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1 Physiology, Medical College of Georgia, Augusta, GA, USA
2 Neuroscience, University of Pittsburgh, Pittsburgh, PA, USA
* To whom correspondence should be addressed. E-mail: ASchreihofer{at}mail.mcg.edu.
Interruption of the baroreceptor reflex by transection of afferent nerves (sino-aortic denervation; SAD) or lesions of nucleus tractus solitarius (NTS) elevates sympathetic nerve activity (SNA) and arterial pressure (AP). However, within a week mean AP returns to normal despite the absence of baroreflexes. Here we examine central mechanisms that control AP in chronic baroreceptor denervated rats. In urethane-anesthetized rats (1.5g/kg, iv) after autonomic ganglionic blockade (chlorisondamine, 5mg/kg, iv), 
1-adrenergic-mediated pressor responses (phenylephrine; 1-100 µg/kg, iv) were not altered by chronic lesions of NTS, indicating vascular reactivity to sympathetic stimulation is normal. Transection of the spinal cord at T1 profoundly decreased AP and was not further reduced by chlorisondamine in control or denervated rats. Inhibition of the rostral ventrolateral medulla (RVLM) by microinjections of muscimol (100 pmol/side) decreased AP to levels not further reduced by chlorisondamine in control rats, rats with SAD, or rats with NTS lesions. Blockade of GABAA receptors in the RVLM (bicuculline, 50 pmol/side) increased AP similarly in control rats and denervated rats. In agreement, inhibition of the caudal ventrolateral medulla (CVLM) by microinjections of muscimol or blockade of glutamatergic inputs (kynurenate, 2.7 nmol/side) produced comparable increases in AP in control and denervated rats. These data suggest the RVLM continues to drive the SNA that regulates AP in the chronic absence of baroreceptor inputs. In addition, despite the absence of a tonic excitatory input from NTS, in chronic baroreceptor denervated rats glutamatergic inputs drive the CVLM to tonically inhibit the RVLM. Baroreceptor-independent regulation of the VLM may underlie central mechanisms contributing to the long-term control of AP.
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