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 longterm 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.hippocampus ͉ long-term depression ͉ long-term potentiation ͉ anxiety T he neurotransmitter serotonin (5-HT) exerts a wide spectrum of actions in the nervous system by modulating neural development, synaptic plasticity, pain sensation, rhythm, food intake, and a variety of behaviors (1-4). It has been proposed that perturbation of the 5-HT level in the brain contributes to depression and anxiety, and posttraumatic stress disorder (PTSD) (5-8), which are often accompanied by learning and memory deficits (9-11). The hippocampus is known to be critical for the formation of spatial and contextual fear memories (12-15), and the retrieval of hippocampus-dependent memories was found to be impaired in patients with depression and PTSD (16)(17)(18). Recent studies have also implicated the hippocampus as one of the primary sites for antidepressants (6,(19)(20)(21)(22)(23). Aversive stimuli such as foot shock that led to anxiety, depression, and fear memory in rodents also altered activity-dependent hippocampal synaptic plasticity (24)(25)(26)(27)(28)(29). Because the modulation of 5-HT activity altered hippocampal long-term potentiation (LTP) and long-term depression (LTD) (30-32), it is possible that perturbation of 5-HT level in the brain may affect hippocampusdependent learning and memory, and changes in hippocampal synaptic plasticity may also contribute to mental disorders, including anxiety and fear memory.Inhibitors of 5-HT biosynthesis that...

The expression profile of Lim homeodomain transcription factor Lmx1b in the mouse brain was investigated at different postnatal stages by immunohistochemistry and in situ hybridization. At postnatal day (P) 7, many Lmx1b-expressing neurons were found in the posterior hypothalamic area, supramammillary nucleus, ventral premammillary nucleus, and subthalamic nucleus. In the midbrain, numerous Lmx1b-expressing neurons were present in the substantia nigra pars compacta and ventral tegmental area. In the hindbrain, Lmx1b-expressing neurons were primarily observed in the raphe nuclei, parabrachial nuclei, principal sensory trigeminal nucleus, nucleus of the solitary tract, and laminae I-II of the medullary dorsal horn as well as spinal dorsal horn. Although expression levels diminished as postnatal life progressed, persistent expression throughout the first year of life was observed in many of these regions. In contrast, Lmx1b was present in a few brain regions (e.g., principal sensory trigeminal nucleus) only in early life with expression expiring by P60. Lmx1b was observed in dopaminergic neurons in the midbrain and serotonergic neurons in the hindbrain, as determined by double labeling with specific markers. In addition, we found that Lmx1b-expressing neurons are not GABAergic, and Lmx1b was colocalized with Tlx3 in the parabrachial nuclei, principal sensory trigeminal nucleus, nucleus of the solitary tract. as well as the medullary and spinal dorsal horns, suggesting that Lmx1b-expressing cells in these areas are excitatory neurons. Our data suggest that Lmx1b is involved in the postnatal maturation of certain types of neurons and maintenance of their normal functions in the adult brain.

BackgroundNeurotropic virus-based tracers have been extensively applied in mapping and manipulation of neural circuits. However, their neurotropic and neurotoxic properties remain to be fully characterized.MethodsThrough neural circuit tracing, we systematically compared the neurotropism discrepancy among different multi-trans-synaptic and mono-synaptic retrograde viral tracers including pseudorabies virus (PRV), rabies virus (RV), and the newly engineered retro adeno-associated virus (rAAV2-retro) tracers. The (single-cell) RNA sequencing analysis was utilized for seeking possible attribution to neurotropism discrepancy and comparing cell toxicity caused by viral infection between glycoprotein-deleted RV (RV-∆G) and rAAV2-retro. Viral toxicity induced microglia activation and neuronal protein change were evaluated by immunohistochemistry.ResultsMulti-trans-synaptic retrograde viral tracers, PRV and RV, exhibit differential neurotropism when they were used for central neural circuit tracing from popliteal lymph nodes. Mono-synaptic retrograde tracers, including RV-∆G and rAAV2-retro, displayed discrepant neurotropic property, when they were applied to trace the inputs of lateral hypothalamic area and medial preoptic nucleus. rAAV2-retro demonstrated preference in cerebral cortex, whereas RV-∆G prefers to label basal ganglia and hypothalamus. Remarkably, we detected a distinct preference for specific cortical layer of rAAV2-retro in layer 5 and RV-∆G in layer 6 when they were injected into dorsal lateral geniculate nucleus to label corticothalamic neurons in primary visual cortex. Complementation of TVA receptor gene in RV-resistant neurons enabled EnvA-pseudotyped RV infection, supporting receptors attribution to viral neurotropism. Furthermore, both RV-∆G and rAAV2-retro exerted neurotoxic influence at the injection sites and retrogradely labeled sites, while the changes were more profound for RV-∆G infection. Finally, we demonstrated a proof-of-concept strategy for more comprehensive high-order circuit tracing of a specific target nucleus by combining rAAV2-retro, RV, and rAAV tracers.ConclusionsDifferent multi-trans-synaptic and mono-synaptic retrograde viral tracers exhibited discrepant neurotropism within certain brain regions, even cortical layer preference. More neurotoxicity was observed under RV-∆G infection as compared with rAAV2-retro. By combining rAAV2-retro, RV, and rAAV tracers, high-order circuit tracing can be achieved. Our findings provide important reference for appropriate application of viral tracers to delineate the landscape and dissect the function of neural network.Electronic supplementary materialThe online version of this article (10.1186/s13024-019-0308-6) contains supplementary material, which is available to authorized users.

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