Neurotransmitter release is well known to occur at specialized synaptic regions that include presynaptic active zones and postsynaptic densities. At cholinergic synapses in the chick ciliary ganglion, however, membrane formations and physiological measurements suggest that release distant from postsynaptic densities can activate the predominantly extrasynaptic α7 nicotinic receptor subtype. We explored such ectopic neurotransmission with a novel model synapse that combines Monte Carlo simulations with high-resolution serial electron microscopic tomography. Simulated synaptic activity is consistent with experimental recordings of miniature excitatory postsynaptic currents only when ectopic transmission is included in the model, broadening the possibilities for mechanisms of neuronal communication.Throughout the nervous system, release of synaptic vesicles from presynaptic nerve terminals is thought to be associated with pre-and post-synaptic specializations, including active zones (AZs) and postsynaptic densities (PSDs). Release of neurotransmitter vesicles at extrasynaptic sites (ectopic release) has been suggested by the presence of morphologically docked vesicles distant from PSDs in electron micrographs from tissues, including the ribbon synapses of bipolar neurons (1) and saccular hair cells (2). Recently, direct measurements of quantal release have been made from climbing fibers in the cerebellar cortex onto the closely apposed Bergmann glia (3). Despite these findings, there † To whom correspondence should be addressed: terry@salk.edu. * These authors contributed equally to this work. HHMI Author Manuscript HHMI Author Manuscript HHMI Author Manuscripthas been no demonstration of the participation of ectopic release of neurotransmitter in the course of interneuronal synaptic transmission.At the structurally complex and umbrella-like calyceal synapse of the ciliary ganglion (CG), the case for ectopic release has been growing. Two major classes of kinetically distinct nicotinic acetylcholine receptors (nAChRs) are spatially segregated in the CG (4-6). The α7-nAChRs are expressed on matted spines but are largely excluded from PSDs regardless of where they occur (7-9). The α3*-nAChRs (6) are primarily localized to PSDs (whether on spines or somatic membrane) but are present at lower density on non-PSD membrane (4,9,10). The α7-nAChRs exhibit profound desensitization, an order of magnitude faster decay time, and an open probability lower by a factor of 30 than that of α3*-nAChRs (11-13).The segregation of the two nAChR subtypes, especially the exclusion of α7-nAChRs from PSDs, has made it difficult to interpret physiological measurements that show that the α7-nAChRs account for the majority of current in evoked EPSCs (11,12), are necessary to sustain higher frequency throughput (11,14), and produce distinct Ca signals localized to spines (15). Images of presynaptic vesicles within docking distance (ready to release), as well as Ω profiles (the image capture of fusing vesicles), are seen throughout the ...

Glucose is the main energy substrate in the adult brain under normal conditions. Accumulating evidence, however, indicates that lactate produced in astrocytes (a type of glial cell) can also fuel neuronal activity. The quantitative aspects of this so-called astrocyte-neuron lactate shuttle (ANLS) are still debated. To address this question, we developed a detailed biophysical model of the brain’s metabolic interactions. Our model integrates three modeling approaches, the Buxton-Wang model of vascular dynamics, the Hodgkin-Huxley formulation of neuronal membrane excitability and a biophysical model of metabolic pathways. This approach provides a template for large-scale simulations of the neuron-glia-vasculature (NGV) ensemble, and for the first time integrates the respective timescales at which energy metabolism and neuronal excitability occur. The model is constrained by relative neuronal and astrocytic oxygen and glucose utilization, by the concentration of metabolites at rest and by the temporal dynamics of NADH upon activation. These constraints produced four observations. First, a transfer of lactate from astrocytes to neurons emerged in response to activity. Second, constrained by activity-dependent NADH transients, neuronal oxidative metabolism increased first upon activation with a subsequent delayed astrocytic glycolysis increase. Third, the model correctly predicted the dynamics of extracellular lactate and oxygen as observed in vivo in rats. Fourth, the model correctly predicted the temporal dynamics of tissue lactate, of tissue glucose and oxygen consumption, and of the BOLD signal as reported in human studies. These findings not only support the ANLS hypothesis but also provide a quantitative mathematical description of the metabolic activation in neurons and glial cells, as well as of the macroscopic measurements obtained during brain imaging.

Nicotinic acetylcholine receptors are widely distributed throughout the nervous system, but their functions remain largely unknown. One of the most abundant is a class of receptors that contains the alpha 7 gene product, has a high relative permeability to calcium, and binds alpha-bungarotoxin. Here, we report that receptors sensitive to alpha-bungarotoxin, though concentrated in perisynaptic clusters on neurons, can generate a large amount of the synaptic current. Residual currents through other nicotinic receptors are sufficient to elicit action potentials, but with slower rise times. This demonstrates a postsynaptic response for alpha-bungarotoxin-sensitive receptors on neurons and suggests that the functional domain of the postsynaptic membrane is broader than previously recognized.