Alban de Kerchove d'Exaerde

D2R striatopallidal neurons inhibit both locomotor and drug reward processes.

The specific functions of dopamine D2 receptor–positive (D2R) striatopallidal neurons remain poorly understood. Using a genetic mouse model, we found that ablation of D2R neurons in the entire striatum induced hyperlocomotion, whereas ablation in the ventral striatum increased amphetamine conditioned place preference. Thus D2R striatopallidal neurons limit both locomotion and, unexpectedly, drug reinforcement.

Differential regulation of motor control and response to dopaminergic drugs by D1R and D2R neurons in distinct dorsal striatum subregions

The dorsal striatum is critically involved in a variety of motor behaviours, including regulation of motor activity, motor skill learning and motor response to psychostimulant and neuroleptic drugs, but contribution of D2R‐striatopallidal and D1R‐striatonigral neurons in the dorsomedial (DMS, associative) and dorsolateral (DLS, sensorimotor) striatum to distinct functions remains elusive. To delineate cell type‐specific motor functions of the DMS or the DLS, we selectively ablated D2R‐ and D1R‐expressing striatal neurons with spatial resolution. We found that associative striatum exerts a population‐selective control over locomotion and reactivity to novelty, striatopallidal and striatonigral neurons inhibiting and stimulating exploration, respectively. Further, DMS‐striatopallidal neurons are involved only in early motor learning whereas gradual motor skill acquisition depends on striatonigral neurons in the sensorimotor striatum. Finally, associative striatum D2R neurons are required for the cataleptic effect of the typical neuroleptic drug haloperidol and for amphetamine motor response sensitization. Altogether, these data provide direct experimental evidence for cell‐specific topographic functional organization of the dorsal striatum.

Kit-negative fibroblast-like cells expressing SK3, a Ca2+-activated K+ channel, in the gut musculature in health and disease.

Abstract. The apamin-sensitive component of the inhibitory response of the gastrointestinal musculature involves the small conductance Ca2+-activated K+ channel SK3. Kit-immunoreactive (ir) interstitial cells of Cajal appear to be involved in nitrergic inhibition while the role of the recently described CD34-ir fibroblast-like cells adjacent to, but distinct from, the cells of Cajal remains elusive. The distribution of SK3 was studied by immunohistochemistry in the normal human gut, in motility disorders with a lack of cells of Cajal (infantile hypertrophic pyloric stenosis and Hirschsprungs disease) and in mice deficient in cells of Cajal. SK3 immunoreactivity was observed exclusively in Kit-negative interstitial cells adjacent to, but distinct from, the Kit-ir interstitial cells of Cajal in the normal gut. The distribution of SK3-ir cells was not altered in conditions where cells of Cajal were lacking. These cells were CD34-ir fibroblast-like cells in the human gut and in the mouse stomach, while SK3-ir cells in the mouse intestine were CD34 negative. As SK channels are reportedly involved in inhibitory neurotransmission, our morphological observations suggest that SK3-ir interstitial cells, distinct from the Kit-ir interstitial cells of Cajal, may represent a novel cellular component in the control of excitability of the digestive musculature. Further studies will be required to directly address the function of these cells.

Aminopyridines correct early dysfunction and delay neurodegeneration in a mouse model of spinocerebellar ataxia type 1.

The contribution of neuronal dysfunction to neurodegeneration is studied in a mouse model of spinocerebellar ataxia type 1 (SCA1) displaying impaired motor performance ahead of loss or atrophy of cerebellar Purkinje cells. Presymptomatic SCA1 mice show a reduction in the firing rate of Purkinje cells (both in vivo and in slices) associated with a reduction in the efficiency of the main glutamatergic synapse onto Purkinje cells and with increased A-type potassium current. The A-type potassium channel Kv4.3 appears to be internalized in response to glutamatergic stimulation in Purkinje cells and accumulates in presymptomatic SCA1 mice. SCA1 mice are treated with aminopyridines, acting as potassium channel blockers to test whether the treatment could improve neuronal dysfunction, motor behavior, and neurodegeneration. In acutely treated young SCA1 mice, aminopyridines normalize the firing rate of Purkinje cells and the motor behavior of the animals. In chronically treated old SCA1 mice, 3,4-diaminopyridine improves the firing rate of Purkinje cells, the motor behavior of the animals, and partially protects against cell atrophy. Chronic treatment with 3,4-diaminopyridine is associated with increased cerebellar levels of BDNF, suggesting that partial protection against atrophy of Purkinje cells is possibly provided by an increased production of growth factors secondary to the reincrease in electrical activity. Our data suggest that aminopyridines might have symptomatic and/or neuroprotective beneficial effects in SCA1, that reduction in the firing rate of Purkinje cells can cause cerebellar ataxia, and that treatment of early neuronal dysfunction is relevant in neurodegenerative disorders such as SCA1.

Distribution and compartmental organization of GABAergic medium-sized spiny neurons in the mouse nucleus accumbens

The nucleus accumbens (NAc) is a critical brain region involved in many reward-related behaviors. The NAc comprises major compartments the core and the shell, which encompass several subterritories. GABAergic medium-sized spiny neurons (MSNs) constitute the output neurons of the NAc core and shell. While the functional organization of the NAc core outputs resembles the one described for the dorsal striatum, a simple classification of the NAc shell neurons has been difficult to define due to the complexity of the compartmental segregation of cells. We used a variety of BAC transgenic mice expressing enhanced green fluorescence (EGFP) or the Cre-recombinase (Cre) under the control of the promoter of dopamine D1, D2, and D3 receptors and of adenosine A2a receptor to dissect the microanatomy of the NAc. Moreover, using various immunological markers we characterized in detail the distribution of MSNs in the mouse NAc. In addition, cell-type specific extracellular signal-regulated kinase (ERK) phosphorylation in the NAc subterritories was analyzed following acute administration of SKF81297 (a D1R-like agonist), quinpirole (a D2 receptors (D2R)-like agonist), apomorphine (a non-selective DA receptor agonist), raclopride (a D2R-like antagonist), and psychostimulant drugs, including cocaine and d-amphetamine. Each drug generated a unique topography and cell-type specific activation of ERK in the NAc. Our results show the existence of marked differences in the receptor expression pattern and functional activation of MSNs within the shell subterritories. This study emphasizes the anatomical and functional heterogeneity of the NAc, which will have to be considered in its further study.

The prolactin-releasing peptide antagonizes the opioid system through its receptor GPR10

Prolactin-releasing peptide (PrRP) and its receptor G protein–coupled receptor 10 (GPR10) are expressed in brain areas involved in the processing of nociceptive signals. We investigated the role of this new neuropeptidergic system in GPR10-knockout mice. These mice had higher nociceptive thresholds and stronger stress-induced analgesia than wild-type mice, differences that were suppressed by naloxone treatment. In addition, potentiation of morphine-induced antinociception and reduction of morphine tolerance were observed in mutants. Intracerebroventricular administration of PrRP in wild-type mice promoted hyperalgesia and reversed morphine-induced antinociception. PrRP administration had no effect on GPR10-mutant mice, showing that its effects are mediated by GPR10. Anti-opioid effects of neuropeptide FF were found to require a functional PrRP-GPR10 system. Finally, GPR10 deficiency enhanced the acquisition of morphine-induced conditioned place preference and decreased the severity of naloxone-precipitated morphine withdrawal syndrome. Altogether, our data identify the PrRP-GPR10 system as a new and potent negative modulator of the opioid system.

Inhibition of constitutive inward rectifier currents in cerebellar granule cells by pharmacological and synaptic activation of GABAB receptors

γ‐Aminobutyric acid (GABA)B receptors are known to enhance activation of Kir3 channels generating G‐protein‐dependent inward rectifier K+‐currents (GIRK). In some neurons, GABAB receptors either cause a tonic GIRK activation or generate a late K+‐dependent inhibitory postsynaptic current component. However, other neurons express Kir2 channels, which generate a constitutive inward rectifier K+‐current (CIRK) without requiring G‐protein activation. The functional coupling of CIRK with GABAB receptors remained unexplored so far. About 50% of rat cerebellar granule cells in the internal granular layer of P19–26 rats showed a sizeable CIRK current. Here, we have investigated CIRK current regulation by GABAB receptors in cerebellar granule cells, which undergo GABAergic inhibition through Golgi cells. By using patch‐clamp recording techniques and single‐cell reverse transcriptase‐polymerase chain reaction in acute cerebellar slices, we show that granule cells co‐express Kir2 channels and GABAB receptors. CIRK current biophysical properties were compatible with Kir2 but not Kir3 channels, and could be inhibited by the GABAB receptor agonist baclofen. The action of baclofen was prevented by the GABAB receptor blocker CGP35348, involved a pertussis toxin‐insensitive G‐protein‐mediated pathway, and required protein phosphatases inhibited by okadaic acid. GABAB receptor‐dependent CIRK current inhibition could also be induced by repetitive GABAergic transmission at frequencies higher than the basal autorhythmic discharge of Golgi cells. These results suggest therefore that GABAB receptors can exert an inhibitory control over CIRK currents mediated by Kir2 channels. CIRK inhibition was associated with an increased input resistance around rest and caused a ∼ 5 mV membrane depolarization. The pro‐excitatory action of these effects at an inhibitory synapse may have an homeostatic role re‐establishing granule cell readiness under conditions of strong inhibition.

Spatial distribution of D1R- and D2R-expressing medium-sized spiny neurons differs along the rostro-caudal axis of the mouse dorsal striatum.

The striatum projection neurons are striatonigral and striatopallidal medium-sized spiny neurons (MSNs) that preferentially express D1 (D1R) and D2 (D2R) dopamine receptors, respectively. It is generally assumed that these neurons are physically intermingled, without cytoarchitectural organization although this has not been tested. To address this question we used BAC transgenic mice expressing enhanced green fluorescence (EGFP) under the control of Drd1a or Drd2 promoter and spatial point pattern statistics. We demonstrate that D1R- and D2R-expressing MSNs are randomly distributed in most of the dorsal striatum, whereas a specific region in the caudal striatum, adjacent to the GPe, lacks neurons expressing markers for indirect pathway neurons. This area comprises almost exclusively D1R-expressing MSNs. These neurons receive excitatory inputs from the primary auditory cortex and the medial geniculate thalamic nucleus and a rich dopamine innervation. This area contains cholinergic and GABAergic interneurons but apparently no D2R/A2aR modulation because no fluorescence was detected in the neuropil of Drd2-EGFP or Drd2-Cre, and Adora-Cre BAC transgenic mice crossed with reporter mice. This striatal area that expresses calbindin D28k, VGluT1 and 2, is poor in μ opiate receptors and preproenkephalin. Altogether, the differences observed in D1R-MSNs, D2R-MSNs, and interneurons densities, as well as the anatomical segregation of D1R- and D2R/A2aR-expressing MSNs suggest that there are regional differences in the organization of the striatum.

Targeting neuronal populations of the striatum.

The striatum is critically involved in motor and motivational functions. The dorsal striatum, caudate–putamen, is primarily implicated in motor control and the learning of habits and skills, whereas the ventral striatum, the nucleus accumbens, is essential for motivation and drug reinforcement. The GABA medium-sized spiny neurons (MSNs, about 95% of striatal neurons), which are targets of the cerebral cortex and the midbrain dopaminergic neurons, form two pathways. The dopamine D1 receptor-positive (D1R) striatonigral MSNs project to the medial globus pallidus and substantia nigra pars reticulata (direct pathway) and co-express D1R and substance P, whereas dopamine D2 receptor-positive (D2R) striatopallidal MSNs project to the lateral globus pallidus (indirect pathway) and co-express D2R, adenosine A2A receptor (A2AR) and enkephalin (Enk). The specific role of the two efferent pathways in motor and motivational control remained poorly understood until recently. Indeed, D1R striatonigral and D2R striatopallidal neurons, are intermingled and morphologically indistinguishable, and, hence, cannot be functionally dissociated with techniques such as chemical lesions or surgery. In view of the still debated respective functions of projection D2R striatopallidal and D1R striatonigral neurons and striatal interneurons, both in motor control and learning but also in more cognitive processes such as motivation, the present review sum up the development of new models and techniques (bacterial artificial chromosome transgenesis, optogenetic, viral transgenesis) allowing the selective targeting of these striatal neuronal populations in adult animal brain to understand their specific roles.

Interstitial cells of Cajal in the striated musculature of the mouse esophagus

Abstract. Interstitial cells of Cajal (ICC) are important regulatory cells in the smooth muscle coats of the digestive tract. Expression of the Kit receptor tyrosine kinase was used in this study as a marker to study their distribution and development in the striated musculature of the mouse esophagus. Sections and whole-mounts were studied by immunohistochemistry. KitW-lacZ transgenic mice, which carry the lacZ reporter gene inserted in place of the first exon of the Kit gene, were processed for Xgal histochemistry, for quantitative analysis and for ultrastructural studies. Spindle-shaped ICC were scarce in both muscle layers of the thoracic esophagus, while their number increased steeply toward the cardia in the striated portion of the intraabdominal esophagus. They did not form networks and had no relationship with intrinsic myenteric ganglia and motor end-plates. They were often close to nerve fibers immunoreactive for neuronal nitric oxide synthase (nNOS), vasoactive intestinal polypeptide (VIP) or neuropeptide Y (NPY), but not to fibers immunoreactive for substance P (SP), calcitonin gene related peptide (CGRP), enkephalin, or the capsaicin receptor VR1. They were present in the fetus but absent in adult ICC-deficient KitW-lacZ/KitWv mice. Interstitial cells of Cajal were identified by electron microscopy by their ultrastructure in the striated muscle of the esophagus and exhibited Xgal labeling, while fibroblasts and muscle cells were unlabeled. Interstitial cells of Cajal are scattered between striated muscle cells in the mouse esophagus. They are close to nerves with defined neurochemical coding and could possibly represent specialized esophageal spindle proprioceptors.