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Am J Physiol Heart Circ Physiol 276: H1482-H1492, 1999;
0363-6135/99 $5.00
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Vol. 276, Issue 5, H1482-H1492, May 1999

Glutamatergic projection to RVLM mediates suppression of reflex bradycardia by parabrachial nucleus

Wen-Bin Len1 and Julie Y. H. Chan1,2

1 Institute of Physiology, National Yang-Ming University, Taipei 11221, Taiwan; and 2 Department of Medical Education and Research, Veterans General Hospital-Kaohsiung, Kaohsiung 81346, Taiwan, Republic of China

We investigated the role of glutamatergic projection from the parabrachial nucleus (PBN) complex to the rostral ventrolateral medulla (RVLM) in the PBN-induced suppression of reflex bradycardia in adult Sprague-Dawley rats that were maintained under pentobarbital anesthesia. Under stimulus conditions that did not appreciably alter the baseline systemic arterial pressure and heart rate, electrical (10-s train of 0.5-ms pulses, at 10-20 µA and 10-20 Hz) or chemical (L-glutamate, 1 nmol) stimulation of the ventrolateral regions and Köelliker-Fuse (KF) subnucleus of the PBN complex significantly suppressed the reflex bradycardia in response to transient hypertension evoked by phenylephrine (5 µg/kg iv). The PBN-induced suppression of reflex bradycardia was appreciably reversed by bilateral microinjection into the RVLM of the N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (500 pmol) or the non-NMDA-receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (50 pmol). Anatomically, most of the retrogradely labeled neurons in the ventrolateral regions and KF subnucleus of the ipsilateral PBN complex after microinjection of fast blue into the RVLM were also immunoreactive to anti-glutamate antiserum. These results suggest that a direct glutamatergic projection to the RVLM from topographically distinct regions of the PBN complex may participate in the suppression of reflex bradycardia via activation of both NMDA and non-NMDA receptors at the RVLM.

rostral ventrolateral medulla; N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors; immunohistochemistry; retrograde tract tracing; rat


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