Long-lasting effects of naltrexone, an opioid receptor antagonist, on cell proliferation in developing rat forebrain

Addiction and the cerebellum with a focus on actions of opioid receptors

Increasing evidence suggests that the cerebellum could play a role in the higher cognitive processes involved in addiction as the cerebellum contains anatomical and functional pathways to circuitry controlling motivation and saliency. In addition, the cerebellum exhibits a widespread presence of receptors, including opioid receptors which are known to play a prominent role in synaptic and circuit mechanisms of plasticity associated with drug use and development of addiction to opioids and other drugs of abuse. Further, the presence of perineural nets (PNNs) in the cerebellum which contain proteins known to alter synaptic plasticity could contribute to addiction. The role the cerebellum plays in processes of addiction is likely complex, and could depend on the particular drug of abuse, the pattern of use, and the stage of the user within the addiction cycle. In this review, we discuss functional and structural modifications shown to be produced in the cerebellum by opioids that exhibit dependency-inducing properties which provide support for the conclusion that the cerebellum plays a role in addiction.

A meta-analysis of the potential antidepressant effects of buprenorphine versus placebo as an adjunctive pharmacotherapy for treatment-resistant depression

BACKGROUND

Numerous reports have suggested that buprenorphine may have antidepressant effects. Many individuals with depressive disorders don't respond to first-line treatment and are classified with treatment-resistant depression (TRD). Novel therapies for depression are required to better treat this population. This meta-analysis of randomized placebo-controlled trials sought to evaluate the potential antidepressant effects of buprenorphine as an adjunctive pharmacological treatment for individuals with TRD.

METHODS

PubMed, Embase, CINAHL, Web of Science, and ClinicalTrials.gov databases were searched until June 2019 for original peer-reviewed reports of buprenorphine used for the treatment of depression. Standardized mean differences (SMD) were generated from random effects models. Risk of publication bias was assessed using a funnel plot. Potential sources of heterogeneity were explored in subgroup analyses.

RESULTS

In six studies that met inclusion criteria, depression symptom severity in individuals with TRD was not significantly decreased after an adjunctive intervention with buprenorphine when compared to placebo (SMD = -0.07, 95% CI: -0.21-0.06, p = 0.30). Five of the six studies utilized a combination of buprenorphine/samidorphan. In these studies, depression symptom severity was also not significantly reduced after intervention compared to placebo (SMD = -0.08, 95% CI: -0.21 - 0.05, p = 0.23).

LIMITATIONS

Five included studies were performed by the same research group with significant conflicts of interest.

CONCLUSIONS

This meta-analysis did not reveal a significant reduction in depression symptom severity in individuals with TRD after an adjunctive intervention with buprenorphine when compared to placebo. However, more optimal doses of buprenorphine (2 mg/day) and longer treatment lengths should be explored.

Opiate Drugs with Abuse Liability Hijack the Endogenous Opioid System to Disrupt Neuronal and Glial Maturation in the Central Nervous System

The endogenous opioid system, comprised of multiple opioid neuropeptide and receptor gene families, is highly expressed by developing neural cells and can significantly influence neuronal and glial maturation. In many central nervous system (CNS) regions, the expression of opioid peptides and receptors occurs only transiently during development, effectively disappearing with subsequent maturation only to reemerge under pathologic conditions, such as with inflammation or injury. Opiate drugs with abuse liability act to modify growth and development by mimicking the actions of endogenous opioids. Although typically mediated by μ-opioid receptors, opiate drugs can also act through δ- and κ-opioid receptors to modulate growth in a cell-type, region-specific, and developmentally regulated manner. Opioids act as biological response modifiers and their actions are highly contextual, plastic, modifiable, and influenced by other physiological processes or pathophysiological conditions, such as neuro-acquired immunodeficiency syndrome. To date, most studies have considered the acute effects of opiates on cellular maturation. For example, activating opioid receptors typically results in acute growth inhibition in both neurons and glia. However, with sustained opioid exposure, compensatory factors become operative, a concept that has been largely overlooked during CNS maturation. Accordingly, this article surveys prior studies on the effects of opiates on CNS maturation, and also suggests new directions for future research in this area. Identifying the cellular and molecular mechanisms underlying the adaptive responses to chronic opiate exposure (e.g., tolerance) during maturation is crucial toward understanding the consequences of perinatal opiate exposure on the CNS.

Effect of maternal morphine sulfate exposure on neuronal plasticity of dentate gyrus in Balb/c mice offspring

This study carried out to evaluate the effects of maternal morphine exposure during gestational and lactation period on the neuronal cells of dentate gyrus in 18 and 32 days Balb/c mice offspring. In this experimental study 10 female mice were randomly allocated into cases and controls. In experimental group, animals were received morphine sulfate 10 mg/kg/body weight intraperitoneally during 7 days before mating, gestational period (GD0-21), 18 and 32 days after delivery. The control animals were received an equivalent volume normal saline. Cerebrum of six infant for each group were removed and stained with cresyl violet and monoclonal anti-neuronal nuclei (NeuN) antibody. Quantitative computer-assisted morphometric study was done on dentate gyrus of hippocampus. In the P18 mice, the numbers of granular cells in dentate gyrus medial blade and dentate gyrus lateral blade significantly reduced from 171.45 +/- 4.2 and 174.51 +/- 3.1 cells in control group to 153.32 +/- 2.8 and 151.23 +/- 3.2 cells in 10000 microm2 area of granular layer in treated group (p < 0.001). In P32 mice the numbers of granular cells in mb and lb of dentate gyrus significantly decreased from 155.31 +/- 4.1 and 153.77 +/- 3.4 in control group to 138.33 +/- 4.5 and 135.13 +/- 4.3 in treated group, respectively (p < 0.001). The granular layer thickness in mb and lb area of dentate gyrus significantly reduced in treated mice in compared to controls in P18 and P32 mice (p < 0.05). This study revealed that morphine administration before, during pregnancy and lactation period causes neuronal cells loss of dentate gyrus in 18 and 32 days old infant mice.

HIV-1 alters neural and glial progenitor cell dynamics in the central nervous system: coordinated response to opiates during maturation

HIV-associated neurocognitive disorders (HANDs) are common sequelae of human immunodeficiency virus (HIV) infection, even when viral titers are well controlled by antiretroviral therapy. Evidence in patients and animal models suggests that neurologic deficits are increased during chronic opiate exposure. We have hypothesized that central nervous system (CNS) progenitor cells in both adult and developing CNS are affected by HIV infection and that opiates exacerbate these effects. To examine this question, neural progenitors were exposed to HIV-1 Tat(1-86) in the developing brain of inducible transgenic mice and in vitro. We examined whether Tat affected the proliferation or balance of progenitor populations expressing nestin, Sox2, and Olig2. Disease relevance was further tested by exposing human-derived progenitors to supernatant from HIV-1 infected monocytes. Studies concentrated on striatum, a region preferentially targeted by HIV and opiates. Results were similar among experimental paradigms. Tat or HIV exposure reduced the proliferation of undifferentiated (Sox2(+)) progenitors and oligodendroglial (Olig2(+)) progenitors. Coexposure to morphine exacerbated the effects of Tat or HIV-1(SF162) supernatant, but partially reversed HIV-1(IIIB) supernatant effects. Populations of Sox2(+) and Olig2(+) cells were also reduced by Tat exposure, although progenitor survival was unaffected. In rare instances, p24 immunolabeling was detected in viable human progenitors by confocal imaging. The vulnerability of progenitors is likely to distort the dynamic balance among neuron/glial populations as the brain matures, perhaps contributing to reports that neurologic disease is especially prevalent in pediatric HIV patients. Pediatric disease is atypical in developed regions but remains a serious concern in resource-limited areas where infection occurs commonly at birth and through breast feeding.

Purkinje cells loss in off spring due to maternal morphine sulfate exposure: a morphometric study

The toxic effects of morphine sulfate in the adult cerebral cortex and one-day neonatal cerebellum have been studied. This study was carried out to evaluate the effect of maternal morphine exposure during gestational and lactation period on the Purkinje cells and cerebellar cortical layer in 18- and 32-day-old mice offspring. Thirty female mice were randomly allocated into cases and controls. In cases, animals received morphine sulfate (10 mg/kg/body weight intraperitoneally) during the 7 days before mating, gestational day (GD 0-21) 18 or 32. The controls received an equivalent volume of saline. The cerebellum of six infants for each group was removed and each was stained with cresyl violet. Quantitative computer-assisted morphometric study was done on cerebellar cortex. The linear Purkinje cell density in both experimental groups (postnatal day [P]18, 23.40±0.5; P32, 23.45±1.4) were significantly reduced in comparison with the control groups (P18, 28.70±0.9; P32, 28.95±0.4) (P<0.05). Purkinje cell area, perimeter and diameter at apex and depth of simple lobules in the experimental groups were significantly reduced compared to the controls (P<0.05). The thickness of the Purkinje layer of the cerebellar cortex was significantly reduced in morphine treated groups (P<0.05). This study reveals that morphine administration before pregnancy, during pregnancy and during the lactation period causes Purkinje cells loss and Purkinje cell size reduction in 18- and 32-day-old infant mice.

Chronic pain-induced emotional dysfunction is associated with astrogliosis due to cortical delta-opioid receptor dysfunction

It has been widely recognized that chronic pain could cause physiological changes at supraspinal levels. The delta-opioidergic system is involved in antinociception, emotionality, immune response and neuron-glia communication. In this study, we show that mice with chronic pain exhibit anxiety-like behavior and an increase of astrocytes in the cingulate cortex due to the dysfunction of cortical delta-opioid receptor systems. Using neural stem cells cultured from the mouse embryonic forebrain, astrocyte differentiation was clearly observed following long-term exposure to the selective delta-opioid receptor antagonist, naltrindole. We also found that micro-injection of either activated astrocyte or astrocyte-conditioned medium into the cingulate cortex of mice aggravated the expression of anxiety-like behavior. Our results indicate that the chronic pain process promotes astrogliosis in the cingulate cortex through the dysfunction of cortical delta-opioid receptors. This phenomenon may lead to emotional disorders including aggravated anxiety under chronic pain-like state.

Differential regulation of hippocampal progenitor proliferation by opioid receptor antagonists in running and non-running spontaneously hypertensive rats

Voluntary running in mice and forced treadmill running in rats have been shown to increase the amount of proliferating cells in the hippocampus. Little is known as yet about the mechanisms involved in these processes. It is well known that the endogenous opioid system is affected during running and other forms of physical exercise. In this study, we evaluated the involvement of the endogenous opioids in the regulation of hippocampal proliferation in non-running and voluntary running rats. Nine days of wheel running was compared with non-running in spontaneously hypertensive rats (SHR), a rat strain known to run voluntarily. On the last 2 days of the experimental period all rats received two daily injections of the opioid receptor antagonists naltrexone or naltrindole together with injections of bromodeoxyuridine to label dividing cells. Brain sections from the running rats showed approximately a five-fold increase in newly generated cells in the hippocampus, and this increase was partly reduced by naltrexone but not by naltrindole. By contrast, both naltrexone and naltrindole increased hippocampal proliferation in non-running rats. In non-running rats the administration of naltrexone decreased corticosterone levels and adrenal gland weights, whereas no significant effects on these parameters could be detected for naltrindole. However, adrenal gland weights were increased in naltrexone- but not in naltrindole-administered running rats. In addition, in voluntary running rats there was a three-fold increase in the hippocampal levels of Met-enkephalin-Arg-Phe compared with non-runners, indicating an increase in opioid activity in the hippocampus during running. These data suggest an involvement of endogenous opioids in the regulation of hippocampal proliferation in non-running rats, probably through hypothalamic-pituitary-adrenal axis modulation. During voluntary running in SHR naltrexone altered hippocampal proliferation via as yet unknown mechanisms.

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.

Opioid-induced proliferation through the MAPK pathway in cultures of adult hippocampal progenitors

Administration of opioid agonists or antagonists has been reported to regulate proliferation or survival of neural progenitors in vivo. Here we report that beta-endorphin and selective mu-opioid receptor (MOR) and delta-opioid receptor (DOR) agonists stimulate proliferation of isolated rat adult hippocampal progenitors (AHPs). The AHPs were found to express DORs and MORs, but not kappa-opioid receptors. Incubation with beta-endorphin for 48 h increased the number of AHPs found in mitosis, the total DNA content, and the expression of proliferating cell nuclear antigen. This proliferative effect from beta-endorphin on AHPs was antagonized by naloxone. The beta-endorphin-induced proliferation was mediated through phosphorylation of extracellular signal-regulated kinases 1 and 2 and dependent on phosphatidylinositol 3-kinase and both intra- and extracellular calcium. These data suggest a role for the opioid system in the regulation of proliferation in progenitors from the adult hippocampus.

Mu- and delta-opioid receptor antagonists decrease proliferation and increase neurogenesis in cultures of rat adult hippocampal progenitors

Opioids have previously been shown to affect proliferation and differentiation in various neural cell types. In the present study, cultured rat adult hippocampal progenitors (AHPs) were shown to release beta-endorphin. Membrane preparations of AHPs were found to bind [125I]beta-endorphin, and immunoreactivity for mu- and delta-opioid receptors (MORs and DORs), but not for kappa-opioid receptors (KORs), was found on cells in culture. Both DNA content and [3H]thymidine incorporation were reduced after a 48-h incubation with 100 microM naloxone, 10 micro m naltrindole or 10 microM beta-funaltrexamine, but not nor-binaltorphimine, suggesting proliferative actions of endogenous opioids against MORs and DORs on AHPs. Furthermore, analysis of gene and protein expression after incubation with MOR and DOR antagonists for 48 h using RT-PCR and Western blotting suggested decreased signalling through the mitogen-activated protein kinase (MAPK) pathway and lowered levels of genes and proteins that are important in cell cycling. Cultures were incubated with naloxone (10 or 100 microM) for 10 days to study the effects on differentiation. This resulted in an approximately threefold increase in neurogenesis, a threefold decrease in astrogliogenesis and a 50% decrease in oligodendrogenesis. In conclusion, this study suggests that reduced signalling through MORs and DORs decreases proliferation in rat AHPs, increases the number of in vitro-generated neurons and reduces the number of astrocytes and oligodendrocytes in culture.

The effects of morphine on cell proliferation

There is increasing evidence that endogenous opioid peptides ("enkephalins") and other neurotransmitters have widespread, receptor-mediated roles as growth regulators in non-neuronal cells and tissues. For example, it is now believed that enkephalins produced in placental trophoblast giant cells have multiple roles in supporting embryo growth, and in maternal adaptation to pregnancy. Since plant and synthetic narcotics (e.g., morphine) bind to the same receptors, the questions immediately arise: Do narcotics also have actions as growth regulators? If so, do these actions have physiological significance in addicts? Recent work on the first of these questions is covered in this review. While the greatest volume of research has been focused on the proliferative effects of narcotics for cells of the immune system, the roles of opioid peptides and narcotics on the growth of a variety of other cells has come under study recently.

Differential expression of delta opioid receptors and mRNA in proliferating astrocytes during the cell cycle

Previous immunohistochemical and radioligand binding studies have shown a cell cycle-dependent regulation of the delta opioid receptor (DOR). The relationship between DOR expression and mitosis in primary astroglial cultures of rat cerebral cortex was investigated in this study. The cultures were arrested during the G(1)/S transition or during mitosis. The DOR protein level increased twofold (P = 0.009) during mitosis and DOR mRNA level increased threefold (P = 0.002) during the G(1)/S transition compared to nonsynchronized cultures. DOR mRNA was also elevated (1.6-fold, P = 0.008) during the G(1)/S transition compared with mitotic cells. A premitotic increase in DOR mRNA suggests that elevated DOR protein levels during mitosis might be regulated during transcription.

The opioid growth factor, [Met5]-enkephalin, inhibits DNA synthesis during recornification of mouse tail skin

Opioid peptides serve as tonically active negative growth regulators in renewing and regenerating epithelia. To examine the involvement of opioids in renewal of the stratum corneum after tape stripping of tail skin, C57BL/6 J mice were given systemic injections of the potent opioid antagonist, naltrexone (NTX, 20 mg/kg i.p.) following injury. Blockade of opioid-receptor interaction by NTX for 4 h resulted in an elevation of 36-66% in basal cell DNA synthesis measured 24 h after injury. Injection of the endogenous opioid peptide, [Met5]-enkephalin (OGF, 10 mg/kg i.p.) 4 h before termination, suppressed radiolabelled thymidine incorporation in the basal cell layer by 37-46% at 24 h after wounding. The magnitude of the effects on DNA synthesis of OGF, but not NTX, depended on the timing of administration with respect to injury. OGF maximally depressed basal cell labelling (72%) when given 16 h after tape stripping. Concomitant administration of naloxone (10 mg/kg) with OGF blocked the inhibition of DNA synthesis; naloxone alone at the dosage utilized had no effect on cell labelling. Both OGF and its receptor, OGFr, were detected by immunocytochemistry in the basal and suprabasal cell layers, but not the cornified layer of tape stripped and uninjured tail skin. These results indicate: (a) a native opioid peptide and its receptor are expressed in epidermal cells of injured and uninjured mouse tail skin; (b) removal of the stratum corneum by tape stripping does not disrupt the function of the endogenous opioid growth system; (c) the proliferative response to wounding of the tail is tonically inhibited by the receptor-mediated action of an endogenous opioid peptide; and (d) DNA synthesis by basal cells can be elevated by disrupting opioid peptide receptor interactions.

Cloning, sequencing, chromosomal location, and function of cDNAs encoding an opioid growth factor receptor (OGFr) in humans

The native opioid growth factor (OGF), [Met(5)]-enkephalin, is a tonic inhibitory peptide that modulates cell proliferation and tissue organization during development, cancer, cellular renewal, wound healing, and angiogenesis. OGF action is mediated by a receptor mechanism. We have cloned and sequenced cDNAs encoding multiple spliced forms of a human OGF receptor. The open reading frame in the longest cDNA was found to encode a protein of 697 amino acids, and 8 imperfect repeats of 20 amino acids each were a prominent feature. Altogether, five alternatively spliced forms were observed. The cDNA hybridized to mRNA from a variety of normal and neoplastic cells and tissues. Functional studies using antisense oligonucleotides to OGFr demonstrated an enhancement in cell growth. Fluorescent in situ hybridization (FISH) experiments showed the chromosomal location to be 20q13.3. This OGF receptor has no homology to classical opioid receptors. These results provide molecular validity for the interaction of OGF and OGF receptor in the regulation of growth processes in humans.

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.

Delta-opioid receptor immunoreactivity on astrocytes is upregulated during mitosis

Endogenous opioid peptides and opioid receptors are expressed by brain cells early during normal development, and exogenous opiate exposure in this period is known to affect brain cell proliferation and maturation. Despite the abundant evidence that opioids affect brain development, little is known about the mechanisms involved. In this study cortical astrocytes in primary culture were examined immunohistochemically by using antibodies against the opioid receptors. The immunoreactivity for delta-opioid receptors was strongly upregulated during mitosis with an increase in immunostaining that started in early prophase and lasted through the M-phase to cytokinesis. Similar effects could not be observed when antibodies against the mu- or kappa-opioid receptor subtypes were used. Cultured neurons and microglia presented a strong and homogenous immunostaining for the delta-opioid receptor and no further upregulation of immunoreactivity could be detected in these cells. The presence of functional delta-opioid receptors on the mitotic astrocytes was verified by using microspectrofluorometry for detection of delta-opioid agonist induced changes in intracellular free calcium concentrations ([Ca2+]i). In these experiments fluo-3/AM incubated cells showed a rapidly induced delta-opioid agonist (DPDPE, 10(-6) M) evoked increase in [Ca2+]i. These results suggest an upregulation of the delta-opioid receptors that could represent a mechanism involved in the response to opioids in the developing brain.

mu-Opioid receptor activation enhances DNA synthesis in immature oligodendrocytes

Opioids disrupt nervous system development by inhibiting the proliferation of neuronal and glial progenitors. These studies explored the hypothesis that mu opioid receptors are expressed by immature oligodendrocytes (OLs) and are functionally related to growth. Antibodies identifying the cloned mu opioid receptor demonstrated that cultured OLs expressed mu opioid receptor immunoreactivity very early during development. Cultures were treated with the selective mu opioid receptor agonist H-Tyr-Pro-Phe (N-Me)-D-Pro-NH2 (PL017; 1 microM), or PL017 (1 microM) plus the antagonist naloxone (3 microM). Opioid-dependent changes in DNA synthesis were assessed by determining the proportion of bromodeoxyuridine (BrdU)-labeled O4-immunoreactive OLs. Treatment with PL017 caused a 311% increase in the proportion of O4-immunoreactive OLs incorporating BrdU compared to untreated controls, and these effects were prevented by co-administering naloxone. These preliminary results indicate that (i) immature OLs express mu opioid receptors and that (ii) the activation of this receptor type is functionally coupled to DNA synthesis and the cell division cycle. The expression of opioid receptors by OLs suggests that the endogenous opioid system is widely distributed among glial types.

mu-Opioid receptor-induced Ca2+ mobilization and astroglial development: morphine inhibits DNA synthesis and stimulates cellular hypertrophy through a Ca(2+)-dependent mechanism

Morphine, a preferential mu-opioid receptor agonist, alters astroglial development by inhibiting cell proliferation and by promoting cellular differentiation. Although morphine affects cellular differentiation through a Ca(2+)-dependent mechanism, few studies have examined whether Ca2+ mediates the effect of opioids on cell proliferation, or whether a particular Ca2+ signal transduction pathway mediates opioid actions. Moreover, it is uncertain whether one or more opioid receptor types mediates the developmental effects of opioids. To address these questions, the present study examined the role of mu-opioid receptors and Ca2+ mobilization in morphine-induced astrocyte development. Morphine (1 microM) and non-morphine exposed cultures enriched in murine astrocytes were incubated in Ca(2+)-free media supplemented with < 0.005, 0.3, 1.0, or 3.0 mM Ca2+ ([Ca2+]o), or in unmodified media containing Ca2+ ionophore (A23187), nifedipine (1 microM), dantrolene (10 microM), thapsigargin (100 nM), or L-glutamate (100 microM) for 0-72 h. mu-Opioid receptor expression was examined immunocytochemically using specific (MOR1) antibodies. Intracellular Ca2+ ([Ca2+]i) was measured by microfluorometric analysis using fura-2. Astrocyte morphology and bromodeoxyuridine (BrdU) incorporation (DNA synthesis) were assessed in glial fibrillary acidic protein (GFAP) immunoreactive astrocytes. The results showed that morphine inhibited astroglial growth by activating mu-opioid receptors. Astrocytes expressed MOR1 immunoreactivity and morphine's actions were mimicked by the selective mu agonist PL017. In addition, morphine inhibited DNA synthesis by mobilizing [Ca2+]i in developing astroglia. At normal [Ca2+]o, morphine attenuated DNA synthesis by increasing [Ca2+]i; low [Ca2+]o (0.3 mM) blocked this effect, while treatment with Ca2+ ionophore or glutamate mimicked morphine's actions. At extremely low [Ca2+]o (< 0.005 mM), morphine paradoxically increased BrdU incorporation. Although opioids can increase [Ca2+]i in astrocytes through several pathways, not all affect DNA synthesis or cellular morphology. Nifedipine (which blocks L-type Ca2+ channels) did not prevent morphine-induced reductions in BrdU incorporation or cellular differentiation, while thapsigargin (which depletes IP3-sensitive Ca2+ stores) severely affected inhibited DNA synthesis and cellular differentiation-irrespective of morphine treatment. However, dantrolene (an inhibitor of Ca(2+)-dependent Ca2+ release) selectively blocked the effects of morphine. Collectively, the findings suggest that opioids suppress astroglial DNA synthesis and promote cellular hypertrophy by inhibiting Ca(2+)-dependent Ca2+ release from dantrolene-sensitive intracellular stores. This implies a fundamental mechanism by which opioids affect central nervous system maturation.

The opioid growth factor, [Met5]-enkephalin, and the zeta opioid receptor are present in human and mouse skin and tonically act to inhibit DNA synthesis in the epidermis

Opioid peptides serve as tonically active negative growth factors in neural and non-neural cells, in addition to being neuromodulators. To investigate the involvement of opioids in homeostatic renewal of epithelial cells in the epidermis, mice were given systemic injections of the potent opioid antagonist, naltrexone (NTX) (20 mg/kg). Disruption of opioid-receptor interaction by NTX resulted in an elevation of 42 and 72% in DNA synthesis in skin from the dorsum and plantar surface of the hindfoot, respectively, within 2 h; response to NTX was dependent on the circadian rhythm in each region examined. Injection of the naturally occurring and potent opioid growth factor (OGF), [Met5]-enkephalin, at 1 mg/kg depressed DNA synthesis in the dorsum and plantar surface by 42 and 19%, respectively, within 2 h; the effects of OGF complied with the pattern of circadian rhythm in each area of skin. The decreases in labeling index evoked by OGF were blocked by concomitant administration of the opioid antagonist, naloxone (10 mg/kg); naloxone alone at the dosage utilized had no influence on cell replicative processes. In tissue culture studies, OGF and NTX respectively depressed and elevated DNA synthesis. Both OGF and its receptor, zeta, were detected in all but the cornified layer of the epidermis in murine skin from the dorsum, plantar surface, pinnae, and tail. In addition, both peptide and receptor were observed in basal and suprabasal cells of the human epidermis. These results lead to the suggestion that an endogenous opioid peptide and its receptor are present and govern cellular renewal processes in the skin in a direct manner, regulating DNA synthesis in a tonically inhibitory, circadian rhythm-dependent fashion.

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.

Postsynaptic and extrasynaptic localization of kappa-opioid receptor in selected brain areas of young rat and chick using an anti-receptor monoclonal antibody

kappa-opioid receptors were visualized by light and electron microscopical immunohistochemistry in young rat and chick brains, using a monoclonal antibody KA8 (IgG1, kappa) raised against a kappa-opioid receptor preparation from frog brain, which recognizes selectively the kappa-type receptor with preference for the kappa-2 subtype. The most pronounced kappa-opioid receptor-like immunoreactivity was observed in the hypothalamic nuclei of the rat brain and in the chick optic tectum, in regions where the functional significance of kappa-opioid receptors is well documented. Both neurons and glia were stained, the former on both somata and dendrites. At the ultrastructural level, the receptor-like immunoreactivity was similar in both species. Immunoprecipitate decorated the inner surface of the plasma membrane of glial cells, neuronal somata and dendrites, in a discontinuous arrangement. In the cytoplasm, labelling was associated with ribosomes, polyribosomes and rough endoplasmic reticulum membranes but not with Golgi cisternae. In the neuropil, the immunoprecipitate was observed along the dendritic microtubules and was also associated with postsynaptic sites. Nuclei and axons were devoid of label and immunoreactivity was never visible presynaptically. Our findings indicate that the antibody used in the present study marks various forms of the kappa-opioid receptor protein including those synthesised in ribosomes, transported along dendritic microtubules and incorporated into postsynaptic and non-synaptic membranes. The antibody also recognizes glial opioid receptors. The observed subcellular distribution appears to be conserved in phylogenetically distant species.

The fetal expression of proenkephalin mRNAs and Met-enkephalin immunoreactivity in the hypothalamoseptal tract and adjacent hypothalamic areas of the guinea pig brain

The development of the enkephalinergic hypothalamoseptal tract in the guinea pig brain was studied from embryonic day 30 until birth. Proenkephalin (PE) mRNAs were detected in the hypothalamic magnocellular dorsal nucleus (MDN) by in situ hybridization with a synthetic 35S-labeled oligonucleotide. The Met-enkephalin-like immunoreactivity (Met-enk-LI) in the MDN and the lateral septum (LS) was detected with antibodies against Met-enkephalin, on adjacent cryostat sections. At the same time, an immunohistochemical study of the arrangement of enkephalinergic axon terminals in the LS at birth was performed at the electron microscopic level. PE mRNAs were first found to be expressed in the MDN at embryonic day 32 (E32) and increased to reach a maximal level at E48. Met-enk-LI was consistently detectable from E38 in numerous perikarya of the MDN as well as in nerve terminals of the LS. The number of Met-enk-LI cells of the MDN decreased after this stage until birth, whereas positive nerve endings in the LS increased. At the electron microscopic level, numerous cell bodies of the LS at birth were consistently surrounded by Met-enk immunoreactive nerve terminals. Cells expressing the PE gene and Met-enk-LI were also observed from E38 to E44 in the periventricular area. Some of these cells were found double-labeled with Met-enkephalin and Somatostatin antisera. The enkephalinergic system of the hypothalamoseptal tract appears at early embryonic stages and may be essential in regulating septal neuronal functions early in gestation. Differing ontogenic onsets of the enkephalinergic hypothalamoseptal and periventricular-median eminence tracts suggest possible developmental and functional differences.

Posthatching development of preproenkephalin mRNA-expressing cell populations in the pigeon telencephalon

Enkephalin peptides are highly expressed in the vertebrate telencephalon. Our previous investigations in the pigeon and in the chicken [26] suggested that the cellular distribution of these peptides is conserved in phylogenetically 'old' telencephalic regions (e.g. the basal ganglia), while it has species-specific organizations in areas (e.g. dorsomedial forebrain and bulbus olfactorius) that are likely to play important roles in species-specific behaviors. In the present study, we investigated the posthatching development of preproenkephalin (PPE) mRNA-containing cells in the pigeon forebrain using in situ hybridization histochemistry. These cells are densely distributed in the paleostriatal complex (corresponding to the mammalian caudate-putamen) at hatching, and their density progressively decreases during the first 9 days posthatching, when it is similar to that of adult pigeons. In the dorsomedial forebrain (corresponding to the mammalian hippocampus), PPE mRNA-expressing cells are present at hatching, and their density reaches a peak around the 6th day posthatching. In the bulbus olfactorius, the first PPE mRNA-containing cells are observed after 9 days posthatching. The developmental profile of PPE mRNA expression in these areas of the pigeon telencephalon shows remarkable similarities with the development of enkephalinergic cells in corresponding brain areas of mammals. As in the mammalian caudate-putamen, the developmental expression of enkephalin peptides in the paleostriatal complex is likely to be related to neuronal withdrawal from the mitotic cycle. The developmental pattern of expression of PPE mRNA in the dorsomedial forebrain suggests that enkephalin peptides contribute to the maturation of the behavioral functions of this area.

Stimulatory effect of opioid peptides and naloxone on rat spinal cord cells in primary dissociated culture

Opioid peptides leu-enkephalin, a synthetic analog of enkephalin dalargin and an opiate receptor blocker naloxone were studied for their morphological effect on the cells of dissociated cultures of rat spinal cord. Low density seeding of cells (3.10(5);6.10(5) cells/ml) on collagen substrate was performed to document that opioid peptides increase the number of cultured cells and neurite outgrowth and lead to the activation of the initiated processes of aggregate formation. Upon higher density of plating (5.10(6) cells/ml) with poly-L-lysine as a substrate, activation of the aggregate formation process was demonstrated, both opioid peptides and naloxone leading to an increase in the size of aggregates. Statistical treatment of the results obtained in this set of experiments documented that leu-enkephalin, dalargin and naloxone increased 2.2-, 2.2-2.6-, 2.4-fold, respectively, the size of aggregates compared to the control, i.e. the reaction of spinal cord cells to opioid peptides and opiate receptor blocker naloxon was unidirected. The total effect of opioid peptides and naloxon resulted in a 3.6-fold increase in the size of the aggregates compared to the control. The data obtained in this study allow the assumption that opioid peptides and naloxone, while activating spinal cord cells via receptors of a different type, manifest the properties of factors thus increasing survival and adhesion of spinal cord cells in culture.

Morphine does not affect astrocyte survival in developing primary mixed-glial cultures

In mixed-glial cultures, high concentrations of morphine (1 microM) have previously been shown to completely inhibit any increase in glial numbers, although DNA synthesis continues in flat, polyhedral astrocytes (type 1 astrocytes). This suggests that high concentrations of morphine are toxic to glia. Morphine toxicity was assessed in mixed-glial cultures using calcein-AM and ethidium homodimer dyes as viability markers to identify live and dead cells, respectively. At 3, 5, and 7 days in vitro there was no significant difference in the number of dead cells between untreated and opiate-treated groups. Comparable numbers of ethidium homodimer-labeled cells were present in all groups. The greatest amount of cell death (16-19%) occurred at 3 days in vitro, while fewer cells (8-12%) were dying at 7 days in vitro. To further characterize the dying glia, glial fibrillary acidic protein (GFAP) and A2B5 immunocytochemistry were combined with viability markers. Only GFAP immunoreactive process-bearing cells and A2B5 immunoreactive cells (process-bearing cells and possibly some neurons) were dying in culture, whereas the death of flat, polyhedral GFAP-positive cells was not observed. Cell survival was not affected by morphine, but may be affected by culture conditions. Thus, morphine-induced reductions in glial numbers did not result from an increased rate of cell death. Collectively, the present and previous findings suggest that morphine inhibits the production of flat, polyhedral astrocytes solely by decreasing their rate of proliferation.

Ontogeny of proenkephalin mRNA and enkephalin peptide expression in the cerebellar cortex of the rat: spatial and temporal patterns of expression follow maturational gradients in the external granular layer and in Purkinje cells

Proenkephalin mRNA and peptide products were examined in developing cells of the postnatal rat cerebellar cortex using in situ hybridization and immunocytochemistry. On day 7, proenkephalin mRNA was first detected as discrete cellular labeling in Golgi cells and as a diffuse hybridization signal over the Purkinje cell layer. On day 14, proenkephalin mRNA and peptide products primarily appeared in distinct subpopulations of Purkinje cells present in the posterior and lateral cerebellum. Similarly, in the external granular layer (EGL), enkephalin immunoreactivity was present only in the posterior and lateral portions of the cerebellum on day 14. However, proenkephalin mRNA was not detected in enkephalin-immunoreactive EGL cells. On day 21, the subset of Purkinje cells that expressed proenkephalin mRNA and peptides were distributed more uniformly throughout the cerebellum. On day 28, a few enkephalin-immunoreactive Purkinje cells were uniformly present throughout the cerebellum, but proenkephalin mRNA was not detected in most of these cells. The spatial gradients in proenkephalin mRNA expression evident in the Purkinje cells of younger rats were no longer present in 28-day-old rats. These findings are important, because endogenous opioids such as enkephalin have been previously shown to inhibit the growth of Purkinje cell dendrites and dendritic spines, and inhibit the rate of mitosis in EGL neuroblasts. Cells do not develop at uniform rates within the cerebellum. There are regional differences in the timing of the formation of the EGL, and in the morphogenesis of Purkinje cells. In conjunction with previous work, the present findings suggest that during development, the pattern of enkephalin immunoreactivity in Purkinje and EGL cells closely follows the spatial and temporal gradients of maturation in both these cell types. The emergence and disappearance of enkephalin immunoreactivity in Purkinje and EGL cells is spatially and temporally related, and coincides with proenkephalin mRNA expression in Purkinje cells. Thus, the transient and coordinated appearance of enkephalin in cerebellar Purkinje and EGL cells may contribute to regional differences in the rate of cerebellar maturation, and may help synchronize the developmental interactions between these two cell types.

Immunocytochemical visualization of kappa-opioid receptors on chick embryonic neurons differentiating in vitro

The present paper is the first immunocytochemical demonstration of kappa-opioid receptors in neurons isolated from seven-day-old chick embryonic forebrains and cultivated for one to seven days. The monoclonal antibody KA8 (IgG1-k) [Maderspach et al. (1991) J. Neurochem. 56, 1897-1904] was raised against the frog brain kappa-opioid receptor as an antigen and recognizes an epitope in or near the ligand binding site. The KA8 immunostaining of the neurons displayed individual variations and changed with the in vitro differentiation. Receptors often appeared at the pole of the primary outgrowing process, later on in the whole soma and finally on the branched processes. Specific radioligand binding and KA8 immunocytochemistry both presented an increase in the receptor concentration with development. The equilibrium binding values that were measured at 1 nM [3H]naloxone concentration were 2.9 and 6.1 fmol/10(6) cells on the first and sixth cultivation days, respectively. Neurons were treated with 10(-7) M bremazocine or dynorphine (agonists with relative specificity to kappa-opioid receptors) on the second and third cultivation days. The agonist promoted the morphological differentiation which was already visible within 24 h. It also promoted the expression of the 200,000 mol. wt neurofilament protein, this became pronounced after two to three days. The changes provoked by the agonist were reduced by the opioid antagonist norbinaltorphimine (10(-7) M) or naloxone (10(-5) M) indicating that the effect was receptor-mediated. The hypothesis that kappa-opioid agonists through their receptors may function as regulatory signals in the early neuronal differentiation is discussed.

Endogenous opioid systems and the growth of oligodendrocyte progenitors: paradoxical increases in oligodendrogenesis as an indirect mechanism of opioid action

Endogenous opioids inhibit nervous system development by inhibiting the proliferation of certain neuronal and glial progenitors. To determine whether opioids affect the growth of preoligodendrocytes, the effects of the endogenous opioid [Met5]-enkephalin were examined in preoligodendrocytes in primary mixed-glial and preoligodendrocyte-enriched (> 98% pure) cultures. Proliferating preoligodendrocytes in mixed-glial or preoligodendrocyte-enriched cultures were continuously treated for a total of 40 h with either basal growth media (controls), 1 microM [Met5]-enkephalin, 1 microM [Met5]-enkephalin plus the opioid antagonist naloxone (3 microM), or naloxone alone (3 microM), and incubated in [3H]-thymidine (0.2 microCi/ml/4-6 h) after 34-36 h of opioid exposure. Opioid-dependent changes in DNA synthesis were assessed autoradiographically in O4-immunoreactive oligodendrocyte progenitors. Naloxone alone significantly decreased the rate of DNA synthesis and number of O4-immunoreactive preoligodendrocytes in mixed-glial cultures. However, naloxone and/or [Met5]-enkephalin did not affect DNA synthesis or the number of O4-immunoreactive preoligodendrocytes in cultures enriched in preoligodendrocytes. The results suggest that astrocytes, or perhaps another cell type, play a permissive role in opioid-dependent alterations in preoligodendrocyte proliferation. Endogenous opioids affect the genesis of neural cells by both direct and indirect mechanisms.

Ontogeny of prodynorphin gene expression in the rat hypothalamus

Opioid peptides, deriving from prodynorphin, proenkephalin and proopiomelanocortin genes, have been shown to modulate brain development. Prodynorphin gene expression was studied here by in situ hybridization in the developing rat hypothalamus using oligodeoxynucleotide probes. Prodynorphin mRNA-synthetizing cells were observed in the ventromedial hypothalamic nucleus, the supraoptic and the paraventricular nuclei from embryonic days 16, 18 and 21, respectively. We detected no transient expression of prodynorphin gene in the rat hypothalamus. Prodynorphin mRNA-containing cells were also observed prenatally in the striatum, the cortex, the hippocampus and the amygdala. When compared with data from the literature, our results suggest that translation may immediately follow transcription of prodynorphin gene in the supraoptic nucleus. The presence of prodynorphin mRNA in the developing rat hypothalamus also raises the possibility of an involvement of prodynorphin-derived peptides in developmental processes and/or in the maturation of adult neural regulations.

Morphine suppresses DNA synthesis in cultured murine astrocytes from cortex, hippocampus and striatum

To determine whether there are regional differences in the ability of opiates to affect astrocyte proliferation, the effects of morphine were examined in astrocyte-enriched cultures from striatum, hippocampus and cerebral cortex derived from newborn mouse brains. Cultures from each region were continuously incubated in media alone (controls), or in media treated with 1 microM morphine, 1 microM morphine plus 3 microM naloxone, or 3 microM naloxone alone. Before harvesting at 6 days in vitro, cultures were exposed to [3H]thymidine (0.24 mu CI/ml for 16 h). Thymidine-labeling index was determined autoradiographically in flat, polyhedral (type 1) glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes. Morphine significantly inhibited [3H]thymidine incorporation in astrocytes from all three brain regions, although regional differences in labeling indices were noted. The results show that opiates can intrinsically affect the proliferative rate of astrocytes from diverse brain regions.

kappa-Opioid agonist modulation of [3H]thymidine incorporation into DNA: evidence for the involvement of pertussis toxin-sensitive G protein-coupled phosphoinositide turnover

A body of evidence has indicated that mu-opioid agonists can inhibit DNA synthesis in developing brain. We now report that kappa-selective opioid agonists (U69593 and U50488) modulate [3H]thymidine incorporation into DNA in fetal rat brain cell aggregates in a dose- and developmental stage-dependent manner, kappa agonists decreased thymidine incorporation by 35% in cultures grown for 7 days, and this process was reversed by the kappa-selective antagonist, norbinaltorphimine, whereas in 21-day brain cell aggregates a 3.5-fold increase was evident. Cell labeling by [3H]thymidine was also inhibited by the kappa-opioid agonist as shown by autoradiography. In addition, U69593 reduced basal rates of phosphoinositide formation in 7-day cultures and elevated it in 21-day cultures. Control levels were restored by norbinaltorphimine. Pertussis toxin blocked U69593-mediated inhibition of DNA synthesis. The action of kappa agonists on thymidine incorporation in the presence of chelerythrine, a protein kinase C (PKC) inhibitor, or in combination with LiCl, a noncompetitive inhibitor of inositol phosphatase, was attenuated in both 7- and 21-day cultures. These results suggest that kappa agonists may inhibit DNA synthesis via the phosphoinositide system with a pertussis toxin-sensitive G protein as transducer. In mixed glial cell aggregates, U50488 increased thymidine incorporation into DNA 3.1-fold, and this stimulation was reversed by the opioid antagonist naltrexone.

Nerve growth factor and its receptor are differentially modified by chronic naltrexone treatment during rat brain development

In order to examine the relationship between the action of opioid neurotransmitters and growth factors in the regulation of brain development, we have studied the long-term effect of the opiate antagonist naltrexone (NTX) on the content of nerve growth factor (NGF) in cortex, hippocampus, septum and neostriatum, and on NGF receptor (NGFRs) levels in cortical membranes. 50 mg/kg NTX treatment induced a decrease in the number of 125I-NGF high-affinity binding sites, without detectable changes in NGF levels. However, low doses of NTX (1 mg/kg) produced no differences in 125I-NGF binding sites, but induced a decrease in NGF levels in hippocampus, septum and neostriatum. These results suggest that NGF and NGFRs could be involved in the trophic effects of opioids during brain development.

Morphine regulates DNA synthesis in rat cerebellar neuroblasts in vitro

The effects of morphine on DNA synthesis by external granular layer (EGL) neuroblasts was examined in whole-mount organotypic cultures isolated from 10-day-old rat cerebella using bromodeoxyuridine (BrdU). After 24 h in vitro, explants were treated for 24 h with 10 nM, 1 or 100 microM morphine, morphine plus 30 nM, 3 or 300 microM of the opiate antagonist naloxone, respectively, or those concentrations of naloxone alone. BrdU was added during the last 4 h of drug treatment. EGL neuroblasts were unambiguously identified by size and morphology, location and by protein kinase C II immunocytochemistry. The proportion of EGL neuroblasts incorporating BrdU was significantly reduced in the presence of 1 microM morphine, while 100 microM morphine had little additional effect. The concentration of morphine predicted to cause a half-maximal reduction in BrdU labeling index was 22.5 nM. Morphine's ability to reduce BrdU incorporation by EGL neuroblasts was concentration dependent and was prevented by concomitant treatment with naloxone, implicating the involvement of opioid receptors. The results suggest that morphine can directly regulate the growth of the developing cerebellum by inhibiting neuroblast proliferation within the EGL.

Evidence for the implication of phosphoinositol signal transduction in mu-opioid inhibition of DNA synthesis

An opioid receptor agonist, [D-Ala2,Me-Phe4,Glyol5]enkephalin (DAMGE), decreased [3H]thymidine incorporation into DNA of fetal rat brain cell aggregates. This action proved to depend on the dose of this enkephalin analog and the interval the aggregates were maintained in culture. The opioid antagonist naltrexone and the mu-specific antagonist cyclic D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr amide (CTOP) reversed the DAMGE effect, arguing for a receptor-mediated mechanism. The mu-opioid nature of this receptor was further established by inhibiting DNA synthesis with the highly mu-selective agonist morphiceptin and blocking its action with CTOP. Several other opioids, pertussis toxin, and LiCl also diminished DNA synthesis, whereas cholera toxin elicited a modest increase. Naltrexone completely reversed the inhibition elicited by the combination of DAMGE and low doses of LiCl but not by that of high levels of LiCl alone. The enkephalin analog also reduced basal [3H]inositol trisphosphate and glutamate-stimulated [3H]inositol monophosphate and [3H]inositol bisphosphate accumulation in the aggregates. These DAMGE effects were reversed by naltrexone and were temporally correlated with the inhibition of DNA synthesis. A selective protein kinase C inhibitor, chelerythrine, also inhibited thymidine incorporation dose-dependently. The effect of DAMGE was not additive in the presence of chelerythrine but appeared to be consistent with their actions being mediated via a common signaling pathway. These results suggest the involvement of the phosphoinositol signal transduction system in the modulation of thymidine incorporation into DNA by DAMGE.

Differential development of beta-endorphin and mu opioid binding sites in mouse brain

Mouse brains of various ages from embryonal day 14 (E14) to adult were analyzed for opioid receptor binding using the enkephalin analog Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAMGE) and the opiate alkaloid dihydromorphine (DHM) as mu-selective radioligands. Binding parameters were estimated from homologous and heterologous competition binding curves. During the postnatal period, Kd values for [3H]DAMGE did not change but Bmax values (fmol/mg protein) increased 2.7 fold from postnatal day 3 (P3) to P7. Minor receptor density fluctuations were evident from P7 to adult. Similar results were obtained with [3H]DHM. In contrast, estimation of total mu binding sites (fmol/brain) revealed a continuous rise from P3 to the adult. The postnatal developmental profile of total mu binding sites was comparable to the weight gain of mouse brain and the increase in protein content. In contrast, during the same period beta-endorphin immunoreactivity (IR) levels undergo an increase that is inversely proportional to mu opioid receptor Bmax values. [3H]DAMGE binding to E14 membrane preparations was inhibited to a greater extent by Gpp(NH)p than that to P1 or adult. Additional characterization of mu receptors was accomplished by heterologous competition binding assays. IC50 values for beta-endorphin in competition with [3H]DHM and [3H]DAMGE were age dependent and differed for the two radioligands. These results suggest that mu receptor selectivity for mu-specific peptide and alkaloid ligands changes as a function of age.

An opioid growth factor regulates the replication of microorganisms

An opioid growth factor (OGF), [Met5]-enkephalin, interacts with the zeta (zeta) opioid receptor to modulate development of eukaryotes. We have found that [Met5]-enkephalin, an endogenous opioid peptide serves to inhibit the growth of S. aureus. This effect on growth involves cell proliferative events and is under tonic control, since potent opioid antagonists accelerate cell replication. Both the OGF and zeta opioid receptor were associated with these microorganisms. Other opioid receptors (mu, delta and kappa) were not detected. OGF also controlled the growth of other bacteria: P. aeruginosa and S. marcesans. These results indicate that OGF and its receptor, known to be important in the regulation of mammalian development, also function in the growth of simple unicellular organisms. We suggest that the endogenous opioid system related to growth originated billions of years ago.

Characterization of opioid-dependent glial development in dissociated and organotypic cultures of mouse central nervous system: critical periods and target specificity

Opioid-dependent changes in glial growth were characterized in primary dissociated and organotypic explant cultures of the developing mouse central nervous system (CNS) continuously grown in the presence of an endogenous opioid, [Met5]enkephalin, or an opiate drug, morphine. The glia in dissociated, astrocyte-enriched cultures derived from the cerebra of postnatal day 1, 3, or 5 mice, respectively, displayed age-dependent reductions in glial numbers that occurred at 3, 7, or 9 days in vitro (DIV) in response to continuous [Met5]enkephalin (10(-6) M) exposure. In contrast, in cultures derived from gestational day 19 mice, glial numbers were not reduced following continuous exposure to 10(-6) M [Met5]enkephalin during the first 7 days in vitro. An examination of [3H]thymidine incorporation by glial fibrillary acidic protein-(GFAP) immunoreactive astrocytes with flat (type 1) morphology in dissociated cultures derived from postnatal day 1 mice revealed that the reduction in glial numbers at 3 DIV was not immediately preceded by a reduction in the rate of [3H]thymidine incorporation at 2 DIV, although previous studies have shown that opioids inhibit the rate of [3H]thymidine incorporation by more mature astrocytes at 4 or 6 DIV. Early (i.e., at 2 to 3 DIV) changes in glial numbers may result from an inhibition of the proliferative rate of non-GFAP-containing glia or astrocyte precursors, or an enhanced rate of glial death. The rate of [3H]thymidine incorporation by GFAP-immunoreactive astrocytes with process-bearing (type 2) morphology was unchanged by opioid treatment. In separate experiments, a comparison of the area of growth of GFAP-immunoreactive astrocytes in paired symmetrical (right vs left) organotypic explant cultures demonstrated that opiates (i.e., 10(-5) M morphine) can inhibit astrocyte growth when the normal histiotypic organization of neurons and glia are maintained, and that there are regional differences in astrocyte responsiveness. Opioid-dependent alterations in astrocyte growth were mediated through specific opioid receptors since they were prevented by simultaneous treatment with (-)naloxone. The results suggest that the ability of opioids to modify glial growth is highly selective and varies depending on astrocyte type, as well as temporal and regional factors. Spatial and temporal differences in the response of developing glia to opioids may determine critical periods of CNS vulnerability to opioids in the maturing brain.

Ontogeny of proenkephalin gene expression in the rat hypothalamus

In the rat hypothalamus, proenkephalin (PE) mRNA synthetizing cells were detected by in situ hybridization, using synthetic oligodeoxy-nucleotides, from embryonic day 14 (E14) in the presumptive anterior hypothalamic area (AHA) and preoptic part of the bed nucleus of the stria terminalis (BST), and from E18 in the developing median preoptic area, perifornical area, suprachiasmatic nucleus, dorsomedial and ventromedial hypothalamic nuclei. In the paraventricular nucleus, cells expressed PE gene in the late prenatal stages; both parvo- and magnocellular neurons synthetized PEmRNA in the early postnatal stages. Cells expressing PE gene were observed after birth in the lateral preoptic area, lateral hypothalamus, medial and lateral parts of the BST. PEmRNA was also found from E14 in the striatum, from E18 in the central and medial amygdaloid nuclei, the medial group of the thalamic nuclei, and postnatally in a second more anterior structure of the thalamus. In the hypothalamus, a clear similarity was observed between adult and developmental distributions of PE gene expressing cells. The early onset of PE gene expression in the developing rat diencephalon suggests an involvement of PE in developmental processes, such as cell proliferation and differentiation; the presence of PE during the perinatal period may also indicate the appearance of adult neural regulations.

Glial growth is regulated by agonists selective for multiple opioid receptor types in vitro

To determine whether one or more opioid receptor types might be preferentially involved in gliogenesis, primary mixed glial cultures derived from mouse cerebra were continuously treated with varying concentrations of opioid agonists selective for mu (mu), i.e., DAGO ([D-Ala2, MePhe4, Gly(ol)5]enkephalin), delta (delta), i.e., DPDPE ([D-PEN2,D-PEN5]enkephalin), or kappa (kappa), i.e., U69,593, opioid receptor types. In addition, a group of cultures was treated with [Met5]-enkephalin, an agonist for delta opioid receptors as well as putative zeta (zeta) opioid receptors. Opioid-dependent changes in growth were assessed by examining alterations in (1) the number of cells in mixed glial cultures at 3, 6, and 8 days in vitro (DIV), (2) [3H]thymidine incorporation by glial fibrillary acidic protein (GFAP) immunoreactive, flat (type 1) astrocytes at 6 DIV, and (3) the area and form factor of GFAP-immunoreactive, flat (type 1) astrocytes. DPDPE at 10(-8) or 10(-10) M, as well as [Met5]-enkephalin at 10(-6), 10(-8), or 10(-10) M, significantly reduced the total number of glial cells in culture; but this effect was not observed with DAGO or U69,593 (both at 10(-6), 10(-8), or 10(-10) M). Equimolar concentrations (i.e., 10(-6) M) of [Met5]enkephalin or U69,593, but not DPDPE or DAGO, suppressed the rate of [3H]thymidine incorporation by GFAP-immunoreactive, flat (type 1) astrocytes. DAGO had no effect on growth, although in previous studies morphine was found to inhibit glial numbers and astrocyte DNA synthesis. [Met5]enkephalin (10(-6) M) was the only agonist to significantly influence astrocyte area. Collectively, these results indicate that delta (and perhaps mu) opioid receptor agonists reduce the total number of cells in mixed glial cultures; while [Met5]enkephalin-responsive (and perhaps kappa-responsive) opioid receptors mediate DNA synthesis in astrocytes. This implies that delta opioid receptors, as well as [Met5]enkephalin-sensitive, non-delta opioid receptors, mediate opioid-dependent regulation of astrocyte and astrocyte progenitor growth. These data support the concept that opioid-dependent changes in central nervous system growth are the result of endogenous opioid peptides acting through multiple opioid receptor types.

Morphine alters astrocyte growth in primary cultures of mouse glial cells: evidence for a direct effect of opiates on neural maturation

To determine whether exogenous opiate drugs with abuse liability directly modify neural growth, the present study investigated the effects of morphine on astrocyte proliferation and differentiation in primary cultures of murine glial cells. The results indicate that morphine decreases glial cell production in a dose-dependent, naloxone-reversible manner. Most notably, gliogenesis virtually ceased in the presence of 10(-6) M morphine during the first week in culture, whereas 10(-8) M or 10(-10) M morphine caused an intermediate suppression of growth compared to control or 10(-6) M morphine treated cultures. Moreover, morphine treatment inhibited [3H]thymidine incorporation by glial fibrillary acidic protein (GFAP) immunoreactive, flat (type 1) astrocytes, suggesting that the decrease in glial cell production was due in part to an inhibition of astrocyte proliferation. Morphine also caused significant increases in both cytoplasmic area and process elaboration in flat (type 1) astrocytes indicating greater morphologic differentiation. In the above experiments, morphine-dependent alterations in astrocyte growth were antagonized by naloxone, indicating that morphine action was mediated by specific opioid receptors. These observations suggest that opiate drugs can directly modify neural growth by influencing two critical developmental events in astrocytes, i.e., inhibiting proliferation and inducing morphologic differentiation.

Identification of opioid peptides regulating proliferation of neurons and glia in the developing nervous system

Endogenous opioid systems (i.e. opioids and opioid receptors) play a role in regulating neural development. Using the cerebellar cortex of 6-day-old rats, the most potent opioid peptides involved with cell proliferation were assessed. In both the external germinal (granule) layer (EGL), a germinative matrix giving rise to neurons, and the medullary layer (MED), a pool of cells that are the precursors of glia (astrocytes and oligodendrocytes), [Met5]enkephalin and peptide F were extremely potent in depressing the labeling index (LI) using [3H]thymidine and autoradiographic techniques; concentrations as low as 100 micrograms/kg reduced the LI of EGL cells by 24% and MED cells by 43%. This inhibition of DNA synthesis by opioid peptides was blocked by concomitant exposure to to naloxone, an opioid antagonist. Peptide action was apparent 2 h following drug administration, and concentrations of 80 micrograms/kg but not 1 or 10 micrograms/kg [Met5]enkephalin depressed the LI. These results identify a selective group of opioid peptides, derived from proenkephalin A, as the potent, natural, inhibitory factors targeted to cell proliferation of cells destined to be neurons and glia in the developing nervous system.

Effects of central administration of beta-endorphin on brain and liver DNA synthesis in preweanling rats

We have previously shown that central administration of beta-endorphin results in a reduction of ornithine decarboxylase activity. Ornithine decarboxylase catalyses the rate-limiting step in the biosynthesis of the polyamines putrescine, spermidine and spermine, thought to modulate nucleic acid synthesis. The present study examines the effects of intracisternal injection of beta-endorphin on brain and liver DNA synthesis in preweanling rats. In six-day-old rats, beta-endorphin (0.75 micrograms/g brain wt) produced approximately a 70% inhibition in brain and liver DNA synthesis 1 h after injection, and values were still subnormal in both tissues 10 h later. Subcutaneous administration of beta-endorphin did not alter liver DNA synthesis. Thus, it is most likely that the suppressed liver DNA synthesis observed in animals given beta-endorphin intracisternally is mediated by central mechanisms. Co-administration of naloxone plus beta-endorphin intracisternally prevented the response, indicating an opioid receptor-mediated phenomenon. Naloxone alone caused small but significant increases in brain and liver DNA synthesis, suggesting a tonic influence on tissue DNA by endogenous opioids in the CNS. Acute inhibition of ornithine decarboxylase activity by alpha-difluoromethylornithine did not alter DNA synthesis, indicating that the decreases in DNA synthesis induced by beta-endorphin are unrelated to the ornithine decarboxylase/polyamine system. The effect appears to be restricted to early development as no significant changes in DNA synthesis were obtained in 20-day-old animals. The results from these studies indicate that CNS beta-endorphin has the ability to influence DNA synthesis in central as well as in peripheral tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient appearance of beta-endorphin immunoreactive cells within the germinal zone of neonatal rat forebrain

Recent evidence implicates endogenous opioid systems in basic processes which underlie morphogenesis. The present report describes a population of cells within the germinal zone of the neonatal rat forebrain which are immunoreactive for the opioid peptide beta-endorphin and other peptides derived from the proopiomelanocortin precursor. These cells are present at the time of birth, but are no longer detectable by the sixth postnatal day. They have medially and laterally directed processes which extend to the ventricular wall and across the caudate putamen to its lateral border. Cells of similar morphology and distribution which are immunoreactive for two other proopiomelanocortin peptides, alpha-melanocyte stimulating hormone and adrenocorticotrophic hormone, were also observed in similar distributions during the same developmental period. These data are consistent with the hypothesis that cells within the germinal zone transiently synthesize proopiomelanocortin, which is further processed to yield these three peptide products. This finding may be important in understanding the role of proopiomelanocortin-derived peptides in neural development.

Effects of chronic postnatal opioid receptor blockade by naltrexone upon proliferation capacity in the prenatally X-irradiated brain of the rat

We recently reported that in rats prenatally X-irradiated on gestation day 14 with 1 Gy, postnatal chronic application of the opioid antagonist naltrexone (NX) led to a remarkable growth spurt of the microencephalic brain. In the present study we present histological and autoradiographic results found in the subependymal layer (SEL) of the forebrain lateral ventricles. NX led to an intermittent augmentation of the mitotic index of the X-irradiated brains within a postnatal observation period of 24 weeks. The most conspicuous finding was transient hyperplasia of the SEL at 4-6 weeks of age which occurred in close proximity to an intact ependymal lining. Districts of the lateral ventricles which were denuded from ependyme and where the rest of the ependymal layer (EL) was dislocated peripherally showed upon NX treatment a long-lasting SEL hyperplasia with a tendency towards dysplasia. These results revealed that repair proliferation of embryotoxic X-irradiation is normally under strong control by the opioid system. If that system, which exerts a suppressing effect upon glial growth, is blocked by NX, prominent hyperplastic reactions occur which may be useful for repairing the lesion pattern.

Effects of beta-endorphin on DNA synthesis in brain regions of preweanling rats

Recent studies of whole brain in rat pups have shown a marked decrease in DNA synthesis following intracisternal (i.c.) administration of beta-endorphin (BE). This investigation examines DNA synthesis in the cerebral cortex and cerebellum to determine whether the effect shows regional selectivity. Two- to twenty-day-old rats were given a single ic injection of BE, and DNA synthesis was assessed 1 h later. In the cerebral cortex, a region that undergoes major phases of cell multiplication in the immediate pre- and postnatal periods, BE significantly decreased DNA synthesis in 2-day-old rats, and a maximal inhibition was obtained by 4 days of age. In contrast, the cerebellum, a region that grows predominantly after birth, showed less sensitivity to BE during the early postnatal days, and a maximal effect was not attained until 10 days of age. While at 15 days of age the inhibition began to diminish in the cortex, a maximal effect was still seen in the cerebellum. Naloxone prevented the response in both brain regions, indicating the participation of opioid receptors. These results indicate that CNS BE is apparently able to alter DNA synthesis throughout the brain, with the greatest sensitivity occurring in those regions with highest mitotic rates at the time of exposure to BE.

Cellular localization of proenkephalin mRNA and enkephalin peptide products in cultured astrocytes

To identify the possible cellular sites of opioid gene expression during ontogeny, proenkephalin mRNA and enkephalin peptide expression were examined, respectively, by in situ hybridization and immunocytochemistry in organotypic explants of rat cerebellum and in astrocyte-enriched cultures of murine cerebral hemispheres. High levels of proenkephalin mRNA and enkephalin immunoreactivity were detected in immature cells identified as astrocytes. Double-labeling studies combining in situ hybridization and immunocytochemical localization of the astrocytic marker, glial fibrillary acidic protein, provided direct evidence that proenkephalin mRNA is expressed by astrocytes in culture. Based on previous studies that Met-enkephalin can inhibit astrocyte growth in vitro, the present results suggest that proenkephalin gene expression by astrocytes is important during central nervous system maturation.