Striatal interneurones: chemical, physiological and morphological characterization

Local disinhibition of neocortical neuronal circuits causes augmentation of glutamatergic and GABAergic synaptic transmission in the rat neostriatum in vitro

Intra- and extracellular recordings were performed to investigate the influence of local disinhibition of neocortical circuits on corticostriatal synaptic transmission. In rat brain slices with preserved corticostriatal connections, electrical stimulation of the neocortex elicited composed postsynaptic responses in neostriatal neurons consisting of glutamatergic excitatory postsynaptic potentials (EPSPs) and weakly expressed GABAA receptor-mediated inhibitory postsynaptic potentials (IPSPs). Following local application of the GABAA receptor antagonist bicuculline to the neocortex, neocortical neurons responded to intracortical stimulation with transient paroxysmal depolarizations. Simultaneously, the amplitude of neocortically evoked EPSPs recorded from neostriatal neurons was found to be enhanced without changes in duration. Similarly, the amplitude of IPSPs increased following disinhibition of neocortical circuits. In addition and in contrast to EPSPs, the duration of the IPSPs was found to be markedly prolonged. The results demonstrate that local disinhibition of neocortical neuronal circuits potentiates both excitatory and inhibitory synaptic transmission in striatal neurons. However, compared to AMPA receptor-mediated excitation, GABAA receptor-mediated inhibition becomes more efficient due to a marked prolongation of IPSPs. The pronounced augmentation of inhibition can be attributed to a strong activation of inhibitory interneurons within the striatum.

A critical role of the nitric oxide/cGMP pathway in corticostriatal long-term depression

High-frequency stimulation (HFS) of corticostriatal glutamatergic fibers induces long-term depression (LTD) of excitatory synaptic potentials recorded from striatal spiny neurons. This form of LTD can be mimicked by zaprinast, a selective inhibitor of cGMP phosphodiesterases (PDEs). Biochemical analysis shows that most of the striatal cGMP PDE activity is calmodulin-dependent and inhibited by zaprinast. The zaprinast-induced LTD occludes further depression by tetanic stimulation and vice versa. Both forms of synaptic plasticity are blocked by intracellular 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ), a selective inhibitor of soluble guanylyl cyclase, indicating that an increased cGMP production in the spiny neuron is a key step. Accordingly, intracellular cGMP, activating protein kinase G (PKG), also induces LTD. Nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) and 7-nitroindazole monosodium salt (7-NINA) block LTD induced by either HFS or zaprinast, but not that induced by cGMP. LTD is also induced by the NO donors S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine. SNAP-induced LTD occludes further depression by HFS or zaprinast, and it is blocked by intracellular ODQ but not by L-NAME. Intracellular application of PKG inhibitors blocks LTD induced by HFS, zaprinast, and SNAP. Electron microscopy immunocytochemistry shows the presence of NOS-positive terminals of striatal interneurons forming synaptic contacts with dendrites of spiny neurons. These findings represent the first demonstration that the NO/cGMP pathway exerts a feed-forward control on the corticostriatal synaptic plasticity.

Phencyclidine induces D-1 dopamine receptor mediated Fos-like immunoreactivity in discretely localised populations of striatopallidal and striatoentopeduncular neurons in the rat

Phencyclidine (PCP), a non-competitive antagonist of the NMDA subtype of glutamate receptor, which also acts as an indirect dopamine agonist and at sigma sites, can induce a long lasting psychotic state when taken acutely. It is well established that PCP is toxic to specific limbic structures and we have recently demonstrated that it induces apoptosis of a subpopulation of striatal neurons. These neurons lie predominantly in the dorsomedial striatum and project to the globus pallidus. The mechanisms mediating this neuronal death are unclear though manipulations of dopamine transmission can induce striatal c-fos expression and continuous c-fos expression has been implicated in the molecular cascades controlling apoptosis. We accordingly undertook a series of experiments to determine the action of PCP on striatal Fos-like immunoreactivity (FLI). PCP (80 mg/kg, s.c.) elicited FLI in three distinct striatal areas, namely dorsomedial, dorsolateral and the nucleus accumbens. The level of PCP-induced FLI was consistently attenuated by the co-administration of the D-1 antagonist, SCH 23390. Vehicle injections also induced modest levels of FLI in the dorsomedial striatum and the nucleus accumbens which again were attenuated by SCH 23390. The type of striatal neuron in which PCP-induced FLI was determined by the use of a retrograde anatomical tracer. A colloidal gold tracer was thus injected into the major areas of termination of striatal projection neurons prior to the administration of PCP. This procedure demonstrated that the majority of the FLI positive striatal cells were striatopallidal neurons, though some FLI positive striatoentopeduncular neurons were also seen. The potential pharmacological mechanisms underlying the results are discussed. It is argued that the complex pattern of PCP-induced striatal FLI might be accounted for by a differential action upon extracellular dopamine levels whereby they are elevated in some striatal areas and simultaneously reduced in others.

Immunohistochemical localization of the somatostatin sst2(b) receptor splice variant in the rat central nervous system

Somatostatin is a neuromodulator in the mammalian CNS. To date, genes for at least five different somatotrophin release inhibiting factor receptors, termed sst1-sst5, have been cloned. The rat sst2 receptor exists in two splice variants, sst(alpha)a) and sst2(b), which differ in their carboxy-termini. When heterologously expressed in Chinese hamster ovary-K1 cells, these splice variants show little difference in their operational characteristics. Recently, the distribution of the sst2(a) receptor was documented, yet at present no data are available about the distribution of the sst2(b) receptor in the CNS. Here, we present the characterization of a novel polyclonal anti-peptide antibody that is selective for the sst2(b) receptor splice variant. The antibody was raised against the unique intracellular carboxy-terminal portion of the receptor protein. Using this affinity-purified antibody in western blotting experiments, the sst2(b) receptor expressed in Chinese hamster ovary-K1 cells was shown to be a glycoprotein with a molecular weight centred at about 85,000. The antibody showed no cross-reactivity to any of the recombinant human sst1-5 receptors, the rat sst2(a) receptor or wild-type Chinese hamster ovary-K1 cells. Employing immunohistochemistry, we investigated the distribution of the sst2(b) receptor in the brain and spinal cord of adult rats. A distinct distribution was found throughout the rostrocaudal axis of the CNS. Somatodendritic as well as axonal staining was observed. Somatodendritic labelling was particularly obvious in the olfactory bulb, cerebral cortex, hippocampal formation, mesencephalic trigeminal nucleus and cerebellum, as well as in cranial and spinal motor areas. The results show that the distribution of the sst2(b) receptor partially overlaps with that of the sst2(b) receptor, although there were differences in a number of brain areas. The location of the sst2(b) receptor implies that it may mediate a modulatory role of somatostatin inhibitory releasing factor on sensory as well as motor functions.

The effects of the N-terminal tripeptide of insulin-like growth factor-1, glycine-proline-glutamate in different regions following hypoxic-ischemic brain injury in adult rats

Insulin-like growth factor-1 has pleiotropic effects in the central nervous system and can act both as a survival and a differentiation factor. Insulin-like growth factor-1 can be proteolytically cleaved into des-N-(1-3)-insulin-like growth factor-1 and a N-terminal tripeptide fragment, glycine-proline-glutamate. Both insulin-like growth factor-1 and des-N-(1-3)-insulin-like growth factor-1 can improve neuronal survival after hypoxic-ischemic brain injury in vivo. The present study investigates the effects of glycine-proline-glutamate on different brain regions and neuronal populations after hypoxic-ischemic injury. Unilateral hypoxic-ischemic injury was induced in adult rats. Glycine-proline-glutamate (3 microg) was administered centrally 2 h after the injury and the extent of brain damage determined five days later. In a separate trial immunohistochemical techniques were used to determine the effects of glycine-proline-glutamate on specific populations of neurons in the striatum after the injury. Compared to the vehicle treatment, glycine-proline-glutamate (n=19) treatment reduced the extent of cortical damage and neuronal loss in the CA1-2 subregions of the hippocampus (P<0.05). In the striatum, there was a trend towards a reduction in neuronal loss after glycine-proline-glutamate treatment (P=0.053) compared to the vehicle (n=21)-treated animals. In a separate study, glycine-proline-glutamate (n=8) treatment prevented the loss of choline acetyltransferase (P<0.05), glutamate acid decarboxylase (P<0.05) and somatostatin (P<0.05) containing neurons in the ipsilateral striatum following hypoxic-ischemic brain injury and also increased the numbers of neuronal nitric oxide synthase (P<0.05) containing neurons in the contralateral side. These studies suggest that in addition to neuroprotective effects, glycine-proline-glutamate can influence neuronal activity after hypoxic-ischemic injury.

Cortically driven immediate-early gene expression reflects modular influence of sensorimotor cortex on identified striatal neurons in the squirrel monkey

Current understanding of basal ganglia function emphasizes their involvement in the focal, context-dependent release of motor and cognitive circuits in the brainstem and frontal lobes. How such selective action can arise despite the existence of massively convergent inputs from the cerebral cortex is unknown. However, anatomical work has suggested that specificity could be achieved in corticostriatal circuits by modular patterns of convergent and divergent cortical inputs to striatal projection neurons. To test for such modular activation of striatal neurons, we electrically microstimulated physiologically identified sites in the primary somatosensory (SI) and primary motor (MI) cortex of the squirrel monkey. We compared the efferent fiber distributions anterogradely traced from these sites to the distributions of striatal neurons activated by microstimulation to express Fos- and Jun B-like immediate-early gene proteins. We show that the microstimulation of sensorimotor cortex induces Fos and Jun B expression in localized cell clusters in the putamen and that these clusters match the anatomical input fiber clusters (matrisomes). The modular activation of striatal neurons by sensorimotor cortex seems likely. Unexpectedly, >75% of the Fos-positive nuclei in densely labeled cell clusters were in enkephalin-immunoreactive neurons. This expression pattern suggests that the primate sensorimotor cortex exerts a differential influence on the enkephalinergic (indirect pathway) as opposed to the substance P/dynorphin (direct pathway) projection neurons of the putamen. The densely labeled clusters of Fos-labeled enkephalinergic neurons occurred within larger zones containing sparsely distributed Fos-labeled parvalbumin neurons. Moreover, when the cortical stimulation induced expression of Fos-like protein only in sparsely distributed neurons, almost every putamenal neuron expressing Fos was a parvalbumin-containing (GABAergic) interneuron. These patterns suggest a model in which the primate sensorimotor cortex can target parvalbumin-containing inhibitory interneurons, which in turn depress the remaining neuronal activity within and around matrisomes in a feed-forward manner until sufficient coherent cortical input can overcome the inhibition to influence selectively enkephalinergic projection neurons in the activated matrisomes. Tuning of cortical input by striatal interneurons thus may be an important mechanism by which broader anatomical connections are dynamically adjusted to achieve selective flow of information through the basal ganglia.

XCE, a new member of the endothelin-converting enzyme and neutral endopeptidase family, is preferentially expressed in the CNS

In the present study, we have isolated a cDNA encoding a novel member of the family of zinc metallopeptidases that includes neutral endopeptidase and endothelin-converting enzyme. The predicted amino-acid sequence of this enzyme, termed XCE, consists of 775 amino-acids with a single putative membrane-spanning region, an N-terminal cytoplasmic domain of 59 residues, and a large luminal domain that contains a characteristic zinc-binding motif. Western blot analysis of cells stably expressing this new metallopeptidase revealed a glycosylated protein of approximately 95 kDa. XCE mRNA was found to be predominantly expressed in the central nervous system, sympathetic ganglia and in uterine subepithelial cells. In the rat and human CNS, a very specific pattern of neuronal labelling (in presumptive cholinergic interneurons of basal ganglia, basal forebrain neurons, as well as brainstem and spinal cord motoneurons) was detected by in situ hybridization histochemistry. The enzyme substrate, as yet unidentified, might be found among the numerous neuropeptide transmitters which are colocalized with acetylcholine in these neurons.

Role of arachidonic acid in the regulation of the NMDA-evoked release of acetylcholine in striatal compartments

The role of endogenously released arachidonic acid in the control of the NMDA (50 microM)-evoked release of [3H]-acetylcholine previously formed from [3H]-choline was investigated in striosome-enriched areas and in the matrix of the rat striatum using a microsuperfusion procedure in vitro. Experiments were performed with either mepacrine (0.2 microM) or bovine serum albumin (BSA, 0.02%) which inhibits phospholipase A2 activity or binds endogenously released arachidonic acid, respectively. Both treatments similarly reduce the NMDA-evoked release of [3H]-acetylcholine, this effect being more pronounced in striosomes than in the matrix. These reductions result from a facilitation of dopamine release, since they were not observed in the presence of (-)sulpiride, the D2 dopamine receptor antagonist. Moreover, the superfusion with BSA was shown to enhance the release of [3H]-dopamine (formed from [3H]-tyrosine), this effect being of larger amplitude in striosomes than in the matrix. In control conditions, due to the blockade of the presynaptic inhibitory effect of GABA on dopamine release, bicuculline (GABA(A) receptor antagonist) reduces the NMDA-evoked release of [3H]-acetylcholine in both striatal compartments. Bicuculline was no longer effective following superfusions with either mepacrine or BSA, suggesting that these treatments eliminate the GABAergic presynaptic inhibitory control on dopamine transmission and thus lead to the dopamine-mediated inhibition of [3H]-acetylcholine release. These results indicate that arachidonic acid endogenously formed under weak stimulation of NMDA receptors contributes to the regulation of the evoked release of [3H]-acetylcholine by facilitating GABAergic transmission and that this process is more important in striosomes than in the matrix.

Distribution of histamine H3-receptor binding in the normal human basal ganglia: comparison with Huntington's and Parkinson's disease cases

It is now widely recognized that histamine acts as a neurotransmitter in the mammalian central nervous system. Three selective histamine receptors have been described, all of which are present in the basal ganglia. This study is a detailed, quantitative, autoradiographical examination of the densities of histamine H3-receptors in coronal sections of human basal ganglia, using the selective ligand [3H]-(R)-alpha-methylhistamine. [3H]-(R)-alpha-methylhistamine binding was highest within the external and internal segments of the globus pallidus together with the substantia nigra. High levels were also found in the striatum, where density was significantly higher (P < 0.05) at a pre-, as opposed to post-, anterior commissure coronal level. Within the striatum, binding was noticeably higher in both the nucleus accumbens and acetylcholinesterase-deficient striosomes, while being undetectable in the subthalamic nucleus and very low in both the ventroanterior and ventrolateral thalamic nuclei. An intermediate level of binding, often with a laminar distribution, was seen in the insular cortex. [3H]-(R)-alpha-methylhistamine binding was also examined in both Parkinson's disease and Huntington's disease. No difference from control receptor density was found in any area examined in Parkinson's disease, while values were significantly lower in caudate (P < 0.001), putamen (P < 0.001), external (P < 0.001) and internal (P < 0.05) globus pallidus, although not the insular cortex, in Huntington's disease cases. These data suggest that H3-receptors are present upon striatonigral projection neurons of the direct and indirect movement pathways thus providing histaminergic control over the activity of both these circuits.

In vitro evidence for increased facilitation of striatal acetylcholine release via pre- and postsynaptic NMDA receptors in hemiparkinsonian rats

The NMDA-evoked acetylcholine release from striatal slices and synaptosomes was investigated in rats subjected to unilateral injection of 6-hydroxydopamine into the substantia nigra. In slices prepared from the striatum contralateral to the lesion, the NMDA-evoked endogenous acetylcholine release was not significant at 10 microM NMDA and maximal at 100 microM NMDA (124 +/- 19%). Conversely, in slices taken from the dopamine-depleted striatum, NMDA was effective even at 10 microM (41 +/- 4%), and at 100 microM (196 +/- 24%) efficacy was nearly doubled. In synaptosomes prepared from the contralateral striatum, NMDA maximally stimulated 20 mM KCl-induced endogenous acetylcholine release at 1 microM (66 +/- 5.1%), with lower concentrations (0.01-0.1 microM) being ineffective. Conversely, in synaptosomes prepared from the dopamine-depleted striatum, NMDA maximally enhanced the K+/--evoked acetylcholine release at 0.1 microM (118 +/- 12.4%). Concentration-response curves of NMDA-evoked acetylcholine release in sham-operated rats could be superimposed on those observed in the contralateral striatum of the 6-hydroxydopamine-lesioned animals. The present data support the view of an increased glutamatergic regulation of striatal acetylcholine release via pre- and postsynaptic NMDA receptors during Parkinson's disease.

Expression of NMDA glutamate receptor subunit mRNAs in neurochemically identified projection and interneurons in the striatum of the rat

NMDA receptors are composed of proteins from two families: NMDAR1 and NMDAR2. We used quantitative double-label in situ hybridization to examine in rat brain the expression of NMDAR1, NMDAR2A, NMDAR2B, and NMDAR2C mRNA in six neurochemically defined populations of striatal neurons: preproenkephalin (ENK) and preprotachykinin (SP) expressing projection neurons, and somatostatin (SOM), glutamic acid decarboxylase 67 (GAD67), parvalbumin (PARV), and choline acetyltransferase (ChAT) expressing interneurons. NMDAR1 was expressed by all striatal neurons: strongly in ENK, SP, PARV and ChAT neurons, and less intensely in SOM and GAD67 positive cells. NMDAR2A mRNA was present at moderate levels in all striatal neurons except those containing ChAT. Labeling for NMDAR2B was strong in projection neurons and ChAT interneurons, and only moderate in SOM, GAD67 and PARV interneurons. NMDAR2C was scarce in striatal neurons, but a low level signal was detected in GAD67 positive cells. NMDAR2C expression was also observed in small cells not labeled by any of the markers, most likely glia. These data suggest that all striatal neurons have NMDA receptors, but different populations have different subunit compositions which may affect function as well as selective vulnerability.

The 5-HT3 receptor antagonist, MDL 72222, dose-dependently potentiates morphine-induced immediate-early gene expression in the rat caudate putamen

Previous studies from this laboratory have demonstrated that acute administration of morphine induces the immediate-early genes (IEGs) c-Fos and JunB in the rat caudate putamen (CPu). In the present study, we tested the hypothesis that the serotonin-3 receptor (5-HT3R) is involved in morphine-induced IEG expression, using the selective antagonist to the 5-HT3R, MDL 72222. Rats were divided into three pretreatment groups: MDL 72222, 1 mg/kg or 10 mg/kg; or vehicle (DMSO). Thirty minutes following the pretreatment, the rats were administered either morphine (10 mg/kg) or vehicle. Morphine significantly induced c-Fos expression in the dorsomedial CPu, as we have reported previously. Whereas MDL 72222 alone did not induce c-Fos, it potentiated the morphine-induced c-Fos expression. Morphine also induced JunB expression in the same region of the dorsomedial CPu. At 1 mg/kg, MDL 72222 both induced JunB expression and potentiated the response induced by morphine. At 10 mg/kg, MDL 72222 had no effect on basal JunB levels, but augmented the response to morphine. These findings demonstrate that the 5-HT3R antagonist, MDL 72222, can positively modulate morphine-induced IEG expression in the rat CPu in a dose dependent manner, in contrast to the reported suppressive effect observed when this antagonist is administered prior to amphetamine.

Selective innervation of neostriatal interneurons by a subclass of neuron in the globus pallidus of the rat

A subpopulation of neurons in the globus pallidus projects to the neostriatum, which is the major recipient of afferent information to the basal ganglia. Given the moderate nature of this projection, we hypothesized that the pallidostriatal projection might exert indirect but powerful control over principal neuron activity by targeting interneurons, which comprise only a small percentage of neostriatal neurons. This was tested by the juxtacellular labeling and recording of pallidal neurons in combination with immunolabeling of postsynaptic neurons. In addition to innervating the subthalamic nucleus and output nuclei, 6 of 23 labeled pallidal neurons projected to the neostriatum. Both the firing characteristics and the extent of the axonal arborization in the neostriatum were variable. However, light and electron microscopic analysis of five pallidostriatal neurons revealed that each neuron selectively innervated neostriatal interneurons. A large proportion of the boutons of an individual axon (19-66%) made contact with parvalbumin-immunoreactive interneurons. An individual parvalbumin-immunoreactive neuron (n = 27) was apposed on average by 6.7 boutons (SD = 6.1) from a single pallidal axon (n = 2). Individual pallidostriatal boutons typically possessed more than one symmetrical synaptic specialization. In addition, 3-32% of boutons of axons from four of five pallidal neurons contacted nitric oxide synthase-immunoreactive neurons. Descending collaterals of pallidostriatal neurons were also found to make synaptic contact with dopaminergic and GABAergic neurons of the substantia nigra. These data imply that during periods of cortical activation, individual pallidal neurons may influence the activity of GABAergic interneurons of the neostriatum (which are involved in feed-forward inhibition and synchronization of principle neuron activity) while simultaneously patterning neuronal activity in basal ganglia downstream of the neostriatum.

Morphological and functional study of dwarf neurons in the rat striatum

Combination of morphological and electrophysiological techniques provided data, suggesting existence in the young rat striatum of a peculiar class of neurons, the neurogliaform or dwarf neurons. Striatal neurons (n = 92), intracellularly recorded from rat brain slices, were filled (one in each slice) with the intracellular marker biocytin, to compare physiological and morphological properties in the same cell. Moreover, some neurons (n = 7) were filled with biocytin plus the fluorescent calcium indicator fura-2, identifying cells during electrophysiological recording. Electrophysiological recording showed that striatal neurons had different firing patterns, suggestive in most cases (n = 80) of spiny neuron class and in others (n = 12) of interneuron class. Fura-2 injection clearly identified the body of six medium-sized cells and of one distinctive tiny cell. This small cell, however, showed a resting membrane potential and spontaneous and evoked firing pattern characteristic of striatal interneurons. Moreover, the fura-2 injected in such small neuron also completely filled the cell body of a near large neuron; the fura-2 fluorescence changed synchronously in the two paired neurons after electrical stimulation of the impaled small one. Accordingly, the biocytin staining identified the morphology of the small recorded neuron as a neurogliaform-like cell apposed to a dendrite of an aspiny neuron, suggesting that the dye injected in one neuron had diffused to the other of a different type. Furthermore, such heterologous dye coupling unexpectedly involved seven pairs of cells detected with biocytin staining (7.6% of the recorded neurons), invariably represented by a medium or large neuron on one side, and on the other side by a small (5.44 +/- 0.15 x 9.14 +/- 0.7 microns, mean +/- SD; n = 7) neurogliaform cell, roundish in shape with few slender and short processes, usually apposed to a dendrite of the companion neurons (six out of seven). In the other cases, the biocytin staining revealed in each slice either the morphology of single spiny or aspiny neurons (80.4% of recorded neurons), or of two-three medium-sized spiny neurons detected near to each other, suggesting that dye coupling had occurred typically between similar neurons (11.9% of the recorded neurons). These data suggest that some neurogliaform cells in the striatum of young rat can be identified as dwarf interneurons, that may be dye-coupled with neurons of different classes.

Characterization of the mechanism of action of tachykinins in rat striatal cholinergic interneurons

The mechanism by which substance P depolarizes cholinergic interneurons in the rat striatum was studied using whole-cell recording techniques. In all cases the effects of substance P were mimicked by the neurokinin1 receptor agonist [Sar9, Met(O2)11] substance P and were antagonized by the neurokinin1 receptor antagonist SR140333. [Sar9, Met(O2)11] substance P was found to depolarize cholinergic interneurons by the induction of a calcium-activated inward current at -60 mV. This inward current was irreversibly potentiated by photolysis of caged GTPgammaS within neurons implicating the involvement of a G-protein. The [Sar9, Met(O2)11] substance P-induced inward current was inhibited by the phospholipase C inhibitor U-73122, and by the inclusion of the inositol-1,4,5-triphosphate receptor antagonist heparin in the electrode solution. These findings suggest that neurokinin1 receptors depolarize cholinergic interneurons in the rat striatum primarily through a phosphoinositide signalling pathway.

Chemical phenotype of calretinin interneurons in the human striatum

We recently reported the existence of a new class of aspiny interneurons characterized by their immunoreactivity for the calcium-binding protein calretinin (CR) in human striatum. This group is composed of numerous medium-sized (10-20 microm) neurons with poorly branched dendrites and a smaller number of large-sized (24-42 microm) neurons with highly ramified dendrites. We further demonstrated the selective sparing of the medium-sized, but not all the large-sized, CR+ striatal neurons in Huntington's disease. In the present study, we applied a double-antigen localization method to postmortem striatal tissue obtained from normal individuals to further characterize the chemical phenotype of these two subsets of CR+ neurons. Our results reveal that in the medium-sized neurons, CR is not colocalized with any of the following current markers of striatal neurons: calbindin, parvalbumin, beta-nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), or choline acetyltransferase (ChAT). Furthermore, quantitative estimates show that the medium-sized CR+ neurons are by far the most abundant type of interneurons in the human striatum. In contrast, CR is colocalized with ChAT in about 80% of the large-sized CR+ neurons. Thus, the medium-sized CR+ neurons appear to form a distinct class of striatal interneurons, whereas most of the large-sized CR+ neurons belong to the population of giant cholinergic neurons. This study has provided the first exhaustive characterization of the chemical phenotype of the CR + neurons in the human striatum.

Evolution of the basal ganglia in tetrapods: a new perspective based on recent studies in amphibians

It has been postulated frequently that the fundamental organization of the basal ganglia (BG) in vertebrates arose with the appearance of amniotes during evolution. An alternative hypothesis, however, is that such a condition was already present in early anamniotic tetrapods and, therefore, characterizes the acquisition of the tetrapod phenotype rather than the anamniotic-amniotic transition. Re-examination of the BG organization in tetrapods in the light of recent findings in amphibians strongly supports the notion that elementary BG structures were present in the brain of ancestral tetrapods and that they were organized according to a general plan shared today by all extant tetrapods.

Effects of intrastriatal cannabinoids on rotational behavior in rats: interactions with the dopaminergic system

The effect of unilateral intrastriatal cannabinoid receptor stimulation on rotational behavior in rats was explored. The potent cannabinoid agonist CP 55,940 (5 microg/0.5 microl) induced contralateral turning when microinjected unilaterally into the striatum. The D2 dopamine agonist quinpirole reversed this contralateral rotation but failed to affect motor behavior on its own. Finally, the D1 dopamine agonist SKF 82958 inhibited movement when administered into the striatum and this inhibition was reversed by co-administration of the cannabinoid agonist. Surprisingly, microinjections of the cannabinoid agonist into the striatum induced movement through activation of the striatonigral pathway and/or inhibition of the striatopallidal pathway, while the D1 dopamine agonist produced the opposite effect.

Distribution of calbindin D-28k and parvalbumin neurons and fibers in the rat basal ganglia

This review deals with the distribution of immunoreactivity for calbindin D-28k (CB) and parvalbumin (PV) in the different nuclei of the rodent basal ganglia analyzed with the data available after the use of single and double antigen procedures applied to single sections. These findings reveal that CB and PV are distributed according to a highly heterogeneous pattern in the caudate putamen complex (CPu), globus pallidus (GP), entopeduncular nucleus (EP), subthalamic nucleus (STh) and substantia nigra (SN) of the rat. In each basal ganglia structure, the two calcium-binding proteins label different neuronal subsets. Therefore, the use of CB and PV immunohistochemistry may be considered as an excellent tool to define distinct chemoarchitectonic and functional domains within the complex organization of the basal ganglia. Double immunohistochemical methods are also useful to illustrate the relationships between the different chemical subdivisions of the CPu, GP, EP, STh and SN and the chemically characterized connections with each other and with other forebrain and brainstem structures. However, specific rules should be followed when combining single and double immunostaining procedures, and the results of such studies must be evaluated with caution. When they are used properly, these methods can reveal hitherto unknown principles of organization of the basal ganglia and thus shed new light on the anatomical and functional organization of this set of subcortical structures involved in the control of motor behavior.

Increased opioid inhibition of GABA release in nucleus accumbens during morphine withdrawal

The nucleus accumbens is a key component of the reward pathway that plays a role in addiction to many drugs of abuse, including psychostimulants and opioids. The effects of withdrawal from chronic morphine were examined in the nucleus accumbens using brain slices from morphine-treated animals. Recordings were made from interneurons in the shell of nucleus accumbens, and the presynaptic inhibition of GABA-A IPSCs by opioids was examined. In slices from control animals, opioids caused a maximal inhibition of 50%, forskolin increased the IPSC amplitude by less than twofold, and the maximal inhibition by opioids in the presence of forskolin was not changed. During withdrawal, however, forskolin caused approximately a fourfold increase in the amplitude of the IPSC, and the maximal inhibition by opioids was increased to 80%. The results indicate that transmitter release is increased during opioid withdrawal, particularly after the activation of adenylyl cyclase. The cAMP-dependent increase in transmitter release is potently inhibited by opioids, such that the overall effect of opioids is augmented during withdrawal. The induction of an opioid-sensitive cAMP-dependent mechanism that regulates transmitter release may be a critical component of acute opioid withdrawal.

D1 and D2 dopamine receptor mediation of amphetamine-induced acetylcholine release in nucleus accumbens

To assess the interaction of dopamine and acetylcholine systems in the rat nucleus accumbens in response to direct D-amphetamine administration, in vivo microdialysis measures of acetylcholine were used during reverse dialysis of amphetamine alone and in combination with D1 and D2 receptor antagonists SCH 23390 and sulpiride, respectively. During a 15-min exposure to amphetamine (50 microM) in the nucleus accumbens, acetylcholine increased to 33% above pre-infusion levels, became maximal at 15 min post-infusion (+41%) and gradually returned to baseline levels by 60 min post-amphetamine. Conversely, amphetamine (1 mM) administration caused a biphasic change in acetylcholine release with a trend toward a decrease (-14%) during exposure followed by a significant increase (+36%) at 30 min post-amphetamine that returned to baseline levels by 60 min after infusion. The increases observed during amphetamine (50 microM) exposure and during recovery from amphetamine (1 mM) were both blocked by co-administration with the D1 antagonist, SCH 23390 (10 microM), but not with the D2 antagonist, sulpiride (10 microM). Co-infusion of sulpiride eliminated the trend toward reduced acetylcholine release observed during 1 mM amphetamine whereas co-administration of SCH 23390 potentiated this decrease. A possible tonic D1 facilitation of nucleus accumbens acetylcholine release was indicated by the consistent reductions in acetylcholine release observed during infusion of SCH 23390. These results suggest that amphetamine administration in the nucleus accumbens induces a bidirectional change in acetylcholine release that is dependent on dose and opposing effects of nucleus accumbens D1 and D2 activation. In general, relatively low doses of amphetamine administered into the nucleus accumbens caused an increase in acetylcholine release that was dependent on dopamine D1 receptors whereas higher doses of amphetamine resulted in a D2-mediated decrease.

Endogenous ACh enhances striatal NMDA-responses via M1-like muscarinic receptors and PKC activation

Cortical glutamatergic fibres and cholinergic inputs arising from large aspiny interneurons converge on striatal spiny neurons and play a major role in the control of motor activity. We have investigated the interaction between excitatory amino acids and acetylcholine (ACh) on striatal spiny neurons by utilizing intracellular recordings, both in current- and in voltage-clamp mode in rat brain slices. Muscarine (0.3-10 microM) produced a reversible and dose-dependent increase in the membrane depolarizations/inward currents induced by brief applications of N-methyl-D-aspartate (NMDA), while it did not affect the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-induced responses. These concentrations of muscarine did not alter the membrane potential and the current-voltage relationship of the recorded cells. Neostigmine (0.3-10 microM), an ACh-esterase inhibitor, mimicked this facilitatory effect. The facilitatory effects of muscarine and neostigmine were antagonized either by scopolamine (3 microM) or by pirenzepine (10-100 nM), an antagonist of M1-like muscarinic receptors, but not by methoctramine (300 nM), an antagonist of M2-like muscarinic receptor. Accordingly, these facilitatory effects were mimicked by McN-A-343 (1-10 microM), an agonist of M1-like muscarinic receptors, but not by oxotremorine (300 nM), an agonist of M2-like receptors. Tetrodotoxin (TTX) did not block the facilitatory effect produced by the activation of muscarinic receptors suggesting that this effect is postsynaptically mediated. The action of neostigmine was prevented either by the intracellular calcium (Ca2+) chelator BAPTA (200 mM) or by preincubating the slices with inhibitors of protein kinase C (PKC) (staurosporine 100 nM or calphostin C 1 microM). McN-A-343 did not alter the excitatory post synaptic potentials (EPSPs) evoked by corticostriatal stimulation in the presence of physiological concentration of magnesium (Mg2+ 1.2 mM), while it enhanced the duration of these EPSPs recorded in the absence of external magnesium. Our data show that endogenous striatal ACh exerts a positive modulatory action on NMDA responses via M1-like muscarinic receptors and PKC activation.

Brain functions and psychiatric disorders. A clinical view

Psychiatric disorders are viewed as brain disorders affecting five circuits of the brain: the brainstem, the hypothalamus, the motor striatum, the limbic system (ventral striatum), and the neocortex. This five-circuit model combines neuroscientific, psychopharmacologic, evolutionary, introspective, and behavioral data. The article presents a broad speculative overview of some brain functions and dysfunctions relevant for clinicians.

Blockade of M2-like muscarinic receptors enhances long-term potentiation at corticostriatal synapses

Acetylcholine (ACh) exerts a crucial role in learning and memory. The striatum contains the highest concentration of this transmitter in the brain. This structure expresses two different forms of synaptic plasticity, long-term depression (LTD) and long-term potentiation (LTP), which might contribute to the storage of motor skills and some cognitive processes. We have investigated the role of M2-like muscarinic receptors in striatal LTP by utilizing intracellular recordings in vitro from a rat corticostriatal slice preparation. Methoctramine (250 nM), an antagonist of M2-like muscarinic receptors, enhanced striatal LTP induced in the absence of external magnesium (Mg2+) by high-frequency stimulation (HFS) of corticostriatal fibres. Methoctramine did not affect the amplitude of excitatory postsynaptic potentials (EPSPs) when bath applied either before or after the conditioning tetanus suggesting that a critical increase of ACh concentrations is produced only during HFS. Methoctramine per se failed to enhance the NMDA-mediated EPSPs recorded in the absence of external Mg2+ and in the presence of 10 microM CNQX. Methoctramine antagonized the presynaptic inhibitory action of neostigmine, an inhibitor of ACh-esterase, and oxotremorine, an agonist of M2-like muscarinic receptors. These data indicate that the activation of M2-like muscarinic receptors exerts a negative influence on striatal LTP, probably by reducing the release of glutamate from corticostriatal fibres and they suggest a complex modulatory effect of ACh in striatal synaptic plasticity.

Cholinergic switching within neocortical inhibitory networks

Differential actions of acetylcholine on the excitability of two subtypes of interneurons in layer V of the rat visual cortex were examined. Acetylcholine excited low-threshold spike (LTS) cells through nicotinic receptors, whereas it elicited hyperpolarization in fast spiking (FS) cells through muscarinic receptors. Axons of LTS cells were mainly distributed vertically to upper layers, and those of FS cells were primarily confined to layer V. Thus, cortical cholinergic activation may reduce some forms of intralaminar inhibition, promote intracolumnar inhibition, and change the direction of information flow within cortical circuits.

Distinct modifications by neurokinin1 (SR140333) and neurokinin2 (SR48968) tachykinin receptor antagonists of the N-methyl-D-aspartate-evoked release of acetylcholine in striosomes and matrix of the rat striatum

The effects of SR140333 and SR48968 (neurokinin1 and neurokinin2 tachykinin receptor antagonists, respectively) on the N-methyl-D-aspartate-evoked release of [3H]acetylcholine (previously formed from [3H]choline) were investigated in striosome-enriched areas and in the matrix of the rat striatum using an in vitro microsuperfusion method. In both striatal compartments, SR140333 and SR48968 did not modify the 50 microM N-methyl-D-aspartate-evoked release of [3H]acetylcholine. However, in low concentrations, both SR140333 (0.1 microM to 1 pM) and SR48968 (0.1 microM to 0.1 nM) markedly enhanced the 1 mM N-methyl-D-aspartate (+10 microM D-serine)-evoked release of [3H]acetylcholine in striosome-enriched areas. These responses were dopamine-dependent since they were not observed any more following the local blockade of D2 receptors by sulpiride or of dopamine synthesis by alpha-methyl-p-tyrosine. A dopamine-dependent disinhibitory effect (of lower amplitude) on the 1 mM N-methyl-D-aspartate (+10 microM D-serine)-evoked release of [3H]acetylcholine was also induced by SR48968 (0.1 microM to 0.1 nM) (but not by SR140333) in the matrix. In addition, in the matrix, as shown only in the presence of alpha-methyl-p-tyrosine, both SR140333 and SR48968 reduced the 1 mM N-methyl-D-aspartate (+10 microM D-serine)-evoked response and these non-dopamine-mediated inhibitory effects only occurred at the highest tested concentration (0.1 microM) of the antagonists. Indicating the specificity of these responses, the effects of SR140333 were reproduced by RP67580, another neurokinin1 receptor antagonist and, as expected from previous binding studies, corresponding SR140333 and SR48968 enantiomers were without effect. These results suggest that under potent stimulation of N-methyl-D-aspartate receptors, endogenously released substance P and neurokinin A (or related tachykinins) regulate differently the N-methyl-D-aspartate-evoked release of [3H]acetylcholine in striosomes and in the matrix. The inhibitory effects of these tachykinins on the evoked release of [3H]acetylcholine are mediated by dopamine. On the contrary, their facilitatory responses are only observed in the matrix under blockade of dopamine transmission.

Identification of an ATP-sensitive potassium channel current in rat striatal cholinergic interneurones

1. Whole-cell patch-clamp recordings were made from rat striatal cholinergic interneurones in slices of brain tissue in vitro. In the absence of ATP in the electrode solution, these neurones were found to gradually hyperpolarize through the induction of an outward current at -60 mV. This outward current and the resultant hyperpolarization were blocked by the sulphonylureas tolbutamide and glibenclamide and by the photorelease of caged ATP within neurones. 2. This ATP-sensitive outward current was not observed when 2 mM ATP was present in the electrode solution. Under these conditions, 500 microM diazoxide was found to induce an outward current that was blocked by tolbutamide. 3. Using permeabilized patch recordings, neurones were shown to hyperpolarize in response to glucose deprivation or metabolic poisoning with sodium azide (NaN3). The resultant hyperpolarization was blocked by tolbutamide. 4. In cell-attached recordings, metabolic inhibition with 1 mM NaN3 revealed the presence of a tolbutamide-sensitive channel exhibiting a unitary conductance of 44.1 pS. 5. Reverse transcription followed by the polymerase chain reaction using cytoplasm from single cholinergic interneurones demonstrated the expression of the ATP-sensitive potassium (KATP) channel subunits Kir6.1 and SUR1 but not Kir6.2 or SUR2. 6. It is concluded that cholinergic interneurones within the rat striatum exhibit a KATP channel current and that this channel is formed from Kir6.1 and SUR1 subunits.

Muscarinic IPSPs in rat striatal cholinergic interneurones

1. Intracellular recordings were made from neurones in slice of rat striatum in vitro. 2. The forty-nine neurones studied were immunoreactive for choline acetyltransferase and had the electrophysiological characteristics typical of large aspiny interneurones. 3. Focal stimulation of the slice elicited a hyperpolarizing inhibitory postsynaptic potential in thirty-five neurones. This IPSP lasted 0.5-1 s and reversed polarity at a membrane potential which was dependent on the logarithm of the extracellular potassium concentration. 4. The IPSP was reversibly blocked by scopolamine and methoctramine, which has some selectivity for M2 subtype of muscarinic receptor. It was unaffected by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM), DL-2-amino-phosphonovaleric acid (30 microM) and bicuculline (30 microM). 5. Exogenous acetylcholine and muscarine also hyperpolarized the neurones, and this was blocked by methoctramine by not by pirenzepine, which is an M1 receptor-selective antagonist. 6. The findings demonstrate that muscarinic IPSPs occur in the central nervous system. The IPSP may mediate an 'autoinhibition' of striatal cholinergic neurone activity.

Inhibition of GABAergic inhibitory postsynaptic currents by cannabinoids in rat corpus striatum

Electrophysiological consequences of activation of cannabinoid receptors have been mostly investigated on neuronal cell lines and on cells transfected with cannabinoid receptors. The aim of the present experiments was to study cannabinoid effects on identified neurons in situ. Electrically-evoked postsynaptic currents and voltage-dependent calcium currents were investigated in the principal neurons of the corpus striatum, the medium spiny neurons, with the patch-clamp method for brain slices. These neurons were chosen because they produce messenger RNA for cannabinoid receptors and because the density of cannabinoid binding sites in the striatum is high. Activation of muscarinic receptors by carbachol (10(-5) M) reduced inhibitory postsynaptic current amplitude by 67%. The synthetic cannabinoid receptor agonist R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4- benzoxazin-yl]-(1-naphtalenyl)methanone (WIN55212-2; 10(-8) to 10(-5) M) dose-dependently reduced striatal inhibitory postsynaptic currents; the maximum effect, inhibition by 52%, was observed at 10(-6) M. Another cannabinoid agonist, (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydr oxypropyl)cyclohexanol (CP55940; 10(-6) M), also reduced inhibitory postsynaptic currents, by 50%. The CB1 cannnabinoid receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)4-methyl-3-pyra zolecarboxamide (SR141716A; 10(-6) M) had no effect when given alone but abolished the effect of WIN55212-2 (10(-6) M). WIN55212-2 (10(-6) M) did not change the current evoked by the GABA(A)-receptor agonist muscimol (10(-6) M). Activation of muscarinic receptors by carbachol (10(-5) M) inhibited voltage-dependent calcium currents by 21%, but the cannabinoid receptor agonist WIN55212-2 (10(-6) M) was without effect. The results show that activation of CB1 cannabinoid receptors reduces GABAergic inhibitory postsynaptic currents in medium spiny neurons of the corpus striatum: the likely mechanism is presynaptic inhibition of GABA release from terminals of recurrent axons of the medium spiny neurons themselves.

Drugs of abuse and immediate-early genes in the forebrain

A diverse array of chemical agents have been self administered by humans to alter the psychological state. Such drugs of abuse include both stimulants and depressants of the central nervous system. However, some commonalties must underlie the neurobiological actions of these drugs, since the desire to take the drugs often crosses from one drug to another. Studies have emphasized a role of the ventral striatum, especially the nucleus accumbens, in the actions of all drugs of abuse, although more recent studies have implicated larger regions of the forebrain. Induction of immediate-early genes has been studied extensively as a marker for activation of neurons in the central nervous system. In this review, we survey the literature reporting activation of immediate-early gene expression in the forebrain, in response to administration of drugs of abuse. All drugs of abuse activate immediate-early gene expression in the striatum, although each drug induces a particular neuroanatomical signature of activation. Most drugs of abuse activate immediate-early gene expression in several additional forebrain regions, including portions of the extended amygdala, cerebral cortex, lateral septum, and midline/intralaminar thalamic nuclei, although regional variations are found depending on the particular drug administered. Common neuropharmacological mechanisms responsible for activation of immediate-early gene expression in the forebrain involve dopaminergic and glutamatergic systems. Speculations on the biological significance and clinical relevance of immediate-early gene expression in response to drugs of abuse are presented.

Early specification of striatal projection neurons and interneuronal subtypes in the lateral and medial ganglionic eminence

The striatum is thought to be generated from two transient swellings in the ventral telencephalon, the lateral and medial ganglionic eminences, present at mid-stages of embryonic rat development. We have studied the relative contribution of these structures to the specific generation of striatal neuronal subtypes such as projection neurons and cholinergic and somatostatin-containing interneurons at an early stage and a mid stage in striatal neurogenesis. Dissociated progenitors isolated from the embryonic day 12.5 and embryonic day 15.5 rat lateral ganglionic eminence grafted into the previously ibotenic acid lesioned adult striatum, produce grafts containing extensive numbers of neurons expressing messenger RNA for the striatal projection neuron marker, DARPP-32, whereas grafts of the embryonic day 12.5 and embryonic day 15.5 medial ganglionic eminences do not. While preprosomatostatin messenger RNA-expressing neurons were observed in grafts from each of the lateral ganglionic eminence and medial ganglionic eminence at both embryonic day 12.5 and embryonic day 15.5, choline acetyltransferase messenger RNA-expressing cholinergic neurons were largely found in grafts derived from the embryonic day 12.5 medial ganglionic eminence. These results suggest that the neuronal diversity of the adult striatum may derive both from the lateral ganglionic eminence, providing DARPP-32-expressing projection neurons as well as somatostatin-containing interneurons, and the early stage medial ganglionic eminence specifically contributing the cholinergic interneurons.

Focal cerebral ischemia in rats induces expression of P75 neurotrophin receptor in resistant striatal cholinergic neurons

Expression of p75 neurotrophin receptor and survival of medium-sized spiny projection neurons and cholinergic interneurons in the rat striatum were studied using immunocytochemistry at different times after transient, unilateral middle cerebral artery occlusion. Thirty minutes of middle cerebral artery occlusion caused a major loss of projection neurons, identified by their immunoreactivity to dopamine- and adenosine 3':5'-monophosphate-regulated phosphoprotein with a molecular weight of 32,000, in the lateral part of the striatum, as observed at 48 h following the insult with no further change at one week. In contrast, no reduction of the number of choline acetyltransferase-positive, cholinergic interneurons, which also expressed TrkA, was detected at either time-point. At 48 h following middle cerebral artery occlusion, expression of p75 neurotrophin receptor was observed in striatal cells which, by the use of double-label immunostaining, were identified as the cholinergic interneurons. No p75 neurotrophin receptor immunoreactivity remained in cholinergic cells after one week of reperfusion. Based on current hypotheses regarding the function of the p75 neurotrophin receptor, the transient expression of this receptor in striatal cholinergic interneurons might contribute to their high resistance to ischemic neuronal death. However, the expression of p75 neurotrophin receptor could also be a first step in a pathway leading to apoptosis, which is inhibited after the present insult due to concomitant activation of TrkA.

Morphological and electrophysiological characteristics of noncholinergic basal forebrain neurons

Cholinergic neurons in the basal forebrain are the focus of considerable interest because they are severely affected in Alzheimer's disease. However, both cholinergic and noncholinergic neurons are intermingled in this region. The goal of the present study was to characterize the morphology and in vivo electrophysiology of noncholinergic basal forebrain neurons. Neurons in the ventral pallidum and substantia innominata were recorded extracellularly, labeled juxtacellularly with biocytin and characterized for the presence of choline acetyltransferase immunoreactivity. Two types of ventral pallidal cells were observed. Type I ventral pallidal neurons had axons that rarely branched near the cell body and tended to have smaller somata and lower spontaneous firing rates than did type II ventral pallidal neurons, which displayed extensive local axonal arborizations. Subtypes of substantia innominata neurons could not be distinguished based on axonal morphology. These noncholineregic neurons exhibited local axon arborizations along a continuum that varied from no local collaterals to quite extensive arbors. Substantia innominata neurons had lower spontaneous firing rates, more variable interspike intervals, and different spontaneous firing patterns than did type II ventral pallidal neurons and could be antidromically activated from cortex or substantia nigra, indicating that they were projection neurons. Ventral pallidal neurons resemble, both morphologically and electrophysiologically, previously described neurons in the globus pallidus, whereas the substantia innominata neurons bore similarities to isodendritic neurons of the reticular formation. These results demonstrate the heterogeneous nature of noncholinergic neurons in the basal forebrain.

Facilitation of striatal acetylcholine release by dopamine D1 receptor stimulation: involvement of enhanced nitric oxide production via neurokinin-2 receptor activation

The regulation of striatal cholinergic function by dopamine D1 receptor activation was examined in vivo in urethane-anaesthetized rats with microdialysis probes. Extracellular acetylcholine levels were enhanced by activation of D1 receptors either directly by a striatal application of the D1 receptor agonist (+)-SKF-38393 (3 microM) or indirectly by the release of dopamine evoked by striatal application of neurotensin (0.1 microM) under D2 receptor blockade. SR 144190, a new potent and selective non-peptide neurokinin-2 receptor antagonist (0.03-1 mg/kg, i.p.), dose-dependently reduced the acetylcholine release induced by (+)-SKF-38393 or neurotensin. Furthermore, intrastriatal application of SR 144190 (1 nM) blocked the increase in acetylcholine release induced by the local application of (+)-SKF-38393 (3 microM), neurokinin A (1 microM) or substance P (1 microM). Finally, a role for nitric oxide in mediating the effects of D1 neurokinin-2 receptor activation on acetylcholine release is proposed since local infusion of the competitive inhibitor of nitric oxide synthase, N(G)-monomethyl-L-arginine (0.01-10 microM), blocked the increase in acetylcholine release induced by (+)-SKF-38393 (3 microM), neurotensin (0.1 microM) or neurokinin A (1 microM) without affecting the enhancing effect of the neurokinin-1 agonist septide (0.1 microM).

Striatal spiny neurons and cholinergic interneurons express differential ionotropic glutamatergic responses and vulnerability: implications for ischemia and Huntington's disease

Striatal spiny neurons are selectively vulnerable in Huntington's disease (HD) and ischemia, whereas large aspiny (LA) cholinergic interneurons of the striatum are spared in these pathological conditions. We have investigated whether a different sensitivity to ionotropic glutamatergic agonists might account for this differential vulnerability. Intracellular recordings were obtained from morphologically identified striatal spiny neurons and LA cholinergic interneurons by using a rat brain slice preparation. The two striatal neuronal subtypes had strikingly different intrinsic membrane properties. Both subtypes responded to cortical stimulation with excitatory postsynaptic potentials: these potentials, however, had a different time course and pharmacology in the two classes of cells. Interestingly, membrane depolarizations and inward currents produced by exogenous glutamate receptor agonists (AMPA, kainate, and NMDA) were remarkably larger in spiny neurons than in LA interneurons. Moreover, concentrations of agonists producing reversible membrane changes in LA interneurons caused irreversible depolarizations in spiny cells. Our data suggest that the different physiological responses induced by the activation of ionotropic glutamate receptors may account for the cell type-specific vulnerability of striatal neurons in ischemia and HD.

Phencyclidine and corticosteroids induce apoptosis of a subpopulation of striatal neurons: a neural substrate for psychosis?

Phencyclidine, a non-competitive N-methyl-D-aspartate receptor antagonist and indirect dopamine agonist, has neuroprotective properties. Phencyclidine, however, can also exert toxic effects and causes degeneration of neurons in the retrosplenial cortex. In this paper we demonstrate that acute administration of a high dose of phencyclidine to rats, (80 mg/kg), also causes death of a subpopulation of striatal neurons. The dying cells exhibited many of the morphological and biochemical features of cells undergoing apoptosis as revealed by a silver methenamine stain, propidium iodide fluorescence histochemistry and a TUNEL procedure. The majority of the dying cells tended to be clustered within the dorsomedial aspect of the striatum. The type of striatal cell undergoing apoptosis was determined by stereotaxically injecting a colloidal gold retrograde anatomical tracer into the major areas of striatal termination prior to the administration of phencyclidine. This procedure demonstrated that phencyclidine induced striatal apoptosis is almost exclusively limited to striatopallidal neurons. A similar series of experiments was conducted to determine whether the synthetic corticosteroid, dexamethasone, also induces apoptosis of striatal neurons. Corticosteroids are known to be toxic to hippocampal neurons and interact with striatal dopamine transmission. Acute administration of dexamethasone, (20 mg/kg), induced apoptosis of a subpopulation of striatal cells. As was the case with phencyclidine, most of the dexamethasone-induced apoptotic striatal cells were striatopallidal neurons located within the dorsomedial striatum. The pathology during the early stages of Huntington's disease is restricted to an equivalent subpopulation of striatal neurons. Many Huntington's patients are extremely psychotic during this stage in the progression of the disease. Psychosis is also associated with the acute administration of both phencyclidine and dexamethasone to humans. We accordingly speculate that the selective loss of striatopallidal neurons in the dorsomedial striatum may represent the neural substrate of many forms of psychosis.

Proton MR spectroscopic findings in paroxysmal kinesigenic dyskinesia

Although paroxysmal kinesigenic dyskinesia (PKD) has characteristic clinical features, the pathophysiology of PKD has remained unknown. The purpose of this study was to investigate the pathophysiology of idiopathic PKD by performing proton magnetic resonance spectroscopy (1H-MRS) in five patients with idiopathic PKD. Three patients were familial and two sporadic. Single-voxel 1H-MRS was performed on a GE 1.5-T SIGNA MR system. Localized 1H-MR spectra were obtained from the basal ganglia (n = 5), thalamus (n = 3), and supplementary motor area (SMA; n = 4) using STEAM sequence (stimulated echo acquisition mode; TR = 3.0 sec, TE = 30 msec, 64 AVG, volume = 8 mL) or PRESS (point resolved spectroscopy; TR = 3.0 sec, TE = 135 msec, volume = 4 mL). Peak ratios of Cho/Cr (Cho: choline, Cr: creatine) and mI/Cr (mI: myoinositol) were decreased significantly in the unilateral basal ganglia of two patients. In one, decreased peak ratio of mI/Cr in the unilateral basal ganglia was the only abnormality. In the remaining two, there was no significant abnormality. 1H-MR spectra obtained from the thalamus and SMA were all within normal limits. In conclusion, these results suggest that underlying pathophysiological mechanism of PKD may be at least partially associated with the dysfunction of cholinergic system in the basal ganglia.

Fibrillar beta-amyloid induces microglial phagocytosis, expression of inducible nitric oxide synthase, and loss of a select population of neurons in the rat CNS in vivo

To determine the stability of beta-amyloid peptide (Abeta) and the glial and neuronal changes induced by Abeta in the CNS in vivo, we made single injections of fibrillar Abeta (fAbeta), soluble Abeta (sAbeta), or vehicle into the rat striatum. Injected fAbeta is stable in vivo for at least 30 d after injection, whereas sAbeta is primarily cleared within 1 d. After injection of fAbeta, microglia phagocytize fAbeta aggregates, whereas nearby astrocytes form a virtual wall between fAbeta-containing microglia and the surrounding neuropil. Similar glial changes are not observed after sAbeta injection. Microglia and astrocytes near the injected fAbeta show a significant increase in inducible nitric oxide synthase (iNOS) expression compared with that seen with sAbeta or vehicle injection. Injection of fAbeta but not sAbeta or vehicle induces a significant loss of parvalbumin- and neuronal nitric oxide synthase-immunoreactive neurons, whereas the number of calbindin-immunoreactive neurons remains unchanged. These data demonstrate that fAbeta is remarkably stable in the CNS in vivo and suggest that fAbeta neurotoxicity is mediated in large part by factors released from activated microglia and astrocytes, as opposed to direct interaction between Abeta fibrils and neurons.

Adenosine A2A antagonist: a novel antiparkinsonian agent that does not provoke dyskinesia in parkinsonian monkeys

Treatment of Parkinson's disease with L-dopa therapy leads to long-term complications, including loss of drug efficacy and the onset of dyskinesia. Adenosine A2A receptors in striatum are selectively localized to GABAergic output neurons of the striato-pallidal pathway and may avoid such problems. The novel adenosine A2A receptor antagonist KW-6002 has been examined for antiparkinsonian activity in MPTP-treated primates. Oral administration of KW-6002 reversed motor disability in MPTP-treated common marmosets in a dose-dependent manner. However, KW-6002 only modestly increased overall locomotor activity and did not cause abnormal movement, such as stereotypy. The ability of KW-6002 to reverse motor disability was maintained on repeated daily administration for 21 days, and no tolerance was observed. KW-6002 induced little or no dyskinesia in MPTP-treated primates previously primed to exhibit dyskinesia by prior exposure to L-dopa. These results suggest that selective adenosine A2A receptor antagonists represent a new class of antiparkinsonian agents that improve disability without producing hyperactivity and without inducing dyskinesia.

Postsynaptic integration of cholinergic and dopaminergic signals on medium-sized GABAergic projection neurons in the neostriatum

The effects of cholinergic drugs and the interaction between cholinergic and dopaminergic compounds were studied on electrically evoked [3H]gamma-aminobutyric acid (GABA) overflow in slices of the rat neostriatum. Slices were prepared and loaded with [3H]GABA in the presence of beta-alanine and then superfused with Krebs-bicarbonate buffer containing aminooxyacetic acid and nipecotic acid to inhibit GABA uptake and metabolism, respectively. The nonselective muscarinic agonist oxotremorine (0.1-10 microM) increased the release of [3H]GABA and the selective M1 receptor agonist McN-A-343 (0.1-10 microM) exerted similar effect. The stimulatory effect of oxotremorine (10 microM) on [3H][GABA overflow was antagonized by the nonselective muscarinic antagonist atropine (1 microM) and the selective M1 receptor antagonist pirenzepine (0.1-1.0 microM). The M2 receptor antagonist methoctramine (1.0 microM) did not alter the stimulatory effect of oxotremorine. Of the muscarinic receptor antagonists atropine, pirenzepine, and methoctramine (1.0 microM) failed to affect [3H]GABA overflow. The M3 receptor antagonist p-F-HHSiD (1 microM) increased [3H]GABA overflow and p-F-HHSiD and oxotremorine were found to be additive in increasing this effect. The D2 dopamine receptor antagonist sulpiride (10 microM) increased the electrical stimulation-induced [3H]GABA overflow, and this stimulation was counteracted by concomitant administration of atropine (1 microM). McN-A-343 and sulpiride also increased the KCl-induced [3H]GABA overflow from superfused neostriatal slices and tetrodotoxin (1 microM) did not affect these stimulations. These data indicate that the release of GABA in the neostriatum is under the control of M1 stimulatory and M3 inhibitory muscarinic receptors. Dopamine, which exerts inhibition on GABA release via D2 receptors, may counteract the M1 facilitation, and M1 and D2 receptors involved in the cholinergic-dopaminergic interaction may be located postsynaptically on medium-sized spiny GABAergic projection neurons.

Effects of the globus pallidus lesion on the induction of c-Fos by dopaminergic drugs in the striatum possibly via pallidostriatal feedback loops

c-Fos is one of the transcription factors contributing to the regulation of the downstream gene expression. Administration of dopamine D1 receptor agonist or D2 receptor antagonist have been known to induce c-Fos expressions in striatal projection neurons. We examined the effects of unilateral ablation of the globus pallidus (GP) on apomorphine- or haloperidol-induced expression of c-Fos in the striatum. Haloperidol induced a high level of c-Fos expression in the striatal neurons, predominantly those in the dorsal part, and the unilateral GP lesion caused by ibotenic acid increased the number of neurons exhibiting haloperidol-induced c-Fos expression in the striatum on the side with the GP lesion by about 2- or 3-fold. On the other hand, the unilateral GP lesion had no significant effect on the apomorphine-induced c-Fos expression in the striatal neurons. The present study provides evidence indicating that the activity of GP neurons may have an inhibitory influence on the induction of the immediate early genes by haloperidol in the striatal neurons, suggesting a function of the pallido-striatal feedback loops which have been identified anatomically.

Up and down states in striatal medium spiny neurons simultaneously recorded with spontaneous activity in fast-spiking interneurons studied in cortex-striatum-substantia nigra organotypic cultures

In vivo intracellular spontaneous activity in striatal medium spiny (MS) projection neurons is characterized by "up" and "down" states. How this type of activity relates to the neuronal activity of striatal fast-spiking (FS) interneurons was examined in the presence of nigral and cortical inputs using cortex-striatum-substantia nigra organotypic cultures grown for 45 +/- 4 d. The nigrostriatal projection was confirmed by tyrosine hydroxylase immunoreactivity. Corticostriatal (CS) projection neurons, striatal MS neurons, and FS neurons were intracellularly recorded and morphologically and electrophysiologically characterized. Intracellular spontaneous activity in the cultures consisted of intermittent depolarized periods of 0.5-1 sec duration. Spontaneous depolarizations in MS neurons were restricted to a narrow membrane potential range (up state) during which they occasionally fired single spikes. These up states were completely blocked by the glutamate antagonist CNQX. In FS interneurons, depolarized periods were characterized by large membrane potential fluctuations that occupied a wide range between rest and spike threshold. Also, FS interneurons spontaneously fired at much higher rates than did MS neurons. Simultaneous intracellular recordings established that during spontaneous depolarizations MS neurons and FS interneurons displayed correlated subthreshold neuronal activity in the low frequency range. These results indicate that (1) the CS projection neurons, striatal MS neurons, and FS interneurons grown in cortex-striatum-substantia nigra organotypic cultures show morphological and electrophysiological characteristics similar to those seen in vivo; (2) striatal MS neurons but not FS interneurons show an up state; (3) striatal MS neurons and FS interneurons receive common, presumably cortical inputs in the low frequency range. Our results support the view that the cortex provides a feedforward inhibition of MS neuron activity during the up state via FS interneurons.

Localization of mGluR1a-like immunoreactivity and mGluR5-like immunoreactivity in identified populations of striatal neurons

Metabotropic glutamate receptors are important mediators of excitatory amino acid neurotransmission in the striatum. Two-color immunofluorescence histochemistry and immunohistochemistry in combination with retrograde tract-tracing techniques were used to examine the distribution of metabotropic glutamate receptor subtypes 1a and 5 (mGluR1a and mGluR5) among identified subpopulations of striatal projection neurons and interneurons. The majority of striatopallidal and striatonigral neurons were double-labeled for both mGluR1a or mGluR5. Approximately 60% to 70% of either striatonigral or striatopallidal neurons expressed mGluR1a- or mGluR5-like immunoreactivity. The percentage of double-labeled striatopallidal or striatonigral projection neurons did not differ among striatal quadrants. Striatal interneurons expressing parvalbumin or somatostatin or choline acetyltransferase exhibited varying degrees of expression of mGluR1a or mGluR5. Virtually all (94%) parvalbumin-immunoreactive striatal neurons expressed mGluR1a-like immunoreactivity with a majority (79%) of these neurons expressing mGluR5-like immunoreactivity. A high percentage (89%) of striatal choline acetyltransferase-immunoreactive neurons were double-labeled for mGluR1a-like immunoreactivity. Approximately 65% of striatal choline acetyltransferase-immunoreactive neurons expressed mGluR5-like immunoreactivity. A majority (65%) of somatostatin-immunoreactive striatal interneurons expressed mGluR1a-like immunoreactivity with a slightly lower percentage (55%) expressing mGluR5-like immunoreactivity. These findings indicate considerable heterogeneity among striatal projection and interneurons with respect to mGluR1a and mGluR5 expression. There may be subpopulations of striatonigral and striatopallidal projection neurons. These results are consistent as well with prior data indicating subpopulations of the different classes of striatal interneurons.