title: alpha2-adrenoceptors inhibit the intracellular Ca2+ response to electrical stimulation in normal and injured sensory neurons, with increased inhibition of calcitonin gene-related peptide expressing neurons after injury.
abstract: Nerve injury resulting in chronic pain is associated with novel excitatory effects of norepinephrine on injured peripheral nerve terminals and their cell bodies, due to actions on alpha2-adrenoceptors. Paradoxically, alpha2-adrenoceptor agonists administered near peripheral terminals or their cell bodies results in analgesia, not pain. This study tested, using intracellular Ca2+ response to stimulation, the effects of alpha2-adrenoceptor agonists on injured sensory neurons and classified their neuronal phenotype. Dorsal root ganglion cells from normal and spinal nerve-ligated rats were dissociated and activated twice with electrical field stimulation, while measuring Fura-2 fluorescence. Cells were perfused between stimulations with vehicle or alpha2-adrenoceptor agonists alone or with antagonists. Cells were considered inhibited if the ratio of their peak Ca2+ response to the second stimulus divided by the first was less than the 2.5th percentile for vehicle controls. alpha2-, But not alpha1-adrenoceptor agonists inhibited the Ca2+ response in a concentration related fashion, and this inhibition was blocked by alpha2-adrenoceptor antagonists. Clonidine inhibited a similar percentage of cells in the normal and spinal nerve-ligated group. In both groups, the large majority of clonidine-inhibited cells stained for isolectin B4. Spinal nerve ligation resulted in a 4-10-fold increase in the percentage of clonidine inhibited cells which immunostained for calcitonin gene-related peptide. These data are consistent with the known inhibition of Ca2+ currents by alpha2-adrenoceptors and suggest that, at the level of intracellular Ca2+, the key determinant of neurotransmitter release, alpha2-adrenoceptors are inhibitory after nerve injury, not excitatory. There is a shift in phenotype of sensory neurons which are inhibited by clonidine after nerve injury, which may explain clonidine's increased potency in the treatment of neuropathic compared with acute pain.
authors.affiliation: Department of Anesthesiology, Center for the Study of Pharmacologic Plasticity in the Presence of Pain, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157, USA. eisenach@wfubmc.edu
authors.forename: J C
authors.initials: JC
authors.lastname: Eisenach
authors.forename: Y
authors.initials: Y
authors.lastname: Zhang
authors.forename: F
authors.initials: F
authors.lastname: Duflo
mesh_terms: D000818:Animals; D015740:Calcitonin Gene-Related Peptide / Q000235:genetics*; D002118:Calcium / Q000502:physiology*; D004558:Electric Stimulation; D005727:Ganglia, Spinal / Q000502:physiology / Q000503:physiopathology; D005786:Gene Expression Regulation; D008297:Male; D018715:Microscopy, Video; D009475:Neurons, Afferent / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D018343:Receptors, Adrenergic, beta-2 / Q000502:physiology*; D012016:Reference Values; D013119:Spinal Cord Injuries / Q000503:physiopathology*
chemical_list: D018343:Receptors, Adrenergic, beta-2; D015740:Calcitonin Gene-Related Peptide; D002118:Calcium