title: Morphine decreases enteric neuron excitability via inhibition of sodium channels.
abstract: Gastrointestinal peristalsis is significantly dependent on the enteric nervous system. Constipation due to reduced peristalsis is a major side-effect of morphine, which limits the chronic usefulness of this excellent pain reliever in man. The ionic basis for the inhibition of enteric neuron excitability by morphine is not well characterized as previous studies have mainly utilized microelectrode recordings from whole mount myenteric plexus preparations in guinea pigs. Here we have developed a Swiss-Webster mouse myenteric neuron culture and examined their electrophysiological properties by patch-clamp techniques and determined the mechanism for morphine-induced decrease in neuronal excitability. Isolated neurons in culture were confirmed by immunostaining with pan-neuronal marker, β-III tubulin and two populations were identified by calbindin and calretinin staining. Distinct neuronal populations were further identified based on the presence and absence of an afterhyperpolarization (AHP). Cells with AHP expressed greater density of sodium currents. Morphine (3 µM) significantly reduced the amplitude of the action potential, increased the threshold for spike generation but did not alter the resting membrane potential. The decrease in excitability resulted from inhibition of sodium currents. In the presence of morphine, the steady-state voltage dependence of Na channels was shifted to the left with almost 50% of channels unavailable for activation from hyperpolarized potentials. During prolonged exposure to morphine (two hours), action potentials recovered, indicative of the development of tolerance in single enteric neurons. These results demonstrate the feasibility of isolating mouse myenteric neurons and establish sodium channel inhibition as a mechanism for morphine-induced decrease in neuronal excitability.
authors.affiliation: Department of Pharmacology and Toxicology, Virginia Commonwealth University, Richmond, Virginia, USA.
authors.forename: Tricia H
authors.initials: TH
authors.lastname: Smith
authors.forename: John R
authors.initials: JR
authors.lastname: Grider
authors.forename: William L
authors.initials: WL
authors.lastname: Dewey
authors.forename: Hamid I
authors.initials: HI
authors.lastname: Akbarali
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D015415:Biomarkers / Q000378:metabolism; D064032:Calbindin 2; D064026:Calbindins; D008297:Male; D051379:Mice; D008839:Microelectrodes; D009020:Morphine / Q000494:pharmacology*; D009197:Myenteric Plexus / Q000166:cytology / Q000187:drug effects* / Q000502:physiology; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000502:physiology; D018408:Patch-Clamp Techniques; D061251:Primary Cell Culture; D064030:S100 Calcium Binding Protein G / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000187:drug effects* / Q000502:physiology; D014404:Tubulin / Q000378:metabolism
chemical_list: D000701:Analgesics, Opioid; D015415:Biomarkers; C576511:Calb2 protein, mouse; D064032:Calbindin 2; D064026:Calbindins; D064030:S100 Calcium Binding Protein G; D015222:Sodium Channels; D014404:Tubulin; D009020:Morphine; D012964:Sodium