Zinc protoporphyrin-IX blocks the effects of metabotropic glutamate receptor activation in the rat nucleus tractus solitarii

Steven R. Glaum*, Richard J. Miller

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

69 Scopus citations


The effects of the metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] on ionic current responses produced by ionotropic glutamate and γ-aminobutyric acid (GABA)A receptor activation in the nucleus of the tractus solitarius (NTS) were examined. Recordings were made in the dorsomedial subdivision of the NTS adjacent to the area postrema in transverse brainstem slices of the rat. (1S,3R)-ACPD produced a small inward current (IACPD) associated with a decrease in conductance in approximately 50% of recordings. Monosynaptic excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation in the region of the tractus solitarius in the presence of D-amino-5-phosphonopentanoic acid and bicuculline were reversibly reduced by (15,3R)-ACPD in >90% of cells. The inward current evoked by pressure application of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) (IAMPA) was potentiated in the presence of (1S,3R)-ACPD, whereas the outward current evoked by the GABAA receptor agonist muscimol (IMUSC) was inhibited. We have previously demonstrated that these effects may involve the activation of soluble guanylate cyclase. The diffusible second messengers nitric oxide and carbon monoxide are known to activate soluble guanylate cyclase. The nitric oxide synthase inhibitor L-ω-nitroarginine failed to inhibit responses to (1S,3R)-ACPD. The selective heme oxygenase inhibitor Zn-protoporphyrin-IX, which would be expected to block the production of carbon monoxide, antagonized the effects of (1S,3R)-ACPD on EPSCs, IAMPA, and IMUSC. However, IACPD was not blocked. A relatively inactive metalloprotoporphyrin, Cu-protoporphyrin-IX, was ineffective. A cell-permeant form of cGMP, 8-Br-cGMP, inhibited EPSCs, IAMPA, and IMUSC in the presence of Zn-protoporphyrin-IX but did not induce an inward current. These results further support the hypothesis that multiple metabotropic glutamate receptors exist in the NTS, and they suggest that one of these may be coupled to the activation of a soluble guanylate cyclase via the liberation of an easily diffusible second messenger such as carbon monoxide.

Original languageEnglish (US)
Pages (from-to)965-969
Number of pages5
JournalMolecular pharmacology
Issue number6
StatePublished - Jun 1993

ASJC Scopus subject areas

  • Molecular Medicine
  • Pharmacology


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