TLR4 in Learning Figure 4. Pharmacological TLR4 inhibition does not affect fear-learning and memory. (A) aCSF infused mice (n = 10) show similar association curves in the fear-conditioning paradigm compared with TLR4 antagonist infused mice (n = 10). (B) aCSF infused mice (n = 10) show similar freezing levels in the fear-conditioning paradigm compared with TLR4 antagonist infused mice (n = 10). (C) Average freezing during contextual fear. (D) aCSF infused mice (n = 10) show similar freezing in the fear-conditioning paradigm compared with TLR4 antagonist infused mice (n = 10) in the presence of tone. doi:10.1371/journal.pone.0047522.g004 the platform and enhanced swimming strategy. The fact that developmental deficiency in TLR4 impacted spatial, contextual and motor learning is in line with previous studies reporting effects of TLR4 deficiency on the proliferation of NPC in the embryonic telencephalon as well as the postnatal and adult brain [6,8]. In the latter studies, TLR4 deficiency promoted NPC proliferation, while TLR4 activation using LPS inhibited NPC proliferation. Further, TLR4 was shown to affect in-vivo neurogenesis in the dentate gyrus of adult mice by promoting differentiation of NPC into neurons. As TLR4 was implicated in adult dentate gyrus neurogenesis, and neurogenesis was implicated in cognitive learning and memory, we cannot rule out the possibility that the Figure 5. CREB and p-CREB are upregulated in the hippocampus of TLR42/2 mice. Brains from TLR4+/+ (n = 8) and TLR42/2 (n = 8) mice were dissected and hippocampi were removed. Tissues were then lysed, electrophoresed and immunoblotted against GluR1, CREB, ERK and their phosphorylated forms. Representative blots demonstrate that levels of CREB and pCREB were upregulated in TLR42/2 mice compared to TLR4+/+ mice, whereas GluR1, ERK and their phosphorylated forms were not changed. * p,0.05. doi:10.1371/journal.pone.0047522.g005 PLOS ONE | www.plosone.org 6 October 2012 | Volume 7 | Issue 10 | e47522