Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic chemicals comprising more than 9000 substances that have been used in industry and consumer products worldwide about seven decades [1,2]. For the classification of PFAS, the length of the fluorocarbon chain (ranging from C4–C17) is often used as the primary discriminant and is a good predictor of physicochemical properties, bioaccumulation, protein binding, and environmental fate distribution [3]. Most of the commercial focus historically has been on these two groups: perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs). Specifically, PFCAs include PFBA, PFPA, PFHxA, PFHpA, PFOA, PFNA, PFDA, PFUA, PFDoA, PFTrDA, and PFTeDA, and PFSAs include PFBS, PFHxS, PFHpS, PFDS, 6:2 FTS, and 8:2 FTS (Fig. 1A and Table S1). [4]. PFAS are used as oil repellents and coatings in many different industrial and consumer products, including the manufacture of nonstick cookware, waterproof clothing, antifouling fabrics and carpets, cosmetics, firefighting foams, and products resistant to grease, water, and oil [5].

PFAS molecules have linked chains of carbon and fluorine atoms. Since the carbon-fluorine bond is one of the strongest bonds and is chemically inert and resistant to high temperature, these chemicals are not easily degraded in the environment. Due to their bio-persistence, they are often referred to as "forever chemicals" [1]. Human exposure to PFAS is widespread due to the variety of PFAS. Most PFAS, including PFOA and PFOS, do not decompose, so they remain in the environment for a long time [5]. Due to their widespread use and persistence in the environment, PFAS are present in the blood of humans and animals, and in a variety of food products [4,6,7]. With repeated exposure over time, some PFAS can accumulate in humans and animals [8]. Experimental animal studies suggest that certain PFAS are endocrine disrupting chemicals, possibly affecting many systems including reproduction, metabolism, and immune system [2,[9], [10], [11]].

There is growing evidence that PFAS exposure may also adversely impact neurodevelopment and neurological health across the lifespan. Several epidemiological studies have associated higher PFAS exposure with reduced cognitive function and academic achievement in children and adolescents [12]. Increasing odds of attention deficit hyperactivity disorder is consistent with increased in children with higher serum PFAS levels [13] and PFAS concentrations in the maternal serum is associated with lower Full Scale Intelligence Quotient in offspring [13]. In adults, elevated PFAS levels have been linked to lower IQ scores and faster cognitive decline with advanced age in longitudinal studies [14]. Animal studies also indicate that developmental PFAS exposure can lead to neurobehavioral deficits [15]. However, the mechanisms of neurodevelopmental toxicity of PFAS remain unclear.

Brain development and function is regulated by many factors including neuroactive steroids, named as neurosteroids. These neurosteroids are formed by steroid 5α-reductase 1 (SRD5A1) and subsequent catalysis of 3α-hydroxysteroid dehydrogenase. SRD5A1 catalyze the rate-limiting irreversible step and it is encoded by the SRD5A1 gene and is expressed in numerous tissues, including the brain [16,17]. SRD5A1 is a smooth endoplasmic reticulum nicotinamide adenine dinucleotide phosphate (NADPH)-dependent enzyme that catalyzes the conversion of pregnenolone or testosterone to the neuroactive intermediate dihydropregnenolone or dihydrotestosterone (DHT) (Fig. 1B) [18].

The activity of SRD5A1 plays a key role in determining the levels of neurosteroids. Inhibition of SRD5A1 activity may lead to some neurological diseases. In this study, we investigated whether PFAS can inhibit human and rat SRD5A1 activity, what is the inhibitory strength and structure-activity relationship (SAR) by testing 18 common PFAS.