Complex cardiovascular actions of α-adrenergic receptors expressed in the nucleus tractus solitarii of rats

Although both α 1 - and α 2 -adrenergic receptors (ARs) are known to be expressed in the nucleus of the solitary tract (NTS), the functional significance of these receptors is still not fully established. In this study, we microinjected α 1 - and α 2 -AR agonists into the NTS of urethane-anaesthetized Wister rats to study the cardiovascular effects in response to their activation. When the α 1 -AR agonist phenylephrine was microinjected into the area where barosensitive neurons are dominantly located (baro-NTS), mean arterial pressure (MAP) and heart rate (HR) were significantly elevated. When tested in the area where chemosensitive neurons are dominantly located (chemo-NTS), however, MAP and HR were significantly decreased. Pretreatment with the non-specific α-AR antagonist phentolamine into the NTS inhibited the phenylephrine-induced cardiovascular responses. In contrast, microinjection of the α 2 -AR agonist clonidine into either the baro-NTS or the chemo-NTS decreased MAP and HR; they were also inhibited by the α 2 -adrenergic antagonist yohimbine. Moreover, we immunohistochemically identified that cardiovascular responses induced by α 1 -ARs may be mediated by NTS neurons while those induced by α 2 -ARs may be mediated by astrocytes located in the barosensitive and chemosensitive areas of the NTS. These results suggest that both types of α-AR expressed in the NTS may be involved in regulating cardiovascular homeostasis via modulation of input signals from baroreceptor and chemoreceptor afferents; however, cardiovascular responses produced by stimulation of α 1 -ARs are strictly location specific within the NTS.