Activation of noradrenergic mechanism attenuates glutamate-induced vasopressor responses in the pons and medulla of cats in vivo

Abstract

1. Using anesthetized cats, the authors examined the noradrenergic modulation of the glutamate induced pressor and depressor responses in various brainstem areas, including pontine gigantocellular tegmental field (FTG), dorsomedial medulla (DM), rostral ventrolateral medulla (RVLM), and caudal ventrolateral medulla (CVLM). 2. Unilateral microinjection of L-glutamate (Glu, 3 nmol in 30 nL saline) into FTG, DM and RVLM produced an increased in systemic arterial presure (SAP) and a decrease in heart rate (HR), while into CVLM produced decreases of SAP and HR. 3. Application of norepinephrine (NE) into the pressor areas (005 to 5 nmol) did not alter the resting SAP and HR, but significantly attenauted the Glu-induced pressor response with an order of potency: FTG > DM > RVLM. In the depressor CVLM, NE alone produced a dose-dependent decrease of resting SAP and HR, but did not affect the Glu-induced depressor responses. 4. The involvement of different adrenoceptor subtypes was further investigated by application of selective adrenoceptor agonists including phenylephrine (α1), clonidine (α2), and isoproterenol (β). Responses to these agonists are similar to those elicited by NE, except that only α-adrenoceptor agonists could antagonize the Glu-induced pressor responses of the RVLM. 5. Our observations indicate that NE not only inhibits the pressor mechanisms in various brainstem areas but also elicits a direct depressor response in CVLM. These findings also suggest that NE acts more likely a neurotransmitter, rather than a modulator, in the CVLM.