title: Biphasic modulation of visceral nociception by neurotensin in rat rostral ventromedial medulla.
abstract: A potential role for neurotensin in the rostral ventromedial medulla (RVM) in modulation of visceral nociceptive transmission was examined in this study. Microinjection of neurotensin (3-3000 pmol) into the RVM of awake rats produced a dose-dependent inhibition of the visceromotor response (VMR) to noxious colorectal distension (CRD) that lasted 30 to 120 min. Additionally, intra-RVM injection of neurotensin (300 pmol) significantly reduced the slope of the stimulus-response function to graded CRD (20-80 mm Hg), whereas the greatest dose of neurotensin (3000 pmol) completely inhibited the VMR at all intensities of CRD. General motor function was unaffected after intra-RVM injection of neurotensin (3000 pmol). Intra-RVM injection of lesser doses of neurotensin (0.03-0.30 pmol) resulted an enhancement of the VMR to noxious CRD that had a short duration (18-30 min), and produced a leftward shift of the stimulus-response function to graded CRD without a change in the slope of the function. Additionally, intra-RVM injection of the neurotensin-receptor antagonist SR48692 (0.3-300 fmol) in naive animals produced dose-dependent inhibition of VMR to noxious CRD, whereas a lesser dose (0.03 fmol) enhanced the VMR. These data support a role for neurotensin in the RVM in biphasic modulation of visceral nociception. The results obtained with SR48692 suggest that endogenous neurotensin in the RVM modulates VMR to noxious CRD via a prominent interaction with neurotensin receptors that mediate facilitatory influences and a lesser interaction with neurotensin receptors that mediate masked inhibitory influences.
authors.affiliation: Department of Pharmacology, College of Medicine, The University of Iowa, Iowa City, Iowa, USA. murban@blue.weeg.uiowa.edu
authors.forename: M O
authors.initials: MO
authors.lastname: Urban
authors.forename: S V
authors.initials: SV
authors.lastname: Coutinho
authors.forename: G F
authors.initials: GF
authors.lastname: Gebhart
mesh_terms: D000818:Animals; D008297:Male; D008526:Medulla Oblongata / Q000033:anatomy & histology / Q000187:drug effects*; D008845:Microinjections; D009496:Neurotensin / Q000008:administration & dosage / Q000494:pharmacology / Q000502:physiology*; D009619:Nociceptors / Q000187:drug effects*; D010147:Pain Measurement / Q000187:drug effects; D010812:Physical Stimulation; D011597:Psychomotor Performance / Q000187:drug effects; D011720:Pyrazoles / Q000494:pharmacology; D011804:Quinolines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018028:Receptors, Neurotensin / Q000037:antagonists & inhibitors
chemical_list: D011720:Pyrazoles; D011804:Quinolines; D018028:Receptors, Neurotensin; C079087:SR 48692; D009496:Neurotensin