Time course of early motor and neuropathological anomalies in a knock-in mouse model of Huntington's disease with 140 CAG repeats

Huntington's disease (HD) is caused by an abnormal expansion of CAG repeats in the gene encoding huntingtin. The development of therapies for HD requires preclinical testing of drugs in animal models that reproduce the dysfunction and regionally specific pathology observed in HD. We have developed a new knock-in mouse model of HD with a chimeric mouse/human exon 1 containing 140 CAG repeats inserted in the murine huntingtin gene. These mice displayed an increased locomotor activity and rearing at 1 month of age, followed by hypoactivity at 4 months and gait anomalies at 1 year. Behavioral symptoms preceded neuropathological anomalies, which became intense and widespread only at 4 months of age. These consisted of nuclear staining for huntingtin and huntingtin-containing nuclear and neuropil aggregates that first appeared in the striatum, nucleus accumbens, and olfactory tubercle. Interestingly, regions with early pathology all receive dense dopaminergic inputs, supporting accumulating evidence for a role of dopamine in HD pathology. Nuclear staining and aggregates predominated in striatum and layer II/III and deep layer V of the cerebral cortex, whereas neuropil aggregates were found in the globus pallidus and layer IV/superficial layer V of the cerebral cortex. The olfactory system displayed early and marked aggregate accumulation, which may be relevant to the early deficit in odor discrimination observed in patients with HD. Because of their early behavioral anomalies and regionally specific pathology, these mice provide a powerful tool with which to evaluate the effectiveness of new therapies and to study the mechanisms involved in the neuropathology of HD.

Early motor dysfunction and striosomal distribution of huntingtin microaggregates in Huntington's disease knock-in mice

Huntington's disease (HD) is characterized by a progressive loss of neurons in the striatum and cerebral cortex and is caused by a CAG repeat expansion in the gene encoding huntingtin. Mice with the mutation inserted into their own huntingtin gene (knock-in mice) are, genetically, the best models of the human disease. Here we show for the first time that knock-in mice with 94 CAG repeats develop a robust and early motor phenotype at 2 months of age, characterized by increased rearing at night. This initial increase in repetitive movements was followed by decreased locomotion at 4 and 6 months, despite a normal life span. The decrease in striatal enkephalin mRNA that is known to occur at 4 months was not present at 2 months, when increased rearing was observed. Both the hyperactive and hypoactive phases of motor dysfunction preceded the detection of nuclear microaggregates of mutated huntingtin in striatal neurons. Nuclear microaggregates, defined as small huntingtin-positive punctas detected by light microscopy, were very rare at 4 months but became widely distributed in striatal neurons at 6 months. Nuclear inclusions did not appear until 18 months. When present, nuclear microaggregates predominated in the striosomal compartment of the striatum, providing a possible explanation for the different neuronal vulnerability of striatal compartments observed in humans. The early motor phenotype observed in the knock-in mouse is reminiscent of repetitive movements often observed in early HD and provides a novel opportunity to assess the ability of therapies to prevent the initial effects of the mutation in vivo.

Neuronal differentiation of stem cells isolated from adult muscle

Lineage uncommitted pluripotent stem cells reside in the connective tissue of skeletal muscle. The present study was carried out with pluripotent stem cells (PPSCs) isolated from 6-month old rat muscle. Before differentiation, these cells were vimentin+, CD90+, CD45-, and varied in their expression of CD34. The PPSCs were expanded as non-adherent aggregates under similar conditions to those used to generate neurospheres from embryonic or neural stem cells. The PPSC-derived neurospheres were positive for nestin, an early marker present in neuronal precursors, and expressed the two alternative mRNA forms of the neuroectodermal marker Pax-6, as well as mRNA for Oct-4, a gene related to the pluripotentiality of stem cells. To confirm their neural potential, PPSC-derived neurospheres were plated on coated coverslips under varying conditions: Neurobasal medium with N2 or B27, and either NT3 or BDNF. After 4-6 days the cells expressed neuronal (Tuj1+, NF68), astrocytic (GFAP) and oligodendrocytic (MOSP+, MBP+) markers, both by immunocytochemistry and RT-PCR. In addition, PPSCs were cultured as monolayers under adherent conditions, exposed to growth factors and defined differentiating conditions for 5 hr, and subsequently kept for 2 days in a maturation medium. At this point they gave rise to a mixed population of early neural progenitors (Nestin+ or NG2+), immature and mature neurons (Tuj1+ and NF145+) and myelin producing oligodendrocytes (CNPase + and MOSP+). Our study shows that PPSCs present in adult muscle can overcome germ lineage restrictions and express the molecular characteristics of brain cells. Therefore, PPSCs isolated from adult muscle could provide a novel source for autologous cell replacement in neurodegenerative and demyelinating diseases.

Ultrastructural evidence for differential axonal sprouting in the striatum after thermocoagulatory and aspiration lesions of the cerebral cortex in adult rats

Thermocoagulation of pial blood vessels overlying the cerebral cortex induces an ischemic degeneration of the cortex. We have previously shown with anatomical tracing techniques that thermocoagulatory lesions of the sensorimotor cortex trigger a robust axonal sprouting of contralateral cortical neurons into the denervated striatum. Similar sprouting was not observed after acute aspiration lesions of the same cortical region. We have now examined immunostaining for the growth-associated protein (GAP)-43 at the ultrastructural level after both types of lesions. A modest increase in growth cone-like structures was observed just below the corpus callosum after both lesions. However, GAP-43-positive growth cone-like structures were markedly increased in the denervated dorsolateral striatum only after thermocoagulatory lesions. In contrast, no significant increase in growth cone immunostaining was found in the dorsolateral striatum after aspiration lesions, confirming the absence of axonal sprouting in the dorsolateral striatum in this condition. Corticostriatal inputs make asymmetric synapses with dendritic spines of striatal neurons. As expected, the density of asymmetric synapses was markedly decreased in the dorsolateral striatum after aspiration lesions. However, it was not different from control after thermocoagulatory lesions that removed the same cortical area. The density of symmetric synapses was decreased after both types of lesions at 16 but not 42 days post-surgery. These data reveal that robust axonal and synaptic remodeling can occur in the dorsolateral striatum of adult rats after ischemic lesions of the cerebral cortex and further demonstrate marked differences in the degree of anatomical plasticity induced by two different types of cortical lesions.

Mouse models of Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder. In 1993 the mutation that causes HD was identified as an unstable expansion of CAG repeats in the IT15 gene. Since then one of the most important advances in HD research has been the generation of various mouse models that enable the exploration of early pathological, molecular and cellular abnormalities produced by the mutation. In addition, these models have made it possible to test different pharmacological approaches to delay the onset or slow the progression of HD. In this article, insights gained from mouse models towards the understanding of HD and the design of new therapeutic strategies are discussed.

Electrophysiological and morphological changes in striatal spiny neurons in R6/2 Huntington's disease transgenic mice

We examined passive and active membrane properties and synaptic responses of medium-sized spiny striatal neurons in brain slices from presymptomatic (approximately 40 days of age) and symptomatic (approximately 90 days of age) R6/2 transgenics, a mouse model of Huntington's disease (HD) and their age-matched wild-type (WT) controls. This transgenic expresses exon 1 of the human HD gene with approximately 150 CAG repeats and displays a progressive behavioral phenotype associated with numerous neuronal alterations. Intracellular recordings were obtained using standard techniques from R6/2 and age-matched WT mice. Few electrophysiological changes occurred in striatal neurons from presymptomatic R6/2 mice. The changes in this age group were increased neuronal input resistance and lower stimulus intensity to evoke action potentials (rheobase). Symptomatic R6/2 mice exhibited numerous electrophysiological alterations, including depolarized resting membrane potentials, increased input resistances, decreased membrane time constants, and alterations in action potentials. Increased stimulus intensities were required to evoke excitatory postsynaptic potentials (EPSPs) in neurons from symptomatic R6/2 transgenics. These EPSPs had slower rise times and did not decay back to baseline by 45 ms, suggesting a more prominent component mediated by activation of N-methyl-D-aspartate receptors. Neurons from both pre- and symptomatic R6/2 mice exhibited reduced paired-pulse facilitation. Data from biocytin-filled or Golgi-impregnated neurons demonstrated decreased dendritic spine densities, smaller diameters of dendritic shafts, and smaller dendritic fields in symptomatic R6/2 mice. Taken together, these findings indicate that passive and active membrane and synaptic properties of medium-sized spiny neurons are altered in the R6/2 transgenic. These physiological and morphological alterations will affect communication in the basal ganglia circuitry. Furthermore, they suggest areas to target for pharmacotherapies to alleviate and reduce the symptoms of HD.

Detection of DNA damage in tissue sections by in situ nick translation

The technique of in situ nick translation (ISNT) is used to detect DNA strand breaks in tissue sections at the cellular level with great sensitivity. In fact, ISNT can be used to detect DNA damage in a single cell, which is particularly useful to assess programmed cell death during development. One crucial advantage of ISNT is the anatomical resolution that permits a detailed topographical analysis of DNA damage. This can be useful to identify cells that are more vulnerable to an experimental insult. Furthermore, cells with DNA damage can be identified morphologically with this method. This can be of interest to determine whether cells that exhibit DNA damage already exhibit clear features of dying cells or are still relatively intact morphologically. This can be useful to identify the mode of cell death involved. This unit provides a protocol that describes tissue preparation, in situ nick translation and emulsion autoradiography.

Endomorphin-1: induction of motor behavior and lack of receptor desensitization

The endomorphins are recently discovered endogenous agonists for the mu-opioid receptor (Zadina et al., 1997). Endomorphins produce analgesia; however, their role in other brain functions has not been elucidated. We have investigated the behavioral effects of endomorphin-1 in the globus pallidus, a brain region that is rich in mu-opioid receptors and involved in motor control. Bilateral administration of endomorphin-1 in the globus pallidus of rats induced orofacial dyskinesia. This effect was dose-dependent and at the highest dose tested (18 pmol per side) was sustained during the 60 min of observation, indicating that endomorphin-1 does not induce rapid desensitization of this motor response. In agreement with a lack of desensitization of mu-opioid receptors, 3 hr of continuous exposure of the cloned mu receptor to endomorphin-1 did not diminish the subsequent ability of the agonist to inhibit adenylate cyclase activity in cells expressing the cloned mu-opioid receptor. Confirming the involvement of mu-opioid receptors, the behavioral effect of endomorphin-1 in the globus pallidus was blocked by the opioid antagonist naloxone and the mu-selective peptide antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7) amide (CTOP). Furthermore, the selective mu receptor agonist [d-Ala(2)-N-Me-Phe(4)-Glycol(5)]-enkephalin (DAMGO) also stimulated orofacial dyskinesia when infused into the globus pallidus, albeit transiently. Our findings suggest that endogenous mu agonists may play a role in hyperkinetic movement disorders by inducing sustained activation of pallidal opioid receptors.

Tissue-specific proteolysis of Huntingtin (htt) in human brain: evidence of enhanced levels of N- and C-terminal htt fragments in Huntington's disease striatum

Proteolysis of mutant huntingtin (htt) has been hypothesized to occur in Huntington's disease (HD) brains. Therefore, this in vivo study examined htt fragments in cortex and striatum of adult HD and control human brains by Western blots, using domain-specific anti-htt antibodies that recognize N- and C-terminal domains of htt (residues 181-810 and 2146-2541, respectively), as well as the 17 residues at the N terminus of htt. On the basis of the patterns of htt fragments observed, different "protease-susceptible domains" were identified for proteolysis of htt in cortex compared with striatum, suggesting that htt undergoes tissue-specific proteolysis. In cortex, htt proteolysis occurs within two different N-terminal domains, termed protease-susceptible domains "A" and "B." However, in striatum, a different pattern of fragments indicated that proteolysis of striatal htt occurred within a C-terminal domain termed "C," as well as within the N-terminal domain region designated "A". Importantly, striatum from HD brains showed elevated levels of 40-50 kDa N-terminal and 30-50 kDa C-terminal fragments compared with that of controls. Increased levels of these htt fragments may occur from a combination of enhanced production or retarded degradation of fragments. Results also demonstrated tissue-specific ubiquitination of certain htt N-terminal fragments in striatum compared with cortex. Moreover, expansions of the triplet-repeat domain of the IT15 gene encoding htt was confirmed for the HD tissue samples studied. Thus, regulated tissue-specific proteolysis and ubiquitination of htt occur in human HD brains. These results suggest that the role of huntingtin proteolysis should be explored in the pathogenic mechanisms of HD.

Increased m-CPP-induced oral dyskinesia after lesion of serotonergic neurons

Peripheral administration of the 5-hydroxytryptamine (5-HT)(2C/1B) agonist 1-(m-chlorophenyl)piperazine (m-CPP) produces abnormal orofacial movements in rats. We have previously shown that this behavior is mediated by 5-HT(2C) receptors in the subthalamic nucleus [Neuroscience 72 (1996) 117]. The present studies examined this effect after serotonin depletion to determine whether removal of endogenous serotonin affected this behavioral response and/or subthalamic 5-HT(2C) receptors. Rats received an intraventricular infusion of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT, 100 mg/10 ml) or vehicle after desipramine pretreatment (25 mg/kg ip). The efficacy of serotonin depletion was confirmed by a decrease in serotonin uptake sites measured by autoradiography. Oral dyskinesia induced by peripheral administration of m-CPP (1.0 mg/kg ip) was markedly increased in lesioned rats compared to sham-operated controls 4 and 8 but not 12 days after the lesion. A subset of lesioned rats that displayed transient seizures after m-CPP injection did not prevent the measurement of oral dyskinesia during the observation period. No differences in 5-HT(2C) receptor levels were found with ligand-binding autoradiography in the subthalamic nucleus, or in other brain regions that express this receptor, in rats sacrificed 5 days following 5,7-DHT lesions. The data indicate that lesion of serotonergic neurons in adult rats induces a transient increase in motor responses mediated by subthalamic 5-HT(2C) receptors. These data suggest that functional alterations in serotonergic transmission in the subthalamic nucleus may be involved in the pathophysiology of hyperkinetic movement disorders.

Increased behavioral response to dopaminergic stimulation of the subthalamic nucleus after nigrostriatal lesions

Local infusions of the nonselective dopaminergic agonist apomorphine into the subthalamic nucleus of rats has been shown to elicit orofacial dyskinesia which can be blocked by D1 but not D2 receptor antagonists. In the present study, we show that the selective D1 agonist A77636 also induces orofacial dyskinesia when injected into the subthalamic nucleus of awake rats, thus confirming a role for D1 receptors in this effect. We also examined the dyskinesia induced by intrasubthalamic injections of apomorphine in rats with an ipsilateral lesion of the nigrostriatal pathway. The orofacial response to local administration of apomorphine (1.0 microg) into the subthalamic nucleus was markedly increased in the lesioned rats. As in control rats, the enhanced behavioral response seen in lesioned rats was blocked by peripheral administration of D1 antagonists. Although D1 receptor binding autoradiography revealed no difference in D1 receptor binding in the subthalamic nucleus on the side of the lesion compared to controls, D1 binding was higher in the subthalamic nucleus on the side of the lesion compared to the contralateral side. The increased behavioral response observed after unilateral dopamine denervation suggests that the subthalamic nucleus is tonically regulated by dopaminergic projections from the substantia nigra. Furthermore, the data suggest that subthalamic D1 receptors may be involved in the development of dyskinesia induced by dopaminergic drugs.

A role for N-methyl-D-aspartate receptors in the regulation of synaptogenesis and expression of the polysialylated form of the neural cell adhesion molecule in the developing striatum

Striatal development proceeds during a protracted postnatal period in rats. In the dorsolateral striatum, the number of asymmetric synapses, formed mostly by glutamatergic afferents innervating the dendritic spines of medium-sized striatal neurons, increases during the 3rd postnatal week and then rapidly declines before reaching adult levels. The polysialylated form of the neural cell adhesion molecule (PSA-NCAM), which is widely expressed along neuronal membranes early in development, becomes progressively localized to synapses, and is no longer detectable in remaining synapses after synaptic pruning has occurred. Administration of MK-801, an antagonist of N-methyl-D-aspartate receptors, on day 20, either peripherally or locally into the striatum, decreases asymmetric synapse number by 30% and totally abolishes immunolabelling for PSA-NCAM in the dorsolateral striatum.

Nigrostriatal lesions alter oral dyskinesia and c-Fos expression induced by the serotonin agonist 1-(m-chlorophenyl)piperazine in adult rats

The loss of dopaminergic innervation of the basal ganglia, a group of subcortical regions involved in motor control, is the hallmark of Parkinson's disease. The resulting molecular and cellular alterations mediate behavioral deficits and may modify neuronal responses to other neurotransmitters. In the present study, we sought to determine the effects of chronic dopamine (DA) depletion on responses mediated by stimulation of serotonergic 2C (5-HT(2C)) receptors, a serotonergic receptor subtype present in discrete regions of the basal ganglia. Specifically, the effects of unilateral lesions of nigrostriatal DA neurons on oral dyskinesia and Fos protein expression induced by the non-selective 5-HT(2C) agonist 1-(m-chlorophenyl)piperazine (m-CPP) were examined. Confirming previous findings, both peripheral and local injections of m-CPP into the subthalamic nucleus elicited oral dyskinesia. Nigrostriatal lesions markedly enhanced oral bouts induced by peripheral but not intrasubthalamic administration of m-CPP. In intact rats, Fos expression was increased by m-CPP (1 mg/kg, i.p.) in the striatum and the subthalamic nucleus. After nigrostriatal lesions, m-CPP-induced Fos expression remained unchanged in the subthalamic nucleus but was reduced in the medial quadrants of the striatum and was markedly enhanced in the entopeduncular nucleus. These data demonstrate regionally specific alterations in behavioral and cellular responses to a serotonergic agonist in an animal model of Parkinson's disease.

Decrease in striatal enkephalin mRNA in mouse models of Huntington's disease

Huntington's disease is a devastating progressive neurodegenerative illness characterized by massive neuronal loss in the striatum. It is caused by the presence of an expanded CAG repeat in the gene encoding huntingtin, a protein of unknown function. We have examined the expression of neurotransmitters and other antigens present in striatal neurons with immunohistochemistry, and the level of expression of mRNAs encoding enkephalin, substance P, and glutamic acid decarboxylases with quantitative in situ hybridization histochemistry, in the striatum of two mouse models of Huntington's disease: transgenic animals expressing exon 1 of the human huntingtin gene with 144 CAG repeats and "knock-in" mice containing a chimeric mouse/human exon 1 with 71 or 94 CAG repeats inserted by homologous targeting. Although the transgenic (but not the knock-in) mice were previously shown to display prominent huntingtin- and ubiquitin-containing nuclear inclusions in striatal neurons, in situ nick translation followed by emulsion autoradiography did not reveal any DNA damage in striatum or cortex in these mice. Immunolabeling for calbindin D 28K, enkephalin, substance P, glutamic acid decarboxylases (M(r) 65,000 or 67,000, GAD65 and GAD67), somatostatin, choline acetyltransferase, parvalbumin, and glial fibrillary acidic protein were remarkably similar in transgenic, knock-in, and wild-type mice. Both transgenic and knock-in mice, however, showed a marked decrease in the level of expression of enkephalin mRNA in striatal neurons without significant decreases in mRNAs encoding substance P, GAD65, or GAD67. The data indicate that decreased expression of enkephalin mRNA may be an early sign of neuronal dysfunction due to the Huntington's disease mutation.

Enhanced sensitivity to N-methyl-D-aspartate receptor activation in transgenic and knockin mouse models of Huntington's disease

We used two mouse models of Huntington's disease (HD) to examine changes in glutamate receptor sensitivity and striatal electrophysiology. One model, a transgenic, consisted of mice expressing exon 1 of the human HD gene and carrying 141-157 CAG repeat sequences (R6/2 line). The second model, a CAG repeat "knockin," consisted of mice with different lengths of CAG repeats (CAG71 and CAG94 repeats). The effects of glutamate receptor activation were examined by visualizing neurons in brain slices with infrared videomicroscopy and differential interference contrast optics to determine changes in somatic area (cell swelling). Striatal and cortical neurons in both models (R6/2 and CAG94) displayed more rapid and increased swelling to N-methyl-D-aspartate (NMDA) than those in controls. This effect was specific as there were no consistent group differences after exposure to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) or kainate (KA). Intracellular recordings revealed that resting membrane potentials (RMPs) in the R6/2 transgenics were significantly more depolarized than those in their respective controls. RMPs in CAG94 mice also were more depolarized than those in CAG71 mice or their controls in a subset of striatal neurons. Confirming previous results, R6/2 mice expressed behavioral abnormalities and nuclear inclusions. However, CAG71 and CAG94 knockins did not, suggesting that increased sensitivity to NMDA may occur early in the disease process. These findings imply that NMDA antagonists or compounds that alter sensitivity of NMDA receptors may be useful in the treatment of HD.

Morphology and compartmental location of cells exhibiting DNA damage after quinolinic acid injections into rat striatum

Although excitotoxic injury is thought to play a role in many pathologic conditions, the type of cell death induced by excitotoxins in vivo and the basis for the differential vulnerability of neurons to excitotoxic injury are still poorly understood. Morphologic alterations and the presence of DNA damage were examined in adult rat striatum after an intrastriatal injection of low doses of quinolinic acid, a N-methyl-D-aspartate receptor agonist. Rats were killed 6, 8, 10, or 12 hours after quinolinate or vehicle injection. Numerous neurons with necrotic morphologies were detected in the quinolinate-injected striata. In addition, few neurons with apoptotic morphologies were found in the dorsomedial striatum. DNA strand breaks were detected in tissue sections by in situ nick translation with (35)S-radiolabeled nucleotides and emulsion autoradiography. Labeled cells were first detected outside the needle track 10 hours after quinolinate injection and, on average, 20% of neurons exhibited DNA damage by 12 hours after surgery. DNA damage was found in cells with both apoptotic and necrotic morphologies. A marked differential vulnerability to DNA damage at this time was observed in two striatal compartments, the striosomes, identified as regions of dense [(3)H]naloxone binding, and the extrastriosomal matrix: the great majority of labeled cells were found in the extrastriosomal matrix and extremely few were seen in the striosomes. This preferential distribution was not due to premature cell death in the striosomes which contained numerous unlabeled neurons. The results suggest a greater vulnerability of neurons in the matrix, versus the striosomes, to early excitotoxin-induced DNA damage in rat striatum.

Early effects of intrastriatal injections of quinolinic acid on microtubule-associated protein-2 and neuropeptides in rat basal ganglia

The long-term effects of intrastriatal injections of the agonist of N-methyl-D-aspartate receptors, quinolinic acid, have been extensively characterized. Much less is known, however, about the early molecular and neurochemical changes which occur within a few hours of the toxin injection. In the present study, we have performed intrastriatal injections of low doses of quinolinic acid which induce DNA damage 10-12 h post-lesion, and selective death of striatal projection neurons two weeks later. We examined the time-course of alterations in the microtubule-associated protein 2, an early marker of cytoskeletal disruption, and enkephalin and substance P, two neuropeptides present in largely distinct subpopulations of striatal efferent neurons projecting to the globus pallidus and entopeduncular nucleus, respectively. Immunoreactivity for microtubule-associated protein 2 was decreased at the periphery of the lesion 10 h after quinolinate injection. Levels of enkephalin messenger RNA were markedly decreased as early as 6 h post-lesion; however, a significant decrease in enkephalin immunoreactivity was not observed in the globus pallidus (external pallidum) until 12 h post-injection. Levels of substance P messenger RNA were decreased 12 h post-injection in striatal neurons. However, in contrast to enkephalin immunoreactivity, immunolabeling for substance P was not significantly decreased at this time-point in the internal pallidum, a finding reminiscent of early grades of Huntington's disease. The results reveal the time-course of change in messenger RNA and peptide levels in striatal efferent neurons shortly after an excitotoxic insult. These data have implications for the interpretation of findings in post mortem brain and mouse models of Huntington's disease.

N-methyl-D-aspartate receptor blockade affects polysialylated neural cell adhesion molecule expression and synaptic density during striatal development

Glutamatergic neurons innervate the striatum and form asymmetric synapses with the dendritic spines of striatal efferent neurons. The role of glutamate in striatal development, however, remains largely unknown. Previous studies have shown a dramatic increase in the density of asymmetric synapses in the rat striatum during the third postnatal week, followed by a decrease to adult levels by postnatal day 25. At the same time, the highly polysialylated form of the neural cell adhesion molecule becomes progressively restricted to synaptic regions and then disappears. We have now examined the effects of antagonists of the N-methyl-D-aspartate subtype of glutamatergic receptors on the expression of the polysialylated form of the neural cell adhesion molecule and on synaptic density during this late period of striatal development. Peripheral administration of the N-methyl-D-aspartate receptor antagonist dizocilpine maleate markedly decreased immunoreactivity for the highly polysialylated form of the neural cell adhesion molecule in the dorsolateral striatum and cerebral cortex when drug treatment included postnatal day 20, but not earlier in development. This effect was regionally specific and loss of the polysialylated neural cell adhesion molecule in the striatum was reproduced by the local administration of dizocilpine maleate, DL-2-amino-5-phosphonovalerate or ketamine on postnatal day 20. Quantitative ultrastructural studies of synaptic density with the physical disector method performed after one of the regimens inducing loss of the polysialylated neural cell adhesion molecule (postnatal days 18-20) revealed a 30% decrease in asymmetric synapses in the dorsolateral striatum of treated rats. Symmetric synapses, which presumably do not use glutamate, were not affected. The data indicate that N-methyl-D-aspartate receptors play a role in the late stages of synaptogenesis in the striatum and suggest that a subset of synapses expressing immunoreactivity for the highly polysialylated form of the neural cell adhesion molecule may be dependent on N-methyl-D-aspartate receptor stimulation during a critical period of striatal development.

Synaptogenesis and ultrastructural localization of the polysialylated neural cell adhesion molecule in the developing striatum

The polysialylated neural cell adhesion molecule (PSA-NCAM) plays a role in axonal development and synaptic plasticity. Its pattern of expression is regulated temporally and topographically in the brain during development. However, it is unclear whether or not its subcellular location also changes. We have examined PSA-NCAM expression in relation to synapse formation in the developing rat striatum with immunohistochemistry and electron microscopy. Early in development, PSA-NCAM was present along the cytoplasmic membranes of neurons and in growth cones. PSA-NCAM expression became progressively confined to pre- and postsynaptic elements as neurons matured morphologically. Confirming previous results, a marked increase in the density of asymmetric synapses determined by using the physical dissector method was observed in the dorsolateral striatum between postnatal day 14 (P14) and P18. It was followed by a reduction between P18 and P25, when asymmetric synapse density reached adult levels. In contrast, the density of symmetric synapses had surpassed adult levels by P14. In the dorsomedial striatum, the density of asymmetric and symmetric synapses was similar at P18, at P25, and in adults. PSA-NCAM was associated with most asymmetric and symmetric synapses at P14 and P18 and was expressed in both pre- and postsynaptic elements of a majority (P14) or approximately half (P18) of the synapses. Most synapses lost PSA-NCAM expression between P18 and P25 in the dorsolateral striatum and between P25 and adult in the dorsomedial striatum. The data indicate that PSA-NCAM expression becomes restricted topographically during neuronal maturation but remains strategically associated with developing synapses during late postnatal development in the striatum.

Motor and somatosensory deficits following uni- and bilateral lesions of the cortex induced by aspiration or thermocoagulation in the adult rat

We have previously shown that lesions of the sensorimotor cortex induced by either thermocoagulation or aspiration produce different effects on axonal plasticity. We have now investigated whether these methods of lesion also influence the behavioral outcome. The behavioral effects of unilateral and bilateral lesions of the sensorimotor cortex induced by either aspiration or by thermocoagulation of pial blood vessels were examined in adult Sprague-Dawley rats. Rats were tested to determine limb use asymmetry by analyzing (1) coordinated forelimb placement and (2) paw use preference when rearing. Their responsiveness to somatosensory stimulation was tested by analyzing (1) the latency to remove sticky tape on the ventral surface of the paw, and (2) vibrissae-stimulated forelimb placing. Behavioral tests were performed prior to surgery and on day 4, 8, 12, 16, and 20 after surgery. Both unilateral lesions resulted in an over-reliance on the nonimpaired forelimb as early as 4 days after the surgery; functional recovery occurred after 16 days. Animals with bilateral lesions did not use either forelimb for support in postural support behaviors. However, this effect was more apparent in the animals with a thermocoagulatory lesion and, in contrast to the animals with an aspiration lesion, these animals did not show functional recovery. Animals with a unilateral aspiration, but not a thermocoagulatory lesion, showed a slowed response to tactile stimulation applied to the contralateral forelimb. After bilateral lesions, animals showed a slowed response to tactile stimulation applied to either forelimb at early time points after the lesion and recovery of function at later time points. These data indicate that, for the most part, lesions of the sensorimotor cortex by aspiration or thermocoagulation produce very similar effects on the behaviors examined in this study. However, unexpectedly, thermocoagulatory lesions induced a more severe (unilateral lesion) or prolonged (bilateral lesion) deficit in forelimb use than aspiration lesions. Conversely, the effect on tactile stimulation is more prominent after unilateral aspiration than thermocoagulatory lesions.

Methylmalonate toxicity in primary neuronal cultures

Several inhibitors of mitochondrial complex II cause neuronal death in vivo and in vitro. The goal of the present work was to characterize in vitro the effects of malonate (a competitive blocker of the complex) which induces neuronal death in a pattern similar to that seen in striatum in Huntington's disease. Exposure of striatal and cortical cultures from embryonic rat brain for 24 h to methylmalonate, a compound which produces malonate intracellularly, led to a dose-dependent cell death. Methylmalonate (10 mM) caused >90% mortality of neurons although cortical cells were unexpectedly more vulnerable. Cell death was attenuated in a medium containing antioxidants. Further characterization revealed that DNA laddering could be detected after 3 h of treatment. Morphological observations (videomicroscopy and Hoechst staining) showed that both necrotic and apoptotic cell death occurred in parallel; apoptosis was more prevalent. A decrease in the ATP/ADP ratio was observed after 3 h of treatment with 10 mM methylmalonate. In striatal cultures it occurred concomitantly with a decline in GABA and a rise in aspartate content and the aspartate/glutamate ratio. Changes in ion concentrations were measured in similar cortical cultures from mouse brain. Neuronal [Na+]i increased while [K+]i and membrane potential decreased after 20 min of continuous incubation in 10 mM methylmalonate. These changes progressed with time, and a rise in [Ca2+]i was also observed after 1 h. The results demonstrate that malonate collapses cellular ion gradients, restoration of which imposes an additional load on the already compromised ATP-generation machinery. An early elevation in [Ca2+]i may trigger an increase in activity of proteases, lipases and endonucleases and production of free radicals and DNA damage which, ultimately, leads to cells death. The data also suggest that maturational and/or extrinsic factors are likely to be critical for the increased vulnerability of striatal neurons to mitochondrial inhibition in vivo.

Toxicity of dopamine to striatal neurons in vitro and potentiation of cell death by a mitochondrial inhibitor

Intrastriatal injections of the mitochondrial toxins malonate and 3-nitropropionic acid produce selective cell death similar to that seen in transient ischemia and Huntington's disease. The extent of cell death can be attenuated by pharmacological or surgical blockade of cortical glutamatergic input. It is not known, however, if dopamine contributes to toxicity caused by inhibition of mitochondrial function. Exposure of primary striatal cultures to dopamine resulted in dose-dependent death of neurons. Addition of medium supplement containing free radical scavengers and antioxidants decreased neuronal loss. At high concentrations of the amine, cell death was predominantly apoptotic. Methyl malonate was used to inhibit activity of the mitochondrial respiratory chain. Neither methyl malonate (50 microM) nor dopamine (2.5 microM) caused significant toxicity when added individually to cultures, whereas simultaneous addition of both compounds killed 60% of neurons. Addition of antioxidants and free radical scavengers to the incubation medium prevented this cell death. Dopamine (up to 250 microM) did not alter the ATP/ADP ratio after a 6-h incubation. Methyl malonate, at 500 microM, reduced the ATP/ADP ratio by approximately 30% after 6 h; this decrease was not augmented by coincubation with 25 microM dopamine. Our results suggest that dopamine causes primarily apoptotic death of striatal neurons in culture without damaging cells by an early adverse action on oxidative phosphorylation. However, when combined with minimal inhibition of mitochondrial function, dopamine neurotoxicity is markedly enhanced.

Effects of intrastriatal injection of quinolinic acid on electrical activity and extracellular ion concentrations in rat striatum in vivo

Changes in neuronal activity and extracellular concentrations of ions were measured in rat striatum for 60-90 min after intrastriatal injection of quinolinic acid, an agonist of the N-methyl-D-aspartate receptor. The excitotoxin induced bursts of synchronous electrical activity which were accompanied by rises in [K+]e (to approximately 6 mM) and decreases in [Ca2+]e (by less than 0.1 mM); [H+]e usually increased (0.1-0.3 pH unit) after a short and small (< 0.1 pH unit) alkaline shift. The magnitude and frequency of these periodic changes decreased with time; after 90 min the amplitudes fell to 10-20% of the early values and the frequency to about one every 8 min as compared to one every 2-3 min immediately after quinolinate injection. By 90 min there was an increase in [K+]e from 3.3 mM to 4.2 mM and a decrease in [Ca2+]e from 1.34 mM to 1.30 mM. It is postulated that activation of the N-methyl-D-aspartate receptor causes disturbances in neuronal activity and ion gradients; restoration of the original ionic balances raises utilization of ATP and places an additional demand on energy-producing pathways. Increased influx of calcium into neurons may lead to an enhanced accumulation and subsequent overload of mitochondria with the cation. This, in turn, could result in dysfunction of the organelles and account for the decrease in respiration and [ATP]/[ADP] that have been observed previously in this model. The results of the present study lead to the conclusion that quinolinic acid produces early changes in activity of striatal neurons and movements of several cations which may contribute to subsequent abnormalities in energy metabolism and ultimately, cell death.

Differential expression of glutamate decarboxylase messenger RNA in cerebellar Purkinje cells and deep cerebellar nuclei of the genetically dystonic rat

The genetically dystonic rat exhibits a motor syndrome that closely resembles the human disease, generalized idiopathic dystonia. Although in humans dystonia is often the result of pathology in the basal ganglia, previous studies have revealed electrophysiological abnormalities and alterations in glutamate decarboxylase, the synthetic enzyme for GABA, in the cerebellum of dystonic rats. In this study, we further characterized the alterations in cerebellar GABAergic transmission in these mutants by examining the expression of the messenger RNA encoding glutamate decarboxylase (67000 mol. wt) with in situ hybridization histochemistry at the single cell level in Purkinje cells and neurons of the deep cerebellar nuclei. Glutamate decarboxylase (67000 mol. wt) messenger RNA levels were increased in the Purkinje cells and decreased in the deep cerebellar nuclei of dystonic rats compared to control littermates, suggesting opposite changes in GABAergic transmission in Purkinje cells and in their target neurons in the deep cerebellar nuclei. In contrast, levels of glutamate decarboxylase (67000 mol. wt) messenger RNA in the pallidum, and of enkephalin messenger RNA in the striatum, were unaffected in dystonic rats. The data indicate that both the Purkinje cells and GABAergic neurons of the deep cerebellar nuclei are the site of significant functional abnormality in the dystonic rat.