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Am J Physiol Regul Integr Comp Physiol (December 19, 2002). doi:10.1152/ajpregu.00559.2002
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Submitted on September 11, 2002
Accepted on December 16, 2002

Sympathoinhibitory pathway from caudal midline medullato RVLM is independent of baroreceptor reflex pathway

Jason R Potas1 and Roger A. L Dampney1*

1 Physiology, University of Sydney, Sydney, NSW, Australia

* To whom correspondence should be addressed. E-mail: rogerd{at}physiol.usyd.edu.au.

Glutamate stimulation of the caudal midline medulla (CMM) causes profound sympathoinhibition, due to GABAergic inhibition of presympathetic neurons in the rostral ventrolateral medulla (RVLM). We investigated whether the sympathoinhibitory pathway from CMM to RVLM, like the central baroreceptor reflex pathway, includes a glutamatergic synapse in the caudal ventrolateral medulla (CVLM). In pentobarbitone-anesthetised rats, the RVLM on one side was inhibited by a muscimol microinjection. Then, the response evoked by glutamate microinjections into the CMM or by baroreceptor stimulation was determined before and after (1) microinjection of the GABA receptor antagonist bicuculline into the RVLM on the other side, or (2) microinjections of the glutamate receptor antagonist kynurenate bilaterally into the CVLM. Bicuculline in the RVLM greatly reduced both CMM- and baroreceptor-evoked sympathoinhibition. Compared with the effect of vehicle solution, kynurenate in the CVLM greatly reduced baroreceptor-evoked sympathoinhibition, whereas its effect on CMM-evoked sympathoinhibition was not different to that of the vehicle solution. These findings indicate that the output pathway from CMM sympathoinhibitory neurons, unlike the baroreceptor and other reflex sympathoinhibitory pathways, does not include a glutamatergic synapse in the CVLM.




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