Fig 1.
A low (10 ng/ml) concentration of LPS inhibits CA1 LTP via activation of TLR4.
A. The graph shows the time course of changes in EPSPs and the ability of a single HFS (arrow) to induce robust LTP (white circles). Incubation of slices with 10 ng/ml LPS for 2–4 hours prior to study completely inhibits LTP (black circles) as shown previously [9] (p = 0.0007 vs. control LTP). B, C, D. Co-treatment of hippocampal slices with three different TLR4 inhibitors, TAK-242 (B) (p = 0.0006 vs. LPS alone); IAXO-102 (C) (p = 0.0031 vs. LPS alone); and LPS-RS (D) (p = 0.0008 vs. LPS alone), prevented the effects of LPS. Traces in this and other figures show representative EPSPs during baseline (dashed traces) and 60 min following HFS (solid traces). Calibration: 1 mV, 5ms.
Fig 2.
LTP inhibition by low LPS involves the oxysterol, 25-HC.
A. In slices from wild type mice, a single HFS readily induces LTP (white circles). LTP in wild type slices is blocked by incubation with 10 ng/ml LPS x 2–4 hours (black circles) (p = 0.0132 vs. control LTP), consistent with our previous observations using brief high (μg/ml) LPS [9]. B. HFS readily induced LTP in slices from Ch25h KO mice (white circles) and LPS failed to block LTP in these slices (black circles) (p = 0.5273 vs. control LTP). Traces show EPSPs as in Fig 1. Calibration 1 mV, 5 ms.
Fig 3.
Adverse effects of acute high (1 μg/ml) LPS on LTP involve 5α-reduced neurosteroids, but are not prevented by the neurosteroid, AlloP, alone.
A. In the presence of 1 μg/ml LPS administered for 15 min prior to HFS (black bar), CA1 LTP is completely inhibited as we showed previously (black circles) [9]. This LTP inhibition is prevented by the 5α-reductase inhibitor, finasteride (white circles) (p = 0.0038 vs. LPS alone). B,C. Pretreatment with 1 μM exogenous AlloP for 15 min before and during 1 μg/ml LPS failed to promote LTP (p = 0.0937 vs. LPS alone) vs. LPS alone (B) as did 2–4 h preincubation with AlloP (p = 0.2503 vs. LPS alone) (C). Calibration: 1 mV, 5 ms.
Fig 4.
In contrast to high LPS, the effects of low LPS on LTP are prevented by preincubation with AlloP.
A. In slices preincubated with 1 μM AlloP plus 10 ng/ml LPS, robust LTP is readily induced by a single HFS (p = 0.0026 vs. LPS alone). B. At 1 μM, the unnatural enantiomer of AlloP had variable effects on LTP inhibition by low LPS but allowed LTP in 6 of 10 slices as described in the text (p = 0.0554 vs LPS alone, N = 10). C. ent-AlloP, akin to nat-AlloP, failed to alter LTP inhibition by high LPS (p = 0.7929 vs. LPS alone). Traces show representative EPSPs. Calibration: 1 mV, 5 ms.
Fig 5.
The enantiomers of AlloP prevent the effects of stimulation of TLR7 and TLR2.
A. In slices preincubated for 3–4 hours with 0.5 μM imiquimod, an agonist of endosomal TLR7/8 that signals via the adaptor protein MyD88, LTP is completely inhibited (black circles) (p = 0.0003 vs. control LTP). AlloP co-incubation with imiquimod allowed robust LTP (white circles) (p = 0.0119 vs. imiquimod). B. Similar to nat-AlloP, ent-AlloP prevent the effects of imiquimod on LTP (gray circles and bar) (p = 0.0012 vs. imiquimod). C. Preincubation of slices with 10 ng/ml Pam3Cys, an agonist of TLR2 completely inhibits CA1 LTP (black circles) (p = 0.0003 vs. control LTP), and AlloP prevented the LTP inhibition (white circles) (p = 0.0001 vs. Pam3Cys alone). D. ent-AlloP also prevented LTP inhibition by Pam3Cys (gray circles) (p = 0.0024 vs. Pam3Cys alone; p = 0.0398 vs natural AlloP). Calibration: 1 mV, 5 ms.
Fig 6.
Stimulation of TLR3, a pattern recognition receptor that signals from endosomes in a MyD88-independent fashion, shows reverse AlloP enantioselectivity.
A. In slices preincubated with the TLR3 agonist Poly I:C (25 μg/ml) for 3–4 hours, LTP is completely inhibited (p = 0.0005 vs. control LTP). B. Co-incubation of slices with nat-AlloP failed to overcome LTP inhibition (p = 0.2228 vs. Poly(I:C) alone) C. In contrast, incubation of slices with ent-AlloP allowed small but persisting potentiation following HFS in the presence of Poly I:C (p = 0.0117 vs. Poly(I:C) alone). The degree of potentiation in the presence of Poly I:C and ent-AlloP was less than control LTP in naïve slices (p = 0.0009). Calibration 1 mV, 5 ms.
Fig 7.
AlloP enantiomers and finasteride prevent the effects of LPS on one-trial inhibitory avoidance learning in vivo.
Left graph: control rats, exposed to vehicle (DMSO) one hour prior to conditioning show robust learning 24 hours later and do not leave the lit chamber of the test apparatus. In contrast, animals pre-treated with vehicle followed by LPS (1 mg/kg ip) one hour later, show defective learning 24 hours following conditioning (p < 0.01 vs. controls). Effects of LPS on learning are completely prevented by pretreatment with 3 mg/kg (ip) AlloP or ent-AlloP one hour prior to LPS when tested 24 hours following conditioning. Right graph: finasteride pretreatment also prevented the effects of LPS on one-trial learning. The solvent for finasteride was 5% β-CDX (left bar). ** P < 0.01 by Dunn’s multiple comparison test.
Fig 8.
The diagram depicts mechanisms involved in LPS-induced LTP inhibition, highlighting the roles of AlloP as a mediator and a modulator of changes in synaptic plasticity.
25-HC and AlloP are highlighted in red as endogenous mediators of LTP inhibition. Text in green depicts the step at which exogenous AlloP appears to prevent the effects of LPS.