Feed-forward excitation of interneurons in the cerebellar granule cell layer.

The cerebellum is involved in maintenance of posture and balance and coordination of voluntary movements. It has previously been shown that the inhibition of granule cells by Golgi cells, in the input layer of the cerebellar cortex, is important for normal cerebellar function. However, little is known about what determines the firing of spontaneously active Golgi cells and how intrinsic activity interacts with sensory input. In particular, the excitation of the interneuron by mossy fibres, which may mediate feed-forward inhibition of granule cells, has not been characterized. I have used immuno-histochemistry, patch-clamp recordings and imaging in acute cerebellar slices of rats to study feed-forward mossy fibre input onto Golgi cells and its downstream effects. I confirm that mossy fibres, Golgi cells and granule cells form a functional feed-forward inhibitory circuit. Anatomical analysis of the circuitry suggests that only spatially correlated inputs result in feed-forward inhibition. Activation of the pathway required synchronous activity in 4 out of the approximately 10 mossy fibres contacting a Golgi cell. These inputs can reset the timing of spontaneous Golgi cell firing with remarkably high temporal precision. I found that an interaction between fast EPSC kinetics, electronic compactness and pacemaker conductances allowed precise temporal signaling while integrating only 6 quanta across the dendritic tree of a Golgi cell. Golgi cell mean firing rate was only weakly modulated by mossy fibre input due to dominant pacemaker conductances. These results suggest that the properties of the feed-forward mossy fibre - Golgi cell - granule cell pathway are tuned to detect and signal coincident synaptic activity with high temporal precision. This provides a likely synaptic basis for precisely timed Golgi cell responses observed in vivo, which may signal the onset of sensory stimulation producing spatiotemporally correlated mossy fibre activity. These findings are discussed in the context of current models of granule cell layer processing

Cerebellar Globular Cells Receive Monoaminergic Excitation and Monosynaptic Inhibition from Purkinje Cells

Inhibitory interneurons in the cerebellar granular layer are more heterogeneous than traditionally depicted. In contrast to Golgi cells, which are ubiquitously distributed in the granular layer, small fusiform Lugaro cells and globular cells are located underneath the Purkinje cell layer and small in number. Globular cells have not been characterized physiologically. Here, using cerebellar slices obtained from a strain of gene-manipulated mice expressing GFP specifically in GABAergic neurons, we morphologically identified globular cells, and compared their synaptic activity and monoaminergic influence of their electrical activity with those of small Golgi cells and small fusiform Lugaro cells. Globular cells were characterized by prominent IPSCs together with monosynaptic inputs from the axon collaterals of Purkinje cells, whereas small Golgi cells or small fusiform Lugaro cells displayed fewer and smaller spontaneous IPSCs. Globular cells were silent at rest and fired spike discharges in response to application of either serotonin (5-HT) or noradrenaline. The two monoamines also facilitated small Golgi cell firing, but only 5-HT elicited firing in small fusiform Lugaro cells. Furthermore, globular cells likely received excitatory monosynaptic inputs through mossy fibers. Because globular cells project their axons long in the transversal direction, the neuronal circuit that includes interplay between Purkinje cells and globular cells could regulate Purkinje cell activity in different microzones under the influence of monoamines and mossy fiber inputs, suggesting that globular cells likely play a unique modulatory role in cerebellar motor control

Local Field Potential Modeling Predicts Dense Activation in Cerebellar Granule Cells Clusters under LTP and LTD Control

Local field-potentials (LFPs) are generated by neuronal ensembles and contain information about the activity of single neurons. Here, the LFPs of the cerebellar granular layer and their changes during long-term synaptic plasticity (LTP and LTD) were recorded in response to punctate facial stimulation in the rat in vivo. The LFP comprised a trigeminal (T) and a cortical (C) wave. T and C, which derived from independent granule cell clusters, co-varied during LTP and LTD. To extract information about the underlying cellular activities, the LFP was reconstructed using a repetitive convolution (ReConv) of the extracellular potential generated by a detailed multicompartmental model of the granule cell. The mossy fiber input patterns were determined using a Blind Source Separation (BSS) algorithm. The major component of the LFP was generated by the granule cell spike Na+ current, which caused a powerful sink in the axon initial segment with the source located in the soma and dendrites. Reproducing the LFP changes observed during LTP and LTD required modifications in both release probability and intrinsic excitability at the mossy fiber-granule cells relay. Synaptic plasticity and Golgi cell feed-forward inhibition proved critical for controlling the percentage of active granule cells, which was 11% in standard conditions but ranged from 3% during LTD to 21% during LTP and raised over 50% when inhibition was reduced. The emerging picture is that of independent (but neighboring) trigeminal and cortical channels, in which synaptic plasticity and feed-forward inhibition effectively regulate the number of discharging granule cells and emitted spikes generating “dense” activity clusters in the cerebellar granular layer

Calcium-induced aggregation of archaeal bipolar tetraether liposomes derived from the thermoacidophilic archaeon Sulfolobus acidocaldarius

Previously, we showed that the proton permeability of small unilamellar vesicles (SUVs) composed of polar lipid fraction E (PLFE) from the thermoacidophilic archaeon Sulfolobus acidocaldarius was remarkably low and insensitive to temperature (Komatsu and Chong 1998). In this study, we used photon correlation spectroscopy to investigate the time dependence of PLFE SUV size as a function of Ca2+ concentration. In the absence of Ca2+, vesicle diameter changed little over 6 months. Addition of Ca2+, however, immediately induced formation of vesicle aggregates with an irregular shape, as revealed by confocal fluorescence microscopy. Aggregation was reversible upon addition of EDTA; however, the reversibility varied with temperature as well as incubation time with Ca2+. Freeze-fracture electron microscopy showed that, after a long period of incubation (2 weeks) with Ca2+, the PLFE vesicles had not just aggregated, but had fused or coalesced. The initial rate of vesicle aggregation varied sigmoidally with Ca2+ concentration. At pH 6.6, the threshold calcium concentration (Cr) for vesicle aggregation at 25 and 40 °C was 11 and 17 mM, respectively. At pH 3.0, the Cr at 25 °C increased to 25 mM. The temperature dependence of Cr may be attributable to changes in membrane surface potential, which was –22.0 and –13.2 mV at 25 and 40 °C, respectively, at pH 6.6, as determined by 2-(p-toluidinyl)naphthalene-6-sulfonic acid fluorescence. The variation in surface potential with temperature is discussed in terms of changes in lipid conformation and membrane organization

Estimation of the time course of neurotransmitter release at central synapses from the first latency of postsynaptic currents

► Method for estimating vesicular release time course from PSC first latencies. ► Analytical derivation of binomial model of release at central synapses. ► Systematic tests of robustness with biologically realistic simulations. ► Generalization of existing first latency correction based on Poisson model

Pressure Perturbation and Differential Scanning Calorimetric Studies of Bipolar Tetraether Liposomes Derived from the Thermoacidophilic Archaeon Sulfolobus acidocaldarius

Differential scanning calorimetry (DSC) and pressure perturbation calorimetry (PPC) were used to characterize thermal phase transitions, membrane packing, and volumetric properties in multilamellar vesicles (MLVs) composed of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius grown at different temperatures. For PLFE MLVs derived from cells grown at 78°C, the first DSC heating scan exhibits an endothermic transition at 46.7°C, a small hump near 60°C, and a broad exothermic transition at 78.5°C, whereas the PPC scan reveals two transitions at ∼45°C and 60°C. The endothermic peak at 46.7°C is attributed to a lamellar-to-lamellar phase transition and has an unusually low ΔH (3.5 kJ/mol) and ΔV/V (0.1%) value, as compared to those for the main phase transitions of saturated diacyl monopolar diester lipids. This result may arise from the restricted trans-gauche conformational changes in the dibiphytanyl chain due to the presence of cyclopentane rings and branched methyl groups and due to the spanning of the lipid molecules over the whole membrane. The exothermic peak at 78.5°C probably corresponds to a lamellar-to-cubic phase transition and exhibits a large and negative ΔH value (−23.2 kJ/mol), which is uncommon for normal lamellar-to-cubic phospholipid phase transformations. This exothermic transition disappears in the subsequent heating scans and thus may involve a metastable phase, which is irreversible at the scan rate used. Further, there is no distinct peak in the plot of the thermal expansion coefficient α versus temperature near 78.5°C, indicating that this lamellar-to-cubic phase transition is not accompanied by any significant volume change. For PLFE MLVs derived from cells grown at 65°C, similar DSC and PPC profiles and thermal history responses were obtained. However, the lower growth temperature yields a higher ΔV/V (∼0.25%) and ΔH (14 kJ/mol) value for the lamellar-to-lamellar phase transition measured at the same pH (2.1). A lower growth temperature also generates a less negative temperature dependence of α. The changes in ΔV/V, ΔH, and the temperature dependence of α can be attributed to the decrease in the number of cyclopentane rings in PLFE at the lower growth temperature. The relatively low ΔV/V and small ΔH involved in the phase transitions help to explain why PLFE liposomes are remarkably thermally stable and also echo the proposal that PLFE liposomes are generally rigid and tightly packed. These results help us to understand why, despite the occurrence of thermal-induced phase transitions, PLFE liposomes exhibit a remarkably low temperature sensitivity of proton permeation and dye leakage