S.E. Lappi

Loss of neonatal hypoxia tolerance after prenatal nicotine exposure: Implications for sudden infant death syndrome

Maternal cigarette smoking has a high correlation with sudden Infant Death Syndrome, a condition in which cardiorespiratory failure occurs during an hypoxic episode, as in sleep apnea. Pregnant rats were given nicotine infusions of 2 or 6 mg/kg/day throughout gestation, regimens that produce plasma nicotine levels spanning the range in smokers. The day after birth, animals in the high dose group displayed excessive mortality during hypoxic challenge. These animals were found to be deficient in an essential response component, namely adrenomedullary catecholamine release that is required to maintain neonatal cardiac rhythm during hypoxia; the defect was in adrenal cell function rather than in altered innervation or nicotinic receptor desensitization. We also examined brainstem and forebrain noradrenergic mechanisms that are involved in neonatal respiratory control. The nicotine group showed suppressed spontaneous neuronal activity, but were hyperresponsive to hypoxia. As these projections are inhibitory for respiration, the nicotine-induced sensitization would be expected to contribute to respiratory arrest during hypoxia. Prenatal nicotine exposure may thus provide a useful animal model with which to study the physiological mechanisms that underlie Sudden Infant Death Syndrome, while at the same time providing a biological explanation for the association of the syndrome with smoking.

Glucocorticoids and the development of neuronal function : effects of prenatal dexamethasone exposure on central noradrenergic activity

Although glucocorticoids slow the development of most cell types, they have been hypothesized to promote the differentiation of catecholaminergic cells. In the current study, pregnant rats were given dexamethasone on gestational days 17, 18 and 19, and the functional state of noradrenergic synaptic activity was assessed throughout postnatal development by measurements of transmitter levels and turnover, and receptor binding capabilities. Despite growth inhibition caused by dexamethasone, the steroid treatment had little or no effect on transmitter levels or receptor binding and accelerated the maturation of norepinephrine turnover in a regionally selective manner. Effects were most notable in the midbrain and brainstem, where turnover rose to maximum levels 1-2 weeks in advance of controls. Turnover also leveled off prematurely in the dexamethasone group, leading to deficits in the postweaning period and into young adulthood. Although similar patterns were obtained in other, later-developing regions, the effects were less consistent and robust; the smaller effects also extended to dopamine turnover. These results suggest that glucocorticoids have a specific promotional effect on the development of central catecholaminergic activity and that administration of exogenous steroids during critical periods of development can lead to lasting functional abnormalities.

Fetal nicotine exposure ablates the ability of postnatal nicotine challenge to release norepinephrine from rat brain regions

Exposure of the fetus to nicotine is known to affect the function of noradrenergic pathways in the central nervous system. In the current study, synaptic mechanisms underlying the functional defects were evaluated in the offspring of pregnant rats given nicotine infusions of 2 mg/kg/day throughout gestation, administered by osmotic minipumps. At 30 days postpartum, norepinephrine levels in brain regions of the offspring were significantly reduced. More importantly, acute challenge with either 0.1 mg/kg or 0.3 mg/kg of nicotine evoked significant norepinephrine release from brain regions of control animals, but failed to do so in the fetal nicotine cohort. These results suggest that prenatal exposure to nicotine produces a deficit in subsequent noradrenergic responsiveness, deficits which may participate in behavioral and neuroendocrine abnormalities.

Deficits in Development of Central Cholinergic Pathways Caused by Fetal Nicotine Exposure: Differential Effects on Choline Acetyltransferase Activity and (3H)Hemicholinium-3 Binding

Nicotine has been hypothesized to induce neurobehavioral teratology by mimicking prematurely the natural developmental signals ordinarily communicated by the ontogeny of cholinergic synaptic transmission. In the current study, the effects of fetal nicotine exposure (2 mg/kg/day or 6 mg/kg/day) on development of central cholinergic pathways were examined in striatum and hippocampus of animals exposed from gestational days 4 through 20, using maternal infusions with osmotic minipumps. Brain region weights and choline acetyltransferase activity, an enzymatic marker for development of cholinergic nerve terminals, were within normal limits in the nicotine-exposed animals. However, development of [3H]hemicholinium-3 binding which labels the presynaptic high affinity cholinergic transporter, was deficient in both striatum and hippocampus. Abnormalities occurred during two distinct phases; in the early neonatal period, when [3H]hemicholinium-3 binding sites are transiently overexpressed, and during or after the period of rapid synaptogenesis, when binding in controls is rising consequent to the increase in nerve impulse activity. These data thus indicate that fetal nicotine exposure, even at doses that do not cause overt signs of maternal/fetal/neonatal toxicity or growth impairment, influences both specific gene expression of cholinergic nerve terminal markers, as well as indices of neuronal function. Comparison of regional selectivity at the two dose levels indicated greater sensitivity of the striatum, a region with a prenatal peak of neuronal mitosis, as compared to hippocampus, where mitosis peaks postnatally; the regional differences are consistent with vulnerability to nicotine during a critical phase of cell development.

Prenatal nicotine exposure impairs β-adrenergic function: Persistent chronotropic subsensitivity despite recovery from deficits in receptor binding

Gestational exposure to nicotine has been shown to interfere with biochemical markers of development of central and peripheral noradrenergic activity. The current study examines the development and function of cardiac beta-adrenergic receptors in the offspring of pregnant rats given nicotine infusions of 6 mg/kg/day from gestational days 4 through 20, administered by subcutaneously implanted osmotic minipumps. Prenatal nicotine exposure delayed the development of beta-adrenergic receptor binding capabilities, as assessed with [125I]pindolol in membrane preparations from heart and kidney. The deficits in receptor binding were associated with marked subsensitivity of chronotropic responses to administration of a beta-adrenergic agonist, isoproterenol. Although the effects on receptor binding resolved after weaning, functional deficiencies in responsiveness to isoproterenol or to preganglionic electrical stimulation of sympathetic nerves to the heart persisted into adulthood. These results indicate that prenatal exposure to nicotine produces long-term alterations in adrenergic responsiveness of sympathetic target tissues.

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)

Impact of fetal nicotine exposure on development of rat brain regions: Critical sensitive periods or effects of withdrawal?

Fetal nicotine exposure evokes alterations in central nervous system structural, neurochemical, and behavioral development. In the current study, the relative importance of critical developmental exposure periods and withdrawal were examined by infusing nicotine to pregnant rats using osmotic minipumps beginning on the fourth day of gestation. Infusions were confined to either the first 8 days (withdrawal on gestational day 13), to nearly all of gestation (withdrawal on gestational day 21), or throughout gestation and continued into the first 2 postnatal weeks. Maternal weight gain was retarded by nicotine, with a hierarchy corresponding to the duration of nicotine exposure. Similarly, fetal and neonatal body weights were unaffected in the group receiving the shortest duration of nicotine exposure, and were less affected by the intermediate infusion regimen than by the longest regimen; brain region weights were reduced significantly only with the longest regimen. Using ODC activity, a sensitive marker for altered brain cell development, we found little change in animals exposed to nicotine in early gestation and undergoing withdrawal on day 13. However, in the groups receiving nicotine through the end of gestation or through gestation and into the postnatal period, ODC activity was significantly elevated. These results indicate that withdrawal from nicotine contributes little, if any, effect either to the growth deficits or to abnormalities of brain cell development. Instead, the most important factor appears to be exposure within the developmental period corresponding to the proliferation of nicotinic receptors and the timing of receptor control of cell replication and differentiation.

Altered development of basal and forskolin-stimulated adenylate cyclase activity in brain regions of rats exposed to nicotine prenatally

Exposure of the fetus to nicotine is known to affect cellular development, synaptogenesis and synaptic activity of a wide variety of neurotransmitter pathways in the central nervous system. In the current study, pregnant rats received nicotine infusions of 6 mg/kg/day throughout gestation, administered by osmotic minipumps. After birth, offspring of the nicotine infused dams displayed marked alterations in membrane-associated adenylate cyclase activity; the regional selectivity correlated both with nicotinic cholinergic receptor concentration and the maturational timetable of each region. In the midbrain and brainstem, which display relatively high receptor concentrations and earliest cell development, basal adenylate cyclase activity in the nicotine group was elevated in the immediate period postpartum, returned to normal by the end of the first month, but then became subnormal in young adulthood. The initial promotion of basal activity was mirrored by forskolin-stimulated activity, suggesting that in this phase, the alterations were occurring at the level of the adenylate cyclase catalytic unit itself. The lack of effect on forskolin stimulation in the later phase, where basal activity was subnormal in the nicotine group, suggests that some alterations in regulatory subunits are responsible for the maturational switch in nicotines effects on adenylate cyclase. In the cerebellum, where cell replication occurs primarily after birth and receptor concentrations are low, basal adenylate cyclase showed only a deficit in the nicotine group; again, although forskolin stimulation was significantly affected, the actions on basal activity were much more prominent, suggesting defects at the level of G-proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Fetal terbutaline exposure causes selective postnatal increases in cerebellar α-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.