The role of the a7 subunit of the nicotinic acetylcholine receptor on motor coordination in mice treated with methyllcaconitine and anabasine

Authors: Welch, Kevin; Pfister, James; Gardner, Dale; Green, Benedict - Ben; Panter, Kip

Submitted to: Journal of Applied Toxicology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/9/2013
Publication Date: 9/1/2013
Publication URL: http://handle.nal.usda.gov/10113/5451334
Citation: Welch, K.D., Pfister, J.A., Gardner, D.R., Green, B.T., Panter, K.E. 2013. The role of the a7 subunit of the nicotinic acetylcholine receptor on motor coordination in mice treated with methyllcaconitine and anabasine. Journal of Applied Toxicology. 33(9):1017-26.

Interpretive Summary: The toxicity of larkspur plants (Delphinium spp.) is due to the more than 18 norditerpenoid alkaloids, each with varying degrees of affinity, and potency at nicotinic acetylcholine receptors (nAChR). Research has demonstrated that the physiological effects of methyllycaconitine (MLA), one of the more abundant toxic alkaloids in larkspur, were attributable to its high affinity for nAChR ligand binding sites in muscle and nervous systems. Methyllycaconitine has been regarded as a potent and selective competitive antagonist with nanomolar affinity at a7 nAChR and micromolar affinity at other nAChR subunits. Research has demonstrated that different strains of mice had differences in susceptibility to the acute toxicity of MLA, with a direct correlation between the expression of several nAChR subunits, including the a7 subunit, and susceptibility. In a recent study to further characterize the role of the a7 subunit of the nAChR in the acute toxicosis of MLA, the LD50 values for MLA and anabasine in wild-type mice were compared to mice lacking the a7 subunit. Interestingly, there were no differences in the LD50 values for MLA or anabasine for wild-type, heterozygous knockout, and homozygous knockout mice. The results of that study suggest that a7 nAChR does not play an integral role in the acute lethality of MLA or anabasine in mice. However, there is still a possibility that the a7 nAChR subunit may play a role in more subtle aspects of MLA toxicosis, including effects on motor function and coordination. In a recent study, we characterized the motor coordination deficiencies that occur upon exposure to a non-lethal dose of nAChR antagonists and agonists using a balance beam, grip strength meter, rotarod, open field analysis and tremor monitor. The objective of this study was to determine if the a7 nAChR subunit plays a role in the motor coordination deficiencies elicited by nAChR agonists and antagonists by comparing the motor function and coordination of wild type mice to mice lacking the a7 nAChR subunit after exposure to a non-lethal dose of MLA and anabasine. The data presented in this study clearly demonstrate that treatment of mice with both a nAChR antagonist and agonist results in decreased motor function and coordination. These motor function deficits were highlighted by the inability of poisoned mice to traverse a balance beam, a decrease in limb grip strength, a decreased ability to maneuver on a rotating rod, a lack of normal activity, and a measureable difference in muscle tremors and movement energy. However, the results of this study suggest that the a7 subunit of nAChRs does not play an integral role in the motor coordination deficits elicited by MLA or anabasine, prototypical antagonist and agonist of nAChRs, respectively, as there was no difference in motor coordination deficiencies between wild-type mice and mice lacking the a7 nAChR subunit. This suggests that the mechanism of action for the motor coordination deficiencies and acute toxicity of these compounds is via interaction with other nAChR subunits, or that the knockout mice have developed a compensatory mechanism such that they were no longer dependent upon the a7 subunit. We postulate that the mechanism of action of these types of toxins is via direct inhibition of nAChRs at the neuromuscular junction, which does not involve the a7 subunit.

Technical Abstract: The adverse effects of methyllycaconitine (MLA) have been attributed to competitive antagonism of nicotinic acetylcholine receptors (nAChR). Research has indicated a correlation between the LD50 of MLA and the amount of a7 nAChR in various mouse strains, suggesting that mice with more a7 nAChR require more MLA to be poisoned. However, recent research demonstrated that there was no difference in the acute lethality (LD50) to MLA in mice lacking the a7 nAChR subunit compared to wild-type mice. The objective of this study was to determine if the a7 nAChR subunit plays a role in motor coordination deficiencies that result from exposure to nAChR antagonists and agonists. We compared the motor function and coordination in wild-type mice to mice lacking the a7 subunit of the nAChR, after treating them with a non-lethal dose of MLA or anabasine, using the following tests: balance beam, grip strength, rotarod, open field, and tremor monitor. Analysis of the data indicated that overall there was no difference between the wild-type and knockout mice (P = 0.39 for grip strength; P = 0.21 for rotarod; P = 0.41 for balance beam; P = 0.22 for open field; and P = 0.62 for tremors). Thus results from this study suggest that a7 nAChR does not play an integral role in the acute effects of MLA or anabasine on motor function/coordination. Consequently other subunits of nAChRs found in the neuromuscular junction are likely the primary target for MLA and anabasine resulting in motor coordination deficiencies and acute toxicosis.