Adrenergic control of DNA synthesis in developing rat brain regions: effects of intracisternal administration of isoproterenol

Norepinephrine, neurodevelopment and behavior

Neurotransmitters play critical roles in the developing nervous system. Among the neurotransmitters, norepinephrine (NE) is in particular postulated to be an important regulator of brain development. NE is expressed during early stages of development and is known to regulate both the development of noradrenergic neurons and the development of target areas. NE participates in the shaping and the wiring of the nervous system during the critical periods of development, and perturbations in this process can alter the brain's developmental trajectory, which in turn can cause long-lasting and even permanent changes in the brain function and behavior later in life. Here we will briefly review evidence for the role of noradrenergic system in neurodevelopmental processes and will discuss about the potential disruptors of noradrenergic system during development and their behavioral consequences.

Developmental neurotoxicity resulting from pharmacotherapy of preterm labor, modeled in vitro: Terbutaline and dexamethasone, separately and together

Terbutaline and dexamethasone are used in the management of preterm labor, often for durations of treatment exceeding those recommended, and both have been implicated in increased risk of neurodevelopmental disorders. We used a variety of cell models to establish the critical stages at which neurodifferentiation is vulnerable to these agents and to determine whether combined exposures produce a worsened outcome. Terbutaline selectively promoted the initial emergence of glia from embryonic neural stem cells (NSCs). The target for terbutaline shifted with developmental stage: at later developmental stages modeled with C6 and PC12 cells, terbutaline had little effect on glial differentiation (C6 cells) but impaired the differentiation of neuronotypic PC12 cells into neurotransmitter phenotypes. In contrast to the specificity shown by terbutaline, dexamethasone affected both neuronal and glial differentiation at all stages, impairing the emergence of both cell types in NSCs but with a much greater impairment for glia. At later stages, dexamethasone promoted glial cell differentiation (C6 cells), while shifting neuronal cell differentiation so as to distort the balance of neurotransmitter phenotypes (PC12 cells). Finally, terbutaline and dexamethasone interacted synergistically at the level of late stage glial cell differentiation, with dexamethasone boosting the ability of terbutaline to enhance indices of glial cell growth and neurite formation while producing further decrements in glial cell numbers. Our results support the conclusion that terbutaline and dexamethasone are directly-acting neuroteratogens, and further indicate the potential for their combined use in preterm labor to worsen neurodevelopmental outcomes.

Alpha 1-adrenoceptor agonists protect against stress-induced death of neural progenitor cells

Here, we show that alpha(1)-adrenoceptor agonists suppress stress-induced death of mouse embryonic brain-derived neural progenitor cells (NPCs). NPCs highly expressed both alpha(1A)- and alpha(1B)-adrenoceptor genes, whereas the gene encoding alpha(1D)-adrenoceptor was expressed at low levels. Application of the alpha(1)-adrenoceptor agonists phenylephrine and cirazoline significantly promoted cell survival of embryonic NPCs that had been exposed to stress, as measured by a lactate dehydrogenase release assay, but had no remarkable effect on differentiation of the NPCs. Both phenylephrine and cirazoline protected NPCs from death induced by growth factor deprivation, N2 nutrient deprivation, tunicamycin treatment or staurosporine treatment. Phenylephrine and cirazoline treatments both maximally reduced stress-induced cell death by approximately 60% but did not change the percentage of undifferentiated cells as measured by nestin staining. Moreover, phenylephrine and cirazoline treatments did not affect the cellular activities of caspase-3 and caspase-7 but markedly reduced propidium iodide penetration into the cytoplasm, suggesting that alpha(1)-adrenoceptor agonists inhibit caspase-3/7-independent death of the embryonic NPCs.

Disruption of rat forebrain development by glucocorticoids: critical perinatal periods for effects on neural cell acquisition and on cell signaling cascades mediating noradrenergic and cholinergic neurotransmitter/neurotrophic responses

Glucocorticoids are the consensus treatment for the prevention of respiratory distress in preterm infants, but there is evidence for increased incidence of neurodevelopmental disorders as a result of their administration. We administered dexamethasone (Dex) to developing rats at doses below or within the range of those used clinically, evaluating the effects on forebrain development with exposure in three different stages: gestational days 17-19, postnatal days 1-3, or postnatal days 7-9. At 24 h after the last dose, we evaluated biomarkers of neural cell acquisition and growth, synaptic development, neurotransmitter receptor expression, and synaptic signaling mediated by adenylyl cyclase (AC). Dex impaired the acquisition of neural cells, with a peak effect when given in the immediate postnatal period. In association with this defect, Dex also elicited biphasic effects on cholinergic presynaptic development, promoting synaptic maturation at a dose (0.05 mg/kg) well below those used therapeutically, whereas the effect was diminished or lost when doses were increased to 0.2 or 0.8 mg/kg. Dex given postnatally also disrupted the expression of adrenergic receptors known to participate in neurotrophic modeling of the developing brain and evoked massive induction of AC activity. As a consequence, disparate receptor inputs all produced cyclic AMP overproduction, a likely contributor to disrupted patterns of cell replication, differentiation, and apoptosis. Superimposed on the heterologous AC induction, Dex impaired specific receptor-mediated cholinergic and adrenergic signals. These results indicate that, during a critical developmental period, Dex administration leads to widespread interference with forebrain development, likely contributing to eventual, adverse neurobehavioral outcomes.

Transiently overexpressed alpha2-adrenoceptors and their control of DNA synthesis in the developing brain

During brain development, neurotransmitters act as trophic factors controlling the patterns of cell replication and differentiation. Alpha2-adrenoceptors (alpha2ARs) are transiently overexpressed in zones with high mitotic activity and we evaluated whether these receptors are linked to DNA synthesis in the perinatal rat brain. Acute administration of clonidine (2 mg/kg), an alpha2AR agonist, elicited dramatic decreases in DNA synthesis in the forebrain, brainstem, and cerebellum whether given on gestational day (GD) 21, or on postnatal days (PN) 1 or 8. However, alpha2AR blockade elicited by yohimbine (2.5 mg/kg) also resulted in decreased DNA synthesis on GD21 and PN8, albeit to a smaller extent than with clonidine. Yohimbine was able to blunt the effects of clonidine, verifying that both drugs are acting through the same receptor population. Because betaARs are also known to regulate DNA synthesis, we used propranolol (10 mg/kg) blockade of betaARs to evaluate whether the alpha2AR effects were mediated by presynaptic autoreceptors that regulate the release of norepinephrine and consequent betaAR responses; the effects of yohimbine were still discernible in the presence of propranolol. Accordingly, transiently overexpressed alpha2ARs in the developing brain participate in the control of DNA synthesis in a biphasic manner, with promotional actions at low, endogenous levels of stimulation, but inhibitory effects when stimulation is high. Effects on alpha2ARs are likely to contribute to long-term consequences of adrenergic agents used in obstetrics or neurotoxicants that affect adrenergic activity.

Structural effects and neurofunctional sequelae of developmental exposure to psychotherapeutic drugs: experimental and clinical aspects

The advent of psychotherapeutic drugs has enabled management of mental illness and other neurological problems such as epilepsy in the general population, without requiring hospitalization. The success of these drugs in controlling symptoms has led to their widespread use in the vulnerable population of pregnant women as well, where the potential embryotoxicity of the drugs has to be weighed against the potential problems of the maternal neurological state. This review focuses on the developmental toxicity and neurotoxicity of five broad categories of widely available psychotherapeutic drugs: the neuroleptics, the antiepileptics, the antidepressants, the anxiolytics and mood stabilizers, and a newly emerging class of nonprescription drugs, the herbal remedies. A brief review of nervous system development during gestation and following parturition in mammals is provided, with a description of the development of neurochemical pathways that may be involved in the action of the psychotherapeutic agents. A thorough discussion of animal research and human clinical studies is used to determine the risk associated with the use of each drug category. The potential risks to the fetus, as demonstrated in well described neurotoxicity studies in animals, are contrasted with the often negative findings in the still limited human studies. The potential risk fo the human fetus in the continued use of these chemicals without more adequate research is also addressed. The direction of future research using psychotherapeutic drugs should more closely parallel the methodology developed in the animal laboratories, especially since these models have already been used extremely successfully in specific instances in the investigation of neurotoxic agents.

Does terbutaline damage the developing heart?

BACKGROUND

Beta(2)-Adrenoceptor (betaAR) agonists, such as terbutaline, are widely used to arrest preterm labor. They also cross the placenta where they stimulate receptors in fetal tissues, which in turn use betaAR input for trophic control of cell replication and differentiation.

METHODS

As rats are altricial, we administered terbutaline in two different postnatal exposure periods (10 mg/kg given daily on Days 2-5 or 11-14).

RESULTS

Hearts were examined twenty-four hours after the last dose and on postnatal day 30 for cardiac damage. Neither treatment paradigm caused an increase in cardiac abnormalities compared to controls but quantitative analysis of the number of nuclei indicated reductions in females.

CONCLUSIONS

These findings do not support earlier case reports of outright myocardial necrosis after terbutaline tocolysis in human infants. Nevertheless, the significant statistical association between terbutaline and cardiac anomalies in epidemiological studies suggest that terbutaline may sensitize the developing heart to other insults that affect development.

Alpha2-adrenergic receptor subtype alterations in the brainstem in the sudden infant death syndrome

BACKGROUND

The sudden infant death syndrome (SIDS) is still the main cause of postneonatal infant death. However, the causes and mechanisms of SIDS have never been completely elucidated. Catecholamines, via alpha2-adrenergic receptor (alpha2-AR) interactions, are known to influence brainstem autonomic and respiratory activity.

AIMS

To examine the catecholaminergic system abnormalities in SIDS victims, we investigated the alterations of alpha2-AR subtypes.

SUBJECTS AND METHODS

We examined the developmental changes of alpha2-AR subtypes in the brainstem, especially in cardiorespiratory nuclei, in 21 SIDS victims and 17 age-matched controls by means of immunohistochemical methods. For statistical analysis, the chi2-test or Fisher's exact probability test was performed.

RESULTS

There was a significant decrease in alpha2A-AR immunoreactivity in the solitary nucleus and ventrolateral medulla (VLM) in the medulla oblongata in SIDS victims compared with in control cases, but there were no significant differences of the alpha2B and alpha2C-AR immunoreactivity in the brainstem between SIDS victims and controls.

CONCLUSION

Alpha2A-AR immunoreactivity was selectively decreased in the solitary nucleus and VLM in the medulla oblongata in SIDS victims, so there was no possibility that it was secondary to chronic hypoxia or repeated ischemia. It may be related to some impairment of the cardiorespiratory neuronal system. Therefore, SIDS victims may be vulnerable to asphyxia, hypoxia, and/or hypercapnia, and fail to exhibit brainstem responses.

Developmental toxicity of terbutaline: critical periods for sex-selective effects on macromolecules and DNA synthesis in rat brain, heart, and liver

beta-Adrenoceptors (betaARs) control cell replication/differentiation, and during development, signaling is not subject to desensitization. We examined the effects of terbutaline, a beta(2)AR agonist used as a tocolytic, on development in rat brain regions and peripheral tissues with high betaAR concentrations. Prenatal terbutaline (gestational days 17-20) decreased cell numbers (DNA content) in the fetal brain and liver. Early postnatal exposure (PN2-5) reduced DNA synthesis in early-developing brain regions of females, with sensitization of the effect upon repeated terbutaline administration; after multiple terbutaline injections, DNA content was reduced in male cerebellum. The cerebellum was targeted later (PN11-14), exhibiting decreased DNA synthesis in both sexes; in contrast, cardiac DNA synthesis decreased after one injection but increased after the fourth daily injection. Our results suggest that excessive betaAR stimulation by terbutaline alters cell development in brain regions and peripheral tissues, with the net effect depending on sex and the timing of exposure. These effects may contribute to neuropsychiatric, cognitive, cardiovascular, and metabolic abnormalities reported in the offspring of women treated with beta-agonist tocolytics.

Expression of alpha(2)-adrenergic receptor subtypes in prenatal rat spinal cord

The results of molecular cloning have revealed three subtypes of the alpha(2)-adrenergic receptors (alpha(2) AR) that have been defined alpha(2)C10 (alpha(2A)), alpha(2)C2 (alpha(2B)) and alpha(2)C4 (alpha(2C)). The differential expression of alpha(2) AR subtypes is affected by developmental factors in rat submandibular gland, lung and brain. In the spinal cord of postnatal and adult rats, alpha(2A) and alpha(2C) AR subtypes are expressed and appear to mediate pain perception. However, the relative expression of alpha(2) AR subtypes in the prenatal spinal cord is unknown. In the present study subtype-specific antibodies and reverse transcription-polymerase chain reaction (RT-PCR) were used to determine the expression and localization of the alpha(2) AR subtypes in sections of embryonic day 14 rat spinal cords and primary cultures of cells isolated from these cords. Spinal cords were removed from day 14 embryos, and were sectioned or used for the preparation of cell cultures. After 9 days in culture, neurons were examined by immunofluorescence microscopy or used for preparation of total RNA. In both intact spinal cords and isolated cells, positive immunoreactivity was detected with antibodies against alpha(2A) and alpha(2B) subtypes, but not with antibodies against the alpha(2C) subtype. Using a dual-labeling approach, anti-alpha(2A) and anti-alpha(2B) immunoreactivity was present on the same population of neurons. RT-PCR results were consistent with immunofluorescence studies, and showed that mRNA encoding the alpha(2A) and alpha(2B) subtypes was present in total RNA prepared from primary cultures of rat spinal cord neurons. In contrast to spinal cords of postnatal or adult rats that express alpha(2A) and alpha(2C) AR subtypes on different neurons, prenatal spinal cords contain alpha(2A) and alpha(2B) AR subtypes, and these two subtypes appear to be co-expressed in the same cells.

Regulation of fetal cardiac and hepatic beta-adrenoceptors and adenylyl cyclase signaling: terbutaline effects

Terbutaline (Ter), a beta(2)-adrenergic agonist used in preterm labor, stimulates fetal beta-adrenoceptors (beta-ARs). We administered Ter to pregnant rats on gestational days 17-20 and examined beta-ARs and adenylyl cyclase (AC) signaling in heart and liver. Ter produced less downregulation of cardiac beta-ARs than in adults, despite a higher proportion of the beta(2)-subtype, and failed to elicit desensitization of the receptor-mediated AC response. AC stimulants acting at different points indicated an offsetting of homologous desensitization at the level of the beta-AR by heterologous sensitization at the level of AC: induction of total AC catalytic activity and a shift in the catalytic profile or AC isoform. In fetal liver, Ter produced downregulation of beta-ARs, in keeping with the predominance of the beta(2)-subtype; hepatic receptor downregulation was equivalent in fetus and adult. Nevertheless, there was still no desensitization of beta-AR-mediated AC responses and again AC was induced. Our results indicate that, unlike in the adult, fetal beta-AR signaling is not desensitized by beta-agonists and, in fact, displays heterologous sensitization, thus sustaining responses during parturition. At the same time, the inability to desensitize beta-AR AC responses may lead to disruption of cardiac, hepatic, or neural cell development as a consequence of tocolytic therapy with beta-agonists.

Behavioral and neural consequences of prenatal exposure to nicotine

OBJECTIVE

To review evidence for the neurodevelopmental effects of in utero exposure to nicotine. Concerns about long-term cognitive and behavioral effects of prenatal exposure to nicotine arise from reports of increased rates of disruptive behavioral disorders in children whose mothers smoked during pregnancy. The relatively high rate of tobacco smoking among pregnant women (25% of all pregnancies in the U.S.) underlines the seriousness of these concerns.

METHOD

This review examines the largest and most recent epidemiological and clinical studies that investigated the association of prenatal nicotine exposure with health, behavioral, and cognitive problems. Because of the numerous potential confounding variables in human research, findings from animal studies, in which environmental factors are strictly controlled, are also discussed. Finally, neural and molecular mechanisms that are likely to underlie neurodevelopmental disruptions produced by prenatal nicotine exposure are outlined.

RESULTS

A dose-response relationship between maternal smoking rates and low birth weight (potentially associated with lower cognitive ability) and spontaneous abortion is consistently found, whereas long-term developmental and behavioral effects in the offspring are still controversial, perhaps because of the difficulty of separating them from other genetic and environmental factors. Despite the wide variability of experimental paradigms used in animal studies, common physical and behavioral effects of prenatal exposure to nicotine have been observed, including low birth weight, enhanced locomotor activity, and cognitive impairment. Finally, disturbances in neuronal pathfinding, abnormalities in cell proliferation and differentiation, and disruptions in the development of the cholinergic and catecholaminergic systems all have been reported in molecular animal studies of in utero exposure to nicotine.

CONCLUSIONS

Prenatal exposure to nicotine may lead to dysregulation in neurodevelopment and can indicate higher risk for psychiatric problems, including substance abuse. Knowledge of prenatal exposure to nicotine should prompt child psychiatrists to closely monitor at-risk patients.

Beta-adrenoceptor signaling and its control of cell replication in MDA-MB-231 human breast cancer cells

MDA-MB-231 human breast cancer cells express high beta-adrenoceptor levels, predominantly the beta2 subtype. Receptor stimulation by isoproterenol evoked immediate reductions in DNA synthesis which were blocked completely by propranolol and were of the same magnitude as effects elicited by high concentrations of 8-Br-cAMP. Isoproterenol-induced inhibition of DNA synthesis was maintained throughout several days of exposure, resulting in a decrement in total cell number, and the effects were augmented by cotreatment with dexamethasone; an even greater effect was seen when cAMP breakdown was inhibited by theophylline, with or without addition of isoproterenol. Despite the persistent effect of isoproterenol, receptor downregulation was evident with as little as 1 h of treatment, and over 90% of the receptors were lost within 24 h. Receptor downregulation was paralleled by homologous desensitization of the adenylyl cyclase response to beta-adrenoceptor stimulation. Dexamethasone augmented the effects of isoproterenol on DNA synthesis but did not prevent receptor downregulation or desensitization. These results indicate that beta-adrenoceptors are effectively linked, through cAMP, to the termination of cell replication in MDA-MB-231 human breast cancer cells, and that activation of only a small number of receptors is sufficient for a maximal effect. Novel pharmacologic strategies that focus on cell surface receptors operating through adenylyl cyclase may offer opportunities to combat cancers that are unresponsive to hormonal agents, or that have developed multidrug resistance.

Neonatal chlorpyrifos exposure alters synaptic development and neuronal activity in cholinergic and catecholaminergic pathways

After routine home application of chlorpyrifos (CPF), infant and child exposures can exceed acceptable levels. We treated neonatal rats daily on postnatal days (PN) 1-4 (1 mg/kg) or days 11-14 (5 mg/kg), treatments that evoked no overt signs of toxicity. Effects on the development of cholinergic neuronal function were assessed using choline acetyltransferase (ChAT) activity and hemicholinium-3 (HC-3) binding as indices of synaptic proliferation and synaptic activity, respectively. In the forebrain, early CPF treatment caused a decrease in ChAT without affecting HC-3 binding; late treatment decreased HC-3 binding without affecting ChAT. In the brainstem, early treatment had no effect on either parameter but late treatment decreased both ChAT and HC-3 binding. Effects of CPF were not limited to development of cholinergic synapses but also involved catecholamine pathways. For norepinephrine or dopamine, either early or late CPF treatment evoked an increase in synaptic activity (transmitter turnover). The cerebellum, a region with sparse cholinergic innervation, was affected the most. Effects on catecholamine systems were unrelated to the magnitude or temporal pattern of cholinesterase inhibition. Our results suggest that CPF exposure during the postnatal period of synaptogenesis elicits widespread disruption of cholinergic and catecholaminergic pathways. As this is the period in which patterns of synaptic responsiveness is programmed by neural input, the period of developmental vulnerability to CPF is likely to extend into childhood.

Cellular mechanisms for developmental toxicity of chlorpyrifos: targeting the adenylyl cyclase signaling cascade

Developmental neurotoxicity caused by chlorpyrifos exposure is generally thought to target cholinesterase but chlorpyrifos may also act on cellular intermediates, such as adenylyl cyclase, that serve global functions in the coordination of cell development. In the current study, neonatal rats were exposed to apparently subtoxic doses of chlorpyrifos (no weight loss, no mortality) either on Postnatal Days 1-4 or on Postnatal Days 11-14, and the effects on components of the adenylyl cyclase cascade were evaluated in brain regions that are enriched (forebrain) or sparse (cerebellum) in cholinergic innervation, as well as in a nonneural tissue (heart). In all three, chlorpyrifos evoked deficits in multiple components of the adenylyl cyclase cascade: expression and activity of adenylyl cyclase itself, functioning of G-proteins that link neurotransmitter and hormone receptors to cyclase activity, and expression of neurotransmitter receptors that act through this cascade. Disruption of signaling function was not restricted to transduction of cholinergic signals but rather extended to adrenergic signals as well. In most cases, the adverse effects were not evident during the immediate period of chlorpyrifos administration, but appeared after a delay of several days. These results suggest that chlorpyrifos can affect cell development by altering the activity and reactivity of the adenylyl cyclase signaling cascade, a major control point for trophic regulation of cell differentiation. The effects are not restricted to cholinergic targets, nor even to the central nervous system. Hence, disruption of cell development by chlorpyrifos is likely to be more widespread than previously thought.

Catecholaminergic regulation of proliferation and survival in rat forebrain paraventricular germinal cells

We have investigated the possible role of alpha1-adrenoreceptors in regulating the germination of progenitor cells cultured from embryonic rat neocortex. High binding levels of the alpha1-selective radioligand 3[H]prazosin were detected in the forebrain of the rat embryo at E13, and the greatest density of binding sites was localized to the ventricular and subventricular zones. Catecholamine-containing axon terminals were present in these zones in the same period. Germinal neuroepithelial cells retained specific 3[H]prazosin binding in culture. Approximately 25% of cells in culture displayed complex intracellular Ca2+ transients in response to phenylephrine, many of which were abolished with the alpha1B antagonist, chloroethylclonidine. Cultures exhibited concentration-dependent catecholamine stimulation of DNA synthesis mediated by alpha1 receptors in serum-limited conditions. Neuroepithelial cells were labelled via their ventricular processes by intraventricular injection of Fast blue in E13 embryos prior to transfer of the neocortex to dissociated cell culture. Many of labelled cells were present in culture in germinal foci. Some cells which migrated from these foci underwent apoptosis, as determined by TUNEL in situ hybridization. During a transitory period of up to 48 h in culture, alpha1-adrenoreceptor activation by phenylephrine or noradrenaline increased the number of surviving cells. Apoptosis was observed in vivo in both ventricular and subventricular zones of the neocortex from E13 to E15 in increasing numbers. We propose that both the supply of noradrenaline to forebrain germinal cells, and the expression of alpha1-adrenoreceptors on their surface could act to determine whether they die or continue to proliferate.

Neuronal control of cardiac and hepatic macromolecule synthesis in the neonatal rat: effects of sympathectomy

Neurotransmitters are thought to influence cell development in their target tissues. In the current study, neonatal rats were given 6-hydroxydopamine to produce permanent sympathetic denervation, and the effects on cardiac and hepatic DNA and protein synthesis were assessed. Lesioned animals showed deficits in cardiac DNA synthesis over the first 8 d postpartum, a period in which sympathetic innervation is sparse and synaptic norepinephrine concentrations are low; the effect of lesioning was also evident for protein synthesis. Subsequently, DNA synthesis in control animals declined precipitously during the second to third postnatal week, the phase associated with ingrowth of the majority of sympathetic terminals and sympathetic hyperactivity. Neonatal lesioning delayed the ontogenetic decline in DNA synthesis: this effect was not shared by protein synthesis. In the liver, a tissue whose cells, unlike the heart, maintain the ability to divide into adulthood, there was no effect of 6-hydroxydopamine on DNA synthesis and only minor changes in protein synthesis. These results suggest that neural input provides two distinct trophic signals to the developing heart: an early promotion of cell replication associated with low levels of stimulation, and a subsequent promotion of the switchover from cell replication, to cell differentiation and enlargement, associated with high levels of stimulation. In light of the precipitous rise in circulating catecholamines at parturition, and of the subsequent development of sympathetic innervation, catecholamines are likely to play a trophic role in the establishment of the proper pattern of cardiac cell development.

Expression of alpha 2 adrenoceptors during rat brain development--I. Alpha 2A messenger RNA expression

The distribution of alpha 2A adrenoceptor messenger RNA expression in developing rat brain was characterized using in situ hybridization with 35S-labeled riboprobes. Intense hybridization signal was detected as early as embryonic day 14 in several areas adjacent to the forebrain and hindbrain germinal zones and in central noradrenergic neurons. A marked increase in messenger RNA expression was observed throughout the brain during late prenatal development, consistent with the migration and maturation of neurons in developing brain structures. In embryonic brain, there was a temporal and spatial correspondence in the appearance of alpha 2A messenger RNA expression and binding sites labeled with [3H]idazoxan or p-[125I]iodoclonidine, indicating translation into receptor protein at an early stage of development. Whereas the presynaptic expression remained constant throughout development, there was an early postnatal decline of alpha 2A receptor expression in many brain regions, including the olfactory bulb, cortex, caudate-putamen, hippocampus, thalamus, hypothalamus and medulla. Thereafter, messenger RNA expression increased, establishing an adult-like pattern during the second postnatal week, but remained low in areas such as the caudate-putamen, thalamus and hippocampus, which do not exhibit extensive expression in the adult. The transient perinatal expression of this alpha 2 adrenoceptor type, which coincides with a period of hyperreactivity to sensory stimuli in the locus coeruleus, may indicate a specific functional role for the alpha 2A receptor in the developing rat brain. The early and intense expression in olfactory structures suggests an involvement in early olfactory learning. The pattern of widespread, transient expression of alpha 2A receptors in the fetal brain is in marked contrast to the postnatal development of the alpha 2C receptor type.

Cell-Cell Signaling in Early Cortical Development

The remarkable process of neuronal migration has long been viewed as the key event of corticogenesis. Before neurons complete their final cell division and begin to migrate out of the proliferative zones, however, many important decisions concerning their ultimate fate have already been made. These critical decisions are, in part, under local environmental control. The signals by which proliferating cortical precursor cells interact with each other and with their environment are still largely unknown, but, as these factors are dis covered, they will alter our view of corticogenesis and most likely provide new insights into causes of cerebral malformations. This review is intended to describe the repertoire of cellular interactions detected thus far among proliferating cortical cells and to discuss possible roles of cell-cell signaling pathways. The Neurosci entist 1:268-276, 1995

Development and isoproterenol-induced regulation of adrenoceptor binding in cultured rat neocortical explants is seen only with the beta-1, not with the beta-2 subtype

The presence and time-course of beta-adrenoceptor density in cultured explants of neocortex obtained from 6-day-old rat pups were investigated using a [125I]ICYP binding assay. A delayed, but more pronounced, increase in the receptor expression was observed as compared to the situation previously described in vivo. These changes only occurred for the beta 1-subtype of the receptor, whereas the beta 2-subtype binding remained constant up to 3 weeks in vitro. The delay of beta 1-adrenoceptor expression may be due to the incomplete presence of the proper maturational input, and the late enhancement of receptor expression to upregulation related to the absence in vitro of noradrenergic input. Decreased beta-adrenoceptor levels could be induced by chronic treatment of the beta-agonist isoproterenol (1 microM) introduced either for 3 or 13 days. Again, changes in density were found only for the beta 1-adrenoceptor binding sites. There is no reduction of receptor density following return to control conditions for 10 days after a 3-day treatment with isoproterenol, demonstrating the ability of this model to attain its final receptor density notwithstanding the developmental insult.

Early postnatal appearance of enhanced noradrenaline content in the brain of vasopressin-deficient Brattleboro rat; normal adrenoceptor densities and aberrant influences of vasopressin treatment

The course of postnatal development of noradrenaline (NA) and its unconjugated free metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG), as well as the influence on early chronic vasopressin treatment, were investigated in various brain regions of the hereditary vasopressin-deficient (homozygous di/di) Brattleboro rat. In addition, the densities of the adrenergic receptor subtypes were measured in adult brain. Brain NA levels of di/di pups appeared enhanced already at 7 days of age when compared with data of heterozygous (+/di) controls. This was also seen in areas not known to receive a vasopressinergic input, e.g. the frontal cortex. Levels of MHPG also differed between genotypes, but changes were slight and either a decrease or increase, depending on age and region tested. Saturation analyses of alpha 1-, alpha 2-, and beta-adrenoceptor binding on crude membrane preparations of some brain regions revealed no differences in adulthood. Chronic treatment with vasopressin between 6 and 13 days of age reduced the enhanced NA brain levels throughout the brain of the di/di Brattleboro pups. The known vasopressin-mediated enhancement of NA turnover in adult brain was also measurable in +/di pups of this neonatal period (MHPG/NA ratios), indicating the early maturation of the interaction of vasopressinergic and NAergic systems. However, the dose-response in the di/di Brattleboro rat was biphasic with a decrease at a low dose of vasopressin. Since changes were found throughout the brain, it was concluded that vasopressin deficiency had altered the maturation of NA neurons of the locus coeruleus which may be due to the absence of a presumed inhibitory control of vasopressin on synthesis and storage mechanisms at the perikaryal level.

Development and regulation of alpha adrenoceptors in kitten visual cortex

Alpha-1 and alpha-2 adrenergic receptors were localized in developing cat visual cortex by using [3H]prazosin and [3H]rauwolscine, respectively as selective ligands. The effects of neuronal input on the development of the two receptor subtypes were also studied in animals with lesions at various sites within the central visual pathways. Binding densities for both ligands increased during the first few postnatal weeks and declined thereafter. For both receptor subtypes, the highest concentration of binding sites was found in the subplate zone of the cortex in neonatal animals. Both ligands showed their highest concentrations in cortical layer IV beginning at postnatal day 30 and in the superficial cortical layers in adulthood. However, the developmental redistribution of alpha-1 receptors began at earlier ages than that of the alpha-2 sites. The alpha-1 sites were still concentrated in the subplate zone up to 60 days postnatal, while the alpha-2 sites in this region disappeared much earlier. Receptor binding densities were also examined in animals with quinolinic acid lesions within cortex, lesions of the lateral geniculate nucleus and lesions of the optic tract. The results indicate that both alpha-adrenoceptor subtypes were mainly located on cortical cells, and that the absence of neuronal activity during development resulted in a reduction of the binding density for both subtypes in the visual cortex. An additional major reduction in alpha-2 but not alpha-1 binding sites was observed following the lateral geniculate nucleus lesion, suggesting that the development of alpha-2 receptors is also dependent on input from the lateral geniculate nucleus. Removal of the lateral geniculate nucleus early in life resulted in a significant increase in alpha-1 receptors in the subplate region, indicating that receptor densities in this zone may be negatively regulated by the lateral geniculate nucleus afferents. These results show that adrenergic receptors reorganize during postnatal cortical development with a strong temporary concentration in the subplate zone. The reorganization process is heavily influenced by cortical inputs.

In search of a mechanism for receptor-mediated neurobehavioral teratogenesis by nicotine: catecholamine release by nicotine in immature rat brain regions

Nicotine disrupts central nervous system development through interactions with nicotinic cholinergic receptors found in immature brain, leading to discoordination of target cell replication and differentiation. However, it is unclear whether the net result is achieved by nicotine's actions on its specific target cells, or indirectly through receptor-mediated release of other neurotransmitters, such as catecholamines, that possess neurotrophic properties. In the current study, developing rats (1, 7, 14 and 21 days old) were challenged acutely with nicotine (0.3 mg/kg) and the release of catecholamines was evaluated in vivo (AMPT method) in three brain regions that differ in nicotinic receptor concentrations. Nicotine did not stimulate catecholamine release at birth, but developed the capacity to do so in parallel with the ontogeny of nicotinic cholinergic receptors in the midbrain+brainstem and in the forebrain. In the cerebellum, which remains poor in nicotinic receptors, no response was obtained at any age. Superimposed on this general pattern, changes in sensitivity to nicotine were also seen that corresponded to ontogenetic changes in endogenous cholinergic tone, suggesting that receptor desensitization occurs normally during developmental stages in which neuronal activity is high. The absence of a catecholamine response to nicotine at birth in the rat indicates that neurobehavioral teratology associated with fetal nicotine exposure does not reflect secondary actions mediated through catecholamines. However, because brain development in the neonatal rat corresponds to fetal stages in man, the onset of these mechanisms may be relevant to human fetal exposure.

Epinephrine induces beta-adrenergic desensitization and differentiation of HL-60 cells

The HL-60 cell line was cultured in a serum-free medium and exposed to various concentrations of EPI. The effects on cell growth, differentiation and beta-adrenergic response were followed during the culture period of 72 h. Short-term exposure (3 min) to EPI (1 nM-1 mM) in the presence of theophylline (4 mM) caused a dose-dependent increase of cAMP levels with a maximum of 1500% above basal levels. When the cells were exposed to EPI (1 nM-10 microM) for 72 h, a dose-dependent increase of cAMP levels with a maximum of 60% above basal levels. Sustained exposure to EPI generated a dose-dependent desensitization of the beta-adrenergic signal system. After EPI treatment for 72 h, IPR (10 microM for 3 min) in the presence of theophylline (4 mM) increased cAMP-levels by only 80% above baseline level (cAMP levels after maintained exposure to EPI), compared to 1080% above unstimulated level in control cells. The alpha-adrenergic receptor blocker PHENT (10 microM) did not affect baseline cAMP level or IPR-dependent cAMP response, but a mixture of EPI and PHENT increased the response to IPR. The HL-60 cell growth was not influenced by EPI. However, after repeated exposure to EPI for 72 h a concentration-dependent increase of HL-60 differentiation was demonstrated. Differentiation was not influenced by PHENT. These results suggest a differentiation induction due to a beta-adrenergic-induced cAMP elevation.

Septohippocampal cholinergic changes after destruction of the A10-septal dopaminergic pathways

Mice were injected bilaterally into the septum with 6-hydroxydopamine and 6 weeks later the hippocampi were assayed for activity of choline acetyltransferase, muscarinic receptor binding capabilities and for formation of inositol phosphate in response to direct (carbachol) or presynaptically elicited (K+) stimulation of the postsynaptic receptors. Levels of dopamine in the septum were reduced by 70% in the lesioned animals and hippocampal choline acetyltransferase was elevated by the same amount. The Bmax of muscarinic binding was significantly reduced without changes in Kd; nevertheless, carbachol-induced stimulation of formation of inositol phosphate was unaffected. The response to K+ was markedly elevated in the 6-hydroxydopamine-treated animals. Thus, the regulatory effect of A10-septal dopaminergic pathways on the septohippocampal cholinergic innervations is both on the presynaptic and postsynaptic levels.

Differential development of cholinergic nerve terminal markers in rat brain regions: implications for nerve terminal density, impulse activity and specific gene expression

During critical developmental periods, cholinergic activity plays a key role in programming the development of target cells. In the current study, ontogeny of cholinergic terminals and their activity were contrasted in 4 brain regions of the fetal and neonatal rat using choline acetyltransferase activity, which is unresponsive to changes in impulse flow, and [3H]hemicholinium-3 binding, which labels the high-affinity choline transporter that upregulates in response to increased neuronal stimulation. In all 4 regions (cerebral cortex, midbrain + brainstem, striatum, hippocampus) choline acetyltransferase activity increased markedly from late gestation through young adulthood, but generally did so in parallel with the expansion of total membrane protein, reflective of axonal outgrowth and synaptic proliferation. In contrast, [3H]hemicholinium-3 binding was extremely high in late gestation and immediately after birth, declined in the first postnatal week and then rose again into young adulthood. The ontogenetic changes reflected alterations primarily in the number of binding sites (Bmax) and not in binding affinity. Only the latter phase of development of [3H]hemicholinium-3 binding corresponded to the ontogenetic changes in choline acetyltransferase activity; in the hippocampus, there were disparities even in young adulthood, where [3H]hemicholinium-3 binding showed a spike of activity centered around the 5th to 6th postnatal week, whereas choline acetyltransferase did not. Correction of binding for membrane protein development did not eliminate any of the major differences in developmental patterns between the two markers. These results suggest that development of the choline transporter binding site is regulated independently of the outgrowth of the bulk of cholinergic nerve terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal exposure to the beta-adrenoceptor agonist clenbuterol has regionally selective direct and long-term effects on rat brain beta-adrenoceptors and monoamine metabolism

The effects of chronic postnatal beta 2-adrenoceptor activation on the maturation of the rat brain noradrenergic system have been studied. For that purpose, rat pups have been treated twice daily during the first 10 days of life with the beta 2-agonist clenbuterol-HCl (2.5 mg/kg s.c.), and the effects on the beta-adrenoceptor number and monoamine metabolism have been determined directly after the treatment and in adulthood. On postnatal day 10, 90 min after the last clenbuterol injection 4.5 micrograms/g of the drug was present in the brain. At the end of the treatment the beta-receptor binding had decreased in the cerebellum (35%), but not in the frontal cortex or mesolimbic system. Clenbuterol significantly increased the steady-state brain levels of noradrenaline (NA) in the striatum 90 min after the last injection, whereas the levels in the frontal cortex, meso-limbic system, medulla pons and cerebellum were unaffected. The NA metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG), had significantly increased in the frontal cortex and striatum. The serotonergic (5-HT) and dopaminergic (DA) system were not altered. In general, no long-lasting effects on beta-adrenoceptor number and affinity or monoamine metabolism were measurable, except for the frontal cortex which showed a sustained increase of MHPG, a decrease of 5-HT and an increase of 5-HIAA/5-HT on PN 60. In conclusion, chronic postnatal activation of beta 2-adrenoceptors by clenbuterol treatment selectively causes changes in the setting of the neurochemical parameters investigated in the frontal cortex.

Prenatal cocaine and cell development in rat brain regions: effects on ornithine decarboxylase and macromolecules

Prenatal cocaine exposure has been shown to cause neurobehavioral abnormalities. To determine whether effects on basic patterns of cell development underlie these functional deficits, we examined the aftermath of acute and chronic cocaine exposure on ontogenetic patterns of ornithine decarboxylase (ODC), a key regulator of cell replication/differentiation, DNA synthesis as monitored by [3H]thymidine incorporation, and markers of cell number (DNA content) and cell size (protein/DNA ratio). Administration of 30 mg/kg SC of cocaine to pregnant rats on gestational day 20 resulted in acute increases of ODC throughout the brain. When the same dose of cocaine was given daily from gestational days 8 through 20, ODC elevations persisted into the neonatal period but disappeared by the middle of the first postnatal week. Although this treatment regimen retarded maternal weight gain, there was little or no effect on pup body or brain region weights. Similarly, minor changes in DNA synthesis were seen in two brain regions (forebrain, cerebellum), but DNA content was largely unaffected. Postnatal cell growth was significantly reduced in the forebrain, as evidenced by deficits in protein/DNA but, again, the magnitude of effect was quite small. Raising the daily dose of cocaine to 100 mg/kg resulted in significant maternal mortality and fetal resorptions in surviving dams. Shortening the treatment regimen to a 3-day period (gestational days 18 through 20) eliminated the effects on maternal weight gain and on postnatal pup brain region ODC.(ABSTRACT TRUNCATED AT 250 WORDS)

Fetal dexamethasone exposure alters macromolecular characteristics of rat brain development: a critical period for regionally selective alterations?

Fetal glucocorticoid exposure retards postnatal growth and evokes abnormalities of nervous system structure and function. To examine the underlying mechanisms, we administered 0.2 or 0.8 mg/kg of dexamethasone to pregnant rats on gestational days 17, 18, and 19 and assessed brain region cell development with indices of DNA content (total cell numbers), DNA concentration (cell packing density), and protein/DNA ratio (relative cell size). Dexamethasone evoked deficits of pup body and brain region weights, but the brain regions displayed growth-sparing associated initially with preservation of cell numbers (normal or elevated DNA content and concentration), at the expense of relative cell size (decreased protein/DNA). Subsequently, brain cell acquisition lagged behind that of controls, with deficits in DNA and elevations of protein/DNA. In midbrain + brainstem and in cerebellum, cell markers returned to normal by weaning. However, the forebrain showed persistent elevations of DNA and reduced protein/DNA, indicative of replacement of neurons with glia. Because the treatment period coincided with the timing of neuronal cell replication in the forebrain, but not in the other regions, these results suggest that the critical period for lasting deficits of dexamethasone coincides with the peak of neuronal mitosis.

Fetal dexamethasone exposure sensitizes neonatal rat brain to hypoxia: effects on protein and DNA synthesis

Fetal exposure to glucocorticoids is known to produce long-term alterations in cell development within the central nervous system. The current study examines whether some of the adverse effects of prenatal dexamethasone treatment on brain development represent sensitization to hypoxia-induced damage. Pregnant rats were given 0.2 or 0.8 mg/kg of dexamethasone on gestational days 17, 18 and 19 and their offspring were challenged by exposure to 7% O2 on postnatal days 1 and 8. In control rats at 1 day of age, hypoxia evoked an acute decrease in protein synthesis, assessed by [3H]leucine incorporation, in both the midbrain + brainstem and forebrain. The decrease was also seen in animals receiving the low dose of dexamethasone, but was of smaller magnitude in the midbrain + brainstem than in the control cohort. At the higher dose of dexamethasone, hypoxia failed to evoke a decrease in protein synthesis; instead, protein synthesis was significantly increased. By 8 days of age, the animals receiving the lower dose of dexamethasone also displayed the anomalous increment in [3H]leucine incorporation during hypoxic challenge, whereas the effect in the high dose group was less notable. Similarly, parallel examination of incorporation of [3H]thymidine into DNA on postnatal day 1 indicated that control animals would reduce their macromolecule synthetic rate in a hypoxic environment, but that animals exposed to the high dose of dexamethasone would not; unlike the case with protein synthesis, however, the dexamethasone group never showed an increase in DNA synthesis during hypoxia. By 8 days of age, the interaction between the high dose of dexamethasone and hypoxia was no longer apparent for DNA synthesis.2

Presynaptic input regulates development of beta-adrenergic control of rat brain ornithine decarboxylase: effects of 6-hydroxydopamine or propranolol

beta-Adrenergic control of ornithine decarboxylase (ODC) activity is exerted only during a critical period in central nervous system development, playing an important role in neurotransmitter modulation of cell replication and differentiation. The current study examines the effects of lesions caused by 6-hydroxydopamine administration to neonatal rats, or of gestational exposure to propranolol, on the subsequent development of the ODC response to beta-adrenergic stimulation elicited by an acute intracisternal challenge with isoproterenol. 6-Hydroxydopamine treatment severely attenuated the ability of isoproterenol to stimulate ODC in the cerebellum, a tissue that shows a postnatal peak of ODC reactivity. In contrast, much smaller effects were seen in the cerebral cortex, which has an earlier (pre/perinatal) peak of ODC, despite the fact that norepinephrine depletion was more persistent in the cortex. On the other hand, blockade of fetal beta-receptors by maternal propranolol infusions resulted in immediate postnatal attenuation of the ODC response in cerebral cortex, but not cerebellum. These data suggest that early exposure of beta-receptors to norepinephrine "programs" the subsequent efficiency of the receptor linkage to ODC during a critical ontogenetic period that occurs prenatally in the cerebral cortex and postnatally in the cerebellum.

Inhibition of DNA synthesis in neonatal rat brain regions caused by acute nicotine administration

Perinatal exposure to nicotine has been shown to cause morphological and neurobehavioral abnormalities in developing brain. In the current study, neonatal rats were given an acute injection of nicotine (3 mg/kg) at 1, 3, 8, 10 or 15 days of age, and [3H]thymidine incorporation into DNA examined over the 30-min period after drug administration. Three brain/regions were used that differ in their timetables of cell maturation and in their concentrations of nicotinic receptors. Nicotine inhibited DNA synthesis in all brain regions but with a rank order of effect corresponding to the concentration of nicotinic receptors, namely midbrain + brainstem greater than or equal to cerebral cortex greater than cerebellum. Superimposed on this hierarchy, periods of rapid cell replication were more sensitive to nicotine, so that drug effects in the cerebellum, which develops last, became significant past the point at which nicotine no longer affected DNA synthesis in the other regions. The inhibitory effect of nicotine was also found in fetal brain on gestational day 20 after injection of nicotine to pregnant rats. Studies with adrenergic and ganglionic blocking agents and with 100% O2 indicated that autonomic and respiratory actions of nicotine, including ischemia, cardiac arrhythmias and hypoxia, could not solely account for the inhibition of DNA synthesis in neonatal brain. In contrast, injection of a small amount (2 micrograms) of nicotine directly into the central nervous system readily caused inhibition; the same small dose given systemically had no effect. These data suggest that nicotine damages the developing brain, in part, through direct actions on cell replication.

Effects of MK-801 on DNA synthesis in neonatal rat brain regions under normoxic and hypoxic conditions

Global metabolic insults such as ischemia/hypoxia, damage neural cells through release of excitatory amino acids and their subsequent actions at the N-methyl-D-aspartate (NMDA) receptor. NMDA receptors are highly expressed in neonatal rat brain, and the current study examines the effects of receptor blockade with MK-801 on DNA synthesis under normoxic and hypoxic conditions. At one day of age, hypoxia alone caused a decrease in [3H]thymidine incorporation into DNA throughout the brain, whereas MK-801 alone decreased incorporation selectively in regions known to be enriched in NMDA receptors. MK-801 afforded no protection from hypoxia and instead exacerbated the effects of hypoxia in the cerebellum. At 8 days of age, hypoxia alone or MK-801 alone still produced the same patterns of inhibition of DNA synthesis, but MK-801 neither prevented nor exacerbated the effects of hypoxia; animals receiving MK-801 showed a significant incidence of hypoxia-induced mortality. These data suggest that excitatory actions exerted at the NMDA receptor serve to maintain cell replication in neonatal brain and, as distinct from the situation for excitatory amino acid-induced cell death, these receptors do not participate in adverse effects of hypoxia on DNA synthesis.

Cocaine acutely inhibits DNA synthesis in developing rat brain regions: evidence for direct actions

Perinatal exposure to cocaine has been shown to cause morphological and neurobehavioral abnormalities. In the current study, neonatal rats were given an acute injection of cocaine (30 mg/kg s.c.) at 1, 3, 5, 8, 11 or 15 days of age, and [3H]thymidine incorporation into DNA examined over the ensuing 30 min period. Three brain regions were used that differ in their timetables of cell maturation: cerebellum, cerebral cortex and midbrain + brainstem. Cocaine inhibited DNA synthesis in all brain regions, with diminishing impact as the animals matured; by 15 days of age, the effect of cocaine was no longer significant. Inhibition of macromolecule synthesis was selective for DNA, as [3H]leucine incorporation into protein was much less affected by cocaine. Although inhibition of [3H]thymidine incorporation by a single injection of cocaine was short-lived, repeated administration could have cumulative effects: chronic treatment on days 2, 3 and 4 did not desensitize the adverse effect of a subsequent dose administered on day 5. Additionally, with chronic cocaine, the cerebellum displayed a pronounced rebound elevation of DNA synthesis 24 h after the last dose, a characteristic finding in delayed cell maturation. Inhibition of DNA synthesis by cocaine in developing brain was not secondary to ischemia, nor to local anesthesia, as alpha-adrenergic blockade with phenoxybenzamine afforded no protection, and lidocaine could not substitute for cocaine. In contrast, a small amount (15 micrograms) of cocaine injected directly into the central nervous system readily caused inhibition of DNA synthesis; the same dose given systemically had no effect. These data suggest that cocaine damages the developing brain, in part, through direct interference with DNA synthesis.

Dual control of DNA synthesis by alpha- and beta-adrenergic mechanisms in normoxic and hypoxic neonatal rat brain

To examine how catecholamines influence cell replication in the developing brain, we examined regional [3H]thymidine incorporation into DNA after acute challenge with an alpha-adrenergic blocking agent (phenoxybenzamine) or a beta-blocker (propranolol). Phenoxybenzamine inhibited DNA synthesis in 1-day-old rat pups but the effect was less pronounced at 8 days; regional differences corresponded to transient expression of alpha-receptors and their subsequent maturational decline. Propranolol given at 1 day of age exerted a regionally selective, promotional effect on DNA synthesis; in contrast, at 8 days, propranolol inhibited DNA synthesis in all brain regions. Propranolol, but not phenoxybenzamine, also exacerbated the reduction in DNA synthesis caused by neonatal hypoxia, and again the effect was limited to the 1-day-old group. These results indicate that catecholamines exert a dual action on DNA synthesis; the effects are dependent upon maturational profiles of specific receptor populations which are either transiently expressed or which couple to cell replication only during a critical period.

Fetal terbutaline exposure causes selective postnatal increases in cerebellar alpha-adrenergic receptor binding

beta-Adrenergic agonists used in therapy of premature labor and asthma cross the placenta and can affect development of the fetal nervous system. In the current study, pregnant rats were given 10 mg/kg of terbutaline on gestational days 17, 18 and 19 and adrenergic receptor binding capabilities examined in brain regions of the offspring. Despite the absence of body or brain growth impairment, selective increases were seen postnatally in cerebellar alpha 1- and alpha 2-receptor subtypes, whereas the same receptor populations were decreased by small amounts in cerebral cortex and midbrain + brainstem. beta-Adrenergic receptors showed little or no change in any region. The regional and subtype selectivity are compatible with primary deficits in the development of noradrenergic projections to the cerebellum identified in previous studies and provide further evidence that therapeutic use of beta-adrenergic agonists may produce neurobehavioral teratology.

Prenatal terbutaline exposure in the rat: selective effects on development of noradrenergic projections to cerebellum

Terbutaline, used in the treatment of premature labor and asthma, crosses the placenta and can stimulate beta 2-adrenergic receptors in the fetus. This study examines the effects of prenatal exposure to terbutaline (10 mg/kg SC on gestational days 17, 18 and 19) on the development of noradrenergic projections in brain regions of the fetal and neonatal rat, using synaptosomal uptake of [3H]norepinephrine as a marker for synaptogenesis. Although terbutaline exposure did not compromise body or brain region growth, uptake was adversely affected selectively in the cerebellum, a region which also displays close coupling of fetal beta 2-receptors to control of cell development near term. These results thus provide biochemical evidence that terbutaline may be a neurobehavioral teratogen.

Are glial cells targets of the central noradrenergic system? A review of the evidence

It has been suggested by a number of investigators that glial cells as well as neurons are targets of the central noradrenergic system. This important hypothesis, however, has not been presented previously in a systematic and unified manner. The present review was therefore undertaken to accomplish this. The evidence supporting noradrenergic action on glia consists primarily of findings that beta-adrenoceptors, norepinephrine (NE)-stimulated cyclic AMP (cAMP) responses and glycogen are localized preferentially in glial cells and that beta-receptor density and glycogen hydrolysis are under the control of neuronally released NE. While there is some disagreement as to the extent to which beta-receptors are preferentially localized in glia, there is a consensus that most glycogen in the forebrain is localized in this cellular compartment. The presumed function of the noradrenergic action on glia appears to be the release of glucose for production of energy, the synthesis of neurotrophic factors such as nerve growth factor, and the release of substances which may affect local neurotransmission including taurine, cAMP and its metabolites. These glial responses may be intimately related to the electrophysiological actions of NE on neurons.

Prenatal exposure to nicotine impairs nervous system development at a dose which does not affect viability or growth

Prenatal exposure to high doses of nicotine (greater than 6 mg/kg/day) via maternal infusions has been shown to impair nervous system development and to decrease viability and growth. In the current study, we have examined the effects of infusing pregnant rats with 2 mg/kg of nicotine per day from gestational days 4 through 20. At this lower dose, there was neither interference with maternal weight gain nor any increase in resorption rate. Intrauterine and postnatal growth was maintained at normal or supranormal rates in the exposed offspring. Nevertheless, sufficient nicotine penetrated the fetal brain to cause persistent alterations in [3H]nicotine binding sites, abnormalities of cellular development [assessed by measurements of ornithine decarboxylase (ODC) activity and deoxyribonucleic acid (DNA)], and impairment of development of peripheral noradrenergic projections (assessed by kidney norepinephrine levels); in each case, the neural alterations were virtually equivalent to those obtained previously at the higher, growth-suppressant dosage. These findings indicate that growth impairment alone is insufficient to predict the adverse effects of nicotine on development.