Beta-endorphin is a potent inhibitor of thymidine incorporation into DNA via mu- and kappa-opioid receptors in fetal rat brain cell aggregates in culture

CRF and stress in fish

The endocrine stress response is pivotal in vertebrate physiology. The stress hormone cortisol-the end product of the endocrine stress axis-(re-)directs energy flows for optimal performance under conditions where homeostasis may be or become at risk. Key players in the continuous adaptation process are corticotropin-releasing factor (CRF) from the hypothalamic nucleus preopticus (NPO), pituitary adrenocorticotropic hormone (ACTH) and cortisol produced by the interrenal cells in the headkidney (adrenal equivalent of fish). CRF is a member of a large family of related peptides that signals through CRF-receptor subtypes specific for central and peripheral actions of the peptide. CRF is "chaperoned" by a unique and phylogenetically very well-conserved binding protein (CRFBP); the functions of the CRFBP can only be speculated on so far, but its mRNA and protein abundance are important indicators of the central CRF-system activity, and indeed its mRNA levels are altered by restraint stress. Moreover, the unique structure and size of the CRFBP provide good tools in phylogenetic studies, that date the CRF-system to at least one billion years old. Pro-opiomelanocortin is produced and processed to ACTH and endorphin in the hypothalamic NPO and pituitary pars distalis ACTH-cells, to MSH and acetylated endorphins in the pituitary pars intermedia MSH-cells. ACTH is the prime corticotrope in acute stress conditions. In carp, MSH, considered a mild corticotrope in chronic stress responses in other fish, lacks corticotropic effects (in line with the absence of the melanocortin-5 receptor in headkidney); yet, an unknown corticotropic signal substance in the pars intermedia of carp awaits elucidation. Interesting observations were made on the CRF control of pituitary cells. CRF stimulates ACTH-cells, but only when these cells experience a mild dopaminergic block. Endorphin, produced in the NPO and transported via axons to the pituitary gland in vivo, reverses the stimulatory CRF action on MSH-cells to a differential inhibition of N-acetyl beta-endorphin release in vitro (MSH release is not affected). We speculate that the consistently observed elevation of plasma MSH during chronic stress may exert central actions related to feeding and leptin regulated processes. A BOLD-fMRI study revealed the functional anatomy of the stress response at work in a paradigm, where carp were exposed to a sudden water temperature drop. In carp (and other fish), the endocrine stress axis is already operational in very early life stages, viz., around hatching and comprises hypothalamic, pituitary, and interrenal signaling to adjust the physiology of the hatchling to its dynamically changing environment. Understanding of stress during early life stages is critical as the consequent rises in cortisol may have long lasting effects on survival and fish quality.

delta-Opioid agonist induced regulation of E2F1 DNA binding activity in NG108-15 cells

Activation of opioid receptors have been implicated in the modulation of cell proliferation and the E2F family of transcription factors may play a role in opioid inhibition of DNA synthesis. Gel shift assays and Western blotting of nuclear extracts from NG108-15 cells revealed increased E2F1 DNA binding activity and higher levels of E2F1 following activation of delta-opioid receptors. It is suggested that DADLE-induced regulation of E2F DNA binding activity involves ERKs.

Selective vulnerability of cerebellar granule neuroblasts and their progeny to drugs with abuse liability

Cerebellar development is shaped by the interplay of genetic and numerous environmental factors. Recent evidence suggests that cerebellar maturation is acutely sensitive to substances with abuse liability including alcohol, opioids, and nicotine. Assuming substance abuse disrupts cerebellar maturation, a central question is: what are the basic mechanisms underlying potential drug-induced developmental defects? Evidence reviewed herein suggests that the maturation of granule neurons and their progeny are intrinsically affected by several classes of substances with abuse liability. Although drug abuse is also likely to target directly other cerebellar neuron and glial types, such as Purkinje cells and Bergmann glia, findings in isolated granule neurons suggest that they are often the principle target for drug actions. Developmental events that are selectively disrupted by drug abuse in granule neurons and/or their neuroblast precursors include proliferation, migration, differentiation (including neurite elaboration and synapse formation), and programmed cell death. Moreover, different classes of drugs act through distinct molecular mechanisms thereby disrupting unique aspects of development. For example, drug-induced perturbations in: (i) neurotransmitter biogenesis; (ii) ligand and ion-gated receptor function and their coupling to intracellular effectors; (iii) neurotrophic factor biogenesis and signaling; and (iv) intercellular adhesion are all likely to have significant effects in shaping developmental outcome. In addition to identifying therapeutic strategies for drug abuse intervention, understanding the mechanisms by which drugs affect cellular maturation is likely to provide a better understanding of the neurochemical events that normally shape central nervous system development.

Chronic exposure to the opioid growth factor, [Met5]-enkephalin, during pregnancy: maternal and preweaning effects

The opioid peptide, [Met(5)]-enkephalin (termed opioid growth factor, OGF), is an autocrine growth factor that serves as a constitutively active inhibitory agent. OGF crosses the placenta and depresses DNA synthesis in the fetus. The role of OGF in pregnancy and parturition, and the influence exerted on prenatal and neonatal features of the offspring, were studied in rats. Females received daily injections of 10 mg/kg OGF throughout gestation; all offspring were cross-fostered to lactating noninjected dams at birth. No effects on the length of gestation, course of pregnancy, behavior of the pregnant dam, maternal weight gain, or food and water intake throughout gestation were recorded in OGF-treated mothers. Moreover, nociceptive response in these females was not altered by chronic OGF exposure, and no signs of physical dependence or withdrawal could be observed following a challenge by the opioid antagonist naloxone. Litter size and the number of live births per litter of OGF-treated mothers were reduced by 25% from control subjects and a fourfold increase in stillborns was noted for mothers receiving OGF compared to control levels. Histopathologic analysis confirmed the stillborns to have died in utero. OGF-exposed neonates were normal in body weight and crown-to-rump length, but these pups were observed to be lethargic and cyanotic, and had subnormal weights of many organs. Body weights of 10-, 15-, and 21-day-old OGF-exposed rats were reduced 11-27% from control levels. Wet and dry organ weights of the rats maternally subjected to OGF were decreased from control values in six of the eight organs evaluated at 10 days. At weaning, some organs were subnormal in weight. These data lead us to hypothesize that a native opioid peptide-OGF-is integral to certain aspects of maternal, neonatal, and postnatal well-being, and that disruptions in this opioid peptide have serious repercussions on the course of pregnancy and fetal outcome.

Opioid system diversity in developing neurons, astroglia, and oligodendroglia in the subventricular zone and striatum: impact on gliogenesis in vivo

Accumulating evidence, obtained largely in vitro, indicates that opioids regulate the genesis of neurons and glia and their precursors in the nervous system. Despite this evidence, few studies have assessed opioid receptor expression in identified cells within germinal zones or examined opioid effects on gliogenesis in vivo. To address this question, the role of opioids was explored in the subventricular zone (SVZ) and/or striatum of 2-5-day-old and/or adult ICR mice. The results showed that subpopulations of neurons, astrocytes, and oligodendrocytes in the SVZ and striatum differentially express mu-, delta-, and/or kappa-receptor immunoreactivity in a cell type-specific and developmentally regulated manner. In addition, DNA synthesis was assessed by examining 5-bromo-2'-deoxyuridine (BrdU) incorporation into glial and nonglial precursors. Morphine (a preferential mu-agonist) significantly decreased the number of BrdU-labeled GFAP(+) cells compared with controls or mice co-treated with naltrexone plus morphine. Alternatively, in S100beta(+) cells, morphine did not significantly decrease BrdU incorporation; however, significant differences were noted between mice treated with morphine and those treated with morphine plus naltrexone. Most cells were GFAP(-)/S100beta(-). When BrdU incorporation was assessed within the total population (glia and nonglia), morphine had no net effect, but naltrexone alone markedly increased BrdU incorporation. This finding suggests that DNA synthesis in GFAP(-)/S100beta(-) cells is tonically suppressed by endogenous opioids. Assuming that S100beta and GFAP, respectively, distinguish among younger and older astroglia, this implies that astroglial replication becomes increasingly sensitive to morphine during maturation, and suggests that opioids differentially regulate the development of distinct subpopulations of glia and glial precursors.

Transplacental transfer of the opioid growth factor, [Met(5)]-enkephalin, in rats

Placental transfer of the pentapeptide [Met5]-enkephalin, known to function as a growth regulating factor and neuromodulatory agent, was studied in pregnant Sprague-Dawley rats. Using separation by reversed phase high-performance liquid chromatography, and analysis by derivative spectroscopy, [Met5]-enkephalin was detected in 20-day-old fetal tissue including brain, heart, lung, and kidney. Fetal tissues from pregnant rats given an injection of 40 mg/kg [Met5]-enkephalin on gestation day 20 had markedly elevated levels of peptide within 1 h, indicating the transplacental transfer of this opioid. [Met5]-enkephalin levels were increased from control samples at 1, 2, 4, and 14 h post-injection of peptide, but not at 24 h. Evaluation of breakdown products of [Met5]-enkephalin, along with the related peptide [Leu5]-enkephalin, revealed that elution times differed substantially from [Met5]-enkephalin. These data indicate that [Met5]-enkephalin is present in fetal organs, crosses the placenta, does not appear to be restrictive in organ specificity, and is sustained in fetal tissues at detectable levels for at least 14 h. Given that [Met5]-enkephalin tonically inhibits DNA synthesis in the fetus, these results raise the question of whether an elevated level of this peptide (either maternally or from the fetus) may be detrimental to cellular ontogeny in the fetus, and perhaps have long-term implications for postnatal development.

Regulation of mouse kappa opioid receptor gene expression by retinoids

The effect of retinoids on the expression of kappa opioid receptor (KOR) gene was examined in normal and transgenic animals. KOR-lacZ transgene expression was specifically elevated in KOR-positive areas of the developing CNS by depleting vitamin A from animal diets. The endogenous KOR mRNA species, including all three isoforms, were also upregulated by depleting vitamin A in developing animals. Change in the expression of isoforms a and b is similar in prenatal stages but differs during postnatal development. Interestingly, upregulation of isoform c is most significant postnatally. The regulation of KOR gene by vitamin A was substantiated in a mouse embryonal carcinoma P19 culture system in which retinoic acid (RA), the most potent ingredient of vitamin A, was able to suppress the expression of all the three KOR isoforms and KOR protein. The RA-mediated suppression was blocked by an RA receptor antagonist and a histone deacetylase (HDAC) inhibitor. By using a reporter transfection assay in P19 cells, the potential genetic element responsible for RA-mediated suppression of KOR gene expression was located to intron 1 of the mouse KOR gene, which could also be blocked by HDAC inhibitor. Furthermore, suppression of KOR gene expression by RA in P19 cells appeared to be an indirect event and required protein synthesis. A role of RA in KOR gene regulation during developmental stages was discussed.

Regulation of mouse kappa opioid receptor gene expression by retinoids

The effect of retinoids on the expression of kappa opioid receptor (KOR) gene was examined in normal and transgenic animals. KOR-lacZ transgene expression was specifically elevated in KOR-positive areas of the developing CNS by depleting vitamin A from animal diets. The endogenous KOR mRNA species, including all three isoforms, were also upregulated by depleting vitamin A in developing animals. Change in the expression of isoforms a and b is similar in prenatal stages but differs during postnatal development. Interestingly, upregulation of isoform c is most significant postnatally. The regulation of KOR gene by vitamin A was substantiated in a mouse embryonal carcinoma P19 culture system in which retinoic acid (RA), the most potent ingredient of vitamin A, was able to suppress the expression of all the three KOR isoforms and KOR protein. The RA-mediated suppression was blocked by an RA receptor antagonist and a histone deacetylase (HDAC) inhibitor. By using a reporter transfection assay in P19 cells, the potential genetic element responsible for RA-mediated suppression of KOR gene expression was located to intron 1 of the mouse KOR gene, which could also be blocked by HDAC inhibitor. Furthermore, suppression of KOR gene expression by RA in P19 cells appeared to be an indirect event and required protein synthesis. A role of RA in KOR gene regulation during developmental stages was discussed.

Opioids intrinsically inhibit the genesis of mouse cerebellar granule neuron precursors in vitro: differential impact of mu and delta receptor activation on proliferation and neurite elongation

Although opioids are known to affect neurogenesis in vivo, it is uncertain the extent to which opioids directly or indirectly affect the proliferation, differentiation or death of neuronal precursors. To address these questions, the intrinsic role of the opioid system in neurogenesis was systematically explored in cerebellar external granular layer (EGL) neuronal precursors isolated from postnatal mice and maintained in vitro. Isolated neuronal precursors expressed proenkephalin-derived peptides, as well as specific mu and delta, but negligible kappa, opioid receptors. The developmental effects of opioids were highly selective. Morphine-induced mu receptor activation inhibited DNA synthesis, while a preferential delta2-receptor agonist ([D-Ala2]-deltorphin II) or Met-enkephalin, but not the delta1 agonist [D-Pen2, D-Pen5]-enkephalin, inhibited differentiation within the same neuronal population. If similar patterns occur in the developing cerebellum, spatiotemporal differences in endogenous mu and delta opioid ligand-receptor interactions may coordinate distinct aspects of granule neuron maturation. The data additionally suggest that perinatal exposure to opiate drugs of abuse directly interfere with cerebellar maturation by disrupting normal opioid signalling and inhibiting the proliferation of granule neuron precursors.

Mu-opioid agonist inhibition of kappa-opioid receptor-stimulated extracellular signal-regulated kinase phosphorylation is dynamin-dependent in C6 glioma cells

In previous studies we found that mu-opioids, acting via mu-opioid receptors, inhibit endothelin-stimulated C6 glioma cell growth. In the preceding article we show that the kappa-selective opioid agonist U69,593 acts as a mitogen with a potency similar to that of endothelin in the same astrocytic model system. Here we report that C6 cell treatment with mu-opioid agonists for 1 h results in the inhibition of kappa-opioid mitogenic signaling. The mu-selective agonist endomorphin-1 attenuates kappa-opioid-stimulated DNA synthesis, phosphoinositide turnover, and extracellular signal-regulated kinase phosphorylation. To investigate the role of receptor endocytosis in signaling, we have examined the effects of dynamin-1 and its GTPase-defective, dominant suppressor mutant (K44A) on opioid modulation of extracellular signal-regulated kinase phosphorylation in C6 cells. Overexpression of dynamin K44A in C6 cells does not affect kappa-opioid phosphorylation of extracellular signal-regulated kinase. However, it does block the inhibitory action on kappa-opioid signaling mediated by the kappa-opioid receptor. Our results are consistent with a growing body of evidence of the opposing actions of mu- and kappa-opioids and provide new insight into the role of opioid receptor trafficking in signaling.

Opioid growth factor and organ development in rat and human embryos

In addition to neurotransmission, the native opioid peptide, [Met5]enkephalin, is a tonically active inhibitory growth molecule that is termed opioid growth factor (OGF). OGF interacts with the zeta (zeta) opioid receptor to influence cell proliferation and tissue organization. We now identify OGF and the zeta receptor in embryonic derivatives including ectoderm, mesoderm, and endoderm of the rat on gestation day 20. Messenger RNA for preproenkephalin (PPE), the precursor of OGF, was detected in the developing cells, suggesting an autocrine production of this peptide. Acute exposure of the pregnant female to OGF resulted in a decrease in DNA synthesis in cells of organs representing all three germ layers, and did so in a receptor-mediated fashion. The influence of OGF was direct, as evidenced in organ culture studies. Blockade of endogenous opioid interaction using naltrexone (NTX) produced an increase in DNA synthesis, indicating the constitutive and functional nature of opioid activity on growth during prenatal life. Human fetal cells contained OGF and the zeta receptor. These data support the hypothesis that endogenous opioid modulation of organ development is a fundamental principle of mammalian embryogenesis, and that OGF has a profound influence on ontogeny. Irregularities in the role of opioids as growth regulators in relationship to the more than 500,000 newborns suffering from birth defects each year in the US needs to be examined.

Opioids modulate cell division in the germinal zone of the late embryonic neocortex

Opioid effects on cell division in the embryonic cerebral cortex were examined using two experimental approaches: (i) the presence of opioid receptors in the embryonic day 16 mouse neocortex was tested using immunohistochemical techniques; (ii) the values of the indices of [3H]thymidine pulse labelled cells and mitotic indices were estimated in the ventricular zone of the embryonic day 16 mouse neocortex 2.5, 4.5 and 8.5 h after administration to pregnant females of selected opioid receptor agonists or the opioid antagonist naloxone. The immunohistochemical study demonstrated that distinct subpopulations of the ventricular zone cells express mu, delta or kappa opioid receptors. Acute exposure of mouse embryos to mu, delta and kappa opioid receptor agonists or naloxone differentially affects the indices of [3H] thymidine pulse labelled cells and mitotic indices indicating changes in the cell cycle composition. Treatment with the mu opioid receptor agonist D-Ala2-MePhe4, Gly-ol5-enkephalin (DAGO), or the partially selective kappa opioid receptor agonist bremazocine, increased the [3H]thymidine labelling and mitotic indices. In contrast, the delta receptor agonist (D-Ser8)-leucine enkephalin-Thr (DSLET) produced a decrease in the labelled cell indices and mitotic indices. Naloxone provided a biphasic effect: a decrease in the values of labelled cell indices 2.5 h after naloxone administration, followed by an increase in the values of the indices at 4.5 and 8.5 h. These results suggest that the endogenous embryonic/maternal opioid systems are involved in the regulation of cell division in the ventricular zone of the late embryonic cortex.

Promoter activity of mouse kappa opioid receptor gene in transgenic mouse

The biological activity of mouse kappa opioid receptor (KOR) gene promoter was examined in transgenic mice using a beta-galactosidase (lacZ) reporter strategy for the first time. A lacZ cDNA was inserted at the 5th amino acid in the coding region of a mouse KOR genomic segment containing 3 kb of the 5' regulatory region, to generate a Kor-lacZ fusion gene which was then used to generate transgenic mice. The expression of transgene was demonstrated at the RNA level by reverse transcription-polymerase chain reaction (RT-PCR), and at the protein level by in situ lacZ enzyme assay. From studying three independent transgenic mouse lines that express this transgene, it is concluded that Kor-lacZ expression begins at embryonic day 9.5 (E9.5) and increases in several brain areas and neural tube as embryos develop. At E12.5 and E13.5, Kor-lacZ expression is found primarily in the mantle layer of midbrain, hindbrain and medulla oblongata, cranial ganglion and vagus nerve. At E15.5 and E17.5, the transgene is expressed in eye, ear, neopallial cortex, caudate putamen, lateral ventricle, thalamus, hypothalamus and pons. Therefore, the 3 kb 5' regulatory sequence of the mouse KOR gene is functional in transgenic animals and directs a specific expression pattern recapitulating that of the endogenous KOR gene expression during developmental stages. However, in adult animals, this transgene is only expressed in the brain, indicating that the regulatory information for peripheral expression in the adult is not encoded within this 3 kb upstream sequence.

Evidence for kappa- and mu-opioid receptor expression in C6 glioma cells

The astrocytoma cell line rat C6 glioma has been used as a model system to study the mechanism of various opioid actions. Nevertheless, the type of opioid receptor(s) involved has not been established. Here we demonstrate the presence of high-affinity U69,593, endomorphin-1, morphine, and beta-endorphin binding in desipramine (DMI)-treated C6 cell membranes by performing homologous and heterologous binding assays with [3H]U69,593, [3H]morphine, or 125I-beta-endorphin. Naive C6 cell membranes displayed U69,593 but neither endomorphin-1, morphine, nor beta-endorphin binding. Cross-linking of 125I-beta-endorphin to C6 membranes gave labeled bands characteristic of opioid receptors. Moreover, RT-PCR analysis of opioid receptor expression in control and DMI-treated C6 cells indicate that both kappa- and mu-opioid receptors are expressed. There does not appear to be a significant difference in the level of mu nor kappa receptor expression in naive versus C6 cells treated with DMI over a 20-h period. Collectively, the data indicate that kappa- and mu-opioid receptors are present in C6 glioma cells.

Developmental expression of the mu, kappa, and delta opioid receptor mRNAs in mouse

To characterize further the establishment of the opioid system during prenatal mouse development, we have examined the spatial and temporal expression patterns of mu, kappa, and delta opioid receptor mRNAs and find that the expression patterns of these mRNAs are distinct at all ages. Within the embryo, kappa is the first opioid receptor expressed, with transcripts detected in the gut epithelium as early as embryonic day 9.5 (E9.5). By E10.5, mu receptor expression is first detected in the facial-vestibulocochlear preganglion complex, whereas delta receptor mRNA is first detected at E12.5 in several peripheral tissues, including the olfactory epithelium, heart, limb bud, and tooth. In the brain, both mu and kappa mRNAs are first detected at E11.5 in the basal ganglia and midbrain, respectively. During mid-gestation and late gestation, the expression of both mu and kappa receptors extends to other brain regions that exhibit high expression in the adult, including the medial habenula, hypothalamus, pons, and medulla for mu and the basal ganglia, thalamus, hypothalamus, raphe, and ventral tegmental area for kappa. Thus by E17.5, many aspects of the adult expression patterns of mu and kappa receptors already have been established. Compared with mu and kappa, delta receptor mRNA expression in the brain begins relatively late, and the expression levels remain very low even at E19.5. In contrast to its late appearance in the brain, however, delta is the first opioid receptor expressed in the dorsal root ganglion, at E12.5, before its expression in the spinal cord begins at E15.5. Mu receptor is the first opioid receptor expressed in the spinal cord, at E11.5. These results extend previous ligand-binding data to significantly earlier ages and suggest that early developmental events in both neural and non-neural tissues may be modulated by opioid receptors. Several examples of possible autocrine and paracrine loops of opioid peptide and receptor expression have been identified, suggesting a role for these local circuits in developmental processes.

Developmental expression of the mu, kappa, and delta opioid receptor mRNAs in mouse

To characterize further the establishment of the opioid system during prenatal mouse development, we have examined the spatial and temporal expression patterns of mu, kappa, and delta opioid receptor mRNAs and find that the expression patterns of these mRNAs are distinct at all ages. Within the embryo, kappa is the first opioid receptor expressed, with transcripts detected in the gut epithelium as early as embryonic day 9.5 (E9.5). By E10.5, mu receptor expression is first detected in the facial-vestibulocochlear preganglion complex, whereas delta receptor mRNA is first detected at E12.5 in several peripheral tissues, including the olfactory epithelium, heart, limb bud, and tooth. In the brain, both mu and kappa mRNAs are first detected at E11.5 in the basal ganglia and midbrain, respectively. During mid-gestation and late gestation, the expression of both mu and kappa receptors extends to other brain regions that exhibit high expression in the adult, including the medial habenula, hypothalamus, pons, and medulla for mu and the basal ganglia, thalamus, hypothalamus, raphe, and ventral tegmental area for kappa. Thus by E17.5, many aspects of the adult expression patterns of mu and kappa receptors already have been established. Compared with mu and kappa, delta receptor mRNA expression in the brain begins relatively late, and the expression levels remain very low even at E19.5. In contrast to its late appearance in the brain, however, delta is the first opioid receptor expressed in the dorsal root ganglion, at E12.5, before its expression in the spinal cord begins at E15.5. Mu receptor is the first opioid receptor expressed in the spinal cord, at E11.5. These results extend previous ligand-binding data to significantly earlier ages and suggest that early developmental events in both neural and non-neural tissues may be modulated by opioid receptors. Several examples of possible autocrine and paracrine loops of opioid peptide and receptor expression have been identified, suggesting a role for these local circuits in developmental processes.

Ontogeny of the opioid growth factor, [Met5]-enkephalin, preproenkephalin gene expression, and the zeta opioid receptor in the developing and adult aorta of rat

Opioid peptides are known to play a role in the function of the mammalian cardiovascular system in the newborn. To learn about mechanisms underlying the morphogenesis of the developing vasculature and the principles contributing to the organization and maintenance of adult blood vessels, we have investigated the expression of the opioid growth factor (OGF), [Met5]-enkephalin, and its receptor, zeta. Moreover, gene expression for preproenkephalin mRNA, which encodes OGF, was studied to determine the source(s) of this inhibitory growth factor. By using immunocytochemistry, both OGF and the zeta opioid receptor were detected at embryonic day (E) 16 in the mesenchymal cells of the aortic wall. Staining appeared to be abundant in endothelial cells, smooth muscle cells, and fibroblasts at E20 and in the neonate. Immunoreactivity was noted to decrease progressively from day 5 to 10, but by weaning (day 21) and continuing into adulthood intense staining for both the peptide and receptor were observed. Preproenkephalin mRNA was detected throughout the aortic wall at E16, and the number of silver grains increased up today. Message was progressively reduced at days 5, 10, and 21, but signal in the adult aorta was comparable to that observed at day 5. These results indicate that components related to an endogenous opioid system regulating growth are present in the embryo, display a distinct spatial and temporal pattern of ontogeny, and persist into adulthood. In addition, these data indicate that OGF is an autocrine produced growth factor that is related to the emergence of vascular architecture and the maintenance of homeostasis in blood vessels.

Naltrexone is not detected in preweaning rats following transplacental exposure: implications for growth modulation

Extracts of brain and heart from rats at birth and postnatal days 2 and 10 were evaluated for naltrexone following maternal injection of 50 mg/kg opioid antagonist throughout gestation. Samples were prepared by ultrafiltration, lyophilized, reconstituted in mobile phase, and separated by reversed-phase high performance liquid chromatography with ultraviolet detection. Qualitative analysis revealed the presence of naltrexone in tissues from neonates, but not in rats of 2 and 10 days, that were transplacentally exposed to drug. These results confirm earlier reports showing that naltrexone, maternally administered, passes through the placenta and enters the fetus. Moreover, the data suggest that the somatic and neurobiological acceleration observed in offspring exposed to naltrexone during gestation is not due to opioid receptor blockade during the postnatal period.

Opioid receptor blockade during prenatal life modifies postnatal behavioral development

The ontogeny of physical characteristics, spontaneous motor, and sensorimotor behaviors of preweaning rats, as well as ambulation and emotionality at weaning (day 21) were studied in rats exposed to 50 mg/kg naltrexone (NTX) or saline (controls) daily throughout gestation by maternal administration; all animals were cross-fostered to untreated mothers at birth. Morphine challenge tests and nociceptive measures revealed that this dosage of opioid antagonist blocked opioid receptors for 24 h. At birth and weaning, animals in the NTX group weighed 12 and 20%, respectively, more than control offspring. The age at which a specific physical characteristic, spontaneous motor behavior, or reflex initially appeared and the age at which 100% of the animals demonstrated a particular characteristic/behavior often were accelerated in animals prenatally exposed to NTX. The frequency of ambulation was subnormal in the NTX group, and the frequency and/or incidence of rearing, grooming, wet-dog shakes, and defecation were reduced from normal levels in these opioid antagonist-exposed rats. These results imply that interactions of endogenous opioid systems during embryogenesis are determinants of somatic, physical, and behavioral development in postnatal life.

Transplacental transfer of naltrexone in rats

Extracts of fetal (20 days gestation) brain, heart, and liver were evaluated for naltrexone in rats 1 hour following maternal injection of 50 mg/kg opioid antagonist; adult plasma from the pregnant rats was analyzed. Samples were prepared by ultrafiltration, lyophilized, reconstituted in mobile phase, and separated by reversed phase high-performance liquid chromatography with ultraviolet detection. This qualitative analysis revealed the presence of naltrexone in all fetal tissues, as well as in adult plasma. These results indicate naltrexone, maternally administered, passes through the placenta and enters the fetus. The data would suggest that reports concerning somatic and neurobiological acceleration in offspring exposed to naltrexone during gestation may be the result of a direct opioid antagonist action in the fetus.

Immunoneutralization of beta-endorphin blocks prolactin release during suckling without affecting tuberoinfundibular dopaminergic neural activity

The effect of immunoneutralization of beta-endorphin on the suckling-induced prolactin increase and on the activity of the tuberoinfundibular dopaminergic (TIDA) neurons was determined in lactating female rats between days 8 - 12 post-partum. Two antisera were used in the immunoneutralization studies. Both were specific for beta-endorphin, exhibiting little cross reactivity with met- or leu-enkephalin or dynorphin. Antisera to beta-endorphin completely abolished the suckling-induced prolactin increase indicating that this endogenous opioid peptide is involved in this response. Suckling significantly inhibited DOPA accumulation in the median eminence and antiserum to beta-endorphin did not prevent this inhibition. Additionally, 5-endorphin antiserum significantly reduced TIDA neural activity even in pup-deprived dams. These results indicate that beta-endorphin is involved in the prolactin secretory response to suckling but that inhibition of TIDA neuronal activity is not its mechanism of action. Other possible mechanisms are discussed.

Identification of [Met5]-enkephalin in developing, adult, and renewing tissues by reversed-phase high performance liquid chromatography and radioimmunoassay

Extracts of adult corneal epithelium, and developing and adult cerebellum, aorta, and heart, from rats were evaluated for [Met5]-enkephalin. Samples were prepared by ultrafiltration and solid phase extraction with a C-18 Sep-pak, separated by reversed phase high-performance liquid chromatography, and analyzed by radioimmunoassay (RIA). This qualitative analysis revealed the presence of [Met5]-enkephalin in all tissues but the adult cerebellum. These results confirm and extend earlier reports that have used RIA or immunohistochemistry with regard to the presence of this opioid peptide in developing and renewing tissues, and indicate that [Met5]-enkephalin is indeed being recognized by immunological assays.

Transcription-dependent and -independent regulation of hepatic ornithine decarboxylase activity by CNS beta-endorphin in rat pups

We have previously shown that intracerebroventricular administration of relatively low doses of beta-endorphin suppresses basal levels of hepatic ODC activity as well as tissue ODC responsiveness to administered insulin in developing rats. Using Northern blotting analysis, the current studies examine whether these effects of CNS beta-endorphin may be mediated by changes in ODC gene expression. Subcutaneous administration of insulin (20 IU/kg body weight) rapidly and profoundly increased liver ODC activity. The time course of the response was characterized by proportionally increased levels of ODC mRNA, suggesting that insulin-induced stimulation of ODC activity is due to an increased transcription of ODC mRNA. Pretreatment with actinomycin D (2 mg/kg body weight, intraperitoneally) completely prevented the insulin-induced increase in ODC activity, confirming the requirement for the de novo synthesis of ODC mRNA for the effect. More importantly, intracerebroventricular but not subcutaneous injection of beta-endorphin (1 microgram) markedly diminished the stimulatory effect of insulin on hepatic ODC mRNA accumulation. The time course and magnitude of the inhibition of mRNA accumulation essentially mirrored that of the peptide on ODC activity. On the other hand, contrary to the inhibitory effect of beta-endorphin on basal ODC activity, the peptide did not lower basal ODC mRNA levels when given alone. Taken together, the results from these studies provide evidence for the existence of at least two separate mechanisms through which CNS beta-endorphin might downregulate ODC activity in peripheral organs of rat pups. The peptide can suppress insulin-induced ODC activity in the liver tissue by decreasing the rate of transcription of the ODC gene, whereas the inhibition of basal ODC activity appears to involve posttranscriptional mechanisms.

Dynorphins modulate DNA synthesis in fetal brain cell aggregates

Previously, opioid peptide analogues, beta-endorphin, and synthetic opiates were found to inhibit DNA synthesis in 7-day fetal rat brain cell aggregates via kappa- and mu-opioid receptors. Here dynorphins and other endogenous opioid peptides were investigated for their effect on DNA synthesis in rat and guinea pig brain cell aggregates. At 1 microM, all dynorphins tested and beta-endorphin inhibited [3H]thymidine incorporation into DNA by 20-38% in 7-day rat brain cell aggregates. The putative epsilon-antagonist beta-endorphin (1-27) did not prevent the effect of beta-endorphin, suggesting that the epsilon-receptor is not involved in opioid inhibition of DNA synthesis. The kappa-selective antagonist norbinaltorphimine blocked dynorphin A or B inhibition of DNA synthesis, implicating a kappa-opioid receptor. In dose-dependency studies, dynorphin B was three orders of magnitude more potent than dynorphin A in the attenuation of thymidine incorporation, indicative of the mediation of its action by a discrete kappa-receptor subtype. The IC50 value of 0.1 nM estimated for dynorphin B is in the physiological range for dynorphins in developing brain. In guinea pig brain cell aggregates, the kappa-receptor agonists U50488, U69593, and dynorphin B reduced thymidine incorporation by 40%. When 21-day aggregates were treated with dynorphins, a 33-86% enhancement of thymidine incorporation was observed. Because both 7- and 21-day aggregates correspond to stages in development when glial cell proliferation is prevalent and glia preferentially express kappa-receptors in rat brain, these findings support the hypothesis that dynorphins modulate glial DNA synthesis during brain ontogeny.