# Does Nanosilver Have a Pronounced Toxic Effect on Humans?
## 2 Results of Investigations Performed by Other Researchers
### Effect of AgNPs on the Brain and Behavior  
AgNPs along with other metal NPs are able to cross not just the placental barrier but the blood-brain barrier as well [8; 9]. [PERSON] et al. [36] proposed two mechanisms of AgNP penetration across the blood-brain barrier, i.e., the transcytosis of endothelial cells of the brain-blood capillary and the reduction in the close connection between endothelial cells, or dissolution of endothelial cell membranes. The main mechanisms of nanoparticles for crossing the blood-brain barrier according to the literature are summarized in Figure 1.  
Therefore, upon contact with NPs, there is a risk of biochemical, histological, and functional disorders in the brain, including cognitive dysfunctions. Studies of the effect of NPs on brain functions have received much attention in recent years. After a single dose injection of AgNPs, the parameters of oxidative stress and the antioxidant potential of the brain on gene expression and the level of protein (superoxide dismutase and glutathione reductase) activity were altered [37]. The shape of astrocytes was disturbed, cerebral capillaries were deformed, and edema was developed in adjacent areas [10; 13; 38]. In addition, the permeability of the blood-brain barrier increased in direct proportion to the dose of AgNPs received by animals [10]. After seven daily injections, impairments of working memory were noted in rats: The animals meaningfully more often made mistakes by repeatedly looking into the arm of the maze they had just examined; however, referential memory and the long-term understanding of the structure of space were not disturbed [38]. After three weeks of injections, the social behavior of the mice differed significantly from the normal behavior in the study by [PERSON] et al. [42] The mice administrated with AgNPs preferred to stay in an empty chamber rather than to familiarize themselves with new animals. In experimental animals, motor coordination and balance were impaired, but the swimming speed in the Morris test was preserved, which revealed the absence of spatial memory formation in individuals treated with AgNPs. The emotional state, the level of anxiety, and the ability to conditioned-reflex learning, in which fear is the motivation, were considered in the study by [PERSON] et al. [43]. After long-term (1-6 months) oral contact with small doses of AgNPs (50 \(\upmu\)g) through drinking water, the authors recorded and interpreted the test results as two attempts of the brain to adapt to the effects of AgNPs. Thus, after 2 months of contact, anxiety and fear increased, and after 4 months, they decreased with a simultaneous increase in exploratory behavior. After continued contact with AgNPs (up to 6 months), the impairment of long-term memory and conditioned-reflex learning was observed. The fading effect was noted in the research by [PERSON] et al. [34]. Although several days after exposure to AgNPs, zebrafish larvae were hyperactive at changes in illumination, no fluctuations in their activity were detected after 5 days. The influence of prenatal contact with AgNPs on the brain and behavior is described in a limited number of studies. After regular injections of AgNPs to future mothers during pregnancy, the spatial memory of their offspring was impaired, while conditioned-reflex learning and the emotional state of young rats did not differ from the offspring of the control group [44]. The experimental offspring demonstrated signs of depressive-like behavior, i.e., passivity and lack of interest in food [45]. After zebrafish embryos were exposed to AgNP solution, their avoidance motor response to the touch was disrupted; the membrane potentials of their motoneurons were decreased, and the expression of many genes connected with neurogenesis was low [46]. In studies of the effect of AgNP coating material (BSA, polyethylene glycol, and citrate) on cognition, spatial memory, and neurotransmitter levels in the rat hippocampus, only rats administrated with citrate-coated NPs maintained long-term spatial memory.