Olfactory Transcriptional Analysis of Salmon Exposed to Mixtures of Chlorpyrifos and Malathion Reveal Novel Molecular Pathways of Neurobehavioral Injury

Abstract

Pacific salmon exposed to sublethal concentrations of organophosphate pesticides (OP) have impaired olfactory function that can lead to loss of behaviors that are essential for survival. These exposures often involve mixtures and can occur at levels below those which inhibit acetylcholinesterase (AChE). In this study, juvenile Coho salmon were exposed for 24 h to either 0.1, 0.5, or 2.5 ppb chlorpyrifos (CPF), 2, 10, or 50 ppb malathion (MAL), or binary mixtures of 0.1 CPF:2 ppb MAL, 0.5 CPF:10 ppb MAL, or 2.5 CPF:10 ppb MAL to mimic single and binary environmental exposures. Microarray analysis of olfactory rosettes from pesticide-exposed salmon revealed differentially expressed genes involved in nervous system function and signaling, aryl hydrocarbon receptor signaling, xenobiotic metabolism, and mitochondrial dysfunction. Coho exposed to OP mixtures exhibited a more pronounced loss in detection of a predatory olfactory cue relative to those exposed to single compounds, whereas respirometry experiments demonstrated that exposure to OPs, individually and in mixtures, reduced maximum respiratory capacity of olfactory rosette mitochondria. The observed molecular, biochemical, and behavioral effects occurred largely in the absence of effects on brain AChE. In summary, our results provide new insights associated with the sublethal neurotoxic effects of OP mixtures relevant to environmental exposures involving molecular and cellular pathways of injury to the salmon olfactory system that underlie neurobehavioral injury.

Effects of Chlorpyrifos on Cholinesterase and Serine Lipase Activities and Lipid Metabolism in Brains of Rainbow Trout (Oncorhynchus mykiss)

“…However, cholinergic synapses have not been identified in olfactory rosettes (Scholz et al, 2006), and reduced reception to predatory cues occurs in the absence of AChE inhibition, suggesting a non-AChE mechanism of action. Other studies have indicated inhibition of olfactory receptor neuron signaling pathways, alterations in downstream cAMP-catalyzed events, and altered Ca 2þ regulation following CPF, each independent of neuromuscular impairment and AChE inhibition (Maryoung et al, 2014;Wang et al, 2016).…”

“…In order to evaluate the molecular initiating event for impaired signal transduction observed in previous sublethal studies with CPF in fish (Maryoung et al, 2014;Wang et al, 2016), a lipidomic screen and evaluation of serine hydrolase enzymes important in the cleavage of FFAs and other signaling lipids from glycerol backbones was carried out. Lipidomic evaluations were conducted at the sublethal concentration of 1.38 lg/l CPF, where AChE, MAGL, and FAAH activities were significantly reduced, without inducing acute lethality.…”

“…Overactivation of eCB signaling suppresses excitatory transmission in the nervous system via reduced glutamate release (Pouille and Schoppa, 2018), and cannabinoid receptors are highly expressed in excitatory corticofugal feedback (CFF) neurons in the olfactory bulb that make connections onto glutamatergic neurons involved in odor-evoked output and odor-driven behaviors (Boyd et al, 2015;Martin et al, 2004;Otazu et al, 2015). Thus, overactivation of eCB signaling and suppression of neurotransmission between CFF and glutamatergic neurons could play a direct role in diminished olfactory behavioral responses and inhibition of olfactory receptor neuron signaling pathways in CPF-exposed fish (Maryoung et al, 2014;Wang et al, 2016). Cannabinoid receptor 1 (CB1) agonist suppression of transmission is greater in juvenile rats than adults, suggesting juvenile trout may also be more susceptible than older trout (Kang-Park et al, 2007).…”

“…Olfactory function in zebrafish is disrupted by CPF through inhibition of G-protein-coupled receptor signaling and odorant binding, leading to reduced olfactory responsiveness (Tilton et al, 2011). Genes involved in second messenger signaling, neurotransmitter signaling, and calcium signaling were consistently downregulated in salmon with reduced odor avoidance behaviors (Wang et al, 2016), and the magnitude of overevoked field potentials was inhibited in electrophysiological recordings in response to CPF (Sandahl et al, 2004). Previous studies in our laboratory using rainbow trout have indicated that reduced physiological and behavioral responses following CPF exposure correspond with transcriptomic changes in the brain for genes involved in neuronal signal transduction and are associated with diminished cAMP and calcium signaling (Maryoung et al, 2014).…”

Systematic developmental neurotoxicity assessment of a representative PAH Superfund mixture using zebrafish

“…Embryonic photomotor response (EPR) was evaluated in 24 hpf embryos using a custom built photomotor response assay tool (PRAT) (Kokel et al , 2013; Noyes et al , 2015). After exposures, embryos were not exposed to light until administration of the EPR test.…”