Interaction of opioid peptide-containing terminals with dopaminergic perikarya in the rat hypothalamus

The Emerging Therapeutic Potential of Kisspeptin and Neurokinin B

Kisspeptin (KP) and neurokinin B (NKB) are neuropeptides that govern the reproductive endocrine axis through regulating hypothalamic gonadotropin-releasing hormone (GnRH) neuronal activity and pulsatile GnRH secretion. Their critical role in reproductive health was first identified after inactivating variants in genes encoding for KP or NKB signaling were shown to result in congenital hypogonadotropic hypogonadism and a failure of pubertal development. Over the past 2 decades since their discovery, a wealth of evidence from both basic and translational research has laid the foundation for potential therapeutic applications. Beyond KP's function in the hypothalamus, it is also expressed in the placenta, liver, pancreas, adipose tissue, bone, and limbic regions, giving rise to several avenues of research for use in the diagnosis and treatment of pregnancy, metabolic, liver, bone, and behavioral disorders. The role played by NKB in stimulating the hypothalamic thermoregulatory center to mediate menopausal hot flashes has led to the development of medications that antagonize its action as a novel nonsteroidal therapeutic agent for this indication. Furthermore, the ability of NKB antagonism to partially suppress (but not abolish) the reproductive endocrine axis has supported its potential use for the treatment of various reproductive disorders including polycystic ovary syndrome, uterine fibroids, and endometriosis. This review will provide a comprehensive up-to-date overview of the preclinical and clinical data that have paved the way for the development of diagnostic and therapeutic applications of KP and NKB.

Partial loss of arcuate kisspeptin neurons in female rats stimulates luteinizing hormone and decreases prolactin secretion induced by estradiol

Kisspeptin, neurokinin, and dynorphin (KNDy) neurons in the arcuate nucleus (ARC) control luteinizing hormone (LH) and prolactin (PRL) release, although their role in conveying the effects of estradiol (E₂ ) to these hormones is not well understood. We performed a longitudinal evaluation of female rats in which KNDy neurons were ablated using a neurokinin-3 receptor agonist conjugated with saporin (NK3-SAP) to investigate the impact of the reduction of KNDy neurons on the E₂ regulation of gonadal and PRL axes. NK3-SAP rats, bearing a moderate loss of ARC kisspeptin-immunoreactive (-IR) neurons (50%-90%), displayed irregular estrous cycles but essentially unaltered follicular development and a normal number of corpora lutea. Rats were then ovariectomized (OVX) and treated with a positive-feedback dose of E₂ (OVX + E₂ ). LH and PRL were measured in the tail blood by an enzyme-linked immunosorbent assay. The E₂ -induced LH surge was amplified, whereas the PRL rise was decreased in NK3-SAP rats compared to Blank-SAP control. After 10 days of no hormonal treatment, basal LH levels were equally elevated in NK3-SAP and controls. Tyrosine hydroxylase (TH) phosphorylation in the median eminence, in turn, was increased in NK3-SAP rats, with no change in the number of ARC TH-IR neurons. Thus, KNDy neurons exert concurrent and opposite roles in the E₂ -induced surges of LH and PRL. The partial loss of KNDy neurons disrupts ovarian cyclicity but does not preclude ovulation, consistent with the disinhibition of the LH preovulatory surge. Conversely, KNDy neurons tonically inhibit the enzymatic activity of tuberoinfundibular dopaminergic neurons, which appears to facilitate PRL release in response to E₂ .

Kisspeptin Stimulation of Prolactin Secretion Requires Kiss1 Receptor but Not in Tuberoinfundibular Dopaminergic Neurons

Kisspeptin has been shown to stimulate prolactin secretion. We investigated whether kisspeptin acts through the Kiss1 receptor (Kiss1r) to regulate dopamine and prolactin. Initially, we evaluated prolactin response in a Kiss1r-deficient mouse line, in which Kiss1r had been knocked into GnRH neurons (Kiss1r-/-R). Intracerebroventricular kisspeptin-10 (Kp-10) increased prolactin release in wild-type but not in Kiss1r-/-R female mice. In ovariectomized, estradiol-treated rats, the Kiss1r antagonist kisspeptin-234 abolished the Kp-10-induced increase in prolactin release but failed to prevent the concomitant reduction in the activity of tuberoinfundibular dopaminergic (TIDA) neurons, as determined by the 3,4-dihydroxyphenylacetic acid/dopamine ratio in the median eminence. Using whole-cell patch clamp recordings in juvenile male rats, we found no direct effect of Kp-10 on the electrical activity of TIDA neurons. In addition, dual-label in situ hybridization in the hypothalamus of female rats showed that Kiss1r is expressed in the periventricular nucleus of the hypothalamus (Pe) and arcuate nucleus of the hypothalamus (ARC) but not in tyrosine hydroxylase (Th)-expressing neurons. Kisspeptin also has affinity for the neuropeptide FF receptor 1 (Npffr1), which was expressed in the majority of Pe dopaminergic neurons but only in a low proportion of TIDA neurons in the ARC. Our findings demonstrate that Kiss1r is necessary to the effect of kisspeptin on prolactin secretion, although TIDA neurons lack Kiss1r and are electrically unresponsive to kisspeptin. Thus, kisspeptin is likely to stimulate prolactin secretion via Kiss1r in nondopaminergic neurons, whereas the colocalization of Npffr1 and Th suggests that Pe dopaminergic neurons may play a role in the kisspeptin-induced inhibition of dopamine release.

Coordination or Coincidence? The Relationship between Prolactin and Gonadotropin Secretion

A recent paper demonstrates that kisspeptin can stimulate the secretion of both prolactin and luteinizing hormone (LH). This provides novel insight into mechanisms that could account for previous observations of concordant patterns of secretion of the two hormones, such as pulsatile secretion with simultaneous pulses, as well as the preovulatory surges of both prolactin and LH.

Influence of dynorphin on estradiol- and cervical stimulation-induced prolactin surges in ovariectomized rats

Prolactin is an anterior pituitary hormone necessary for fertility, pregnancy maintenance, lactation, and aspects of maternal behavior. In rodents, there is a surge of prolactin on the afternoon of proestrus, and a semi-circadian pattern of prolactin surges during early pregnancy, with a diurnal and nocturnal surge every day. Both of these patterns can be replicated in ovariectomized rats. A prior study demonstrated that central antagonism of κ-opioid receptors, the target of dynorphin, largely abolished the nocturnal prolactin surge in pregnant rats. We build on this to determine whether dynorphin, perhaps from the arcuate population that co-express kisspeptin, neurokinin B, and dynorphin (KNDy neurons), also contributes to the estradiol- or cervical stimulation-induced surges in ovariectomized rats. Ovariectomized rats were treated with either estradiol or cervical stimulation to induce prolactin surge(s). Blood samples were taken around the expected surge time to determine the effect of either acute κ-opioid receptor antagonism or previous chemical ablation of the KNDy population on prolactin levels. Dynorphin antagonism does significantly disrupt the nocturnal prolactin surge, but it does not contribute to the estradiol-induced surge. Chemical ablation of KNDy neurons had opposite effects; ablation of 40 % of the KNDy neurons had no impact on the nocturnal prolactin surge, while a somewhat larger ablation significantly reduced the size of the estradiol-induced surge. We conclude that dynorphin is likely a controlling factor for the nocturnal surge induced by cervical stimulation, and that other KNDy neuron products must play a role in the estradiol-induced surge.

Neuroendocrine regulation of lactation and milk production

Prolactin (PRL) released from lactotrophs of the anterior pituitary gland in response to the suckling by the offspring is the major hormonal signal responsible for stimulation of milk synthesis in the mammary glands. PRL secretion is under chronic inhibition exerted by dopamine (DA), which is released from neurons of the arcuate nucleus of the hypothalamus into the hypophyseal portal vasculature. Suckling by the young activates ascending systems that decrease the release of DA from this system, resulting in enhanced responsiveness to one or more PRL-releasing hormones, such as thyrotropin-releasing hormone. The neuropeptide oxytocin (OT), synthesized in magnocellular neurons of the hypothalamic supraoptic, paraventricular, and several accessory nuclei, is responsible for contracting the myoepithelial cells of the mammary gland to produce milk ejection. Electrophysiological recordings demonstrate that shortly before each milk ejection, the entire neurosecretory OT population fires a synchronized burst of action potentials (the milk ejection burst), resulting in release of OT from nerve terminals in the neurohypophysis. Both of these neuroendocrine systems undergo alterations in late gestation that prepare them for the secretory demands of lactation, and that reduce their responsiveness to stimuli other than suckling, especially physical stressors. The demands of milk synthesis and release produce a condition of negative energy balance in the suckled mother, and, in laboratory rodents, are accompanied by a dramatic hyperphagia. The reduction in secretion of the adipocyte hormone, leptin, a hallmark of negative energy balance, may be an important endocrine signal to hypothalamic systems that integrate lactation-associated food intake with neuroendocrine systems.

Sex differences in acute hormonal and subjective response to naltrexone: The impact of menstrual cycle phase

Women often exhibit larger hormonal and subjective responses to opioid receptor antagonists than men, but the biological mechanisms mediating this effect remain unclear. Among women, fluctuations in estradiol (E2) and progesterone (P4) across the menstrual cycle (MC) affect the endogenous opioid system. Therefore, the goal of the current study was to compare acute naltrexone response between women in the early follicular phase of the MC (low E2 and P4), women in the luteal phase of the MC (high E2 and P4), and men. Seventy healthy controls (n=46 women) participated in two morning sessions in which they received 50mg naltrexone or placebo in a randomized, counterbalanced order. Women were randomized to complete both sessions in either the early follicular (n=23) or luteal phase of the MC. Serum cortisol, salivary cortisol, prolactin, luteinizing hormone (LH), and subjective response were assessed upon arrival to the laboratory and at regular intervals after pill administration. In luteal and early follicular women but not men, naltrexone (vs. placebo) increased serum cortisol and prolactin levels from baseline; however, the naltrexone-induced increases in these hormones were significantly greater in luteal women than early follicular women. Additionally, only luteal women demonstrated an increase from baseline in salivary cortisol levels and the severity of adverse drug effects in response to naltrexone. In sum, the results indicate that luteal phase women are more sensitive to acute hormonal and subjective effects of naltrexone than early follicular women and men. These findings may have important implications for the use of naltrexone in women.

Identification of prolactin-sensitive GABA and kisspeptin neurons in regions of the rat hypothalamus involved in the control of fertility

High levels of circulating prolactin are known to cause infertility, but the precise mechanisms by which prolactin influences the neuroendocrine axis are yet to be determined. We used dual-label in situ hybridization to investigate whether prolactin-receptor (PRLR) mRNA is expressed in GnRH neurons. In addition, because γ-aminobutyric acidergic and kisspeptin neurons in the rostral hypothalamus are known to regulate GnRH neurons and, hence, might mediate the actions of prolactin, we investigated whether these neurons coexpress PRLR mRNA. (35)S-labeled RNA probes to detect PRLR mRNA were hybridized together with digoxigenin-labeled probes to detect either GnRH, Gad1/Gad2, or Kiss1 mRNA in the rostral hypothalamus of ovariectomized (OVX), estradiol-treated rats. Additional sets of serial sections were cut through the arcuate nucleus of OVX rats, without estradiol replacement, to examine coexpression of PRLR mRNA in the arcuate population of kisspeptin neurons. PRLR mRNA was highly expressed throughout the rostral preoptic area, particularly in periventricular regions surrounding the third ventricle, and there was a high degree of colocalization of PRLR mRNA in both Gad1/Gad2 and Kiss1 mRNA-containing cells (86 and 85.5%, respectively). In contrast, only a small number of GnRH neurons (<5%) was found to coexpress PRLR mRNA. In the arcuate nucleus of OVX rats, the majority of Kiss1 mRNA-containing cells also coexpressed PRLR mRNA. These data are consistent with the hypothesis that, in addition to a direct action on a small subpopulation of GnRH neurons, prolactin actions on GnRH neurons are predominantly mediated indirectly, through known afferent pathways.

Tuberoinfundibular peptide of 39 residues is activated during lactation and participates in the suckling-induced prolactin release in rat

Tuberoinfundibular peptide of 39 residues (TIP39) and the PTH-2 receptor (PTH2R) constitute a peptide-receptor neuromodulator system. Based on the abundance of TIP39 fibers and axonal terminals as well as PTH2R-containing neurons and their processes in the hypothalamic para- and periventricular and arcuate nuclei TIP39 has been suggested to play a role in neuroendocrine regulation. We showed previously that TIP39 expression decreased dramatically by adulthood. In the present study, using in situ hybridization histochemistry, real-time RT-PCR, and immunohistochemistry, we found that TIP39 mRNA and peptide expression levels are markedly elevated in the posterior intralaminar complex of the thalamus (PIL) of lactating dams, one of the three locations of TIP39-containing cell bodies in the brain. In addition, in mother rats, these TIP39 neurons showed Fos expression in response to pup exposure. Transection of TIP39 fibers originating in the PIL resulted in an ipsilateral disappearance of TIP39 immunoreactivity throughout the mediobasal hypothalamus of mother rats, suggesting that TIP39 fibers there arise from the PIL. To elucidate the function of TIP39 activation in dams, mothers separated from their pups for 4 h on postpartum d 9 received injection of a PTH2R antagonist into the lateral ventricle 5 min before returning the pups. Blood samples were taken seven times during the experimental period through jugular cannulae. The PTH2R antagonist administered in two different concentrations markedly inhibited suckling-induced elevation of plasma prolactin levels in a dose-dependent manner. These results suggest that TIP39 neurons in the PIL may regulate suckling-induced prolactin release in rat dams.

Kisspeptin regulates prolactin release through hypothalamic dopaminergic neurons

Prolactin (PRL) is tonically inhibited by dopamine (DA) released from neurons in the arcuate and periventricular nuclei. Kisspeptin plays a pivotal role in LH regulation. In rodents, kisspeptin neurons are found mostly in the anteroventral periventricular and arcuate nuclei, but the physiology of arcuate kisspeptin neurons is not completely understood. We investigated the role of kisspeptin in the control of hypothalamic DA and pituitary PRL secretion in adult rats. Intracerebroventricular kisspeptin-10 (Kp-10) elicited PRL release in a dose-dependent manner in estradiol (E2)-treated ovariectomized rats (OVX+E2), whereas no effect was found in oil-treated ovariectomized rats (OVX). Kp-10 increased PRL release in males and proestrous but not diestrous females. Associated with the increase in PRL release, intracerebroventricular Kp-10 reduced Fos-related antigen expression in tyrosine hydroxylase-immunoreactive (ir) neurons of arcuate and periventricular nuclei in OVX+E2 rats, with no effect in OVX rats. Kp-10 also decreased 3,4-dihydroxyphenylacetic acid concentration and 3,4-dihydroxyphenylacetic acid-DA ratio in the median eminence but not striatum in OVX+E2 rats. Double-label immunofluorescence combined with confocal microscopy revealed kisspeptin-ir fibers in close apposition to and in contact with tyrosine hydroxylase-ir perikarya in the arcuate. In addition, Kp-10 was not found to alter PRL release from anterior pituitary cell cultures regardless of E2 treatment. We provide herein evidence that kisspeptin regulates PRL release through inhibition of hypothalamic dopaminergic neurons, and that this mechanism is E2 dependent in females. These findings suggest a new role for central kisspeptin with possible implications for reproductive physiology.

A direct neuronal connection between the subparafascicular and ventrolateral arcuate nuclei in non-lactating female rats. Could this pathway play a role in the suckling-induced prolactin release?

The neuronal pathways, through which prolactin secretion is regulated during lactation, have still not been fully explored. Studies indicate that the suckling stimulus travels through the spinal cord, the brain stem, and then reaches the hypothalamus. The focus of this present experiment is to further explore the neuronal connections between the brain stem and the arcuate nucleus that may be involved in suckling-induced prolactin release. Ante- and retrograde tracing techniques were used. To chemically characterize the explored neurons neuropeptide immunohistochemistry was applied. Previous studies have indicated that the peripeduncular nucleus is a relay of the suckling stimulus in the midbrain, conveying the information to the hypothalamus. In our experiments, we have found an additional cell group in the subparafascicular parvocellular nucleus located just behind the posterior thalamus that projects to the arcuate neurons. The injection of the retrograde tracer into the ventrolateral part of the arcuate nucleus labeled cells in the lateral subdivision of the subparafascicular parvocellular nucleus. Anterograde tracing from the subparafascicular parvocellular nucleus resulted in fiber labeling in the arcuate nucleus in close apposition with dynorphin immunopositive neurons. Double labeling revealed that a subpopulations of the subparafascicular parvocellular neurons projecting to the arcuate nucleus contained tuberoinfundibular peptide of 39 residues or calcitonin gene-related peptide. The presented findings suggest that the ascending fibers from the subparafascicular parvocellular nucleus might be in the pathway involved in the suckling-induced prolactin release.

Pregnancy-induced adaptation in the neuroendocrine control of prolactin secretion

During pregnancy, neuroendocrine control of prolactin secretion is markedly altered to allow a state of hyperprolactinaemia to develop. Prolactin secretion is normally tightly regulated by a short-loop negative-feedback mechanism, whereby prolactin stimulates activity of tuberoinfundibular dopamine (TIDA) neurones to increase dopamine secretion into the pituitary portal blood. Dopamine inhibits prolactin secretion, thus reducing prolactin concentrations in the circulation back to the normal low level. Activation of this feedback secretion by placental lactogen during pregnancy maintains relatively low levels of prolactin secretion during early and mid-pregnancy. Despite the continued presence of placental lactogen, however, dopamine secretion from TIDA neurones is reduced during late pregnancy. Moreover, the neurones become completely unresponsive to endogenous or exogenous prolactin at this time, allowing a large nocturnal surge of prolactin to occur from the maternal pituitary gland during the night before parturition. In this review, we describe the changing patterns of prolactin secretion during pregnancy in the rat, and discuss the neuroendocrine mechanisms controlling these changes. The loss of response to prolactin is an important maternal adaptation to pregnancy, allowing the prolonged period of hyperprolactinaemia required for mammary gland development and function and for maternal behaviour immediately after parturition, and possibly also contributing to a range of other adaptive responses in the mother.

Sexual dimorphism in the organization of the rat hypothalamic infundibular area

The hypothalamic infundibular area is located outside the blood-brain barrier and includes, the ventromedial arcuate nucleus (vmARC) sensing circulating substances, and the median eminence (ME) where neurohormones are released into the hypothalamo-hypophysial vasculature. This integrated functional unit, pivotal in endocrine control, adjusts neuroendocrine output to feedback information. Despite a differing physiology in males and females, this functional unit has not appeared differently organized between sexes. Using immunocytochemistry, we describe here for the first time in adult rats, a conspicuous sex-difference in its axonal wiring by intrinsic glutamatergic neurons containing the neuropeptides neurokinin B (NKB) and dynorphin. In the male, NKB neurons send axons to capillary vessels of the vmARC and of the ME (only where gonadotropin-releasing hormone (GnRH) axons terminate). Electron microscopy revealed that NKB axons target the barrier of tanycytes around fenestrated capillary vessels (in addition to GnRH axons), suggesting a control of regional bidirectional permeability. In the female, NKB neurons send axons to the neuropile of the vmARC, suggesting a direct control of its sensor neurons. The other projections of NKB neurons, studied by surgical isolation of the ARC-ME complex and confocal microscopy, are not sexually dimorphic and target both integrative and neuroendocrine centers controlling reproduction and metabolism, suggesting a broad influence over endocrine function. These observations demonstrate that the mechanisms subserving hypothalamic permeability and sensitivity to feedback information are sexually dimorphic, making the infundibular area a privileged site of generation of the male-to-female differences in the adult pattern of pulsatile hormonal secretions.

Morphologic evidence that neurokinin B modulates gonadotropin-releasing hormone secretion via neurokinin 3 receptors in the rat median eminence

Recent studies suggest that arcuate neurokinin B (NKB) neurons play a role in the regulation of gonadotropin secretion, but there is little information on the relationship between these neurons and the hypothalamic reproductive axis. In the present study, dual-label fluorescent immunohistochemistry was used to visualize the relationship between gonadotropin-releasing hormone (GnRH) neurons and either proNKB or NK3 receptor (NK3R) immunoreactivity. Immunocytochemistry was also combined with i.p. injections of the fluorescent retrograde tracer aminostilbamidine to determine whether arcuate neuroendocrine neurons expressed either proNKB or NK3R. A dense interweaving and close apposition of GnRH and proNKB-immunoreactive (ir) fibers was observed within the rat median eminence, where GnRH axons expressed NK3R immunoreactivity. These data provide morphological evidence that NKB neurons could influence GnRH secretion via interaction with NK3R in the rat median eminence. Colocalization of GnRH and NK3R was also identified in fiber tracts converging within the organum vasculosum of the lamina terminalis. In contrast, only a small number (16%) of GnRH-ir somata exhibited NK3R staining. ProNKB and NK3R-ir somata were identified within the arcuate nucleus, but none of these neurons were labeled by aminostilbamidine. Thus, we found no evidence that arcuate NKB neurons project to the primary capillary plexus of the portal system. Arcuate neuroendocrine neurons, however, were surrounded and closely apposed by proNKB-ir puncta and fibers. These data suggest that NKB neurons could indirectly influence anterior pituitary function by inputs to arcuate neuroendocrine neurons, but through a receptor other than NK3R. Our results provide an anatomic framework for putative interactions between NKB neurons and the hypothalamic reproductive axis.

From galactorrhea to osteopenia: rethinking serotonin-prolactin interactions

The widespread use of the selective serotonin reuptake inhibitors (SSRIs) has been accompanied by numerous reports describing a potential association with hyperprolactinemia. Antipsychotics are commonly known to elevate serum prolactin (PRL) through blockade of dopamine receptors in the pituitary. However, there is little awareness of the mechanisms by which SSRIs stimulate PRL release. Hyperprolactinemia may result in overt symptoms such as galactorrhea, which may be accompanied by impaired fertility. Long-term clinical sequelae include decreased bone density and the possibility of an increased risk of breast cancer. Through literature review, we explore the possible pathways involved in serotonin-induced PRL release. While the classic mechanism of antipsychotic-induced hyperprolactinemia directly involves dopamine cells in the tuberoinfundibular pathway, SSRIs may act on this system indirectly through GABAergic neurons. Alternate pathways involve serotonin stimulation of vasoactive intestinal peptide (VIP) and oxytocin (OT) release. We conclude with a comprehensive review of clinical sequelae associated with hyperprolactinemia, and the potential role of SSRIs in this phenomenon.

Opioid receptor subtypes involved in the regulation of prolactin secretion during pregnancy and lactation

Afferent endogenous opioid neuronal systems facilitate prolactin secretion in a number of physiological conditions including pregnancy and lactation, by decreasing tuberoinfundibular dopamine (TIDA) inhibitory tone. The aim of this study was to investigate the opioid receptor subtypes involved in regulating TIDA neuronal activity and therefore facilitating prolactin secretion during early pregnancy, late pregnancy and lactation in rats. Selective opioid receptor antagonists nor-binaltorphimine (kappa-receptor antagonist, 15 micro g/5 micro l), beta funaltrexamine (mu-receptor antagonist, 5 microg/5 microl) and naltrindole (delta-receptor antagonist, 5 microg/5 microl) or saline were administered intracerebroventricularly (i.c.v.) on day 8 of pregnancy during a nocturnal prolactin surge, on day 21 of pregnancy during the ante partum prolactin surge or on day 7 of lactation before the onset of a suckling stimulus. Serial blood samples were collected at regular time intervals, via chronic indwelling jugular cannulae, before and after drug administration and plasma prolactin was determined by radioimmunoassay. TIDA neuronal activity was measured using the 3,4-dihydroxyphenylacetic acid (DOPAC) : dopamine ratio in the median eminence 2 h 30 min after i.c.v. drug injection. In each experimental condition, plasma prolactin was significantly inhibited by both kappa- and mu-receptor antagonists, whereas the delta-receptor antagonist had no effect compared to saline-injected controls. Similarly, nor-binaltorphimine and beta funaltrexamine significantly increased the median eminence DOPAC : dopamine ratio during early and late pregnancy, and lactation whereas naltrindole had no effect compared to saline-injected controls. These data suggest that TIDA neuronal activity, and subsequent prolactin secretion, is regulated by endogenous opioid peptides acting at both kappa- and mu-opioid receptors during prolactin surges of early pregnancy, late pregnancy and lactation.

Behavioral and neuroendocrine actions of the Met-enkephalin-related peptide MERF

The effects and the mediation of the action of the proenkephalin derivative Met(5)-enkephalin-Arg(6)-Phe(7) (MERF) on the hypothalamo-pituitary-adrenal (HPA) system and open-field behavior were investigated in mice. Intracerebroventricular injection of the heptapeptide increased square crossing, rearing, and plasma corticosterone level. To characterize the receptors involved in these neuroendocrine processes, animals were pretreated either with the nonselective opioid antagonist naloxone or the kappa-antagonist nor-binaltorphimine (nor-BNI). Both antagonists dose-dependently attenuated the HPA activation elicited by MERF. Naloxone also blocked the behavioral responses, but nor-binaltorphimine did not elicit a significant inhibition. The dopamine antagonist haloperidol and a corticotropin-releasing hormone (CRH) antagonist were also preadministered to shed light on the transmission of the actions of MERF. Both the motor responses and the HPA activation were diminished by the preadministration of the CRH antagonist, while haloperidol attenuated only square crossing and rearing. To investigate the direct effect of MERF on the dopaminergic system, dopamine release of striatal slices was measured in a superfusion system. Neither the basal nor the electric impulse-evoked dopamine release was modified by MERF. The results suggest that opioid-mediation predominate in the neuroendocrine actions of MERF, and the effect of the heptapeptide on the HPA system seems to be mediated by kappa-receptors. In the behavioral responses evoked by MERF, both CRH release and the action of the dopaminergic neurons of the subcortical motor system might be involved. MERF also appears to activate the paraventricular CRH neurons, but dopaminergic transmission does not seem to play a significant role in its hypothalamic action.

Opioid control of prolactin secretion in late pregnant rats is mediated by tuberoinfundibular dopamine neurons

Prolactin (PRL) secretion from the anterior pituitary is tonically inhibited by tuberoinfundibular dopamine (TIDA) neurons in the arcuate nucleus of the hypothalamus. During late pregnancy, TIDA neuronal activity is reduced allowing the expression of an antepartum PRL surge. We show here that continuous infusion of the opioid receptor antagonist naloxone (10 mg/h) during the night preceding parturition completely abolished the antepartum PRL surge and significantly increased TIDA neuronal activity. These data indicate that endogenous opioid neurons facilitate PRL secretion at the end of pregnancy by suppressing TIDA neuronal activity.

The effects of heroin on prolactin levels in male rhesus monkeys: use of cumulative-dosing procedures

There is considerable evidence that mu opioid receptors are involved in the regulation of anterior pituitary function. For example, in nonhuman primates and humans, mu agonists generally increase prolactin (PRL) levels. In contrast, mu antagonists decrease or have no effect on PRL levels. The goal of this study was to assess the potential utility of cumulative-dosing procedures to evaluate the endocrine effects of mu opioid receptor ligands. The effects of single and multiple, cumulative doses of the mu agonist heroin and the mu-selective antagonist quadazocine on PRL levels were investigated in four male rhesus monkeys. Cumulative dose-response curves were determined by infusing increasing drug doses at 60 min intervals over 290 min. Blood samples for PRL analysis were collected at 25 and 50 min after each cumulative infusion. Samples were collected at similar time points following single drug dose administration. Heroin (0.01-0.32 mg/kg, IV) administration dose-dependently increased PRL levels. Maximum levels of heroin-induced PRL levels were equivalent after single and cumulative doses. Quadazocine alone (0.032-1.0 mg/kg, IM) did not alter PRL levels significantly. However, quadazocine (0.1 mg/kg, IM) antagonized heroin-stimulated increases in PRL levels and produced a significant rightward shift in the heroin dose-effect curve. These data suggest that a cumulative-dosing procedure similar to that used in behavioral pharmacology may be useful to study the endocrine pharmacology of mu opioids in rhesus monkeys.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Involvement of endogenous opioidergic neurons in modulation of prolactin secretion in response to mating in the female rat

Mating in female rats induces an acute prolactin (PRL) release within 60 min and twice-daily surges of PRL throughout the first 10 days of pregnancy to maintain luteal function. Little is known about the brain mechanism whereby the vaginocervical stimulation is processed to induce PRL release. Our recent results revealed an increase in Fos expression in the arcuate nucleus (ARC) following mating in the intact estrous rat, suggesting that a neuronal network in the brain area may participate in conveying and integrating the genitosensory stimulation. To further investigate the phenotype of activated neurons in the ARC, the present study examined whether beta-endorphin (beta-END) and/or dopamine (DA) neurons are activated by mating, and if so, whether activation is involved in the mating-induced acute release of PRL and the establishment of the twice-daily surges of PRL. In experiment 1, proestrous rats receiving intromissions (mated group) from males or mounts without intromission (mounted group) were sacrificed along with rats taken directly from their home cage (control group) 60 min after the beginning of mating or mounting. Expression of Fos in beta-END neurons and expression of fos-related antigen (FRA) in DA neurons, which were labeled by tyrosine hydroxylase (TH) antibody in the ARC were examined by double-label immunocytochemistry. In experiment 2, proestrous females with indwelling atrial catheters were mated with males. Naloxone (10 microl/min, 2 mg/10 min), an opiate antagonist, or saline was infused before, during and after mating. Blood samples were collected during the mating session and also at several times 3 days after mating. The results showed that mating induced a significant increase in the percentage of beta-END/Fos colabeled neurons and a significant decrease in the number of beta-END cells in all subdivisions of the ARC. In contrast, neither the percentage of FRA/TH colabeled cells nor the number of TH cells was influenced by mating. Mating induced an acute increase in PRL release in saline-treated control animals within 30 min and a subsequent diurnal surge (18.00 h) and a nocturnal surge of PRL (2.00 h) 3 days after mating. Naloxone infusion during mating blocked the mating-induced acute PRL response and the diurnal surge of PRL 3 days after mating, but affected neither the nocturnal surge of PRL nor the incidence of pregnancy. These results demonstrate that (1) beta-END neurons but not DA neurons in the ARC are activated in response to mating in proestrous rats, and (2) the mating-induced activation of beta-END neurons may participate in the acute response of PRL release to mating and the memory mechanism for the establishment of the diurnal PRL surge, but not the nocturnal PRL surge in early pregnancy. These results lead to a conclusion that endogenous opioid peptides may be involved in the neuronal transmission of genitosensory stimulation to induce PRL secretion.

Immunoneutralization of endogenous opioid peptides prevents the suckling-induced prolactin increase and the inhibition of tuberoinfundibular dopaminergic neurons

Previous studies have shown that the endogenous opioid peptides, acting at specific opiate receptor subtypes, are involved in the suckling-induced prolactin secretory response. The prolactin increase elicited by suckling is due, at least in part, to an inhibition of tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus. We investigated the effects of immunoneutralization of dynorphin, leu-enkephalin and met-enkephalin on the suckling-induced prolactin increase and on the activity of the TIDA neurons in lactating female rats between days 7 and 12 postpartum. Rats were injected into the right lateral ventricle with antiserum specific for one of these three peptides. Control rats were administered equal amounts of immunoglobulin proteins. Suckling produced a profound and significant increase in prolactin levels, as well as a decrease in DOPA accumulation in the median eminence of lactating rats. Administration of immunoglobulin concentrations of up to 3.6 microg did not inhibit the prolactin secretory response to the suckling stimulus and did not prevent the suckling-induced inhibition of TIDA neurons. Antisera to all three endogenous opioid peptides abolished the suckling-induced prolactin increase and prevented the inhibition in DOPA accumulation in the median eminence. Thus, the endogenous opioid peptides, dynorphin, leu-enkephalin and met-enkephalin, are essential for the prolactin secretory response to suckling and inhibition of TIDA neuronal activity is at least one of the mechanisms of action utilized by these peptides.

An opioid pathway in the hypothalamus of the turkey that stimulates prolactin secretion

Circulating prolactin (PRL) levels increase when dynorphin is infused into the turkey brain. This study tested the hypothesis that centrally infused dynorphin requires an intact vasoactive intestinal peptide (VIP) system in order to stimulate turkey PRL secretion. It also investigated the roles of the dopaminergic and serotonergic systems in dynorphin-induced PRL release. Drugs were infused into the third ventricle of anesthetized laying turkeys via stereotaxically guided cannulae and circulating blood was assayed for changes in PRL levels. When a highly selective kappa opioid receptor antagonist was given prior to dynorphin injection, the PRL response to dynorphin was almost totally blocked. The coinfusion of either a serotonin (5-HT) or a D(1) dopamine (DA) receptor antagonist with dynorphin prevented the increase in PRL observed in birds when dynorphin was infused alone. On the other hand, the kappa opioid receptor antagonist failed to prevent the 5-HT-induced release of PRL. In hens actively immunized against VIP, infused dynorphin was unable to increase plasma PRL levels and infused VIP gave a muted PRL rise, while large increases in PRL were seen in nonimmunized birds receiving the same infusions. These data show that: (1) dynorphin stimulates PRL secretion by activating kappa opioid receptors in the avian hypothalamus, and (2) dynorphin, 5-HT, DA, and VIP stimulate avian PRL secretion via a common pathway expressing kappa opioid, serotonergic, dopaminergic, and VIPergic receptors at synapses arranged serially in that functional order, with the VIPergic system as the final mediator (releasing factor).

Periventricular preoptic area neurons coactivated with luteinizing hormone (LH)-releasing hormone (LHRH) neurons at the time of the LH surge are LHRH afferents

Earlier studies demonstrated coactivation of the periventricular preoptic area (pePOA) with LHRH neurons at the time of an induced or spontaneous LH surge, suggesting that the pePOA might regulate LHRH neurons. To investigate this hypothesis, studies were conducted to determine the temporal pattern of pePOA Fos activation during the rat estrous cycle and establish the connections of the pePOA neurons with LHRH neurons. Fos activation within LHRH and pePOA neurons showed the same temporal pattern. Both were absent during diestrous I, diestrous II, and the morning of proestrus. Fos was induced in the pePOA and LHRH neurons beginning on the afternoon of proestrus (4 h before lights off), with a decline 8 h later on proestrous evening. Tract-tracing studies then established the relationship between LHRH and pePOA neurons. Retrograde labeling with fluorogold determined that a portion of the Fos-positive pePOA neurons present at the time of the LH surge sent a projection to regions that contain LHRH cells. Anterograde tracer (neurobiotin) injections established that the pePOA neurons sent axons to the LHRH cells. Taken together, these data indicate that the pePOA provides direct input to LHRH neurons that is likely to stimulate LHRH neurons at the time of the LH surge.

Neuropeptide Y and tuberoinfundibular dopamine activities are altered during lactation: role of prolactin

During lactation the suckling stimulus increases the activity of two populations of neuropeptide Y (NPY) neurons in the hypothalamus, the caudal portion of the arcuate nucleus (ARH) and the dorsomedial hypothalamus (DMH), and suppresses the activity of TIDA neurons in the ARH. In the present study, an acute resuckling model was used to examine the role of suckling-induced hyperprolactinemia in modulating the activity of these systems. Lactating rats were deprived of their eight-pup litters on day 9 postpartum, and 48 h later, the animals served either as nonsuckled controls (0 pups) or were suckled for 24 h. In addition, some of the resuckled animals received two s.c. injections of bromocriptine (0.5 mg/rat x injection), a dopamine D2 agonist, to inhibit suckling-induced PRL secretion. In situ hybridization was performed for rat NPY messenger RNA (mRNA) and tyrosine hydroxylase (TH) mRNA to provide an index for NPY and TIDA neuronal activities, respectively. Resuckling for 24 h induced a significant increase in NPY mRNA levels in the caudal portion of the ARH and in the DMH. Bromocriptine treatment did not alter the increase in NPY mRNA levels in the ARH, whereas the treatment greatly attenuated the increase in NPY mRNA in the DMH. TH mRNA levels in the rostral ARH area returned to basal levels in the nonsuckled control animals, and 24 h of resuckling significantly suppressed TH mRNA expression in this area. Bromocriptine treatment caused a significant increase in TH mRNA levels compared with those in the eight-pup suckled group. Thus, the results from the present study demonstrate that the suckling stimulus activated the two populations of NPY neurons and suppressed TIDA activity. Suckling-induced hyperprolactinemia did not participate in the increase in ARH NPY activity, whereas it played a major stimulatory role in suckling-induced activation of NPY neurons in the DMH and an inhibitory role in suckling-induced suppression of TIDA activity. The increase in TIDA activity after bromocriptine treatment was unexpected and suggests that the role of PRL in the regulation of TIDA activity is significantly altered during lactation.

Endogenous opioid peptides contribute to suckling-induced prolactin release by suppressing tyrosine hydroxylase activity and messenger ribonucleic acid levels in tuberoinfundibular dopaminergic neurons

The endogenous opioid peptides have been implicated in the control of the suckling-induced PRL rise during lactation. This study examined the role of the endogenous opioid peptides in suppressing tuberoinfundibular dopaminergic neuronal activity during lactation. In the first experiment, lactating rats were constantly exposed to pups. Naloxone (NAL; 60 mg/kg x h; i.v.), an opioid receptor antagonist, or saline was infused for 12 h. Blood was collected before and at 2-h intervals during the infusion. NAL suppressed circulating PRL levels to less than 36% of control values at 4, 6, 8, and 12 h after the onset of the infusion. Tyrosine hydroxylase (TH) activity in the stalk-median eminence and TH messenger RNA signal levels in the arcuate nucleus were determined at the end of the NAL infusion. TH activity and TH messenger RNA signal levels were increased 2.5- and 2.7-fold, respectively, after the 12-h NAL infusion. Even though the time spent with their pups was similar between the two groups, the pups in the NAL-treated group failed to gain weight during the 12-h NAL infusion period, whereas the control litters (8 pups) gained 5 g. In a second experiment, pups were removed from the dams before the 12-h NAL infusion and were returned after 11 h. Blood was collected before the infusion, at 3-h intervals during the pup separation period, and at 15-min intervals after reunion with the pups. Plasma PRL in control and NAL-treated rats was low (1-15 ng/ml) and similar during the separation period. The suckling-induced PRL surge in NAL-treated rats was markedly attenuated to 9-25% of control levels (350-650 ng/ml). After a 1-h suckling episode, TH activity in the stalk-median eminence of NAL-treated rats was 4.5-fold greater than controls. Litter weight gains were significantly less in NAL-treated rats during the 1-h suckling episode. These data indicate that the endogenous opioid peptides are an integral component for increasing PRL release in response to suckling and they act to decrease tuberoinfundibular dopaminergic neuronal activity during lactation, in part, by suppressing TH gene expression.

Nicotinic control of tuberoinfundibular dopaminergic neuron activity and prolactin secretion: diurnal rhythm and involvement of endogenous opioidergic system

The possible involvement of cholinergic and opioidergic neurons in the control of diurnal changes of tuberoinfundibular dopaminergic (TIDA) neuronal activity was reported. Adult Sprague-Dawley rats ovariectomized and treated with estrogen were used. All drugs were administered centrally through preimplanted intracerebroventricular cannula, and both TIDA neuronal activity and serum prolactin level were determined. Nicotine (10 ng/3 microl/rat) given at 10:00 h significantly inhibited TIDA neuronal activity from 5 to 30 min and stimulated serum PRL levels at 5 and 15 min. Co-administration of either mecamylamine (1 microg) or naloxone (2.5 microg) prevented both nicotine's effects. A dose-related (0.1-100 ng) effect of nicotine on TIDA neuronal activity and serum PRL level was also observed in the morning when TIDA neuronal activity is high and serum PRL level is low, but not in the afternoon when the former activity is low and the latter is high. When atropine (20 microg), naloxone (25 microg) or Nor-BNI (20 microg) was given at 14:00 h all increased the lowered TIDA neuronal activity in the afternoon. When atropine was co-administered with either naloxone or Nor-BNI, however, no additive effect was observed. Submaximal doses of atropine (0.2 microg), mecamylamine (0.1 microg) or naloxone (0.25 microg) was also effective in stimulating the afternoon levels of TIDA neuronal activity and inhibiting serum PRL, and no additive effect was observed either. Moreover, simultaneous injection of morphine (15 microg) prevented atropine's effect in the afternoon. These results indicate that cholinergic neurons may act through activating the endogenous opioidergic neurons to exhibit an inhibitory effect on TIDA neuronal activity and a stimulatory one on prolactin secretion. A diurnal difference in its endogenous activity between morning and afternoon was also implicated.

Cold exposure attenuates effects of secretagogues on serum prolactin and growth hormone levels in male rats

The stimulatory effect of morphine, dexmedetomidine (an alpha 2-adrenoceptor agonist), 1-(3-chlorophenyl)-piperazine (m-CPP, a 5-HT1B agonist), U-50488H (a kappa-opioid receptor agonist), pimozide (a dopamine antagonist), and restraint stress on prolactin and growth hormone (GH) secretion was compared during cold exposure (4 degrees C) and under basal conditions (30 degrees C) in male rats. Rectal temperature was also measured. The stimulatory effect of morphine, dexmedetomidine, m-CPP, and partially U-50488H on prolactin secretion was attenuated in rats kept at 4 degrees C. Cold exposure did not abolish prolactin release induced by pimozide and restraint stress. Cold exposure also antagonized the effect of morphine and dexmedetomidine on GH secretion. The stimulatory effect of morphine on prolactin and GH secretion was restored in the warm environment despite the sustained hypothermia. Cold exposure blocked the stimulatory effect of morphine on prolactin secretion in rats that were tolerant to the hypothermic effect of morphine. Thus hypothermia caused by morphine, dexmedetomidine, and m-CPP during cold exposure is not the sole factor in the antagonistic effect of cold. We suggest that cold exposure releases some compound(s) modulating hypothalamic neural pathways.

Neurochemical evidence that AMPA receptor-mediated tonic inhibition of hypothalamic dopaminergic neurons occurs via activation of inhibitory interneurons

Blockade of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors with 6-nitro-7-sulfamoyl-benzo-[f]quinoxaline-2,3(1H,4H)-dione (NBQX) activates tuberoinfundibular (TIDA) and periventricular-hypophysial dopaminergic (PHDA) neurons suggesting that these neurons are tonically inhibited by endogenous excitatory amino acid neurotransmitters acting at AMPA receptors. The purpose of the present study was to identify prospective inhibitory neurotransmitters mediating this effect by examining the ability of gamma-aminobutyric acid (GABA)A and kappa-opioid receptor agonists to reverse the stimulatory effects of NBQX on the activity of these neurons (estimated from concentrations of 3,4-dihydroxyphenylacetic acid [DOPAC] in the median eminence and intermediate lobe of the pituitary). The GABAA receptor agonist isoguvacine prevented the NBQX-induced increase in DOPAC concentrations in the median eminence but not in the intermediate lobe. The kappa-opioid receptor agonist U-50,488 had no effect in the median eminence, but attenuated the NBQX-induced increase of DOPAC concentrations in the intermediate lobe. These results suggest that excitatory amino acid neurotransmitters activate AMPA receptors and increase release of GABA, which by acting at GABAA receptors tonically inhibits TIDA neurons. On the other hand, AMPA receptor-mediated tonic inhibition of PHDA neurons occurs, at least in part, by a mechanism involving endogenous kappa-opioids.

Evidence that amplification of norepinephrine-induced LH release by morphine is indirectly due to suppression of tuberoinfundibular dopamine secretion

We previously observed that morphine markedly amplifies LH secretion following intracerebroventricular (i.c.v.) norepinephrine (NE) infusions. Based on additional evidence, we hypothesized that perhaps these morphine effects were due to suppression of tuberoinfundibular dopamine (TIDA) secretion thus allowing NE to evoke a greater release of LHRH from axon terminals in the median eminence than would otherwise occur. In the present studies, we examined whether apomorphine (a DA receptor agonist) would suppress and haloperidol (a DA receptor antagonist) would mimic these enhancing effects of morphine on NE-induced LH secretion. Estrogen-treated ovariectomized rats were used in these studies. NE, when infused i.c.v. (45 micrograms) evoked a modest increase in plasma LH (1.1 +/- 0.2 to 2.2 +/- 0.2 ng/ml) within 15 min. When morphine sulfate (10 mg/kg s.c.) was given 15 min prior to NE, LH peak values of 11 +/- 2 ng/ml were obtained by 60 min. Treatment of rats with apomorphine (1.5 mg/kg s.c.) at -15 min, morphine at 0 min and i.c.v. NE at 15 min resulted in a significant blunting of morphine's effect on NE-induced LH release. Moreover, in all morphine-treated rats, plasma prolactin (PRL) increased significantly within 10 min, peaked at 30 min and declined towards basal values by 90 min. Apomorphine completely blocked this morphine effect of PRL release. Haloperidol (HAL; 2.5 mg/kg s.c.) treatment had no effect on basal LH release but resulted in a significant increase in PRL which remained elevated up to 180 min.(ABSTRACT TRUNCATED AT 250 WORDS)