Insulin secretion is impaired in type 2 diabetes (T2D). The insulin and glucose responses to central autonomic activation induced by excitation of brain medullary TSH-releasing hormone (TRH) receptors were studied in T2D Goto-Kakizaki (GK) rats. Blood glucose levels in normally fed, pentobarbital-anesthetized GK and non-diabetic Wistar rats were 193 and 119 mg% in males and 214 and 131 mg% in females. Intracisternal injection (ic) of the stable TRH analog, RX 77368 (10 ng), induced significantly higher insulin response in both genders of overnight-fasted GK rats compared with Wistar rats and slightly increased blood glucose in female Wistar rats but significantly decreased it from 193 to 145 mg% in female GK rats. RX 77368 (50 ng) ic induced markedly greater glucose and relatively weaker insulin responses in male GK rats than Wistar rats. Bilateral vagotomy blocked ic RX 77368-induced insulin secretion while adrenalectomy abolished its hyperglycemic effect. In adrenalectomized male GK but not Wistar rats, ic RX 77368 (50 ng) dramatically increased serum insulin levels by 6.5-fold and decreased blood glucose levels from 154 to 98 mg%; these changes were prevented by vagotomy. GK rats had higher basal pancreatic insulin II mRNA levels but lower response to ic RX 77368 (50 ng) compared with Wistar rats. These results indicate that central-vagal activation-induced insulin secretion is susceptible in T2D GK rats. However, the dominant sympathetic-adrenal response to medullary TRH plays a suppressing role on vagal-mediated insulin secretion. This unbalanced vago-sympathetic activation by medullary TRH may contribute to the impaired insulin secretion in T2D.