title: Substance P targets sympathetic control neurons in the paraventricular nucleus.
abstract: The paraventricular nucleus (PVN) contains spinally-projecting neurons implicated in fine-tuning the cardiovascular system. In vivo activity of "presympathetic" parvocellular neurons is suppressed by tonic inhibition from GABA-ergic inputs, inhibition of which increases sympathetic pressor activity and heart rate. Targeting of this specific neuronal population could potentially limit elevations of heart rate and blood pressure associated with disease. Here we show, for the first time, that "presympathetic" PVN neurons are disinhibited by the neuropeptide substance P (SP) acting via tachykinin NK1 receptor inhibition of GABA(A) currents. Application of SP to the paraventricular nucleus of rats increases heart rate and blood pressure. In in vitro brain slice experiments, in the presence of GABA, 1 micromol/L SP increased action current frequency by a factor of 2.7+/-0.6 (n=5, P< or =0.05, ANOVA). Furthermore, 1 micromol/L SP inhibited GABA(A) currents by 70+/-8% (n=8, P< or =0.005 paired t test). These effects were abolished by NK1 antagonists, but not NK2 and NK3 antagonists. GABA(A) inhibition was not reproduced by NK2 or NK3 agonists. The inhibition of parvocellular GABA(A) currents by SP was also abolished by a protein kinase C (PKC) inhibitor peptide and mimicked by application of phorbol-12-myristate-13-acetate (PMA), implicating a PKC-dependent mechanism. Single-channel analysis indicates that SP acts through reduction of channel mean open-time (cmot): GABA(A) cmot being reduced by approximately 60% by SP (P< or =0.05 ANOVA, Bonferroni). These data suggest that tachykinins mediate their pressor activity by increasing the excitability of spinally-projecting neurons and identifies NK1 receptors as potential targets for therapeutic modulation of the cardiovascular system.
authors.affiliation: Veterinary Sciences Faculty, Brownlow Hill, University of Liverpool, UK.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Womack
authors.forename: Richard
authors.initials: R
authors.lastname: Morris
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Gent
authors.forename: Richard
authors.initials: R
authors.lastname: Barrett-Jolley
mesh_terms: D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D020536:Enzyme Activators / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006339:Heart Rate / Q000187:drug effects; D007668:Kidney / Q000294:innervation; D008297:Male; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009924:Organ Culture Techniques; D010286:Paraventricular Hypothalamic Nucleus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D011493:Protein Kinase C / Q000187:drug effects / Q000378:metabolism; D011812:Quinuclidines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000187:drug effects / Q000378:metabolism; D018040:Receptors, Neurokinin-1 / Q000187:drug effects / Q000378:metabolism; D013373:Substance P / Q000494:pharmacology / Q000502:physiology*; D013564:Sympathetic Nervous System / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
chemical_list: D020536:Enzyme Activators; D004791:Enzyme Inhibitors; D011812:Quinuclidines; D011963:Receptors, GABA-A; D018040:Receptors, Neurokinin-1; C076851:L 703606; D013373:Substance P; D005680:gamma-Aminobutyric Acid; D011493:Protein Kinase C