This AOP is licensed under a Creative Commons Attribution 4.0 International License.
Inhibition of CYP7B activity leads to decreased reproductive success via decreased locomotor activity
- Florence Pagé-Larivière
|Author status||OECD status||OECD project||SAAOP status|
|Not under active development||Under Development|
This AOP was last modified on June 15, 2020 15:09
|CYP7B activity, inhibition||May 17, 2017 12:21|
|7α-hydroxypregnenolone synthesis in the brain, decreased||May 17, 2017 13:08|
|Dopamine release in the brain, decreased||May 17, 2017 13:05|
|Locomotor activity, decreased||May 18, 2017 10:54|
|Decreased, Reproductive Success||December 03, 2016 16:37|
|Decreased, Population trajectory||April 18, 2017 16:19|
|CYP7B activity, inhibition leads to 7α-hydroxypregnenolone synthesis in the brain, decreased||May 17, 2017 21:09|
|CYP7B activity, inhibition leads to Locomotor activity, decreased||May 17, 2017 21:13|
|7α-hydroxypregnenolone synthesis in the brain, decreased leads to Locomotor activity, decreased||May 17, 2017 21:14|
|7α-hydroxypregnenolone synthesis in the brain, decreased leads to Dopamine release in the brain, decreased||May 25, 2017 14:22|
|Dopamine release in the brain, decreased leads to Locomotor activity, decreased||May 02, 2017 22:01|
|Locomotor activity, decreased leads to Decreased, Reproductive Success||June 26, 2017 15:06|
|Decreased, Reproductive Success leads to Decreased, Population trajectory||May 09, 2017 10:02|
|Ketoconazole||May 02, 2017 11:08|
This AOP details the downstream events of CYP7B inhibition leading to a decreased locomotor activity that adversely impacts reproductive success. CYP7B is expressed in the brain and catalyzes the conversion of pregnenolone to 7α-hydroxypregnenolone, a neurosteroid that stimulates the release of dopamine in the telencephalon. When released through this pathway, dopamine binds D2 receptor which is involved in locomotor activity induction. Ketoconazole and other azole fungicides are potent inhibitor of cytochrome P450s, including CYP7B. They bind to the heme site of the enzyme preventing its catalytic activity. Exposure to one of these molecules induces a decrease in 7α-hydroxypregnenolone synthesis which, in turn, reduces dopamine release in the telencephalon and limits locomotor activity. Since locomotor activity is closely associated to reproductive success through courtship enhancement (newt), expansion of territory (bird) and homing migration (salmon), its inhibition negatively affects the fitness of animals.
7α-hydroxypregnenolone was recently discovered and its function and regulation remain unclear. The few studies that focused on this neurosteroid and that were used for this AOP are based on in vitro and in vivo experiments in salmon, quail and newt. At present, it is believed that the function of this neurosteroid differs in mammals, which suggest that this AOP is only applicable to non-mammalian vertebrates. Also, the sex applicability of the AOP varies according to species.
The stressor identified for this AOP is used as fungicide both in the field for crop protection and in animal against fungus infection. Because it can inhibit various cytochrome P450 enzymes activity, a family of enzymes involved in a plethora of pathways including steroidogenesis, it has the potential to induce many different side effects for animal exposed indirectly through the environment or directly through medical treatment. This AOP targets one of these side effects.
Summary of the AOP
Molecular Initiating Events (MIE)
Key Events (KE)
Adverse Outcomes (AO)
|Sequence||Type||Event ID||Title||Short name|
|1||MIE||1386||CYP7B activity, inhibition||CYP7B activity, inhibition|
|2||KE||1387||7α-hydroxypregnenolone synthesis in the brain, decreased||7α-hydroxypregnenolone synthesis in the brain, decreased|
|3||KE||1388||Dopamine release in the brain, decreased||Dopamine release in the brain, decreased|
|4||KE||1389||Locomotor activity, decreased||Locomotor activity, decreased|
|5||KE||1141||Decreased, Reproductive Success||Decreased, Reproductive Success|
|6||AO||442||Decreased, Population trajectory||Decreased, Population trajectory|
Relationships Between Two Key Events (Including MIEs and AOs)
|CYP7B activity, inhibition leads to Locomotor activity, decreased||non-adjacent||Low||Low|
|7α-hydroxypregnenolone synthesis in the brain, decreased leads to Locomotor activity, decreased||non-adjacent||High||Moderate|
Life Stage Applicability
|Adult, reproductively mature||High|
Overall Assessment of the AOP
Domain of Applicability
Taxons: This AOP is supported with evidence from studies conducted with newt, quail, and salmon. Based on anticipated conservation of the biology associated with the KEs and KERs described, it is presumed to be applicable to all amphibian, bird and migratory teleost fish.
Previous evidence suggest that this AOP is not applicable to mammal. All the key events of this AOP are described or are biologically plausible in mammal, but the relationship between them might differ, as suggested by Yau et al. (2006).
Sex: The sex applicability of this AOP is species-specific. Female quail and newt are insensitive to this MIE in regard to locomotor activity whereas male are highly sensitive. In salmon, both male and female exhibit a decreased locomotor activity with induction of the MIE.
Life Stage: This AOP applies to sexually mature animals since the endpoint is related to reproduction. However, all the key events except “reproductive success, decreased” (event 1141) and and the adverse outcome (event 442) are known to occur in juveniles which suggest that an AOP connecting CYP7B inhibition and decreased locomotor activity in juvenile to an endpoint not sexually-oriented could be built.
Essentiality of the Key Events
Few studies measured multiple key events of this AOP. For this reason, the evidence for essentiality of the key events is mainly indirect and provided by a series of antagonist/exogenous supplementation experiments. The animal models used for these investigations were newt, quail, and salmon.
Inhibition of CYP7B
At present, no CYP7B knock-out experiments were conducted in species of interest. However, several indirect evidences linking CYP7B inhibition to a decreased locomotor activity suggest an important correlation between the two events.
Numerous direct evidences connecting this neurosteroid to locomotor activity were described.
Dopamine release, decreased
There is strong evidence demonstrating the involvement of dopamine in locomotor activity among all vertebrates. However, only indirect evidence relates CYP7B inhibition to a decreased dopamine release. The rational is stronger for 7α-hydroxypregnenolone in relation to dopamine release, although this neurosteroid receptor remains to be identified.
Locomotor activity, decreased
All the previous key events can decrease locomotor activity in salmon and male quail, chicken, and newt.
This AOP connects the cyp7b catalyzed synthesis on an important neurosteroid to a well characterized sequence of events. For instance, the involvement of dopamine in locomotor activity that in turn impacts on reproductive success is well described and undisputed (Bardo M.T. et al., 1999; Levens et al., 2000). What is less characterized is the relation between 7α-hydroxypregnenolone and dopamine release. Since the neurosteroid receptor has yet to be identified, no direct interaction between 7α-hydroxypregnenolone and dopaminergic neuron has been demonstrated. It is thus possible that an intermediate event takes place in between to indirectly connect the neurosteroid to dopamine release.
In terms of structural plausibility, the brain expresses the steroidogenic enzymes required for pregnenolone synthesis, the main substrate of CYP7B. It also expresses CYP7B which synthesizes high concentration of 7α-hydroxypregnenolone in the diencephalon. This region of the brain is populated by neurons projecting into the striatum which is known to express a high quantity of D1- and D2-like dopamine receptor and control motor activity (Orgen S. et al., 1986; Mezey S. et al., 2002; Callier S. et al., 2003).
Uncertainties or inconsistencies
At present, there are no inconsistencies reported in the literature, but some gaps remain to be filled.
The most important ones are 7α-hydroxypregnenolone receptor localization and the connection between 7α-hydroxypregnenolone and dopamine release discussed in the previous section.
In addition, mammalian CYP7B not only catalyzes the 7α-hydroxylation of pregnenolone but also that of dehydroepiandrosterone (DHEA). Although no clear information reported this enzymatic reaction in the bird, it is plausible that CYP7B catalyzes the hydroxylation of DHEA. Thus, the phenotypic effect of CYP7B inhibition in the brain cannot be uniquely attributed to a depletion in 7α-hydroxypregnenolone. Additionally, ketoconazole is known to inhibit a variety of CYPs, which suggest that animal exposed to it are likely to have several other enzymes inhibited. It is plausible that the impacts of ketoconazole are the result of multiple CYPs inhibition that all converge towards the same phenotype. These off target effects greatly limit the investigations on 7α-hydroxypregnenolone since its concentration cannot be specifically decreased.
If a CYP7B knock-out in the brain was to be performed in an animal species, 7α-hydroxyDHEA supplementation would be required to properly study 7α-hydroxypregnenolone function.
This information is not available for the moment.