|Xue, Neil -|
|Liao, Shenfga -|
|Boling, James -|
|Matthews, James -|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 12, 2011
Publication Date: July 25, 2011
Citation: Xue, N., Liao, S.R., Strickland, J.R., Boling, J.A., Matthews, J.C. 2011. Bovine neuronal vesicular glutamate transporter activity is inhibited by ergovaline and other ergopeptines. J Dairy Sci. 94:3331-3341. Interpretive Summary: Cattle grazing endophyte-infected tall fescue have reduced productive and reproductive capacities. This research found that ergopeptines contained in endophyte-infected tall fescue can inhibit the function of a glutamate transport activity that is specific for glutamate-signaling in nervous and peripheral tissues. Thus, this research may have identified a causative mechanism for the wide-spread physiologic effects of fescue toxicosis.
Technical Abstract: L-Glutamate (Glu) is the major excitatory neurotransmitter responsible for neurotransmission in the vertebrate central nervous system, including the gastrointestinal tract (GIT) of cattle. Vesicular Glu transporters VGLUT1 and VGLUT2 concentrate (50 mM) Glu (Km = 1 to 4 mM) into synaptic vesicles (SV) for subsequent release into the synaptic cleft of glutamatergic neurons. VGLUT activity is dependent on vacuolar H+-ATPase function. Previous research has shown that ergopeptines contained in endopyte-infected tall fescue interact with dopaminergic and serotoninergic receptors, thereby affecting physiology regulated by these neuron types. To test the hypothesis that ergopeptine alkaloids inhibit VGLUT activity of bovine cerebral SV, SV were isolated from cerebral tissue of Angus-cross steers that were naive to ergot alkaloids. Immunoblot analysis validated the enrichment of VGLUT1, VGLUT2, synaptophysin 1, and vacuolar H+-ATPase in purified SV. Glutamate uptake assays demonstrated the dependence of SV VGLUT-like activity on the presence of ATP, H+-gradients, and H+-ATPase function. The effect of ergopeptines on VGLUT activity was evaluated by ANOVA. Inhibitory competition (IC50) experiments revealed that VGLUT-mediated Glu uptake (n = 9) was inhibited by ergopeptine alkaloids: bromocriptine (2.83 ± 0.59 µM) < ergotamine (20.5 ± 2.77 µM) < ergocornine (114 ± 23.1 µM) < ergovaline (137 ± 6.55 µM). Subsequent ergovaline kinetic inhibition analysis (n = 9; Glu = 0.05, 0.10, 0.50, 1, 2, 4, 5 mM) demonstrated no change in apparent Km. However, the Vmax of Glu uptake was decreased when evaluated in the presence of 50, 100, and 200 µM ergovaline, suggesting that ergovaline inhibited SV VGLUT activity through a non-competitive mechanism. The findings of this study suggest cattle with fescue toxicosis may have a reduced glutamatergic neurotransmission capacity, resulting in impaired GIT function due to consumption of ergopeptine alkaloids.