Changes in the processing of beta-endorphin in the hypothalamus and pituitary gland of female rats during sexual maturation

Effects of N-terminal fragments of beta-endorphin on feeding in chicks

It is known that N-terminal fragments of beta-endorphin have biological activities, such as an antagonism effect of beta-endorphin (1-31) on the secretion of hormones or thermoregulation in mammals. We studied the effects of the N-terminal fragments on feeding behavior in male broiler chicks. Intracerebroventricular administration of beta-endorphin (1-27) (0.4nmol) stimulated feeding behavior compared with saline control during the 60-min experimental period. beta-Endorphin (1-17) (2.0nmol) also increased food intake at 30min postinjection. Co-injection of either beta-endorphin (1-27) or (1-17) was effective in reducing full-length beta-endorphin-induced feeding in chicks. These data suggest that the N-terminal fragments of beta-endorphin act as a partial agonist, and may regulate the activity of the central opioidergic system in chicks.

Two distinct populations of neurons expressing nitric oxide synthase mRNA in the female rat preoptic area: site specific changes induced by sex steroids

Non-isotopic in situ hybridization histochemistry in the basal forebrain of gonadectomized juvenile female rats visualized neuronal nitric oxide synthase (nNOS) mRNA in two distinct cellular populations, one in the organum vasculosum of the lamina terminals (OVLT) and the other in the rostral preoptic area at the level of the anteroventral periventricular nucleus (rPOA). In the rPOA, digoxigenin-labeled nNOS mRNA positive cells were in close proximity to the cell body of gonadotropin-releasing hormone (GnRH) -immunoreactive neurons. In the OVLT, the labeled cells were in an area rich in GnRH fibers. In the frontal section of the rPOA, the labeled cells were distributed in an inverted V-shaped area over the third ventricle. Combined treatment with estradiol and progesterone caused a significant reduction in the number of nNOS mRNA positive cells in the inverted V-shaped area in the female rat rPOA. The treatment induced a luteinizing hormone surge at the time of sacrifice. In the OVLT, ovarian steroids had no effect on nNOS mRNA expression. The results indicate that nNOS mRNA expression in the rPOA is regulated by ovarian steroids in a site-specific manner.

Endocrine-disrupting chemicals: prepubertal exposures and effects on sexual maturation and thyroid activity in the female rat. A focus on the EDSTAC recommendations

In 1996, the US Environmental Protection Agency was given a mandate by Congress to develop a screening program that would evaluate whether variously identified compounds could affect human health by mimicking or interfering with normal endocrine regulatory functions. Toward this end, the Agency chartered the Endocrine Disruptor Screening and Testing Advisory Committee in October of that year that would serve to recommend a series of in vitro and in vivo protocols designed to provide a comprehensive assessment of a chemical's potential endocrine-disrupting activity. A number of these protocols have undergone subsequent modification by EPA, and this review focuses specifically on the revised in vivo screening procedure recommended under the title Research Protocol for Assessment of Pubertal Development and Thyroid Function in Juvenile Female Rats. Background literature has been provided that summarizes what is currently known about pubertal development in the female rat and the influence of various forms of pharmaceutical and toxicological insult on this process and on thyroid activity. Finally, a section is included that discusses technical issues that should be considered if the specified pubertal endpoints are to be measured and successfully evaluated.

Effect of opioid antagonism on beta-endorphin processing and proopiomelanocortin-peptide release in the hypothalamus

Previous studies have shown that chronic opioid receptor blockade has significant effects on POMC gene expression and peptide levels in the hypothalamus. We have now examined the effects of the opioid antagonist naltrexone on beta-EP processing in the hypothalamus and on the release of 2 POMC-derived peptides, beta-EP and gamma 3-MSH, from the perifused hypothalamus in vitro. The beta-EP immunoactivity in the medial basal hypothalamus (MBH) of 7 rats infused for 1 week with naltrexone by osmotic minipump, was individually analyzed by HPLC and compared to 7 control rats. The mean ratio of beta-EP1-31 compared to beta-EP1-27 plus beta-EP1-26 was 2.34 +/- 0.41 in the naltrexone treated rats, significantly higher than the ratio of 1.26 +/- 0.09 in the control rats (P < 0.02). Thus in the setting of chronic opioid antagonism although beta-EP content decreases, there is relatively more beta-EP1-31, the biologically active opioid form of the peptide, compared to the C-terminally cleaved forms of beta-EP which have reduced biological activity. To study the effects of naltrexone on beta-EP and gamma 3-MSH release, hypothalami were perifused in vitro with 10(-6) M naltrexone. Basal release of gamma 3-MSH was significantly higher from the naltrexone treated brains compared to the controls (221 +/- 20 pg/60 min vs. 161 +/- 6.7 pg/60 min) (P < 0.01); KCl stimulated gamma 3-MSH was also significantly higher in the naltrexone group (951 +/- 94 vs. 543 +/- 85 pg/60 min) (P < 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Current concepts of beta-endorphin physiology in female reproductive dysfunction

beta-Endorphin has a role in the regulation of the normal menstrual cycle and possibly in the onset of puberty. We have reviewed the evidence pointing to an alteration in this neuropeptide that may contribute to the pathogenesis of various reproductive dysfunctions. Elevated or high levels of beta-endorphin have been associated with exercise-associated amenorrhea, stress-associated amenorrhea, and polycystic ovarian syndrome. Depressed or low levels of beta-endorphin have been associated with PMS and menopause. Alterations in the levels of beta-endorphin may change the pulsatile release of GnRH via noradrenergic and/or dopaminergic pathways. We have primarily focused on beta-endorphin as representative of the endogenous opioid peptides, but other opioid peptides may also contribute to the pathogenesis of various types of reproductive dysfunction. Perhaps it will become possible to characterize and hone our understanding of the function of beta-endorphin and the other substances composing the endogenous opioid peptides. A better understanding of their role in physiological as well as pathophysiological processes may allow for the development of rational approaches to the treatment of specific disorders pertaining to reproduction. Many questions remain unanswered. Among the most relevant are: what is the precise mechanism of action by which beta-endorphin exerts its influence on pulsatile GnRH release? Is there a functional relationship between CNS and peripheral (serum) levels of beta-endorphin? Are the detected changes in beta-endorphin levels merely associated, or are they a cause of a particular disorder? Since it took almost 40 years between the time prostaglandins were first discovered and eventual realization of their clinical application, it may take some time before the beta-endorphin story is complete.

Opioids and the developing organism: a comprehensive bibliography, 1984-1988

A comprehensive bibliography of the literature concerned with opioids and the developing organism for 1984-1988 is presented. Utilized with companion papers (Neurosci. Biobehav. Rev. 6:439-479; 1982; 8:387-403; 1984), these articles cover the clinical and laboratory references beginning in 1875. For the years 1984, 1985, 1986, 1987, and 1988, a total of 877 citations were recorded. A series of indexes accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics, and subdivided into such topics as the type of opioid explored and the general area of biological interest (e.g., physiology).

Gonadal steroid and chronic morphine treatment do not change the posttranslational processing of beta-endorphin in the rat brain

The present study examines whether two treatments known to induce refractoriness to exogenous morphine produce this desensitization through a change in the posttranslational processing of brain beta-endorphin (beta-End). The first experiment examined whether an ovarian steroid regimen which produces a transient desensitization of brain opiate receptor mechanisms alters beta-End processing in the preoptic area (POA), medial basal hypothalamus (MBH), and brainstem (BS). The second experiment monitored the effects of morphine pellet treatment, known to produce morphine dependency, on immunoreactive beta-End forms in the hypothalamus and periaquaductal gray area of the midbrain (PAG). The individual molecular forms of beta-End were separated using ion exchange chromatography and collection fractions were quantitated for beta-End immunoreactivity by RIA. The results show that regional differences occur in the posttranslational processing of beta-End. In the hypothalamus, MBH and POA, beta-End-(1-31) and its non-acetylated C-terminal cleavage products, beta-End-(1-27) and beta-End-(1-16) were the predominant forms of beta-End. The PAG pools produced a beta-End peptide elution profile similar to the hypothalamus, although small amounts of N-acetyl-beta-End-(1-31) were also identified. The BS exhibited the least posttranslational processing of beta-End; beta-End-(1-31) was the primary product with smaller amounts of beta-End-(1-27) and beta-End-(1-26) observed. However, neither ovarian steroid treatment nor chronic morphine produced any changes in posttranslational processing of beta-End or in total beta-End concentration in any of the brain regions examined in these experiments. These data indicate that the refractoriness or tolerance to exogenous morphine associated with steroid or chronic morphine treatment cannot be explained by alterations in the biological activity of beta-End resulting from the differential regulation of its posttranslational processing products.

The effects of simultaneous or separate infusions of some pro-opiomelanocortin-derived peptides (beta-endorphin, melanocyte stimulating hormone, and corticotrophin-like intermediate polypeptide) and their acetylated derivatives upon sexual and ingestive behaviour of male rats

Intraneuronal post-translational cleavage of pro-opiomelanocortin yields a variety of peptides including beta-endorphin, melanocyte stimulating hormone and corticotrophin-like intermediate polypeptide, some of which are subsequently N-acetylated. Such peptides may be co-released from neuronal terminals, and so these experiments explored the effects of co-administration of some of them on sexual behaviour in the male rat, which is known to be sensitive to hypothalamic infusions of beta-endorphin. Peptides were infused into the pre-optic-anterior hypothalamic area bilaterally in doses up to 320 pmol, and males allowed access to a sexually receptive female and/or a sweet solution (0.1% Acesulfame-K) for 15 min, so that both sexual and ingestive behaviour could be studied. beta-Endorphin(1-31) by itself inhibited sexual interaction, confirming our previous data. Acesulfame-K ingestion was inhibited in control-infused rats in the presence of a female, but this inhibition was released when sexual behaviour was itself diminished by beta-endorphin(1-31). Both the acetylated and non-acetylated forms of melanocyte stimulating hormone (alpha-melanocyte stimulating hormone and des-acetyl melanocyte stimulating hormone) stimulate sexual behaviour; latencies both to ejaculation and to resumption of copulatory behaviour after an ejaculation (post-ejaculatory interval) were reduced. However, infusion of either corticotrophin-like intermediate peptide or N-acetylated beta-endorphin (1-31) had no effect on either sexual or ingestive behaviour. Infusion of either acetylated melanocyte stimulating hormone or des-acetyl melanocyte stimulating hormone mixed with beta-endorphin(1-31) prevented the inhibitory effect of the latter on sexual behaviour. Dose-response studies showed that the behavioural effect of such mixtures depended upon the molar ratios of the two peptides, rather than their absolute concentrations. The higher the ratio in favour of alpha-melanocyte stimulating hormone or des-acetyl melanocyte stimulating hormone, the greater the display of sexual behaviour. Infusing either corticotrophin-like intermediate polypeptides or N-acetyl beta-endorphin(1-31) with beta-endorphin(1-31) did not prevent the inhibition of sexual activity expected with beta-endorphin(1-31) alone. These results are discussed in terms of the functional consequences of co-release of proopiomelanocortin peptides from hypothalamic nerve terminals.

Selective effects of beta-endorphin infused into the hypothalamus, preoptic area and bed nucleus of the stria terminalis on the sexual and ingestive behaviour of male rats

beta-Endorphin was infused bilaterally into the medial preoptic area-anterior hypothalamic continuum at doses of 5, 10 and 40 pmol each side. The highest dose selectively abolished mounting, intromitting and ejaculating in sexually experienced male rats paired with an oestrous female. Males infused with 40 pmol beta-endorphin still followed the female, investigated her anogenital region and other parts of her body, but made abortive attempts to mount. A dose of 5 pmol beta-endorphin had no effect, but 10 pmol proved partially effective. The same males, in other tests, were allowed to ingest a highly preferred, sweet, non-calorific solution (acesulfame-K) in the absence of a female. beta-Endorphin infusions (up to 40 pmol) into the same area of the hypothalamus had no effect on this behaviour. Control males allowed simultaneous access both to an oestrous female and to the sweet solution copulated normally but reduced their ingestive behaviour, despite there being sufficient time during tests for both to occur. beta-Endorphin (40 pmol) infused into the preoptic area-anterior hypothalamic continuum under these conditions suppressed sexual interaction, but ingestion of acesulfame-K increased to values observed when the female was absent. beta-Endorphin infused into neighbouring areas of the brain had different behavioural effects. Sexual behaviour was not inhibited, and ingestion of acesulfame-K was unaltered, when beta-endorphin was infused either into the bed nucleus of the stria terminalis or the rostral ventromedial hypothalamus. However, infusions of cholecystokinin-8 into the ventromedial hypothalamus suppressed acesulfame-K ingestion in most animals, showing that the cannulae were placed in an area regulating ingestive behaviour. The inhibition of sexual behaviour after preoptic area-anterior hypothalamic continuum infusions of beta-endorphin was prevented by either pretreating rats with 1 mg/kg naloxone intraperitoneally, or by infusing a putative delta opiate receptor blocker (0.5 pmols ICI 174864) into the preoptic area-anterior hypothalamic continuum 5 min prior to beta-endorphin treatment. ICI 174864 administered alone significantly increased mount rate and reduced the post-ejaculatory refractory period in copulating males. These experiments suggest that there is both neurochemical and neuroanatomical specificity relating beta-endorphin to sexual behaviour in the male rat.

The effects of castration, testosterone replacement and photoperiod upon hypothalamic beta-endorphin levels in the male Syrian hamster

Syrian hamsters kept in long day-lengths have active gonads and high circulating levels of gonadal steroids. Under the influence of the pineal gland, animals exposed to short photoperiods undergo testicular regression, have low circulating levels of testosterone and gonadotrophins and elevated levels of beta-endorphin within the hypothalamus. This paper describes the interaction between testosterone and photoperiod in the regulation of beta-endorphin levels in three regions of the hypothalamus. Hypothalamic beta-endorphin levels were measured by a combination of high-performance liquid chromatography and radioimmunoassay techniques that allows separation of the beta-endorphin (1-31) peptide from its metabolites and precursors. All of the beta-endorphin-like immunoreactivity in the hypothalamus of the male hamster, in both photoinhibited and photostimulated conditions, was found to represent the 31-amino-acid peptide. In photostimulated hamsters, chronic castration was associated with a significant increase of beta-endorphin levels in the anterior hypothalamus and mediobasal hypothalamus, which was reversed by treatment with exogenous testosterone. Castration prevented the ability of naloxone, an opiate receptor antagonist, to release luteinizing hormone, and this effect was also reversed by exogenous steroid. In photoinhibited hamsters, however, castration had no effect upon beta-endorphin levels in the preoptic area or mediobasal hypothalamus, and there was only a small increment in the anterior hypothalamus. Significantly, beta-endorphin levels in all areas of the hypothalamus of photoinhibited castrates were not decreased by testosterone treatment. In addition, administration of exogenous testosterone did not restore sensitivity to naloxone in these animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of neonatal testosterone upon opioid receptors and the content of beta-endorphin, neuropeptide Y and neurotensin in the medial preoptic and the mediobasal hypothalamic areas of the rat brain

The content of beta-endorphin, neuropeptide Y and neurotensin-like immunoreactivity (beta-End, NPY and NT), and the total number of opioid binding sites, were measured in the medial preoptic area (MPOA) and mediobasal hypothalamus (MBH) of ovariectomized adult rats which were oestrogen-primed. The rats had been injected neonatally with either testosterone propionate (TP) or vehicle (oil). NPY content was found to be higher in the MPOA of animals which received TP, whereas no significant difference was observed in the MBH NPY content. However, the NT concentration in the MBH of TP-treated rats was almost twice the amount detected in oil-treated rats and with this peptide no significant changes were detected in the MPOA. Finally, beta-End and the total number of opioid binding sites were reduced in both the MPOA and MBH of the rats which were exposed to TP neonatally. Since exposure to testosterone neonatally masculinises the rat hypothalamus, to the extent that female rats cannot generate oestrogen-stimulated prolactin and luteinizing hormone surges, we suggest that the neurochemical changes reported in this paper reflect some aspects of this sexual differentiation of the rat hypothalamus.

beta-Endorphin levels in the cerebrospinal fluid of male talapoin monkeys in social groups related to dominance status and the luteinizing hormone response to naloxone

beta-Endorphin-like immunoreactivity was measured in the cerebrospinal fluid of 20 male talapoin monkeys living in mixed-sex social groups. It was shown that beta-endorphin was the major immunoreactive peptide; there was no evidence for high molecular weight precursors, or for either N-acetyl or C-shortened metabolites. Dominant males (those at the top of the social hierarchy) had lower levels of beta-endorphin than those of intermediate rank; subordinate males had higher levels than either of the other two ranks--about three times those measured in dominants. There were significant negative correlations between beta-endorphin in cerebrospinal fluid and both the amount of aggression given and sexual behaviour shown towards females. The response of the hypothalamo-pituitary system to opiate blockade was tested by giving the males naloxone in doses of 0.125, 0.25, 0.5, 1.0 and 5.0 mg/kg and assaying serum levels of luteinizing hormone 20 min later. Dominant males released significant amounts of luteinizing hormone at doses of 0.25 and higher; there was no release in either intermediate or subordinate monkeys at any dose. These findings show that an animal's rank in the social group in which it lives is strongly correlated with beta-endorphin levels in the cerebrospinal fluid, and with changes in the neuroendocrine response to opiate blockade. Altered opiate neural activity may be responsible for the depressed levels of sexual behaviour and gonadal function observed in monkeys at the bottom of the hierarchy.

Annual reproductive rhythms in mammals: mechanisms of light synchronization

Animals restrict the time of birth of offspring to the most advantageous time of year, usually spring or summer. This is achieved by controlling the preceding period of fertility and, in some cases, by delaying implantation of the zygote. Seasonal changes in daylength, the principal, though not the only cue, regulate pulsatile release of hypothalamic releasing factors that in turn activates the pituitary-gonadal axis. The role of the neuroendocrine system is therefore to translate the photoperiodic stimulus into an endocrine signal (Figure 12). The measurement of day length is a function of the circadian system, environmental light being sampled on a 24-hour basis. Experimental manipulations of the photoperiodic response have revealed the existence of a rhythm of sensitivity to the presence of light that is entrained by the prevailing photoperiod. Light falling within the period of maximal sensitivity results in an LD type response. It is important to note that although different species measure day length in a similar manner, the gonadal response to a given photoperiod will vary between species depending upon the nature of their seasonal reproductive strategy. Photic information is conveyed from the retina to the pineal gland by way of the suprachiasmatic nuclei of the hypothalamus and the cervical sympathetic trunk. The central connections between these structures are poorly understood. The pineal is an essential mediator of the photoperiodic response. The effects of pinealectomy vary between species, but in all cases the responses to changes in day length are blocked. The gland is neither anti- nor progonadotrophic; it merely provides a signal. This signal is probably the nocturnal release of melatonin. Studies on in vivo melatonin production and the responses of photoperiodic species to timed administration of exogenous melatonin have suggested that the duration of nocturnal melatonin production by the pineal is read by the CNS as an indicator of the length of darkness. This model for PTM provides a physiological basis to the observed rhythm of sensitivity to light. This period of sensitivity is probably a parallel to the nocturnal rhythm of melatonin production. Light falling in this phase blocks melatonin production, truncates the pineal signal, and hence produces an LD response by the CNS. The site of the signal detector is not known, although the anterior hypothalamus may be involved. How the pineal signal triggers changes in the hypothalamic LHRH pulse generator is not known. The endogenous opioids, however, especially beta-END may have a major role in exercising photoperiodic control over pituitary action.