Striatal Low-threshold Spike Interneurons Research Articles

Emergence of novel functions in striatal low-threshold spike interneurons (Commentary on Gazan et al., 2019).

LTSIs selectively target distal dendrites of spiny projection neurons (SPNs, top). In this article by Gazan et al., the authors performed a selective ablation of LTSIs using Cre-dependent diphtheria toxin in SST-Cre mice, injected in the striatum. They demonstrate that the ablation of LTSIs (bottom) induced a decrease in the number of distal dendritic spines on SPNs associated with an increased intrinsic excitability of these cells. Behaviorally, this is associated with a potentiation of cocaine effect on locomotion. Whether these behavioral effects are the direct result of the loss of LTSIs or whether it involves rearrangements in glutamatergic (Glu) or dopaminergic (DA) synapses occurring normally on these dendritic spines remains to be explored. Commentary on Gazan A, Rial D, Schiffmann SN. Ablation of striatal somatostatin interneurons affects MSN morphology and electrophysiological properties, and increases cocaine induced hyperlocomotion in mice. Eur J Neurosci. 2020;51:1388-1402. https://doi.org/10.1111/ejn.14581.

Striatal Low-Threshold Spiking Interneurons Regulate Goal-Directed Learning

The dorsomedial striatum (DMS) is critically involved in motor control and reward processing, but the specific neural circuit mediators are poorly understood. Recent evidence highlights the extensive connectivity of low-threshold spiking interneurons (LTSIs) within local striatal circuitry; however, the invivo function of LTSIs remains largely unexplored. We employed fiber photometry to assess LTSI calcium activity in a range of DMS-mediated behaviors, uncovering specific reward-related activity that is down-modulated during goal-directed learning. Using two mechanistically distinct manipulations, we demonstrated that this down-modulation of LTSI activity is critical for acquisition of novel contingencies, but not for their modification. In contrast, continued LTSI activation slowed instrumental learning. Similar manipulations of fast-spiking interneurons did not reproduce these effects, implying a specific function of LTSIs. Finally, we revealed a role for the γ-aminobutyric acid (GABA)ergic functions of LTSIs in learning. Together, our data provide new insights into this striatal interneuron subclass as important gatekeepers of goal-directed learning.

Dichotomous Effects of Mu Opioid Receptor Activation on Striatal Low-Threshold Spike Interneurons.

Striatal low-threshold spike interneurons (LTSIs) are tonically active neurons that express GABA and nitric oxide synthase and are involved in information processing as well as neurovascular coupling. While mu opioid receptors (MORs) and their ligand encephalin are prominent in the striatum, their action on LTSIs has not been investigated. We addressed this issue carrying out whole-cell recordings in transgenic mice in which the NPY-expressing neurons are marked with green fluorescent protein (GFP). The MOR agonist (D-Ala(2), N-MePhe(4), Gly-ol)-enkephalin (DAMGO) produced dual effects on subpopulations of LTSIs. DAMGO caused inhibitory effects, accompanied by decreases of spontaneous firing, in 62% of LTSIs, while depolarizing effects (accompanied by an increase in spontaneous firing) were observed in 23% of LTSIs tested. The dual effects of DAMGO persisted in the presence of tetrodotoxin (TTX), a sodium channel blocker or in the presence of the nicotinic acetylcholine receptor antagonist mecamylamine. However, in the presence of either the GABAA receptor antagonist picrotoxin or the muscarinic cholinergic receptor antagonist atropine, DAMGO only elicited inhibitory effects on LTSIs. Furthermore, we found that DAMGO decreased the amplitude and frequency of spontaneous GABAergic events. Unexpectedly, these effects of DAMGO on spontaneous GABAergic events disappeared after blocking of the muscarinic and nicotinic cholinergic blockers, showing that GABA inputs to LTSIs are not directly modulated by presynaptic MORs. These finding suggest that activation of MORs affect LTSIs both directly and indirectly, through modulation of GABAergic and cholinergic tones. The complex balance between direct and indirect effects determines the net effect of DAMGO on LTSIs.

The ionic mechanism of membrane potential oscillations and membrane resonance in striatal LTS interneurons.

Striatal low-threshold spiking (LTS) interneurons spontaneously transition to a depolarized, oscillating state similar to that seen after sodium channels are blocked. In the depolarized state, whether spontaneous or induced by sodium channel blockade, the neurons express a 3- to 7-Hz oscillation and membrane impedance resonance in the same frequency range. The membrane potential oscillation and membrane resonance are expressed in the same voltage range (greater than -40 mV). We identified and recorded from LTS interneurons in striatal slices from a mouse that expressed green fluorescent protein under the control of the neuropeptide Y promoter. The membrane potential oscillation depended on voltage-gated calcium channels. Antagonism of L-type calcium currents (CaV1) reduced the amplitude of the oscillation, whereas blockade of N-type calcium currents (CaV2.2) reduced the frequency. Both calcium sources activate a calcium-activated chloride current (CaCC), the blockade of which abolished the oscillation. The blocking of any of these three channels abolished the membrane resonance. Immunohistochemical staining indicated anoctamin 2 (ANO2), and not ANO1, as the CaCC source. Biophysical modeling showed that CaV1, CaV2.2, and ANO2 are sufficient to generate a membrane potential oscillation and membrane resonance, similar to that in LTS interneurons. LTS interneurons exhibit a membrane potential oscillation and membrane resonance that are both generated by CaV1 and CaV2.2 activating ANO2. They can spontaneously enter a state in which the membrane potential oscillation dominates the physiological properties of the neuron.

Complex autonomous firing patterns of striatal low-threshold spike interneurons.

During sensorimotor learning, tonically active neurons (TANs) in the striatum acquire bursts and pauses in their firing based on the salience of the stimulus. Striatal cholinergic interneurons display tonic intrinsic firing, even in the absence of synaptic input, that resembles TAN activity seen in vivo. However, whether there are other striatal neurons among the group identified as TANs is unknown. We used transgenic mice expressing green fluorescent protein under control of neuronal nitric oxide synthase or neuropeptide-Y promoters to aid in identifying low-threshold spike (LTS) interneurons in brain slices. We found that these neurons exhibit autonomous firing consisting of spontaneous transitions between regular, irregular, and burst firing, similar to cholinergic interneurons. As in cholinergic interneurons, these firing patterns arise from interactions between multiple intrinsic oscillatory mechanisms, but the mechanisms responsible differ. Both neurons maintain tonic firing because of persistent sodium currents, but the mechanisms of the subthreshold oscillations responsible for irregular firing are different. In LTS interneurons they rely on depolarization-activated noninactivating calcium currents, whereas those in cholinergic interneurons arise from a hyperpolarization-activated potassium conductance. Sustained membrane hyperpolarizations induce a bursting pattern in LTS interneurons, probably by recruiting a low-threshold, inactivating calcium conductance and by moving the membrane potential out of the activation range of the oscillatory mechanisms responsible for single spiking, in contrast to the bursting driven by noninactivating currents in cholinergic interneurons. The complex intrinsic firing patterns of LTS interneurons may subserve differential release of classic and peptide neurotransmitters as well as nitric oxide.

Autonomous firing of low-threshold spike interneurons in the striatum

Specific striatal neuronal subtypes are classified as either phasically or tonically active. Of the four major neuronal subtypes of the striatum, the firing patterns of three have been studied in detail. Spiny projection neurons and fast-spiking interneurons fire phasically in response to synaptic input and are silent when input is absent. Cholinergic (ACh) interneurons are tonically active even in the absence of synaptic input. These ACh interneurons were the only known tonically active neurons in the striatum until recently. We have found that the low-threshold spike (LTS) interneurons also belong to this classification of striatal tonically active neurons, presenting a new aspect to the striatal oscillatory network. Striatal LTS interneurons are GABAergic interneurons that also contain neuronal nitric oxide synthase (nNOS), neuropeptide Y (NPY), and somatostatin. We used transgenic mice expressing green fluorescent protein (GFP) under control of the nNOS or NPY promoters to help identify LTS interneurons. GFP positive neurons were examined using whole-cell and cell-attached recording techniques to investigate firing patterns and possible mechanisms of such activity. GFP positive neurons displayed the morphological properties characteristic of LTS interneurons, and fired strong rebound bursts at the offset of hyperpolarizing current pulses. LTS interneurons were spontaneously active and this activity persisted in the presence of synaptic blockers. When compared with ACh interneurons, LTS interneurons fired at higher rates and with less regularity. Similar to ACh interneurons, LTS interneurons were capable of firing in a variety of firing patterns, including regular single spiking, irregular single spiking, and bursting combined with periods of regular and irregular single spiking. LTS interneurons were observed spontaneously shifting between these firing patterns. Whole-cell voltage clamp recordings revealed a tetrodotoxin-sensitive region of negative slope conductance in the subthreshold range consistent with a persistent sodium current. An absence of a zero current crossing in the I-V curve combined with a linear F-I curve suggest these interneurons possess a Hodgkin’s class 1 oscillation mechanism. Tonically active neurons in the striatum have been shown to become strongly synchronized with the beta oscillation seen in Parkinson’s disease. These strongly synchronized neurons may include LTS interneurons. In addition, both ACh and LTS interneurons are capable of endogenous oscillations in the beta frequency range. Our findings place LTS interneurons in the subtype of tonically active neurons in the striatum and in turn change our understanding of how synchronization may occur in the striatum.