Firing Differences Between Adult Intralaminar Thalamo-striatal Neurons.

Differences in the intrinsic properties of intralaminar thalamo-striatal neurons such as expressing low-threshold-spikes (LTS) or after hyperpolarizing potentials (AHPs) of different duration have been attributed to different maturation stages. However, two morphological types: "diffuse" and "bushy" have been described. Therefore, we explored whether electrophysiological differences persist in adult mice using whole cell recordings. Some recorded neurons were identified by intracellular labeling with biocytin and double labeling with retrograde or anterograde tracings using Cre-mice. We classified these neurons by their AHPs during spontaneous firing. Neurons with long duration AHPs, with fast and slow components, were mostly found in the parafascicular (Pf) nucleus. Neurons with brief AHPs were mainly found in the central lateral (CL) nucleus. However, neurons with both AHPs were found in both nuclei in different proportions. Firing frequency adaptation differed between these neuron classes: those with prolonged AHPs exhibited firing frequency adaptation with fast and slow time constants whereas those with brief AHPs were slow adapters. Neurons with more prolonged AHPs had significant higher input resistances than neurons with brief AHPs. Both cell classes could fire in two modes: trains of single action potentials at depolarized potentials or high frequency bursts on top of LTS at more hyperpolarized potentials. LTS were probably generated by Cav3 calcium channels since they were blocked by the selective antagonist TTA-P2. About 11% of neurons with brief AHPs and 55% of neurons with prolonged AHPs do not show LTS and bursts, even when potassium currents are blocked.

Activation of parvalbumin-expressing neurons reconfigures neuronal ensembles in murine striatal microcircuits.

The striatum is the largest entrance to the basal ganglia. Diverse neuron classes make up striatal microcircuit activity, consisting in the sequential activation of neuronal ensembles. How different neuron classes participate in generating ensemble sequences is unknown. In control mus musculus brain slices in vitro, providing excitatory drive generates ensemble sequences. In Parkinsonian microcircuits captured by a highly recurrent ensemble, a cortical stimulus causes a transitory reconfiguration of neuronal groups alleviating Parkinsonism. Alternation between neuronal ensembles needs interconnectivity, in part due to interneurons, preferentially innervated by incoming afferents. One main class of interneuron expresses parvalbumin (PV+ neurons) and mediates feed-forward inhibition. However, its more global actions within the microcircuit are unknown. Using calcium imaging in ex vivo brain slices simultaneously recording dozens of neurons, we aimed to observe the actions of PV+ neurons within the striatal microcircuit. PV+ neurons in active microcircuits are 5%-11% of the active neurons even if, anatomically, they are <1% of the total neuronal population. In resting microcircuits, optogenetic activation of PV+ neurons turns on circuit activity by activating or disinhibiting, more neurons than those actually inhibited, showing that feed-forward inhibition is not their only function. Optostimulation of PV+ neurons in active microcircuits inhibits and activates different neuron sets, resulting in the reconfiguration of neuronal ensembles by changing their functional connections and ensemble membership, showing that neurons may belong to different ensembles at different situations. Our results show that PV+ neurons participate in the mechanisms that generate alternation of neuronal ensembles, therefore provoking ensemble sequences.

Localization of chloride co-transporters in striatal neurons.

The ionic driving force for the chloride-permeable GABAA receptor is subject to spatial control and distribution of chloride transporters. NKCC1 and KCC2 are mostly expressed in neurons in a specific manner. In the striatum, the localization of these transporters in identified neurons is unknown. In this study, the expression of these transporters was found to be different between projection neurons and interneurons. NKCC1 immunoreactivity was observed in the soma of adult BAC-D1-eGFP+ and D2-eGFP+ striatal projection neurons (SPNs). KCC2 was not expressed in either projection neuron and immunoreactivity to this transporter was observed only in the neuropile. However, NKCC1 and KCC2 co-transporters were not localized in intracellular biocytin-injected dendrites of SPNs of the direct or indirect pathways (D1-SPNs and D2-SPNs). Experiments with PV Cre transgenic mice transfected with Cre-dependent adeno-associated viruses containing tdTomato in the striatum showed a cell-type-specific distribution of KCC2 chloride transporter co-expression associated with PV interneurons. Thus, depolarizing actions of GABA responses in adult projection neurons can be explained by the expression and somatic localization of the NKCC1 transporters. A somato/dendritic distribution of KCC2 expression was observed only in striatal interneurons and corresponds to the hyperpolarizing action of GABA recorded in these cells. This correlates the different roles for GABA actions in striatal neuronal excitability with the expression of specific chloride transporters.

Activation of Glutamatergic Fibers in the Anterior NAc Shell Modulates Reward Activity in the aNAcSh, the Lateral Hypothalamus, and Medial Prefrontal Cortex and Transiently Stops Feeding.

Although the release of mesoaccumbal dopamine is certainly involved in rewarding responses, recent studies point to the importance of the interaction between it and glutamate. One important component of this network is the anterior nucleus accumbens shell (aNAcSh), which sends GABAergic projections into the lateral hypothalamus (LH) and receives extensive glutamatergic inputs from, among others, the medial prefrontal cortex (mPFC). The effects of glutamatergic activation of aNAcSh on the ingestion of rewarding stimuli as well as its effect in the LH and mPFC are not well understood. Therefore, we studied behaving mice that express a light-gated channel (ChR2) in glutamatergic fibers in their aNAcSh while recording from neurons in the aNAcSh, or mPFC or LH. In Thy1-ChR2, but not wild-type, mice activation of aNAcSh fibers transiently stopped the mice licking for sucrose or an empty sipper. Stimulation of aNAcSh fibers both activated and inhibited single-unit responses aNAcSh, mPFC, and LH, in a manner that maintains firing rate homeostasis. One population of licking-inhibited pMSNs in the aNAcSh was also activated by optical stimulation, suggesting their relevance in the cessation of feeding. A rewarding aspect of stimulation of glutamatergic inputs was found when the Thy1-ChR2 mice learned to nose-poke to self-stimulate these inputs, indicating that bulky stimulation of these fibers are rewarding in the sense of wanting. Stimulation of excitatory afferents evoked both monosynaptic and polysynaptic responses distributed in the three recorded areas. In summary, we found that activation of glutamatergic aNAcSh fibers is both rewarding and transiently inhibits feeding. SIGNIFICANCE STATEMENT: We have established that the activation of glutamatergic fibers in the anterior nucleus accumbens shell (aNAcSh) transiently stops feeding and yet, because mice self-stimulate, is rewarding in the sense of wanting. Moreover, we have characterized single-unit responses of distributed components of a hedonic network (comprising the aNAcSh, medial prefrontal cortex, and lateral hypothalamus) recruited by activation of glutamatergic aNAcSh afferents that are involved in encoding a positive valence signal important for the wanting of a reward and that transiently stops ongoing consummatory actions, such as licking.

Modulation of direct pathway striatal projection neurons by muscarinic M4-type receptors.

Models of basal ganglia (BG) function posit a dynamic balance between two classes of striatal projection neurons (SPNs): direct pathway neurons (dSPNs) that facilitate movements, and indirect pathway neurons (iSPNs) that repress movement execution. Two main modulatory transmitters regulate the output of these neurons: dopamine (DA) and acetylcholine (ACh). dSPNs express D1-type DA, M1-and M4-type ACh receptors, while iSPNs express D2-type DA and M1-type ACh receptors. Actions of M1-, D1-, and D2-receptors have been extensively reported, but we still ignore most actions of muscarinic M4-type receptors. Here, we used whole-cell recordings in acutely dissociated neurons, pharmacological tools such as mamba-toxins, and BAC D(1 or 2)-eGFP transgenic mice to show that activation of M4-type receptors with bath applied muscarine enhances Ca(2+)-currents through CaV1-channels in dSPNs and not in iSPNs. This action increases excitability of dSPNs after both direct current injection and synaptically driven stimulation. The increases in Ca(2+)-current and excitability were blocked specifically by mamba toxin-3, suggesting mediation via M4-type receptors. M4-receptor activation also increased network activity of dSPNs but not of iSPNs as seen with calcium-imaging techniques. Moreover, actions of D1-type and M4-type receptors may add to produce a larger enhancement of excitability of dSPNs or, paradoxically, oppose each other depending on the order of their activation. Possible implications of these findings are discussed.