Upstream eventCYP7B activity, inhibition
7α-hydroxypregnenolone synthesis in the brain, decreased
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Inhibition of CYP7B activity leads to decreased reproductive success via decreased locomotor activity||adjacent||High||Moderate|
|Inhibition of CYP7B activity leads to decreased reproductive success via decreased sexual behavior||adjacent|
|Japanese quail||Coturnix japonica||NCBI|
Life Stage Applicability
|All life stages|
Key Event Relationship Description
Neurosteroids are steroids synthesized in the brain that interact with cell surface receptors or ligand-gated ion channels in order to modify the neuronal excitability (Paul and Purdy, 1992). They are involved in numerous biological functions including locomotor activity, memory, learning, sexually-dimorphic behaviors and anxiety.
Neurosteroids are synthesized from pregnenolone or its derivatives by different cytochromes P450. Among these CYPs is CYP7B hydroxylase which synthesizes the neurosteroid 7α-hydroxypregnenolone. CYP7B is the only enzyme responsible for the synthesis of this neurosteroid. Therefore, its inhibition induces a decrease in 7α-hydroxypregnenolone concentration in the brain.
The expression of CYP7B and the synthesis of its molecular product vary cyclically on a daily and/or seasonal basis. In male quail, a diurnal animal, CYP7B expression and 7α-hydroxypregnenolone are inhibited by melatonin secretion, a hormone involved in circadian rhythm and sleep regulation. Oppositely, in a nocturnal animal such a newt, melatonin acts as an inducer of CYP7B expression and 7α-hydroxypregnenolone synthesis. These results indicate that CYP7B expression and therefore 7α-hydroxypregnenolone synthesis follow a circadian rhythm regulation.
In addition to this daily variation, CYP7B and its product are regulated by seasons in salmon and male newt where it peaks during homing migration (salmon) and breeding (newt) period (Haraguchi et al., 2009). It is plausible that the same seasonal variation occurs in avian.
Evidence Supporting this KER
The vertebrate brain expresses all the enzymes involved in the different steroidogenic pathways, including CYP7B (review do Rego and Vaudry, 2016; Tsutsui and Yamazaki, 1995). These enzymes in the brain are known to convert cholesterol into pregnenolone, the precursor of 7α-hydroxypregnenolone Therefore, the brain possesses both the molecular precursor and the enzyme required to synthesized 7α-hydroxypregnenolone. Since CYP7B is the only enzyme known to synthesize 7α-hydroxypregnenolone, its inhibition is assumed to decrease 7α-hydroxypregnenolone concentration in the brain.
In the quail brain, the precise localization of CYP7B protein was explored and the results were as followed: nucleus preopticus medialis (POM), the nucleus paraventricularis magnocellularis (PVN), the nucleus ventrodedialis hypothalami (VMN), the nucleus dorsolateralis anterior thalami (DLA) and the nucleus lateralis anterior thalami (LA) (Tsutsui et al., 2008).
In the salmon, cells expressing CYP7B are mainly localized in the magnocellular preoptic nucleus, oculomotor nucleus, nucleus lateralis valvulae, and nucleus lateralis valvulae (Haraguchi et al., 2015).
In the newt brain, CYP7B cells are mainly localized in the anterior preoptic area, the magnocellular preoptic nucleus, and the tegmental area. It was also detected in the lateral and dorsal pallium, the suprachiasmatic nucleus, the ventral hypothalamic nucleus, and the tectum mesencephali (Haraguchi et al., 2010).
- CYP7B inhibitor (ketoconazole, 10-4 M) decreased the synthesis of 7α-hydroxypregnenolone in vitro (Tsutsui et al., 2008; Matsunaga et al., 2008; Toyoda et al., 2012).
- CYP7B inhibitor (intracerebroventricular injection of ketoconazole, 5 μg, from 5 AM to 6 AM) decreased the synthesis of 7α-hydroxypregnenolone in the brain (quail) in vivo (Tsutsui et al., 2008).
- CYP7B activity is regulated (inhibited) by melatonin in male quail. When male quail brains were injected (intracerebroventricular) with a melatonin receptor antagonist (luzindole), the production of 7α-hydroxypregnenolone significantly increased (Tsutsui et al., 2008). The opposite effect was observed on newt where melatonin stimulated 7α-hydroxypregnenolone synthesis (Koyama et al., 2009).
- Similarly, orbital enucleation and pinealectomy performed on male quail, which abolished melatonin synthesis, induces a significant increase in 7α-hydroxypregnenolone concentration (Tsutsui et al., 2008).
Uncertainties and Inconsistencies
Quantitative Understanding of the Linkage
Little is known about the dose-response of CYP7B inhibitors. This information is lacking in the literature.
One experiment conducted in vitro with mouse recombinant enzyme showed that 1 µM clotriconazole significantly decreased CYP7B activity while 10 µM abolished it (Rose et al., 1997). However, the subtrate used in the experiment was DHEA meaning that the measured product was 7α-hydroxyDHEA. It is highly plausible that the same result would have been observed with pregnenolone as a substrate for CYP7B and 7α-hydroxypregnenolone as a product.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
The vertebrate brain expresses all the enzymes involved in the different steroidogenic pathways (do Rego and Vaudry, 2016; Tsutsui et al., 1999).
The physiological function of 7α-hydroxypregnenolone is more understood in birds, newts, and rats than in human. However, the direct causal effect between CYP7B inhibition and the decrease in 7α-hydroxyPREG was demonstrated in human, fish and other vertebrates (Haraguchi et al., 2015; Yantsevich et al., 2014; Yau et al., 2006).
Therefore, it is plausible that this KER is applicable to all vertebrates.
do Rego, J.L., and Vaudry, H. (2016). Comparative aspects of neurosteroidogenesis: From fish to mammals. Gen Comp Endocrinol 227, 120-129.
Haraguchi, S., Matsunaga, M., Koyama, T., Do Rego, J.L., and Tsutsui, K. (2009). Seasonal changes in the synthesis of the neurosteroid 7alpha-hydroxypregnenolone stimulating locomotor activity in newts. Ann N Y Acad Sci 1163, 410-413.
Haraguchi, S., Koyama, T., Hasunuma, I., Vaudry, H., and Tsutsui, K. (2010). Prolactin increases the synthesis of 7α-hydroxypregnenolone, a key factor for induction of locomotor activity, in breeding male newts. Endocrinology 151, 2211–2222.
Haraguchi, S., Yamamoto, Y., Suzuki, Y., Hyung Chang, J., Koyama, T., Sato, M., Mita, M., Ueda, H., and Tsutsui, K. (2015). 7alpha-Hydroxypregnenolone, a key neuronal modulator of locomotion, stimulates upstream migration by means of the dopaminergic system in salmon. Sci Rep 5, 12546.
Koyama, T., Haraguchi, S., Vaudry, H., and Tsutsui, K. (2009). Diurnal changes in the synthesis of the neurosteroid 7alpha-hydroxypregnenolone stimulating locomotor activity in newts. Ann N Y Acad Sci 1163, 444-447.
Paul, S.M., and Purdy, R.H. (1992). Neuroactive steroids. FASEB J 6, 2311-2322.
Tsutsui, K., Inoue, K., Miyabara, H., Suzuki, S., Ogura, Y., and Haraguchi, S. (2008). 7Alpha-hydroxypregnenolone mediates melatonin action underlying diurnal locomotor rhythms. J Neurosci 28, 2158-2167.
Tsutsui, K., Ukena, K., Takase, M., Kohchi, C., and Lea, R.W. (1999). Neurosteroid biosynthesis in vertebrate brains. Comp Biochem Physiol C Pharmacol Toxicol Endocrinol 124, 121-129.
Tsutsui, K., and Yamazaki, T. (1995). Avian neurosteroids. I. Pregnenolone biosynthesis in the quail brain. Brain Res 678, 1-9.
Yantsevich, A.V., Dichenko, Y.V., Mackenzie, F., Mukha, D.V., Baranovsky, A.V., Gilep, A.A., Usanov, S.A., and Strushkevich, N.V. (2014). Human steroid and oxysterol 7alpha-hydroxylase CYP7B1: substrate specificity, azole binding and misfolding of clinically relevant mutants. FEBS J 281, 1700-1713.
Yau, J.L., Noble, J., Graham, M., and Seckl, J.R. (2006). Central administration of a cytochrome P450-7B product 7 alpha-hydroxypregnenolone improves spatial memory retention in cognitively impaired aged rats. J Neurosci 26, 11034-11040.