of the contacts of a connection varied between 80 and 585 ,um (mean, 147 /sm; median, 105 ,um). The correlation between EPSP amplitude and the number of morphologically determined synaptic contacts or the mean geometric distances from the soma was only weak (correlation coefficients were 0-2 and 0-26, respectively).7. Compartmental models constructed from camera lucida drawings of eight target neurones showed that synaptic contacts were located at mean electrotonic distances between 0 07 and 0 33 from the soma (mean, 0-13). Simulations of unitary EPSPs, assuming quantal conductance changes with fast rise time and short duration, indicated that amplitudes of quantal EPSPs at the soma were attenuated, on average, to < 10% of dendritic EPSPs and varied in amplitude up to 10-fold depending on the dendritic location of synaptic contacts. The inferred quantal peak conductance increase varied between 1.5 and 5.5 nS (mean, 3 nS).8. The combined physiological and morphological measurements in conjunction with EPSP simulations indicated that the 20-fold range in efficacy of the synaptic connections between thick tufted pyramidal neurones, which have their synaptic contacts preferentially located on basal and apical oblique dendrites, was due to differences in transmitter release probability of the projecting neurones and, to a lesser extent, to differences in the number of release sites per bouton or quantal size. 9. The continuum of efficacies in their synaptic connections implies that layer 5 pyramidal neurones can be recruited to ensemble electrical activity via their axon collaterals if as few as five of the strongly and reliably connected neighbouring neurones are active synchronously, whereas coincident APs of as many as 100 of the weakly connected pyramidal neurones are necessary.

Gene-targeted mice lacking the L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR-A exhibited normal development, life expectancy, and fine structure of neuronal dendrites and synapses. In hippocampal CA1 pyramidal neurons, GluR-A-/- mice showed a reduction in functional AMPA receptors, with the remaining receptors preferentially targeted to synapses. Thus, the CA1 soma-patch currents were strongly reduced, but glutamatergic synaptic currents were unaltered; and evoked dendritic and spinous Ca2+ transients, Ca2+-dependent gene activation, and hippocampal field potentials were as in the wild type. In adult GluR-A-/- mice, associative long-term potentiation (LTP) was absent in CA3 to CA1 synapses, but spatial learning in the water maze was not impaired. The results suggest that CA1 hippocampal LTP is controlled by the number or subunit composition of AMPA receptors and show a dichotomy between LTP in CA1 and acquisition of spatial memory.

Networks of GABAergic interneurons are of critical importance for the generation of gamma frequency oscillations in the brain. To examine the underlying synaptic mechanisms, we made paired recordings from ''basket cells'' (BCs) in different subfields of hippocampal slices, using transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the parvalbumin promoter. Unitary inhibitory postsynaptic currents (IPSCs) showed large amplitude and fast time course with mean amplitudeweighted decay time constants of 2.5, 1.2, and 1.8 ms in the dentate gyrus, and the cornu ammonis area 3 (CA3) and 1 (CA1), respectively (33-34°C). The decay of unitary IPSCs at BC-BC synapses was significantly faster than that at BC-principal cell synapses, indicating target cell-specific differences in IPSC kinetics. In addition, electrical coupling was found in a subset of BC-BC pairs. To examine whether an interneuron network with fast inhibitory synapses can act as a gamma frequency oscillator, we developed an interneuron network model based on experimentally determined properties. In comparison to previous interneuron network models, our model was able to generate oscillatory activity with higher coherence over a broad range of frequencies (20 -110 Hz). In this model, high coherence and flexibility in frequency control emerge from the combination of synaptic properties, network structure, and electrical coupling.G amma frequency oscillations are thought to be of key importance for higher brain functions, such as feature binding and temporal encoding of information (1-5). Experimental and theoretical evidence suggests that local networks of synaptically connected GABAergic interneurons are critically involved in the generation of these oscillations (6-19). First, perisomatic inhibitory interneurons (basket cells) fire action potentials at high frequency during gamma activity in vivo, with single spikes phase-locked to the oscillations of the field potential (6, 7). Second, pharmacologically isolated networks of inhibitory interneurons in vitro can oscillate at gamma frequency in response to metabotropic glutamate receptor activation (8). Finally, models of mutually connected interneurons generate coherent action potential activity in the gamma frequency range in the presence of a tonic excitatory drive (9-19).The mechanisms leading to the generation of coherent gamma oscillations in interneuron networks, however, have remained unclear. Although gamma frequency oscillations can be generated in interneuron network models, coherence is fragile against variation in amplitude and time course of the inhibitory postsynaptic conductance, against heterogeneity of the tonic excitatory drive, and against sparseness of connectivity (11-14). The mechanisms contributing to the control of network frequency are also poorly understood. It is thought that the time course of the inhibitory synaptic conductance change is a major factor (8-14), but the significance of other parameters remains undetermined. Some models suggest t...

Apolipoprotein E receptor 2 (Apoer2), a member of the LDL receptor gene family, and its ligand Reelin control neuronal migration during brain development. Apoer2 is also essential for induction of long-term potentiation (LTP) in the adult brain. Here we show that Apoer2 is present in the postsynaptic densities of excitatory synapses where it forms a functional complex with NMDA receptors. Reelin signaling through Apoer2 markedly enhances LTP through a mechanism that requires the presence of amino acids encoded by an exon in the intracellular domain of Apoer2. This exon is alternatively spliced in an activity-dependent manner and is required for Reelin-induced tyrosine phosphorylation of NMDA receptor subunits. Mice constitutively lacking the exon perform poorly in learning and memory tasks. Thus, alternative splicing of Apoer2, a novel component of the NMDA receptor complex, controls the modulation of NMDA receptor activity, synaptic neurotransmission, and memory by Reelin.

Gene therapy in the central nervous system (CNS) is hindered by the presence of the blood-brain barrier, which restricts access of serum constituents and peripheral cells to the brain parenchyma. Expression of exogenously administered genes in the CNS has been achieved in vivo using highly invasive routes, or ex vivo relying on the direct implantation of genetically modified cells into the brain. Here we provide evidence for a novel, noninvasive approach for targeting potential therapeutic factors to the CNS. Genetically-modified hematopoietic cells enter the CNS and differentiate into microglia after bone-marrow transplantation. Up to a quarter of the regional microglial population is donor-derived by four months after transplantation. Microglial engraftment is enhanced by neuropathology, and gene-modified myeloid cells are specifically attracted to the sites of neuronal damage. Thus, microglia may serve as vehicles for gene delivery to the nervous system.

The three-dimensional morphology of the axosomatic synaptic structures between a calyx of Held and a principal neuron in the medial nucleus of the trapezoid body (MNTB) in the brainstem of young postnatal day 9 rats was reconstructed from serial ultrathin sections. In the apposition zone between the calyx and the principal neuron two types of membrane specializations were identified: synaptic contacts (SCs) with active zones (AZs) and their associated postsynaptic densities (PSDs) constituted approximately 35% (n = 554) of the specializations; the remaining 65% (n = 1010) were puncta adherentia (PA). Synaptic contacts comprised approximately 5% of the apposition area of presynaptic and postsynaptic membranes. A SC had an average area of 0.100 microm(2), and the nearest neighbors were separated, on average, by 0.59 microm. Approximately one-half of the synaptic vesicles in the calyx were clustered within a distance of 200 nm of the AZ membrane area, a cluster consisting of approximately 60 synaptic vesicles (n = 52 SCs). Approximately two synaptic vesicles per SC were "anatomically docked." Comparing the geometry of the synaptic structure with its previously studied functional properties, we find that during a single presynaptic action potential (AP) (1) approximately 35% of the AZs release a transmitter quantum, (2) the number of SCs and anatomically docked vesicles is comparable with the low estimates of the readily releasable pool (RRP) of quanta, and (3) the broad distribution of PSD areas [coefficient of variation (CV) = 0.9] is likely to contribute to the large variability of miniature EPSC peaks. The geometry of the reconstructed synapse suggests that each of the hundreds of SCs is likely to contribute independently to the size and rising phase of the EPSC during a single AP.

Estrogens have been described to induce synaptogenesis in principal neurons of the hippocampus and have been shown to be synthesized and released by exactly these neurons. Here, we have focused on the significance of local estrogen synthesis on spine synapse formation and the synthesis of synaptic proteins. To this end, we reduced hippocampal estrogen synthesis in vitro with letrozole, a reversible nonsteroidal aromatase inhibitor. In hippocampal slice cultures, letrozole treatment resulted in a dose-dependent decrease of 17␤-estradiol as quantified by RIA. This was accompanied by a significant decrease in the density of spine synapses and in the number of presynaptic boutons. Quantitative immunohistochemistry revealed a downregulation of spinophilin, a marker of dendritic spines, and synaptophysin, a protein of presynaptic vesicles, in response to letrozole. Surprisingly, no increase in the density of spines, boutons, and synapses and in spinophilin expression was seen after application of estradiol to the medium of cultures that had not been treated with letrozole. However, synaptophysin expression was upregulated under these conditions. Our results point to an essential role of endogenous hippocampal estrogen synthesis in the maintenance of hippocampal spine synapses.

The adult mouse subependymal zone (SEZ) harbours neural stem cells that are thought to generate exclusively GABAergic interneurons of the olfactory bulb. Here we describe the adult generation of glutamatergic juxtaglomerular neurons, with dendritic arborizations that project into adjacent glomeruli identifying them as short-axon cells. Fate mapping revealed that these originate from Neurogenin2- and Tbr2-expressing progenitors located in the dorsal region of the SEZ. Progenitors of these glutamatergic interneurons recapitulate the sequential expression of transcription factors that hallmark glutamatergic neurogenesis in the developing cerebral cortex and adult hippocampus. Indeed, the molecular specification of these SEZ progenitors allows for their recruitment into the cerebral cortex upon lesion. Taken together, our data show that SEZ progenitors not only produce a novel population of adult-born glutamatergic juxtaglomerular neurons, but may also provide a new source of progenitors for endogenous repair.