Minocycline Activates the Nucleus of the Solitary Tract-Associated Network to Alleviate Lipopolysaccharide-Induced Neuroinflammation.

Objective To examine the neuroanatomical substrates underlying the effects of minocycline in alleviating lipopolysaccharide (LPS)-induced neuroinflammation. Methods Forty C57BL/6 male mice were randomly and equally divided into eight groups. Over three conse-cutive days, saline was administered to four groups of mice and minocycline to the other four groups. Immediately after the administration of saline or minocycline on the third day, two groups of mice were additionally injected with saline and the other two groups were injected with LPS. Six or 24 hours after the last injection, mice were sacrificed and the brains were removed. Immunohistochemical staining across the whole brain was performed to detect microglia activation via Iba1 and neuronal activation via c-Fos. Morphology of microglia and the number of c-Fo-positive neurons were analyzed by Image-Pro Premier 3D. One-way ANOVA and Fisher's least-significant differences were employed for statistical analyses. Results Minocycline alleviated LPS-induced neuroinflammation as evidenced by reduced activation of microglia in multiple brain regions, including the shell part of the nucleus accumbens (Acbs), paraventricular nucleus (PVN) of the hypothalamus, central nucleus of the amygdala (CeA), locus coeruleus (LC), and nucleus tractus solitarius (NTS). Minocycline significantly increased the number of c-Fo-positive neurons in NTS and area postrema (AP) after LPS treatment. Furthermore, in NTS-associated brain areas, including LC, lateral parabrachial nucleus (LPB), periaqueductal gray (PAG), dorsal raphe nucleus (DR), amygdala, PVN, and bed nucleus of the stria terminali (BNST), minocycline also significantly increased the number of c-Fo-positive neurons after LPS administration. Conclusion Minocycline alleviates LPS-induced neuroinflammation in multiple brain regions, possibly due to increased activation of neurons in the NTS-associated network.

Enhanced contextual fear memory in central serotonin-deficient mice.

Central serotonin (5-HT) dysregulation contributes to the susceptibility for mental disorders, including depression, anxiety, and posttraumatic stress disorder, and learning and memory deficits. We report that the formation of hippocampus-dependent spatial memory is compromised, but the acquisition and retrieval of contextual fear memory are enhanced, in central 5-HT-deficient mice. Genetic deletion of serotonin in the brain was achieved by inactivating Lmx1b selectively in the raphe nuclei of the brainstem, resulting in a near-complete loss of 5-HT throughout the brain. These 5-HT-deficient mice exhibited no gross abnormality in brain structures and had normal locomotor activity. Spatial learning in the Morris water maze was unaffected, but the retrieval of spatial memory was impaired. In contrast, contextual fear learning and memory induced by foot-shock conditioning was markedly enhanced, but this enhancement could be prevented by intracerebroventricular administration of 5-HT. Foot shock impaired long-term potentiation and facilitated long-term depression in hippocampal slices in WT mice but had no effect in 5-HT-deficient mice. Furthermore, bath application of 5-HT in 5-HT-deficient mice restored foot shock-induced alterations of hippocampal synaptic plasticity. Thus, central 5-HT regulates hippocampus-dependent contextual fear memory, and 5-HT modulation of hippocampal synaptic plasticity may be the underlying mechanism. The enhanced fear memory in 5-HT-deficient mice supports the notion that 5-HT deficiency confers susceptibility to posttraumatic stress disorder in humans.

Adult raphe-specific deletion of Lmx1b leads to central serotonin deficiency.

The transcription factor Lmx1b is essential for the differentiation and survival of central serotonergic (5-HTergic) neurons during embryonic development. However, the role of Lmx1b in adult 5-HTergic neurons is unknown. We used an inducible Cre-LoxP system to selectively inactivate Lmx1b expression in the raphe nuclei of adult mice. Pet1-CreER(T2) mice were generated and crossed with Lmx1b(flox/flox) mice to obtain Pet1-CreER(T2); Lmx1b(flox/flox) mice (which termed as Lmx1b iCKO). After administration of tamoxifen, the level of 5-HT in the brain of Lmx1b iCKO mice was reduced to 60% of that in control mice, and the expression of tryptophan hydroxylase 2 (Tph2), serotonin transporter (Sert) and vesicular monoamine transporter 2 (Vmat2) was greatly down-regulated. On the other hand, the expression of dopamine and norepinephrine as well as aromatic L-amino acid decarboxylase (Aadc) and Pet1 was unchanged. Our results reveal that Lmx1b is required for the biosynthesis of 5-HT in adult mouse brain, and it may be involved in maintaining normal functions of central 5-HTergic neurons by regulating the expression of Tph2, Sert and Vmat2.