The olfactory system is the system related to the sense of smell (olfaction). Many fish activities are dependent on olfaction, such as: mating, discriminating kin, avoiding predators, locating food, contaminant avoidance, imprinting and homing. These activities are referred to as "olfactory-mediated". Impairment of the olfactory system threatens survival and has been used as an ecologically relevant sub-lethal toxicological endpoint for fish within studies. Olfactory information is received by sensory neurons, like the olfactory nerve, that are in a covered cavity separated from the aquatic environment by mucus. Since they are in almost direct contact with the surrounding environment, these neurons are vulnerable to environmental changes. Fish can detect natural chemical cues in aquatic environments at concentrations as low as parts per billion (ppb) or parts per trillion (ppt). Studies have shown that exposures to metals, pesticides, or surfactants can disrupt fish olfaction, which can impact their survival and reproductive success. Many studies have indicated copper as a source of olfactory toxicity in fishes, among other common substances. Olfactory toxicity can occur by multiple, complex Modes of Toxic Action.
History Early investigation by Hasler and Wisby (1951) examined how fish use olfactory imprinting to discriminate smells in order for fish to find their natal streams. This research provided the framework for testing synthetic chemicals used by hatcheries to examine homing and straying by hatchery fish. The investigation of the toxicity of mercury and copper to the olfactory systems in fish began in the early 1970s. Where they found that solutions of mercury chloride (HgCl2) and copper sulfate (CuSO4) depressed olfactory response during exposure to the two toxicants and found that toxicant concentration and olfactory response had an inverse relationship to each other.
Olfactory system Olfaction begins with an interaction between an odorant molecule and an olfactory sensory neuron (OSN) located within the epithelium of the Glomerulus bulb. Odorants bind to receptor proteins that are held within individual OSNs. It is important to note that not all fish have the same types or number of receptor proteins making olfactory toxicity and the subsequent effects species specific. There are three types of OSN cells: (1) ciliated cells, microvillus cells, and crypt cells. These cells are distributed across the olfactory epithelium (OE), OSNs that express common binding receptor proteins are connected to the olfactory bulb (OB) by axons. The changes in olfactory function can be placed into three categories: (1) anosmia, the inability to smell; (2) hyposmia, a reduced ability to smell; or (3) dysosmia, where olfactory signals are incorrectly processed. Most chemicals at lower concentrations cause a degree of hyposmia while at higher concentrations anosmia is the result. Dysomia is less commonly observed, however, cases of fish becoming attracted to metal-contaminated waters have been studied and examined.
Metals Metals are a necessary and important trace element that most organisms need to function properly. They are often used as coenzymes or interact with biological enzymes to form complexes inside organisms. However, if the metals in question are in too high of concentrations it can be fatal. Different parameters such as pH, alkalinity, temperature, fish size, or salinity can alter how the metals interact or are metabolized by the organism. Fish are oftentimes less tolerant to metals than terrestrial animals are. Their gills are sensitive to changes in their environment and highly susceptible to metal toxicity. Before a metal may have toxic effects it can also cause a change in olfactory response, or other responses, within fish. If the exposure is short in length or low in concentration the effects can be reversed, but at high enough concentrations it becomes toxic to the organism leading to death. Copper, cadmium, lead, and zinc are common metals that cause olfactory toxicity in fish. Copper is a metal looked at in more detail than others. This is because it is commonly used in fish hatcheries as an algaecide as it is an effective way to prevent parasitic and fungal infections within fish populations at hatcheries. It can also be released from industrial or agricultural sources. Either applied in a chemical spray or deposit, or used within copper netting on the outside of aquaculture, copper kills algae and bacteria that can cause fish to become sick. However, it does induce olfactory toxicity at relevant concentrations to aquaculture.
Mechanism of action Metals mechanism of action has been hypothesized to inhibit the electrical properties of olfactory neurons by blocking ligand-gated or voltage-gated ion channels in the nervous system of fish. However, direct mechanisms of action for metals are not fully understood and still need to be researched further.
Past studies
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