Date of Degree
Access restricted until 01/31/2020
PhD (Doctor of Philosophy)
Pharmaceutical Sciences and Experimental Therapeutics
Donovan, Maureen D.
First Committee Member
Rohlman, Diane S.
Second Committee Member
Third Committee Member
Fourth Committee Member
Smith, Ryan M.
Atrazine and 2,4-dichlorophenoxy acetic acid (2,4-D) are two of the most commonly used herbicides in the United States with more than 75 million pounds of atrazine and 48 million pounds of 2,4-D applied annually. Studies report that exposure to atrazine and 2,4-D causes damage to the nervous system manifested by an increased incidence of Parkinson’s Disease-like symptoms and neuronal inflammation. Current toxicological evaluations of these herbicides, however, describe them as “safe” under the application conditions described in the approved packaging.
Direct aerial spraying of aerosolized herbicides can result in spray drift up to 2-10 km away from the treated area, rendering thousands of individuals susceptible to involuntary exposure and potential human health risks. Aerosolized herbicides are usually applied as sprays with average droplet sizes of ~150 µm. Studies have shown that aerosolized herbicides with droplet sizes >10 µm can deposit ~100 % of the inhaled droplets within the nasal cavity. Following deposition, the herbicides may be absorbed from the nasal mucosa or they may be absorbed by the olfactory mucosa, located in the upper region of the nasal cavity and subsequently transferred directly to the brain. The objective of this study is to characterize atrazine and 2,4-D absorption via the nasal respiratory and olfactory mucosae with specific attention to the potential for herbicide transfer to the brain via the olfactory system.
Uptake of atrazine and 2,4-D from aqueous solutions and from commercial herbicide products was investigated across excised nasal mucosal tissues using, and the role of active uptake or efflux pathways was probed using pharmacologic inhibitors. Immunohistochemistry (IHC) studies were also conducted to assess the localization of any efflux transporters which play a role in atrazine and 2,4-D uptake across nasal tissues. In vivo studies were also conducted to compare the concentration of atrazine and its active metabolites in mouse brain and plasma following intranasal, intravenous administrations, and acute inhalation exposure of herbicide products.
Atrazine was transported across the nasal tissues in a manner consistent with passive diffusion, yet MRP1, BCRP, and MDR1 efflux transporters were also found to play a role in limiting its uptake across respiratory and olfactory tissue. For 2,4-D a linear increase in its overall uptake was observed across the olfactory tissues. However, for the respiratory tissues, a saturation in 2,4-D uptake was observed. This saturation in 2,4-D uptake is attributed to the expression of OATP transporters in the respiratory tissues. MDR1, MRP1 and BCRP also limited the uptake of 2,4-D across olfactory tissues. For respiratory tissues, only MRP1 effluxed either compound. IHC studies confirmed the expression of MRP1 and BCRP in the respiratory and olfactory mucosa.
High 2,4-D and co-solvent concentrations in commercial herbicide products resulted in significant transport of 2,4-D across the nasal tissues. Histological evaluations showed significant changes in the nasal epithelium following exposure to these products.
In vivo studies showed that the brain tissue concentrations of atrazine in mice were found to be comparable following intranasal and intravenous administration; a relatively short tmax was observed in mouse brain following intranasal administration. The brain concentration of atrazine and its active metabolites was significantly higher than plasma concentrations. Inhalation exposure to a commercial atrazine-containing herbicide product (Atrazine 4L®) resulted in measurable concentrations of atrazine and its active metabolites being distributed to the brain following inhalation exposure.
These results suggest that lipophilic herbicide compounds like atrazine and 2,4-D can freely permeate across the nasal mucosa despite the presence of many protective barriers, including the nasal epithelial cells and their associated efflux transporters. For both herbicides, significant passive permeability of the compounds across the olfactory tissues suggests there may be increased CNS distribution of these agents with the potential for increased risk of CNS-related toxicities.
xxiv, 201 pages
Includes bibliographical references (pages 187-201).
Copyright © 2018 Wisam Saad Hasan Al-Bakri
Al-Bakri, Wisam Saad Hasan. "The role of the membrane influx and efflux transporters in the brain distribution of environmental chemicals following nasal inhalation." PhD (Doctor of Philosophy) thesis, University of Iowa, 2018.
Available for download on Friday, January 31, 2020