The role of nutrition in the regulation of LH secretion during anestrus by the serotoninergic and dopaminergic systems in Mediterranean ewes treated with melatonin

The steroid-dependent inhibition of LH secretion involves dopaminergic and serotoninergic systems but it is unclear how the plane of nutrition affects this inhibition during anestrus in melatonin treated ewes. Melatonin implants (18 mg) were inserted (Day 0) into ovariectomized, estradiol treated adult Rasa Aragonesa ewes on a high (H; n = 8) or low energy diet (L; n = 6) which were applied in early anestrus (Day 29-57) and late anestrus (Day 90-104). Cyproheptadine (0.1 mg/ kg), a serotoninergic (SHT2) receptor antagonist, was administered in early and late anestrus (Day 50 and 107) followed by pimozide (0.08 mg/kg), a dopaminergic2 receptor antagonist (Day 57 and 114). The H ewes had significantly higher LH concentrations (P < 0.05) before cyproheptadine treatment in early anestrus. The H and L ewes responded in a similar way to the antagonists in both early and late anestrus, except for L ewes who had a higher LH pulse amplitude after pimozide treatment in both periods (P < 0.05). During early anestrus, cyproheptadine tended to increase (P = 0.06) LH pulse frequency in L ewes and LH concentrations in H ewes. The LH secretion also increased in L ewes after pimozide administration during early anestrus (P < 0.05 for mean LH concentrations and LH pulse frequency and amplitude). However, pimozide dramatically increased LH secretion during late anestrus (Day 114) irrespective of the plane of nutrition (P = 0.06-0.08 for LH pulse frequency and P < 0.05 for LH concentrations and pulse amplitude). In melatonin treated Mediterranean ewes, the plane of nutrition appeared to modify the effect of dopaminergic and serotoninergic systems on the steroid-dependent inhibition of LH secretion throughout anestrus.     

Opioidergic, dopaminergic and adrenergic regulation of LH secretion in prepubertal heifers

Studies have shown inhibitory effects of endogenous opioids on LH secretion in early post-natal heifers. However, it is not clear whether these effects change during the rest of the prepubertal period or whether the inhibitory influences on the GnRH neurones are direct or by way of other neuronal systems. Two experiments were performed in heifer calves to study the developmental patterns of opioidergic, dopaminergic and adrenergic regulation of LH and the possible interactions between opioids and dopaminergic and adrenergic neuronal systems, in the regulation of LH secretion. In Expt 1 four groups each of five heifer calves were used. Blood samples were taken every 15 min for 10 h and each calf received one of the following treatments as a single injection at 4, 14, 24, 36 and 48 weeks of age: (i) naloxone (opioid antagonist, 1 mg kg(-1), i. v.); (ii) sulpiride (dopamine D2 antagonist, 0.59 mg kg(-1), s.c.); (iii) naloxone and sulpiride combined; or (iv) vehicle (control group). Treatments began after the first blood sample was taken. The design of Expt 2 was similar; a separate group of heifer calves was assigned to receive one of the following treatments as a single injection at 4, 14, 24, 36 and 48 weeks of age: (i) naloxone; (ii) phenoxybenzamine (an alpha-adrenoreceptor blocker, 0.8 mg kg(-1), i. v.); (iii) naloxone and phenoxybenzamine; (iv) or vehicle. Results from Expt 1 showed that the maximum concentration of LH and the number of calves responding to treatments with an LH pulse was higher in the first hour after treatments at 36 and 48 weeks of age in the naloxone group compared with the control or sulpiride groups (P < 0.05). These values in the naloxone group also increased over time and were greatest at 48 weeks of age (P < 0.05). In heifers given naloxone + sulpiride treatment at 36 and 48 weeks of age, maximum concentrations of LH in the first hour after treatment did not differ from the naloxone and control groups. In Expt 2, at 36 and 48 weeks of age, treatment with naloxone with or without phenoxybenzamine resulted in higher concentrations of LH than in the controls (P < 0.05). No pulses were seen over the first hour of treatment at 36 and 48 weeks of age in heifers treated with phenoxybenzamine. The 10 h periods of blood sampling at 48 weeks of age revealed that phenoxybenzamine alone suppressed LH pulse frequency and mean serum concentrations of LH compared with the control group (P < 0.05). It was concluded that a strong or more acute inhibition of LH secretion by endogenous opioids developed in mid- to late prepubertal heifers, or alternatively, that removal of opioidergic inhibition at the GnRH neurone unmasked stimulatory inputs that were greater in heifers close to first ovulation. Since sulpiride appeared to negate in part the effects of naloxone on LH release, the suppressive effects of opioids could be exerted in part through the inhibition or blocking of a stimulatory dopaminergic system. alpha-Adrenergic neuronal systems have stimulatory effects on LH release, especially during the late prepubertal period, but do not appear to mediate opioidergic inhibition of LH secretion in prepubertal heifer calves.     

Ovarian steroid involvement in endogenous opioid modulation of LH secretion in seasonally anoestrous mature ewes

In June, 16 mature ewes were ovariectomized and allocated to four groups: 1, saline; 2, naloxone; 3, progesterone implant plus naloxone; 4, oestrogen implant plus naloxone. Steroids were implanted at the time of ovariectomy. At 5 days after ovariectomy, the animals were intravenously infused with saline for 8 h and naloxone (50 mg/h) in saline for 8 h the following day. Three intact ewes were given naloxone in a similar way. During infusions and for 8 h on the day after naloxone, jugular venous blood samples were taken every 15 min and assayed for LH. Naloxone resulted in significant increases in mean LH concentration (P less than 0.01), LH episode frequency and episode height (P less than 0.05) in Group 3 ewes, but was without effect in any other group. These results provide evidence that the progesterone status of the ewe affects its response to naloxone, that progesterone negative feedback on LH release may be mediated by an opioid system, and that increased oestradiol negative feedback during seasonal anoestrus is unlikely to work via increased opioid inhibition of LH.     

The dopaminergic inhibition of LH secretion during the menstrual cycle

To assess the potential inhibitory role of hypothalamic dopaminergic input on the LRF-LH system, the gonadotropin response to a dopamine receptor antagonist, metoclopramid (MCP, 10 mg iv bolus) was examined during different phases of the menstrual cycle in 12 women. In addition, the role of dopamine infusion on naloxone (opiate receptor antagonist) induced LH increments was examined. MCP induced an abrupt increase in circulating LH levels in the mid-luteal phases but not in the early and late follicular phase subjects. No significant changes in serum FSH levels were observed. Dopamine, when infused concomitantly with naloxone, completely suppressed the naloxone induced pulsatile increments of LH in mid-luteal subjects. These findings support the contention that an increased dopaminergic inhibition of LRF-LH system occurs during the high estrogen-progesterone phase of the menstrual cycle, and provide preliminary evidence that the inhibitory role of endogenous opioids on LRF release may involve the dopaminergic system.     

Involvement of neurotransmitters in the action of apelin-13 on passive avoidance learning in mice

The widespread distribution of apelin-13 and apelin receptors in the brain suggests an important function of this neuropeptide in the brain that has not been explored extensively so far. In the present work, apelin-13 was found to facilitate the consolidation of passive avoidance learning in mice. In order to assess the possible involvement of transmitters in this action, the animals were pretreated with the following receptor blockers in doses which themselves did not influence the behavioral paradigm: phenoxybenzamine (a nonselective α-adrenergic receptor antagonist), propranolol (a β-adrenergic receptor antagonist), cyproheptadine (a nonselective 5-HT2 serotonergic receptor antagonist), atropine (a nonselective muscarinic acetylcholine receptor antagonist), haloperidol (a D2, D3 and D4 dopamine receptor antagonist), bicuculline (a γ-aminobutyric acid subunit A (GABA-A) receptor antagonist), naloxone (a nonselective opioid receptor antagonist), and nitro-l-arginine (a nitric oxide synthase inhibitor). Phenoxybenzamine, cyproheptadine, atropine, haloperidol, bicuculline and nitro-l-arginine prevented the action of apelin-13. Propranolol and naloxone were ineffective. The data suggest that apelin-13 elicits its action on the consolidation of passive avoidance learning via α-adrenergic, 5-HT2 serotonergic, cholinergic, dopaminergic, GABA-A-ergic and nitric oxide mediations.     

Modulation of luteinizing hormone and follicle-stimulating hormone in circulation by interactions between endogenous opioids and oestradiol during the peripubertal period of heifers.

The aim of this study was to determine whether the decline in oestradiol inhibition of circulating luteinizing hormone (LH) and follicle-stimulating hormone (FSH) during the peripubertal period of heifers is associated with a change in opioid modulation of LH and FSH secretion. Opioid inhibition of LH secretion was determined by response to administration of the opioid antagonist naloxone. Prepubertal heifers (403 days old) were left as intact controls, ovariectomized or ovariectomized and chronically administered oestradiol. Control heifers were used to determine time of puberty. Three weeks after ovariectomy, four doses of naloxone (0.13-0.75 mg kg-1 body weight) or saline were administered to heifers in the treatment groups in a latin square design (one dose per day). Blood samples were collected at intervals of 10 min for 2 h before and 2 h after administration of naloxone. This procedure was repeated four times at intervals of 3 weeks during the time intact control heifers were attaining puberty. All doses of naloxone induced a similar increase in concentration of serum LH within a bleeding period. During the initial bleeding period (before puberty in control heifers), administration of naloxone induced an increase in LH concentration, but the response was greater for heifers in the ovariectomized and oestradiol treated than in the ovariectomized group. At the end of the study when control heifers had attained puberty (high concentrations of progesterone indicated corpus luteum function), only heifers in the ovariectomized and oestradiol treated group responded to naloxone. Opioid inhibition of LH appeared to decline in heifers during the time control heifers were attaining puberty. Heifers in the ovariectomized group responded to naloxone at the time of administration with an increase in FSH, but FSH did not respond to naloxone at any other time. Administration of naloxone did not alter secretion of FSH in ovariectomized heifers. These results suggest that opioid neuropeptides and oestradiol are involved in regulating circulating concentrations of LH and possibly FSH during the peripubertal period. Opioid inhibition of gonadotrophin secretion appeared to decline during the peripubertal period but was still present in ovariectomized heifers treated with oestradiol after the time when age-matched control heifers had attained puberty. We conclude that opioid inhibition is important in regulating LH and FSH in circulation in heifers during the peripubertal period. However, opioids continue to be involved in regulation of circulating concentrations of LH after puberty.     

Opioidergic control of gonadotrophin secretion in the prepubertal gilt during restricted feeding and realimentation

Prepubertal gilts, having undergone a 7-day period of feed restriction to a maintenance ration, were allocated to one of 4 treatments; restricted feeding at 09:00 and 17:00 h for an 8th day both with (Group RN) and without (Group R) administration of the opioid antagonist naloxone hydrochloride (1 mg.kg-1 at 09:30 h followed by 0.5 mg.kg-1 at hourly intervals for 7 h), or feed to appetite with (Group ALN) and without (Group AL) naloxone administration. Gilts were bled at 10-min intervals on Day 8 from morning to evening feed and plasma LH, FSH and prolactin concentrations were measured by radioimmunoassay. Compared with Group R gilts, Group AL gilts exhibited significantly (P less than or equal to 0.05) higher mean and maximum LH concentrations and pulsatility, lower prolactin concentrations (P less than 0.05) but no significant difference in FSH secretion. Naloxone significantly depressed the increase in LH after re-feeding (Group ALN) (P less than 0.05). Once again there were no significant effects on FSH secretion. Naloxone also significantly depressed prolactin secretion in feed-restricted gilts (P less than 0.05). These results confirm that re-feeding of feed-restricted prepubertal gilts stimulates an immediate increase in LH secretion and that this elevation is not mediated via a suppression of inhibitory endogenous opioidergic tone. Rather, naloxone treatment appeared to expose a latent inhibition of LH secretion. The control of LH secretion is distinct from that of FSH in this model.     

Modification of the effects of naloxone in morphine-dependent mice

Mice were rendered dependent on morphine by mixing morphine with their food (2 mg/g) for three days. Increasing doses of naloxone precipitated dose-dependent withdrawal reactions such as weight loss and jumping. These withdrawal reactions were antagonized by morphine pretreatment. Effects of morphine, such as increased locomotor activity, inhibition of intestinal transport, and analgesia were antagonized by naloxone in both non-dependent and dependent subjects. The antagonist actions of naloxone were increased in dependent subjects; lower doses of naloxone were sufficient to antagonize effects of morphine. The present results confirm earlier studies indicating that precipitation of withdrawal can be antagonized by morphine pretreatment suggesting that withdrawal reactions are due to actions of naloxone at the same receptor at which opioid agonists act. The increased antagonist potency of naloxone in dependent subjects extends earlier results obtained with analgesic effects to several other agonist effects of morphine and is consistent with the interpretation that exposure to an opioid agonist induces a change in the conformation of opioid receptors.     

Reduction of the developmental competence of sheep oocytes by inhibition of LH pulses during the follicular phase with a GnRH antagonist

A GnRH antagonist (Antarelix) treatment was used during the breeding season of Romanov ewes, to investigate whether LH pulses are required the day before the preovulatory surge for normal early embryo development in vivo (Expt 1) and in vitro (Expt 2). In Expt 1, at the onset of oestrus after removal of a fluorogestone acetate sponge, group A0.5 (n = 22) received a subcutaneous injection of 0.5 mg Antarelix, and ovulation was induced with an intravenous injection of 3 mg pig LH 24 h later. The control group (group C, n = 20) were untreated. All ewes were mated naturally at 36 and 48 h after oestrus and embryos were recovered 8 days after sponge removal. There were significant differences in the decrease in LH and in the increase in FSH concentration after Antarelix treatment between treated and control groups. The ovulation rate and embryo recovery rate were not significantly different between the two groups but the blastocyst rate was lower (P < 0.0001) in group A0.5 than in group C, with more unfertilized or degenerated oocytes in group A0.5 (69.2%). In Expt 2, 24 h after sponge removal, group A (n = 10) and group B (n = 10) received one subcutaneous injection of 0.5 mg Antarelix. The control group (group C, n = 10) was left untreated. LH pulsatility was re-established in group B with hourly intravenous injections of 5 micrograms ovine LH for 24 h. Oocytes were collected by flushing the oviducts 28 h after the LH surge, and were fertilized and cultured in vitro for 7 days. Ovulation and cleavage rates were not significantly different among the three groups but a higher rate of blastocysts (P < 0.01) was obtained after Antarelix treatment when LH pulsatility was re-established (group B). Oestradiol concentration was strongly depressed (P < 0.0003) after Antarelix treatment in group A, but was maintained after injection of LH pulses in group B, although at a lower value than before the preovulatory surge in the control group. In conclusion, inhibition of endogenous LH pulses 1 day before the preovulatory surge was not essential for ovulation and in vitro fertilization but was associated with a decrease in plasma oestradiol concentrations and inferior embryo development both in vivo and in vitro. When LH pulsatility was re-established, oestradiol concentrations increased and embryo development was restored.     

Unexpected effects of nalmefene, a new opiate antagonist, on the hypothalamic-pituitary-gonadal axis in the male rat

In order to gain additional information on the role of brain opioid peptides in the regulation of the hypothalamic-pituitary-gonadal axis, we studied the effects of nalmefene, a new opiate antagonist, on gonadotropin and testosterone secretion in male rats. The results were compared with those obtained with naloxone, a well-studied antagonist. Acute injections of either nalmefene or naloxone (2 mg/kg) produced 4-fold increases in LH and testosterone secretion. In castrated male rats treated with testosterone propionate (TP), nalmefene (10 mg/kg) reversed the androgen negative feedback on LH secretion; surprisingly, when higher doses (25 and 50 mg/kg) were injected, the compound lost its ability to antagonize the testosterone-induced inhibition of LH levels. In contrast, naloxone was able to increase LH levels in TP-treated castrated rats even at the highest dose tested (50 mg/kg). Chronic administration of these antagonists resulted in suppression of the acute release of LH and T secretion in nalmefene-treated but not in naloxone-injected animals. These data are consistent with previous observations suggesting that opioid peptides a) exert a tonic inhibitory effect on LH and testosterone production and b) participate in the negative androgen-induced feedback control of LH secretion. Our results also show that the antagonistic action of nalmefene, but not naloxone, is reversed when higher doses are used or following chronic administration.     

Effects of melatonin on luteinizing hormone secretion in anestrous ewes following dopamine and opiate receptor blockade

In the present investigation we have examined the ability of melatonin to modify the pulsatile LH secretion induced by treatment with a DA antagonist (sulpiride, SULP) or opioid antagonist (naloxone, NAL) in intact mid-anestrous ewes. The experimental design comprised the following treatments-in experiment 1: (1) intracerebroventricular (i.c.v.) infusion of vehicle (control I); (2) pretreatment with SULP (0.6 mg/kg subcutaneously) and then i.c.v. infusion of vehicle (SULP + veh); (3) pretreatment with SULP and then i.c.v. infusion of melatonin (SULP + MLT, 100 microg per 100 microl/h, total 400 microg). In experiment 2: (4) i.c.v. infusion of vehicle (control II); (5) i.c.v. infusion of NAL (NAL-alone, 100 microg per 100 microl/h, total 300 microg); (6) i.c.v. infusion of NAL in combination with MLT (NAL + MLT, 100 microg + 100 microg per 100 microl/h). All infusions were performed during the afternoon hours. Pretreatment with SULP induced a significant (P < 0.01) increase in LH pulse frequency, but not in mean LH concentration, compared with control I. In SULP + MLT-treated animals, the LH concentration was significantly (P < 0.01) higher during MLT infusion, but due to highly increased LH secretion in only one ewe. The significant changes in the SULP + MLT group occurred in LH pulse frequency. A few LH pulses were noted after melatonin administration compared with the number during the infusion (P < 0.05) and after vehicle infusion in the SULP + MLT group (P < 0.05). The i.c.v. infusion of NAL evoked a significant increase in the mean LH concentration (P < 0.001) and amplitude of LH pulses (P < 0.01) compared with these before the infusion. The enhanced secretion of LH was also maintained after i.c.v. infusion of NAL (P < 0.01) with a concomitant decrease in LH pulse frequency (P < 0.05). In NAL + MLT-treated ewes, mean plasma LH concentrations increased significantly during and after the infusion compared with that noted before ( P < 0.001). No difference in the amplitude of LH pulses was found in the NAL + MLT group, but this parameter was significantly higher in ewes during infusion of both drugs than during infusion of the vehicle (P < 0.01). The LH pulse frequency differed significantly (p < 0.05), increasing slightly during NAL + MLT administration and decreasing after the infusion. In conclusion, these results demonstrate that: (1) in mid-anestrous ewes EOPs, besides DA, are involved in the inhibition of the GnRH/LH axis; (2) brief administration of melatonin in long-photoperiod-inhibited ewes suppresses LH pulse frequency after the elimination of the inhibitory DA input, but seems to not affect LH release following opiate receptor blockade.     

Separate neural systems mediate the steroid-dependent and steroid-independent suppression of tonic luteinizing hormone secretion in the anestrous ewe

In the ewe, two types of seasonal fluctuations in secretion of tonic luteinizing hormone (LH) have been described: a steroid-dependent change whereby estradiol gains the capacity to suppress LH pulse frequency in anestrus, and a steroid-independent decrease in pulse frequency in ovariectomized animals during anestrus. We have proposed that the former reflects activation, in anestrus, of estradiol-sensitive catecholaminergic neurons that inhibit gonadotropin-releasing hormone (GnRH). Three results reported here support this hypothesis: dopaminergic (pimozide) and alpha-adrenergic (phenoxybenzamine) antagonists increased LH in intact anestrous ewes without altering pituitary responses to GnRH; other dopaminergic (fluphenazine) and alpha-adrenergic (dibenamine) antagonists also increased LH in anestrus; agonists for dopaminergic (apomorphine) and alpha-adrenergic (clonidine) receptors suppressed LH secretion in both seasons, suggesting that the appropriate receptors are present in breeding-season ewes. In contrast, catecholamines do not appear to mediate the steroid-independent suppression of pulse frequency; neither pimozide nor phenoxybenzamine increased LH pulse frequency in ovariectomized ewes during anestrus. When antagonists for 6 other neurotransmitter receptors (muscarinic and nicotinic cholinergic, GABAnergic, serotonergic, opioid, and beta-adrenergic) were tested in anestrus, only cyproheptadine, the serotonergic antagonist, increased pulse frequency in ovariectomized ewes. Cyproheptadine had no effect on frequency during the breeding season. On the basis of these results, we propose that the steroid-dependent and -independent actions of anestrous photoperiod occur via catecholaminergic and serotonergic neurons, respectively.     

The response of luteinizing hormone secretion to photoperiod is modified by the level of nutrition in female Mediterranean goats

This paper reports the influence of nutrition on the photoperiodic control of luteinizing hormone (LH) secretion in female Mediterranean goats (i.e., goats from the Mediterranean area in general). Ovariectomized, oestradiol-treated goats were subjected to two consecutive intervals of 3 months of long days followed by 3 months of short days (group LDSD, N=20), or vice versa (group SDLD, N=20). The LDSD and SDLD does were also randomly assigned to one of two nutrition groups that received either 1.1 (H group, N=10) or 0.7 (L group, N=10) times their maintenance requirements. Live weight and body condition score were determined weekly and LH concentrations twice per week. To establish the pulsatility of secretion of LH, three periods of intensive sampling were undertaken. Melatonin was determined after a period of 45 short or long days. All photoperiod/nutrition groups showed large variations in LH concentrations according to photoperiod, with nutrition having a significant effect (P<0.001). The mean time between the shift from long to short days and the stimulation of LH secretion, and between the shift from short to long days and the inhibition of LH secretion, was different in each nutrition group (at least P<0.05). No differences were seen in the frequency of LH pulses between the nutrition groups, but differences between sampling periods were observed (P<0.001). Melatonin secretion was not affected by food supply. These results confirm: (1) that Mediterranean female goats are sensitive to photoperiod, (2) that this environmental cue may control the timing of pituitary activity under natural conditions, and (3) suggest that nutrition plays an important role in the effect of photoperiod on LH secretion.     

Opioid regulation of luteinising hormone and prolactin release in the horse—identical or independent endocrine pathways?

In order to determine if endogenous opioids regulate luteinising hormone (LH) and prolactin secretion via a common, gonadotropin-releasing hormone (GnRH) dependent pathway in the horse, effects of the opioid antagonist naloxone (300 mg) and the GnRH agonist buserelin (20 μg) on prolactin and LH secretion were investigated in stallions (n = 22), long-term castrated geldings (n = 15) and non-lactating mares during the luteal phase of the oestrous cycle (n = 16). Blood samples for determination of LH and prolactin concentrations were withdrawn at 15 min intervals for 120 min. After 60 min of sampling, animals were treated with either naloxone, buserelin or saline. In stallions, naloxone significantly increased LH as well as prolactin release (P < 0.05), indicating an opioid inhibition of both hormones, whereas in mares, naloxone stimulated only LH secretion (P < 0.05). No changes in plasma LH or prolactin concentrations after injection of naloxone were found in geldings. In all animal groups, buserelin induced a significant release of LH (P < 0.05) without affecting prolactin. We conclude that endogenous opioids inhibit LH and prolactin release in the horse but the regulation of these two hormones involves independent opioid pathways. These are activated differentially in stallions, geldings and mares. The opioid regulation of prolactin secretion is not mediated via GnRH.     

Maturational changes in opioidergic control of luteinizing hormone and follicle-stimulating hormone in ram lambs

Stimulation by naloxone, an opioid antagonist, of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion was examined in spring-born crossbred ram lambs raised under natural photoperiod. Vehicle (n = 6) or 1 mg naloxone/kg vehicle (n = 6) was injected (i.m.) 3 times at 2-h intervals at 5, 10 and 15 weeks of age and 4 times at 2-h intervals at 20, 25, 30 and 35 weeks of age. Blood samples were taken every 12 min for 6 h at 5, 10 and 15 weeks of age and for 8 h at 20, 25, 30 and 35 weeks of age. Naloxone had no effect on age at sexual maturity (controls 239 +/- 23 days; naloxone 232 +/- 33 days). The only significant (P less than 0.05) effect of naloxone on FSH was a greater pulse amplitude in 10-week-old treated lambs than in control lambs. Naloxone treatment resulted in greater LH pulse amplitude at 5 and 10 weeks of age (P less than 0.05), lower basal serum concentration of LH at 10 weeks of age (P less than 0.05), greater LH pulse frequency at 25 weeks of age (P less than 0.05), and greater mean serum concentrations of LH, basal LH and LH pulse amplitude at 35 weeks of age (P less than 0.01) than in the controls. In both groups of lambs, mean and basal FSH, and LH and FSH pulse amplitude were highest at 5 weeks of age and fell with age. LH pulse amplitude was lowest at 35 weeks of age (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Restoration of normal gonadotropin responses to naloxone by chronic bromocriptine treatment in elderly men.

Naloxone is unable to stimulate FSH and LH secretion in elderly men, suggesting a reduced endogenous opioid control of gonadotropin secretion in senescence. In the present study, we examined whether in elderly men a chronic dopaminergic stimulation with bromocriptine (5 mg/day for 7 days) modifies the gonadotropin response to naloxone (4 mg as an i.v. bolus plus 10 mg infused in 2 h). Eleven younger men (group 1, 22-40 years old) participated as controls. Twenty-two elderly men were selected from a larger population and were divided into two groups: subjects with compensated gonadal failure (normal blood testosterone and elevated gonadotropin concentrations; group 2, n = 11; 62-80 years old) and men with normal gonadal function (normal blood testosterone and gonadotropin levels; group 3, n = 11; 61-82 years old). Naloxone induced a striking LH and a slight but significant FSH increase in group 1, but was unable to change serum gonadotropin concentrations in elderly subjects of both groups 2 and 3. When experiments were repeated after bromocriptine treatment, no significant differences in LH and FSH responses to naloxone were observed in the younger subjects. On the other hand, bromocriptine restored significant gonadotropin responses to naloxone in elderly men. In fact, after bromocriptine, naloxone-induced FSH and LH increments in groups 2 and 3 were indistinguishable from those observed in group 1. These data suggest that in men age-related dopaminergic alterations may underlie the defective endogenous opioid control of gonadotropin secretion.     

Effects of ovariectomy and estradiol replacement on naloxone induced LH secretion in the female rabbit

The effects of an opioid antagonist, naloxone, on the secretion of gonadotrophins were investigated in the long term ovariectomized rabbit. In the intact and acutely ovariectomized rabbit (2 days p.o.) naloxone at 10 mg/kg induced an increase of 260-300% in LH secretion at 40 min post-injection. From days 33-66 post-surgery naloxone at 10 mg/kg caused significant elevations in LH release even when animals were treated with estradiol benzoate 24 h previously. By contrast, treatment with oestradiol benzoate 3 h before naloxone abolished the LH increase. An LH surge could be elicited in these rabbits with GnRH treatment. These studies indicated that long term ovariectomy in the female rabbit does not completely remove the opioid control of GnRH release and that the LH response to naloxone is influenced by circulating estradiol levels.     

Inhibition of luteinizing hormone secretion by delta9-tetrahydrocannabinol in the ovariectomized rat: effect of pretreatment with neurotransmitter or neuropeptide receptor antagonists.

Acute treatment with delta9-tetrahydrocannabinol [delta9-THC; 0.5 or 1.0 mg/kg b.w. intravenously (i.v.)], the major psychoactive constituent of marijuana, produces a dose-related suppression of pulsatile luteinizing hormone (LH) secretion in ovariectomized rats. To determine whether delta9-THC produces this response by altering neurotransmitter and/or neuropeptide systems involved in the regulation of LH secretion, ovariectomized rats were pretreated with antagonists for dopamine, norepinephrine, serotonin, or opioid receptors, and the effect of delta9-THC on LH release was determined. Pretreatment with the D2 receptor antagonists butaclamol (1.0 mg/kg b.w., intraperitoneally) or pimozide [0.63 mg/kg, subcutaneously (s.c.)], the opioid receptor antagonists naloxone (1-4 mg/kg, i.v.) or naltrexone (2 mg/kg, i.v.), the noradrenergic alpha2-receptor antagonist idazoxan (10 microg/kg, i.v.), or the serotonin 5-HT(1C/2) receptor antagonist ritanserin (1 or 5 mg/kg b.w., i.p.), did not alter delta9-THC-induced inhibition of pulsatile LH secretion. Pretreatment with a relatively high dose of the beta-adrenergic receptor blocker propranolol (6 mg/kg, i.v.) attenuated the ability of the low THC dose to inhibit LH release; however, lower doses of propranolol were without effect. Furthermore, the ability of a relatively nonspecific serotonin 5-HT(1A/1B) receptor antagonist pindolol (4 mg/kg, s.c.) or the specific 5-HT1A receptor antagonist WAY-100635 (1 mg/kg, s.c.) to significantly attenuate THC-induced LH suppression indicates that activation of serotonergic 5-HT1A receptors may be an important mode by which THC causes inhibition of LH release in the ovariectomized rat.     

Opioidergic and nutritional involvement in the control of luteinizing hormone secretion of postpartum Rasa Aragonesa ewes lambing in the mid-breeding season

The role of endogenous opioids and nutrition on the inhibition of luteinizing hormone (LH) secretion during the postpartum period was investigated in a Spanish breed of sheep lambing in the mid-late breeding season. Two groups of adult Rasa Aragonesa ewes housed in individual pens and lambing on 30 December were fed during the suckling period to provide maintenance requirements and the production of 1.1 (M; n=8) or 0.55 (L; n=8) kg of milk per day. On days 10, 20 and 30 after lambing, the effect of a treatment with the opiate receptor antagonist naloxone (1 mg/kg at four hourly intervals) on LH secretion was assessed in half of the ewes of each group, the remaining females receiving four saline injections. After weaning, animals were fed to provide requirements for maintenance of liveweight. Blood samples were collected twice a week from day 20 postpartum until the end of March, and assayed for progesterone and prolactin. Although underfed ewes showed significantly lower mean plasma concentrations during the control period on day 20 postpartum, nutrition did not seem to modify LH secretion before naloxone or saline injections. Moreover, no differences between nutritional groups in the response to naloxone injections on pattern of LH secretion were found. In fact, naloxone treatment induced an increase of mean LH concentrations on days 10, 20 and 30 postpartum (at least, P<0.05), of LH pulse frequency on days 20 and 30 (P<0.05), and of LH pulse amplitude on days 10 and 20 (P<0.05). Underfed ewes during the postpartum period showed a slower decline in plasma prolactin levels, with significant differences on days 29, 36 and 39 after lambing (P<0.05). Only 3 M ewes ovulated before the onset of the seasonal anoestrus period. It is concluded that endogenous opioids are involved in the inhibition of LH secretion during the early suckling period of a reduced seasonality breed of sheep without any influence of nutrition on the response to naloxone treatment; however, ewes underfed before weaning failed to reactivate their cyclicity prior to the onset of the seasonal anoestrus.     

Muscarinic cholinergic and histaminergic H1 and H2 receptors are not involved in the LH response to naloxone in man

The role of muscarinic-cholinergic and H1-, H2-histaminergic receptors as possible mediators of the LH response to the opioid antagonist naloxone was evaluated in 18 normal men. Subjects were divided in 3 groups of 6 men; the increment of LH in the plasma elicited by naloxone was evaluated after giving naloxone alone or together with dexchlorpheniramine, cimetidine or pirenzepine (respectively H1-, H2-histaminergic and muscarinic-cholinergic receptor antagonists). LH release was significantly stimulated by naloxone in all subjects; this response was not altered by histaminergic or cholinergic blockade. These results confirm the stimulatory effect of naloxone on LH release in man, without evidence of the involvement of H1-, H2-histaminergic or muscarinic-cholinergic pathways.