Fatuel Tecuapetla

Glutamatergic Signaling by Mesolimbic Dopamine Neurons in the Nucleus Accumbens

Recent evidence suggests the intriguing possibility that midbrain dopaminergic (DAergic) neurons may use fast glutamatergic transmission to communicate with their postsynaptic targets. Because of technical limitations, direct demonstration of the existence of this signaling mechanism has been limited to experiments using cell culture preparations that often alter neuronal function including neurotransmitter phenotype. Consequently, it remains uncertain whether glutamatergic signaling between DAergic neurons and their postsynaptic targets exists under physiological conditions. Here, using an optogenetic approach, we provide the first conclusive demonstration that mesolimbic DAergic neurons in mice release glutamate and elicit excitatory postsynaptic responses in projection neurons of the nucleus accumbens. In addition, we describe the properties of the postsynaptic glutamatergic responses of these neurons during experimentally evoked burst firing of DAergic axons that reproduce the reward-related phasic population activity of the mesolimbic projection. These observations indicate that, in addition to DAergic mechanisms, mesolimbic reward signaling may involve glutamatergic transmission.

Basal ganglia subcircuits distinctively encode the parsing and concatenation of action sequences

Chunking allows the brain to efficiently organize memories and actions. Although basal ganglia circuits have been implicated in action chunking, little is known about how individual elements are concatenated into a behavioral sequence at the neural level. Using a task in which mice learned rapid action sequences, we uncovered neuronal activity encoding entire sequences as single actions in basal ganglia circuits. In addition to neurons with activity related to the start/stop activity signaling sequence parsing, we found neurons displaying inhibited or sustained activity throughout the execution of an entire sequence. This sustained activity covaried with the rate of execution of individual sequence elements, consistent with motor concatenation. Direct and indirect pathways of basal ganglia were concomitantly active during sequence initiation, but behaved differently during sequence performance, revealing a more complex functional organization of these circuits than previously postulated. These results have important implications for understanding the functional organization of basal ganglia during the learning and execution of action sequences.

Encoding Network States by Striatal Cell Assemblies

Correlated activity in cortico-basal ganglia circuits plays a key role in the encoding of movement, associative learning and procedural memory. How correlated activity is assembled by striatal microcircuits is not understood. Calcium imaging of striatal neuronal populations, with single-cell resolution, reveals sporadic and asynchronous activity under control conditions. However, N-methyl-d-aspartate (NMDA) application induces bistability and correlated activity in striatal neurons. Widespread neurons within the field of observation present burst firing. Sets of neurons exhibit episodes of recurrent and synchronized bursting. Dimensionality reduction of network dynamics reveals functional states defined by cell assemblies that alternate their activity and display spatiotemporal pattern generation. Recurrent synchronous activity travels from one cell assembly to the other often returning to the original assembly; suggesting a robust structure. An initial search into the factors that sustain correlated activity of neuronal assemblies showed a critical dependence on both intrinsic and synaptic mechanisms: blockage of fast glutamatergic transmission annihilates all correlated firing, whereas blockage of GABAergic transmission locked the network into a single dominant state that eliminates assembly diversity. Reduction of L-type Ca(2+)-current restrains synchronization. Each cell assembly comprised different cells, but a small set of neurons was shared by different assemblies. A great proportion of the shared neurons was local interneurons with pacemaking properties. The network dynamics set into action by NMDA in the striatal network may reveal important properties of striatal microcircuits under normal and pathological conditions.

Electrophysiological and Morphological Characteristics and Synaptic Connectivity of Tyrosine Hydroxylase-Expressing Neurons in Adult Mouse Striatum

Whole-cell recordings were obtained from tyrosine hydroxylase-expressing (TH+) neurons in striatal slices from bacterial artificial chromosome transgenic mice that synthesize enhanced green fluorescent protein (EGFP) selectively in neurons expressing TH transcriptional regulatory sequences. Stereological cell counting indicated that there were ∼2700 EGFP–TH+ neurons/striatum. Whole-cell recordings in striatal slices demonstrated that EGFP–TH+ neurons comprise four electrophysiologically distinct neuron types whose electrophysiological properties have not been reported previously in striatum. EGFP–TH+ neurons were identified in retrograde tracing studies as interneurons. Recordings from synaptically connected pairs of EGFP–TH+ interneurons and spiny neurons showed that the interneurons elicited GABAergic IPSPs/IPSCs in spiny neurons powerful enough to significantly delay evoked spiking. EGFP–TH+ interneurons responded to local or cortical stimulation with glutamatergic EPSPs. Local stimulation also elicited GABAA IPSPs, at least some of which arose from identified spiny neurons. Single-cell reverse transcription-PCR showed expression of VMAT1 in EGFP–TH+ interneurons, consistent with previous suggestions that these interneurons may be dopaminergic as well as GABAergic. All four classes of interneurons were medium sized with modestly branching, varicose dendrites, and dense, highly varicose axon collateral fields. These data show for the first time that there exists in the normal rodent striatum a substantial population of TH+/GABAergic interneurons comprising four electrophysiologically distinct subtypes whose electrophysiological properties differ significantly from those of previously described striatal GABAergic interneurons. These interneurons are likely to play an important role in striatal function through fast GABAergic synaptic transmission in addition to, and independent of, their potential role in compensation for dopamine loss in experimental or idiopathic Parkinsons disease.

A Novel Functionally Distinct Subtype of Striatal Neuropeptide Y Interneuron

We investigated the properties of neostriatal neuropeptide Y (NPY)-expressing interneurons in transgenic GFP (green fluorescent protein)-NPY reporter mice. In vitro whole-cell recordings and biocytin staining demonstrated the existence of a novel class of neostriatal NPY-expressing GABAergic interneurons that exhibit electrophysiological, neurochemical, and morphological properties strikingly different from those of previously described NPY-containing, plateau-depolarization low-threshold spike (NPY–PLTS) interneurons. The novel NPY interneuron type (NPY–neurogliaform) differed from previously described NPY–PLTS interneurons by exhibiting a significantly lower input resistance and hyperpolarized membrane potential, regular, nonaccommodating spiking in response to depolarizing current injections, and an absence of plateau depolarizations or low-threshold spikes. NPY–neurogliaform interneurons were also easily distinguished morphologically by their dense, compact, and highly branched dendritic and local axonal arborizations that contrasted sharply with the sparse and extended axonal and dendritic arborizations of NPY–PLTS interneurons. Furthermore, NPY–neurogliaform interneurons did not express immunofluorescence for somatostatin or nitric oxide synthase that was ubiquitous in NPY–PLTS interneurons. IPSP/Cs could only rarely be elicited in spiny projection neurons (SPNs) in paired recordings with NPY–PLTS interneurons. In contrast, the probability of SPN innervation by NPY–neurogliaform interneurons was extremely high, the synapse very reliable (no failures were observed), and the resulting postsynaptic response was a slow, GABAA receptor-mediated IPSC that has not been previously described in striatum but that has been elicited from NPY–GABAergic neurogliaform interneurons in cortex and hippocampus. These properties suggest unique and distinctive roles for NPY–PLTS and NPY–neurogliaform interneurons in the integrative properties of the neostriatum.

Balanced activity in basal ganglia projection pathways is critical for contraversive movements

The basal ganglia, and the striatum in particular, have been implicated in the generation of contraversive movements. The striatum projects to downstream basal ganglia nuclei through two main circuits, originating in striatonigral and striatopallidal neurons, and different models postulate that the two pathways can work in opposition or synergistically. Here we show striatonigral and striatopallidal neurons are concurrently active during spontaneous contraversive movements. Furthermore, we show that unilateral optogenetic inhibition of either or both projection pathways disrupts contraversive movements. Consistently, simultaneous activation of both neuron types produces contraversive movements. Still, we also show that imbalanced activity between the pathways can result in opposing movements being driven by each projection pathway. These data show that balanced activity in both striatal projection pathways is critical for the generation of contraversive movements and highlights that imbalanced activity between the two projection pathways can result in opposing motor output.

Dopaminergic modulation of short-term synaptic plasticity at striatal inhibitory synapses.

Circuit properties, such as the selection of motor synergies, have been posited as relevant tasks for the recurrent inhibitory synapses between spiny projection neurons of the neostriatum, a nucleus of the basal ganglia participating in procedural learning and voluntary motor control. Here we show how the dopaminergic system regulates short-term plasticity (STP) in these synapses. STP is thought to endow neuronal circuits with computational powers such as gain control, filtering, and the emergence of transitory net states. But little is known about STP regulation. Employing unitary and population synaptic recordings, we observed that activation of dopamine receptors can modulate STP between spiny neurons. A D1-class agonist enhances, whereas a D2-class agonist decreases, short-term depression most probably by synaptic redistribution. Presynaptic receptors appear to be responsible for this modulation. In contrast, STP between fast-spiking interneurons and spiny projection neurons is largely unregulated despite expressing presynaptic receptors. Thus, the present experiments provide an explanation for dopamine actions at the circuit level: the control of STP between lateral connections of output neurons and the reorganization of the balance between different forms of inhibitory transmission. Theoretically, D1 receptors would promote a sensitive, responsive state for temporal precision (dynamic component), whereas D2 receptors would sense background activity (static component).

Differential Dopaminergic Modulation of Neostriatal Synaptic Connections of Striatopallidal Axon Collaterals

Recent studies have demonstrated that GABAergic synaptic transmission among neostriatal spiny projection neurons (SPNs) is strongly modulated by dopamine with individual connections exhibiting either D1 receptor (D1R)-mediated facilitation or D2 receptor (D2R)-mediated inhibition and, at least in some preparations, a subset of connections exhibiting both of these effects. In light of the cell type-specific expression of D1aR in striatonigral and D2R in striatopallidal neurons and the differential expression of the other D1 and D2 family dopamine receptors, we hypothesize that the nature of the dopaminergic modulation is specific to the types of SPNs that participate in the connection. Here the biophysical properties and dopaminergic modulation of intrastriatal connections formed by striatopallidal neurons were examined. Contrary to previous expectation, synapses formed by striatopallidal neurons were biophysically and pharmacologically heterogeneous. Two distinct types of axon collateral connections could be distinguished among striatopallidal neurons. The more common, small-amplitude connections (80%) exhibited mean IPSC amplitudes several times smaller than their less frequent large-amplitude counterparts, principally because of a smaller number of release sites involved. The two types of connections were also differentially regulated by dopamine. Small-amplitude connections exhibited strong and exclusively D2R-mediated presynaptic inhibition, whereas large-amplitude connections were unresponsive to dopamine. Synaptic connections from striatopallidal to striatonigral neurons exhibited exclusively D2R-mediated presynaptic inhibition that was similar to the regulation of small-amplitude connections between pairs of striatopallidal cells. Together, these findings demonstrate a previously unrecognized complexity in the organization and dopaminergic control of synaptic communication among SPNs.

Activation of the Cholinergic System Endows Compositional Properties to Striatal Cell Assemblies

Striatal cell assemblies are thought to encode network states related to associative learning, procedural memory, and the sequential organization of behavior. Cholinergic neurotransmission modulates memory processes in the striatum and other brain structures. This work asks if the activity of striatal microcircuits observed in living nervous tissue, with attributes similar to cell assemblies, exhibit some of the properties proposed to be necessary to compose memory traces. Accordingly, we used whole cell and calcium-imaging techniques to investigate the cholinergic modulation of striatal neuron pools that have been reported to exhibit several properties expected from cell assemblies such as synchronous states of activity and the alternation of this activity among different neuron pools. We analyzed the cholinergic modulation of the activity of neuron pools with multidimensional reduction techniques and vectorization of network dynamics. It was found that the activation of the cholinergic system enables striatal cell assemblies with properties that have been posited for recurrent neural artificial networks with memory storage capabilities. Graph theory techniques applied to striatal network states revealed sequences of vectors with a recursive dynamics similar to closed reverberating cycles. The cycles exhibited a modular architecture and a hierarchical organization. It is then concluded that, under certain conditions, the cholinergic system enables the striatal microcircuit with the ability to compose complex sequences of activity. Neuronal recurrent networks with the characteristics encountered in the present experiments are proposed to allow repeated sequences of activity to become memories and repeated memories to compose learned motor procedures.

Direct and indirect dorsolateral striatum pathways reinforce different action strategies.

Summary The basal ganglia, and the striatum in particular, are critical for action reinforcement 1, 2. The dorsal striatum, which can be further subdivided into dorsomedial (DMS) and dorsolateral (DLS) striatum, is mainly composed of two subpopulations of striatal medium spiny projection neurons (MSNs): dopamine D1 receptor-expressing MSNs that constitute the striatonigral or direct pathway (dMSNs); and dopamine D2 receptor-expressing MSNs that constitute the striatopallidal or indirect pathway (iMSNs) [3]. It has been suggested that each pathway has opposing roles in reinforcement, with dMSNs being important to learn positive reinforcement and iMSNs to learn to avoid undesired actions (Go/No-Go) [1]. Furthermore, optogenetic self-stimulation of dMSNs in DMS leads to reinforcement of actions, while self-stimulation of iMSNs leads to avoidance of actions [2]. However, in DLS, which has been implicated in the consolidation of well-trained actions and habits in mice 4, 5, both pathways are active during lever-pressing for reward [6]. Furthermore, extensive skill training leads to long-lasting potentiation of glutamatergic inputs into both dMSNs and iMSNs [4]. We report here that, in DLS, both dMSNs and iMSNs are involved in positive reinforcement, but support different action strategies.