No abstract

Precise identification of neuronal populations is a major challenge in neuroscience. In the striatum, more than 95% of neurons are GABAergic medium-sized spiny neurons (MSNs), which form two intermingled populations distinguished by their projections and protein content. Those expressing dopamine D1-receptors (D1Rs) project preferentially to the substantia nigra pars reticulata (SNr), whereas those expressing dopamine D2- receptors (D2Rs) project preferentially to the lateral part of the globus pallidus (LGP). The degree of segregation of these populations has been a continuous subject of debate, and the recent introduction of bacterial artificial chromosome (BAC) transgenic mice expressing fluorescent proteins driven by specific promoters was a major progress to facilitate striatal neuron identification. However, the fraction of MSNs labeled in these mice has been recently called into question, casting doubt on the generality of results obtained with such approaches. Here, we performed an in-depth quantitative analysis of striatal neurons in drd1a-EGFP and drd2-EGFP mice. We first quantified neuronal and non-neuronal populations in the striatum, based on nuclear staining with TO-PRO-3, and immunolabeling for NeuN, DARPP-32 (dopamine- and cAMP-regulated phosphoprotein Mr∼32,000), and various markers for interneurons. TO-PRO-3 staining was sufficient to identify MSNs by their typical nuclear morphology and, with a good probability, interneuron populations. In drd1a-EGFP/drd2-EGFP double transgenic mice all MSNs expressed EGFP, which was driven in about half of them by drd1a promoter. Retrograde labeling showed that all MSNs projecting to the SNr expressed D1R and very few D2R (<1%). In contrast, our results were compatible with the existence of some D1R-EGFP-expressing fibers giving off terminals in the LGP. Thus, our study shows that nuclear staining is a simple method for identifying MSNs and other striatal neurons. It also unambiguously confirms the degree of segregation of MSNs in the mouse striatum and allows the full exploitation of results obtained with BAC-transgenic mice.

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.

The prelimbic-medial orbital areas (PL/MO) of the prefrontal cortex are connected to the medial part of the subthalamic nucleus (STN) through a direct projection and an indirect circuit that involves the core of the nucleus accumbens (NAcc) and the ventral pallidum (VP). In the present study, the influence of the PL/MO on the discharge of STN cells has been characterized. The major pattern of the responses observed after stimulation of PL/MO consisted of two excitatory peaks often separated by a brief inhibitory period. The early excitation was most likely to be caused by the activation of direct cortical inputs because its latency matches the conduction time of the prefrontal STN projections. The late excitation resulted from the activation of the indirect PL/MO-STN pathway that operates through a disinhibitory process. Indeed, the late excitation was no longer observed after acute blockade of the glutamatergic corticostriatal transmission by CNQX application into the NAcc. A similar effect was obtained after the blockade of the GABAergic striatopallidal transmission by bicuculline application into the VP. Finally, the brief inhibition that followed the early excitation was likely to result from the activation of a feedback inhibitory loop through VP because this inhibition was no longer observed after the blockade of STN inputs by CNQX application into the VP. This study further indicates the implication of STN in prefrontal basal ganglia circuits and underlines that in addition to a direct excitatory input, medial STN receives an indirect excitatory influence from PL/MO through an NAcc-VP-STN disinhibitory circuit. Key words: basal ganglia circuits; prefrontal cortex; subthalamic nucleus; ventral striatum; nucleus accumbens; ventral pallidum; in vivo single unit recordings; ratThe subthalamic nucleus (STN) is a major component of the basal ganglia, and its critical role in the control of movement is well established. Pathological damage to the STN in humans or STN lesions in monkeys induce hemiballism (Whittier, 1947;Carpenter et al., 1950;Crossman, 1987). In 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys, an animal model of Parkinson's disease, abnormal activity of STN neurons has been observed, and lesions or high-frequency stimulation of the STN ameliorates akinesia and rigidity (Bergman et al., 1990;Benazzouz et al., 1993). High-frequency stimulation of the STN is now successf ully applied to improve akinesia and rigidity in Parkinsonian patients (Limousin et al., 1995).In current models of the basal ganglia circuitry, the striatum and the STN are the two major structures through which cortical signals are transmitted to the output structures of the basal ganglia, i.e., the substantia nigra pars reticulata (SN R) and the internal segment of the globus pallidus (GPi) (Alexander and Crutcher, 1990; Parent and Hazrati, 1995a,b). Indeed, both the striatum and the STN receive direct excitatory cortical inputs and send projections to the SN R and the GPi. Because the projection neurons of the stri...

Cortical afferents to the basal ganglia, and in particular the corticostriatal projections, are critical in the expression of basal ganglia function in health and disease. The corticostriatal projections are topographically organized but also partially overlap and interdigitate. To determine whether projections from distinct cortical areas converge at the level of single interneurons in the striatum, double anterograde labeling from the primary motor (M1) and primary somatosensory (S1) cortices in the rat, was combined with immunolabeling for parvalbumin (PV), to identify one population of striatal GABAergic interneurons. Cortical afferents from M1 and S1 gave rise to distinct, but partially overlapping, arbors of varicose axons in the striatum. PV-positive neurons were often apposed by cortical terminals and, in many instances, apposed by terminals from both cortical areas. Frequently, individual cortical axons formed multiple varicosities apposed to the same PV-positive neuron. Electron microscopy confirmed that the cortical terminals formed asymmetric synapses with the dendrites and perikarya of PV-positive neurons as well as unlabelled dendritic spines. Correlated light and electron microscopy revealed that individual PV-positive neurons received synaptic input from axon terminals derived from both motor and somatosensory cortices. These results demonstrate that, within areas of overlap of functionally distinct projections, there is synaptic convergence at the single cell level. Sensorimotor integration in the basal ganglia is thus likely to be mediated, at least in part, by striatal GABAergic interneurons. Furthermore, our findings suggest that the pattern of innervation of GABAergic interneurons by cortical afferents is different from the cortical innervation of spiny projection neurons.

1. In vivo extracellular and intracellular recordings were performed from thalamocortical (TC) neurones in a genetic model of absence epilepsy (genetic absence epilepsy rats from Strasbourg) during spontaneous spike and wave discharges (SWDs). 2. Extracellularly recorded single units (n = 14) fired either a single action potential or a high frequency burst of up to three action potentials, concomitantly with the spike component of the spike-wave complex. 3. Three main events characterized the intracellular activity of twenty-six out of twenty-eight TC neurones during SWDs: a small amplitude tonic hyperpolarization that was present throughout the SWD, rhythmic sequences of EPSP/IPSPs occurring concomitantly with the spike-wave complexes, and a small tonic depolarization at the end of the SWD. The rhythmic IPSPs, but not the tonic hyperpolarization, were mediated by activation of GABAA receptors since they reversed in polarity at -68 mV and appeared as depolarizing events when recording with KCl-filled electrodes. 4. The intracellular activity of the remaining two TC neurones consisted of rhythmic low threshold Ca2+ potentials, with a few EPSP/IPSP sequences present at the start of the SWD. 5. These results obtained in a well-established genetic model of absence epilepsy do not support the hypothesis that the intracellular activity of TC neurones during SWDs involves rhythmic sequences of GABAB IPSPs and low threshold Ca2+ potentials.

Corticostriatal projections constitute the main input to the basal ganglia, an ensemble of interconnected subcortical nuclei involved in procedural learning. Thus, long-term plasticity at corticostriatal synapses would provide a basic mechanism for the function of basal ganglia in learning and memory. We had previously reported the existence of a corticostriatal anti-Hebbian spike timing-dependent plasticity (STDP) at synapses onto striatal output neurons, the medium-sized spiny neurons. Here, we show that the blockade of GABAergic transmission reversed the time dependence of corticostriatal STDP. We explored the receptors and signalling mechanisms involved in the corticostriatal STDP. Although classical models for STDP propose NMDA receptors as the unique coincidence detector, the involvement of multiple coincidence detectors has also been demonstrated. Here, we show that corticostriatal STDP depends on distinct coincidence detectors. Specifically, long-term potentiation is dependent on NMDA receptor activation, while long-term depression requires distinct coincidence detectors: the phospholipase Cβ (PLCβ) and the inositol-trisphosphate receptor (IP 3 R)-gated calcium stores. Furthermore, we found that PLCβ activation is controlled by group-I metabotropic glutamate receptors, type-1 muscarinic receptors and voltage-sensitive calcium channel activities. Activation of PLCβ and IP 3 Rs leads to robust retrograde endocannabinoid signalling mediated by 2-arachidonoyl-glycerol and cannabinoid CB1 receptors. Interestingly, the same coincidence detectors govern the corticostriatal anti-Hebbian STDP and the Hebbian STDP reported at cortical synapses. Therefore, LTP and LTD induced by STDP at corticostriatal synapses are mediated by independent signalling mechanisms, each one being controlled by distinct coincidence detectors.

The functions of the basal ganglia are achieved through excitation of striatal output neurons (SONs) by converging cortical glutamergic afferents. We assessed the relationship between different patterns of activity in cortico-striatal (C-S) cells and the electrical behavior of SONs in vivo. Intracellular activities of rat C-S neurons in the orofacial motor cortex and of SONs, located in the projection field of this cortical region, were recorded under different anesthetics, which generate various temporal patterns of cortical activity. A surface electroencephalogram (EEG) of the orofacial motor cortex was simultaneously performed with intracellular recordings and EEG waves were used as correlates of a coherent synaptic activity in cortical neurons. Under barbiturate anesthesia C-S neurons showed rhythmic (5--7 Hz) supra-threshold depolarizations in phase with large amplitude EEG waves. The correlative activity of SONs was characterized by large amplitude oscillation-like synaptic depolarizations that could trigger action potentials. Under ketamine-xylazine anesthesia C-S neurons exhibited a step-like behavior consisting of depolarizing plateaus (up states), leading to multiple spike discharges, interrupted by hyperpolarizing periods (down states). The related activity of SONs was step-like membrane potential fluctuations with firing confined to the early part of the striatal up state. In C-S neurons and SONs up states coincided with slow recurrent EEG waves (approximately 1 Hz). Finally, under neurolept-analgesia an apparently disorganized EEG activity was associated with a lack of rhythmic discharge in C-S neurons. This uncorrelated activity in C-S neurons resulted in an absence of spontaneous firing as well as of large amplitude synaptic depolarizations in SONs. In the present study we demonstrate that SONs shape their input-output relationship by filtering out uncorrelated synaptic activity and that a minimal synchronization in the cortico-striatal afferents is required to produce significant synaptic depolarization in SONs.