title: Oscillation patterns are enhanced and firing threshold is lowered in medullary respiratory neuron discharges by threshold doses of a μ-opioid receptor agonist.
abstract: μ-Opioid receptors are distributed widely in the brain stem respiratory network, and opioids with selectivity for μ-type receptors slow in vivo respiratory rhythm in lowest effective doses. Several studies have reported μ-opioid receptor effects on the three-phase rhythm of respiratory neurons, but there are until now no reports of opioid effects on oscillatory activity within respiratory discharges. In this study, effects of the μ-opioid receptor agonist fentanyl on spike train discharge properties of several different types of rhythm-modulating medullary respiratory neuron discharges were analyzed. Doses of fentanyl that were just sufficient for prolongation of discharges and slowing of the three-phase respiratory rhythm also produced pronounced enhancement of spike train properties. Oscillation and burst patterns detected by autocorrelation measurements were greatly enhanced, and interspike intervals were prolonged. Spike train properties under control conditions and after fentanyl were uniform within each experiment, but varied considerably between experiments, which might be related to variability in acid-base balance in the brain stem extracellular fluid. Discharge threshold was shifted to more negative levels of membrane potential. The effects on threshold are postulated to result from opioid-mediated disinhibition and postsynaptic enhancement ofN-methyl-d- aspartate receptor current. Lowering of firing threshold, enhancement of spike train oscillations and bursts and prolongation of discharges by lowest effective doses of fentanyl could represent compensatory adjustments in the brain stem respiratory network to override opioid blunting of CO2/pH chemosensitivity.
authors.affiliation: Department of Neuroscience, University of Wisconsin Medical Sciences Center, Madison, Wisconsin; and pmlalley@facstaff.wisc.edu.
authors.forename: Peter M
authors.initials: PM
authors.lastname: Lalley
authors.affiliation: Institute for Cardiovascular and Metabolic Disease, University of North Texas Health Science Center, Fort Worth, Texas.
authors.forename: Steve W
authors.initials: SW
authors.lastname: Mifflin
mesh_terms: D000200:Action Potentials / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D001683:Biological Clocks / Q000187:drug effects / Q000502:physiology*; D002415:Cats; D063226:Central Pattern Generators / Q000502:physiology; D004056:Differential Threshold / Q000187:drug effects / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D005283:Fentanyl / Q000008:administration & dosage*; D008526:Medulla Oblongata / Q000187:drug effects / Q000502:physiology*; D009292:Narcotic Antagonists / Q000008:administration & dosage; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D017450:Receptors, Opioid, mu / Q000037:antagonists & inhibitors* / Q000378:metabolism; D012119:Respiration / Q000187:drug effects*
chemical_list: D009292:Narcotic Antagonists; D017450:Receptors, Opioid, mu; D005283:Fentanyl
keywords: burst and oscillation patterns; opioid actions bursts and oscillations; opioid lowers respiratory neuron discharge threshold; respiratory neuron discharges