Dagoberto Tapia

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.

Inhibitory action of dopamine involves a subthreshold Cs+-sensitive conductance in neostriatal neurons

Intracellular recordings in in vitro slice preparations of rat brain were used to compare the actions of dopamine and dopamine receptor agonists on the subthreshold membrane properties of neostriatal neurons. A reproducible response for dopaminergic agonists was evoked after firing produced by current ramp injections that induced a subthreshold voltage displacement. Dopamine (10–100 μM) decreased both firing rate and membrane slope input resistance in virtually all cells tested. Input resistance change appeared as an increase in inward rectification. Approximate reversal potential was around -87 mV. The D1 receptor agonists SKF 38393 and C1-APB (1–10 μM) mimicked both dopamine effects with a reversal potential around -89 mV. The effects were blocked by the presence of 5–10 μM caesium (Cs+) but not by 1 μM tetrodotoxin, suggesting that main D1 effects on input resistance are due to subthreshold Cs+ sensitive conductances. cAMP analogues mimicked the actions of D1 receptor agonists. The D2 agonist, quinpirole (1–10 μM), did not produce any input resistance change, nonetheless, it still produced a decrease in firing rate. This suggests that the main D2 effect on firing is due to actions on suprathreshold ion conductances. All effects were blocked by D1 and D2 antagonists, respectively. D1 or D2 effects were found in the majority of cells tested.

Different corticostriatal integration in spiny projection neurons from direct and indirect pathways

The striatum is the principal input structure of the basal ganglia. Major glutamatergic afferents to the striatum come from the cerebral cortex and make monosynaptic contacts with medium spiny projection neurons (MSNs) and interneurons. Also: glutamatergic afferents to the striatum come from the thalamus. Despite differences in axonal projections, dopamine (DA) receptors expression and differences in excitability between MSNs from “direct” and “indirect” basal ganglia pathways, these neuronal classes have been thought as electrophysiologically very similar. Based on work with bacterial artificial chromosome (BAC) transgenic mice, here it is shown that corticostriatal responses in D1- and D2-receptor expressing MSNs (D1- and D2-MSNs) are radically different so as to establish an electrophysiological footprint that readily differentiates between them. Experiments in BAC mice allowed us to predict, with high probability (P > 0.9), in rats or non-BAC mice, whether a recorded neuron, from rat or mouse, was going to be substance P or enkephalin (ENK) immunoreactive. Responses are more prolonged and evoke more action potentials in D1-MSNs, while they are briefer and exhibit intrinsic autoregenerative responses in D2-MSNs. A main cause for these differences was the interaction of intrinsic properties with the inhibitory contribution in each response. Inhibition always depressed corticostriatal depolarization in D2-MSNs, while it helped in sustaining prolonged depolarizations in D1-MSNs, in spite of depressing early discharge. Corticostriatal responses changed dramatically after striatal DA depletion in 6-hydroxy-dopamine (6-OHDA) lesioned animals: a response reduction was seen in substance P (SP)+ MSNs whereas an enhanced response was seen in ENK+ MSNs. The end result was that differences in the responses were greatly diminished after DA depletion.

Presynaptic α4β2 nicotinic acetylcholine receptors increase glutamate release and serotonin neuron excitability in the dorsal raphe nucleus.

Several behavioral effects of nicotine are mediated by changes in serotonin (5-HT) release in brain areas that receive serotonergic afferents from the dorsal raphe nucleus (DRN). In vitro experiments have demonstrated that nicotine increases the firing activity in the majority of DRN 5-HT neurons and that DRN contains nicotinic acetylcholine receptors (nAChRs) located at both somata and presynaptic elements. One of the most common presynaptic effects of nicotine is to increase glutamate release. Although DRN receives profuse glutamatergic afferents, the effect of nicotine on glutamate release in the DRN has not been studied in detail. Using whole-cell recording techniques, we investigated the effects of nicotine on the glutamatergic input to 5-HT DRN neurons in rat midbrain slices. Low nicotine concentrations, in the presence of bicuculline and tetrodotoxin (TTX), increased the frequency but did not change the amplitude of glutamate-induced EPSCs, recorded from identified 5-HT neurons. Nicotine-induced increase of glutamatergic EPSC frequency persisted 10–20 min after drug withdrawal. This nicotinic effect was mimicked by exogenous administration of acetylcholine (ACh) or inhibition of ACh metabolism. In addition, the nicotine-induced increase in EPSC frequency was abolished by blockade of α4β2 nAChRs, voltage-gated calcium channels, or intracellular calcium signaling but not by α7 nAChR antagonists. These data suggest that both nicotine and endogenous ACh can increase glutamate release through activation of presynaptic α4β2 but not α7 nAChRs in the DRN. The effect involves long-term changes in synaptic function, and it is dependent on voltage-gated calcium channels and presynaptic calcium stores.

Serotoninergic dorsal raphe neurons possess functional postsynaptic nicotinic acetylcholine receptors

Very few neurons in the telencephalon have been shown to express functional postsynaptic nicotinic acetylcholine receptors (nAChRs), among them, the noradrenergic and dopaminergic neurons. However, there is no evidence for postsynaptic nAChRs on serotonergic neurons. In this study, we asked if functional nAChRs are present in serotonergic (5‐HT) and nonserotonergic (non‐5‐HT) neurons of the dorsal raphe nucleus (DRN). In rat midbrain slices, field stimulation at the tegmental pedunculopontine (PPT) nucleus evoked postsynaptic currents (eEPSCs) with different components in DRN neurons. After blocking the glutamatergic and GABAergic components, the remaining eEPSCs were blocked by mecamylamine and reduced by either the selective α7 nAChR antagonist methyllycaconitine (MLA) or the selective α4β2 nAChR antagonist dihydro‐β‐eritroidine (DHβE). Simultaneous addition of MLA and DHβE blocked all eEPSCs. Integrity of the PPT‐DRN pathway was assessed by both anterograde biocytin tracing and antidromic stimulation from the DRN. Inward currents evoked by the direct application of acetylcholine (ACh), in the presence of atropine and tetrodotoxin, consisted of two kinetically different currents: one was blocked by MLA and the other by DHβE; in both 5‐HT and non‐5‐HT DR neurons. Analysis of spontaneous (sEPSCs) and evoked (eEPSCs) synaptic events led to the conclusion that nAChRs were located at the postsynaptic membrane. The possible implications of these newly described nAChRs in various physiological processes and behavioral events, such as the wake‐sleep cycle, are discussed. Synapse 62:601–615, 2008.

Dopaminergic Modulation of the Striatal Microcircuit: Receptor-Specific Configuration of Cell Assemblies

Selection and inhibition of motor behaviors are related to the coordinated activity and compositional capabilities of striatal cell assemblies. Striatal network activity represents a main step in basal ganglia processing. The dopaminergic system differentially regulates distinct populations of striatal medium spiny neurons (MSNs) through the activation of D1- or D2-type receptors. Although postsynaptic and presynaptic actions of these receptors are clearly different in MSNs during cell-focused studies, their activation during network activity has shown inconsistent responses. Therefore, using electrophysiological techniques, functional multicell calcium imaging, and neuronal population analysis in rat corticostriatal slices, we describe the effect of selective dopaminergic receptor activation in the striatal network by observing cell assembly configurations. At the microcircuit level, during striatal network activity, the selective activation of either D1- or D2-type receptors is reflected as overall increases in neuronal synchronization. However, graph theory techniques applied to the transitions between network states revealed receptor-specific configurations of striatal cell assemblies: D1 receptor activation generated closed trajectories with high recurrence and few alternate routes favoring the selection of specific sequences, whereas D2 receptor activation created trajectories with low recurrence and more alternate pathways while promoting diverse transitions among neuronal pools. At the single-cell level, the activation of dopaminergic receptors enhanced the negative-slope conductance region (NSCR) in D1-type-responsive cells, whereas in neurons expressing D2-type receptors, the NSCR was decreased. Consequently, receptor-specific network dynamics most probably result from the interplay of postsynaptic and presynaptic dopaminergic actions.

The Balance of Striatal Feedback Transmission Is Disrupted in a Model of Parkinsonism

Inhibitory connections among striatal projection neurons (SPNs) called “feedback inhibition,” have been proposed to endow the striatal microcircuit with computational capabilities, such as motor sequence selection, filtering, and the emergence of alternating network states. These properties are disrupted in models of Parkinsonism. However, the impact of feedback inhibition in the striatal network has remained under debate. Here, we test this inhibition at the microcircuit level. We used optical and electrophysiological recordings in mice and rats to demonstrate the action of striatal feedback transmission in normal and pathological conditions. Dynamic calcium imaging with single-cell resolution revealed the synchronous activation of a pool of identified SPNs by antidromic stimulation. Using bacterial artificial chromosome-transgenic mice, we demonstrate that the activated neuron pool equally possessed cells from the direct and indirect basal ganglia pathways. This pool inhibits itself because of its own GABA release when stimuli are frequent enough, demonstrating functional and significant inhibition. Blockade of GABAA receptors doubled the number of responsive neurons to the same stimulus, revealing a second postsynaptic neuron pool whose firing was being arrested by the first pool. Stronger connections arise from indirect SPNs. Dopamine deprivation impaired striatal feedback transmission disrupting the ability of a neuronal pool to arrest the firing of another neuronal pool. We demonstrate that feedback inhibition among SPNs is strong enough to control the firing of cell ensembles in the striatal microcircuit. However, to be effective, feedback inhibition should arise from synchronized pools of SPNs whose targets are other SPNs pools.

Passive properties of neostriatal neurons during potassium conductance blockade

Abstract Voltage recordings from neostriatal projection neurons were obtained using in vitro intracellular techniques before and during K+-conductance blockade. Neurons were stained with the biocytin technique. Somatic surface area (AS) was determined by both whole-cell recordings in isolated somata and by measuring stained somata recorded in slices. Dendritic measurements were done in reconstructed neurons. Average determinations of dendritic (AD) and neuronal (AN) surface areas coincided with previously reported anatomical data. Thus: AS≈ 6.5 × 10–6 cm2; AD≈ 1.9 × 10–4 cm2; AN≈AD + AS≈ 2 × 10–4 cm2; AD/AS≈ 30. Measurements were done before and after superfusion with K+-conductance blockers (K+-blockers). Cells whose neuronal morphology was not obviously distorted by K+-blockade were chosen for the present study. Electrotonic transients were matched to a somatic shunt equivalent cylinder model adjusted with the generalized correction factor (Fdga) that constrains the parameters for neuronal anatomy. Neuronal input resistance (RN; mean ± SEM) increased when it was corrected for somatic shunt, from 49 ± 2 MΩ (n = 80) to 179 ± 7 MΩ (n = 32). A difference was also obtained between the slowest time constant, τ0 = 16 ± 0.9 ms (n = 49), and the dendritic membrane time constant, τmD = 33 ± 1.6 ms (n = 36). When these electrophysiological measurements were used to calculate AN, the value obtained was similar to the anatomical measurements. Combining anatomical and electrophysiological data, somatic and dendritic input resistances were determined: RD = 182 ± 7 MΩ; RS (with shunt) = 74 ± 4 MΩ (n = 32). The generalized correction factor, Fdga = 0.91 ± 0.007 (n = 10), implied a short effective electrotonic length for dendrites: LD = 0.46 ± 0.014 (n = 32). Saturating concentrations of the K+-blockers tetraethylammonium, Cs+, and Ba2+ increased RN and induced charging curves well fitted by single exponential functions in 56% of neostriatal neurons. Ba2+ greatly decreased the somatic shunt (n = 5): (RN = 216 ± 21 MΩ, τ0 = 46 ± 2 ms, RD = 239 ± 25 MΩ, and RS = 3.2 ± 0.5 GΩ), rendering values similar to those obtained with whole-cell recordings (e.g., RN≈ 198 MΩ, RS≈ 2.62 GΩ) (n = 52). Cs+ (n = 5) had less effect on the somatic shunt (RN = 115 ± 19 MΩ, τ0 = 49 ± 13 ms, RS = 161 ± 8 MΩ), although dendritic conductance was equally blocked (RD = 261 ± 16 MΩ). The Cs+-sensitive conductance exhibited inward rectifying properties not displayed by the Ba2+-sensitive conductance, suggesting that Cs+ preferentially acted upon inward rectifier conductances. In contrast, Ba2+ significantly acted upon linear conductances making up the somatic shunt. This suggests a differential action of different K+-blockers on the somato-dendritic membrane, implying a differential distribution of membrane conductances. Another action of K+-blockers, in about 40% of the cells, was to induce dye and probably electrical coupling between neighboring neurons.

Action of Substance P (Neurokinin-1) Receptor Activation on Rat Neostriatal Projection Neurons

Substance P (SP) acts as a neurotransmitter in the neostriatum through the axon collaterals of spiny projection neurons. However, possible direct or indirect actions of SP on the neostriatal output neurons have not been described. Targets of SP terminals within the neostriatum include interneurons, spiny neurons, afferent fibers and boutons. SP induces the release of both dopamine (DA) and acetylcholine (ACh). Since some postsynaptic actions of both DA and ACh on spiny neurons are known, we asked if activation of neostriatal NK1‐class receptors is able to reproduce them. The SP NK1‐receptor agonist, GR73632 (1 μM), had both excitatory and inhibitory actions on virtually all spiny neurons tested at resting potential. The excitatory action was blocked by atropine and coursed with an increase in firing rate and input resistance (RN). The inhibitory action was blocked by haloperidol and coursed with a reduction in firing rate and RN. Therefore, the release of both DA and ACh induced by NK1‐receptor activation modulates indirectly the excitability of the projection neurons. SP facilitates the actions of these transmitters on the spiny neuron. A residual excitatory response to the NK1‐receptor agonist was observed in 30% of a sample of neurons tested in the presence of both haloperidol and atropine. The increase in RN that accompanied this response could be observed in the presence of 1 μM TTX or 100 μM Cd2+, suggesting a direct effect. Double labeling showed that only SP‐immunoreactive neurons were facilitated by NK1‐receptor activation in these conditions. Synapse 33:26–35, 1999.

Inhibitory Contribution to Suprathreshold Corticostriatal Responses: An Experimental and Modeling Study

Neostriatal neurons may undergo events of spontaneous synchronization as those observed in recurrent networks of excitatory neurons, even when cortical afferents are transected. It is necessary to explain these events because the neostriatum is a recurrent network of inhibitory neurons. Synchronization of neuronal activity may be caused by plateau-like depolarizations. Plateau-like orthodromic depolarizations that resemble up-states in medium spiny neostriatal neurons (MSNs) may be induced by a single corticostriatal suprathreshold stimulus. Slow synaptic depolarizations may last hundreds of milliseconds, decay slower than the monosynaptic glutamatergic synaptic potentials that induce them, and sustain repetitive firing. Because inhibitory inputs impinging onto MSNs have a reversal potential above the resting membrane potential but below the threshold for firing, they conform a type of “shunting inhibition”. This work asks if shunting GABAergic inputs onto MSNs arrive asynchronously enough as to help in sustaining the plateau-like corticostriatal response after a single cortical stimulus. This may help to begin explaining autonomous processing in the striatal micro-circuitry in the presence of a tonic excitatory drive and independently of spatio-temporally organized inputs. It is shown here that besides synaptic currents from AMPA/KA- and NMDA-receptors, as well as L-type intrinsic Ca2+- currents, inhibitory synapses help in maintaining the slow depolarization, although they accomplish the role of depressing firing at the beginning of the response. We then used a NEURON model of spiny cells to show that inhibitory synapses arriving asynchronously on the dendrites can help to simulate a plateau potential similar to that observed experimentally.