups, LH values in serum were above 1,000 ng/ml, whereas the control values were less than 50. This acute rise of serum LH was accompanied by an acute increase of synaptic volume percent, area density, and numerical density in the ARC of EB-treated rats. The numerical density of the control groups was approximately 800 million observed synapses per cubic millimeter, whereas in the EB-treated groups, there were approximately 1.8 billion synapses per cubic millimeter. We found no differences in synaptic profiles of the POA in EB-treated animals as compared to the controls. We conclude from this study that estrogens act through neural mechanisms to accelerate maturation of neuroendocrine processes that govern phasic pituitary gland LH release and that this maturation process entails synaptogenesis in the ARC.     

Interactions of insulin-like growth factors and estradiol in rat pituitary gonadotrophs

Insulin-like growth factors are involved in the regulation of gonadotropin secretion. Insulin-like growth factor I (IGF-I) has an augmenting effect on gonodotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) release from female rat gonadotrophs that is facilitated by estradiol. To identify the underlying mechanisms, we investigated whether IGF-I influences total LH pool and the production of intracellular inositol phosphate. In another series of experiments we tested whether IGF-II and estradiol affect LH release of gonadotrophs. Pituitary cells were incubated with 100 pM IGF-I and/or 100 pM estradiol for 24 h. They were stimulated, partially in the presence of Wortmannin, an inhibitor of phosphoinositide 3-kinase, with 330 pM GnRH for 3 h. Subsequently, total LH pool (released and remaining hormone content in lysed cells) in cultures was measured. Intracellular inositol trisphosphate of alphaT3-1 cells, a gonadotrope cell line, treated with estradiol and IGF-I as described before and stimulated with 100 nM GnRH for 15 min was analyzed by ion exchange chromatography. To determine the interaction of IGF-II and estradiol on GnRH-stimulated LH secretion, cells were treated with increasing concentrations of IGF-II (0.05 pM-10 nM) and 100 pM estradiol. IGF-I significantly increased the accumulation of inositol trisphosphate in basal and GnRH-stimulated cells. IGF-I, estradiol, or their combinations did not change total LH pool, although they enhanced LH secretion. Wortmannin abolished the positive effects of IGF-I and estradiol on LH secretion. IGF-II alone increased basal, but not GnRH-induced LH secretion at low concentrations (0.05 pM). Additional estradiol treatment further increased basal, but not GnRH-induced LH secretion. In conclusion, our results suggest that increased LH secretion from female anterior pituitary cells after IGF treatment is due to the amplification of early signal transduction steps rather than changes in LH pool. The inositol trisphosphate signaling pathway is involved in the regulation of LH secretion from gonadotrophs treated with IGF-I. It is not likely that IGF-II plays an important role in the regulation of gonadotropin secretion.     

How do androgens affect episodic gonadotrophin secretion in postmenopausal women?

In the absence of any significant ovarian oestrogen secretion, as in post-menopausal women, the hypothalamic-pituitary axis may still be influenced by the androgens which continue to be produced. The episodic secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) by post-menopausal women was accordingly assessed following short-term androgen antagonism induced by flutamide, a specific androgen receptor blocker. Blood samples were collected at 10-min intervals for 10 h in nine women before and during flutamide administration (750 mg/day for 6 days) for the determination of gonadotrophin and sex hormone concentrations by radibimmunoassay. On both occasions, 25 μg of gonadotrophin-releasing-hormone (GnRH) was injected intravenously 8 h after initiation of the blood collections. Flutamide administration decreased (P < 0.01 or less) androgen concentrations (testosterone, androstenedione and dehydroepiandrosterone sulphate) in relation to baseline values, but did not alter oestrogen (oestrone and oestradiol) or sex-hormone-binding globulin levels. The LH and FSH pulse characteristics (frequency, amplitude, interpulse interval and transverse mean levels) determined by a cluster algorithm in the gonadotrophin secretory profiles did not differ before and during androgen blockade. By contrast, androgen antagonism increased LH (P < 0.01) and tended to enhance FSH (P = 0.10) FSH release in response to GnRH stimulation. Hence, short-term androgen receptor blockade with flutamide did not greatly affect episodic gonadotrophin secretion. However, the combined evidence of the enhanced gonadotrophin release observed in response to GnRH stimulation and the unchanged gonadotrophin secretion during androgen antagonism suggests that alterations in the magnitude, but not the frequency, of hypothalamic GnRH release had occurred. Even in the presence of substantial serum androgen concentrations, the gonadotrophin pulse rhythm in hypogonadal women consitutes the maximal-rate GnRH-LH release pattern.     

Age- and sex-specific changes in naloxone-induced luteinizing hormone secretion and Fos expression in gonadotropin-releasing hormone neurons of gonadectomized rats

In the present study, we examined sex-specific changes in luteinizing hormone (LH) secretion and Fos expression in gonadotropin-releasing hormone (GnRH) neurons in response to naloxone in young (3 months old) and old (24 months old), gonadectomized male and female rats. We revealed by immunocytochemistry that, regardless of age and sex, naloxone significantly increased the number of GnRH neurons expressing Fos, which was associated with increased LH secretion. Additionally, although the magnitude of the increase in Fos-expressing GnRH neurons did not change in old males compared to young males, it was attenuated by almost half in old females compared to young females. LH levels decreased 60% in old males compared to young males and 15% in old females compared to young females. These results suggest LH secretion is impaired with age, but the ability of GnRH neurons to be stimulated by naloxone is preserved. However, the opioid-controlling mechanism is more fragile in females than males during aging.