Neurotransmitter regulation rather than cell-intrinsic properties shape the high-pass filtering properties of olfactory bulb glomeruli

GABAergic periglomerular (PG) cells in the olfactory bulb are proposed to mediate an intraglomerular “high-pass” filter through inhibition targeted onto a glomerulus. With this mechanism, weak stimuli (e.g., an odor with a low affinity for an odorant receptor) mainly produce PG cell-driven inhibition but strong stimuli generate enough excitation to overcome inhibition. PG cells may be particularly susceptible to being activated by weak stimuli due to their intrinsically small size and high input resistance. Here, we used dual-cell patch-clamp recordings and imaging methods in bulb slices obtained from wild-type and transgenic rats with labeled GABAergic cells to test a number of predictions of the high-pass filtering model. We first directly compared the responsiveness of PG cells with that of external tufted cells (eTCs), which are a class of excitatory interneurons that reside in a parallel but opposing position in the glomerular circuitry. PG cells were in fact found to be no more responsive than eTCs at low levels of sensory neuron activity. While PG cells required smaller currents to be excited, this advantage was offset by the fact that a given level of sensory neuron activity produced much smaller synaptic currents. We did however identify other factors that shaped the excitation/inhibition balance in a manner that would support a high-pass filter, including glial glutamate transporters and presynaptic metabotropic glutamate receptors. We conclude that a single glomerulus may act as a high-pass filter to enhance the contrast between different olfactory stimuli through mechanisms that are largely independent cell intrinsic properties.Key Points SummaryGABAergic periglomerular (PG) cells in the olfactory bulb are proposed to mediate a “high-pass” filter at a single glomerulus that selectively blocks weak stimulus signals. Their efficacy may reflect their intrinsically small size and high input resistance, which allows them to be easily excited.We found that PG cells were in fact no more likely to be activated by weak stimuli than excitatory neurons. PG cells spiked more readily in response to a fixed current input, but this advantage for excitability was offset by small synaptic currents.Glomeruli nevertheless display an excitation/inhibition balance that can support a high pass filter, becoming much more favorable with increasing stimulus strength. Factors shaping the filter include glial glutamate transporters and presynaptic metabotropic glutamate receptors.We conclude that a single glomerulus may act as a high-pass filter to enhance stimulus contrast through mechanisms that are largely independent of cell-intrinsic properties.

Configurable Analog Block Using Current-Mode Approach

This paper introduces a new current-mode approach based configurable analog block (CAB) operating in voltage-mode with high input resistance, employing only three impedances in grounded form and providing a wide range functionality. The fourteen functions realized by the CAB include amplifiers, integrators, differentiators, and filters. The nonideal and parasitic analysis of the proposed configurable block is carried out, along with detailed simulation studies using 0.25[Formula: see text][Formula: see text]m CMOS parameters and a supply of [Formula: see text]0.75[Formula: see text]V for justifying the advance to the field. The active element used for designing the CAB exhibits 1.9 and 0.8[Formula: see text]GHz as voltage and current transfer bandwidths, respectively, while also providing low output resistance of 105[Formula: see text][Formula: see text] in voltage conveying action. Thus, the proposed CAB benefits from high frequency performance and lesser sensitivity to errors due to parasitic at that terminal. Most of the functions utilize beneficial form of grounded components. Presentation of conclusive results for integrator, all-pass filter and band-pass filter is included in form of frequency and transient responses. In-depth intermodulation distortion results for band-pass filter are further given. General conclusive annotations on analog circuit design are made at the end of this study.

Uniquely excitable neurons enable precise and persistent information transmission through the retrosplenial cortex

ABSTRACTThe retrosplenial cortex (RSC) is essential for both memory and navigation, but the neural codes underlying these functions remain largely unknown. Here, we show that the most prominent cell type in layers 2/3 (L2/3) of the granular RSC is a uniquely excitable, small pyramidal cell. These cells have a low rheobase (LR), high input resistance, lack of spike-frequency adaptation, and spike widths intermediate to those of neighboring fast-spiking (FS) inhibitory neurons and regular-spiking (RS) excitatory neurons. LR cells are excitatory but rarely synapse onto neighboring neurons. Instead, L2/3 of RSC is an inhibition-dominated network with dense connectivity between FS cells and from FS to LR neurons. Biophysical models of LR but not RS cells precisely and continuously encode sustained input from afferent postsubicular head-direction cells. Thus, the unique intrinsic properties of LR neurons can support both the precision and persistence necessary to encode information over multiple timescales in the RSC.