Yuan, P.Q. and H. Yang. Hypothyroidism increases Fos immunoreactivity in cholinergic neurons in brain medullary dorsal vagal complex in rats. Am. J. Physiol. Endocrin. Metab. XXX: EXXX-EXXX, 200X.---Hypo- or hyperthyroidism is associated with autonomic disorders. We studied Fos expression in the medullary dorsal motor nucleus of the vagus (DMV), nucleus tractus solitarii (NTS), and area postrema (AP) in four groups of rats with different thyroid states induced by a combination of drinking water and daily intraperitoneal (ip) injection for 1-4 weeks: 1) tap water and vehicle; 2) 0.1% propylthiouracil (PTU) and vehicle; 3) PTU and T4 (2 µg/100 g); 4) tap water and T4 (10 µg/100 g). The numbers of Fos immunoreactive (IR) positive neurons in the DMV, NTS, and AP were low in euthyroid rats but were significantly higher in the 4-week duration in hypothyroid rats, which were prevented by simultaneous T4 replacement. Hyperthyroidism had no effect on Fos expression in these areas. There were significant negative correlations between T4 levels and the numbers of Fos IR positive neurons in the DMV (r=-0.6388, P<0.008), NTS (r=-0.6741, P<0.003), and AP (r=-0.5622, P<0.004). Double staining showed that Fos immunoreactivity in the DMV of hypothyroid rats was mostly localized in choline acetyltransferase-containing neurons. Thyroid hormone receptors 1 and 2 were localized in the observed nuclei. These results indicate that thyroid hormone influences the DMV/NTS/AP neuronal activity, which may contribute to the vagal-related visceral disorders observed in hypothyroidism.