(Hypertension. 2001;37:619.)
© 2001 American Heart Association, Inc.
Scientific Contributions |
-Aminobutyric AcidB Receptor–Mediated Responses in the Nucleus Tractus Solitarius Are Altered in Acute and Chronic Hypertension
From the Department of Pharmacology, The University of Texas Health Science Center at San Antonio.
Correspondence to Steve Mifflin, PhD, Department of Pharmacology, Mail Code 7764, The University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Dr, San Antonio, TX 78229-3900. E-mail mifflin{at}uthscsa.edu
The
increase in mean arterial pressure evoked by injection of
the 
-aminobutyric acid (GABA)B agonist
baclofen into the nucleus tractus solitarius (NTS) is greater in
spontaneously hypertensive rats and renal wrap chronically hypertensive
(CHT) rats compared with normotensive (NT) controls. We report here
that the baclofen-induced pressor response (BIPR) is enhanced after
acute hypertension (AHT) of only 30 minutes. Sprague-Dawley rats were
anesthetized with Inactin, paralyzed, and artificially
ventilated. As we previously reported, after unilateral electrolytic
ablation of the NTS, microinjection of 40 pmol baclofen into the
contralateral NTS of NT rats resulted in a BIPR of 22±1 mm Hg
(n=12). During the infusion of phenylephrine for 30 minutes
(AHT), the BIPR was 39±5 mm Hg (n=10), significantly greater
than the response in NT rats
(P<0.01) and no different from
the response in CHT rats (39±5 mm Hg, n=7). Baclofen has both
presynaptic and postsynaptic effects. To eliminate the presynaptic
component of the baclofen response, sinoaortic denervation (SAD) was
performed before the microinjections. The magnitude of the BIPR was
12±1 mm Hg in NT-SAD rats (n=8), 12±1 mm Hg in AHT-SAD
rats (n=12), and 20±3 mm Hg in CHT-SAD rats (n=7). The BIPR is
enhanced in both CHT and AHT rats. It appears that the increase in
baroreceptor afferent input to NTS during
phenylephrine-induced AHT provides a greater substrate
for presynaptic inhibition by baclofen because the postsynaptic
component of the baclofen response is the same in NT-SAD and AHT-SAD.
The enhanced BIPR in CHT rats appears to be associated with an
enhancement of both the presynaptic and postsynaptic components of the
response.
Key Words: baroreceptors • baroreflex • electrophysiology • hypertension, renal
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