New Pharmacological Approaches in Infants with Hypoxic-Ischemic Encephalopathy.

New knowledge of the pathophysiology and evolution of hypoxic-ischemic brain injuries has made feasible interventions to improve clinical outcomes for newborns surviving birth asphyxia. Brain injury following hypoxic-ischemic insult is a complex process evolving over hours to days, which provides a unique window of opportunity for neuroprotective treatment interventions. The specific pathologic processes preceding the onset of irreversible cerebral injury appear to be a combination of several mechanisms that are variable according to the severity and duration of the insult and to biochemical modifications in the brain. Advances in neuroimaging, brain monitoring techniques, and tissue biomarkers have improved the ability to diagnose, monitor, and care for newborn infants with neonatal encephalopathy, as well as to predict their outcome. The role of oxidative stress in newborn morbidity with respect to the higher risk of free radical damage in these babies is growing. However, challenges remain in early identification of infants at risk for neonatal encephalopathy, determination of timing and extent of hypoxic-ischemic brain injury, as well as optimal management and treatment duration. Potential neuroprotective strategies targeting different pathways leading to neuronal cell death in response to hypoxic-ischemic insult have been investigated: hypothermia, erythropoietin, iminobiotin, deferioxamine, magnesium, allopurinol, xenon, melatonin and statins. Hypothermia is currently the only recognized beneficial therapy. However, many infants still develop significant adverse outcomes. It is becoming evident that the association of moderate hypothermia with neuroprotective drugs may enhance the outcome. By virtue of their pleiotropic effects without toxic effects, melatonin and statins may act at different levels of the multiple mechanisms responsible for the progression of the neurodegenerative process and represent promising neuroprotectants, alone or as additional adjunctive therapy, for reducing brain injury and its long-term sequelae in infants. More clinical studies are needed to clarify the role of these potential neuroprotective drugs.

The use of melatonin in hypoxic-ischemic brain damage: an experimental study.

OBJECTIVE: Oxidative stress (OS) plays a key role in perinatal brain damage. The aim of this study is to evaluate the effectiveness of melatonin as a neuroprotective drug by investigating the influence of melatonin on OS and inflammation biomarkers in an animal model of cerebral hypoxia-ischemia. METHODS: Five minutes after hypoxic-ischemic (HI) injury melatonin was administered to 28 rats (HI-Mel group). At the same time, 28 hypoxic-ischemic rats were vehicle-treated (V-HI group). Five rats were used as sham operated controls (CTL). OS biomarkers: isoprostanes (IsoPs), neuroprostanes (NPs) and neurofurans (NFs), and microglial activation markers (glial fibrillary acidic protein [GFAP] and monoclonal antirat CD68 [ED1]) were measured in the cerebral cortex of the two lobes. RESULTS: A significant increase of IsoPs on the left lobe was observed in V-HI after 1 hour (h) from HI injury (p < 0.001); a significant increase of NPs on both side (p < 0.05) and a significant increase of NFs on the left (p < 0.05) were also observed in V-HI after 24 h. A significant increase of IsoPs on the left (p < 0.05) and of NPs on both lobes (p < 0.05) were observed in HI-Mel after 48 h. The ED1 and GFAP expression was lower in the HI-Mel brain tissue. CONCLUSIONS: Melatonin reduces OS and inflammatory cells recruitment and glial cells activation in cerebral cortex after neonatal HI damage. These results lay the groundwork for future clinical studies in infants.

Inhibition of rapamycin-induced autophagy causes necrotic cell death associated with Bax/Bad mitochondrial translocation.

Rapamycin, a lipophilic macrolide antibiotic, has been found to reduce injury in different models of neurodegenerative disorders. We have previously shown that in neonatal rats subjected to hypoxia-ischemia (HI) the neuroprotective effect of rapamycin was associated with increased autophagy and decreased caspase-3 activation. We show here that the strong reduction of caspase-3 activation after rapamycin was due, at least in part, to its effect on the intrinsic apoptotic mitochondrial pathway because after rapamycin treatment there was a marked reduction of Bax and Bad translocation to mitochondria, cytochrome c release, and caspase-3 activation. Poly (ADP-ribose) polymerase 1 (PARP-1) cleavage and the number of terminal dUDP nick-end labeling (TUNEL)-positive cells were also reduced. To assess how the antiapoptotic effect of rapamycin was linked to the strong autophagy signal induced by the drug, we blocked the formation of autophagosomes with 3-methyladenine (3MA). 3MA administered 10 min after rapamycin, elicited again Bax and Bad translocation to the mitochondria but did not cause cytochrome c release and caspase-3 activation. After 3MA treatment, cells underwent necrotic cell death. These data indicate that rapamycin administered before HI prevents the apoptotic signaling taking place through the mitochondrial pathway. We hypothesize that rapamycin confers a preconditioning-like protection and suggest that caution is necessary before using pharmacological agents targeting autophagy in neuroprotection because they could interfere with endogenous protective mechanisms.

Neuroprotective effect of simvastatin in stroke: a comparison between adult and neonatal rat models of cerebral ischemia.

Statins, the most widely used lipid lowering drugs, have been demonstrated to play a protective role in stroke. Animal studies confirmed the observations obtained in clinical trials and provided additional data on the putative mechanism/s of action underlying this beneficial effect. We have shown that simvastatin reduced the size of the infarct to a different extend, according to the animal model used. Indeed, in the rat neonatal model of hypoxia/ischemia simvastatin affords protection only when is administered before the ischemic insult. In contrast, in adult rats bearing middle cerebral artery occlusion, simvastatin exerted its beneficial effect on brain injury when injected for 3 days either before or after induction of ischemia. Studies carried out to determine the therapeutic window of simvastatin demonstrated that the protective effect is observed after a single dose and when the drug is administered within 3-6 hours after ischemia. Simvastatin-dependent activation of eNOS has been claimed to be one of the main mechanisms responsible for neuroprotection. This hypothesis is confirmed in the adult animal model where eNOS is activated by either pre- or post- simvastatin treatment but is not supported by the data obtained in the neonate where eNOS activity is not affected by drug treatment. These observations suggest that the protective effect of simvastatin on stroke may be mediated by multiple mechanisms as can be expected by its pleiotropic effects.

New therapeutic strategies in perinatal stroke.

Perinatal stroke represents an important cause of severe neurological deficits that span the individual's lifetime, including delayed mental and motor development, epilepsy and major cognitive deficits. Most strokes occurring in term births, infants and children can be caused by thromboembolism from intracranial and extracranial vessels and are associated with a variety of risk factors such as birth asphyxia, cardiac diseases, blood disorders, maternal disorders, trauma. Animal models of perinatal stroke have been developed to examine the nature and the time course of the events occurring after the ischemic insult and the possible therapeutic strategies useful in reducing ischemic damage. The present article addresses the potential pharmacological treatments targeting the inflammatory process and apoptotic cell death, with a specific emphasis on the emerging role of statins as neuroprotective agents in perinatal stroke. As a prelude, we will also review advances in our understanding on the mechanisms underlying the hypoxic-ischemic reperfusion injury in the newborn.

Simvastatin protects against long-lasting behavioral and morphological consequences of neonatal hypoxic/ischemic brain injury.

BACKGROUND AND PURPOSE: Recent studies suggest that statins (3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors) not only reduce the incidence of stroke by lowering cholesterol levels but may also exert neuroprotective effects via a mechanism not related to their lipid-lowering effect. Despite the growing body of evidence, however, the neuroprotective effect of statins in stroke is still controversial. Herein, we studied whether a prophylactic administration of simvastatin (Sim) provides significant protection against brain damage, and we sought to determine its long-lasting behavioral consequences in a neonatal model of hypoxia/ischemia. METHODS: Newborn male rats were injected daily from postnatal days 1 to 7 with activated Sim (20 mg/kg) or an equivalent volume of vehicle. On postnatal day 7, the rats were subjected to ligation of the right common carotid artery, followed by 3 hours of hypoxia or by sham operation. The neuroprotective effect of Sim was evaluated after the rats had achieved adulthood by using a battery of behavioral tests and histological analysis. RESULTS: Sim-treated ischemic rats performed the circular water maze, the radial arm maze, and the multiple-choice water maze significantly better than did vehicle-treated ischemic rats. Furthermore, in contrast to the ischemic rats, hypoxia/ischemia-injured rats pretreated with Sim were not hyperactive at weaning and showed less behavioral asymmetry. Consistently, it was found that brain damage was significantly attenuated. CONCLUSIONS: These findings indicate that prophylactic administration of statins may provide a potential neuroprotective strategy leading to an improvement in functional outcome in ischemic stroke. However, toxicity concern must be addressed before these agents can be directed to the asphyxiated fetus or newborn.

Autoradiographic localization of [3H]thiocolchicoside binding sites in the rat brain and spinal cord.

Thiocolchicoside is used in humans as a myorelaxant drug with anti-inflammatory and analgesic activity. Recently we established the experimental conditions that allowed the identification of [3H]thiocolchicoside binding sites in synaptic membranes of rat spinal cord and cerebral cortex. The pharmacological characterization of these sites indicated that GABA and several of its agonists and antagonists, as well as strychnine, were able to interact with [3H]thiocolchicoside binding in a dose-dependent manner and with different affinities. In order to gain more insight into the nature and the anatomical distribution of the binding sites labeled by [3H]thiocolchicoside, in the present study we examined the localization of these sites on parasagittal and coronal sections of the rat brain and spinal cord, respectively, using receptor autoradiography. In the spinal cord an intense signal was observed in the gray matter, with the highest density occurring in the superficial layers of the dorsal horns. Strychnine completely displaced [3H]thiocolchicoside binding, whereas GABA only partially removed the radioligand from its binding sites. In the brain, specific binding occurred in several areas and was displaced by both GABA and strychnine. The distribution of [3H]thiocolchicoside binding sites in brain sections, however, did not match that found for [3H]muscimol. Furthermore, cold thiocolchicoside was not able to completely displace [3H]muscimol binding, and showed a different efficacy in the various areas labeled by the radioligand. We conclude that thiocolchicoside may interact with a subpopulation of GABA(A) receptors having low-affinity binding sites for GABA. Furthermore, the observed sensitivity to strychnine in the spinal cord indicates an interaction also with strychnine-sensitive glycine receptors, suggesting that the pharmacological effects of thiocolchicoside may be the result of its interaction with different receptor populations.

Long-lasting behavioral alterations following a hypoxic/ischemic brain injury in neonatal rats.

The characterization of motor and cognitive dysfunctions following a neonatal ischemic injury is a prerequisite to investigate putative pharmacological interventions. To this end, in the present study, we evaluated the long-lasting behavioral alterations occurring after a hypoxic/ischemic injury obtained by the combination of monolateral carotid ligation and exposure to 8% oxygen for 3 h in 7-day-old rats. These animals show a different degree of damage in the side ipsilateral to the occluded artery. Motor coordination, tested both before and after weaning, was not affected, whereas spontaneous activity was increased at weaning but not in the adult age. When tested in an open field after apomorphine administration, most ischemic animals showed a marked turning behavior ipsilateral to the lesioned side. They also had a reduced rate of spontaneous alternation and a marked tendency to visit the arm of the T-maze ipsilateral to the lesion. Injured rats were deficient in performing water maze and T-maze acquisition tests but, when evaluated in a passive avoidance paradigm, no difference from controls was observed. These data indicate that an ischemic insult in neonatal rats causes long-lasting learning deficits and motor behavior asymmetry. These behavioral alterations may represent a useful endpoint for studying the efficacy of potential pharmacological treatments that may improve the behavioral consequences of a perinatal hypoxic/ischemic insult in humans.

Characterization of [3H]thiocolchicoside binding sites in rat spinal cord and cerebral cortex.

Thiocolchicoside, a semi-synthetic derivative of the naturally occurring compound colchicoside with a relaxant effect on skeletal muscle, has been found to displace both [3H]gamma-aminobutyric acid ([3H]GABA) and [3H]strychnine binding, suggesting an interaction with both GABA and strychnine-sensitive glycine receptors. In order to gain further insight into the interaction of thiocolchicoside with these receptors, the binding of [3H]thiocolchicoside in rat spinal cord-brainstem and cortical synaptic membranes was characterized. [3H]Thiocolchicoside binding was saturable in both tissues examined. In spinal cord-brainstem membranes, we found a K(D) of 254 +/- 47 nM and a Bmax of 2.39 +/- 0.36 pmol/mg protein, whereas in cortical membranes, a K(D) of 176 nM and a Bmax of 4.20 pmol/mg protein was observed. A similar K(D) value was found in kinetic experiments performed in spinal cord-brainstem membranes. Heterologous displacement experiments showed that GABA and strychnine displaced the binding in a dose-dependent manner, whereas glycine was ineffective. [3H]Thiocolchicoside binding was also displaced by several GABA(A) receptor agonists and antagonists, but not by baclofen, flunitrazepam, guvacine, picrotoxin or by other drugs unrelated to GABA transmission. In spinal cord-brainstem, and to a lower extent, in cortical membranes, GABA and its analogs were not able to completely displace [3H]thiocolchicoside specific binding indicating that, besides GABA(A) receptors, thiocolchicoside can bind to another unidentified site. Unlabelled thiocolchicoside, however, completely displaced [3H]muscimol binding both in cortical and in spinal cord-brainstem synaptic membranes with an IC50 in the low microM range. Neurosteroids were found to modulate the binding in cortical but not in spinal cord-brainstem synaptic membranes. We conclude that [3H]thiocolchicoside binding shows a pharmacological profile indicating an interaction with the GABA(A) receptor. The different affinities for the GABA(A) receptor agonists and antagonists and sensitivity to neurosteroids obtained in the cerebral cortex and in the spinal cord may indicate a preferential interaction of the compound with a subtype of the GABA(A) receptor. The data also indicate that [3H]thiocolchicoside binds to another site(s), whose nature remains to be elucidated.

1-Aminocylopropane-1-carboxylic acid derivatives as ligands at the glycine-binding site of the N-methyl-D-aspartate receptor.

Several 1-aminocyclopropane-1-carboxylic acid derivatives were prepared and tested for activity at the glycine-binding site of the N-methyl-D-aspartate (NMDA) receptor complex. Structural modifications involved the amino group, the carboxylic function or position 2 of the ring. When tested in a [3H]-MK-801 binding assay in the presence of glutamic acid, some of the compounds were able to activate the receptor. Two of them (3e and 6) are selective ligands for the glycine site of the NMDA receptor.

Expression of hexokinase mRNA in human hippocampus.

The expression of hexokinase messenger RNA was evaluated in human hippocampus using in situ hybridization technique. The message showed an uneven distribution with high levels present in the granular cell layer of the dentate gyrus and CA3 region. The detection of specific transcripts was also observed in the lateral geniculate nucleus, the dentate polymorphic cell layer and the parahippocampal gyrus. The data suggest that, in the hippocampus, the expression of hexokinase is higher in neurons than in glial cells and that the rate of glucose metabolism may display considerable variations in the different subregions of this area.

Prenatal exposure to ethanol causes differential effects in nerve growth factor and its receptor in the basal forebrain of preweaning and adult rats.

In this study we investigated nerve growth factor (NGF) levels in the cortex and hippocampus of the offspring of pregnant female Sprague-Dawley rats receiving a single intragastric administration of acute ethanol on the 15th day of gestation and compared them with a control group of rats that received an injection of sucrose. We also examined the distribution of the low-affinity NGF receptor, p75NGFR, on NGF-responsive neurons that are localized in the septum and the nucleus of Meynert, which receive the respective trophic support from the hippocampus and the cortex. In the ethanol-treated group, the results show that at post-natal age 15 days, the NGF septohippocampal pathways were markedly affected. At day 15, the NGF level was significantly higher in the offspring of ethanol-treated rats. By day 40, NGF values in both groups decreased to similar levels. At day 60, however, the NGF level in the ethanol-treated animals decreased to a significantly lower value than that of the control group, which remained essentially unchanged. In parallel, at day 60 the numbers of septal cholinergic neurons expressing p75NGFR were also significantly lower in ethanol-treated rats than in control animals. Because ethanol is known to induce neurological disorders, as well as deficits in cell proliferation and differentiation, the results suggest that one cause of the deleterious effects induced by ethanol is the low availability of NGF during certain stages of postnatal brain development.

Glucose-6-phosphate dehydrogenase activity is higher in the olfactory bulb than in other brain areas.

The activity of antioxidant enzymes was measured in the olfactory bulb (OB) of rat and compared with cortex, hippocampus, striatum and septum. Glutathione reductase, glutathione peroxidase, catalase and superoxide dismutase were not significantly different in the five brain areas, while glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase activities were four times higher in the OB than in the other areas. This picture prompted us to explore the reasons of the marked increase of G6PD, since it is the enzyme that regulates the operation of the hexose monophosphate shunt. A first approach was to analyze the G6PD electrophoretic pattern. The analysis revealed that the high G6PD activity of the bulb was neither due to new isoenzymes nor to a modification of the equilibrium between the G6PD dimers. We secondly hypothesized an induction of G6PD activity in the OB by oxidant stress. The assay of markers of the oxidant stress, such as thiobarbituric acid reactive substances, oxidized and reduced glutathione, did not confirm this hypothesis. A third approach was the cytochemical analysis of cryostat sections of OB. By this method we identified a particular cell type which was very rich in G6PD and located at the border of the glomerular layer. Thus, we attributed the high G6PD activity of the OB to the consistent presence of periglomerular cells, that probably need a high G6PD activity for their regulatory function in the neurochemical transmission.

Modulation of muscarinic receptor-stimulated phosphoinositide breakdown by sulfhydryl group modification is a general response in different rat brain regions and depends on the stage of brain development.

In a previous study we reported that muscarinic receptor-stimulated phosphoinositide breakdown in rat cortical slices is significantly reduced by modification of critical thiols located at the post-receptor level. We have now extended this observation by investigating whether this effect is a general response in different brain areas and is differentially regulated, within the cortex, in the neonatal and adult brain. Experimental results have demonstrated that indeed the effect of sulfhydryl group modification on muscarinic receptor-stimulated phosphoinositide breakdown appears to be mediated by a general mechanism operative in different brain regions (cortex, hippocampus and striatum). In addition, this response is more pronounced in the early stages of brain development, when muscarinic receptor agonists lead to the highest inositol phosphate accumulation.

2-(Substituted)amino-2,8-diazaspiro[4,5]decan-1,3-diones as potential muscarinic agonists: synthesis, modeling and binding studies.

A series of 2-(acyl)amino-8-substituted-2,8-diazaspiro[4,5]decan-1,3-diones (5a-j), structurally related to the muscarinic agonist RS-86, was synthesized and compounds tested for their affinity towards muscarinic receptors. Though all compounds proved to be less active than the model in binding studies, only three derivatives (5a, b, c) being able to significantly displace 3H-QNB at mM concentration, their behaviour could be interpreted in terms of theoretical molecular descriptors computed on the basis of the suggestions coming from Molecular Dynamics simulations of ligand-receptor complexes.

Effect of prenatal treatment with methylazoxymethanol on carbachol-, norepinephrine- and glutamate-stimulated phosphoinositide metabolism in the neonatal, young, and adult offspring.

Carbachol-, norepinephrine- and glutamate-stimulated phosphoinositide metabolism was investigated in the neonatal, young and adult cerebral cortex slices of rats prenatally treated with methylazoxymethanol (MAM) on gestational day 15 (GD15) or GD19. In rat offspring treated on GD15 there was a significant reduction in the accumulation of [3H]inositol phosphates induced by carbachol and a significant increase in the accumulation of [3H]inositol phosphates induced by norepinephrine on day 7, whereas no changes were observed at the other ages. No significant changes, on the other hand, were observed for glutamate-stimulated phosphoinositide metabolism in GD15 treated rats and for carbachol-, norepinephrine- and glutamate-stimulated phosphoinositide metabolism in animals treated on GD19 at any of the different ages evaluated. These results indicate that treatment with MAM on GD15, which results in a marked microencephaly, causes a marked alteration of muscarinic and alpha 1-adrenergic receptor-stimulated phosphoinositide metabolism during brain development and that these alterations undergo adaptive changes in the adult brain.

Interaction of ethanol and anoxia with muscarinic receptor--stimulated phosphoinositide metabolism during brain development.

The mechanism(s) by which ethanol induces alterations in brain development may involve direct actions (e.g. changes in specific biochemical pathways), or indirect effects, such as cerebral hypoxia resulting from ethanol - induced circulatory changes. Since both ethanol and hypoxia are known to affect the metabolism of phosphoinositides, which has been suggested as a possible target for ethanol's developmental neurotoxicity, in the present study we have investigated the in vitro effects of both severe hypoxia (anoxia) and ethanol (alone or in combination) on muscarinic receptor-stimulated phosphoinositide metabolism in cerebral cortex slices from neonatal rats. Anoxia markedly inhibited carbachol - stimulated phosphoinositide metabolism in adult rats (67%), but only slightly (10%) in neonatal animals. Reoxygenation reversed the effect of anoxia at both ages. On the other hand, ethanol's inhibitory effect was pronounced in neonatal rats only, and was additive to that of anoxia. The presence of ethanol did not affect the recovery of carbachol - stimulated phosphoinositide metabolism following anoxia and reoxygenation. These results indicate that ethanol and anoxia differently and independently affect muscarinic receptor - stimulated phosphoinositide metabolism and may mutually contribute to the CNS effects observed following developmental ethanol exposure.

Activity of N-(2-phenylethyl)-N-n-propyl-2-(3-hydroxyphenyl) ethylamine derivatives as dopamine receptor ligands.

The N-n-propyl-N-(2-phenylethyl)-2-(3-hydroxyphenyl)ethylamine (1, RU 24213) had been previously identified as selective agonist of DA D2 receptor subtype. In this paper we describe the synthesis and in vitro binding affinities of several derivatives of 1 substituted with fluorine, chlorine and methyl or hydroxy groups on the phenyl ring of the N-2-phenylethyl moiety. The results obtained indicate that these substitutions do not improve the D2 binding affinity. The introduction on the phenyl ring of two fluorine or chlorine atoms decreases with D1 affinity, and the dichloro derivatives are highly selective for the D2 receptor. Preliminary behavioural tests confirm that the dichloro derivatives behave as D2 selective agonists.

Developmental neurotoxicity of ethanol: further evidence for an involvement of muscarinic receptor-stimulated phosphoinositide hydrolysis.

Various lines of evidence suggest that muscarinic receptor-stimulated phosphoinositide hydrolysis during postnatal development in the rat brain may play a relevant role in glial cell proliferation and neuronal differentiation. We have previously shown that administration of ethanol to developing rats during the brain growth spurt causes microencephaly and selectively decreases muscarinic receptor-stimulated phosphoinositide hydrolysis. In the present study we have investigated the sensitivity of the phosphoinositide system coupled to muscarinic receptors to ethanol inhibition during distinct stages of the brain growth spurt. Different groups of rats were treated for 3 days with ethanol (4 g/kg per day) on postnatal days 2-4 (initial), 6-8 or 10-12 (peak), 13-15 (final stage of the brain growth spurt). The results show that the period of maximal sensitivity to ethanol of muscarinic receptor-stimulated phosphoinositide hydrolysis coincides with the peak of the brain growth spurt and with the period of maximal efficacy of muscarinic receptor agonists to induce inositol phosphates accumulation. Interestingly, only when muscarinic receptor-stimulated phosphoinositide hydrolysis was inhibited, a significant reduction of brain weight was observed. The close parallel between inhibition of this second messenger response and reduction of brain weight suggests that the phosphoinositide system coupled to muscarinic receptors may represent a target for the neurotoxic effects of ethanol during this stage of brain development.

Effects of postnatal or adult chronic acetylcholinesterase inhibition on muscarinic receptors, phosphoinositide turnover and m1 mRNA expression.

Muscarinic receptor number, receptor-stimulated phosphoinositide hydrolysis and m1 mRNA expression were examined in the cerebral cortex and hippocampus of rats treated during postnatal development or in adult age with the organophosphate diisopropylfluorophosphate. Developing rats were treated from postnatal days 4-9 or from postnatal days 4-20 and killed on days 10 and 21, respectively, 24 h after the last administration of diisopropylfluorophosphate. Adult animals were treated for 14 days. Acetylcholinesterase activity and muscarinic receptor number were significantly reduced in all groups of treatment. Muscarinic receptor-stimulated phosphoinositide turnover, however, was significantly reduced in postnatal days 4-20 and adult treated rats but not in the postnatal days 4-9 group. No differences were observed in ED50 values. Conversely, m1 mRNA expression was significantly reduced both in the cerebral cortex and hippocampus of postnatal days 4-9 treated rats, but not of postnatal days 4-20 and adult treated rats. These results indicate that chronic inhibition of acetylcholinesterase in developing rats results in significant alterations in muscarinic neurotransmission. These alterations may delay the maturation of the cholinergic system and, therefore, may account for some of the long-lasting neurotoxic effects observed after developmental exposure to organophosphate pesticides.