In response to decreased activity, neurons make global compensatory increases in excitatory synaptic strength. However, how neuronal maturity affects this process is unclear. We silenced cultured hippocampal neurons with TTX at 7 days in vitro, during rapid synaptogenesis, and at 14 days, when major synaptogenesis is complete. For each age, we have explored the effects of short (1 day) and longer (2 days) periods of silencing. We have confirmed that the changes in synaptic strength depend on 2 main mechanisms, one presynaptic and the other postsynaptic. The presynaptic mechanism involves an increase in the probability of neurotransmitter release, mostly arising through an increase in the number of synaptic vesicles available for release. The postsynaptic mechanism operates through an increase in the number of postsynaptic receptors for the excitatory neurotransmitter glutamate. When neurons are silenced for 1 day, young neurons employ the postsynaptic mechanism, whereas more mature neurons increase their strength through the presynaptic mechanism. The postsynaptic strengthening in young neurons does not depend on gene transcription, whereas the presynaptic mechanism does. If neurons are silenced for 2 days, younger and older neurons employ both pre and postsynaptic mechanisms for synaptic strengthening. We also found evidence for 2 additional mechanisms that increased the effective synaptic coupling between neurons after 2 days of silencing: an increase in the number of synapses, and an increase in the electrotonic length of dendrites. These results expand our basic understanding of neuronal homeostasis, and reveal the developmental regulation of its expression mechanisms.hippocampus ͉ homeostasis ͉ plasticity ͉ synaptic transmission ͉ synaptogenesis H ebbian forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD) are intensively studied examples of how activity can alter synapses locally, but global changes in synaptic strength have long been known to occur after synaptic transmission has been prevented (1, 2), a phenomenon known as ''denervation hypersensitivity.'' More recent investigations have deepened our understanding of the underlying mechanisms through which reductions in activity drive increases in synaptic strength, whereas increased activity has the opposite effect (3, 4). The phenomenon is now termed activity-dependent homeostasis.Homeostatic changes are slow to manifest, with effects first seen after hours and days, and cumulative, with changes increasing steadily over time (5-7). Several groups have found marked postsynaptic strengthening after neuronal silencing, shown by an increase in miniature excitatory postsynaptic current (mEPSC) amplitude, with no increase in presynaptic function, as measured by mEPSC frequency (5,7,8). In contrast, other groups have observed a large increase in presynaptic function with modest or no postsynaptic changes (6, 9, 10).Although homeostasis of synaptic strength is assumed to function throughout life, little is known abou...

Mutations in doublecortin (DCX) are associated with intractable epilepsy in humans, due to a severe disorganization of the neocortex and hippocampus known as classical lissencephaly. However, the basis of the epilepsy in lissencephaly remains unclear. To address potential functional redundancy with murin Dcx, we targeted one of the closest homologues, doublecortin-like kinase 2 (Dclk2). Here, we report that Dcx; Dclk2-null mice display frequent spontaneous seizures that originate in the hippocampus, with most animals dying in the first few months of life. Elevated hippocampal expression of c-fos and loss of somatostatin-positive interneurons were identified, both known to correlate with epilepsy. Dcx and Dclk2 are coexpressed in developing hippocampus, and, in their absence, there is dosagedependent disrupted hippocampal lamination associated with a cellautonomous simplification of pyramidal dendritic arborizations leading to reduced inhibitory synaptic tone. These data suggest that hippocampal dysmaturation and insufficient receptive field for inhibitory input may underlie the epilepsy in lissencephaly, and suggest potential therapeutic strategies for controlling epilepsy in these patients.epilepsy ͉ receptive field ͉ pyramidal neuron ͉ dendrites ͉ delamination A pproximately 1 in 10 people will have a seizure sometime during their life, and the prevalence of epilepsy in the general population is Ϸ3%. Cortical dysplasia, which includes defects in both neocortex and hippocampus, can result from disordered neuronal cell proliferation, migration, or differentiation, and is identified in Ͼ25% of children with intractable epilepsy (1), the type of epilepsy associated with the highest morbidity and mortality.Classical lissencephaly (defined as smooth brain, with simplified or absent gyri and sulci) is due to alterations in neuronal migration and differentiation. The frequency of epilepsy in lissencephaly is probably 100%, and is typically associated with lethality in the first or second decade of life. One of the major causative genes in humans is doublecortin (DCX), which encodes a microtubule binding and stabilizing protein (2).DCX has 2 close homologues in mice, doublecortin-like kinase 1 (Dclk1, also known as Dcamkl1) and doublecortin-like kinase 2 (Dclk2, also known as Dck2), both with broad nervous system expression, to include both mitotic neuroblasts and adult neurons, whereas Dcx is mostly expressed in postmitotic immature neurons, and also transiently in adult neuroblasts (3). Previous analysis of Dcx; Dclk1 double knockouts demonstrated functional redundancy during cortical and hippocampal lamination (4, 5). To further uncover functional redundancy in the DCX gene family, we targeted Dclk2, a gene encoding a highly conserved N-terminal doublecortin domain and a C-terminal kinase domain-containing protein (6). Here, we report that mice deficient in both Dcx and Dclk2 display frequent and spontaneous epileptic seizures, bearing some resemblance to the phenotype observed in humans. Results Dclk2 Knockout Leav...

Network activity is strongly tied to animal movement; however, hippocampal circuits selectively engaged during locomotion or immobility remain poorly characterized. Here we examined whether distinct locomotor states are encoded differentially in genetically defined classes of hippocampal interneurons. To characterize the relationship between interneuron activity and movement, we used , two-photon calcium imaging in CA1 of male and female mice, as animals performed a virtual-reality (VR) track running task. We found that activity in most somatostatin-expressing and parvalbumin-expressing interneurons positively correlated with locomotion. Surprisingly, nearly one in five somatostatin or one in seven parvalbumin interneurons were inhibited during locomotion and activated during periods of immobility. Anatomically, the somata of somatostatin immobility-activated neurons were smaller than those of movement-activated neurons. Furthermore, immobility-activated interneurons were distributed across cell layers, with somatostatin-expressing cells predominantly in stratum oriens and parvalbumin-expressing cells mostly in stratum pyramidale. Importantly, each cell's correlation between activity and movement was stable both over time and across VR environments. Our findings suggest that hippocampal interneuronal microcircuits are preferentially active during either movement or immobility periods. These inhibitory networks may regulate information flow in "labeled lines" within the hippocampus to process information during distinct behavioral states. The hippocampus is required for learning and memory. Movement controls network activity in the hippocampus but it's unclear how hippocampal neurons encode movement state. We investigated neural circuits active during locomotion and immobility and found interneurons were selectively active during movement or stopped periods, but not both. Each cell's response to locomotion was consistent across time and environments, suggesting there are separate dedicated circuits for processing information during locomotion and immobility. Understanding how the hippocampus switches between different network configurations may lead to therapeutic approaches to hippocampal-dependent dysfunctions, such as Alzheimer's disease or cognitive decline.

Synaptic inputs onto distal dendritic tufts are believed to function by modulating time-locked proximal inputs. However, little is known about the function of these inputs when proximal synapses are not coincident or silent. Surprisingly we found that activation of apical tuft synapses alone resulted in heterosynaptic potentiation of proximal synapses. In mouse adult hippocampal CA1 pyramidal neurons, we show that activation of distal inputs from the entorhinal cortex specifically strengthens proximal synapses projecting from CA3. This AMPA receptor-mediated potentiation is accompanied by increased synaptic GluN2B-containing NMDA receptors, which are normally restricted to juvenile animals. These two synaptic modifications interact to generate striking bi-directional metaplastic changes. Heterosynaptically potentiated synapses become resistant to subsequent long-term potentiation (LTP) as the two forms of AMPA receptor-mediated potentiation mutually occlude. However this is only true when the LTP induction protocol is relatively weak. When it is strong and repeated, the magnitude of LTP after heterosynaptic plasticity is greatly increased, specifically through the activation of GluN2B-containing NMDA receptors. Thus in addition to strengthening proximal apical synapses, heterosynaptic plasticity shifts the input-selectivity of CA1 neurons and expands the dynamic range of LTP, resulting in neurons that are tuned to greatly potentiate strong CA3 inputs. These results show that one set of inputs can exert long-lasting heterosynaptic control over another, allowing the interplay of two functionally and spatially distinct pathways, thereby greatly expanding the repertoire of cellular and network plasticity.

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