Changes in the pituitary and hypothalamic content of methionine-enkephalin during the estrous cycle of rats

Puberty onset in female rats: relationship with fat intake, ovarian steroids and the peptides, galanin and enkephalin, in the paraventricular and medial preoptic nuclei

Puberty is a time of rapid change, including a marked increase in fat consumption and body fat accrual, particularly in females. The mechanisms underlying these changes are unknown. Building on the results obtained in adult rats, the present study in pubertal rats focused on the orexigenic peptides, galanin (GAL) and enkephalin (ENK), in the paraventricular nucleus (PVN) and medial preoptic nucleus (MPN), which are known to be responsive to female steroids and have a role in both energy balance and reproductive function. The present study examined female rats maintained on pure macronutrient diets from before weaning (day 15) to day 70. After an initial burst in protein intake (days 21-35), rats showed an increase, specifically in preference for fat, from 15% to 30%. In rats examined at different ages before (day 30) and after (days 45 and 60) puberty, this rise in fat intake was associated with a marked increase, from days 30-45, in levels of oestradiol and progesterone and in GAL and ENK mRNA or peptide levels, specifically in the PVN and MPN, but not other hypothalamic areas examined. This positive relationship with increased fat intake, steroids and peptides across ages was also observed when comparing pubertal rats that naturally preferred fat (> 25% of total diet) with those consuming little fat (< 15%) or rats that reached puberty at an early age (days 30-34) with those that were late (days 37-40). These rats with early puberty onset exhibited a strong fat preference 3-4 days before vaginal opening, which was positively related to steroid levels, GAL, fat intake and body fat accrual after puberty. These findings suggest that, in addition to providing a signal for puberty onset, early fat ingestion acting through mechanisms involving the steroids and orexigenic peptides may be related to long-term patterns of eating and body weight regulation.

Hormone effects on specific and global brain functions

The first demonstration of how biochemical changes in neurons in specific parts of the brain direct a complete mammalian behavior derived from the effects of estrogens in hypothalamic neurons that facilitate lordosis behavior, the primary reproductive behavior of female quadrupeds (Pfaff. Estrogens and Brain Function. 1980; Pfaff. Drive: Neurobiological and Molecular Mechanisms of Sexual Motivation. 1999). Sex behaviors depend on sexual arousal that in turn depends on a primitive function: generalized CNS arousal (Pfaff. Brain Arousal and Information Theory. 2006). Here we summarize one of the ways in which a generalized arousal transmitter, norepinephrine, can influence the electrical excitability of ventromedial hypothalamic cells in a way that will foster female sex behavior.

Perimenopausal mental disorders: epidemiology and phenomenology

Perimenopause, the interval of irregular menstrual activity which directly precedes menopause, is characterized by widely fluctuating hormone levels amidst a large-scale decline in circulating estrogen. This phase in a woman's life is typically accompanied by physical discomforts including vasomotor symptoms, such as headaches, insomnia, and hot flushes, as well as genital atrophy. Not surprisingly, studies suggest a significant increase in mood lability for women during this time. While some evidence points toward an exacerbation of bipolar mood symptoms and an increase in schizophrenic psychosis during perimenopause, the majority of research conducted on perimenopausal mental disorders has focused on unipolar depression. Studies vary widely in methodology, definitions of menopausal status, and degrees of depression among subjects; however, the majority of findings indicate an increased susceptibility to depression during the perimenopausal transition. This greater susceptibility may be due to neuroendocrine effects of declining estrogen levels, the subjective experience of somatic symptoms resulting from this hormonal decline, and/or the more frequent occurrence of "exit" or "loss" events for women during this stage of life. At this time, more research is needed to address questions of prevalence, risk, and etiology for depression and other major mental disorders as related to the physiological and psychosocial changes associated with perimenopause.

Neuromodulation in polycystic ovary syndrome

Although central and peripheral factors have been implicated in the neuromodulation of GnRH in PCOS, there are no definitive or conclusive data to establish a primary causal role for any one factor. Because increased GnRH pulse frequency is at least a contributor to the secretion of excess LH and insufficient FSH that are the proximate cause of chronic anovulation in PCOS, strategies to slow the GnRH pulse generator are likely to promote ovulation in women with PCOS. Several pharmacologic agents, such as dopamine agonists and antagonists, have been tried, but the lack of consistent effects in women with PCOS limits their clinical utility. Current treatment strategies include the use of the combined oral contraceptive pills, antiandrogens or androgen receptor blockers, and insulin sensitizers. Oral contraceptive preparations are effective in suppressing ovarian hyperandrogenemia, regulating menstrual cycles, and reducing the risk of endometrial hyperplasia. Androgen blockade and antiandrogens provide symptomatic relief from androgen-induced acne and hirsutism and have been reported to restore ovulation in women with PCOS. Whether this effect is mediated peripherally or centrally remains to be clarified. The most recent class of pharmacologic agents to gain popularity are the "insulin modifiers." With increasing evidence that insulin resistance constitutes a key metabolic element, it seems logical that improving insulin sensitivity and glucose disposal might wholly, or partially, reverse certain features of PCOS, including anovulation. To date, insulin modifiers have proved most promising in improving the clinical features and promoting fertility, but whether this effect is centrally mediated is yet to be elucidated.

Postmenopausal estrogen therapy and depressive symptoms in older women

BACKGROUND

Evidence regarding the effect of postmenopausal estrogen therapy on mood is limited.

METHODS

To determine whether postmenopausal estrogen therapy is associated with fewer depressive symptoms in elderly women, we conducted a cross-sectional study of 6, 602 white women ages 71 years or older who were recruited from population-based listings in Baltimore, Md; Minneapolis, Minn; Portland, Ore; and the Monongahela Valley, Pa. Use of estrogen and progestin was determined by interview. Participants completed the Geriatric Depression Scale short form (GDS) and were considered depressed if they reported 6 or more of 15 possible symptoms of depression.

RESULTS

A total of 6.3% (72/1,150) of current estrogen users, 7.2% (142/1,964) of past estrogen users, and 9.0% (313/3,488) of never users reported 6 or more symptoms of depression (P =.004). Current estrogen users had a decreased risk of reporting 6 or more depressive symptoms, compared with not current (past or never) users of estrogen (odds ratio [OR], 0.7; 95% CI, 0.5 to 0.9; P =.01], adjusted for living alone, bilateral oophorectomy, current smoking, physical activity, social network, self-perceived health, cognitive function, functional status, and antidepressant use. However, excluding women who use estrogen or progestin alone, we were unable to find an association between current use of combined estrogen plus progestin therapy and depressive symptoms (adjusted OR, 0.8; 95% CI, 0.5 to 1.4; P =.5).

CONCLUSIONS

This cross-sectional study found that current use of unopposed estrogen was associated with a decreased risk of depressive symptoms in older women. Additional studies are needed to understand the effect of combined estrogen and progestin therapy on the prevalence of depressive symptoms in older women.

Opiate receptors modulate estrogen-induced cholecystokinin and tachykinin but not enkephalin messenger RNA levels in the limbic system and hypothalamus

Cholecystokinin, substance P and methionine enkephalin all regulate the display of reproductive behaviour. Their expression is exquisitely regulated by estrogen in the limbic-hypothalamic circuit, a circuit that regulates the display of estrogen-sensitive female reproductive behavior. Relatively little is known, however, about the interaction of endogenous opioid peptides with cholecystokinin and substance P in the limbic-hypothalamic circuit. Opiates antagonize the release of cholecystokinin and substance P in the hypothalamus and periaqueductal gray and stimulate cholecystokinin messenger RNA levels in the amygdala. To determine the effect of endogenous opioid input on estrogen-induced cholecystokinin, enkephalin and substance P expression, in situ hybridization histochemistry was used to examine estrogen-induced messenger RNA levels of these neuropeptides in specific nuclei of the limbic system and hypothalamus in the presence of opiate receptor antagonists. Estrogen treatment of ovariectomized rats significantly elevated cholecystokinin messenger RNA levels in the central portion of the medial preoptic nucleus, the encapsulated portion of the bed nucleus of the stria terminalis and the posterodorsal medial amygdala, as well as increased preproenkephalin and preprotachykinin messenger RNA levels in the ventromedial hypothalamic nucleus and the posterodorsal medial amygdala. The universal opiate receptor antagonist naltrexone and the delta-opiate receptor antagonist naltrindole each potentiated the estrogen-induced increase and elevated cholecystokinin messenger RNA levels an additional 1.9- to 2.8-fold depending on the nucleus examined, but had no effect on the estrogen-induced expression of either preproenkephalin or preprotachykinin messenger RNA. beta-Funaltrexamine, a mu-opiate receptor antagonist, had no effect on the medial preoptic or medial amygdaloid cholecystokinin messenger RNA levels or on the estrogen-induced expression of preproenkephalin messenger RNA but did cause a decrease in estrogen-induced cholecystokinin messenger RNA levels in the bed nucleus of the stria terminalis and a decrease in the preprotachykinin messenger RNA levels in the ventromedial hypothalamic nucleus. These results indicate that endogenous opioids, acting on the delta-opiate receptor within nuclei of the limbic-hypothalamic circuit, restrain the estrogen-induced increase of cholecystokinin messenger RNA expression. Activation of the mu-opiate receptor, however, may facilitate cholecystokinin messenger RNA expression in the bed nucleus of the stria terminalis and preprotachykinin messenger RNA expression in the ventromedial hypothalamic nucleus. Thus, endogenous opioid peptides may act in a site- and receptor-specific manner to modulate estrogen-induced neuropeptide levels in the limbic system and hypothalamus.

Alterations in the estrogen sensitivity of hypothalamic proenkephalin mRNA expression with age and prenatal exposure to alcohol

Studies suggest that exposure to alcohol in utero causes reproductive and neuroendocrine deficits in adult female rats. The ventromedial nucleus of the hypothalamus (VMN) is an estrogen-sensitive brain region which is regarded as a primary locus for modulating female reproduction. Proenkephalin (PE) mRNA expression in the VMN is dramatically increased by estrogen and this elevation is thought to be involved in modulating female reproductive behavior and neuroendocrine function. To examine whether prenatal alcohol exposure has long-term effects on the ability of estrogen to influence hypothalamic PE mRNA levels, female rats at 2-3, 6-7 or 15-18 months of age, derived from alcohol- or control-fed dams, were studied. 7 days following ovariectomy, animals received either estrogen or sham treatment for 2 days prior to sacrifice. PE mRNA levels in the VMN and striatum were determined by in situ hybridization histochemistry. Film autoradiogram density, numbers of PE mRNA-expressing cells and exposed silver grains/cell were analyzed. Estrogen treatment increased hybridization density, the number of PE mRNA-expressing cells and PE mRNA (grains) level/cell in the VMN of normal adult female rats. In old rats, estrogen increased the number of PE mRNA-expressing cells without up-regulating PE mRNA grain density/cell. In fetal alcohol-exposed (FAE) female rats, the number of cells that expressed PE mRNA did not increase following estrogen treatment at any age. Elevation of grain density/cell following estrogen was observed in FAE animals but only at 7-8 months of age. Overall, these data indicate that the estrogen responsiveness of PE mRNA expression in the VMN declines with age and, furthermore, prenatal exposure to alcohol blunts estrogen's effects on PE mRNA expression in the adult VMN. These finding may help to explain the mechanisms underlying the loss of reproductive function observed in FAE females.

Co-expression of steroid hormone receptors in opioid peptide-containing neurons correlates with patterns of gene expression during the estrous cycle

The anteroventral periventricular nucleus (AVPV) of the preoptic region represents an essential component of neural pathways regulating gonadotropin secretion, and contains sexually dimorphic populations of neurons that express dynorphin or enkephalin. In the present study we used in situ hybridization to measure prodynorphin (PDYN) and proenkephalin (PENK) mRNA in the AVPV of intact animals killed on each day of the cycle. Levels of PDYN mRNA were lowest in animals killed on the afternoon of proestrus and then increased by over 60% by the morning of the following day. Expression of PENK mRNA was generally stable during the cycle, but a small yet significant reduction was detected on proestrus relative to levels of PENK mRNA in animals killed on the day of diestrus. In addition, we used double in situ hybridization to demonstrate that the majority of PDYN mRNA-containing neurons express both estrogen (50%) and progesterone receptor (85%) mRNAs. Only one quarter of the PENK-containing neurons also co-express estrogen receptor mRNA, and fewer than 10% of the PENK mRNA neurons express PR mRNA. Thus, the differential expression of PDYN and PENK during the cycle generally correlates with distinct differences in the degree of colocalization of ER and PR mRNA in PDYN and PENK mRNA-containing neurons in the AVPV.

Effect of low and high doses of nitrous oxide on preproenkephalin mRNA and its peptide methionine enkephalin levels in the hypothalamus

The effect of exposure to nitrous oxide (N2O) on the levels of preproenkephalin mRNA in the hypothalamus of rats was examined. In the first experiment, rats were exposed to 1000 ppm N2O for 8 h a day over 4 days. Compared with controls (which were exposed to air over the same duration), the N2O exposed animals exhibited significant elevations in preproenkephalin mRNA levels in the hypothalamus. In a second experiment, rats were exposed to 60% N2O or air for 12, 24 and 48 h duration, and hypothalamic levels of preproenkephalin mRNA as well as methionine enkephalin were analyzed. Compared with controls, N2O exposed rats exhibited significant elevations in preproenkephalin mRNA levels. The levels on preproenkephalin mRNA were significantly higher after 48 h of N2O exposure than after 12 h of N2O exposure. Similarly, the concentration of methionine enkephalin was significantly higher after 24 and 48 h of exposure of N2O than after exposure to 12 h of N2O or air. These results indicate that (a) exposure to N2O results in significant elevations in preproenkephalin mRNA levels, (b) the increased preproenkephalin mRNA levels appear to be proportional to the concentration of N2O exposure as well as the duration of N2O exposure, and (c) N2O-induced elevation in preproenkephalin mRNA levels is associated with corresponding increase in tissue concentrations of methionine enkephalin. In total, these results suggest that N2O selectively stimulates synthesis of methionine enkephalin in the diencephalic region of the brain.

Estrogen regulates preproenkephalin-A mRNA levels in the rat ventromedial nucleus: temporal and cellular aspects

Numerous studies suggest that the ventromedial nucleus of the hypothalamus is a primary locus of control for reproductive behavior in the female rat. The display of lordosis behavior is estrogen dependent and its regulation appears to involve the activity of endogenous opioid circuits in the mediobasal hypothalamus and brainstem. Hypothalamic levels of preproenkephalin-A mRNA and the neuropeptide which it encodes, methionine-enkephalin, are dramatically up-regulated by estrogen. To characterize the temporal and cellular aspects of the effect of an acute exposure to estrogen on preproenkephalin-A mRNA levels in the ventromedial nucleus and arcuate nucleus of the hypothalamus, ovariectomized female rats were injected with 50 micrograms estradiol benzoate and used for quantitative in situ hybridization histochemistry of preproenkephalin-A mRNA. In the ventromedial nucleus, estrogen treatment caused a biphasic increase in the number of preproenkephalin-A mRNA cells. Maximal numbers of cells expressing preproenkephalin-A mRNA were observed at 1 and 48 h after estrogen injection. At 4 and 96 h after estrogen injection, the numbers of preproenkephalin-A mRNA cells were similar to that which was seen in the ovariectomized female. The induction of preproenkephalin-A mRNA expression in cells of the arcuate nucleus followed a similar pattern that peaked 48 h after exposure to estrogen. Levels of preproenkephalin-A mRNA per cell did not change in either nucleus as a function of estrogen treatment. Thus, acute estrogen treatment induces expression of preproenkephalin-A mRNA in populations of cells in the ventromedial hypothalamus and arcuate nucleus in which preproenkephalin-A mRNA expression is undetectable by in situ hybridization in the absence of circulating estrogen.

Changes in preproenkephalin messenger RNA level in the rat ventromedial hypothalamus during the estrous cycle

To gain a better understanding of the relationship between the female rat reproductive system and preproenkephalin (PPE) expressing neurons under physiological conditions, we examined changes in PPE mRNA levels in the mediobasal hypothalamus during the rat estrous cycle by means of northern blotting and in situ hybridization histochemistry (ISHH). In the Northern blot studies, we found that PPE mRNA levels in the mediobasal hypothalamus were significantly increased by noon of proestrus compared to those in the morning and stayed high until diestrus day 1, and returned toward low levels on diestrous day 2. In contrast, measured as controls, glyceraldehyde-3-phosphate-dehydrogenase mRNA levels were significantly higher on proestrus regardless of time of day compared to diestrus day 2, and levels of calcineurin mRNA on proestrous and estrous were significantly lower than diestrous day 1 and day 2. ISHH studies revealed that these changes in PPE mRNA levels were specific in the ventromedial hypothalamic nucleus pars ventrolateralis (VMHVL), since we could not see any significant changes in signal in other parts including ventromedial hypothalamic nucleus pars dorsomedialis and arcuate hypothalamic nucleus. In the VMHVL, PPE mRNA levels in the afternoon of proestrous were significantly higher than those in the afternoon of diestrous day 2 whereas no significant change in PPE mRNA was observed in the caudate-putamen. The present study provides additional information relevant to possible implications of PPE gene expression in female reproductive systems, since changes in PPE mRNA levels may be associated with estrogen as well as progesterone or other hormonal concentrations during the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of halothane and methoxyflurane on regional brain and spinal cord concentrations of methionine-enkephalin in the rat

Rats were exposed to either oxygen (controls), 1.5% halothane in oxygen, or methoxyflurane (0.5%) in oxygen over a period of 2 h, then sacrificed at the end of exposure (2-h group), 4 h after removal from environmental chamber (4-h group), or at 24 h following anesthetic exposure (24-h group). Pituitary (excluding the neural lobe, Pit), brain, and spinal cord areas were isolated and processed with Met-enkephalin tissue concentrations determined. In halothane-exposed animals, Met-enkephalin concentrations in pit and across CNS areas studied were significantly lower at 2 h following anesthetic exposure than in control animals. Concentrations of Met-enkephalin in many areas of CNS and Pit of 4-h group approached control levels. Concentrations of Met-enkephalin in all areas studied except spinal cord returned to basal levels by 24 h following halothane exposure. Exposure to methoxyflurane resulted in less dramatic changes in Met-enkephalin concentrations across CNS regions examined. Exposure to methoxyflurane resulted in significant decreases in Met-enkephalin levels in olfactory bulb, thalamus, and hippocampus only. Met-Enkephalin levels did not change significantly in other areas of the central nervous system following methoxyflurane exposure. These results indicate that halothane and methoxyflurane may have differential effects on the endogenous opioid system.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Time-course analysis of changes in calcitonin gene-related peptide-and methionine-enkephalin-immunoreactivity in the female rat preoptic area after estrogen treatment

The time-course effects of one month of estrogen upon calcitonin gene-related peptide - and methionine-enkephalin-immunoreactivity in the periventricular preoptic nucleus and medial preoptic nucleus were semi-quantitatively investigated with a computer-based image analysis system. Female Wistar rats were ovariectomized and implanted subcutaneously with a 10-mm-long silastic capsule containing estradiol-17 beta, or with a blank capsule, as a control. Estradiol-17 beta-treated rats were killed at days 1, 4, 7, 10, 14 and 28 after the implantation of estradiol-17 beta. To investigate the details of changes in calcitonin gene-related peptide- and methionine-enkephalin-immunoreactive fibers in the periventricular preoptic nucleus and medial preoptic nucleus, a grid, made up of 8 x 16 squares (one square corresponding to 50 x 50 microns in the true section), was set on the wall of the third ventricle, and immunoreactivity within each square was measured with an image analyser. In the control rats, calcitonin gene-related peptide- and methionine-enkephalin-immunoreactive fibers were distributed in the periventricular preoptic nucleus and medial preoptic nucleus. In the estradiol-17 beta-treated rats, calcitonin gene-related peptide-immunoreactive fibers increased prominently at day 1, day 7 and day 10 in the periventricular preoptic nucleus, whereas methionine-enkephalin-immunoreactive fibers increased at day 1, day 14 and day 28 in the periventricular preoptic nucleus and medial preoptic nucleus. These findings suggest that the mechanism underlying the increases in these calcitonin gene-related peptide- and methionine-enkephalin-immunoreactive fibers after estrogen treatment might be different.

Semi-quantitative analysis of the effects of estrogen on CGRP- and methionine-enkephalin-immunoreactivity in the periventricular preoptic nucleus and the medial preoptic area of female rats

The effects of 17 beta-estradiol (E2) on calcitonin gene-related peptide (CGRP)- and methionine-enkephalin (Met-Enk)-immunoreactive (IR) nerve fibers in the periventricular preoptic nucleus (Pe) and the medial preoptic area, including the medial preoptic nucleus (MPN), of the female rat were studied semi-quantitatively by using a computer-based image analysis system. The area occupied by CGRP- and Met-Enk-IR fibers was significantly increased in the Pe and the MPN after 28-day exposure to E2. Computer-based analysis of CGRP- and Met-Enk-IR fibers in an area of 50-microns intervals from the wall of the third ventricle showed a low flat histogram pattern in ovariectomized rats, but E2 treatment caused a diphasic pattern, corresponding to the Pe and the MPN, respectively. Since the Pe and the MPN contain a high population of estrogen receptors, it is suggested that E2 may have an influence on the neuronal configuration of afferent fibers to these areas.

Methionine-enkephalin in a porcine endometrial cell line and its responsiveness to potassium depolarization

Immunoreactive methionine-enkephalin (ir-MENK) has been identified in the porcine uterine fluid and endometrium. Previously, we have established a porcine endometrial cell line of epithelial origin (PE-1) by transfecting primary endometrial cells with temperature sensitive SV40 DNA. The current study was conducted to identify and characterize ir-MENK present in PE-1 cells, and to investigate the effect of KCl depolarization on the kinetics of ir-MENK secretion. PE-1 cells were cultured at 33C until confluency was reached (33C cells), after which they were incubated at 40C for 2 days (40C cells). Ir-MENK in PE-1 cells was analyzed by Sephadex G-15 gel filtration and reverse phase (RP)-HPLC. Analysis of 40C cell extract by Sephadex G-15 and RP-HPLC indicated that the major portion of ir-MENK present in PE-1 cells was eluted at a position similar to that of synthetic MENK. The effect of temperature on ir-MENK synthesis in PE-1 cells was examined by measuring ir-MENK content in 33C and 40C cells over a 14-day culture period. Compared to 33C cells, 40C cells maintained higher and steadier levels of ir-MENK, suggesting that synthesis of ir-MENK is temperature sensitive. KCl stimulated ir-MENK secretion at all concentrations tested (5-60 mM for 60 min), with 30 mM being the optimal concentration. Temporal analysis of ir-MENK secretion showed that incubation for 60 min with 30 mM KCl allowed maximal secretion. Secretion of ir-MENK from PE-1 cells resulted in depletion of ir-MENK in cell content. These results demonstrate that PE-1 cells contain ir-MENK which is biochemically similar to synthetic MENK, PE-1 cells synthesize ir-MENK in a temperature sensitive manner, and these cells secrete ir-MENK upon KCl stimulation.

Endogenous opioid-immunoreactive neurons of the ventromedial hypothalamic nucleus concentrate estrogen in male and female rats

Estrogen stimulates expression of proenkephalin mRNA in neurons of the hypothalamic ventromedial nucleus, and evidence is accumulating that synaptic release of one of the peptide end products, met-enkephalin, influences events that regulate reproductive behavior. To address the question of whether estrogen acts directly on neurons that synthesize met-enkephalin or indirectly through a separate neuronal population, we combined estrogen autoradiography with endogenous opioid peptide (EOP) immunohistochemistry. In agreement with previous studies, the ventrolateral subdivision of the hypothalamic ventromedial nucleus was densely packed with EOP-immunoreactive cells. In males, 48% of the estrogen-concentrating cells of the ventrolateral subdivision of the hypothalamic ventromedial nucleus contained EOP, and, in females, 27% of the estrogen-concentrating cells contained EOP. These findings indicate that estrogen acts directly on neurons that express EOP and suggest a mechanism that underlies sexually differentiated reproductive behavior.

Differential regulation of proenkephalin gene expression by estrogen in the ventromedial hypothalamus of male and female rats: implications for the molecular basis of a sexually differentiated behavior

The ventrolateral aspect of the ventromedial hypothalamic nucleus (VL-VM) contains many estrogen-concentrating neurons which mediate estrogen facilitation of reproductive behavior. Previous studies have shown that estrogen treatment increases proenkephalin (PE) gene expression in neurons of the VL-VM in ovariectomized female rats, and that enkephalin peptides may stimulate lordosis behavior. To determine whether there is a sex difference in steroid hormone regulation of PE gene expression we have examined the effects of estrogen and testosterone on PE mRNA levels in male rats. Slot blot hybridization analysis of RNA isolated from the ventromedial hypothalamus indicated that estrogen treatment increased PE mRNA levels in the VL-VM of ovariectomized female rats (2.2-fold), but had no measurable effect on PE mRNA levels in gonadectomized males. Testosterone treatment of gonadectomized males also had no effect on PE gene expression. To determine whether the sex difference in estrogen-inducibility of PE gene expression is due to the developmental effects of gonadal steroids, we have investigated the effect of estrogen on PE mRNA levels in the VL-VM of neonatally androgenized female rats. Unlike the genetic male, the androgenized females responded to estrogen treatment with a female-typical increase in PE mRNA levels (1.7-fold). Further, although the androgenized rats clearly exhibited signs of defeminization, they did exhibit estrogen-facilitated lordosis behavior when tested with manual stimulation. The PE mRNA induction in estrogen-treated androgenized rats correlated well with the lordosis scores obtained by manual stimulation testing. These results indicate that estrogen regulation of PE gene expression in the VL-VM is sexually differentiated and support the hypothesis that the enkephalinergic neurons of the VL-VM are involved in the regulation of female reproductive behavior.

Effect of early exposure to delta-9-tetrahydrocannabinol on the levels of opioid peptides, gonadotropin-releasing hormone and substance P in the adult male rat brain

The effects of neonatal exposure to delta-9-tetrahydrocannabinol (THC) on the adult animal brain neurochemistry and pain perception were evaluated. Newborn rat pups were culled to a litter size of 8 (males and females) and treated either with THC (2 mg/kg) or oil (control) daily, during days 1-4 after birth. After weaning, the THC-treated males were housed 4 per cage. During the juvenile period (day 50), the THC-treated animals exhibited significantly lower baseline tail-flick values (a measure of pain perception) than the control. However, as adults, the THC-treated animals exhibited significantly higher sensitivity to pain following 5 mg/kg morphine challenge. Furthermore, the THC-treated animals had significantly elevated beta-endorphin and methionine-enkephalin levels in almost all the brain areas sampled for the study. In addition, the neonatally THC-treated rats exhibited significantly higher levels of substance P (SP) and significantly lower levels of gonadotropin releasing hormone (GnRH) in the anterior hypothalamus-preoptic area. The SP and GnRH levels did not differ among the THC-treated and control animals in the medial basal hypothalamus. The results of this study indicate that even a very low dose of THC administered during the neonatal period has a long-lasting effect on the brain neurochemistry. In particular, neonatal administration of THC appears to alter functioning of the endogenous opioid system.

Estradiol induction of proenkephalin messenger RNA in hypothalamus: dose-response and relation to reproductive behavior in the female rat

Hormone effects on proenkephalin (PE) mRNA allow an opportunity to compare a brain region-specific molecular change with a quantifiable behavior. Slot blots were used to measure PE mRNA levels in the ventromedial hypothalamus (VMN) and preoptic area (POA) as a function of the dose of estrogen administered to ovariectomized rats. Every rat used had been characterized for the ability to display lordosis behavior. Estradiol treatment led to a monotonic dose-dependent increase in PE mRNA level in VMN, while only a small effect was observed in POA at the higher estradiol doses. Lordosis behavior, assessed manually and in mating tests, also increased monotonically with estradiol dose. The data indicate that an apparent 'threshold' level of PE mRNA in VMN coincided wit display of behavior, and suggest further that high levels of PE mRNA, alone, are not sufficient for lordosis. While the exact relationship of the eventual product, Met-enkephalin, to female reproductive behavior remains to be determined, the parallel changes in PE mRNA level and behavior encourage further analysis.

Mu-opiate receptor binding in the medial preoptic area is cyclical and sexually dimorphic

The density and distribution of mu- and kappa-opiate receptors in the medial preoptic area (MPOA) of male and female rats across the estrous cycle was examined using quantitative in vitro autoradiography of [3H]D-Ala2,MePhe4,Gly-ol5-enkephalin (DAGO), [3H]naloxone and [3H]bremazocine binding. While no difference in kappa-receptor labeling was observed across sex or estrous stage, selective mu-receptor labeling with [3H]DAGO revealed a significant variation of density and distribution in the MPOA across the estrous cycle and between sexes. A dense concentration of mu-receptors located in the central, sexually dimorphic portion of the MPOA was observed during metestrus and diestrus in females, but not during proestrus nor in males. This region appeared to be the same as that labeled similarly using [3H]naloxone. These results suggest that a regional substrate for functional activation by endogenous opioid peptides (e.g. beta-endorphin) is cyclically regulated in females, which may explain the gonadal steroid-dependent effects of MPOA beta-endorphin on lordosis and luteinizing hormone secretion in females.

Estrogen regulation of proenkephalin gene expression in the ventromedial hypothalamus of the rat: temporal qualities and synergism with progesterone

Estrogen has been shown to increase proenkephalin (PE) mRNA levels in neurons of the ventrolateral aspect of the ventromedial hypothalamic nucleus (VL-VM). In this series of experiments, we examined the temporal qualities of this induction by determining both the latency of the estrogen-induced elevation in PE mRNA levels and the rate at which the message levels decline following removal of estrogen. In addition we have examined the effects of progesterone on PE gene expression in the VL-VM of estrogen-primed rats. The latency of the estrogen-induced elevation in PE mRNA levels was found to be relatively short: PE mRNA levels were increased 2-fold within 1 h of estrogen replacement. Following estrogen removal the levels of PE mRNA declined rapidly. Progesterone treatment attenuated this decline, prolonging the estrogen-induced elevation of PE mRNA levels. These results suggest that estrogen rapidly increases PE mRNA levels through a mechanism that probably involves alterations in both the rate of appearance and the rate of degradation of the message. Together, the short latency of the estrogen-induced elevation and the rapid rate of decay following estrogen removal indicate that PE gene expression is highly sensitive to fluctuating estrogen levels. The effect of progesterone suggests that this enkephalinergic system may be regulated by both estrogen and progesterone during the estrous cycle.

Distribution of kappa opioid receptors in the brain of young and old male rats

The experiments to be described have been designed in order to: (a) provide new information on the concentrations of opioid kappa receptors in different regions of the brain of the male rats; and (b) to analyze whether the density of brain kappa receptors might be modified by the process of aging. The concentration of kappa receptors was investigated in the hypothalamus, amygdala, mesencephalon, corpus striatum, hippocampus, thalamus, frontal poles, anterior and posterior cortex collected from male rats of 2 and 19 months of age. 3H-bremazocine (BRZ) was used as the ligand of kappa receptors, after protection of mu and delta receptors respectively with dihydromorphine and d-ala-d-leu-enkephalin. The results obtained show that: (1) in young male rats, the number of kappa opioid receptors is different in the various brain areas examined: the hypothalamus and the striatum have a concentration of kappa binding sites which is significantly higher than that found in the mesencephalon and in the amygdala; much lower concentrations of kappa binding sites have been found in the thalamus, the frontal poles, the hippocampus, the anterior and posterior cerebral cortex. (2) Aging exerts little influence on the number of kappa receptors in the majority of the brain structures considered. However in the amygdala and in the thalamus the number of kappa receptors was increased in old animals. To the authors' knowledge, the data here presented are the first ones which suggest that age may increase rather than decrease the number of neurotransmitter receptors in the brain.

Ontogeny of a sexually dimorphic opioid system in the preoptic area of the rat

A striking sexual dimorphism exists in the distribution of Met-enkephalin (m-ENK) immunoreactive fibers in the preoptic area of the rat brain. A dense plexus of m-ENK fibers, approximately 100 microns wide, is present in the periventricular part of the preoptic area (pePOA) in adult females, but not in males. In the present study, we have examined the time of first expression of this female-typical system during the course of normal brain development. The female-typical plexus of m-ENK fibers in the pePOA is not expressed prepubertally, but first becomes evident during the late peri-pubertal period (usually by 40 days of age). In adult females, the maintenance of immunohistochemically detectable levels of m-ENK in this fiber system is dependent upon the presence of gonadal steroids, especially estradiol. Therefore, we examined whether exposure of pre-pubertal females to estradiol would result in precocious expression of the m-ENK fiber plexus. The results of this experiment demonstrate that exposure to estradiol for 7 days induces the full expression of the m-ENK fiber system in the pePOA of prepubertal females, such that it was indistinguishable from that seen in mature animals. These results demonstrate that while the sexually dimorphic m-ENK system of the pePOA is not normally expressed prepubertally, the neural substrate is nevertheless in place and capable of being activated by exposure to exogenous estradiol.

Study of the gonadotropic cells in the rat after chronic administration of met-enkephalin: light, electron microscope and image analysis

The effects of a chronic administration of met-enkephalin (40 micrograms/day, for 20 days) on the pituitary gonadotropic cells of Wistar male rats were studied in light (PAS alcian blue-orange G and PAP-immunohistochemical for FSH and LH demonstration techniques) and by electron microscope. Also the D CIRCLE (mean diameter) and FORM PE (circular factor of form; irregularity degree) from secretory granules of these cell types were evaluated. The cellular alterations are variable. Gonadotropic cells show an increase on the cytoplasmic basophilia and immunohistochemical reaction. At the electron microscope level the gonadotropic cells show an enlargement and dilatation of RER and Golgi apparatus. The secretory granules are bigger and more spherical in experimental than in normal and control animals. A lot of cells show a large cytoplasmic vacuole which is formed from dilated RER-cisternae. The authors discuss the possible mechanism of action whereby met-enkephalin exerts control on gonadotropic cells.

Novelty-induced opioid analgesia in deer mice (Peromyscus maniculatus): sex and population differences

Exposure to a new environment elicited significant, naloxone (1.0 mg/kg) reversible analgesic responses in three different populations of deer mice; Peromyscus maniculatus artemisiae from the mainland, and P. m. angustus and P. m. triangularis from small marine islands. In all cases male deer mice displayed significantly greater levels of analgesia than females. In addition, the levels of analgesia were significantly greater in the insular than in the mainland populations. These results indicate that there are substantial sex and population differences in the novelty-induced analgesia displayed by natural and laboratory populations of deer mice.

Stress-induced opioid analgesia and activity in deer mice: sex and population differences

We compared restraint stress-induced opioid, analgesic and locomotory responses of 4 different populations of male and female deer mice, Peromyscus maniculatus artemisiae and P. m. nebrascensis from mainlands, and P. m. angustus and P. m. triangularis from small islands. All of the deer mice displayed immobilization-induced analgesia which was blocked by the prototypical mu-opiate antagonist, naloxone (1.0 mg/kg). In all of the populations males displayed significantly greater levels of analgesia than females. In addition, the levels of opioid-induced analgesia were significantly greater in the insular than in the mainland male and female deer mice. Restraint also induced significant increases in the locomotor activity of the mainland deer mice, while significantly decreasing the activity of the insular animals. Males displayed significantly greater stress-induced changes in locomotor activity than did females. The stress-induced increases in activity were blocked by the delta-opiate antagonist, ICI 154, 129 (10 mg/kg), while the decreases in activity were inhibited by naloxone. These results demonstrate that there are marked sex and population differences in the stress-induced, opioid-mediated responses of deer mice. These 'pharmaco-ecological' findings also suggest that the island-mainland population differences in behavioral responses and ecological characteristics may, in part, be related to differences in the activity of mu-, delta- and possibly other opioid systems.

Characterization of kappa opioid receptors in neurosecretosomes from bovine posterior pituitary

The binding properties of opioid receptors on isolated nerve terminals (neurosecretosomes) from bovine posterior pituitaries were characterized. Both [3H]etorphine and [3H]ethylketocyclazocine ([3H]EKC) showed high-affinity binding with complex binding isotherms, consistent with the presence of multiple classes of binding sites. [D-Ala2,D-Leu5]enkephalin showed no specific binding and failed to displace [3H]etorphine at high concentrations, indicating the absence of mu, delta, or benzomorphan (kappa 2) sites. Mathematical modelling of the data suggested the presence of three classes of binding sites. The first was of high affinity with Kd values of 0.9 and 2.0 nM for etorphine and EKC, respectively. The second class of sites appeared to bind etorphine with a KD of 150 nM, and EKC with extremely low affinity (unmeasurable binding). The third class of sites was characterized by KD values of 7 and 2 microM for etorphine and EKC, respectively. These results indicate that the nerve terminals of bovine posterior pituitary contain opioid binding sites of the kappa type. Furthermore, these binding sites appear heterogeneous, consisting of at least two and possibly more subtypes or states.

Decrease of mu opioid receptors in the brain and in the hypothalamus of the aged male rat

Experiments have been designed in order to analyze whether the binding capability of mu opioid receptors in the brain of the male rat is modified by age. In a first experiment, the number of receptors (Bmax) and the constant of affinity (Ka) for the mu ligand 3H-dihydromorphine (3H-DHM) have been measured in the whole brain of male rats of 2, 15 and 22 months of age. In a second experiment the Bmax and the Ka for 3H-DHM have been evaluated in the hypothalamus of male rats of 2 and 22 months of age. In this experiment it was also investigated whether the administration of exogenous testosterone modifies the number and/or the affinity of the hypothalamic mu receptors. Serum levels of LH, FSH, prolactin and testosterone have been measured by specific RIAs. The results obtained show that: serum testosterone levels are significantly decreased in aged rats, while serum LH and FSH show only a small decline; serum prolactin is higher in old than in young animals; the number of mu receptors in the whole brain of 15 and 22 month old animals and in the hypothalamus of 22 month old rats is significantly lower than in the same tissues of young animals; the administration to old animals of testosterone, in doses able to bring back towards normal serum levels of testosterone, induces a decrease of LH and FSH, but has no effect on serum prolactin titers. Testosterone administration does not modify the number of hypothalamic mu opioid receptors, indicating that the decline of brain mu receptors in old animals is not the consequence of the physiological decline of testosterone secretion; in no instance the Ka for the mu ligand is significantly affected.

Sexually dimorphic opioid distribution in the preoptic area: manipulation by gonadal steroids

A striking sexual dimorphism has been found in the density of Met-enkephalin immunoreactive fibers in the periventricular region of the preoptic area in the rat: the enkephalinergic fiber system is much denser in females. The expression of this female-typical fiber plexus is regulated by the actions of gonadal steroids both during development and in adulthood. In light of abundant evidence demonstrating the ability of the opioid peptides to modulate various sexually differentiated neuroendocrine processes and behaviors, this dimorphic system may represent an important anatomical substrate underlying these functions.

Diurnal rhythm of the in vivo release of enkephalin from the globus pallidus of the rat

The in vivo spontaneous release of enkephalin in the globus pallidus of the rat increases from noon to evening by 100%; during this period the local release of exogenous gamma-aminobutyric acid (GABA) decreases by 60%. These diurnal rhythms are more marked in the K+-stimulated release: enkephalin-induced output increases 6-fold while GABA decreases 10-fold during the afternoon and evening hours. Since pallidal enkephalin and GABA are involved in the control of locomotor activity we suggest that these rhythms may be linked to the circadian changes of activity in the rat.

Changes in the immunoreactivities of an opioid peptide leumorphin in the hypothalamus and anterior pituitary during the estrous cycle of the rat and their relation to sexual behavior

Leumorphin, an opioid peptide whose functions are unknown, is found in mammalian brain and pituitary and stimulates lordosis behavior in estrogen-treated female rats. To elucidate the role of leumorphin in the physiological control of female sexual behavior, the levels of immunoreactive (ir) leumorphin as well as ir dynorphin (dynorphin A) were measured in the rat brain and pituitary during the estrous cycle. There was a clear variation of ir leumorphin in the hypothalamus and anterior pituitary during the estrous cycle. The levels of ir leumorphin in the hypothalamus and anterior pituitary on the afternoon of proestrus were significantly higher (P less than 0.01) than those on the afternoons of estrus and metestrus. The rise in the hypothalamic levels of ir leumorphin on the afternoon of proestrus was correlated with the receptivity of lordosis during the estrous cycle. Furthermore, there was a close correlation with ir dynorphin levels. These findings are in agreement with studies demonstrating a common precursor for leumorphin and dynorphin. Ir leumorphin in the hippocampus and neurointermediate pituitary did not change significantly during the estrous cycle. Because the leumorphin antiserum used recognizes rimorphin (dynorphin B) 1.78 times more than porcine leumorphin on a molar basis, high performance-gel permeation chromatography was done on pooled extracts of hypothalamus taken at proestrus and estrus. The peak in the leumorphin-like substance in the activation of sexual behavior is discussed.

Beta-endorphin and met-enkephalin in peritoneal and ovarian follicular fluids of fertile and postmenopausal women

Immunoreactive (IR) beta-endorphin (beta-EP) and met-enkephalin (MET-ENK) have been found in peritoneal fluid (PF) and ovarian follicular fluid (FF). Gel chromatography also revealed the presence of coeluting IR beta-lipotropin and gamma-lipotropin. IR beta-EP and IR MET-ENK levels in healthy menstruating women were from 10 to 40 times higher than those present in circulating plasma, which indicated a possible local production. The highest concentrations of IR beta-EP in FF were found in the largest follicles, whereas in the PF they correlated with the luteal period of the menstrual cycle and with progesterone concentrations. No relevant changes in IR MET-ENK were detected in the FF or in the PF in relation to the phase of the menstrual cycle. In postmenopausal women, the concentrations of the two IR opioid peptides were undetectable in both fluids.

Effect of oral contraceptives on the rat brain and pituitary opioid peptides

This study was designed to explore the hormonal regulation of CNS opioid peptide levels in female Sprague Dawley rats. Forty-eight animals were divided into 2 equal groups for acute and chronic studies. Each group was further divided into 4 subgroups, each containing 6 animals. Each rat in the control group received an inert pill (in 0.25 ml corn oil daily by gavage); the second group, 15 micrograms norethindrone (NE, a potent progestin present in the oral contraceptive Micronor); the third group, 15 micrograms NE and 1 microgram ethinyl estradiol, EE2 (present in the oral contraceptive Modicon) and the fourth group, 10 times the dose of the third group. Rats were treated either acutely for 5 days or chronically for 7 weeks. Opioid peptides were estimated by radioimmunoassay. Acute administration of 150 micrograms NE + 10 micrograms EE2 decreased the levels of methionine-enkephalin (ME), leucine-enkephalin (LE), dynorphin (DYN) and beta-endorphin like immunoreactivity (beta-EI) by about 50% in the pituitary. The same dose on chronic administration also decreased DYN, but increased the levels of ME and LE in the pituitary by 331 and 69%, respectively. In the hypothalamus, chronic administration of NE + EE2 increased the level of ME (155%) and LE (87%) as well as of DYN (97%). In the striatum, the levels of LE (33%) and DYN (115%) were elevated during chronic administration. It is concluded that the acute administration of NE + EE2, in general, reduces the levels of ME, LE, DYN and beta-EI.(ABSTRACT TRUNCATED AT 250 WORDS)

Impairment of opioid control of luteinizing hormone secretion in menstrual disorders

With the aim of examining central opioid influences on the control of luteinizing hormone (LH) secretion, we evaluated the LH response to naloxone, an opioid receptor antagonist, in patients affected by normo-, hyper-, and hypogonadotropic amenorrhea, polycystic ovarian disease and hyperprolactinemia. The results indicate that opioid influences are altered in well-defined pathologic conditions (hyperprolactinemia, obesity), in addition to being modified by gonadal steroids.

Effect of delta 9-THC on brain and plasma catecholamine levels as measured by HPLC

The effects of chronic administration of delta 9-tetrahydrocannabinol (delta 9-THC) on the plasma and brain catecholamine (CA) levels were measured using high performance liquid chromatography-electrochemical detection (LC-EC) system. Intact male rats were injected daily with vehicle (50 microliter oil) or with delta 9-THC (3 mg/kg body wt) over a period of 25 days. Trunk plasma and tissue from preoptic area (POA) and mediobasal hypothalamus (MBH) were collected and catecholamine levels were detected by LC-EC system coupled to an electronic integrator. Alumina extract of tissue and plasma samples, spiked with the internal standard (dihydroxybenzylamine), were injected into the LC-EC system; the CA were chromatographed and eluted within 12 minutes using sodium phosphate buffer as the mobile phase. delta 9-THC treatment resulted in a significant decrease in plasma and MBH levels of norepinephrine (NE), epinephrine (E), POA levels of NE; and significant increases in MBH levels of dopamine (DA) and dihydroxyphenylacetic acid (DOPAC). Our study indicates for the first time that delta 9-THC treatment significantly alters not only the POA and MBH CA levels, but also the plasma CA levels.

beta-Endorphin withdrawal: a possible cause of premenstrual tension syndrome

The authors propose that premenstrual tension syndrome (PMS) is the result of beta-endorphin withdrawal. Sixteen women were included in this study which measured beta-endorphin levels on the 7th and 24th day of each woman's menstrual cycle. A significant decline in beta-endorphin levels was noted during the progression of the cycle. The severity of symptoms, however, was inversely proportional to the amount of decline in beta-endorphin levels. It is hypothesized that the attenuation of endorphin decline may be a compensatory mechanism to moderate the severity of PMS symptoms.

Effect of short photoperiod on hypothalamic methionine-enkephalin and LHRH content and serum beta-endorphin-like immunoreactivity (beta-end LI) levels in golden hamsters

Adult female golden hamsters were used to study the effect of short photoperiod on the endogenous opioid system and the effect of pinealectomy on the serum beta-endorphin-like immunoreactivity (beta-end LI) levels. Hamsters were housed under either long photoperiod (14L:10D) or short photoperiod (2L:22D) and the regularity of the estrous cycles was determined by daily vaginal exfoliative cytology. Hamsters under short photoperiod became acyclic after about 7 wk. At the end of 8 wk, all the hamsters were decapitated and medial basal hypothalamic (MBH) content of LHRH and methionine-enkephalin (met-enkephalin) were measured by specific radioimmunoassays (RIA). Both LHRH and met-enkephalin levels of the MBH were significantly elevated in the short-photoperiod hamsters as compared to the normally cycling control animals under long photoperiod. In a second experiment, the effect of pinealectomy (PNX) on the serum levels of beta-end LI in the short-photoperiod hamsters was determined. The serum beta-end LI levels were increased approximately threefold in the noncyclic hamsters housed under 8 wk of short-photoperiod conditions. Pinealectomized hamsters kept under 8 wk of short-photoperiod exhibited lower serum beta-end LI levels similar to those of normally cycling hamsters kept under long photoperiod. These results indicate a possible functional relationship between increased pineal activity (as a result of short photoperiod) and increased MBH met-enkephalin, LHRH, and serum beta-end LI levels.

Changes in beta-endorphin content in discrete areas of the hypothalamus throughout proestrus and diestrus of the rat

The aim of the present study is to investigate changes in beta-endorphin content in the hypothalamus during different stages of the estrous cycle. Groups of 9 to 10 Sprague-Dawley rats were sacrificed every two hours on proestrus from 8.00 to 18.00 h and groups of 7 to 8 rats were sacrificed on diestrus at 8.00, 12.00, 14.00 and 18.00 h. Preoptic suprachiasmatic region, posterior hypothalamus, arcuate nucleus and median eminence were dissected and assayed for beta-endorphin. A significant increase in beta-endorphin content was detected in the arcuate nucleus during proestrus (9.00 h: 1.76 +/- .31; 14.00 h: 4.10 +/- .85 microgram/g tissue wet weight). Levels did not change during diestrus (1.18 +/- .06 microgram/g). The increase caused significant differences in beta-endorphin values between both days at 12.00, 14.00 and 18.00 h, while the concentrations at 8.00 h were similar. The opposite pattern was observed in the median eminence with significantly higher proestrous beta-endorphin levels at 8.00 h (11.24 +/- 3.1 vs 3.52 +/- .64 microgram/g) and nonsignificant differences for the rest of the day. No significant change in beta-endorphin concentration was seen in the preoptic suprachiasmatic region over the day of proestrus (1.35 +/- .09 microgram/g). Diestrous beta-endorphin concentrations in this region were higher during the morning (2.60 +/- .65 microgram/g) and lower at 18.00 h (0.94 +/- .12 microgram/g) when compared to proestrous values. This pattern was caused by a 50% increase in beta-endorphin during the afternoon of diestrus. No changes were observed in the posterior hypothalamus on either day with comparable levels of beta-endorphin except at 18.00 h, when values were significantly higher on proestrus (1.66 +/- .30 vs 0.83 +/- .06 microgram/g).

Sex-related difference in the rat pituitary [Met5]-enkephalin level--altered by gonadectomy

Ontogenetic study of the rat pituitary [Met5]-enkephalin-like immunoreactivity (ME-LI) revealed that a sex-related difference in the level of this peptide began at 35 days of age. By 70 days of age, the level of male ME-LI was twice that of the female. Castration reduced the ME-LI in male rats and increased the ME-LI in female rats. These results suggest a possible role of sex hormones in regulating the pituitary enkephalin system.