Neurotoxicology and Teratology, Vol. 20, No. 6, pp. 627–635, 1998 Published by Elsevier Science Inc. Printed in the USA. All rights reserved 0892-0362/98 $0.00 + .00 PII S0892-0362(98)00020-8 627 Development of Cholinergic Neurons in Rat Brain Regions: Dose-Dependent Effects of Propylthiouracil-Induced Hypothyroidism SHANNON SAWIN,* PAUL BRODISH,* CHRISTY S. CARTER,† MARK E. STANTON‡ AND CHRISTOPHER LAU§ *Mantech Environmental Sciences, †Department of Psychology, University of North Carolina, Chapel Hill, NC ‡Neurotoxicology Division and §Reproductive Toxicology Division, NHEERL, US EPA, Research Triangle Park, NC Received 17 December 1997; Accepted 24 April 1998 SAWIN, S., P. BRODISH, C. S. CARTER, M. E. STANTON AND C. LAU. Development of cholinergic neurons in rat brain regions: Dose-dependent effects of propylthiouracil-induced hypothyroidism. NEUROTOXICOL TERATOL 20(6) 627–635, 1998.—The effects of hypothyroidism on development of cholinergic system in brain regions (prefrontal cortex and hippocampus) were evaluated by measuring choline acetyltransferase (ChAT) activity and hemicholinium-3 binding to the high-affinity choline transporter. Various degrees of thyroid deficiency were produced by perinatal exposure to propylthiou- racil (PTU) in drinking water ranging from 5 ppm (mg/l) to 25 ppm beginning at gestational day 18 until postnatal day 21. ChAT, a marker for cholinergic nerve terminals, was reduced by PTU in a dose-dependent manner. Concomitant with the enzyme deficits, hemicholinium-3 binding was elevated, suggesting an increase in neuronal impulse activity. Although similar changes were seen in both brain regions examined, the magnitude and duration of these changes were more definitive in the prefrontal cortex. Nonetheless, these neurochemical alterations appeared to be recoverable when the rats returned to a eu- thyroid state, and no further changes were observed as the animals reached adulthood. In comparison, data reported in a suc- ceeding article indicate that deficits in cognitive function were first seen in weanling hypothyroid rats, but that the behavioral impairments lasted well into adulthood when thyroid status and cholinergic parameters in the brain appeared to have recov- ered to normal. These results suggest that alterations of cholinergic system caused by perinatal hypothyroidism are associated with neurobehavioral deficits at weaning, and these developmental deviations may cause permanent impairment of cognitive function despite recovery from the hormonal imbalance at adult ages. Published by Elsevier Science Inc. Hypothyroidism Cholinergic System Propylthiouracil Brain neurons THE influence of thyroid hormones on brain growth and dif- ferentiation have been well documented (7, 8, 14, 22, 29, 33, 41). Hormonal imbalance during critical periods of develop- ment in both humans and laboratory animals typically leads to a host of neuroanatomical, molecular and neurochemical al- terations in the central nervous system as well as various be- havioral abnormalities, although the precise linkages between these disorders remain poorly understood. The ontogeny of numerous CNS neurotransmitter systems that include cate- cholamines, acetylcholine, GABA and glutamate have been shown to be affected by neonatal thyroid deficiency (17, 24, 30, 31, 34–38, 44). Among these, the cholinergic system in var- ious brain regions is perhaps the most profoundly affected (13, 27, 31, 35, 46). Indeed, the activity of choline acetyltrans- ferase (ChAT), an enzyme marker for cholinergic nerve ter- minals, is exquisitely sensitive to thyroid status. Additionally, hyperthyroidism has been shown to accelerate and hypothy- roidism to delay the ontogenetic profiles of perikaryal sizes and dendritic arborization of the cholinergic neurons in the basal forebrain (13). Patel et al. (32) and Oh et al. (27) further suggested that susceptibility of the developing cholinergic neurons to thyroid hormones may involve differential sensi- Requests for reprints should be addressed to Dr. Christopher Lau, Mail Drop 67, US Environmental Protection Agency, Research Triangle Park, NC 27711. Fax: (919) 541-4017; E-mail: lau.christopher@epamail.epa.gov