Distributional coding of associative learning in discrete populations of midbrain dopamine neurons

Midbrain dopamine neurons are thought to play key roles in learning by conveying the difference between expected and actual outcomes. Recent evidence suggests diversity in dopamine signaling, yet it remains poorly understood how heterogeneous signals might be organized to facilitate the role of downstream circuits mediating distinct aspects of behavior. Here, we investigated the organizational logic of dopaminergic signaling by recording and labeling individual midbrain dopamine neurons during associative behavior. Our findings show that reward information and behavioral parameters are not only heterogeneously encoded but also differentially distributed across populations of dopamine neurons. Retrograde tracing and fiber photometry suggest that populations of dopamine neurons projecting to different striatal regions convey distinct signals. These data, supported by computational modeling, indicate that such distributional coding can maximize dynamic range and tailor dopamine signals to facilitate specialized roles of different striatal regions.

A selective projection from the subthalamic nucleus to parvalbumin-expressing interneurons of the striatum

The striatum and subthalamic nucleus (STN) are considered to be the primary input nuclei of the basal ganglia. Projection neurons of both striatum and STN can extensively interact with other basal ganglia nuclei, and there is growing anatomical evidence of direct axonal connections from the STN to striatum. There remains, however, a pressing need to elucidate the organization and impact of these subthalamostriatal projections in the context of the diverse cell types constituting the striatum. To address this, we carried out monosynaptic retrograde tracing from genetically-defined populations of dorsal striatal neurons in adult male and female mice, quantifying the connectivity from STN neurons to spiny projection neurons, GABAergic interneurons, and cholinergic interneurons. In parallel, we used a combination of ex vivo electrophysiology and optogenetics to characterize the responses of a complementary range of dorsal striatal neuron types to activation of STN axons. Our tracing studies showed that the connectivity from STN neurons to striatal parvalbumin-expressing interneurons is significantly higher (∼ four- to eight-fold) than that from STN to any of the four other striatal cell types examined. In agreement, our recording experiments showed that parvalbumin-expressing interneurons, but not the other cell types tested, commonly exhibited robust monosynaptic excitatory responses to subthalamostriatal inputs. Taken together, our data collectively demonstrate that the subthalamostriatal projection is highly selective for target cell type. We conclude that glutamatergic STN neurons are positioned to directly and powerfully influence striatal activity dynamics by virtue of their enriched innervation of GABAergic parvalbumin-expressing interneurons.Significance StatementPlacing the subthalamostriatal projection within schemes of basal ganglia circuit organization is challenging because of the diversity of cell types within striatum. Here, we shed new light on the structural and electrophysiological substrates by which STN neurons can exert direct and biased influences on the striatal microcircuit. We discovered that STN innervation of parvalbumin-expressing interneurons is relatively enriched and impactful as compared to innervation of other types of striatal neuron. Accordingly, the STN joins a growing list of subcortical structures that, although not considered 'canonical' sources of inputs to striatum, selectively target striatal interneurons. Our results are important in supporting the concept that the glutamatergic subthalamostriatal projection is positioned to fulfil diverse and likely unique roles within basal ganglia circuits.

The aperiodic exponent of subthalamic field potentials reflects excitation/inhibition balance in Parkinsonism

Periodic features of neural time-series data, such as local field potentials (LFPs), are often quantified using power spectra. While the aperiodic exponent of spectra is typically disregarded, it is nevertheless modulated in a physiologically relevant manner and was recently hypothesised to reflect excitation/inhibition (E/I) balance in neuronal populations. Here, we used a cross-species in vivo electrophysiological approach to test the E/I hypothesis in the context of experimental and idiopathic Parkinsonism. We demonstrate in dopamine-depleted rats that aperiodic exponents and power at 30-100 Hz in subthalamic nucleus (STN) LFPs reflect defined changes in basal ganglia network activity; higher aperiodic exponents tally with lower levels of STN neuron firing and a balance tipped towards inhibition. Using STN-LFPs recorded from awake Parkinson's patients, we show that higher exponents accompany dopaminergic medication and deep brain stimulation (DBS) of STN, consistent with untreated Parkinson's manifesting as reduced inhibition and hyperactivity of STN. These results suggest that the aperiodic exponent of STN-LFPs in Parkinsonism reflects E/I balance and might be a candidate biomarker for adaptive DBS.

Input Zone-Selective Dysrhythmia in Motor Thalamus after Dopamine Depletion

The cerebral cortex, basal ganglia and motor thalamus form circuits important for purposeful movement. In Parkinsonism, basal ganglia neurons often exhibit dysrhythmic activity during, and with respect to, the slow (∼1 Hz) and beta-band (15-30 Hz) oscillations that emerge in cortex in a brain state-dependent manner. There remains, however, a pressing need to elucidate the extent to which motor thalamus activity becomes similarly dysrhythmic after dopamine depletion relevant to Parkinsonism. To address this, we recorded single-neuron and ensemble outputs in the basal ganglia-recipient zone (BZ) and cerebellar-recipient zone (CZ) of motor thalamus in anesthetized male dopamine-intact rats and 6-OHDA-lesioned rats during two brain states, respectively defined by cortical slow-wave activity and activation. Two forms of thalamic input zone-selective dysrhythmia manifested after dopamine depletion: (1) BZ neurons, but not CZ neurons, exhibited abnormal phase-shifted firing with respect to cortical slow oscillations prevalent during slow-wave activity; and (2) BZ neurons, but not CZ neurons, inappropriately synchronized their firing and engaged with the exaggerated cortical beta oscillations arising in activated states. These dysrhythmias were not accompanied by the thalamic hypoactivity predicted by canonical firing rate-based models of circuit organization in Parkinsonism. Complementary recordings of neurons in substantia nigra pars reticulata suggested that their altered activity dynamics could underpin the BZ dysrhythmias. Finally, pharmacological perturbations demonstrated that ongoing activity in the motor thalamus bolsters exaggerated beta oscillations in motor cortex. We conclude that BZ neurons are selectively primed to mediate the detrimental influences of abnormal slow and beta-band rhythms on circuit information processing in Parkinsonism.SIGNIFICANCE STATEMENT Motor thalamus neurons mediate the influences of basal ganglia and cerebellum on the cerebral cortex to govern movement. Chronic depletion of dopamine from the basal ganglia causes some symptoms of Parkinson's disease. Here, we elucidate how dopamine depletion alters the ways motor thalamus neurons engage with two distinct oscillations emerging in cortico-basal ganglia circuits in vivo We discovered that, after dopamine depletion, neurons in the thalamic zone receiving basal ganglia inputs are particularly prone to becoming dysrhythmic, changing the phases and/or synchronization (but not rate) of their action potential firing. This bolsters cortical dysrhythmia. Our results provide important new insights into how aberrant rhythmicity in select parts of motor thalamus could detrimentally affect neural circuit dynamics and behavior in Parkinsonism.

GABA uptake transporters support dopamine release in dorsal striatum with maladaptive downregulation in a parkinsonism model

Striatal dopamine (DA) is critical for action and learning. Recent data show that DA release is under tonic inhibition by striatal GABA. Ambient striatal GABA tone on striatal projection neurons can be determined by plasma membrane GABA uptake transporters (GATs) located on astrocytes and neurons. However, whether striatal GATs and astrocytes determine DA output are unknown. We reveal that DA release in mouse dorsolateral striatum, but not nucleus accumbens core, is governed by GAT-1 and GAT-3. These GATs are partly localized to astrocytes, and are enriched in dorsolateral striatum compared to accumbens core. In a mouse model of early parkinsonism, GATs are downregulated, tonic GABAergic inhibition of DA release augmented, and nigrostriatal GABA co-release attenuated. These data define previously unappreciated and important roles for GATs and astrocytes in supporting DA release in striatum, and reveal a maladaptive plasticity in early parkinsonism that impairs DA output in vulnerable striatal regions.

GABA transporters expressed in the striatum may affect behaviour. Here the authors investigate the contribution of GABA transporters on astrocytes to the regulation of dopamine release in the striatum, and show decreased expression of GAT-1 and GAT-3 in a mouse model of Parkinsonism.

Tsc1-mTORC1 signaling controls striatal dopamine release and cognitive flexibility

Tuberous Sclerosis Complex (TSC) is a neurodevelopmental disorder caused by mutations in TSC1 or TSC2, which encode proteins that negatively regulate mTOR complex 1 (mTORC1). TSC is associated with significant cognitive, psychiatric, and behavioral problems, collectively termed TSC-Associated Neuropsychiatric Disorders (TAND), and the cell types responsible for these manifestations are largely unknown. Here we use cell type-specific Tsc1 deletion to test whether dopamine neurons, which modulate cognitive, motivational, and affective behaviors, are involved in TAND. We show that loss of Tsc1 and constitutive activation of mTORC1 in dopamine neurons causes somatodendritic hypertrophy, reduces intrinsic excitability, alters axon terminal structure, and impairs striatal dopamine release. These perturbations lead to a selective deficit in cognitive flexibility, preventable by genetic reduction of the mTOR-binding protein Raptor. Our results establish a critical role for Tsc1-mTORC1 signaling in setting the functional properties of dopamine neurons, and indicate that dopaminergic dysfunction may contribute to cognitive inflexibility in TSC.

Thalamocortical dynamics underlying spontaneous transitions in beta power in Parkinsonism

Parkinson's disease (PD) is a neurodegenerative condition in which aberrant oscillatory synchronization of neuronal activity at beta frequencies (15–35 Hz) across the cortico-basal ganglia-thalamocortical circuit is associated with debilitating motor symptoms, such as bradykinesia and rigidity. Mounting evidence suggests that the magnitude of beta synchrony in the parkinsonian state fluctuates over time, but the mechanisms by which thalamocortical circuitry regulates the dynamic properties of cortical beta in PD are poorly understood. Using the recently developed generic Dynamic Causal Modelling (DCM) framework, we recursively optimized a set of plausible models of the thalamocortical circuit (n = 144) to infer the neural mechanisms that best explain the transitions between low and high beta power states observed in recordings of field potentials made in the motor cortex of anesthetized Parkinsonian rats. Bayesian model comparison suggests that upregulation of cortical rhythmic activity in the beta-frequency band results from changes in the coupling strength both between and within the thalamus and motor cortex. Specifically, our model indicates that high levels of cortical beta synchrony are mainly achieved by a delayed (extrinsic) input from thalamic relay cells to deep pyramidal cells and a fast (intrinsic) input from middle pyramidal cells to superficial pyramidal cells. From a clinical perspective, our study provides insights into potential therapeutic strategies that could be utilized to modulate the network mechanisms responsible for the enhancement of cortical beta in PD. Specifically, we speculate that cortical stimulation aimed to reduce the enhanced excitatory inputs to either the superficial or deep pyramidal cells could be a potential non-invasive therapeutic strategy for PD.

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Coupling changes within and between circuit nodes lead to cortical beta enhancement.

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Input propagation delays play a crucial role in the up-regulation of cortical beta.

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Beta power could be modulated by altering lamina specific inputs.

A Hippocampus-Accumbens Tripartite Neuronal Motif Guides Appetitive Memory in Space

Retrieving and acting on memories of food-predicting environments are fundamental processes for animal survival. Hippocampal pyramidal cells (PYRs) of the mammalian brain provide mnemonic representations of space. Yet the substrates by which these hippocampal representations support memory-guided behavior remain unknown. Here, we uncover a direct connection from dorsal CA1 (dCA1) hippocampus to nucleus accumbens (NAc) that enables the behavioral manifestation of place-reward memories. By monitoring neuronal ensembles in mouse dCA1→NAc pathway, combined with cell-type selective optogenetic manipulations of input-defined postsynaptic neurons, we show that dCA1 PYRs drive NAc medium spiny neurons and orchestrate their spiking activity using feedforward inhibition mediated by dCA1-connected parvalbumin-expressing fast-spiking interneurons. This tripartite cross-circuit motif supports spatial appetitive memory and associated NAc assemblies, being independent of dorsal subiculum and dispensable for both spatial novelty detection and reward seeking. Our findings demonstrate that the dCA1→NAc pathway instantiates a limbic-motor interface for neuronal representations of space to promote effective appetitive behavior.

Propagation of beta/gamma rhythms in the cortico-basal ganglia circuits of the parkinsonian rat

Much of the motor impairment associated with Parkinson’s disease is thought to arise from pathological activity in the networks formed by the basal ganglia (BG) and motor cortex. To evaluate several hypotheses proposed to explain the emergence of pathological oscillations in parkinsonism, we investigated changes to the directed connectivity in BG networks following dopamine depletion. We recorded local field potentials (LFPs) in the cortex and basal ganglia of rats rendered parkinsonian by injection of 6-hydroxydopamine (6-OHDA) and in dopamine-intact controls. We performed systematic analyses of the networks using a novel tool for estimation of directed interactions (nonparametric directionality, NPD). We used a “conditioned” version of the NPD analysis that reveals the dependence of the correlation between two signals on a third reference signal. We find evidence of the dopamine dependency of both low-beta (14–20 Hz) and high-beta/low-gamma (20–40 Hz) directed network interactions. Notably, 6-OHDA lesions were associated with enhancement of the cortical “hyperdirect” connection to the subthalamic nucleus (STN) and its feedback to the cortex and striatum. We find that pathological beta synchronization resulting from 6-OHDA lesioning is widely distributed across the network and cannot be located to any individual structure. Furthermore, we provide evidence that high-beta/gamma oscillations propagate through the striatum in a pathway that is independent of STN. Rhythms at high beta/gamma show susceptibility to conditioning that indicates a hierarchical organization compared with those at low beta. These results further inform our understanding of the substrates for pathological rhythms in salient brain networks in parkinsonism.

NEW & NOTEWORTHY We present a novel analysis of electrophysiological recordings in the cortico-basal ganglia network with the aim of evaluating several hypotheses concerning the origins of abnormal brain rhythms associated with Parkinson’s disease. We present evidence for changes in the directed connections within the network following chronic dopamine depletion in rodents. These findings speak to the plausibility of a “short-circuiting” of the network that gives rise to the conditions from which pathological synchronization may arise.

Structural and molecular heterogeneity of calretinin-expressing interneurons in the rodent and primate striatum

Calretinin‐expressing (CR+) interneurons are the most common type of striatal interneuron in primates. However, because CR+ interneurons are relatively scarce in rodent striatum, little is known about their molecular and other properties, and they are typically excluded from models of striatal circuitry. Moreover, CR+ interneurons are often treated in models as a single homogenous population, despite previous descriptions of their heterogeneous structures and spatial distributions in rodents and primates. Here, we demonstrate that, in rodents, the combinatorial expression of secretagogin (Scgn), specificity protein 8 (SP8) and/or LIM homeobox protein 7 (Lhx7) separates striatal CR+ interneurons into three structurally and topographically distinct cell populations. The CR+/Scgn+/SP8+/Lhx7− interneurons are small‐sized (typically 7–11 µm in somatic diameter), possess tortuous, partially spiny dendrites, and are rostrally biased in their positioning within striatum. The CR+/Scgn−/SP8−/Lhx7− interneurons are medium‐sized (typically 12–15 µm), have bipolar dendrites, and are homogenously distributed throughout striatum. The CR+/Scgn−/SP8−/Lhx7+ interneurons are relatively large‐sized (typically 12–20 µm), and have thick, infrequently branching dendrites. Furthermore, we provide the first in vivo electrophysiological recordings of identified CR+ interneurons, all of which were the CR+/Scgn−/SP8−/Lhx7− cell type. In the primate striatum, Scgn co‐expression also identified a topographically distinct CR+ interneuron population with a rostral bias similar to that seen in both rats and mice. Taken together, these results suggest that striatal CR+ interneurons comprise at least three molecularly, structurally, and topographically distinct cell populations in rodents. These properties are partially conserved in primates, in which the relative abundance of CR+ interneurons suggests that they play a critical role in striatal microcircuits.

A Population of Indirect Pathway Striatal Projection Neurons Is Selectively Entrained to Parkinsonian Beta Oscillations

Classical schemes of basal ganglia organization posit that parkinsonian movement difficulties presenting after striatal dopamine depletion stem from the disproportionate firing rates of spiny projection neurons (SPNs) therein. There remains, however, a pressing need to elucidate striatal SPN firing in the context of the synchronized network oscillations that are abnormally exaggerated in cortical-basal ganglia circuits in parkinsonism. To address this, we recorded unit activities in the dorsal striatum of dopamine-intact and dopamine-depleted rats during two brain states, respectively defined by cortical slow-wave activity (SWA) and activation. Dopamine depletion escalated striatal net output but had contrasting effects on "direct pathway" SPNs (dSPNs) and "indirect pathway" SPNs (iSPNs); their firing rates became imbalanced, and they disparately engaged in network oscillations. Disturbed striatal activity dynamics relating to the slow (∼1 Hz) oscillations prevalent during SWA partly generalized to the exaggerated beta-frequency (15-30 Hz) oscillations arising during cortical activation. In both cases, SPNs exhibited higher incidences of phase-locked firing to ongoing cortical oscillations, and SPN ensembles showed higher levels of rhythmic correlated firing, after dopamine depletion. Importantly, in dopamine-depleted striatum, a widespread population of iSPNs, which often displayed excessive firing rates and aberrant phase-locked firing to cortical beta oscillations, preferentially and excessively synchronized their firing at beta frequencies. Conversely, dSPNs were neither hyperactive nor synchronized to a large extent during cortical activation. These data collectively demonstrate a cell type-selective entrainment of SPN firing to parkinsonian beta oscillations. We conclude that a population of overactive, excessively synchronized iSPNs could orchestrate these pathological rhythms in basal ganglia circuits.SIGNIFICANCE STATEMENT Chronic depletion of dopamine from the striatum, a part of the basal ganglia, causes some symptoms of Parkinson's disease. Here, we elucidate how dopamine depletion alters striatal neuron firing in vivo, with an emphasis on defining whether and how spiny projection neurons (SPNs) engage in the synchronized beta-frequency (15-30 Hz) oscillations that become pathologically exaggerated throughout basal ganglia circuits in parkinsonism. We discovered that a select population of so-called "indirect pathway" SPNs not only fire at abnormally high rates, but are also particularly prone to being recruited to exaggerated beta oscillations. Our results provide an important link between two complementary theories that explain the presentation of disease symptoms on the basis of changes in firing rate or firing synchronization/rhythmicity.

Secretagogin expression delineates functionally-specialized populations of striatal parvalbumin-containing interneurons

Corticostriatal afferents can engage parvalbumin-expressing (PV+) interneurons to rapidly curtail the activity of striatal projection neurons (SPNs), thus shaping striatal output. Schemes of basal ganglia circuit dynamics generally consider striatal PV+ interneurons to be homogenous, despite considerable heterogeneity in both form and function. We demonstrate that the selective co-expression of another calcium-binding protein, secretagogin (Scgn), separates PV+ interneurons in rat and primate striatum into two topographically-, physiologically- and structurally-distinct cell populations. In rats, these two interneuron populations differed in their firing rates, patterns and relationships with cortical oscillations in vivo. Moreover, the axons of identified PV+/Scgn+ interneurons preferentially targeted the somata of SPNs of the so-called 'direct pathway', whereas PV+/Scgn- interneurons preferentially targeted 'indirect pathway' SPNs. These two populations of interneurons could therefore provide a substrate through which either of the striatal output pathways can be rapidly and selectively inhibited to subsequently mediate the expression of behavioral routines.

Properties of Neurons in External Globus Pallidus Can Support Optimal Action Selection

The external globus pallidus (GPe) is a key nucleus within basal ganglia circuits that are thought to be involved in action selection. A class of computational models assumes that, during action selection, the basal ganglia compute for all actions available in a given context the probabilities that they should be selected. These models suggest that a network of GPe and subthalamic nucleus (STN) neurons computes the normalization term in Bayes’ equation. In order to perform such computation, the GPe needs to send feedback to the STN equal to a particular function of the activity of STN neurons. However, the complex form of this function makes it unlikely that individual GPe neurons, or even a single GPe cell type, could compute it. Here, we demonstrate how this function could be computed within a network containing two types of GABAergic GPe projection neuron, so-called ‘prototypic’ and ‘arkypallidal’ neurons, that have different response properties in vivo and distinct connections. We compare our model predictions with the experimentally-reported connectivity and input-output functions (f-I curves) of the two populations of GPe neurons. We show that, together, these dichotomous cell types fulfil the requirements necessary to compute the function needed for optimal action selection. We conclude that, by virtue of their distinct response properties and connectivities, a network of arkypallidal and prototypic GPe neurons comprises a neural substrate capable of supporting the computation of the posterior probabilities of actions.

Choosing an appropriate action as quickly and accurately as possible in a given situation is critical for the survival of animals and humans. One of the brain regions involved in action selection is a set of subcortical nuclei known as the basal ganglia. The importance of understanding information processing in the basal ganglia is further emphasised by the fact that their disturbed interactions in Parkinson’s disease results in profound difficulties in movement. Computational models have suggested how the basal ganglia could select actions in the fastest possible way for the required accuracy level. These models further predict that a part of basal ganglia, called the external globus pallidus (GPe), needs to calculate a particular function of its inputs. This paper proposes how this function could be computed in a mathematical model of a network within GPe. Furthermore, it shows that the experimentally observed connectivity and response properties of GPe neurons fulfil the requirements necessary to support optimal action selection. This suggests the GPe neurons have properties that allow them to contribute to optimal action selection in the whole basal ganglia.

LRRK2 BAC transgenic rats develop progressive, L-DOPA-responsive motor impairment, and deficits in dopamine circuit function

Mutations in leucine-rich repeat kinase 2 (LRRK2) lead to late-onset, autosomal dominant Parkinson's disease, characterized by the degeneration of dopamine neurons of the substantia nigra pars compacta, a deficit in dopamine neurotransmission and the development of motor and non-motor symptoms. The most prevalent Parkinson's disease LRRK2 mutations are located in the kinase (G2019S) and GTPase (R1441C) encoding domains of LRRK2. To better understand the sequence of events that lead to progressive neurophysiological deficits in vulnerable neurons and circuits in Parkinson's disease, we have generated LRRK2 bacterial artificial chromosome transgenic rats expressing either G2019S or R1441C mutant, or wild-type LRRK2, from the complete human LRRK2 genomic locus, including endogenous promoter and regulatory regions. Aged (18-21 months) G2019S and R1441C mutant transgenic rats exhibit L-DOPA-responsive motor dysfunction, impaired striatal dopamine release as determined by fast-scan cyclic voltammetry, and cognitive deficits. In addition, in vivo recordings of identified substantia nigra pars compacta dopamine neurons in R1441C LRRK2 transgenic rats reveal an age-dependent reduction in burst firing, which likely results in further reductions to striatal dopamine release. These alterations to dopamine circuit function occur in the absence of neurodegeneration or abnormal protein accumulation within the substantia nigra pars compacta, suggesting that nigrostriatal dopamine dysfunction precedes detectable protein aggregation and cell death in the development of Parkinson's disease. In conclusion, our longitudinal deep-phenotyping provides novel insights into how the genetic burden arising from human mutant LRRK2 manifests as early pathophysiological changes to dopamine circuit function and highlights a potential model for testing Parkinson's therapeutics.

Distinct developmental origins manifest in the specialized encoding of movement by adult neurons of the external globus pallidus

Transcriptional codes initiated during brain development are ultimately realized in adulthood as distinct cell types performing specialized roles in behavior. Focusing on the mouse external globus pallidus (GPe), we demonstrate that the potential contributions of two GABAergic GPe cell types to voluntary action are fated from early life to be distinct. Prototypic GPe neurons derive from the medial ganglionic eminence of the embryonic subpallium and express the transcription factor Nkx2-1. These neurons fire at high rates during alert rest, and encode movements through heterogeneous firing rate changes, with many neurons decreasing their activity. In contrast, arkypallidal GPe neurons originate from lateral/caudal ganglionic eminences, express the transcription factor FoxP2, fire at low rates during rest, and encode movements with robust increases in firing. We conclude that developmental diversity positions prototypic and arkypallidal neurons to fulfil distinct roles in behavior via their disparate regulation of GABA release onto different basal ganglia targets.

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Arkypallidal and prototypic GPe cells have distinct origins and transcriptional codes

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Arkypallidal neurons rapidly and robustly increase firing rate at movement onset

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Movement is accurately encoded by single arkypallidal or prototypic neurons

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Two GPe cell types are fated to affect different targets in distinct ways in behavior

Large intercalated neurons of amygdala relay noxious sensory information

Various GABAergic neuron types of the amygdala cooperate to control principal cell firing during fear-related and other behaviors, and understanding their specialized roles is important. Among GABAergic neurons, the so-called intercalated cells (ITCcs) are critically involved in the expression and extinction of fear memory. Tightly clustered small-sized spiny neurons constitute the majority of ITCcs, but they are surrounded by sparse, larger neurons (L-ITCcs) for which very little information is known. We report here a detailed neurochemical, structural and physiological characterization of rat L-ITCcs, as identified with juxtacellular recording/labeling in vivo. We supplement these data with anatomical and neurochemical analyses of nonrecorded L-ITCcs. We demonstrate that L-ITCcs are GABAergic, and strongly express metabotropic glutamate receptor 1α and GABAA receptor α1 subunit, together with moderate levels of parvalbumin. Furthermore, L-ITCcs are innervated by fibers enriched with metabotropic glutamate receptors 7a and/or 8a. In contrast to small-sized spiny ITCcs, L-ITCcs possess thick, aspiny dendrites, have highly branched, long-range axonal projections, and innervate interneurons in the basolateral amygdaloid complex. The axons of L-ITCcs also project to distant brain areas, such as the perirhinal, entorhinal, and endopiriform cortices. In vivo recorded L-ITCcs are strongly activated by noxious stimuli, such as hindpaw pinches or electrical footshocks. Consistent with this, we observed synaptic contacts on L-ITCc dendrites from nociceptive intralaminar thalamic nuclei. We propose that, during salient sensory stimulation, L-ITCcs disinhibit local and distant principal neurons, acting as "hub cells," to orchestrate the activity of a distributed network.

Cortical and thalamic excitation mediate the multiphasic responses of striatal cholinergic interneurons to motivationally salient stimuli

Cholinergic interneurons are key components of striatal microcircuits. In primates, tonically active neurons (putative cholinergic interneurons) exhibit multiphasic responses to motivationally salient stimuli that mirror those of midbrain dopamine neurons and together these two systems mediate reward-related learning in basal ganglia circuits. Here, we addressed the potential contribution of cortical and thalamic excitatory inputs to the characteristic multiphasic responses of cholinergic interneurons in vivo. We first recorded and labeled individual cholinergic interneurons in anesthetized rats. Electron microscopic analyses of these labeled neurons demonstrated that an individual interneuron could form synapses with cortical and, more commonly, thalamic afferents. Single-pulse electrical stimulation of ipsilateral frontal cortex led to robust short-latency (<20 ms) interneuron spiking, indicating monosynaptic connectivity, but firing probability progressively decreased during high-frequency pulse trains. In contrast, single-pulse thalamic stimulation led to weak short-latency spiking, but firing probability increased during pulse trains. After initial excitation from cortex or thalamus, interneurons displayed a "pause" in firing, followed by a "rebound" increase in firing rate. Across all stimulation protocols, the number of spikes in the initial excitation correlated positively with pause duration and negatively with rebound magnitude. The magnitude of the initial excitation, therefore, partly determined the profile of later components of multiphasic responses. Upon examining the responses of tonically active neurons in behaving primates, we found that these correlations held true for unit responses to a reward-predicting stimulus, but not to the reward alone, delivered outside of any task. We conclude that excitatory inputs determine, at least in part, the multiphasic responses of cholinergic interneurons under specific behavioral conditions.

Effective connectivity of the subthalamic nucleus-globus pallidus network during Parkinsonian oscillations

In Parkinsonism, subthalamic nucleus (STN) neurons and two types of external globus pallidus (GP) neuron inappropriately synchronise their firing in time with slow (∼1 Hz) or beta (13-30 Hz) oscillations in cortex. We recorded the activities of STN, Type-I GP (GP-TI) and Type-A GP (GP-TA) neurons in anaesthetised Parkinsonian rats during such oscillations to constrain a series of computational models that systematically explored the effective connections and physiological parameters underlying neuronal rhythmic firing and phase preferences in vivo. The best candidate model, identified with a genetic algorithm optimising accuracy/complexity measures, faithfully reproduced experimental data and predicted that the effective connections of GP-TI and GP-TA neurons are quantitatively different. Estimated inhibitory connections from striatum were much stronger to GP-TI neurons than to GP-TA neurons, whereas excitatory connections from thalamus were much stronger to GP-TA and STN neurons than to GP-TI neurons. Reciprocal connections between GP-TI and STN neurons were matched in weight, but those between GP-TA and STN neurons were not; only GP-TI neurons sent substantial connections back to STN. Different connection weights between and within the two types of GP neuron were also evident. Adding to connection differences, GP-TA and GP-TI neurons were predicted to have disparate intrinsic physiological properties, reflected in distinct autonomous firing rates. Our results elucidate potential substrates of GP functional dichotomy, and emphasise that rhythmic inputs from striatum, thalamus and cortex are important for setting activity in the STN-GP network during Parkinsonian beta oscillations, suggesting they arise from interactions between most nodes of basal ganglia-thalamocortical circuits.

Stereological and ultrastructural quantification of the afferent synaptome of individual neurons

Determining the number and placement of synaptic inputs along the distinct plasma membrane domains of neurons is essential for explaining the basis of neuronal activity and function. We detail a strategy that combines juxtacellular labeling, neuronal reconstructions and stereological sampling of inputs at the ultrastructural level to define key elements of the afferent ‘synaptome’ of a given neuron. This approach provides unbiased estimates of the total number and somato-dendritic distribution of synapses made with individual neurons. These organizational properties can be related to the activity of the same neurons previously recorded in vivo, for direct structure–function correlations at the single-cell level. The approach also provides the quantitative data required to develop biologically realistic models that simulate and predict neuronal activity and function.

Temporal coupling with cortex distinguishes spontaneous neuronal activities in identified basal ganglia-recipient and cerebellar-recipient zones of the motor thalamus

Neurons of the motor thalamus mediate basal ganglia and cerebellar influences on cortical activity. To elucidate the net result of γ-aminobutyric acid-releasing or glutamatergic bombardment of the motor thalamus by basal ganglia or cerebellar afferents, respectively, we recorded the spontaneous activities of thalamocortical neurons in distinct identified “input zones” in anesthetized rats during defined cortical activity states. Unexpectedly, the mean rates and brain state dependencies of the firing of neurons in basal ganglia-recipient zone (BZ) and cerebellar-recipient zone (CZ) were matched during slow-wave activity (SWA) and cortical activation. However, neurons were distinguished during SWA by their firing regularities, low-threshold spike bursts and, more strikingly, by the temporal coupling of their activities to ongoing cortical oscillations. The firing of neurons across the BZ was stronger and more precisely phase-locked to cortical slow (∼1 Hz) oscillations, although both neuron groups preferentially fired at the same phase. In contrast, neurons in BZ and CZ fired at different phases of cortical spindles (7–12 Hz), but with similar strengths of coupled firing. Thus, firing rates do not reflect the predicted inhibitory–excitatory imbalance across the motor thalamus, and input zone-specific temporal coding through oscillatory synchronization with the cortex could partly mediate the different roles of basal ganglia and cerebellum in behavior.

Cell-type-specific recruitment of amygdala interneurons to hippocampal theta rhythm and noxious stimuli in vivo

Neuronal synchrony in the basolateral amygdala (BLA) is critical for emotional behavior. Coordinated theta-frequency oscillations between the BLA and the hippocampus and precisely timed integration of salient sensory stimuli in the BLA are involved in fear conditioning. We characterized GABAergic interneuron types of the BLA and determined their contribution to shaping these network activities. Using in vivo recordings in rats combined with the anatomical identification of neurons, we found that the firing of BLA interneurons associated with network activities was cell type specific. The firing of calbindin-positive interneurons targeting dendrites was precisely theta-modulated, but other cell types were heterogeneously modulated, including parvalbumin-positive basket cells. Salient sensory stimuli selectively triggered axo-axonic cells firing and inhibited firing of a disctinct projecting interneuron type. Thus, GABA is released onto BLA principal neurons in a time-, domain-, and sensory-specific manner. These specific synaptic actions likely cooperate to promote amygdalo-hippocampal synchrony involved in emotional memory formation.

K-ATP channels in dopamine substantia nigra neurons control bursting and novelty-induced exploration

Phasic activation of the dopamine (DA) midbrain system in response to unexpected reward or novelty is critical for adaptive behavioral strategies. This activation of DA midbrain neurons occurs via a synaptically triggered switch from low-frequency background spiking to transient high-frequency burst firing. We found that, in medial DA neurons of the substantia nigra (SN), activity of ATP-sensitive potassium (K-ATP) channels enabled NMDA-mediated bursting in vitro as well as spontaneous in vivo burst firing in anesthetized mice. Cell-selective silencing of K-ATP channel activity in medial SN DA neurons revealed that their K-ATP channel-gated burst firing was crucial for novelty-dependent exploratory behavior. We also detected a transcriptional upregulation of K-ATP channel and NMDA receptor subunits, as well as high in vivo burst firing, in surviving SN DA neurons from Parkinson's disease patients, suggesting that burst-gating K-ATP channel function in DA neurons affects phenotypes in both disease and health.

Structural correlates of heterogeneous in vivo activity of midbrain dopaminergic neurons

Dopaminergic neurons of the substantia nigra pars compacta (SNc) exhibit functional heterogeneity that likely underpins their diverse roles in behavior. We examined how the functional diversity of identified dopaminergic neurons in vivo correlates with differences in somato-dendritic architecture and afferent synaptic organization. Stereological analysis of individually recorded and labeled dopaminergic neurons of rat SNc revealed that they received approximately 8,000 synaptic inputs, at least 30% of which were glutamatergic and 40-70% were GABAergic. The latter synapses were proportionally greater in number and denser on dendrites located in the substantia nigra pars reticulata (SNr) than on those located in SNc, revealing the existence of two synaptically distinct and region-specific subcellular domains. We also found that the relative extension of SNc neuron dendrites into the SNr dictated overall GABAergic innervation and predicted inhibition responses to aversive stimuli. We conclude that diverse wiring patterns determine the heterogeneous activities of midbrain dopaminergic neurons in vivo.

Frequency selectivity and dopamine-dependence of plasticity at glutamatergic synapses in the subthalamic nucleus

In Parkinson's disease, subthalamic nucleus (STN) neurons burst fire with increased periodicity and synchrony. This may entail abnormal release of glutamate, the major source of which in STN is cortical afferents. Indeed, the cortico-subthalamic pathway is implicated in the emergence of excessive oscillations, which are reduced, as are symptoms, by dopamine-replacement therapy or deep brain stimulation (DBS) targeted to STN. Here we hypothesize that glutamatergic synapses in the STN may be differentially modulated by low-frequency stimulation (LFS) and high-frequency stimulation (HFS), the latter mimicking deep brain stimulation. Recordings of evoked and spontaneous excitatory post synaptic currents (EPSCs) were made from STN neurons in brain slices obtained from dopamine-intact and chronically dopamine-depleted adult rats. HFS had no significant effect on evoked (e) EPSC amplitude in dopamine-intact slices (104.4±8.0%) but depressed eEPSCs in dopamine-depleted slices (67.8±6.2%). Conversely, LFS potentiated eEPSCs in dopamine-intact slices (126.4±8.1%) but not in dopamine-depleted slices (106.7±10.0%). Analyses of paired-pulse ratio, coefficient of variation, and spontaneous EPSCs suggest that the depression and potentiation have a presynaptic locus of expression. These results indicate that the synaptic efficacy in dopamine-intact tissue is enhanced by LFS. Furthermore, the synaptic efficacy in dopamine-depleted tissue is depressed by HFS. Therefore the therapeutic effects of DBS in Parkinson's disease appear mediated, in part, by glutamatergic cortico-subthalamic synaptic depression and implicate dopamine-dependent increases in the weight of glutamate synapses, which would facilitate the transfer of pathological oscillations from the cortex.

Effects of dopamine depletion on information flow between the subthalamic nucleus and external globus pallidus

Abnormal oscillatory synchrony is increasingly acknowledged as a pathophysiological hallmark of Parkinson's disease, but what promotes such activity remains unclear. We used novel, nonlinear time series analyses and information theory to capture the effects of dopamine depletion on directed information flow within and between the subthalamic nucleus (STN) and external globus pallidus (GPe). We compared neuronal activity recorded simultaneously from these nuclei in 6-hydroxydopamine-lesioned Parkinsonian rats with that in dopamine-intact control rats. After lesioning, both nuclei displayed pronounced augmentations of beta-frequency (∼20 Hz) oscillations and, critically, information transfer between STN and GPe neurons was increased. Furthermore, temporal profiles of the directed information transfer agreed with the neurochemistry of these nuclei, being "excitatory" from STN to GPe and "inhibitory" from GPe to STN. Separation of the GPe population in lesioned animals into "type-inactive" (GP-TI) and "type-active" (GP-TA) neurons, according to definitive firing preferences, revealed distinct temporal profiles of interaction with STN and each other. The profile of GP-TI neurons suggested their output is of greater causal significance than that of GP-TA neurons for the reduced activity that periodically punctuates the spiking of STN neurons during beta oscillations. Moreover, STN was identified as a key candidate driver for recruiting ensembles of GP-TI neurons but not GP-TA neurons. Short-latency interactions between GP-TI and GP-TA neurons suggested mutual inhibition, which could rhythmically dampen activity and promote anti-phase firing across the two subpopulations. Results thus indicate that information flow around the STN-GPe circuit is exaggerated in Parkinsonism and further define the temporal interactions underpinning this.

Alterations in brain connectivity underlying beta oscillations in Parkinsonism

Cortico-basal ganglia-thalamocortical circuits are severely disrupted by the dopamine depletion of Parkinson's disease (PD), leading to pathologically exaggerated beta oscillations. Abnormal rhythms, found in several circuit nodes are correlated with movement impairments but their neural basis remains unclear. Here, we used dynamic causal modelling (DCM) and the 6-hydroxydopamine-lesioned rat model of PD to examine the effective connectivity underlying these spectral abnormalities. We acquired auto-spectral and cross-spectral measures of beta oscillations (10-35 Hz) from local field potential recordings made simultaneously in the frontal cortex, striatum, external globus pallidus (GPe) and subthalamic nucleus (STN), and used these data to optimise neurobiologically plausible models. Chronic dopamine depletion reorganised the cortico-basal ganglia-thalamocortical circuit, with increased effective connectivity in the pathway from cortex to STN and decreased connectivity from STN to GPe. Moreover, a contribution analysis of the Parkinsonian circuit distinguished between pathogenic and compensatory processes and revealed how effective connectivity along the indirect pathway acquired a strategic importance that underpins beta oscillations. In modelling excessive beta synchrony in PD, these findings provide a novel perspective on how altered connectivity in basal ganglia-thalamocortical circuits reflects a balance between pathogenesis and compensation, and predicts potential new therapeutic targets to overcome dysfunctional oscillations.

Effects of dopamine depletion on network entropy in the external globus pallidus

Dopamine depletion in cortical-basal ganglia circuits in Parkinson's disease (PD) grossly disturbs movement and cognition. Classic models relate Parkinsonian dysfunction to changes in firing rates of basal ganglia neurons. However, disturbances in other dynamics of neural activity are also common. Taking both inappropriate firing rates and other dynamics into account and determining how changes in the properties of these neural circuits that occur during PD impact on information coding are thus imperative. Here, we examined in vivo network dynamics in the external globus pallidus (GPe) of rats before and after chronic dopamine depletion. Dopamine depletion led to decreases in the firing rates of GPe neurons and increases in synchronized network oscillations in the beta frequency (13-30 Hz) band. Using logistic regression models, we determined the combined and separate impacts of these factors on network entropy, a measure of the upper bound of information coding capacity. Importantly, changes in these features in dopamine-depleted rats led to a significant decrease in GPe network entropy. Changes in firing rates had the largest impact on entropy, with changes in synchrony also decreasing entropy at the network level. Changes in autocorrelations tended to offset these effects because autocorrelations decreased entropy more in the control animals. Thus it is possible that reduced information coding capacity within basal ganglia networks may contribute to the behavioral deficits accompanying PD.

Sparse but selective and potent synaptic transmission from the globus pallidus to the subthalamic nucleus

The reciprocally connected GABAergic globus pallidus (GP)-glutamatergic subthalamic nucleus (STN) network is critical for voluntary movement and an important site of dysfunction in movement disorders such as Parkinson's disease. Although the GP is a key determinant of STN activity, correlated GP-STN activity is rare under normal conditions. Here we define fundamental features of the GP-STN connection that contribute to poorly correlated GP-STN activity. Juxtacellular labeling of single GP neurons in vivo and stereological estimation of the total number of GABAergic GP-STN synapses suggest that the GP-STN connection is surprisingly sparse: single GP neurons maximally contact only 2% of STN neurons and single STN neurons maximally receive input from 2% of GP neurons. However, GP-STN connectivity may be considerably more selective than even these estimates imply. Light and electron microscopic analyses revealed that single GP axons give rise to sparsely distributed terminal clusters, many of which correspond to multiple synapses with individual STN neurons. Application of the minimal stimulation technique in brain slices confirmed that STN neurons receive multisynaptic unitary inputs and that these inputs largely arise from different sets of GABAergic axons. Finally, the dynamic-clamp technique was applied to quantify the impact of GP-STN inputs on STN activity. Small fractions of GP-STN input were sufficiently powerful to inhibit and synchronize the autonomous activity of STN neurons. Together these data are consistent with the conclusion that the rarity of correlated GP-STN activity in vivo is due to the sparsity and selectivity, rather than the potency, of GP-STN synaptic connections.

Increased electrical and metabolic activity in the dorsal raphe nucleus of Parkinsonian rats

Serotonin (5-HT) containing neurons in the dorsal raphe nucleus (DRN) may play important roles in Parkinson's disease (PD). This study investigated neural and metabolic activity of the DRN in animal PD models based on dopamine depletion. The data show both increased firing rate of DRN 5-HT neurons and increased cytochrome oxidase activity in dopamine-depleted rats, as compared to controls. These data support the hypothesis that the DRN 5-HT system is hyperactive in the dopamine-depleted brain.

Cholinergic brainstem neurons modulate cortical gamma activity during slow oscillations

Cholinergic neurons in the rostral brainstem, including the pedunculopontine nucleus (PPN), are critical for switching behavioural state from sleep to wakefulness, and their presumed inactivity during sleep is thought to promote slow cortical rhythms that are characteristic of this state. However, it is possible that the diminished activity of cholinergic brainstem neurons during slow-wave sleep continues to have a functional impact upon ongoing cortical activity. Here we show that identified cholinergic projection neurons in the PPN fire rhythmically during cortical slow oscillations, and predominantly discharge in time with the phase of the slow oscillations supporting nested gamma oscillations (30-60 Hz). In contrast, PPN non-cholinergic neurons that are linked to cortical activity fire in the opposite phase and independent of nested gamma oscillations. Furthermore, cholinergic PPN neurons emit extensive local axon collaterals (as well as long-range projections), and increasing cholinergic tone within the PPN enhances the nested gamma oscillations without producing sustained cortical activation. Thus, in addition to driving global state transitions in the cortex, cholinergic PPN neurons also play an active role in organizing cortical activity during slow-wave sleep. Our results suggest that the role of the PPN in sleep homeostasis is more diverse than previously conceived. The functions supported by nested gamma oscillations during sleep (i.e. consolidation, plasticity) are critically dependent on the gating of the underlying cortical ensembles, and our data show that cholinergic PPN neurons have an hitherto unappreciated influence on this gating process.

A single-cell analysis of intrinsic connectivity in the rat globus pallidus

GABAergic neurons of the globus pallidus (GP) play critical roles in basal ganglia function by virtue of their widespread axonal projections to all parts of the basal ganglia. They also possess local axon collaterals. In view of the importance of GABAergic inputs in sculpting neuronal activity, we quantitatively characterized the local axon collaterals of individual GP neurons by in vivo recording, juxtacellular labeling, reconstruction, and light and electron microscopic analysis in the rat. All labeled GP neurons had similar firing properties and gave rise to local axon collaterals, the main synaptic targets of which were perikarya and primary dendrites. The neurons could be divided into two populations; neurons located within approximately 100 microm of the striatopallidal border ("outer" neurons), which possess a mean of 264 local axonal boutons, and those located approximately 100 microm or more from the striatopallidal border ("inner" neurons), which possess a mean of 581 local axonal boutons. The local axon collaterals gave rise to arborizations close to, or within, the parent dendritic field and arborizations located caudal, medial, and ventral to the parent neuron. The qualitative and quantitative differences in the connectivity of neurons located in the outer and inner regions of the GP underlie a complex microcircuitry that follows an asymmetric rostral to caudal organization. These data suggest that the GP should no longer be considered as an homogeneous relay nucleus that simply transmits striatal information to the subthalamic nucleus and basal ganglia output nuclei, but rather as a structure that can perform complex computations within its borders.

Novel and distinct operational principles of intralaminar thalamic neurons and their striatal projections

Neurons of the intralaminar thalamus, including central lateral (CL) and parafascicular (Pf) nuclei, innervate the cortex and striatum and are important for cognitive, sensory, and motor processes. We tested the hypothesis that CL and Pf neurons provide functionally distinct inputs to the striatum. We performed recordings of single CL and Pf neurons in anesthetized rats and, after juxtacellularly labeling the neurons, their somatodendritic features and synaptic connections were characterized. All CL neurons (n = 31) discharged classic low-threshold Ca2+ spike bursts during cortical slow-wave activity in vivo. In contrast, Pf neurons (n = 52) rarely fired such bursts, but instead discharged groups of spikes at relatively low frequencies. The activity of CL and Pf neurons was often temporally coupled to cortical slow oscillations. Identified CL neurons possessed archetypal "bushy" dendrites and preferentially established synapses with dendritic spines (91% of synapses) of striatal projection neurons. Pf neurons possessed "reticular-like" dendrites, and, on average, preferentially established synapses with dendritic shafts (63%) in striatum, although connectivity was markedly heterogeneous across neurons. Two of the six Pf neurons studied exclusively targeted dendritic shafts, whereas another neuron almost exclusively (97%) targeted spines. The remaining three neurons preferentially targeted dendritic shafts (53-70%). Thus, the fundamental properties of CL and Pf neurons differ (the latter do not express the typical operational principles of thalamic relay neurons), and they provide different temporally patterned inputs to distinct striatal targets. This mechanistic diversity likely underpins the transmission of specific and discrete information from intralaminar thalamic nuclei to striatal and cortical targets.

Changes in functional connectivity within the rat striatopallidal axis during global brain activation in vivo

The functional organization of the basal ganglia (BG) is often defined according to one of two opposing schemes. The first proposes multiple, essentially independent channels of information processing. The second posits convergence and lateral integration of striatal channels at the level of the globus pallidus (GP). We tested the hypothesis that these proposed aspects of functional connectivity within the striatopallidal axis are dynamic and related to brain state. Local field potentials (LFPs) were simultaneously recorded from multiple sites in striatum and GP in anesthetized rats during slow-wave activity (SWA) and during global activation evoked by sensory stimulation. Functional connectivity was inferred from comparative analyses of the internuclear and intranuclear coherence between bipolar derivations of LFPs. During prominent SWA, as shown in the electrocorticogram and local field potentials in the basal ganglia, intranuclear coherence, and, thus, lateral functional connectivity within striatum or globus pallidus was relatively weak. Furthermore, the temporal coupling of LFPs recorded across these two nuclei involved functional convergence at the level of GP. Global activation, indicated by a loss of SWA, was accompanied by a rapid functional reorganization of the striatopallidal axis. Prominent lateral functional connectivity developed within GP and, to a significantly more constrained spatial extent, striatum. Additionally, functional convergence on GP was no longer apparent, despite increased internuclear coherence. These data demonstrate that functional connectivity within the BG is highly dynamic and suggest that the relative expression of organizational principles, such as parallel, independent processing channels, striatopallidal convergence, and lateral integration within BG nuclei, is dependent on brain state.

Delayed synchronization of activity in cortex and subthalamic nucleus following cortical stimulation in the rat

Oscillations may play a role in the functional organization of cortico-basal ganglia-thalamocortical circuits, and it is important to understand their underlying mechanisms. The cortex often drives basal ganglia (BG) activity, and particularly, oscillatory activity in the subthalamic nucleus (STN). However, the STN may also indirectly influence cortex. The aim of this study was to characterize the delayed (>200 ms) responses of STN neurons to synchronized cortical inputs, focusing on their relationship with oscillatory cortical activity. We recorded the short-latency and delayed responses of STN units and frontal electrocorticogram (ECoG) to cortical stimulation in anaesthetized rats. Similar to previous studies, stimulation of ipsilateral frontal cortex, but not temporal cortex, evoked a short-latency triphasic response, followed by a sustained reduction or pause in firing, in rostral STN units. Caudal STN units did not show the short-latency triphasic response but often displayed a prolonged firing reduction. Oscillations in STN unit activity and ECoG were common after this sustained firing reduction, particularly between 200 and 600 ms after frontal cortical stimulation. These delayed oscillations were significantly coherent in a broad frequency band of 5-30 Hz. Coherence with ECoG at 5-15 Hz was observed throughout STN, though coherence at 15-30 Hz was largely restricted to rostral STN. Furthermore, oscillatory responses at 5-30 Hz in rostral STN predominantly led those in cortex (mean latency of 29 ms) after frontal cortical stimulation. These findings suggest that STN neurons responding to corticosubthalamic inputs may provide a delayed input to cortex, via BG output nuclei, and thence, thalamocortical pathways.

GABA(B) receptors at glutamatergic synapses in the rat striatum

Although multiple effects of GABA(B) receptor activation on synaptic transmission in the striatum have been described, the precise locations of the receptors mediating these effects have not been determined. To address this issue, we carried out pre-embedding immunogold electron microscopy in the rat using antibodies against the GABA(B) receptor subunits, GABA(B1) and GABA(B2). In addition, to investigate the relationship between GABA(B) receptors and glutamatergic striatal afferents, we used antibodies against the vesicular glutamate transporters, vesicular glutamate transporter 1 and vesicular glutamate transporter 2, as markers for glutamatergic terminals. Immunolabeling for GABA(B1) and GABA(B2) was widely and similarly distributed in the striatum, with immunogold particles localized at both presynaptic and postsynaptic sites. The most commonly labeled structures were dendritic shafts and spines, as well as terminals forming asymmetric and symmetric synapses. In postsynaptic structures, the majority of labeling associated with the plasma membrane was localized at extrasynaptic sites, although immunogold particles were also found at the postsynaptic specialization of some symmetric, putative GABAergic synapses. Labeling in axon terminals was located within, or at the edge of, the presynaptic active zone, as well as at extrasynaptic sites. Double labeling for GABA(B) receptor subunits and vesicular glutamate transporters revealed that labeling for both GABA(B1) and GABA(B2) was localized on glutamatergic axon terminals that expressed either vesicular glutamate transporter 1 or vesicular glutamate transporter 2. The patterns of innervation of striatal neurons by the vesicular glutamate transporter 1- and vesicular glutamate transporter 2-positive terminals suggest that they are selective markers of corticostriatal and thalamostriatal afferents, respectively. These results thus provide evidence that presynaptic GABA(B) heteroreceptors are in a position to modulate the two major excitatory inputs to striatal spiny projection neurons arising in the cortex and thalamus. In addition, presynaptic GABA(B) autoreceptors are present on the terminals of spiny projection neurons and/or striatal GABAergic interneurons. Furthermore, the data indicate that GABA may also affect the excitability of striatal neurons via postsynaptic GABA(B) receptors.

Dopamine depletion increases the power and coherence of β-oscillations in the cerebral cortex and subthalamic nucleus of the awake rat

Local field potentials (LFPs) recorded from the subthalamic nucleus (STN) of untreated patients implanted with stimulation electrodes for the treatment of Parkinson's disease (PD) demonstrate strong coherence with the cortical electroencephalogram over the beta-frequency range (15-30 Hz). However, studies in animal models of PD emphasize increased temporal coupling in cortico-basal ganglia circuits at substantially lower frequencies, undermining the potential usefulness of these models. Here we show that 6-hydroxydopamine (6-OHDA) lesions of midbrain dopamine neurons are associated with significant increases in the power and coherence of beta-frequency oscillatory activity present in LFPs recorded from frontal cortex and STN of awake rats, as compared with the healthy animal. Thus, the pattern of synchronization between population activity in the STN and cortex in the 6-OHDA-lesioned rodent model of PD closely parallels that seen in the parkinsonian human. The peak frequency of coherent activity in the beta-frequency range was increased in lesioned animals during periods of spontaneous and sustained movement. Furthermore, administration of the dopamine receptor agonist apomorphine to lesioned animals suppressed beta-frequency oscillations, and increased coherent activity at higher frequencies in the cortex and STN, before producing the rotational behaviour indicative of successful lesion. Taken together, these results support a crucial role for dopamine in the modulation of population activity in cortico-basal ganglia circuits, whereby dopaminergic mechanisms effectively filter out synchronized, rhythmic activity at beta-frequencies at the systems level, and shift temporal couplings in these circuits to higher frequencies. These changes may be important in regulating movement.

Coherent spike-wave oscillations in the cortex and subthalamic nucleus of the freely moving rat

The basal ganglia play a critical role in controlling seizures in animal models of idiopathic non-convulsive (absence) epilepsy. Inappropriate output from the substantia nigra pars reticulata (SNr) is known to exacerbate seizures, but the precise neuronal mechanisms underlying abnormal activity in SNr remain unclear. To test the hypothesis that cortical spike-wave oscillations, often considered indicative of absence seizures, propagate to the subthalamic nucleus, an important afferent of SNr, we simultaneously recorded local field potentials from the frontal cortex and subthalamic nucleus of freely moving rats. Spontaneous spike-wave oscillations in cortex (mean dominant frequency of 7.4 Hz) were associated with similar oscillations in the subthalamic nucleus (mean of 7.9 Hz). The power of oscillations at 5-9 Hz was significantly higher during spike-wave activity as compared with rest periods without this activity. Importantly, spike-wave oscillations in cortex and subthalamic nucleus were significantly coherent across a range of frequencies (3-40 Hz), and the dominant (7-8 Hz) oscillatory activity in the subthalamic nucleus typically followed that in cortex with a small time lag (mean of 2.7 ms). In conclusion, these data suggest that ensembles of subthalamic nucleus neurons are rapidly recruited into oscillations during cortical spike-wave activity, thus adding further weight to the importance of the subthalamic nucleus in absence epilepsy. An increase in synchronous oscillatory input from the subthalamic nucleus could thus partly underlie the expression of pathological activity in SNr that could, in turn, aggravate seizures. Finally, these findings also reiterate the importance of oscillations in these circuits in normal behaviour.

Directional analysis of coherent oscillatory field potentials in the cerebral cortex and basal ganglia of the rat

Population activity in cortico-basal ganglia circuits is synchronized at different frequencies according to brain state. However, the structures that are likely to drive the synchronization of activity in these circuits remain unclear. Furthermore, it is not known whether the direction of transmission of activity is fixed or dependent on brain state. We have used the directed transfer function (DTF) to investigate the direction in which coherent activity is effectively driven in cortico-basal ganglia circuits. Local field potentials (LFPs) were simultaneously recorded in the subthalamic nucleus (STN), globus pallidus (GP) and substantia nigra pars reticulata (SNr), together with the ipsilateral frontal electrocorticogram (ECoG) of anaesthetized rats. Directional analysis was performed on recordings made during robust cortical slow-wave activity (SWA) and "global activation". During SWA, there was coherence at approximately 1 Hz between ECoG and basal ganglia LFPs, with much of the coherent activity directed from cortex to basal ganglia. There were similar coherent activities at approximately 1 Hz within the basal ganglia, with more activity directed from SNr to GP and STN, and from STN to GP rather than vice versa. During global activation, peaks in coherent activity were seen at higher frequencies (15-60 Hz), with most coherence also directed from cortex to basal ganglia. Within the basal ganglia, however, coherence was predominantly directed from GP to STN and SNr. Together, these results highlight a lead role for the cortex in activity relationships with the basal ganglia, and further suggest that the effective direction of coupling between basal ganglia nuclei is dynamically organized according to brain state, with activity relationships involving the GP displaying the greatest capacity to change.

Brain State–Dependency of Coherent Oscillatory Activity in the Cerebral Cortex and Basal Ganglia of the Rat

The nature of the coupling between neuronal assemblies in the cerebral cortex and basal ganglia (BG) is poorly understood. We tested the hypothesis that coherent population activity is dependent on brain state, frequency range, and/or BG nucleus using data from simultaneous recordings of electrocorticogram (ECoG) and BG local field potentials (LFPs) in anesthetized rats. The coherence between ECoG and LFPs simultaneously recorded from subthalamic nucleus (STN), globus pallidus (GP), and substantia nigra pars reticulata (SNr) was largely confined to slow- (∼1 Hz) and spindle- (7–12 Hz) frequency oscillations during slow-wave activity (SWA). In contrast, during cortical activation, coherence was mostly restricted to high-frequency oscillations (15–60 Hz). The coherence between ECoG and LFPs also depended on BG recording site. Partial coherence analyses showed that, during SWA, STN and SNr shared the same temporal coupling with cortex, thereby forming a single functional axis. Cortex was also tightly, but independently, correlated with GP in a separate functional axis. During activation, STN, GP, and, to a lesser extent, SNr shared the same coherence with cortex as part of one functional axis. In addition, GP formed a second, independently coherent loop with cortex. These data suggest that coherent oscillatory activity is present at the level of LFPs recorded in cortico-basal ganglia circuits, and that synchronized population activity is dynamically organized according to brain state, frequency, and nucleus. These attributes further suggest that synchronized activity should be considered as one of a number of candidate mechanisms underlying the functional organization of these brain circuits.

Synchronous Unit Activity and Local Field Potentials Evoked in the Subthalamic Nucleus by Cortical Stimulation

The responses of single subthalamic nucleus (STN) neurons to cortical activation are complex and depend on the relative activation of several neuronal circuits, making theoretical extrapolation of single neuron responses to the population level difficult. To understand better the degree of synchrony imposed on STN neurons and associated neuronal networks by cortical activation, we recorded the responses of single units, pairs of neighboring neurons, and local field potentials (LFPs) in STN to discrete electrical stimulation of the cortex in anesthetized rats. Stimulation of ipsilateral frontal cortex, but not temporal cortex, generated synchronized “multiphasic” responses in neighboring units in rostral STN, usually consisting of a brief, short-latency excitation, a brief inhibition, a second excitation, and a long-duration inhibition. Evoked LFPs in STN consistently mirrored unit responses; brief, negative deflections in the LFP coincided with excitations and brief, positive deflections with inhibitions. This characteristic LFP was dissimilar to potentials evoked in cortex and structures surrounding STN and was resistant to fluctuations in forebrain activity. The short-latency excitation and associated LFP deflection exhibited the highest fidelity to low-intensity cortical stimuli. Unit response failures, which mostly occurred in caudal STN, were not associated with LFPs typical of rostral STN. These data suggest that local populations of STN neurons can be synchronized by both direct and indirect cortical inputs. Synchronized ensemble activity is dependent on topography and input intensity. Finally, the stereotypical, multiphasic profile of the evoked LFP indicates that it might be useful for locating the STN in clinical as well as nonclinical settings.

Uniform inhibition of dopamine neurons in the ventral tegmental area by aversive stimuli

Dopamine neurons play a key role in reward-related behaviors. Reward coding theories predict that dopamine neurons will be inhibited by or will not respond to aversive stimuli. Paradoxically, between 3 and 49% of presumed dopamine neurons are excited by aversive stimuli. We found that, in the ventral tegmental area of anesthetized rats, the population of presumed dopamine neurons that are excited by aversive stimuli is actually not dopaminergic. The identified dopamine neurons were inhibited by the aversive stimulus. These findings suggest that dopamine neurons are specifically excited by reward and that a population of nondopamine neurons is excited by aversive stimuli.

Spike timing of dendrite-targeting bistratified cells during hippocampal network oscillations in vivo

Behavior-contingent network oscillations bring about transient, functionally coherent neuronal assemblies in the cerebral cortex, including the hippocampus. Inhibitory input on and close to the soma is believed to phase intrinsic oscillations and output of pyramidal cells, but the function of GABA release to pyramidal cell dendrites remains unknown. We recorded the oscillation-locked spike timing of identified bistratified interneurons in rats. These cells mainly innervated small dendritic shafts of pyramidal cells co-aligned with the glutamatergic Schaffer collateral/commissural input. During theta oscillations, bistratified cells fired at a phase when, on average, pyramidal cell dendrites are most hyperpolarized. Interneurons targeting the perisomatic domain discharge at an earlier phase. During sharp wave-associated ripples, bistratified cells fired with high frequency and in-phase with basket cells, on average 1-2 ms after the discharges in pyramidal cell somata and dendrites. Our results indicate that bistratified cells rhythmically modulate glutamatergic input to the dendrites of pyramidal cells to actively promote the precise input/output transformation during network oscillations.

Brain-state- and cell-type-specific firing of hippocampal interneurons in vivo

Neural-network oscillations at distinct frequencies have been implicated in the encoding, consolidation and retrieval of information in the hippocampus. Some GABA (gamma-aminobutyric acid)-containing interneurons fire phase-locked to theta oscillations (4-8 Hz) or to sharp-wave-associated ripple oscillations (120-200 Hz), which represent different behavioural states. Interneurons also entrain pyramidal cells in vitro. The large diversity of interneurons poses the question of whether they have specific roles in shaping distinct network activities in vivo. Here we report that three distinct interneuron types--basket, axo-axonic and oriens-lacunosum-moleculare cells--visualized and defined by synaptic connectivity as well as by neurochemical markers, contribute differentially to theta and ripple oscillations in anaesthetized rats. The firing patterns of individual cells of the same class are remarkably stereotyped and provide unique signatures for each class. We conclude that the diversity of interneurons, innervating distinct domains of pyramidal cells, emerged to coordinate the activity of pyramidal cells in a temporally distinct and brain-state-dependent manner.

Oscillatory local field potentials recorded from the subthalamic nucleus of the alert rat

Hitherto, high-frequency local field potential oscillations in the upper gamma frequency band (40-80 Hz) have been recorded only from the region of subthalamic nucleus (STN) in parkinsonian patients treated with levodopa. Here we show that local field potentials recorded from the STN in the healthy alert rat also have a spectral peak in the upper gamma band (mean 53 Hz, range 46-70 Hz). The power of this high-frequency oscillatory activity was increased by 30 +/- 4% (+/-SEM) during motor activity compared to periods of alert immobility. It was also increased by 86 +/- 36% by systemic injection of the D2 dopamine receptor agonist quinpirole. The similarities between the high-frequency activities in the STN of the healthy rat and in the levodopa-treated parkinsonian human argue that this oscillatory activity may be physiological in nature and not a consequence of the parkinsonian state.

Move to the rhythm: oscillations in the subthalamic nucleus-external globus pallidus network

Recent anatomical, physiological and computer modeling studies have revealed that oscillatory processes at the levels of single neurons and neuronal networks in the subthalamic nucleus (STN) and external globus pallidus (GPe) are associated with the operation of the basal ganglia in health and in Parkinson's disease (PD). Autonomous oscillation of STN and GPe neurons underlies tonic activity and is important for synaptic integration, whereas abnormal low-frequency rhythmic bursting in the STN and GPe is characteristic of PD. These recent findings provide further support for the view that the basal ganglia use both the pattern and the rate of neuronal activity to encode information.

Regulation of the timing and pattern of action potential generation in rat subthalamic neurons in vitro by GABA-A IPSPs

The regulation of activity in the subthalamic nucleus (STN) by GABAergic inhibition from the reciprocally connected globus pallidus (GP) plays an important role in normal movement and disorders of movement. To determine the precise manner in which GABAergic synaptic input, acting at A-type receptors, influences the firing of STN neurons, we recorded the response of STN neurons to GABA-A inhibitory postsynaptic potentials (IPSPs) that were evoked by supramaximal electrical stimulation of the internal capsule using the perforated-patch technique in slices at 37 degrees C. The mean equilibrium potential of the GABA-A IPSP (EGABA-A IPSP) was -79.4 +/- 7.0 mV. Single IPSPs disrupted the spontaneous oscillation that underlies rhythmic single-spike firing in STN neurons. As the magnitude of IPSPs increased, the effectiveness of prolonging the interspike interval was related more strongly to the phase of the oscillation at which the IPSP was evoked. Thus the largest IPSPs tended to reset the oscillatory cycle, whereas the smallest IPSPs tended to produce relatively phase-independent delays in firing. Multiple IPSPs were evoked at various frequencies and over different periods and their impact was studied on STN neurons held at different levels of polarization. Multiple IPSPs reduced and/or prevented action potential generation and/or produced sufficient hyperpolarization to activate a rebound depolarization, which generated a single spike or restored rhythmic spiking and/or generated a burst of activity. The pattern of IPSPs and the level of polarization of STN neurons were critical in determining the nature of the response. The duration of bursts varied from 20 ms to several hundred milliseconds, depending on the intrinsic rebound properties of the postsynaptic neuron. These data demonstrate that inhibitory input from the GP can produce a range of firing patterns in STN neurons, depending on the number and frequencies of IPSPs and the membrane properties and voltage of the postsynaptic neuron.

Dopamine regulates the impact of the cerebral cortex on the subthalamic nucleus-globus pallidus network

The subthalamic nucleus-globus pallidus network plays a central role in basal ganglia function and dysfunction. To determine whether the relationship between activity in this network and the principal afferent of the basal ganglia, the cortex, is altered in a model of Parkinson's disease, we recorded unit activity in the subthalamic nucleus-globus pallidus network together with cortical electroencephalogram in control and 6-hydroxydopamine-lesioned rats under urethane anaesthesia. Subthalamic nucleus neurones in control and 6-hydroxydopamine-lesioned animals exhibited low-frequency oscillatory activity, which was tightly correlated with cortical slow-wave activity (approximately 1 Hz). The principal effect of dopamine depletion was that subthalamic nucleus neurones discharged more intensely (233% of control) and globus pallidus neurones developed low-frequency oscillatory firing patterns, without changes in mean firing rate. Ipsilateral cortical ablation largely abolished low-frequency oscillatory activity in the subthalamic nucleus and globus pallidus. These data suggest that abnormal low-frequency oscillatory activity in the subthalamic nucleus-globus pallidus network in the dopamine-depleted state is generated by the inappropriate processing of rhythmic cortical input. A component (15-20%) of the network still oscillated following cortical ablation in 6-hydroxydopamine-lesioned animals, implying that intrinsic properties may also pattern activity when dopamine levels are reduced. The response of the network to global activation was altered by 6-hydroxydopamine lesions. Subthalamic nucleus neurones were excited to a greater extent than in control animals and the majority of globus pallidus neurones were inhibited, in contrast to the excitation elicited in control animals. Inhibitory responses of globus pallidus neurones were abolished by cortical ablation, suggesting that the indirect pathway is augmented abnormally during activation of the dopamine-depleted brain. Taken together, these results demonstrate that both the rate and pattern of activity of subthalamic nucleus and globus pallidus neurones are altered profoundly by chronic dopamine depletion. Furthermore, the relative contribution of rate and pattern to aberrant information coding is intimately related to the state of activation of the cerebral cortex.