Somatostatin-containing Interneurons Research Articles

Calcium-Permeable Presynaptic Kainate Receptors Involved in Excitatory Short-Term Facilitation Onto Somatostatin Interneurons During Natural Stimulus Patterns

Schaffer collateral synapses in hippocampus show target-cell specific short-term plasticity. Using GFP-expressing Inhibitory Neuron (GIN) transgenic mice that express enhanced green fluorescent protein (EGFP) in a subset of somatostatin-containing interneurons (SOM interneurons), we previously showed that Schaffer collateral synapses onto SOM interneurons in stratum (S.) radiatum have unusually large (up to 6-fold) paired-pulse facilitation. This results from a low initial release probability and the enhancement of facilitation by synaptic activation of presynaptic kainate receptors. Here we further investigate the properties of these kainate receptors and examine their effects on short-term facilitation during physiologically derived stimulation patterns, using excitatory postsynaptic currents recorded in S. radiatum interneurons during Schaffer collateral stimulation in acute slices from juvenile GIN mice. We find that GluR5 and GluR6 antagonists decrease short-term facilitation at Schaffer collateral synapses onto SOM interneurons with no additive effects, suggesting that the presynaptic kainate receptors are heteromers containing both GluR5 and GluR6 subunits. The calcium-permeable receptor antagonist 1-napthyl acetyl spermine (NASPM) both mimics and occludes the effect of the kainate receptor antagonists, indicating that the presynaptic kainate receptors are calcium permeable. Furthermore, Schaffer collateral synapses onto SOM interneurons show up to 11-fold short-term facilitation during physiologically derived stimulus patterns, in contrast to other interneurons that have less than 1.5-fold facilitation. Blocking the kainate receptors reduces facilitation in SOM interneurons by approximately 50% during the physiologically derived patterns and reduces the dynamic range. Activation of calcium-permeable kainate receptors containing GluR5/GluR6 causes a dramatic increase in short-term facilitation during physiologically derived stimulus patterns, a mechanism likely to be important in regulating the strength of Schaffer collateral synapses onto SOM interneurons in vivo.

Robust but delayed thalamocortical activation of dendritic-targeting inhibitory interneurons

GABA-releasing cortical interneurons are crucial for the neural transformations underlying sensory perception, providing "feedforward" inhibition that constrains the temporal window for synaptic integration. To mediate feedforward inhibition, inhibitory interneurons need to fire in response to ascending thalamocortical inputs, and most previous studies concluded that ascending inputs activate mainly or solely proximally targeting, parvalbumin-containing "fast-spiking" interneurons. However, when thalamocortical axons fire at frequencies that are likely to occur during natural exploratory behavior, activation of fast-spiking interneurons is rapidly and strongly depressed, implying the paradoxical conclusion that feedforward inhibition is absent when it is most needed. To address this issue, we took advantage of lines of transgenic mice in which either parvalbumin- or somatostatin-containing interneurons express GFP and recorded the responses of interneurons from both subtypes to thalamocortical stimulation in vitro. We report that during thalamocortical activation at behaviorally expected frequencies, fast-spiking interneurons were indeed activated only transiently because of rapid depression of their thalamocortical inputs, but a subset of layer 5 somatostatin-containing interneurons were robustly and persistently activated after a delay, due to the facilitation and temporal summation of their thalamocortical excitatory postsynaptic potentials. Somatostatin-containing interneurons are considered distally targeting. Thus, they are likely to provide delayed dendritic inhibition during exploratory behavior, contributing to the maintenance of a balance between cortical excitation and inhibition while leaving a wide temporal window open for synaptic integration and plasticity in distal dendrites.

Somatodendritic Kv7/KCNQ/M Channels Control Interspike Interval in Hippocampal Interneurons

The M-current (I(M)), comprised of Kv7 channels, is a voltage-activated K+ conductance that plays a key role in the control of cell excitability. In hippocampal principal cells, I(M) controls action potential (AP) accommodation and contributes to the medium-duration afterhyperpolarization, but the role of I(M) in control of interneuron excitability remains unclear. Here, we investigated I(M) in hippocampal stratum oriens (SO) interneurons, both from wild-type and transgenic mice in which green fluorescent protein (GFP) was expressed in somatostatin-containing interneurons. Somatodendritic expression of Kv7.2 or Kv7.3 subunits was colocalized in a subset of GFP+ SO interneurons, corresponding to oriens-lacunosum moleculare (O-LM) cells. Under voltage clamp (VC) conditions at -30 mV, the Kv7 channel antagonists linopirdine/XE-991 abolished the I(M) amplitude present during relaxation from -30 to -50 mV and reduced the holding current (I(hold)). In addition, 0.5 mM tetraethylammonium reduced I(M), suggesting that I(M) was composed of Kv7.2-containing channels. In contrast, the Kv7 channel opener retigabine increased I(M) amplitude and I(hold). When strongly depolarized in VC, the linopirdine-sensitive outward current activated rapidly and comprised up to 20% of the total current. In current-clamp recordings from GFP+ SO cells, linopirdine induced depolarization and increased AP frequency, whereas retigabine induced hyperpolarization and arrested firing. In multicompartment O-LM interneuron models that incorporated I(M), somatodendritic placement of Kv7 channels best reproduced experimentally measured I(M). The models suggest that Kv3- and Kv7-mediated channels both rapidly activate during single APs; however, Kv3 channels control rapid repolarization of the AP, whereas Kv7 channels primarily control the interspike interval.

Electrophysiological Classification of Somatostatin-Positive Interneurons in Mouse Sensorimotor Cortex

Classification of inhibitory interneurons is critical in determining their role in normal information processing and pathophysiological conditions such as epilepsy. Classification schemes have relied on morphological, physiological, biochemical, and molecular criteria; and clear correlations have been demonstrated between firing patterns and cellular markers such as neuropeptides and calcium-binding proteins. This molecular diversity has allowed generation of transgenic mouse strains in which GFP expression is linked to the expression of one of these markers and presumably a single subtype of neuron. In the GIN mouse (EGFP-expressing Inhibitory Neurons), a subpopulation of somatostatin-containing interneurons in the hippocampus and neocortex is labeled with enhanced green fluorescent protein (EGFP). To optimize the use of the GIN mouse, it is critical to know whether the population of somatostatin-EGFP-expressing interneurons is homogeneous. We performed unsupervised cluster analysis on 46 EGFP-expressing interneurons, based on data obtained from whole cell patch-clamp recordings. Cells were classified according to a number of electrophysiological variables related to spontaneous excitatory postsynaptic currents (sEPSCs), firing behavior, and intrinsic membrane properties. EGFP-expressing interneurons were heterogeneous and at least four subgroups could be distinguished. In addition, multiple discriminant analysis was applied to data collected during whole cell recordings to develop an algorithm for predicting the group membership of newly encountered EGFP-expressing interneurons. Our data are consistent with a heterogeneous population of neurons based on electrophysiological properties and indicate that EGFP expression in the GIN mouse is not restricted to a single class of somatostatin-positive interneuron.

Early specification of striatal projection neurons and interneuronal subtypes in the lateral and medial ganglionic eminence

The striatum is thought to be generated from two transient swellings in the ventral telencephalon, the lateral and medial ganglionic eminences, present at mid-stages of embryonic rat development. We have studied the relative contribution of these structures to the specific generation of striatal neuronal subtypes such as projection neurons and cholinergic and somatostatin-containing interneurons at an early stage and a mid stage in striatal neurogenesis. Dissociated progenitors isolated from the embryonic day 12.5 and embryonic day 15.5 rat lateral ganglionic eminence grafted into the previously ibotenic acid lesioned adult striatum, produce grafts containing extensive numbers of neurons expressing messenger RNA for the striatal projection neuron marker, DARPP-32, whereas grafts of the embryonic day 12.5 and embryonic day 15.5 medial ganglionic eminences do not. While preprosomatostatin messenger RNA-expressing neurons were observed in grafts from each of the lateral ganglionic eminence and medial ganglionic eminence at both embryonic day 12.5 and embryonic day 15.5, choline acetyltransferase messenger RNA-expressing cholinergic neurons were largely found in grafts derived from the embryonic day 12.5 medial ganglionic eminence. These results suggest that the neuronal diversity of the adult striatum may derive both from the lateral ganglionic eminence, providing DARPP-32-expressing projection neurons as well as somatostatin-containing interneurons, and the early stage medial ganglionic eminence specifically contributing the cholinergic interneurons.

Anatomical analysis of the neurons expressing the acetylcholinesterase gene in the rat brain, with special reference to the striatum

The localization of the neurons expressing the acetylcholinesterase gene in the rat central nervous system was studied by in situ hybridization. The striatal and nigral neurons containing acetylcholinesterase messenger RNA were especially identified. Acetylcholinesterase messenger RNA was detected in numerous areas of the central nervous system, including cholinergic areas, like striatum, nucleus basalis of Meynert, septum and diagonal band of Broca, but also non-cholinergic areas, like the cerebral cortex, the hippocampus, the cerebellum and the raphe dorsalis. In the striatum, 75% of the neurons expressing the acetylcholinesterase gene were identified as cholinergic neurons and 25% as somatostatin-producing neurons. All dopaminergic neurons of the substantia nigra pars compacta and ventral tegmental area were demonstrated to express the acetylcholinesterase gene. Our results suggest that several neuronal populations could contribute to the presence of acetylcholinesterase in the striatum: the striatal cholinergic and somatostatin-containing interneurons, the nigral dopaminergic neurons and other neurons that may be the corticostriatal, thalamostriatal and raphe-striatal neurons. This demonstrates that, especially in the striatum, acetylcholinesterase is not a specific marker of the cholinergic neurons. The diversity of the origins of striatal acetylcholinesterase suggests a multiplicity of functions for this enzyme: besides its cholinolytic actions, it may also possibly play a non-cholinolytic role in neuromodulation.

Induction of immediate-early gene proteins in dentate granule cells and somatostatin interneurons after hippocampal seizures

The expression of the protein products of the immediate-early genes c-fos, Fos B, Fos-related proteins (FRAs), c-jun, jun B, jun D and krox-24 was investigated in the rat hippocampus at various times after electrically-induced hippocampal seizures. Hippocampal seizures induced all the immediate-early gene proteins in dentate granule cells with differing time-courses. In addition, Krox-24, Fos and Jun D were also induced in somatostatin-containing interneurons throughout the hippocampus and also in a small percentage of parvalbumin-containing interneurons. Thus, hippocampal seizures induce waves of immediate-early gene protein expression in dentate granule cells and a selective expression of krox-24, Fos and Jun D in hippocampal somatostatin interneurons. These results suggest that biochemical and/or morphological changes occurring in dentate granule cells and somatostatin interneurons after seizures may be regulated by immediate-early gene expression, and that these immediate-early gene proteins may be involved in seizure development in the nervous system.

Decreased hippocampal inhibition and a selective loss of interneurons in experimental epilepsy.

The occurrence of seizure activity in human temporal lobe epilepsy or status epilepticus is often associated with a characteristic pattern of cell loss in the hippocampus. An experimental model that replicates this pattern of damage in normal animals by electrical stimulation of the afferent pathway to the hippocampus was developed to study changes in structure and function that occur as a result of repetitive seizures. Hippocampal granule cell seizure activity caused a persistent loss of recurrent inhibition and irreversibly damaged adjacent interneurons. Immunocytochemical staining revealed unexpectedly that gamma-aminobutyric acid (GABA)-containing neurons, thought to mediate inhibition in this region and predicted to be damaged by seizures, had survived. In contrast, there was a nearly complete loss of adjacent somatostatin-containing interneurons and mossy cells that may normally activate inhibitory neurons. These results suggest that the seizure-induced loss of a basket cell-activating system, rather than a loss of inhibitory basket cells themselves, may cause disinhibition and thereby play a role in the pathophysiology and pathology of the epileptic state.

Extra-oral Nerve-block Analgesia for Dental Extractions

Thalamocortical neurons relay sensory and motor information to the neocortex using both single spikes and bursts; bursts prevail during low-vigilance states but also occur during awake behavior. Bursts are suggested to provide an alerting signal to the cortex and enhance stimulus detection, but the synaptic mechanisms underlying these effects are not clear, because the postsynaptic responses of different subtypes of cortical neurons to unitary thalamocortical bursts are mostly unknown. Using optogenetically guided recordings in mouse thalamocortical slices, we achieved the first reported paired intracellular recordings from nine monosynaptically connected thalamic and cortical neurons, including principal cells and two subtypes of inhibitory interneurons, and compared between cortical responses to single thalamocortical spikes and bursts. In 18 additional cortical neurons, we elicited unitary burst responses optogenetically. Short-term dynamics and temporal summation of burst-evoked EPSPs were cell-type dependent: in principal cells and somatostatin-containing (SOM), but not fast-spiking (FS), interneurons, peak response during a burst was on average more than twofold larger than the response to the first spike. Thus, firing a burst instead of a single spike would more than double the probability of firing in postsynaptic excitatory neurons and in SOM, but not FS, interneurons. Consistent with this prediction, FS interneurons held near firing threshold fired most often on the first burst component, whereas SOM interneurons fired only on the second or later components. By increasing excitation of principal cells together with SOM-mediated, distally directed inhibition, thalamocortical bursts could momentarily enhance the saliency of the ascending sensory stimulus over less urgent, top-down inputs. <b>SIGNIFICANCE STATEMENT</b> Thalamocortical neurons relay sensory and motor information to the cerebral cortex using both single spikes and high-frequency bursts, but the function of bursts is not fully understood. Using brain slices from mouse somatosensory thalamus and cortex, we achieved the first dual recordings of directly connected thalamic and cortical neurons and compared between cortical responses to single thalamic spikes and to bursts. We report that bursts enhanced the responses of excitatory neurons and of inhibitory interneurons that preferentially target dendrites. A potential consequence is that bursts will enhance the response to the immediate sensory event over responses to less urgent, modulatory inputs.