Structural and functional cardiac cholinergic deficits in adult neurturin knockout mice.
OBJECTIVE
Previous work provided indirect evidence that the neurotrophic factor neurturin (NRTN) is required for normal cholinergic innervation of the heart. This study used nrtn knockout (KO) and wild-type (WT) mice to determine the effect of nrtn deletion on cardiac cholinergic innervation and function in the adult heart.


RESULTS
Immunohistochemistry, confocal microscopy, and quantitative image analysis were used to directly evaluate intrinsic cardiac neuronal development. Atrial acetylcholine (ACh) levels were determined as an indirect index of cholinergic innervation. Cholinergic function was evaluated by measuring negative chronotropic responses to right vagal nerve stimulation in anaesthetized mice and responses of isolated atria to muscarinic agonists. KO hearts contained only 35% the normal number of cholinergic neurons, and the residual cholinergic neurons were 15% smaller than in WT. Cholinergic nerve density at the sinoatrial node was reduced by 87% in KOs, but noradrenergic nerve density was unaffected. Atrial ACh levels were substantially lower in KO mice (0.013 +/- 0.004 vs. 0.050 +/- 0.011 pmol/microg protein; P < 0.02) as expected from cholinergic neuron and nerve fibre deficits. Maximum bradycardia evoked by vagal stimulation was reduced in KO mice (38 +/- 6% vs. 69 +/- 3% decrease at 20 Hz; P < 0.001), and chronotropic responses took longer to develop and fade. In contrast to these deficits, isolated atria from KO mice had normal post-junctional sensitivity to carbachol and bethanechol.


CONCLUSIONS
These findings demonstrate that NRTN is essential for normal cardiac cholinergic innervation and cholinergic control of heart rate. The presence of residual cardiac cholinergic neurons and vagal bradycardia in KO mice suggests that additional neurotrophic factors may influence this system.
Mabe
Abigail M
AM
Department of Pharmacology, James H. Quillen College of Medicine, East Tennessee State University, Johnson City, TN 37614, USA.
Hoover
Donald B
DB
D000109:Acetylcholine / Q000378:metabolism*; D000320:Adrenergic Fibers / Q000378:metabolism; D000818:Animals; D018723:Bethanechol / Q000494:pharmacology; D001919:Bradycardia / Q000378:metabolism* / Q000503:physiopathology; D002217:Carbachol / Q000494:pharmacology; D018679:Cholinergic Agonists / Q000494:pharmacology; D002799:Cholinergic Fibers / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D006321:Heart / Q000294:innervation*; D006325:Heart Atria / Q000294:innervation; D006339:Heart Rate / Q000187:drug effects; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018721:Muscarinic Agonists / Q000494:pharmacology; D051101:Neurturin / Q000172:deficiency* / Q000235:genetics; D009638:Norepinephrine / Q000378:metabolism; D012849:Sinoatrial Node / Q000294:innervation; D013997:Time Factors; D014630:Vagus Nerve / Q000187:drug effects / Q000378:metabolism*
D018679:Cholinergic Agonists; D018721:Muscarinic Agonists; D051101:Neurturin; C495056:Nrtn protein, mouse; D018723:Bethanechol; D002217:Carbachol; D000109:Acetylcholine; D009638:Norepinephrine