Origin and molecular specification of oligodendrocytes in the telencephalon

Previous studies have demonstrated that oligodendrocytes in the spinal cord and brainstem originate from restricted domains of ventral neuroepithelium under the influence of sonic hedgehog protein (SHH). However, the origin of oligodendrocytes in the telencephalon, the most anterior part of the CNS, has not been established. A recent study has precisely mapped the origin of telencephalic oligodendrocytes to a small region of the ventral telencephalon in rodents, and has presented evidence that telencephalic oligodendrogenesis is, like that of the spinal cord, under the influence of SHH signaling.     

Functional and molecular development of striatal fast-spiking GABAergic interneurons and their cortical inputs

Despite their small number, fast-spiking (FS) GABAergic interneurons play a critical role in controlling striatal output by mediating cortical feed-forward inhibition of striatal medium-sized spiny (MS) projection neurons. We have examined the functional development of FS interneurons and their cortical inputs, and the expression of three of their molecular markers, in the dorsolateral rat striatum between postnatal days (P)12--14 and 19--23, the time of major corticostriatal synaptogenesis. FS interneurons were visualized with infrared differential interference contrast (IR-DIC) optics and examined with current-clamp recording in the presence of the GABA(A) receptor antagonist bicuculline methiodide. FS interneurons displayed action potentials at relatively high frequencies in response to depolarizing current pulses by P12, but developmental changes occurred in action potential and afterhyperpolarization duration and amplitude and input resistance between P12--14 and P19--23, as well as an increase in maximum firing frequency in response to depolarizing current pulses. Maturation in electrophysiological properties was paralleled by increases in Kv 3.1 and parvalbumin mRNA expression, while GAD-67 mRNA levels remained constant. Furthermore, FS interneurons in the younger age group responded to stimulation of cortical afferents with excitatory postsynaptic potentials (EPSPs) of higher amplitudes and received significantly more spontaneous depolarizing inputs than did MS neurons. Thus, FS interneurons are under frequent and continuous cortical influence by the end of the 2nd postnatal week, a time when corticostriatal synapses are sparse, suggesting that they may provide a major inhibitory influence in the striatum during the period of intense developmental maturation.     

Excitatory extrinsic afferents to striatal interneurons and interactions with striatal microcircuitry

The striatum constitutes the main input structure of the basal ganglia and receives two major excitatory glutamatergic inputs, from the cortex and the thalamus. Excitatory cortico‐ and thalamostriatal connections innervate the principal neurons of the striatum, the spiny projection neurons (SPNs), which constitute the main cellular input as well as the only output of the striatum. In addition, corticostriatal and thalamostriatal inputs also innervate striatal interneurons. Some of these inputs have been very well studied, for example the thalamic innervation of cholinergic interneurons and the cortical innervation of striatal fast‐spiking interneurons, but inputs to most other GABAergic interneurons remain largely unstudied, due in part to the relatively recent identification and characterization of many of these interneurons. In this review, we will discuss and reconcile some older as well as more recent data on the extrinsic excitatory inputs to striatal interneurons. We propose that the traditional feed‐forward inhibitory model of the cortical input to the fast‐spiking interneuron then inhibiting the SPN, often assumed to be the prototype of the main functional organization of striatal interneurons, is incomplete. We provide evidence that the extrinsic innervation of striatal interneurons is not uniform but shows great cell‐type specificity. In addition, we will review data showing that striatal interneurons are themselves interconnected in a highly cell‐type‐specific manner. These data suggest that the impact of the extrinsic inputs on striatal activity critically depends on synaptic interactions within interneuronal circuitry.     

Selective inhibition of striatal fast-spiking interneurons causes dyskinesias

Fast-spiking interneurons (FSIs) can exert powerful control over striatal output, and deficits in this cell population have been observed in human patients with Tourette syndrome and rodent models of dystonia. However, a direct experimental test of striatal FSI involvement in motor control has never been performed. We applied a novel pharmacological approach to examine the behavioral consequences of selective FSI suppression in mouse striatum. IEM-1460, an inhibitor of GluA2-lacking AMPARs, selectively blocked synaptic excitation of FSIs but not striatal projection neurons. Infusion of IEM-1460 into the sensorimotor striatum reduced the firing rate of FSIs but not other cell populations, and elicited robust dystonia-like impairments. These results provide direct evidence that hypofunction of striatal FSIs can produce movement abnormalities, and suggest that they may represent a novel therapeutic target for the treatment of hyperkinetic movement disorders.     

Re-emergence of striatal cholinergic interneurons in movement disorders

Twenty years ago, striatal cholinergic neurons were central figures in models of basal ganglia function. But since then, they have receded in importance. Recent studies are likely to lead to their re-emergence in our thinking. Cholinergic interneurons have been implicated as key players in the induction of synaptic plasticity and motor learning, as well as in motor dysfunction. In Parkinson's disease and dystonia, diminished striatal dopaminergic signalling leads to increased release of acetylcholine by interneurons, distorting network function and inducing structural changes that undoubtedly contribute to the symptoms. By contrast, in Huntington's disease and progressive supranuclear palsy, there is a fall in striatal cholinergic markers. This review gives an overview of these recent experimental and clinical studies, placing them within the context of the pathogenesis of movement disorders.     

Muscarinic and dopaminergic receptor subtypes on striatal cholinergic interneurons

Unilateral stereotaxic injection of small amounts of the cholinotoxin, AF64A, caused minimal nonselective tissue damage and resulted in a significant loss of the presynaptic cholinergic markers [3H]hemicholinium-3 (45% reduction) and choline acetyltransferase (27% reduction). No significant change from control was observed in tyrosine hydroxylase or tryptophan hydroxylase activity; presynaptic neuronal markers for dopamine- and serotonin-containing neurons, respectively. The AF64A lesion resulted in a significant reduction of dopamine D2 receptors as evidenced by a decrease in [3H]sulpiride binding (42% reduction) and decrease of muscarinic non-M1 receptors as shown by a reduction in [3H]QNB binding in the presence of 100 nM pirenzepine (36% reduction). Saturation studies revealed that the change in [3H]sulpiride and [3H]QNB binding was due to a change in Bmax not Kd. Intrastriatal injection of AF64A failed to alter dopamine D1 or muscarinic M1 receptors labeled with [3H]SCH23390 and [3H]pirenzepine, respectively. In addition, no change in [3H]forskolin-labeled adenylate cyclase was observed. These results demonstrate that a subpopulation of muscarinic receptors (non-M1) are presynaptic on cholinergic interneurons (hence, autoreceptors), and a subpopulation of dopamine D2 receptors are postsynaptic on cholinergic interneurons. Furthermore, dopamine D1, muscarinic M1 and [3H]forskolin-labeled adenylate cyclase are not localized to striatal cholinergic interneurons.     

Ablation of striatal interneurons influences activities of entopeduncular neurons

To characterize modulatory effects of striatal interneurons upon the output nucleus of the basal ganglia, we ablated striatal interneurons that express substance P receptors by using local injection of a selective neurotoxin, substance P-saporin, in rats. We then made extracellular recordings of the activity of entopeduncular neurons and examined their responses to stimulation in the motor cortex. In the interneuron-ablated animals, the spontaneous discharge rate of entopeduncular neurons was significantly decreased, and the proportion of entopeduncular neurons showing responses to cortical stimulation was significantly larger, in comparison with intact animals. It is suggested that striatal interneurons expressing substance P receptors are important for motor control mechanisms mediated by the cortico-basal ganglia pathways.     

Morphological abnormalities in nitric-oxide-synthase-positive striatal interneurons of schizophrenic patients

Schizophrenia has been suggested to be a neurodevelopmental disorder, and nitric-oxide-synthase (NOS)-positive neurons were shown to be involved in distorted cortical development in schizophrenia. Here we investigated whether nitrinergic neurons in the striatum of schizophrenic patients also display abnormalities regarding distribution or morphology. To do so, postmortem putaminal sections of schizophrenic subjects were examined by means of nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) staining and NOS immunohistochemistry. NOS-positive neurons were counted and analyzed morphologically. Abnormalities regarding morphology or number of NOS-containing neurons could be found in the putamen of schizophrenics (n = 3), but not controls (n = 5). Neurons were either of abnormal size and branching pattern, or they were markedly reduced (130 +/- 44 vs. 54 +/- 62 NADPHd-positive somata/mm(3) putamen; p < 0.0001). Striatal nitrinergic interneurons might thus be involved in the pathogenesis of at least some forms of schizophrenia. Studies on larger samples are however needed to further corroborate this finding.     

TRPC3 channel mediates excitation of striatal cholinergic interneurons

Striatal cholinergic interneurons exhibit tonic firing and more positive membrane potentials, however, the mechanism is unclear. In the present study, we found that intracellular perfusion of TRPC3 antibody induced outward current in striatal cholinergic interneurons identified by electrophysiological characteristics. The TRPC3 channel blocker flufenamic acid induced hyperpolarization, and reduced firing rate and outward current which was similar to the effect of TRPC3 channel antibody. Furthermore, by using single-cell RT-PCR we confirmed the co-existence of TRPC3 channel and D5 receptor mRNA in striatal cholinergic interneurons identified by electrophysiological characteristics and expression of choline acetyltransferase (Chat) mRNA. These results implied that the TRPC3 channel is involved in modulating the depolarization of cholinergic interneurons.     

Inhibition of Ih in striatal cholinergic interneurons early after transient forebrain ischemia.

Striatal cholinergic interneurons are relatively resistant to ischemic insults. These neurons express hyperpolarization-activated cation current (I(h)) that profoundly regulates neuronal excitability. Changes in neuronal excitability early after ischemia may be crucial for determining neuronal injury. Here we report that I(h) in cholinergic interneurons was decreased 3 h after transient forebrain ischemia, which was accompanied by a negative shift of the voltage dependence of activation. The inhibition of I(h) might be due to the tonic activation of adenosine A1 receptors, as blockade of A1 receptors significantly increased I(h) in postischemic neurons, but had no effect on control neurons. Consistent with the inhibition of I(h), postischemic neurons showed a reduction in both spontaneous firing and hyperpolarization-induced rebound depolarization. These findings indicate that I(h) may play excitatory roles in striatal cholinergic interneurons. Postischemic inhibition of I(h) might be a novel mechanism by which adenosine confers neuronal resistance to cerebral ischemia.     

Adenylate cyclase in striatal cholinergic interneurons regulates acetylcholine release

Fractional [³H]ACH efflux from dissociated rat striata tested whether tonic inhibition prevents stimulation of acetylcholine (ACH) release by adenylate cyclase. Forskolin stimulated release from the dissociated cells (threshold at 300 nM; EC₅₀ ≥ 1 μM). Release was also stimulated by 3-isobutyl-l-methylxanthine and was additive with forskolin. The 1,9-dideoxy forskolin analog that lacks cyclase-stimulating activity was ineffective. Thus, stimulation of adenylate cyclase within striatal cholinergic interneurons increases ACH secretion but is tonically inhibited by endogenous striatal transmitters. Disinhibition of the excitatory cyclase by denervation of striatal cholinergic interneurons in situ could contribute to supersensitivity without receptor upregulation.     

Cholinergic modulation of striatal nitric oxide‐producing interneurons

Striatal GABAergic interneurons that express nitric oxide synthase—so‐called low‐threshold spike interneurons (LTSIs)—play several key roles in the striatum. But what drives the activity of these interneurons is less well defined. To fill this gap, a combination of monosynaptic rabies virus mapping (msRVm), electrophysiological and optogenetic approaches were used in transgenic mice in which LTSIs expressed either Cre recombinase or a fluorescent reporter. The rabies virus studies revealed a striking similarity in the afferent connectomes of LTSIs and neighboring cholinergic interneurons, particularly regarding connections arising from the parafascicular nucleus of the thalamus and cingulate cortex. While optogenetic stimulation of cingulate inputs excited both cholinergic interneurons and LTSIs, thalamic stimulation excited cholinergic interneurons, but inhibited LTSIs. This inhibition was dependent on cholinergic interneurons and had two components: a previously described GABAergic element and one that was mediated by M4 muscarinic acetylcholine receptors. In addition to this phasic signal, cholinergic interneurons tonically excited LTSIs through a distinct, M1 muscarinic acetylcholine receptor pathway. This coordinated cholinergic modulation of LTSIs predisposed them to rhythmically burst in response to phasic thalamic activity, potentially reconfiguring striatal circuitry in response to salient environmental stimuli.     

Functional and ultrastructural analysis of group I mGluR in striatal fast-spiking interneurons

Striatal parvalbumin-containing fast-spiking (FS) interneurons provide a powerful feedforward GABAergic inhibition on spiny projection neurons, through a widespread arborization and electrical coupling. Modulation of FS interneuron activity might therefore strongly affect striatal output. Metabotropic glutamate receptors (mGluRs) exert a modulatory action at various levels in the striatum. We performed electrophysiological recordings from a rat striatal slice preparation to investigate the effects of group I mGluR activation on both the intrinsic and synaptic properties of FS interneurons. Bath-application of the group I mGluR agonist, (S)-3,5-dihydroxyphenylglycine (3,5-DHPG), caused a dose-dependent depolarizing response. Both (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385) and 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt), selective mGluR1 antagonists, significantly reduced the amplitude of the membrane depolarization caused by 3,5-DHPG application. Conversely, mGluR5 antagonists, 2-methyl-6-(phenylethylnyl)pyridine hydrochloride (MPEP) and 6-methyl-2-(phenylazo)-3-pyridinol (SIB1757), were unable to affect the response to 3,5-DHPG, suggesting that only mGluR1 contributes to the 3,5-DHPG-mediated excitatory action on FS interneurons. Furthermore, mGluR1 blockade significantly decreased the amplitude of the glutamatergic postsynaptic potentials, whereas the mGluR5 antagonist application produced a small nonsignificant inhibitory effect. Surprisingly, our electron microscopic data demonstrate that the immunoreactivity for both mGluR1a and mGluR5 is expressed extrasynaptically on the plasma membrane of parvalbumin-immunoreactive dendrites of FS interneurons. Together, these results suggest that despite a common pattern of distribution, mGluR1 and mGluR5 exert distinct functions in the modulation of FS interneuron activity.     

Subpopulations of striatal interneurons can be distinguished on the basis of neurotrophic factor expression.

Substantial evidence supports a role for trophic activities in the function and survival of fully mature striatal neurons, but little is known regarding trophic factor expression in adult striatum. In situ hybridization was used to identify the distribution and the neurotransmitter phenotypes (i.e., cholinergic and gamma-aminobutyric acid [GABA]-ergic) of cells expressing acidic fibroblast growth factor (aFGF), glial cell line-derived neurotrophic factor (GDNF), or nerve growth factor (NGF) mRNA in adult rat striatum. Each trophic factor mRNA was localized to large, sparsely scattered striatal cells that corresponded to interneurons. Double-labeling studies demonstrated that NGF mRNA was expressed by GABAergic and never by cholinergic cells, whereas aFGF and GDNF mRNAs were expressed by both cell types. Approximately 75% of aFGF+ and GDNF+ cells in dorsal striatum and 46% of aFGF+ and 61% of GDNF+ cells in ventral striatum were cholinergic. Conversely, about 32% of aFGF+ and 24% of GDNF+ cells in dorsal striatum and 55% of aFGF+ and 27% of GDNF+ cells in ventral striatum were GABAergic. A portion of aFGF+ and NGF+ cells was of the parvalbumin GABAergic subtype. The colocalization of trophic factor expression was also examined. Of aFGF+ cells, 20% and 41% were NGF+ and 67% and 83% were GDNF+ in dorsal and ventral striata, respectively. These findings demonstrate that aFGF, GDNF, and NGF are synthesized by discrete but overlapping populations of striatal interneurons. The expression of these survival factors may contribute to the resistance of striatal interneurons to various insults including excitotoxicity.     

Rat striatal cyclic nucleotide-reactive cells and acetylcholinesterase reactive interneurons are separate populations

The concurrent localization of cyclic nucleotide immunofluorescent cells and acetylcholinesterase-containing neurons in the rat caudate-putamen complex has been examined. Cyclic AMP and cyclic GMP stained elements do not exhibit coincident localization with the enzymatically detectable hydrolysis of acetylcholine. These data add further support for the preferential association of the cyclic nucleotides with striatal efferent projection systems, while the large cholinergic somata are part of the interneuron population of the rat caudate-putamen complex.     

Immunohistochemical localization of DARPP-32 in striatal projection neurons and striatal interneurons: implications for the localization of D1-like dopamine receptors on different types of striatal neurons.

Immunohistochemical double-label techniques were used to study the localization of DARPP-32, a phosphoprotein that is enriched in neurons possessing members of the D1 subfamily of dopamine receptors, in several different types of striatal neurons in the rat basal ganglia. The vast majority (94.1%) of striatonigral projection neurons (the vast majority of which contain substance P), identified by retrograde labeling with fluorogold, were observed to contain DARPP-32. Similarly, the vast majority of striatopallidal projection neurons (87.7%), identified by immunofluorescence labeling for enkephalin (ENK), were found to label for DARPP-32. In contrast, cholinergic and neuropeptide Y-containing striatal interneurons were never observed to contain DARPP-32. These results suggest that essentially all major types of striatal medium spiny projection neurons may possess members of the D1 subfamily of dopamine receptors, but that striatal local circuit neurons do not possess members of the D1 subfamily of receptors.     

Cortical inputs to m2-immunoreactive striatal interneurons in rat and monkey

Previous anatomical studies have been unsuccessful in demonstrating significant cortical inputs to cholinergic and somatostatinergic striatal interneurons in rats. On the other hand, electrophysiological studies have shown that cortical stimulation induces monosynaptic EPSPs in cholinergic interneurons. It has been proposed that the negative anatomical findings might have been the result of incomplete labeling of distal dendrites. In the present study, we reinvestigated this issue using m2 muscarinic receptor antibodies as a selective marker for cholinergic and somatostatinergic interneurons in the striatum. This was combined with injections of either the anterograde tracer biotinylated dextran amine (BDA) in the monkey prefrontal cortex or aspiration lesion of the sensorimotor cortex in rats. The results showed that, in both species, a small percentage (1-2%) of cortical terminals make asymmetric synaptic contacts with m2-immunoreactive interneurons in the striatum. Interestingly, the majority of these synapses are onto small dendritic spines or spine-like appendages, as opposed to dendritic shafts and/or cell bodies. Thus, m2-containing striatal interneurons do receive direct cortical inputs and can, therefore, integrate and modulate cortical information flow through the striatum. Although the density of cortical terminals in contact with individual striatal interneurons is likely to be relatively low compared to the massive cortical input to projection neurons, both cholinergic and somatostatinergic interneurons display intrinsic properties that allow even small and distal inputs to influence their overall state of neuronal activity.     

Muscimol increases acetylcholine release by directly stimulating adult striatal cholinergic interneurons

Because GabaA ligands increase acetylcholine (ACh) release from adult striatal slices, we hypothesized that activation of GabaA receptors on striatal cholinergic interneurons directly stimulates ACh secretion. Fractional [³H]ACh release was recorded during perifusion of acutely dissociated, [³H]choline-labeled, adult male rat striata. The GabaA agonist, muscimol, immediately stimulated release maximally 300% with EC₅₀=1 μM. This action was enhanced by the allosteric GabaA receptor modulators, diazepam and secobarbital, and inhibited by the GabaA antagonist, bicuculline, by ligands for D2 or muscarinic cholinergic receptors or by low calcium buffer, tetrodotoxin or vesamicol. Membrane depolarization inversely regulated muscimol-stimulated secretion. Release of endogenous and newly synthesized ACh was stimulated in parallel by muscimol without changing choline release. Muscimol pretreatment inhibited release evoked by K⁺ depolarization or by receptor-mediated stimulation with glutamate. Thus, GabaA receptors on adult striatal cholinergic interneurons directly stimulate voltage- and calcium-dependent exocytosis of ACh stored in vesamicol-sensitive synaptic vesicles. The action depends on the state of membrane polarization and apparently depolarizes the membrane in turn. This functional assay demonstrates that excitatory GabaA actions are not limited to neonatal tissues. GabaA-stimulated ACh release may be prevented in situ by normal tonic dopaminergic and muscarinic input to cholinergic neurons.     

Involvement of intracellular calcium stores during oxygen/glucose deprivation in striatal large aspiny interneurons.

Striatal large aspiny interneurons were recorded from a slice preparation using a combined electrophysiologic and microfluorometric approach. The role of intracellular Ca2+ stores was analyzed during combined oxygen/glucose deprivation (OGD). Before addressing the role of the stores during energy deprivation, the authors investigated their function under physiologic conditions. Trains of depolarizing current pulses caused bursts of action potentials coupled to transient increases in intracellular calcium concentration ([Ca2+]i). In the presence of cyclopiazonic acid (30 micromol/L), a selective inhibitor of the sarcoendoplasmic reticulum Ca2+ pumps, or when ryanodine receptors were directly blocked with ryanodine (20 [micromol/L), the [Ca2+]i transients were progressively smaller in amplitude, suggesting that [Ca2+]i released from intracellular stores helps to maintain a critical level of [Ca2+]i during physiologic firing activity. As the authors have recently reported, brief exposure to combined OGD induced a membrane hyperpolarization coupled to an increase in [Ca2+]i. In the presence of cyclopiazonic acid or ryanodine, the hyperpolarization and the rise in [Ca2+]i induced by OGD were consistently reduced. These data support the hypothesis that Ca2+ release from ryanodine-sensitive Ca2+ pools is involved not only in the potentiation of the Ca2+ signals resulting from cell depolarization, but also in the amplification of the [Ca2+]i rise and of the concurrent membrane hyperpolarization observed in course of OGD in striatal large aspiny interneurons.