The pregnane xenobiotic receptor, a prominent liver factor, has actions in the midbrain for neurosteroid synthesis and behavioral/neural plasticity of female rats

Abstract

A novel factor of interest for growth/plasticity in the brain is pregnane xenobiotic receptor (PXR). PXR is a liver factor known for its role in xenobiotic clearance and cholesterol metabolism. It is expressed in the brain, suggesting a potential role for plasticity, particularly involving cholesterol-based steroids and neurosteroids. Mating induces synthesis of neurosteroids in the midbrain Ventral Tegmental Area (VTA) of female rodents, as well as other "plastic" regions of the brain, including the hippocampus, that may be involved in the consolidation of the mating experience. Reducing PXR in the VTA attenuates mating-induced biosynthesis of the neurosteroid, 5α-pregnan-3α-ol-20-one (3α,5α-THP). The 18 kDA translocator protein (TSPO) is one rate-limiting factor for 3α,5α-THP neurosteroidogenesis. The hypothesis tested was that PXR is an upstream factor of TSPO for neurosteroidogenesis of 3α,5α-THP in the VTA for lordosis, independent of peripheral glands. First, proestrous rats were administered a TSPO blocker (PK11195) and/or 3α,5α-THP following infusions of PXR antisense oligonucleotides (AS-ODNs) or vehicle to the VTA. Inhibiting TSPO with PK11195 reduced 3α,5α-THP levels in the midbrain and lordosis, an effect that could be reversed with 3α,5α-THP administration, but not AS-ODN+3α,5α-THP. Second, proestrous, ovariectomized (OVX), or ovariectomized/adrenalectomized (OVX/ADX) rats were infused with a TSPO enhancer (FGIN 1-27) subsequent to AS-ODNs or vehicle to the VTA. PXR AS-ODNs blocked actions of FGIN 1-27 for lordosis and 3α,5α-THP levels among proestrous > OVX > OVX/ADX rats. Thus, PXR may be upstream of TSPO, involved in neurosteroidogenesis of 3α,5α-THP in the brain for plasticity. This novel finding of a liver factor involved in behavioral/neural plasticity substantiates future studies investigating factors known for their prominent actions in the peripheral organs, such as the liver, for modulating brain function and its augmentation.

Keywords: allopregnanolone; cognition; midbrain ventral tegmental area; progesterone; reproduction.

Figure 1

Depicts different hormonal conditions and TSPO ligand infusions in rats (top) and timeline of experimental protocol (bottom). In Experiment 1, all rats were in the proestrous phase of the estrous cycle and administered via infusions to the VTA, placebo or PXR AS-ODNs (at hour 0, 24, and 44), followed by an inhibitor of TSPO (PK11195) or saline vehicle and/or 3α,5α-THP (at hour 44.5). In Experiment 2, all rats were in the proestrous phase of the estrous cycle and administered via infusions to the VTA, placebo or PXR AS-ODNs (at hour 0, 24, and 44), followed by an enhancer of TSPO (FGIN) or saline vehicle (at hour 44.5). In Experiment 3, all rats were ovariectomized (OVX) and administered subcutaneous injections of E₂ at hour 0 and 24 and then administered via infusions to the VTA, placebo or PXR AS-ODNs (at hour 0, 24, and 44), followed by an enhancer of TSPO (FGIN) or saline vehicle (at hour 44.5). In Experiment 4, all rats were OVX and adrenalectomized (OVX/ADX), and administered subcutaneous injections of E₂ at hour 0 and 24 and then administered via infusions to the VTA, placebo or PXR AS-ODNs (at hour 0, 24, and 44), followed by an enhancer of TSPO (FGIN) or saline vehicle (at hour 44.5). All rats were tested at hour 45, and euthanized for tissue collection immediately after behavioral testing was complete. In all experiments, knockdown of PXR expression in the VTA was validated and levels of 3α,5α-THP were measured in the midbrain.

Figure 2

(A) Depicts the mean (+s.e.m.) 3α,5α-THP levels in the midbrain of proestrous rats infused first with control and vehicle (veh, n = 5), 3α,5α-THP (n = 7), PK11195 (n = 12), PK11195+3α,5α-THP (n = 11), or pregnane xenobiotic receptor (PXR) antisense deoxynucleotides (AS-ODNs) and veh (n = 10), PXR AS-ODN +3α,5α-THP (n = 8), PXR AS-ODN+ PK11195 (n = 7), PXR AS-ODN + PK11195+ 3α,5α-THP (n = 8) in Experiment 1. (B) Depicts the mean (+s.e.m.) lordosis quotient of proestrous rats in these conditions. Rats infused with PXR AS-ODN had a mean 1.5 fold lower PXR expression in the midbrain compared to control infusions in Experiment 1 (data not shown). ^*above a line indicates a significant effect of PXR AS-ODNs compared to control infusion (P < 0.05). ^*above a bar indicates a significant effect of PK11195 and PK1195+3α,5α-THP compared to vehicle infusions (P < 0.05).

Figure 3

(A) Depicts the mean (±s.e.m.) 3α,5α-THP levels in the midbrain of proestrous non tested rats infused with control and vehicle (veh, n = 5) or control+FGIN (n = 4), or pregnane xenobiotic receptor (PXR) antisense deoxynucleotides (AS-ODNs) and veh (n = 6), or PXR AS-ODN+FGIN (n = 6) or behaviorally-tested rats infused with control+veh (n = 15), control+FGIN (n = 15), PXR AS-ODN+veh (n = 8), or PXR AS-ODN+FGIN (n = 13) in Experiment 2. Rats infused with PXR AS-ODN had a mean 1.8 fold lower PXR expression in the midbrain compared to control infusions in Experiment 2 (data not shown). (B) Depicts the mean (±s.e.m.) 3α,5α-THP levels in the midbrain of ovariectomized (OVX) estradiol (E2)-primed non tested rats infused with control+veh (n = 5), control+FGIN (n = 6), PXR AS-ODN+veh (n = 5), PXR AS-ODN+FGIN (n = 5), or behaviorally-tested rats infused with control+veh (n = 8), control+FGIN (n = 7), PXR AS-ODN+veh (n = 7), or PXR AS-ODN+FGIN (n = 6) in Experiment 3. Rats infused with PXR AS-ODN had a mean 1.5 fold lower PXR expression in the midbrain compared to control infusions in Experiment 3 (data not shown). (C) Depicts the mean (±s.e.m.) 3α,5α-THP levels in the midbrain of OVX/adrenalectomized (ADX), E2-primed rats that were non tested and infused with control+veh (n = 6), control+FGIN (n = 5), PXR AS-ODN+veh (n = 5), PXR AS-ODN+FGIN (n = 5), or behaviorally-tested and infused with control+veh (n = 9), control+FGIN (n = 5), PXR AS-ODN+veh (n = 8), or PXR AS-ODN+FGIN (n = 5) in Experiment 4. Rats infused with PXR AS-ODN had a mean 1.4 fold lower PXR expression in the midbrain compared to control infusions in Experiment 4 (data not shown). ^*above a line indicates a significant effect of PXR AS-ODNs compared to control. ^**above a line indicates significant effect of testing compared to non-tested rats (P < 0.05).

Figure 4

(A) Depicts the mean (±s.e.m.) lordosis quotients of proestrous rats infused with control and vehicle (veh, n = 15), control+FGIN (n = 15), pregnane xenobiotic receptor (PXR) antisense deoxynucleotides (AS-ODNs)+vehicle (n = 8), or PXR AS-ODN+FGIN (n = 13) in Experiment 2. (B) Depicts the mean (±s.e.m.) lordosis quotients of ovariectomized (OVX) estradiol (E2)-primed rats infused with control+veh (n = 8), control+FGIN (n = 7), PXR AS-ODN+veh (n = 7), or PXR AS-ODN+FGIN (n = 6) in Experiment 3. (C) Depicts the mean (±s.e.m.) lordosis quotients of OVX/adrenalectomized (ADX), E2-primed rats infused with control+veh (n = 9), control+FGIN (n = 5), PXR AS-ODN+veh (n = 8), or PXR AS-ODN+FGIN (n = 5) in Experiment 4. ^*above a line indicates a significant effect of PXR AS-ODNs compared to control. (P < 0.05).