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. 2008 Nov;100(5):2640-52.
doi: 10.1152/jn.90691.2008. Epub 2008 Sep 17.

Selective, state-dependent activation of somatostatin-expressing inhibitory interneurons in mouse neocortex

Erika E Fanselow  1 , Kristen A Richardson, Barry W Connors
Affiliations

Selective, state-dependent activation of somatostatin-expressing inhibitory interneurons in mouse neocortex

Erika E Fanselow et al. J Neurophysiol. 2008 Nov.

Abstract

The specific functions of subtypes of cortical inhibitory neurons are not well understood. This is due in part to a dearth of information about the behaviors of interneurons under conditions when the surrounding circuit is in an active state. We investigated the firing behavior of a subset of inhibitory interneurons, identified using mice that express green fluorescent protein (GFP) in a subset of somatostatin-expressing inhibitory cells ("GFP-expressing inhibitory neuron" [GIN] cells). The somata of the GIN cells were in layer 2/3 of somatosensory cortex and had dense, layer 1-projecting axons that are characteristic of Martinotti neurons. Interestingly, GIN cells fired similarly during a variety of diverse activating conditions: when bathed in fluids with low-divalent cation concentrations, when stimulated with brief trains of local synaptic inputs, when exposed to group I metabotropic glutamate receptor agonists, or when exposed to muscarinic cholinergic receptor agonists. During these manipulations, GIN cells fired rhythmically and persistently in the theta-frequency range (3-10 Hz). Synchronous firing was often observed and its strength was directly proportional to the magnitude of electrical coupling between GIN cells. These effects were cell type specific: the four manipulations that persistently activated GIN cells rarely caused spiking of regular-spiking (RS) pyramidal cells or fast-spiking (FS) inhibitory interneurons. Our results suggest that supragranular GIN interneurons form an electrically coupled network that exerts a coherent 3- to 10-Hz inhibitory influence on its targets. Because GIN cells are more readily activated than RS and FS cells, it is possible that they act as "first responders" when cortical excitatory activity increases.

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Figures

FIG. 1.
FIG. 1.
Green fluorescent protein–expressing inhibitory neuron (GIN) cell morphologies A: bitufted GIN cell in layer 2/3 of mouse neocortex filled with and stained for biocytin. B: low-magnification picture of slice in which cell in A was found. C: reconstruction of GIN cell depicted in A created using a camera lucida microscope attachment. Cell body and dendrites are black and axon is red. D: multipolar cell in layer 2/3 of mouse neocortex, also filled with and stained for biocytin. E: low-magnification picture of slice in which cell in D was found. In all cases, pial surface is up. F: laminar distribution of GIN cells recorded in this study.
FIG. 2.
FIG. 2.
Characteristics of layer 2/3 neocortical GIN cells. A: cumulative plots of the lowest current value (pA) that elicited action potentials for each cell type (i.e., firing threshold). B: average firing frequencies of GIN, regular-spiking (RS), and fast-spiking (FS) cells as a function of stimulus current, measured during the last 200 ms of 600-ms constant-current steps. Currents are normalized to the threshold (i.e., the lowest current amplitude that yielded action potentials for each cell). C: GIN cell responses to current steps at −100 and 40 pA. An example of the afterdepolarization (ADP) characteristic of GIN cells is enlarged in the inset (from area above horizontal line in C). With injection of stronger currents (e.g., 300 pA in bottom trace in C), GIN cells display spike frequency adaptation. In addition, characteristic action potential height and afterhyperpolarization (AHP) profiles of GIN cells can be observed in this trace. Note that the peak of the first action potential is the most positive in the trace and the depth of the first AHP is the most negative. D: FS cell responses to current steps at −100 and 180 pA (top) and 500 pA (bottom). The lack of an ADP is shown in the inset of the top panel (enlarged from area above horizontal line in D). Note also the delay in firing in the top panel in D, which is characteristic of FS neurons with injections of just-suprathreshold currents. E: GIN cells have a more positive resting membrane potential (fine dashed line) and a more negative action potential threshold (coarse dashed line) than RS (F) and FS (G) cells. Arrows indicate resting membrane potentials for each cell type. H: average resting membrane potentials and action potential thresholds.
FIG. 3.
FIG. 3.
Dynamic responses at GIN → RS and RS → GIN synapses. A: inhibitory postsynaptic potentials (IPSPs) recorded in an RS cell (high [Cl−] in RS cell) in response to stimulation at 10 Hz show minimal synaptic depression. B: in contrast, excitatory postsynaptic potentials (EPSPs) recorded in GIN cells in response to RS cell stimulation show a large degree of synaptic facilitation. These findings are quantified in C and D for 3, 10, 20, and 40 Hz for each synapse type. The degree of depression at the GIN → RS synapse and the degree of facilitation at the RS → GIN synapse increased with increasing stimulus frequency. E: degree of depression at the 8th IPSP for RS cell responses to presynaptic GIN cell stimulation as a function of stimulus frequency. F: degree of facilitation for 8th EPSP for GIN cell responses to presynaptic RS cell stimulation as a function of stimulus frequency. Number of cell pairs tested at each frequency for GIN → RS synapses: 3 Hz = 12, 10 Hz = 12, 20 Hz = 7, 40 Hz = 15; for RS → GIN synapses: 3 Hz = 6, 10 Hz = 8, 20 Hz = 8, 40 Hz = 11.
FIG. 4.
FIG. 4.
Application of low-divalent artificial cerebrospinal fluid (ACSF) causes synchronous, persistent firing in GIN cells. A: when low-divalent ACSF (1 mM Ca2+ and Mg2+) was washed onto the slice, GIN cells fired persistently for as long as the low-divalent ACSF was presented. B: instantaneous firing frequency during recording in A. C: 76.3% (58/76) of GIN cells in layers 2/3 fired persistently during application of low-divalent ACSF, whereas only 9% of RS cells (4/44) and no FS cells (0/11) fired under these conditions. D: synchronous persistent firing in pairs of GIN cells that were electrically coupled. E: cross-correlogram for spike times across 18 min of low-divalent ACSF application, of which the trace in D is a sample. Bin width was 20 ms.
FIG. 5.
FIG. 5.
Synchrony among coupled GIN cells is proportional to the coupling coefficient. When a measure of firing synchrony (measured as the height of the main peak of the cross-correlogram; see methods) was plotted as a function of the coupling coefficient, there was a positive correlation between these 2 measures (R = 0.85). Data were pooled across activating conditions.
FIG. 6.
FIG. 6.
GIN cell responses following brief trains of extracellular stimulation. A: when a train of stimuli (here, 20 stimuli at 100 Hz, pulse duration 200 μs) was presented using a bipolar stimulating electrode near the layer 4/layer 3 border, GIN cells responded with action potentials during the stimulation (arrow), then were inhibited for an average of 5.4 ± 0.5 s, and subsequently fired for an average of 32.6 ± 2.1 s. The average prestimulus resting membrane potential was −51 mV and the average minimum following the stimulus was −61 mV. B: summary of the average membrane potential prior to stimulation, the average negative-most value during the inhibitory period, the average duration of the inhibitory period, and the average duration of the persistent firing period (open bar; error bars indicate SE). C: at resting membrane potentials, 84.7% (n = 83/98) of GIN cells, but no RS (n = 0/25) or FS (n = 0/5) cells showed persistent firing following trains of extracellular stimuli. D: when we recorded from strongly electrically coupled pairs of GIN cells following extracellular stimulation, the firing in these cells was often synchronous. E: expansion of firing during horizontal line in D. F: cross-correlogram of activity across 2 min of activity following extracellular stimulation. Integration window was 20 ms.
FIG. 7.
FIG. 7.
Persistent activity in GIN cells following trains of extracellular stimuli was not blocked by pharmacological blockade of fast synaptic transmission, but was blocked by the metabotropic glutamate receptor 1a (mGluR1a) antagonist LY367385. A: persistent activity in a GIN cell in normal ACSF. B: persistent activity in the same cell after washing on d-2-amino-5-phosphonovaleric acid (APV, 50 μM), 6,7-dinitroquinoxaline-2,3(1H,4H)-dione (DNQX, 20 μM), and picrotoxin (PTX, 20 μM) for 15 min. Note that the inhibitory period that typically followed the stimulus under normal conditions (A) was eliminated by application of these blockers (B). Scale bar for A also applies to B. C: persistent activity in a GIN cell in normal ACSF (stimulus train presented at arrow above trace). D: membrane depolarization and persistent firing were blocked by the application of LY367385 (200 μM). E: membrane depolarization and persistent firing returned on washout of LY367385. Scale bar for D also applies to C and E. Dotted line in all panels indicates baseline membrane potential.
FIG. 8.
FIG. 8.
Application of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine hydrate (DHPG) causes persistent firing in GIN cells. A: persistent firing in a GIN cell during application of DHPG. B: instantaneous firing frequency during the trace in A. C: 95.0% (n = 19/20) GIN cells, 14.3% of RS (n = 2/12) cells, and no FS cells (n = 0/1) showed tonic firing during application of DHPG. D: during DHPG application, firing in pairs of strongly electrically coupled GIN cells showed a significant degree of synchrony. Action potential peaks were cut off in these traces to best show synchrony. E: cross-correlogram of activity during 7 min of firing in response to DHPG application, from which D is a sample. Bin width was 20 ms. F: simultaneous recording from a GIN cell and an RS cell. The electrode for the RS cell contained 30 mM [Cl−] so IPSPs could easily be observed at resting membrane potentials. G: average IPSPs during intracellular GIN stimulation (thick black line), DHPG application (thin black line; traces triggered from action potentials below long horizontal line for GIN cell in F), and a control trace (gray) triggered by action potential times for the subsequent 30-s period. H: expansion of RS trace in F (above short bar) showing IPSPs.
FIG. 9.
FIG. 9.
Application of cholinergic agonists to slices causes persistent firing in GIN cells. A: persistent firing during carbachol application in a GIN cell. B: instantaneous firing rate during trace in A. C: during carbachol application, 94.4% of GIN cells fired (n = 17/18), and no RS cells did (0/4). FS cells were not tested with carbachol. Similarly, 100% of tested GIN cells (n = 9/9) fired in response to application of the muscarinic agonist, muscarine; no RS cells did (n = 0/1) and FS cells were not tested with muscarine. D: during application of carbachol, firing in strongly electrically coupled GIN cells was synchronous. E: cross-correlogram of 4 min of firing in GIN cells during carbachol application, from which D is a sample. Bin width was 20 ms.
FIG. 10.
FIG. 10.
Firing rates of GIN cells during DHPG or muscarine application show a mild degree of concentration dependence. A: firing rates of GIN cells during DHPG application from 0.25 to 100 μM. Circles indicate individual data points and horizontal lines indicate median values for each concentration. B: histogram of firing frequency during DHPG application at steady-state firing levels for each cell. C: firing rates of GIN cells from 0.25 to 100 μM. Circles indicate individual data points and horizontal lines indicate median values. D: histogram of firing frequency for steady-state firing levels for each cell.
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References

    1. Abbott LF, Regehr WG. Synaptic computation. Nature 431: 796–803, 2004. - PubMed
    1. Agmon A, Connors BW. Thalamocortical responses of mouse somatosensory (barrel) cortex in vitro. Neuroscience 41: 365–379, 1991. - PubMed
    1. Amitai Y, Gibson JR, Beierlein M, Patrick SL, Ho AM, Connors BW, Golomb D. The spatial dimensions of electrically coupled networks of interneurons in the neocortex. J Neurosci 22: 4142–4152, 2002. - PMC - PubMed
    1. Bao W, Wu JY. Propagating wave and irregular dynamics: spatiotemporal patterns of cholinergic theta oscillations in neocortex in vitro. J Neurophysiol 90: 333–341, 2003. - PMC - PubMed
    1. Beierlein M, Gibson JR, Connors BW. A network of electrically coupled interneurons drives synchronized inhibition in neocortex. Nat Neurosci 3: 904–910, 2000. - PubMed

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