To investigate the role of astrocytes in regulating synaptic transmission, we generated inducible transgenic mice that express a dominant-negative SNARE domain selectively in astrocytes to block the release of transmitters from these glial cells. By releasing adenosine triphosphate, which accumulates as adenosine, astrocytes tonically suppressed synaptic transmission, thereby enhancing the dynamic range for long-term potentiation and mediated activity-dependent, heterosynaptic depression. These results indicate that astrocytes are intricately linked in the regulation of synaptic strength and plasticity and provide a pathway for synaptic cross-talk.

Tuberous sclerosis complex (TSC) is an autosomal dominant monogenetic disorder that is characterized by the formation of benign tumors in several organs as well as brain malformations and neuronal defects. TSC is caused by inactivating mutations in one of two genes, TSC1 and TSC2, resulting in increased activity of the mammalian Target of Rapamycin (mTOR). Here, we explore the cytoarchitectural and functional CNS aberrations that may account for the neurological presentations of TSC, notably seizures, hydrocephalus, and cognitive and psychological impairments. In particular, recent mouse models of brain lesions are presented with an emphasis on using electroporation to allow the generation of discrete lesions resulting from loss of heterozygosity during perinatal development. Cortical lesions are thought to contribute to epileptogenesis and worsening of cognitive defects. However, it has recently been suggested that being born with a mutant allele without loss of heterozygosity and associated cortical lesions is sufficient to generate cognitive and neuropsychiatric problems. We will thus discuss the function of mTOR hyperactivity on neuronal circuit formation and the potential consequences of being born heterozygote on neuronal function and the biochemistry of synaptic plasticity, the cellular substrate of learning and memory. Ultimately, a major goal of TSC research is to identify the cellular and molecular mechanisms downstream of mTOR underlying the neurological manifestations observed in TSC patients and identify novel therapeutic targets to prevent the formation of brain lesions and restore neuronal function.

Summary Signaling through GABAA receptors controls neural progenitor cell (NPC) development in vitro and is altered in schizophrenic and autistic individuals. However, the in vivo function of GABAA signaling on neural stem cell proliferation, and ultimately neurogenesis, remains unknown. To examine GABAA function in vivo, we electroporated plasmids encoding short-hairpin RNA against the Na-K-2Cl co-transporter NKCC1 in NPCs of the neonatal subventricular zone (SVZ) in mice to reduce GABAA-induced depolarization. Reduced GABAA depolarization identified by a loss of GABAA-induced calcium responses in most electroporated NPCs led to a 70% decrease in the number of proliferative Ki67+ NPCs and a 60% reduction in newborn neuron density. Premature loss of GABAA depolarization in newborn neurons resulted in truncated dendritic arborization at the time of synaptic integration. However, by 6 weeks the dendritic tree had partially recovered and displayed a small, albeit significant, decrease in dendritic complexity but not total dendritic length. To further examine GABAA function on NPCs, we treated animals with a GABAA allosteric agonist, pentobarbital. Enhancement of GABAA activity in NPCs increased the number of proliferative NPCs by 60%. Combining shNKCC1 and pentobarbital prevented the shNKCC1 and the pentobarbital effects on NPC proliferation, suggesting that these manipulations affected NPCs through GABAA receptors. Thus, dysregulation in GABAA depolarizing activity delayed dendritic development and reduced NPC proliferation resulting in decreased neuronal density.

In the postnatal subventricular zone (SVZ), S phase entry of neural progenitor cells (NPCs) correlates with a local increase in blood flow. However, the cellular mechanism controlling this hemodynamic response remains unknown. We show that a subpopulation of SVZ cells, i.e. astrocyte-like cells or B cells, send projections ensheathing pericytes on SVZ capillaries in young mice. We examined whether calcium increases in pericytes or B cells led to a vascular response in acute slices using the P2Y2/4 receptor (P2Y2/4R) agonist UTP, electrical stimulation, or transgenic mice expressing exogenous Gq-coupled receptors (MrgA1) in B cells. UTP increased calcium in pericytes leading to capillary constrictions. Electrical stimulation induced calcium propagation in SVZ cells followed by capillary constrictions involving purinergic receptors. In transgenic mice, selective calcium increases in B cells induced P2Y2/4R-dependent capillary constrictions, suggesting that B cells release ATP activating purinergic receptors on pericytes. Interestingly, in the presence of a P2Y2/4R blocker, a dilation was observed. In vivo, intraventricular UTP injection transiently decreased blood flow monitored using laser Doppler flowmetry. Using neonatal electroporation, we expressed MrgA1 in slow cycling radial glia-derived B1 cells, i.e. NPCs. Intraventricular injection of a MrgA1 ligand increased blood flow in the SVZ. Thus, upon intracellular calcium increases B cells/NPCs release ATP and vasodilating factors that activate purinergic receptors on pericytes triggering a vascular response and blood flow increase in vivo. Considering that NPCs receive signals from other SVZ cells, these findings further suggest that NPCs act as transducers of neurometabolic coupling in the SVZ.