Frontal cortex stimulation evoked neostriatal potentials in rats: intracellular and extracellular analysis

Coincidence of cholinergic pauses, dopaminergic activation and depolarisation of spiny projection neurons drives synaptic plasticity in the striatum

Dopamine-dependent long-term plasticity is believed to be a cellular mechanism underlying reinforcement learning. In response to reward and reward-predicting cues, phasic dopamine activity potentiates the efficacy of corticostriatal synapses on spiny projection neurons (SPNs). Since phasic dopamine activity also encodes other behavioural variables, it is unclear how postsynaptic neurons identify which dopamine event is to induce long-term plasticity. Additionally, it is unknown how phasic dopamine released from arborised axons can potentiate targeted striatal synapses through volume transmission. To examine these questions we manipulated striatal cholinergic interneurons (ChIs) and dopamine neurons independently in two distinct in vivo paradigms. We report that long-term potentiation (LTP) at corticostriatal synapses with SPNs is dependent on the coincidence of pauses in ChIs and phasic dopamine activation, critically accompanied by SPN depolarisation. Thus, the ChI pause defines the time window for phasic dopamine to induce plasticity, while depolarisation of SPNs constrains the synapses eligible for plasticity.

Pauses in Cholinergic Interneuron Activity Are Driven by Excitatory Input and Delayed Rectification, with Dopamine Modulation

Cholinergic interneurons (ChIs) of the striatum pause their firing in response to salient stimuli and conditioned stimuli after learning. Several different mechanisms for pause generation have been proposed, but a unifying basis has not previously emerged. Here, using in vivo and ex vivo recordings in rat and mouse brain and a computational model, we show that ChI pauses are driven by withdrawal of excitatory inputs to striatum and result from a delayed rectifier potassium current (IKr) in concert with local neuromodulation. The IKr is sensitive to Kv7.2/7.3 blocker XE-991 and enables ChIs to report changes in input, to pause on excitatory input recession, and to scale pauses with input strength, in keeping with pause acquisition during learning. We also show that although dopamine can hyperpolarize ChIs directly, its augmentation of pauses is best explained by strengthening excitatory inputs. These findings provide a basis to understand pause generation in striatal ChIs. VIDEO ABSTRACT.

Origin and properties of striatal local field potential responses to cortical stimulation: temporal regulation by fast inhibitory connections

Evoked striatal field potentials are seldom used to study corticostriatal communication in vivo because little is known about their origin and significance. Here we show that striatal field responses evoked by stimulating the prelimbic cortex in mice are reduced by more than 90% after infusing the AMPA receptor antagonist CNQX close to the recording electrode. Moreover, the amplitude of local field responses and dPSPs recorded in striatal medium spiny neurons increase in parallel with increasing stimulating current intensity. Finally, the evoked striatal fields show several of the basic known properties of corticostriatal transmission, including paired pulse facilitation and topographical organization. As a case study, we characterized the effect of local GABA_A receptor blockade on striatal field and multiunitary action potential responses to prelimbic cortex stimulation. Striatal activity was recorded through a 24 channel silicon probe at about 600 µm from a microdialysis probe. Intrastriatal administration of the GABA_A receptor antagonist bicuculline increased by 65±7% the duration of the evoked field responses. Moreover, the associated action potential responses were markedly enhanced during bicuculline infusion. Bicuculline enhancement took place at all the striatal sites that showed a response to cortical stimulation before drug infusion, but sites showing no field response before bicuculline remained unresponsive during GABA_A receptor blockade. Thus, the data demonstrate that fast inhibitory connections exert a marked temporal regulation of input-output transformations within spatially delimited striatal networks responding to a cortical input. Overall, we propose that evoked striatal fields may be a useful tool to study corticostriatal synaptic connectivity in relation to behavior.

State-dependent plasticity of the corticostriatal pathway

Plasticity at corticostriatal synapses is thought to underlie both normal and aberrant forms of reinforcement-driven learning. Studies in brain slices have found bidirectional, spike-timing dependent plasticity in striatum; however it is not known whether similar rules govern corticostriatal plasticity in awake behaving animals. To assess whether behavioral state is a key regulator of plasticity in this pathway, we examined the effects of 5 Hz cortical stimulation trains on evoked striatal field potentials, in either anesthetized or awake, unrestrained rats. Consistent with prior studies we observed long-term potentiation in intact, barbiturate-anesthetized animals. However, when an identical stimulation pattern was applied to the same animals while awake, long-term depression was observed instead. Our results demonstrate that the rules governing corticostriatal plasticity depend critically on behavioral state, and suggest that the dynamic context of cortical-basal ganglia loops must be considered while investigating synaptic mechanisms underlying reinforcement learning and neurological disorders.

Functional disturbances in the striatum by region-specific ablation of NMDA receptors

To study the role of NMDA receptors in dopamine signaling of the striatum, the brain area that receives glutamatergic inputs from various cortical areas and most dopaminergic inputs, we generated striatum-specific NMDA receptor-deficient mice. The mutant pups showed reduced food intake and retarded growth starting at the second postnatal week and died on approximately postnatal day 20 (P20). The time course of postnatal lethality is similar to that of compound mutant, double knockout of dopamine D1/D2 receptors, or genetically engineered dopamine-deficient mouse. In vivo electrophysiological recordings in the mutant pups showed that frequencies in the range of gamma oscillation were reduced in the striatal circuits. Moreover, the number of functional dopamine receptors in the striatum as measured by D1- and D2-binding experiments was greatly diminished in the mutants as compared with control animals. A consequence of diminished dopamine binding in the striatum manifested in an increase of locomotor activity. The administration of D1/D2 agonists paradoxically reduced the hyperactivity of the mutant mice as compared with an increase in locomotor activity in control mice. These results demonstrate that the NMDA receptor plays an essential role in the integration of dopamine signaling in the striatum and that is required in behavioral function.

Rapid Alterations in Corticostriatal Ensemble Coordination during Acute Dopamine-Dependent Motor Dysfunction

Dopaminergic dysregulation can cause motor dysfunction, but the mechanisms underlying dopamine-related motor disorders remain under debate. We used an inducible and reversible pharmacogenetic approach in dopamine transporter knockout mice to investigate the simultaneous activity of neuronal ensembles in the dorsolateral striatum and primary motor cortex during hyperdopaminergia ( approximately 500% of controls) with hyperkinesia, and after rapid and profound dopamine depletion (<0.2%) with akinesia in the same animal. Surprisingly, although most cortical and striatal neurons ( approximately 70%) changed firing rate during the transition between dopamine-related hyperkinesia and akinesia, the overall cortical firing rate remained unchanged. Conversely, neuronal oscillations and ensemble activity coordination within and between cortex and striatum did change rapidly between these periods. During hyperkinesia, corticostriatal activity became largely asynchronous, while during dopamine-depletion the synchronicity increased. Thus, dopamine-related disorders like Parkinson's disease may not stem from changes in the overall levels of cortical activity, but from dysfunctional activity coordination in corticostriatal circuits.

Interactions of glutamate and dopamine in a computational model of the striatum

A network model of simplified striatal principal neurons with mutual inhibition was used to investigate possible interactions between cortical glutamatergic and nigral dopaminergic afferents in the neostriatum. Glutamatergic and dopaminergic inputs were represented by an excitatory synaptic conductance and a slow membrane potassium conductance, respectively. Neuronal activity in the model was characterized by episodes of increased action potential firing rates of variable duration and frequency. Autocorrelation histograms constructed from the action potential activity of striatal model neurons showed that reducing peak excitatory conductance had the effect of increasing interspike intervals. On the other hand, the maximum value of the dopamine-sensitive potassium conductance was inversely related to the duration of firing episodes and the maximal firing rates. A smaller potassium conductance restored normal firing rates in the most active neurons at the expense of a larger proportion of neurons showing reduced activity. Thus, a homogeneous network with mutual inhibition can produce equally complex dynamics as have been proposed to occur in a striatal network with two neuron populations that are oppositely regulated by dopamine. Even without mutual inhibition it appears that increased dopamine concentrations could partially compensate for the effects of reduced glutamatergic input in individual neurons.

Postsynaptic integration of glutamatergic and dopaminergic signals in the striatum

The aim of this study was to achieve a better understanding of the integration in striatal medium-sized spiny neurons (MSNs) of converging signals from glutamatergic and dopaminergic afferents. The review of the literature in the first section shows that these two types of afferents not only contact the same striatal cell type, but that individual MSNs receive both a corticostriatal and a dopaminergic terminal. The most common sites of convergence are dendritic shafts and spines of MSNs with a distance between the terminals of less than 1-2 microns. The second section focuses on synaptic transmission and second messenger activation. Glutamate, the candidate transmitter of corticostriatal terminals, via different types of glutamate receptors can evoke an increase in intracellular free calcium concentrations. The net effect of dopamine in the striatum is a stimulation of adenylate cyclase activity leading to an increase in cAMP. The subsequent sections present information on calcium- and cAMP-sensitive biochemical pathways and review the regional and subcellular distribution of the components in the striatum. The specific biochemical reaction steps were formalized as simplified equilibrium equations. Parameter values of the model were chosen from published experimental data. Major results of this analysis are: at intracellular free calcium concentrations below 1 microM the stimulation of adenylate cyclase by calcium and dopamine is at least additive in the steady state. Free calcium concentrations exceeding 1 microM inhibit adenylate cyclase, which is not overcome by dopaminergic stimulation. The kinases and phosphatases studied can be divided in those that are almost exclusively calcium-sensitive (PP2B and CaMPK), and others that are modulated by both calcium and dopamine (PKA and PP1). Maximal threonine-phosphorylation of the phosphoprotein DARPP requires optimal concentrations of calcium (about 0.3 microM) and dopamine (above 5 microM). It seems favourable if the glutamate signal precedes phasic dopamine release by approximately 100 msec. The phosphorylation of MAP2 is under essentially calcium-dependent control of at least five kinases and phosphatases, which differentially affect its heterogeneous phosphorylation sites. Therefore, MAP2 could respond specifically to the spatio-temporal characteristics of different intracellular calcium fluxes. The quantitative description of the calcium- and dopamine-dependent regulation of DARPP and MAP2 provides insights into the crosstalk between glutamatergic and dopaminergic signals in striatal MSNs. Such insights constitute an important step towards a better understanding of the links between biochemical pathways, physiological processes, and behavioural consequences connected with striatal function. The relevance to long-term potentiation, reinforcement learning, and Parkinson's disease is discussed.

Functionally distinct subpopulations of striatal neurons are differentially regulated by GABAergic and dopaminergic inputs--I. In vivo analysis

Two subpopulations of striatal neurons, Type I and Type II, are distinguished by their contrasting electrophysiological responses to paired impulse stimulation of cortical afferents. Although both Type I and Type II striatal neurons are excited by the first impulse of any pair of impulses, in response to short interstimulus intervals (10-30 ms) Type I neurons display an increase in probability of spike discharge to the second impulse (facilitation), whereas Type II neurons exhibit a decrease in probability of discharge (inhibition); in response to longer interstimulus intervals (50-250 ms) Type I cells display inhibition, whereas Type II cells show facilitation. The present experiments investigated the possibility that the unique paired impulse responses of Type I and Type II neurons reflect differential regulation by GABAergic and dopaminergic afferents. Extracellular recording techniques were combined with micropressure ejection of specific antagonists for GABAA (bicuculline), GABAB (phaclofen), D1 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-IH-3-benzazepin+ ++-7-ol; SCH23390) or D2 (sulpiride) receptors; the role of dopamine was also examined using the specific neurotoxin, 6-hydroxydopamine. Results showed that bicuculline (250-500 microM) reduced stimulation threshold for spike discharge of both Type I and Type II neurons and completely antagonized the paired impulse inhibition in response to short interstimulus intervals characteristic of Type II neurons. In contrast, phaclofen (2-30 mM) had only a variable influence on spike threshold for Type II cells and no effect on the paired impulse responses of either Type I or Type II neurons. Micropressure ejection of SCH23390 (1 mM) decreased spike thresholds for both cell types and attenuated the inhibition of spike discharge to long interstimulus intervals distinctive of Type I neurons, an effect which was mimicked by dopaminergic denervation. In contrast, sulpiride (1 mM) had little effect on spike thresholds, and no influence on the paired impulse responses of either cell type. These results indicate that the excitability of both Type I and Type II neurons is tonically inhibited by GABAergic and dopaminergic input via stimulation of GABAA and D1 receptors, respectively. Moreover, the bicuculline sensitivity of Type II neurons suggests that GABAergic input to this cell class arises from neurons within a cortically driven feedforward and/or feedback loop, whereas Type I cells receive input from neurons which lie outside of such a loop. In addition, the inhibition to longer interstimulus intervals characteristic of Type I cells is, at least in part, dependent on dopaminergic input through D1 receptor stimulation.

Intracellular response of caudate neurons to variable frequency stimulation of motor cortical areas in dog

The intracellular response to electrical stimulation of motor cortex was studied in 77 neurons recorded in the head of the caudate (Cd) nucleus of dog. Single pulse stimulation of either medial, intermediate or lateral precruciate cortex produced a response in 69 neurons, 59% of which responded to more than one cortical area. Most intracellular responses were complex potentials consisting of an initial depolarization (E) followed by a longer duration hyperpolarization (I) or E-I response complex. When stimulated with trains of low frequency pulses (10 Hz), the stimulus-generated I potentials reduced the absolute amplitude of the evoked E's, often to a level below resting potential. However, at higher frequencies (50 Hz), the I potentials were attenuated and the E potentials summated into a prolonged depolarization lasting the duration of the stimulus train. A computer model of the response to multiple stimuli was generated assuming that the E-I response to each stimulus pulse in the train should temporally summate with previous responses. As the frequency of stimulation was increased, this model consistently predicted greater summation of the I potentials than was experimentally observed. These data suggest that inhibition of Cd neurons is input frequency dependent such that as the frequency of cortical input increases there is a decrease of input-generated inhibition of Cd neurons. Thus, inhibition may modulate the response of Cd neurons such that cortical input must reach a critical firing frequency before being relayed through the Cd nucleus.

Electrophysiological characteristics of cells within mesencephalon suspension grafts

Both spontaneous and evoked extracellular electrophysiological activity of neurons within fetal mesencephalon suspension grafts to the dopamine-depleted striatum of rats were examined. In some cases, extracellular recording was combined with intracellular labeling to identify recorded neurons. Grafted rats displaying a complete cessation of ipsilateral rotations following amphetamine administration were examined at post-implantation time intervals of two, four, five, eight and nine months. Four separate classes of neurons were distinguished within the transplanted striatum based on electrophysiological properties. The first of these groups, the type I cells, appeared to be non-grafted striatal neurons. When spontaneously active, these striatal-like cells fired bursts of action potentials separated by periods of decreased activity. Evoked responses in these cells were characteristic of striatal cells. Type I cells which were intracellularly labeled were found outside the grafts and displayed the characteristic morphology of the medium spiny neuron of the neostriatum. The other three cell classes displayed electrophysiological properties similar to neurons recorded in situ within the reticular formation, substantia nigra pars compacta and substantia nigra pars reticulata. Neurons from these three groups which were labeled with an intracellular marker were found to lie within the suspension grafts. The spontaneous activity of the pars compacta dopaminergic-like neurons was predominantly irregular, with some cells also firing in a regular or pacemaker-like pattern. Infrequently, irregular firing dopaminergic-like neurons displayed episodes of doublet bursting. Many of the grafted neurons responded to electrical stimulation of prefrontal cortex and striatum, indicating that the graft was receiving functional inputs from host neurons. Comparison of the firing rate and pattern of grafted neurons to in situ mesencephalic neurons as a function of time following grafting suggested that the grafted neurons and/or the neuronal circuitry is slowly developing within the host environment. A prolonged time-course for the maturation of the graft may be reflected in the time required to achieve improvements in some behavioral deficits following transplantation. However, the relatively rapid recovery of drug-induced rotational asymmetry following grafting suggests that this form of recovery may not require mature functioning of the grafted neurons.

In vivo release of endogenous glutamate and aspartate in the rat striatum during stimulation of the cortex

The release of endogenous glutamate and aspartate from corticostriate neurons in the anaesthetized rat was investigated. Efflux of glutamate, aspartate and leucine in push-pull cannula perfusates of the striatum was measured in 1 min fractions collected before, during and after a 4 min stimulation period of ipsilateral frontal cortex. Efflux of the putative excitatory amino acid transmitters, glutamate and aspartate, determined during the first minute of stimulation, was significantly elevated above prestimulation resting level (by 167% and 316%, respectively), while efflux of leucine, a non-transmitter amino acid, remained unchanged. Efflux of glutamate and aspartate during the last 3 min of stimulation dropped rapidly, indicating the activation of a regulatory mechanism, presumably re-uptake. The data further support the hypothesis that glutamate and/or aspartate act as transmitters or are metabolites of transmitters in the corticostriate pathway.