Neurotoxicology and Teratology, Vol. 15, pp. 107-115, 1993 0892-0362/93 $6.00 + .00 Printed in the U.S.A. All rights reserved. Copyright © 1993 Pergamon Press Ltd. Fetal Nicotine Exposure Alters Ontogeny of M -Receptors and Their Link to G-Proteins E. A. ZAHALKA, I F. J. SEIDLER, J. YANAI AND T. A. SLOTKIN Department of Pharmacology, Duke University Medical Center, Durham, NC 27710 The Melvin A. and Eleanor Ross Laboratory for Studies in Neural Birth Defects (JY), Department of Anatomy and Embryology, Hebrew University, Hadassah Medical School, Jerusalem, Israel Received 13 October 1992; Accepted 14 December 1992 ZAHALKA, E. A., F. J. SEIDLER, J. YANAI AND T. A. SLOTKIN. Fetal nicotine exposure alters ontogeny of Mrreceptorsand theirlink to G-proteins. NEUROTOXICOL TERATOL 15(2) 107-115, 1993.-Prenatal nicotine exposure has been shown to disrupt the development of cholinergic presynaptic tone and behaviors mediated through muscarinic cholinergic receptors. The current study examines nicotine's effects on ontogeny of postsynaptic muscarinic Mrreceptors in rat striatum and hippocampus after continuous maternal infusions of 2 mg/kg/day or 6 mg/kg/day from gestationai days 4 through 20. Although brain region weights were unaffected by nicotine exposure, significant alterations in receptor develop- ment and receptor regulation by G-proteins were found. Postnatal development of striatal M,-receptor binding, as identified with [3H]pirenzepine, was significantly impaired with either of the fetal nicotine regimens. Treatment with 2 mg/kg/day also produced alterations in striatal receptor affinity state, characterized by enhanced ability of an agonist (oxotremorine-M) to displace [3H]pirenzepine; raising the dose to 6 mg/kg/day masked the affinity shift by affecting G-protein regulatory mecha- nisms, such that addition of the GTP analog, GppNHp, produced a larger decrease in agonist affinity. In the hippocampus, no such effects on receptor binding, affinity state, or G-protein regulation were seen with either regimen. 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 cholinergic receptor development and regulation of cell signaling mediated by G-proteins. The selectivity of effects toward Mm-receptorsin the striatum, a region with a prenatal peak of neuronal mitosis, as compared to hippocampus, where mitosis peaks postnatany, suggests that vulnerability to nicotine involves a critical phase of cell develop- ment, rather than being targeted toward receptors of a given subtype. Cholinergic receptors Effects of nicotine in developing brain G-proteins in developing brain Development of cholinergic innervation Muscarinic receptor in developing brain Nicotine Effects on cholinergic development Striatum Development of cholinergic innervation Hippocampus TOBACCO smoking during pregnancy leads to intrauterine growth retardation, spontaneous abortion and low birth weight (5,12,31,35). However, over and above its general peri- natal toxicity, tobacco targets the fetal nervous system for adverse psychological and behavioral effects (1,4,14,28, 44,58). Cigarette smoke contains a wide variety of potentially injurious chemicals, including carbon monoxide (26) and smoking causes fetal ischemia and hypoxia (8,29,40,49,53,55). Nevertheless, nicotine itself is a neurobehavioral teratogen, as demonstrated conclusively by animal models that use slow infusions of nicotine that are devoid of other tobacco compo- nents and do not cause hypoxia and ischemia (see reviews, 23,51). Because nicotine acts via highly selective and sensitive neurotransmitter receptors that are present in fetal brain (6,15,34,55), the developing nervous system is actually more sensitive than are general indices of growth and development (39), a relationship that runs counter to the standard principle that systemic insults spare the developing brain relative to the rest of the body (3,11). In large measure, the alterations of central nervous system development evoked by nicotine exposure reflect the fact that acetylcholine plays a key role in the maturation of target cells receiving cholinergic innervation, eliciting a switchover from cell replication to differentiation and determining the subse- quent architectural modeling of specific brain regions (17, 30,38). Fetal exposure to nicotine thus evokes the same de- velopmental events, but because the drug administration is occurring prior to the natural spike of cholinergic activity, the processes occur prematurely and in a discoordinated fashion (39,53,56). Accordingly, it is especially important to examine the specific relationship of nicotine induced neuroteratologies to development of cholinergic systems. Several studies have Requests for reprints should be addressed to Dr. T. A. Slotkin, Box 3813, Department of Pharmacology, Duke University Medical Center, Durham, NC 27710. Supported by the SmokelessTobacco Research Council and USPHS DA-OS70. 107