The lamellar organization of the rat substantia nigra pars reticulata: segregated patterns of striatal afferents and relationship to the topography of corticostriatal projections

Deep brain stimulation changes basal ganglia output nuclei firing pattern in the dystonic hamster

Dystonia is a heterogeneous syndrome of movement disorders characterized by involuntary muscle contractions leading to abnormal movements and postures. While medical treatment is often ineffective, deep brain stimulation (DBS) of the internal pallidum improves dystonia. Here, we studied the impact of DBS in the entopeduncular nucleus (EP), the rodent equivalent of the human globus pallidus internus, on basal ganglia output in the dt(sz)-hamster, a well-characterized model of dystonia by extracellular recordings. Previous work has shown that EP-DBS improves dystonic symptoms in dt(sz)-hamsters. We report that EP-DBS changes firing pattern in the EP, most neurons switching to a less regular firing pattern during DBS. In contrast, EP-DBS did not change the average firing rate of EP neurons. EP neurons display multiphasic responses to each stimulation impulse, likely underlying the disruption of their firing rhythm. Finally, neurons in the substantia nigra pars reticulata display similar responses to EP-DBS, supporting the idea that EP-DBS affects basal ganglia output activity through the activation of common afferent fibers.

Neurocomputational models of motor and cognitive deficits in Parkinson's disease

We review the contributions of biologically constrained computational models to our understanding of motor and cognitive deficits in Parkinson's disease (PD). The loss of dopaminergic neurons innervating the striatum in PD, and the well-established role of dopamine (DA) in reinforcement learning (RL), enable neural network models of the basal ganglia (BG) to derive concrete and testable predictions. We focus in this review on one simple underlying principle - the notion that reduced DA increases activity and causes long-term potentiation in the indirect pathway of the BG. We show how this theory can provide a unified account of diverse and seemingly unrelated phenomena in PD including progressive motor degeneration as well as cognitive deficits in RL, decision making and working memory. DA replacement therapy and deep brain stimulation can alleviate some aspects of these impairments, but can actually introduce negative effects such as motor dyskinesias and cognitive impulsivity. We discuss these treatment effects in terms of modulation of specific mechanisms within the computational framework. In addition, we review neurocomputational interpretations of increased impulsivity in the face of response conflict in patients with deep-brain-stimulation.

The ventral basal ganglia, a selection mechanism at the crossroads of space, strategy, and reward

The basal ganglia are often conceptualised as three parallel domains that include all the constituent nuclei. The 'ventral domain' appears to be critical for learning flexible behaviours for exploration and foraging, as it is the recipient of converging inputs from amygdala, hippocampal formation and prefrontal cortex, putatively centres for stimulus evaluation, spatial navigation, and planning/contingency, respectively. However, compared to work on the dorsal domains, the rich potential for quantitative theories and models of the ventral domain remains largely untapped, and the purpose of this review is to provide the stimulus for this work. We systematically review the ventral domain's structures and internal organisation, and propose a functional architecture as the basis for computational models. Using a full schematic of the structure of inputs to the ventral striatum (nucleus accumbens core and shell), we argue for the existence of many identifiable processing channels on the basis of unique combinations of afferent inputs. We then identify the potential information represented in these channels by reconciling a broad range of studies from the hippocampal, amygdala and prefrontal cortex literatures with known properties of the ventral striatum from lesion, pharmacological, and electrophysiological studies. Dopamine's key role in learning is reviewed within the three current major computational frameworks; we also show that the shell-based basal ganglia sub-circuits are well placed to generate the phasic burst and dip responses of dopaminergic neurons. We detail dopamine's modulation of ventral basal ganglia's inputs by its actions on pre-synaptic terminals and post-synaptic membranes in the striatum, arguing that the complexity of these effects hint at computational roles for dopamine beyond current ideas. The ventral basal ganglia are revealed as a constellation of multiple functional systems for the learning and selection of flexible behaviours and of behavioural strategies, sharing the common operations of selection-by-disinhibition and of dopaminergic modulation.

Quantitative assessment of stereotyped and challenged locomotion after lesion of the striatum: a 3D kinematic study in rats

BACKGROUND

Although the striatum is in position to regulate motor function, the role of the structure in locomotor behaviour is poorly understood. Therefore, a detailed analysis of locomotion- and obstacle avoidance-related parameters was performed after unilateral lesion of the striatum in rats.

METHODS AND RESULTS

Using the three dimensional motion capture technology, kinematics of walking and clearing obstacles, head and body orientation were analyzed before and up to 60 days after the lesion. Recordings were performed in treadmill running rats with or without obstacles attached to the treadmill belt. The lesion, which was induced by the direct injection of the mitochondrial toxin malonate into the left caudoputamen resulted in the complete destruction of the dorsal striatum. During the first three days following the lesion, rats were unable to run on the treadmill. Thereafter, rats showed normal looking locomotion, yet the contralesional limbs exhibited changes in length and timing parameters, and were overflexed. Moreover, the head of lesioned rats was orientated towards the side of the lesion, and their postural vertical shifted towards the contralesional side. During obstructed running, the contralesional limbs when they were leading the crossing manoeuvre stepped on the obstacle rather than to overcome obstacle without touching it, yet more frequently with the forelimb than the hindlimb. Unsuccessful crossings appeared to be due to a paw placement farther away from the front of the obstacles, and not to an inappropriate limb elevation. Importantly, deficit in locomotor behaviour did not regress over the time.

CONCLUSION

Our results argue that the striatum of one hemisphere controls kinematics of contralateral limbs during stereotyped locomotion and plays a prominent role in the selection of the right motor program so that these limbs successfully cross over obstacle.

Cocaine-induced stereotypy is linked to an imbalance between the medial prefrontal and sensorimotor circuits of the basal ganglia

The dysfunction of basal ganglia circuits related to stereotyped motor activity was analysed using the well-established model of cocaine-induced stereotypy in the rat. We examined and compared the neurochemical and electrophysiological effects occurring in medial prefrontal and sensorimotor basal ganglia circuits of the dorsal striatum after cocaine injection in sensitized and non-sensitized rats. Acute injections of cocaine (25 mg/kg), not inducing stereotyped behaviour, affected both medial prefrontal and sensorimotor circuits in a similar way: (i) a mild and delayed increase and decrease of N-methyl-D-aspartate-evoked dopamine and acetylcholine release, respectively and (ii) a marked decrease of cortically evoked inhibition of substantia nigra pars reticulata neurons revealing an imbalance of information transmission between the direct and indirect trans-striatal pathways. In contrast, following sensitization to cocaine, a challenge injection of the same dose of cocaine, generating strong stereotyped behaviour, provoked neurochemical and electrophysiological effects only in the medial prefrontal but not in the sensorimotor circuits: (i) a strong increase of dopamine and decrease of acetylcholine release in the medial prefrontal territory of the dorsal striatum and (ii) a reduction of all inhibitory and excitatory components of the responses evoked in substantia nigra pars reticulata by medial prefrontal stimulation. Therefore, these data disclose distinct reactivity of the medial prefrontal and sensorimotor circuits of the basal ganglia to repeated cocaine administration leading to stereotyped behaviour induced by subsequent cocaine challenge. Thus, we suggest that stereotyped behaviour is correlated to an imbalance between the medial prefrontal and sensorimotor circuits of the basal ganglia resulting in a loss of control of motor behaviour.

Changes to interneuron-driven striatal microcircuits in a rat model of Parkinson's disease

Striatal interneurons play key roles in basal ganglia function and related disorders by modulating the activity of striatal projection neurons. Here we have injected rabies virus (RV) into either the rat substantia nigra pars reticulata or the globus pallidus and took advantage of the trans-synaptic spread of RV to unequivocally identify the interneurons connected to striatonigral- or striatopallidal-projecting neurons, respectively. Large numbers of RV-infected parvalbumin (PV+/RV+) and cholinergic (ChAT+/RV+) interneurons were detected in control conditions, and they showed marked changes following intranigral 6-hydroxydopamine injection. The number of ChAT+/RV+ interneurons innervating striatopallidal neurons increased concomitant with a reduction in the number of PV+/RV+ interneurons, while the two interneuron populations connected to striatonigral neurons were clearly reduced. These data provide the first evidence of synaptic reorganization between striatal interneurons and projection neurons, notably a switch of cholinergic innervation onto striatopallidal neurons, which could contribute to imbalanced striatal outflow in parkinsonian state.

Neurocomputational models of basal ganglia function in learning, memory and choice

The basal ganglia (BG) are critical for the coordination of several motor, cognitive, and emotional functions and become dysfunctional in several pathological states ranging from Parkinson's disease to Schizophrenia. Here we review principles developed within a neurocomputational framework of BG and related circuitry which provide insights into their functional roles in behavior. We focus on two classes of models: those that incorporate aspects of biological realism and constrained by functional principles, and more abstract mathematical models focusing on the higher level computational goals of the BG. While the former are arguably more "realistic", the latter have a complementary advantage in being able to describe functional principles of how the system works in a relatively simple set of equations, but are less suited to making specific hypotheses about the roles of specific nuclei and neurophysiological processes. We review the basic architecture and assumptions of these models, their relevance to our understanding of the neurobiological and cognitive functions of the BG, and provide an update on the potential roles of biological details not explicitly incorporated in existing models. Empirical studies ranging from those in transgenic mice to dopaminergic manipulation, deep brain stimulation, and genetics in humans largely support model predictions and provide the basis for further refinement. Finally, we discuss possible future directions and possible ways to integrate different types of models.

The contribution of synaptic plasticity in the basal ganglia to the processing of visual information

A mechanism for the involvement of the basal ganglia in the processing of visual information, based on dopamine-dependent modulation of the efficiency of synaptic transmission in interconnected parallel associative and limbic cortex-basal ganglia-thalamus-cortex circuits, is proposed. Each circuit consists of a visual or prefrontal area of the cortex connected with the thalamic nucleus and the corresponding areas in different nuclei of the basal ganglia. The circulation of activity in these circuits is supported by the recurrent arrival of information in the thalamus and cortex. Dopamine released in response to a visual stimulus modulates the efficiencies of "strong" and "weak" corticostriatal inputs in different directions, and the subsequent reorganization of activity in the circuit leads to disinhibition (inhibition) of the activity of those cortical neurons which are "strongly" ("weakly") excited by the visual stimulus simultaneously with dopaminergic cells. The pattern in each cortical area is the neuronal reflection of the properties of the visual stimulus processed by this area. Excitation of dopaminergic cells by the visual stimulus via the superior colliculi requires parallel activation of the disinhibitory input to the superior colliculi via the thalamus and the "direct" pathway" in the basal ganglia. The prefrontal cortex, excited by the visual stimulus via the mediodorsal nucleus of the thalamus, mediates the descending influence on the activity of dopaminergic cells, simultaneously controlling dopamine release in different areas of the striatum and thus facilitating the mutual selection of neural reflections of the individual properties of the visual stimulus and their binding into an integral image.

Frontal cortical afferents facilitate striatal nitric oxide transmission in vivo via a NMDA receptor and neuronal NOS-dependent mechanism

Striatal nitric oxide (NO) signaling plays a critical role in modulating neural processing and motor behavior. Nitrergic interneurons receive synaptic inputs from corticostriatal neurons and are activated via ionotropic glutamate receptor stimulation. However, the afferent regulation of NO signaling is poorly characterized. The role of frontal cortical afferents in regulating NO transmission was assessed in anesthetized rats using amperometric microsensor measurements of NO efflux and local field potential recordings. Low frequency (3 Hz) electrical stimulation of the ipsilateral cortex did not consistently evoke detectable changes in striatal NO efflux. In contrast, train stimulation (30 Hz) of frontal cortical afferents facilitated NO efflux in a stimulus intensity-dependent manner. Nitric oxide efflux evoked by train stimulation was transient, reproducible over time, and attenuated by systemic administration of either the NMDA receptor antagonist MK-801 or the neuronal NO synthase inhibitors 7-nitroindazole and NG-propyl-L-arginine. The interaction between NO efflux evoked via train stimulation and local striatal neuron activity was assessed using dual microsensor and local field potential recordings carried out concurrently in the contralateral and ipsilateral striatum, respectively. Systemic administration of the non-specific NO synthase inhibitor methylene blue attenuated both evoked NO efflux and the peak oscillation frequency (within the delta band) of local field potentials recorded immediately after train stimulation. Taken together, these observations indicate that feed-forward activation of neuronal NO signaling by phasic activation of frontal cortical afferents facilitates the synchronization of glutamate driven oscillations in striatal neurons. Thus, NO signaling may act to amplify coherent corticostriatal transmission and synchronize striatal output.

Subcellular arrangement of molecules for 2-arachidonoyl-glycerol-mediated retrograde signaling and its physiological contribution to synaptic modulation in the striatum

Endogenous cannabinoids (endocannabinoids) mediate retrograde signals for short- and long-term suppression of transmitter release at synapses of striatal medium spiny (MS) neurons. An endocannabinoid, 2-arachidonoyl-glycerol (2-AG), is synthesized from diacylglycerol (DAG) after membrane depolarization and Gq-coupled receptor activation. To understand 2-AG-mediated retrograde signaling in the striatum, we determined precise subcellular distributions of the synthetic enzyme of 2-AG, DAG lipase-alpha (DAGLalpha), and its upstream metabotropic glutamate receptor 5 (mGluR5) and muscarinic acetylcholine receptor 1 (M1). DAGLalpha, mGluR5, and M1 were all richly distributed on the somatodendritic surface of MS neurons, but their subcellular distributions were different. Although mGluR5 and DAGLalpha levels were highest in spines and accumulated in the perisynaptic region, M1 level was lowest in spines and was rather excluded from the mGluR5-rich perisynaptic region. These subcellular arrangements suggest that mGluR5 and M1 might differentially affect endocannabinoid-mediated, depolarization-induced suppression of inhibition (DSI) and depolarization-induced suppression of excitation (DSE) in MS neurons. Indeed, mGluR5 activation enhanced both DSI and DSE, whereas M1 activation enhanced DSI only. Importantly, DSI, DSE, and receptor-driven endocannabinoid-mediated suppression were all abolished by the DAG lipase inhibitor tetrahydrolipstatin, indicating 2-AG as the major endocannabinoid mediating retrograde suppression at excitatory and inhibitory synapses of MS neurons. Accordingly, CB1 cannabinoid receptor, the main target of 2-AG, was present at high levels on GABAergic axon terminals of MS neurons and parvalbumin-positive interneurons and at low levels on excitatory corticostriatal afferents. Thus, endocannabinoid signaling molecules are arranged to modulate the excitability of the MS neuron effectively depending on cortical activity and cholinergic tone as measured by mGluR5 and M1 receptors, respectively.

Reliable long-lasting depression interacts with variable short-term facilitation to determine corticostriatal paired-pulse plasticity in young rats

Synaptic plasticity at corticostraital synapses is proposed to fine tune movment and improve motor skills. We found paired-pulse plasticity at corticostriatal synapses reflected variably expressed short-term facilitation blended with a consistent background of longer-lasting depression. Presynaptic modulation via neuotransmitter receptor activation was ruled out as a mechanism for long-lasting paired-pulse depression by examining the effect of selective receptor antagonists. EPSC amplitude and paired-pulse plasticity, however, was influenced by block of D2 dopamine receptors. Block of glutamate transport with l-transdicarboxylic acid (PDC) reduced EPSCs, possibly through a mechanism of AMPA receptor desensitization. Removal of AMPA receptor desensitization with cyclothiazide reduced the paired-pulse depression at long-duration interstimulus intervals (ISIs), indicating that AMPA receptor desensitization participates in corticostriatal paired-pulse plasticity. The low-affinity glutamate receptor antagonist cis-2,3-piperidine dicarboxylic acid (PDA) increased paired-pulse depression, suggesting that a presynaptic component also exists for long-lasting paired-pulse depression. Low Ca(2+)-high Mg(2+) or BAPTA-AM dramatically reduced the amplitude of corticostriatal EPSCs and both manipulations increased the expression of facilitation and, to a lesser extent, they reduced long-lasting paired-pulse depression. EGTA-AM produced a smaller reduction in EPSC amplitude and it did not alter paired-pulse facilitation, but in contrast to low Ca(2+) and BAPTA-AM, EGTA-AM increased long-lasting paired-pulse depression. These experiments suggest that facilitation and depression are sensitive to vesicle depletion, which is dependent upon changes in peak Ca(2+) (i.e. low Ca(2+)-high Mg(2+) or BAPTA-AM). In addition, the action of EGTA-AM suggests that basal Ca(2+) regulates the recovery from long-lasting paired-pulse depression, possibly thourgh a Ca(2+)-sensitive process of vesicle delivery.

Morphological and physiological properties of parvalbumin- and calretinin-containing gamma-aminobutyric acidergic neurons in the substantia nigra

Evidence for the existence of different populations of gamma-aminobutyric acid (GABA)-ergic neurons in the substantia nigra comes partially from anatomical studies, which have shown there to be little if any overlap between the calcium-binding proteins parvalbumin and calretinin in individual neurons, suggesting that these may represent neuronal subtypes with distinct electrophysiological and/or anatomical properties. We obtained whole-cell recordings from neurons in the substantia nigra pars reticulata in rat brain slices and labeled them with biocytin, followed by immunocytochemical staining for parvalbumin and calretinin. In other cases, neurons were retrogradely labeled from the thalamus or tectum and immunocytochemically identified to determine their projection sites. Intracellularly stained neurons were found to have a variety of somatic sizes and shapes. Reconstructions revealed that all parvalbumin- and calretinin-positive neurons issued at least one axon collateral, which ramified within the substantia nigra pars reticulata and/or pars compacta. Local collaterals were of medium caliber and branched modestly, expressing many long, smooth segments that then issued numerous en passant or terminal boutons, consistent with previous in vivo studies. There were no clear differences in the electrophysiological or morphological properties of neurons expressing parvalbumin or calretinin. Retrograde tracing experiments revealed that both parvalbumin- and calretinin-containing neurons project nonpreferentially to the thalamus or tectum. In sum, the parvalbumin- and calretinin-containing GABAergic neurons of the substantia nigra pars reticulata cannot be differentiated on the basis of their electrophysiological properties, morphological properties, or target nuclei, and both parvalbumin- and calretinin-containing projection neurons issue local axon collaterals that arborize within the substantia nigra.

Synchronous high-voltage spindles in the cortex-basal ganglia network of awake and unrestrained rats

Synchronous oscillations in various frequency ranges have been recorded in several nuclei of the basal ganglia (BG) and are thought to be an information processing mechanism. High-voltage spindles (HVSs) are 5-13 Hz spike-and-wave oscillations, which are commonly recorded in rats and which have been reported in some recent studies where their occurrence in the BG has been investigated. We recorded single neurons and local field potentials (LFPs) simultaneously in the motor cortex, striatum and substantia nigra pars reticulata (SNr) of the freely moving rat. We took advantage of the high level of synchronization observed during HVSs to study signal transmission in the cortex-BG network in the awake animals. The results show that LFPs are synchronized in the motor cortex, striatum and SNr during HVS episodes and that the latter propagate from the cortex to the SNr via the striatum. Moreover, > 50% of single neurons in each of these structures are triggered by the HVS. Following the discharge of cortical cells, SNr neurons are first inhibited after approximately 19 ms and then activated after approximately 45 ms. This response is probably driven by the direct and indirect pathways, respectively, without any involvement of the hyperdirect pathway. Here, it is shown that cortex-BG connectivity can be studied using physiological signals in the freely moving animal as opposed to artificial stimulation under anaesthetized conditions. This opens the door to further studies under various experimental conditions, such as animal models of basal ganglia disorders.

The pars reticulata of the substantia nigra: a window to basal ganglia output

Together with the internal segment of the globus pallidus (GP(i)), the pars reticulata of the substantia nigra (SNr) provides a main output nucleus of the basal ganglia (BG) where the final stage of information processing within this system takes place. In the last decade, progress on the anatomical organization and functional properties of BG output neurons have shed some light on the mechanisms of integration taking place in these nuclei and leading to normal and pathological BG outflow. In this review focused on the SNr, after describing how the anatomical arrangement of nigral cells and their afferents determines specific input-output registers, we examine how the basic electrophysiological properties of the cells and their interaction with synaptic inputs contribute to the spatio-temporal shaping of BG output. The reported data show that the intrinsic membrane properties of the neurons subserves a tonic discharge allowing BG to gate the transmission of information to motor and cognitive systems thereby contributing to appropriate selection of behavior.

Effect of orofacial motor cortex stimulation on neuronal activity in the red nucleus

We studied modulation of the activities of the red nucleus (RN) neurons under to electrical stimulation of the orofacial motor cortex (OfM) in urethane-anesthetized rats. Of 57 neurons studied, 30 (53%) neurons modulated the firing patterns. The firing patterns of the RN neurons were classified into four types: an excitation (E) type (n=4), a long inhibition (LI) type (n=4), a short inhibition (SI) type (n=22), and a no-effect type (n=27). These modulated neurons were intermingled in the dorso-ventral part of the RN. Our results suggest that the RN neurons receive excitatory or inhibitory inputs from the OfM.

Distinct patterns of striatal medium spiny neuron activity during the natural sleep-wake cycle

Striatal medium-sized spiny neurons (MSNs) integrate and convey information from the cerebral cortex to the output nuclei of the basal ganglia. Intracellular recordings from anesthetized animals show that MSNs undergo spontaneous transitions between hyperpolarized and depolarized states. State transitions, regarded as necessary for eliciting action potential firing in MSNs, are thought to control basal ganglia function by shaping striatal output. Here, we use an anesthetic-free rat preparation to show that the intracellular activity of MSNs is not stereotyped and depends critically on vigilance state. During slow-wave sleep, much as during anesthesia, MSNs displayed rhythmic step-like membrane potential shifts, correlated with cortical field potentials. However, wakefulness was associated with a completely different pattern of temporally disorganized depolarizing synaptic events of variable amplitude. Transitions from slow-wave sleep to wakefulness converted striatal discharge from a cyclic brisk firing to an irregular pattern of action potentials. These findings illuminate different capabilities of information processing in basal ganglia networks, suggesting in particular that a novel style of striatal computation is associated with the waking state.

The somatodendritic domain of substantia nigra pars reticulata projection neurons in the rat

We have examined the morphology of the somatodendritic domain of projection neurons located in different sectors of rat substantia nigra pars reticulata (SNr) or having distinct axonal arborizations. Forty-three neurons - 23 located in the dorsal half and 20 in the ventral half of SNr - were injected with biotinylated dextran amine and their somatodendritic domain was reconstructed from serial sagittal sections with a camera lucida. The axonal arborization of 14 neurons was also reconstructed. Dorsally located SNr neurons had a larger perikaryon, a higher number of primary dendrites and a more extensive dendritic arbor than the ventrally located ones. However, irrespective of their location in the SNr, the somatodendritic domain was always longer along the rostrocaudal axis than along the dorsoventral and mediolateral axes. Specific correlations between somatodendritic morphology and axonal arborization could be established for some SNr neurons, but among SNr neurons with similar efferent projections, those lying dorsally always exhibited a larger perikaryon and a more widespread dendritic arbor than those located ventrally. These results indicate that the morphology of the somatodendritic domain of SNr projection neurons is related to the location of their perikaryon within the structure rather than to the pattern of their axonal projections.

How best to consider the structure and function of the pedunculopontine tegmental nucleus: Evidence from animal studies

This review presents the hypothesis that the best way to consider the pedunculopontine tegmental nucleus is by analogy with the substantia nigra. The substantia nigra contains two main compartments: the pars compacta and the pars reticulata. The former contains dopamine neurons that project widely within the basal ganglia while the latter is in receipt of corticostriatal output. Similarly, the PPTg contains the Ch5 acetylcholine containing neurons that project to the thalamus and corticostriatal systems (notably the pars compacta of substantia nigra and the subthalamic nucleus) while the non-cholinergic neurons of the pedunculopontine are in receipt of corticostriatal output. Assessment of the location, composition and connections of the pedunculopontine tegmental nucleus is made to support the hypothesis that it has structural similarities with substantia nigra. Assessment of the motor, sensory and cognitive functions of the pedunculopontine is also made, suggesting functional similarities exist also. Having a clear model of pedunculopontine structure and function is a matter of some importance. It is clearly involved in Parkinson's disease and could potentially be a target for therapeutic intervention. If this is to be realized it will be best to have as clear an understanding as possible of pedunculopontine structure and function in order to maximize positive benefits.

Modulation of cortically induced rhythmical jaw movements by stimulation of the red nucleus in the rat

We study whether stimulation of the red nucleus (RN) can modulate rhythmical jaw movements in rats anesthetized by urethane. Rhythmical jaw movements were induced by repetitive electrical stimulation of the two cortical masticatory areas (area A: the orofacial motor cortex; area P: the insular cortex). Stimuli applied to the RN did influence rhythmical jaw movements induced by stimulation of the A-area. Stimuli applied in the jaw-closing phase increased the amplitude of the jaw-closing movement. Stimuli applied in the jaw-opening phase disturbed the rhythm of jaw movements and induced a small jaw-closing movement. Stimuli applied to the RN did not influence rhythmical jaw movements induced by stimulation of the P-area. These results indicate that the RN is involved in the modulation of rhythmical jaw movements induced by stimulation of the A-area.

Cortical inputs and GABA interneurons imbalance projection neurons in the striatum of parkinsonian rats

The striatum receives massive cortical excitatory inputs and is densely innervated by dopamine. Striatal projection neurons form either the direct or indirect pathways. Models of Parkinson's disease propose that dopaminergic degeneration imbalances both pathways, although direct electrophysiological evidence is lacking. Here, striatal neurons were identified by electrophysiological criteria and Neurobiotin labeling combined with either immunohistochemistry or in situ hybridization. Their spontaneous discharge activity and spike response to cortical stimulation were recorded in vivo in anesthetized rats rendered hemi-parkinsonian by 6-hydroxydopamine. We showed that striatonigral neurons (direct pathway) were inhibited whereas striatopallidal neurons (indirect pathway) were activated by dopaminergic lesion. We also identified, with antidromic stimulations, corticostriatal neurons that preferentially innervate striatonigral or striatopallidal neurons and showed that dopaminergic depletion selectively decreased the spontaneous activity of the former. Therefore, dopamine degeneration induces a cascade of imbalances that spread out of the basal ganglia and affect the whole basal ganglia-thalamo-cortical circuits. Fast-spiking GABA interneurons provide potent feedforward inhibition of striatal projection neurons. We showed here that these interneurons narrowed the time window of the responses of projection neurons to cortical stimulation. In the dopamine-depleted striatum, because the intrinsic activity of these interneurons was not altered, their feedforward inhibition worsened the striatal imbalance. Indeed, the time window of the evoked responses was narrower for striatonigral neurons and wider for striatopallidal neurons. Therefore, after dopaminergic depletion, cortical inputs and GABA interneurons might imbalance striatal projection neurons and represent two novel nondopaminergic mechanisms that might secondarily contribute to the pathophysiology of Parkinson's disease.

Competition between feedback loops underlies normal and pathological dynamics in the basal ganglia

Experiments performed in normal animals suggest that the basal ganglia (BG) are crucial in motor program selection. BG are also involved in movement disorders. In particular, BG neuronal activity in parkinsonian animals and patients is more oscillatory and more synchronous than in normal individuals. We propose a new model for the function and dysfunction of the motor part of BG. We hypothesize that the striatum, the subthalamic nucleus, the internal pallidum (GPi), the thalamus, and the cortex are involved in closed feedback loops. The direct (cortex-striatum-GPi-thalamus-cortex) and the hyperdirect loops (cortex-subthalamic nucleus-GPi-thalamus-cortex), which have different polarities, play a key role in the model. We show that the competition between these two loops provides the BG-cortex system with the ability to perform motor program selection. Under the assumption that dopamine potentiates corticostriatal synaptic transmission, we demonstrate that, in our model, moderate dopamine depletion leads to a complete loss of action selection ability. High depletion can lead to synchronous oscillations. These modifications of the network dynamical state stem from an imbalance between the feedback in the direct and hyperdirect loops when dopamine is depleted. Our model predicts that the loss of selection ability occurs before the appearance of oscillations, suggesting that Parkinson's disease motor impairments are not directly related to abnormal oscillatory activity. Another major prediction of our model is that synchronous oscillations driven by the hyperdirect loop appear in BG after inactivation of the striatum.

Input-output organization of jaw movement-related areas in monkey frontal cortex

The brain mechanisms underlying mastication are not fully understood. To address this issue, we analyzed the distribution patterns of cortico-striatal and cortico-brainstem axon terminals and the origin of thalamocortical and intracortical fibers by injecting anterograde/retrograde tracers into physiologically and morphologically defined jaw movement-related cortical areas. Four areas were identified in the macaque monkey: the primary and supplementary orofacial motor areas (MIoro and SMAoro) and the principal and deep parts of the cortical masticatory area (CMaAp and CMaAd), where intracortical microstimulation produced single twitch-like or rhythmic jaw movements, respectively. Tracer injections into these areas labeled terminals in the ipsilateral putamen in a topographic fashion (MIoro vs. SMAoro and CMaAp vs. CMaAd), in the lateral reticular formation and trigeminal sensory nuclei contralaterally (MIoro and CMaAp) or bilaterally (SMAoro) in a complex manner of segregation vs. overlap, and in the medial parabranchial and Kölliker-Fuse nuclei contralaterally (CMaAd). The MIoro and CMaAp received thalamic projections from the ventrolateral and ventroposterolateral nuclei, the SMAoro from the ventroanterior and ventrolateral nuclei, and the CMaAd from the ventroposteromedial nucleus. The MIoro, SMAoro, CMaAp, and CMaAd received intracortical projections from the ventral premotor cortex and primary somatosensory cortex, the ventral premotor cortex and rostral cingulate motor area, the ventral premotor cortex and area 7b, and various sensory areas. In addition, the MIoro and CMaAp received projections from the three other jaw movement-related areas. Our results suggest that the four jaw movement-related cortical areas may play important roles in the formation of distinctive masticatory patterns.

Subcellular localization of type 1 cannabinoid receptors in the rat basal ganglia

Endocannabinoids, acting via type 1 cannabinoid receptors (CB1), are known to be involved in short-term synaptic plasticity via retrograde signaling. Strong depolarization of the postsynaptic neurons is followed by the endocannabinoid-mediated activation of presynaptic CB1 receptors, which suppresses GABA and/or glutamate release. This phenomenon is termed depolarization-induced suppression of inhibition (DSI) or excitation (DSE), respectively. Although both phenomena have been reported to be present in the basal ganglia, the anatomical substrate for these actions has not been clearly identified. Here we investigate the high-resolution subcellular localization of CB1 receptors in the nucleus accumbens, striatum, globus pallidus and substantia nigra, as well as in the internal capsule, where the striato-nigral and pallido-nigral pathways are located. In all examined nuclei of the basal ganglia, we found that CB1 receptors were located on the membrane of axon terminals and preterminal axons. Electron microscopic examination revealed that the majority of these axon terminals were GABAergic, giving rise to mostly symmetrical synapses. Interestingly, preterminal axons showed far more intense staining for CB1, especially in the globus pallidus and substantia nigra, whereas their terminals were only faintly stained. Non-varicose, thin unmyelinated fibers in the internal capsule also showed strong CB1-labeling, and were embedded in bundles of myelinated CB1-negative axons. The majority of CB1 receptors labeled by immunogold particles were located in the axonal plasma membrane (92.3%), apparently capable of signaling cannabinoid actions. CB1 receptors in this location cannot directly modulate transmitter release, because the release sites are several hundred micrometers away. Interestingly, both the CB1 agonist, WIN55,212-2, as well as its antagonist, AM251, were able to block action potential generation, but via a CB1 independent mechanism, since the effects remained intact in CB1 knockout animals. Thus, our electrophysiological data suggest that these receptors are unable to influence action potential propagation, thus they may not be functional at these sites, but are likely being transported to the terminal fields. The present data are consistent with a role of endocannabinoids in the control of GABA, but not glutamate, release in the basal ganglia via presynaptic CB1 receptors, but also call the attention to possible non-CB1-mediated effects of widely used cannabinoid ligands on action potential generation.

Neuroleptic-induced catalepsy: electrophysiological mechanisms of functional recovery induced by high-frequency stimulation of the subthalamic nucleus

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) remarkably alleviates motor disorders in parkinsonian patients. The mechanisms by which STN HFS exerts its beneficial effects were investigated in anesthetized rats, using a model of acute interruption of dopaminergic transmission. Combined systemic injections of SCH-23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5,-tetrahydro-1H-3-benzazepine] and raclopride, antagonists of the D1 and D2 classes of dopaminergic receptors, respectively, were performed, and the parameters of STN HFS that reversed the neuroleptic-induced catalepsy were determined in freely moving animals. The effects of neuroleptics and the impact of STN HFS applied at parameters alleviating neuroleptic-induced catalepsy were analyzed in the substantia nigra pars reticulata (SNR), a major basal ganglia output structure, by recording the neuronal firing pattern and the responses evoked by cortical stimulation. Neuroleptic injection altered the tonic and regular mode of discharge of SNR neurons, most of them becoming irregular with bursts of spikes and pauses. The inhibitory component of the cortically evoked response, which is attributable to the activation of the direct striatonigral circuit, was decreased, whereas the late excitatory response resulting from the indirect striato-pallido-subthalamo-nigral circuit was reinforced. During STN HFS, the spontaneous firing of SNR cells was either increased or decreased with a global enhancement of the firing rate in the overall population of SNR cells recorded. However, in all of the cases, SNR firing pattern was regularized, and the bias between the trans-striatal and trans-subthalamic circuits was reversed. By these effects, STN HFS restores the functional properties of the circuits by which basal ganglia contribute to motor activity.

The striatofugal fiber system in primates: a reevaluation of its organization based on single-axon tracing studies

The current model of basal ganglia rests on the idea that the striatofugal system is composed of two separate (direct and indirect) pathways originating from distinct cell populations in the striatum. The striatum itself is divided into two major compartments, the striosomes and the matrix, which differ by their neurochemical makeup and input/output connections. Here, neurons located in either striosomes or the extrastriosomal matrix in squirrel monkeys were injected with biotin dextran amine, and their labeled axons were entirely reconstructed with a camera lucida. Twenty-four of 27 reconstructed axons arborized into the three main striatal targets (external pallidum, globus pallidus, and substantia nigra pars reticulata), a finding that is at odds with the concept of a dual striatofugal system. Axons of striosomal neurons formed several columnar terminal fields in the substantia nigra pars reticulata. These data indicate that the substantia nigra pars compacta is neither the only nor the main target of striosomal neurons, a finding that calls for a reevaluation of the organization of the striatonigral projection system.

Feedforward inhibition of projection neurons by fast-spiking GABA interneurons in the rat striatum in vivo

Discharge activities and local field potentials were recorded in the orofacial motor cortex and in the corresponding rostrolateral striatum of urethane-anesthetized rats. Striatal projection neurons were identified by antidromic activation and fast-spiking GABAergic interneurons (FSIs) by their unique characteristics: briefer spike and burst responses. Juxtacellular injection of neurobiotin combined with parvalbumin immunohistochemistry validated this identification. Spontaneous activities and spike responses to cortical stimulation were recorded during both states of cortical activity: slow waves and desynchronization. Both FSI and projection neurons spontaneously discharged synchronously with slow waves at the maximum of cortical activity, but, on average, FSIs were much more active. Cortical desynchronization enhanced FSI activity and facilitated their spike responses to cortical stimulation, whereas opposite effects were observed regarding projection neurons. Experimental conditions favoring FSI discharge were always associated with a decrease in the firing activity of projection neurons. Spike responses to cortical stimulation occurred earlier (latency difference, 4.6 ms) and with a lower stimulation current for FSIs than for projection neurons. Moreover, blocking GABA(A) receptors by local picrotoxin injection enhanced the spike response of projection neurons, and this increase was larger in experimental conditions favoring FSI responses. Therefore, on average, FSIs exert in vivo a powerful feedforward inhibition on projection neurons. However, a few projection neurons were actually more sensitive to cortical stimulation than FSIs. Moreover, picrotoxin, which revealed FSI inhibition, preferentially affected projection neurons exhibiting the weakest sensitivity to cortical stimulation. Thus, feedforward inhibition by FSIs filters cortical information effectively transmitted by striatal projection neurons.

Basal ganglia neural mechanisms of natural movement sequences

Natural rodent grooming and other instinctive behavior serves as a natural model of complex movement sequences. Rodent grooming has syntactic (rule-driven) sequences and more random movement patterns. Both incorporate the same movements--only the serial structure differs. Recordings of neural activity in the dorsolateral striatum and the substantia nigra pars reticulata indicate preferential activation during syntactic sequences over more random sequences. Neurons that are responsive during syntactic grooming sequences are often unresponsive or have reverse activation profiles during kinematically similar movements that occur in flexible or random grooming sequences. Few neurons could be categorized as strictly movement related--instead they were activated only in the context of particular sequential patterns of movements. Particular sequential patterns included "syntactic chain" grooming sequences of paw, head, and body movements and also "warm-up" sequences, which consist of head and body/limb movements that precede locomotion after a period of quiet resting (Golani 1992). Activation during warm-up was less intense and less frequent than during grooming sequences, but both sequences activated neurons above baseline levels, and the same neurons sometimes responded to both sequences. The fact that striatal neurons code 2 natural sequences which are made up of different constituent movements suggests that the basal ganglia may have a generalized role in sequence control. The basal ganglia are modulated by the context of the sequence and may play an executive function in the complex natural patterns of sequenced behaviour.

Striatal Inhibition of Nociceptive Responses Evoked in Trigeminal Sensory Neurons by Tooth Pulp Stimulation

The noxious evoked response in trigeminal sensory neurons was studied to address the role of striatum in the control of nociceptive inputs. In urethane-anesthetized rats, the jaw opening reflex (JOR) was produced by suprathreshold stimulation of the tooth pulp and measured as electromyographic response in the digastric muscle, with simultaneous recording of noxious responses in single unit neurons of the spinal trigeminal nucleus pars caudalis (Sp5c). The microinjection of glutamate (80 ηmol/0.5 μl) into striatal JOR inhibitory sites significantly decreased the Aδ and C fiber–mediated–evoked response (53 ± 4.2 and 43.6 ± 6.4% of control value, P < 0.0001) in 92% (31/34) of nociceptive Sp5c neurons. The microinjection of the solvent was ineffective, as was microinjection of glutamate in sites out of the JOR inhibitory ones. In another series of experiments, simultaneous single unit recordings were performed in the motor trigeminal nucleus (Mo5) and the Sp5c nucleus. Microinjection of glutamate decreased the noxious-evoked response in Sp5c and Mo5 neurons in parallel with the JOR, without modifying spontaneous neuronal activity of trigeminal motoneurons ( n = 8 pairs). These results indicate that the striatum could be involved in the modulation of nociceptive inputs and confirm the role of the basal ganglia in the processing of nociceptive information.

On the activity of the corticostriatal networks during spike-and-wave discharges in a genetic model of absence epilepsy

Absence seizures are characterized by impairment of consciousness associated with widespread bilaterally synchronous spike-and-wave discharges (SWDs) in the electroencephalogram (EEG), which reflect highly synchronized oscillations in thalamocortical networks. Although recent pharmacological studies suggest that the basal ganglia could provide a remote control system for absence seizures, the mechanisms of propagation of epileptic discharges in these subcortical nuclei remain unknown. In the present study, we provide the first description of the electrical events in the corticostriatal pathway during spontaneous SWDs in the genetic absence epilepsy rats from Strasbourg (GAERS), a genetic model of absence epilepsy. In corticostriatal neurons, the SWDs were associated with suprathreshold rhythmic depolarizations in-phase with local EEG spikes. Consistent with this synchronized firing in their excitatory cortical afferents, striatal output neurons (SONs) exhibited, during SWDs, large-amplitude rhythmic synaptic depolarizations. However, SONs did not discharge during SWDs. Instead, the rhythmic synaptic excitation of SONs was shunted by a Cl(-)-dependent increase in membrane conductance that was temporally correlated with bursts of action potentials in striatal GABAergic interneurons. The reduced SON excitability accompanying absence seizures may participate in the control of SWDs by affecting the flow of cortical information within the basal ganglia circuits.

Patterns of axonal branching of neurons of the substantia nigra pars reticulata and pars lateralis in the rat

Axons from neurons of the rat substantia nigra pars reticulata (SNr) and pars lateralis (SNl) were traced after injecting their cell body with biotinylated dextran amine. Thirty-two single axons were reconstructed from serial sagittal sections with a camera lucida, whereas four other SNr axons were reconstructed in the coronal plane to determine whether they innervate the contralateral hemisphere. Four distinct types of SNr projection neurons were identified based on their main axonal targets: type I neurons that project to the thalamus; type II neurons that target the thalamus, the superior colliculus (SC), and the pedunculopontine tegmental nucleus (PPTg); type III neurons that project to the periaqueductal gray matter and the thalamus; and type IV neurons that target the deep mesencephalic nucleus (DpMe) and the SC. The axons of the SNl showed the same branching patterns as SNr axons of types I, II, and IV. The coronal reconstructions demonstrated that SNr neurons innervate the thalamus, the SC, and the DpMe bilaterally. At the thalamic level, SNr and SNl axons targeted preferentially the ventral medial, ventral lateral, paracentral, parafascicular, and mediodorsal nuclei. Axons reaching the SC arborized selectively within the deep layers of this structure. Our results reveal that the SNr and SNl harbor several subtypes of projection neurons endowed with a highly patterned set of axon collaterals. This organization allows single neurons of these output structures of the basal ganglia to exert a multifaceted influence on a wide variety of diencephalic and midbrain structures.

Control of oro-facio-lingual movements by the substantia nigra pars reticulata: High-frequency electrical microstimulation and GABA microinjection findings in rats

To provide direct evidence for substantia nigra pars reticulata (SNr) control of oro-facio-lingual muscle activity, high-frequency electrical microstimulation (mainly trains of 20, 333-Hz cathodal pulses at 40-60 microA) and GABA microinjection (1-5 microl of 10 mM GABA in saline) were carried out using a three-barreled microelectrode at the same SNr site in lightly anesthetized, chronically decorticated rats (n=39). Decortication eliminated the possibility that SNr microstimulation might activate corticofugal fibers descending in the adjoining cerebral peduncle. When the most ventral layer of the SNr was approached, high-amplitude electromyographic (EMG) activity of up to 6 mV with a distinctive waveform appeared synchronously with electrical stimuli in the anterior digastric, masseter, genioglossus, and levator labii superioris muscles. This EMG activity was evoked bilaterally, with an ipsilateral predominance. Eye movements, mostly rotation of the eyeball vertically down in the orbit, were noted. Infrequent blinking was also noted. Histologic examination localized the effector site to the middle third of the mediolateral extent of the caudal SNr corresponding to between 5.8 mm (level of the oculomotor nerve) and 6.5mm (caudal end of the SNr) caudal to bregma; and to the ventralmost peripeduncular region of the SNr corresponding to 7.7 mm to 8.0 mm beneath the cortical surface. We referred to this site as the substantia nigra pars reticulata oro-facio-lingual (SNr-ofl) region. GABA injection produced tonic EMG discharge with consistent amplitude in all of the four muscles studied. The GABA effect was negated by a preceding microinjection of the GABA-A receptor antagonist bicuculline, whereas saline control injection had no effect. Changes in amplitude of evoked EMG activity according to location of the stimulating microelectrode reflected somatotopic organization of the SNr-ofl region. This extremely localized electrical and receptor microstimulation in the SNr produced synchronized powerful contraction of jaw, tongue, and facial muscles with different neural innervation. These findings advance our understanding of the mechanisms of the SNr concerning oro-facio-lingual movements.

The basal ganglia: anatomy, physiology, and pharmacology

The basal ganglia are perceived as important nodes in cortico-subcortical networks involved in the transfer, convergence, and processing of information in motor, cognitive, and limbic domains. How this integration might occur remains a matter of some debate, particularly given the consistent finding in anatomic and physiologic studies of functional segregation in cortico-subcortical loops. More recent theories, however, have raised the notion that modality-specific information might be integrated not spatially, but rather temporally, by coincident processing in discrete neuronal populations. Basal ganglia neurotransmitters, given their diverse roles in motor performance, learning, working memory, and reward-related activity are also likely to play an important role in the integration of cerebral activity. Further work will elucidate this to a greater extent, but for now, it is clear that the basal ganglia form an important nexus in the binding of cognitive, limbic, and motor information into thought and action.

Corticostriatal plasticity: life after the depression

Experience-dependent changes in corticostriatal transmission efficacy are likely to support the role of the striatum in reinforcement-based motor learning. Whereas long-term depression at glutamatergic corticostriatal synapses has long been regarded as the normal form of striatal plasticity, recent work provides evidence that use-dependent potentiation can naturally occur at these connections through an increase in both synaptic efficacy and postsynaptic intrinsic excitability. By decreasing the weight of cortical inputs required to fire striatal output neurons, short-term and long-term potentiation at corticostriatal connections can jointly participate in the formation of sensorimotor links by which specific context-dependent patterns of cortical activity can engage selected motor programs.

Functional diversity of ventral midbrain dopamine and GABAergic neurons

Recent findings indicate that VTA and SN dopaminergic (DA) and GABAergic neurons form subpopulations that are divergent in their electrophysiological features, vulnerability to neurodegeneration, and regulation by neuropeptides. This diversity can be correlated with the anatomical organization of the VTA and SN and their inputs and outputs. In this review we describe the heterogeneity in ion channels and firing patterns, especially burst firing, in subpopulations of dopamine neurons. We go on to describe variations in vulnerability to neurotoxic damage in models of Parkinson's disease in subgroups of DA neurons and its possible relationship to developmental gene regulation, the expression of different ion channels, and the expression of different protein markers, such as the neuroprotective marker calbindin. The electrophysiological properties of subgroups of GABAergic midbrain neurons, patterns of expression of protein markers and receptors, possible involvement of GABAergic neurons in a number of processes that are usually attributed exclusively to dopaminergic neurons, and the characteristics of a subgroup of neurons that contains both dopamine and GABA are also discussed.

Various synaptic activities and firing patterns in cortico-striatal and striatal neurons in vivo

It is commonly assumed that spontaneous activity of striatal output neurons is characterized by a two-state behavior. This assumption is mainly based on in vivo intracellular recordings under urethane and/or ketamine-xylazine anesthesia showing that striatal neurons oscillate between two preferred membrane potentials, a Down state (hyperpolarized level), resulting from an inwardly rectifying potassium conductance, and an Up state (depolarized level) caused by complex interactions between a barrage of cortical synaptic excitation and voltage-dependent potassium conductances. However, a recent comparative study using different anesthetics showed that striatal neurons can exhibit various shapes of synaptic activity depending on the temporal structure and the degree of synchronization of their cortico-striatal afferents. These new data demonstrate that the "classical" Up and Down states do not provide the unique spontaneous activity that can be encountered in striatal neurons in vivo. Rather we propose that striatal neurons should exhibit various synaptic activities and firing patterns depending on the states of vigilance. This hypothesis would be validated in further experiments in which the intracellular activity of striatal neurons will be recorded during the natural sleep-wake cycle.

Transduction profiles of recombinant adeno-associated virus vectors derived from serotypes 2 and 5 in the nigrostriatal system of rats

We compared the transduction efficiencies and tropisms of titer-matched recombinant adeno-associated viruses (rAAV) derived from serotypes 2 and 5 (rAAV-2 and rAAV-5, respectively) within the rat nigrostriatal system. The two serotypes (expressing enhanced green fluorescent protein [EGFP]) were delivered by stereotaxic surgery into the same animals but different hemispheres of the striatum (STR), the substantia nigra (SN), or the medial forebrain bundle (MFB). While both serotypes transduced neurons effectively within the STR, rAAV-5 resulted in a much larger EGFP-expressing area than did rAAV-2. However, neurons transduced with rAAV-2 vectors expressed higher levels of EGFP. Consistent with this result, EGFP-positive projections emanating from transduced striatal neurons covered a larger area of the SN pars reticulata (SNr) after striatal delivery of rAAV-5, but EGFP levels in fibers of the SNr were higher after striatal injection of rAAV-2. We also compared the potentials of the two vectors for retrograde transduction and found that striatal delivery of rAAV-5 resulted in significantly more transduced dopaminergic cell bodies within the SN pars compacta and ventral tegmental area. Similarly, EGFP-transduced striatal neurons were detected only after nigral delivery of rAAV-5. Furthermore, we demonstrate that after striatal AAV-5 vector delivery, the transduction profiles were stable for as long as 9 months. Finally, although we did not target the hippocampus directly, efficient and widespread transduction of hippocampal neurons was observed after delivery of rAAV-5, but not rAAV-2, into the MFB.

The nitric oxide-guanylyl cyclase signaling pathway modulates membrane activity States and electrophysiological properties of striatal medium spiny neurons recorded in vivo

Nitric oxide (NO)-releasing interneurons are believed to regulate the activity of striatal medium spiny neurons (MSNs) that contain the NO effector enzyme guanylyl cyclase (GC). The involvement of NO-GC signaling in modulating steady-state membrane activity of striatal MSNs was examined using in vivo intracellular recordings in rats. Intrastriatal infusion of a neuronal NO synthase inhibitor or a NO scavenger via reverse microdialysis consistently decreased the amplitude of spontaneously occurring depolarized plateau potentials (up events). Intrastriatal infusion of a NO scavenger also decreased the amplitude of EPSPs evoked during electrical stimulation of the orbital prefrontal cortex. The effect of the NO scavenger on spontaneous up events was partially reversed by coperfusion with a cell-permeable cGMP analog. Intracellular injection of MSNs with a soluble GC inhibitor resulted in large decreases in the following: (1) spontaneous up-event amplitude, (2) responsiveness to depolarizing current, (3) action potential amplitude, and (4) input resistance. These effects were partially reversed by coinjection of cGMP. Conversely, intracellular injection of a phosphodiesterase inhibitor increased MSN neuron membrane excitability. These results indicate that, in the intact animal, the NO signaling pathway exerts a powerful tonic modulatory influence over the membrane activity of striatal MSNs via the activation of GC and stimulation of cGMP production.

Efferent connections of “posterodorsal” auditory area in the rat cortex: Implications for auditory spatial processing

We examined efferent connections of the cortical auditory field that receives thalamic afferents specifically from the suprageniculate nucleus (SG) and the dorsal division (MGD) of the medial geniculate body (MG) in the rat [Neuroscience 117 (2003) 1003]. The examined cortical region was adjacent to the caudodorsal border (4.8-7.0 mm posterior to bregma) of the primary auditory area (area Te1) and exhibited relatively late auditory response and high best frequency, compared with the caudal end of area Te1. On the basis of the location and auditory response property, the cortical region is considered identical to "posterodorsal" auditory area (PD). Injections of biocytin in PD revealed characteristic projections, which terminated in cortical areas and subcortical structures that play pivotal roles in directed attention and space processing. The most noticeable cortical terminal field appeared as dense plexuses of axons in area Oc2M, the posterior parietal cortex. Small terminal fields were scattered in area frontal cortex, area 2 that comprises the frontal eye field. The subcortical terminal fields were observed in the pontine nucleus, the nucleus of the brachium inferior colliculus, and the intermediate and deep layers of the superior colliculus. Corticostriatal projections targeted two discrete regions of the caudate putamen: the top of the middle part and the caudal end. It is noteworthy that the inferior colliculus and amygdala virtually received no projection. Corticothalamic projections terminated in the MGD, the SG, the ventral zone of the ventral division of the MG, the ventral margin of the lateral posterior nucleus (LP), and the caudodorsal part of the posterior thalamic nuclear group (Po). Large terminals were found in the MGD, SG, LP and Po besides small terminals, the major component of labeling. The results suggest that PD is an auditory area that plays an important role in spatial processing linked to directed attention and motor function. The results extend to the rat findings from nonhuman primates suggesting the existence of a posterodorsal processing stream for auditory spatial perception.

Spontaneous and evoked activity of substantia nigra pars reticulata neurons during high-frequency stimulation of the subthalamic nucleus

The subthalamic nucleus (STN), a major component of the basal ganglia, exerts an excitatory influence on the output structures of this system i.e., the substantia nigra pars reticulata (SNR) and the internal segment of the globus pallidus. High-frequency stimulation of the STN is a method currently used to treat parkinsonian symptoms. The aim of the present study was to analyze the effects of STN high-frequency stimulation on the activity of SNR neurons and to investigate its impact on the transfer of information between the cerebral cortex and the SNR. During STN high-frequency stimulation, the activity of SNR cells was decreased at low-intensity stimulation, whereas it was increased at a higher intensity. The decrease in the discharge of SNR cells likely results from the activation of a GABAergic transmission in the SNR because this effect was blocked by local application of bicuculline. The increased activity likely results from the activation of the glutamatergic subthalamonigral projection because the latency of the evoked excitations was consistent with the conduction time of the subthalamonigral neurons. Finally, during STN high-frequency stimulation, the transmission of cortical information along the direct trans-striatal pathway was preserved, whereas the functionality of the trans-subthalamic pathways was partly preserved or completely blocked depending on the stimulation intensity. The present data indicate that STN high-frequency stimulation influences the activity of SNR cells through activation of their excitatory and inhibitory synaptic afferent pathways as well as antidromic activation of the projection neurons.

Three-dimensional organization of the recurrent axon collateral network of the substantia nigra pars reticulata neurons in the rat

The substantia nigra pars reticulata (SNR) constitutes a major output nucleus of the basal ganglia where the final stage of information processing within this system takes place. In this study, using juxtacellular labeling and three-dimensional reconstruction methods, we investigated the spatial organization of the intranigral innervation provided by single GABAergic projection neurons from the sensory-motor subdivision of the rat SNR. Confirming previous observations, most labeled SNR cells were found to possess a local axonal network innervating the pars reticulata and pars compacta (SNC). Within the SNR, axons of these cells were distributed along curved laminas enveloping a dorsolaterally located core, thus mostly respecting the onion-like compartmentalization of this nucleus. Although the axonal projection field mostly remained confined to the dendritic field of the parent neuron, it usually extended beyond its limits in caudal, lateral, and/or dorsal directions. Because SNR cells are GABAergic, this pattern of axonal projection suggests the existence of lateral inhibitory interactions between neurons belonging to the same as well as to adjacent functional subdivisions. Axonal projections of SNR cells to the SNC formed longitudinal bands. These bands partly occupied the SNC region projecting to the striatal sector from which parent SNR cells receive their afferents. These data indicate that SNR cells contribute to an indirect nigrostriatal loop circuit through which the striatum could upregulate its level of dopaminergic transmission via a disinhibition of nigrostriatal neurons. Spatial relationships between elements of this indirect nigrostriatonigral circuit indicate that this circuit operates in both a closed and open loop manner.

Spike-dependent intrinsic plasticity increases firing probability in rat striatal neurons in vivo

The collision of pre- and postsynaptic activity is known to provide a trigger for controlling the gain of synaptic transmission between neurons. Here, using in vivo intracellular recordings of rat striatal output neurons, we analyse the effect of a single action potential, generated by ongoing synaptic activity, on subsequent excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of the cerebral cortex. This pairing induced a short-term increase in the probability that cortically evoked EPSPs caused striatal cells to fire. This enhanced EPSP-spike coupling was associated with a decrease in the voltage firing threshold with no apparent change in the synaptic strength itself. Antidromic action potentials in striatal cells were also able to induce the facilitation while subthreshold EPSPs were ineffective, indicating that the postsynaptic spike was necessary and sufficient for the induction of the plasticity. A prior spontaneous action potential also enhanced the probability with which directly applied current pulses elicited firing, suggesting that the facilitation originated from changes in the intrinsic electrical properties of the postsynaptic cell. Using whole-cell recordings in cortico-striatal slices, we found that the increase in membrane excitability as well as in EPSP-spike coupling was abolished by low concentration of 4-aminopyridine. This suggests that the intrinsic plasticity results from a time-dependent modulation of a striatal voltage-dependent potassium current available close to the firing threshold. Action potentials thus provide a postsynaptic signal, not only for associative synaptic plasticity but also for activity-dependent intrinsic plasticity, which directly controls the efficacy of coupling between pre- and postsynaptic neurons.

Neuroanatomical relationship between type 1 cannabinoid receptors and dopaminergic systems in the rat basal ganglia

Dopamine and endocannabinoids are neurotransmitters known to play a role in the activity of the basal ganglia motor circuit. While a number of studies have demonstrated functional interactions between type 1 cannabinoid (CB1) receptors and dopaminergic systems, we still lack detailed neuroanatomical evidence to explain their relationship. Single- and double-labeling methods (in situ hybridization and immunohistochemistry) were employed to determine both the expression and localization of CB1 receptors and tyrosine hydroxylase (TH) in the basal ganglia. In the striatum, we found an intense signal for CB1 receptor transcripts but low signal for CB1 receptor protein, whereas in the globus pallidus and substantia nigra we found the opposite; no hybridization signal but intense immunoreactivity. Consequently, CB1 receptors are synthesized in the striatum and mostly transported to its target areas. No co-expression or co-localization of CB1 receptors and TH was found. In the caudate-putamen, globus pallidus and substantia nigra, TH-immunoreactive fibers were interwoven with the CB1 receptor-immunoreactive neuropil and fibers. Our data suggest that the majority of the striatal CB1 receptors are located presynaptically on inhibitory GABAergic terminals, in a position to modulate neurotransmitter release and influence the activity of substantia nigra dopaminergic neurons. In turn, afferent dopaminergic fibers from the substantia nigra innervate CB1 receptor-expressing striatal neurons that are known to also express dopamine receptors. In conclusion, these data provide a neuroanatomical basis to explain functional interactions between endocannabinoid and dopaminergic systems in the basal ganglia.

Basal ganglia and processing of cortical information: functional interactions between trans-striatal and trans-subthalamic circuits in the substantia nigra pars reticulata

The substantia nigra pars reticulata (SNR), a major output station of basal ganglia, receives information from the cerebral cortex through three main pathways, i.e. a direct inhibitory trans-striatal pathway, an indirect excitatory trans-striatal pathway that involves the pallidum and the subthalamus and a direct excitatory trans-subthalamic pathway. In order to determine how cortical information flow originating from functionally distinct cortical areas and processed through the trans-striatal and trans-subthalamic pathways is integrated within the SNR, the responses induced by electrical stimulation of prefrontal, motor and auditory cortex in SNR cells were analyzed in anesthetized rats. Further confirming that direct striato-nigral pathways related to these functionally distinct cortical areas are organized in parallel channels, stimulation of the prefrontal, motor and auditory cortex induced an inhibitory response on distinct subpopulations of SNR cells. Within a given channel, the direct trans-striatal and the trans-subthalamic pathways converge on a large number of nigral cells. In addition, the present study reveals that nigral cells receiving an inhibitory input from a given cortical area through the direct trans-striatal pathway can also receive an excitatory input from a functionally distinct cortical area through the trans-subthalamic pathways. Such a convergence mainly occurred between the direct striato-nigral pathway issued from the auditory cortex and the trans-subthalamic pathways issued from the motor cortex. These data reveal the existence of a converging influence of trans-subthalamic and direct striato-nigral pathways not only within but also across channels. Within a given cortico-basal ganglia channel, the trans-subthalamic pathways likely contribute to the temporal shaping of the striato-nigral inhibition and thus of the disinhibition of the related nigral target nuclei in the thalamus and mesencephalon. Across channels, the specific interactions between trans-subthalamic and direct striato-nigral pathways could contribute to prevent inhibition of subpopulations of nigral cells implicated in competing functions.

Effects of dopaminergic cell degeneration on electrophysiological characteristics and GAD65/GAD67 expression in the substantia nigra: different action on GABA cell subpopulations

The motor disturbances occurring in Parkinson's disease have been partially attributed to a hyperactivity of gamma-aminobutyric acid (GABA)-ergic nigral cells largely in the substantia nigra pars reticulata (SNr) secondary to the degeneration of dopaminergic nigrostriatal neurons. However, some aspects of this response remain unclear. In this work, different electrophysiological and neurochemical parameters were studied in GABAergic cells of the SN after unilateral nigrostriatal dopaminergic lesion using 6-hydroxydopamine injection in rats. Our data showed that 1) the SN under normal conditions contains different subsets of GABAergic cells according to their firing pattern and glutamic acid decarboxylase mRNA levels, and 2) the response of these GABAergic cell subgroups was different after the ipsi- and contralateral dopaminergic cell degeneration. These findings indicate a complex regulation of nigral GABAergic activity after nigrostriatal dopaminergic degeneration that probably involves local mechanisms, the nigro-striato-nigral loop, as well as interhemispheric mechanisms whose anatomical basis remains unstudied.

Neural activities in the substantia nigra modulated by stimulation of the orofacial motor cortex and rhythmical jaw movements in the rat

Neurons related to jaw movements in the substantia nigra pars reticulata were explored by examining changes in their neural activities in response to electrical stimulation of the orofacial sensorimotor cortex and during rhythmical jaw movements induced by mechanical stimulation applied to the oral cavity in the rat. Out of 80 neurons tested, 59 showed changes in their firing patterns of activities in response to the electrical stimulation of the cortex. The responding neurons were mainly located in the dorsolateral part of the substantia nigra pars reticulata. The substantia nigra pars reticulata neurons showing responses were classified into the following five types according to their response patterns: (1) an inhibition preceded by an early excitation and followed by a late excitation (n = 26), (2) an inhibition preceded by an early excitation but not followed by a late excitation (n = 7), (3) an inhibition not preceded by an early excitation but followed by a late excitation (n = 2), (4) an inhibition without early or late excitations (n = 7) and (5) an excitation without an inhibition (n = 17). Out of 18 neurons responding to the cortical stimulation, 11 (61.1%) increased or decreased their neural activities during rhythmical jaw movements. Some of these neurons had a projection to the lateral part of the superior colliculus (n = 5) and/or to the parvicellular reticular formation (n = 2). These results provide first neurophysiological evidence for neurons in the dorsolateral part of the substantia nigra pars reticulata with inputs from and outputs to the areas related to jaw movements. These neurons may participate in the control of jaw movements in the rat.

Role of subcortical structures in the pathogenesis of infantile spasms: what are possible subcortical mediators?

Infantile spasms present a constellation of symptoms and laboratory findings that suggest a role of subcortical circuits in the pathogenesis of this illness. The clinical features of spasms and the influence of subcortical circuits in the regulation of the electroencephologram, along with frequent abnormalities in subcortical structure and functional anatomy, brain stem electrophysiology, sleep regulation, and subcortical neurotransmitter levels, point to the importance of subcortical circuits in the generation of spasms. Furthermore, laboratory evidence shows that modulation of subcortical nuclei may attenuate and ameliorate seizures. We review clinical evidence indicating abnormal function in subcortical circuits and present a hypothesis that the development of infantile spasms requires dysfunction in both cortical and subcortical circuits. The confluence of evidence suggesting a role of subcortical structures in the origin of spasms and laboratory data indicating an anticonvulsant role on some subcortical nuclei raise the possibility of novel approaches to the treatment of infantile spasms.