Lasting effects of developmental dexamethasone treatment on neural cell number and size, synaptic activity, and cell signaling: critical periods of vulnerability, dose-effect relationships, regional targets, and sex selectivity.

Glucocorticoids administered to prevent respiratory distress in preterm infants are associated with neurodevelopmental disorders. To evaluate the long-term effects on forebrain development, we treated developing rats with dexamethasone (Dex) at 0.05, 0.2, or 0.8 mg/kg, doses below or spanning the range in clinical use, testing the effects of administration during three different stages: gestational days 17-19, postnatal days 1-3, or postnatal days 7-9. In adulthood, we assessed biomarkers of neural cell number and size, cholinergic presynaptic activity, neurotransmitter receptor expression, and synaptic signaling mediated through adenylyl cyclase (AC), in the cerebral cortex, hippocampus, and striatum. Even at doses that were devoid of lasting effects on somatic growth, Dex elicited deficits in the number and size of neural cells, with the largest effect in the cerebral cortex. Indices of cholinergic synaptic function (choline acetyltransferase, hemicholinium-3 binding) indicated substantial hyperactivity in males, especially in the hippocampus, effectively eliminating the normal sex differences for these parameters. However, the largest effects were seen for cerebrocortical cell signaling mediated by AC, where Dex treatment markedly elevated overall activity while obtunding the function of G-protein-coupled catecholaminergic or cholinergic receptors that stimulate or inhibit AC; uncoupling was noted despite receptor upregulation. Again, the effects on signaling were larger in males and offset the normal sex differences in AC. These results indicate that, during critical developmental periods, Dex administration evokes lasting alterations in neural cell numbers and synaptic function in forebrain regions, even at doses below those used in preterm infants.

Critical periods for the role of oxidative stress in the developmental neurotoxicity of chlorpyrifos and terbutaline, alone or in combination.

The developmental neurotoxicity of chlorpyrifos (CPF) involves mechanisms other than inhibition of cholinesterase. In the current study, we examined the ability of CPF to evoke lipid peroxidation in the developing brain of fetal and neonatal rats. CPF given to pregnant rats on gestational days 17-20 or to neonatal rats on postnatal days 1-4, failed to elicit increases in thiobarbituric acid-reactive species (TBARS) in brain regions even when the dose was raised above the threshold for systemic toxicity and hepatic damage. In contrast, CPF administration during the second postnatal week, the peak period of neuronal cell differentiation and synaptogenesis, did evoke significant increases in TBARS even at a dose devoid of systemic toxicity. Terbutaline, which is chemically unrelated to CPF and which stimulates neuronal cell metabolism through direct actions on beta-adrenoceptors, also elicited oxidative damage in the developing brain with greater sensitivity in the second postnatal week. These results indicate that diverse compounds can exert convergent effects on brain development through their shared potential to elicit oxidative stress, and that the net outcome is dependent upon specific developmental stages in which metabolic demand is especially high. Furthermore, given the common use of terbutaline in the therapy of preterm labor, and the nearly ubiquitous exposure of the human population to organophosphorus pesticides, the combined oxidative burden of exposure to both agents may contribute to the worsened neurodevelopmental outcomes noted in animal models of such dual exposures.

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.