Microinjection of orexin into the rat nucleus tractus solitarius causes increases in blood pressure.

Orexin A (OX-A) and orexin B (OX-B), also known as hypocretin-1 and hypocretin-2, have been suggested to play a role cardiovascular control. The nucleus tractus solitarius (NTS), located in the dorsal medulla plays an essential role in neural control of the cardiovascular system. Orexin-immunoreactive axons have been demonstrated within this nucleus suggesting that NTS may be a site through which OX acts to influence cardiovascular control. We report here that microinjection of OX-A into the NTS of urethane anesthetized rats causes increases in blood pressure (10(-9) M, mean AUC=607.1+/-65.65 mmHg s, n=5) and heart rate (10(-9) M, mean AUC=16.15+/-3.3 beats, n=5) which returns to baseline within 90 s. We show that these effects are dose related and site specific. Microinjection of OX-B into NTS elicited similar increases in BP (mean AUC=680.8+/-128.5 mmHg s, n=4) to that of OX-A suggesting specific actions at the OX(2)R receptor. These observations support the conclusion that orexins act as chemical messengers in the NTS likely influencing the excitability of cardiovascular neurons in this region and thus regulating global cardiovascular function.