Running wheel exercise enhances recovery from nigrostriatal dopamine injury without inducing neuroprotection.

Forced use of the forelimb contralateral to a unilateral injection of the dopaminergic neurotoxin 6-hydroxydopamine can promote recovery of motor function in that limb and can significantly decrease damage to dopamine terminals. The present study was conducted to determine (1) whether a form of voluntary exercise, wheel running, would improve motor performance in rats with such lesions, and (2) whether any beneficial effects of wheel running are attributable to ameliorating the dopaminergic damage. In experiment 1, rats were allowed to run in exercise wheels or kept in home cages for 2 1/2 weeks, then given stereotaxic infusions of 6-hydroxydopamine into the left striatum. The rats were replaced into their original environments (wheels or home cages) for four additional weeks, and asymmetries in forelimb use were quantified at 3, 10, 17, and 24 days postoperatively. After killing, dopaminergic damage was assessed by both quantifying 3 beta-(4-iodophenyl)tropan-2 beta-carboxylic acid methyl ester ([(125)I]RTI-55) binding to striatal dopamine transporters and counting tyrosine hydroxylase-positive cells in the substantia nigra. Exercised 6-hydroxydopamine-infused rats showed improved motor outcomes relative to sedentary lesioned controls, effects that were most apparent at postoperative days 17 and 24. Despite this behavioral improvement, 6-hydroxydopamine-induced loss of striatal dopamine transporters and tyrosine hydroxylase-positive nigral cells in exercised and sedentary groups did not differ. Since prior studies suggested that forced limb use improves motor performance by sparing nigrostriatal dopaminergic neurons from 6-hydroxydopamine damage, experiment 2 used a combined regimen of forced plus voluntary wheel running. Again, we found that the motor performance of exercised rats improved more rapidly than that of sedentary controls, but that there were no differences between these groups in the damage produced by 6-hydroxydopamine. It appears that voluntary exercise can facilitate recovery from partial nigrostriatal injury, but it does so without evident sparing of dopamine nerve terminals.

Repeated administration of methamphetamine damages cells in the somatosensory cortex: overlap with cytochrome oxidase-rich barrels.

Multiple injections of methamphetamine (mAMPH) cause degeneration of neurons in rat primary somatosensory cortex (Par1). These degenerating cells can be labeled histochemically with the fluorochrome dye, Fluoro-Jade (FJ). This area of Par1 also contains the representation of the mystacial vibrissae. Neurons in this area of Par1 receiving projections derived from the vibrissae are arranged in discrete functional units ("barrels"), which are revealed by cytochrome oxidase (CO) histochemistry. Here, rats given mAMPH (four injections of 4 mg/kg, sc, 2-h intervals between injections) showed FJ-positive neurons in Par1 that were located predominantly near the perimeter of the CO-dense barrels. Thus, the Par1 neurons damaged by multiple administration of mAMPH are located within whisker barrels.

Chronic L-dopa alters striatal NMDA receptors in rats with dopaminergic injury.

To examine the effects of chronic L-dopa treatment on excitatory amino acid receptors, rats with unilateral nigrostriatal injury and intact controls were treated for 21 days with Sinemet (L-dopa+carbidopa) or vehicle followed by a 3-day washout period. Nigrostriatal damage induced by unilateral injection of 6-hydroxy-dopamine (6-OHDA) produced an extensive decline in [3H]mazindol binding in the ipsilateral caudate-putamen (CPu) and a small (7%) decline in [3H]glutamate binding to N-methyl-D-aspartate (NMDA) receptors in CPu. Sinemet treatment of 6-OHDA-injected rats further reduced the binding of [3H]glutamate to NMDA receptors. The greatest reductions (-34%) occurred in the denervated CPu, but moderate declines (-18% to -22%) were also observed in the CPu, nucleus accumbens and cingulate cortex of the intact hemisphere. Unexpectedly, chronic Sinemet treatment also caused a decrease in [3H]mazindol binding in both hemispheres of rats receiving unilateral 6-OHDA injections. L-Dopa/ carbidopa treatment did not affect [3H]glutamate or [3H]mazindol binding in neurologically intact rats.

Functional and anatomical evidence for different dopamine dynamics in the core and shell of the nucleus accumbens in slices of rat brain.

The characteristics of dopamine (DA) uptake and release were compared in the core and shell of the nucleus accumbens (NAc). DA release was elicited from rat brain slices by local electrical stimulation, and its extracellular concentration was monitored with fast-scan cyclic voltammetry using Nafion-coated, carbon-fiber microelectrodes. The voltammetric results show that the values of DA release and uptake in the shell NAc are approximately one-third of those measured in the core region, and DA uptake in the shell was less sensitive than the core to inhibition by either cocaine or nomifensine. The density of [3H]mazindol binding sites in the NAc was examined by autoradiography and the shell was found to have an average of half the number of DA uptake sites measured in the core region. This combination of anatomical and functional results shows that DA neurotransmission in the shell NAc is distinct from that in the core region. These data are consistent with the view that multiple functional forms of the DA transporter, exhibiting disparate kinetics and pharmacology, exist in different brain regions that exhibit disparate kinetics and pharmacology. Different forms of the transporter, combined with different release kinetics and auto- and heteroreceptor activity, give a vast range of possibilities for regional variation in DA neurotransmission.

Striatal and cortical NMDA receptors are altered by a neurotoxic regimen of methamphetamine.

Methamphetamine (m-AMPH) treatment produces long-lasting damage to striatal and cortical monoaminergic terminals and may also injure nonmonoaminergic cortical neurons. Evidence suggests that both dopamine (DA) and glutamate (GLU) play crucial roles in producing this damage. We used quantitative autoradiography to examine [3H]mazindol ([3H]MAZ) binding to striatal DA transporters and [3H]GLU binding to N-methyl-D-aspartate (NMDA) receptors in the striatum and cortex 1 week and 1 month after a neurotoxic regimen of m-AMPH. Rats received m-AMPH (4 mg/kg) or saline (SAL) (1 ml/kg) in four s.c. injections separated by 2 h intervals. One week after m-AMPH, the ventral and lateral sectors of the striatum showed the greatest decreases in both [3H]MAZ and [3H]GLU binding, while the nucleus accumbens (NA) showed no significant decreases. One month after m-AMPH, striatal [3H]MAZ binding was still significantly decreased, while NMDA receptor binding had recovered. Surprisingly, the parietal cortex showed a m-AMPH-induced increase in NMDA receptor binding in layers II/III and IV 1 week after m-AMPH and only in layers II/III 1 month after m-AMPH. The prefrontal cortex showed no m-AMPH-induced changes in NMDA receptor binding at either time point. This is the first demonstration that a regimen of m-AMPH that results in long-lasting damage to DA terminals can alter forebrain NMDA receptor binding. Thus, repeated m-AMPH treatments may produce changes in glutamatergic transmission in selected striatal and cortical regions.

Excitotoxic striatal lesions protect against subsequent methamphetamine-induced dopamine depletions.

Repeated administration of methamphetamine (m-AMPH) produces a prolonged elevation of extracellular dopamine (DA) levels in rat striatum and subsequent damage to striatal DA terminals. In the present study, a unilateral striatal infusion of quinolinic acid (QA) (15 ug/0.5 microliter) 2 weeks before repeated m-AMPH treatment (four injections of 4 mg/kg, s.c., at 2-hr intervals) protected that striatum from m-AMPH-induced DA terminal injury. One week after m-AMPH treatments, striatal DA contents were substantially below control values in the vehicle-infused striata, whereas the DA contents of the QA-infused striata were equal to those of animals not exposed to m-AMPH. The QA infusions alone injured striatal neurons, as indicated by decreased [3H]SCH 23390 and [3H]spiroperidol binding to D1 and D2 receptors, respectively. However, QA infusions by themselves did not significantly change the DA content or [3H]mazindol binding to the high-affinity DA transporter of the infused striata 3 weeks later. In vivo microdialysis was performed in the previously QA- or vehicle-infused striata during regimens of repeated m-AMPH or saline treatments. QA infusions that were effective in protecting against m-AMPH neurotoxicity did not significantly reduce stimulant-induced DA overflow, compared with the overflow that occurred in the vehicle-infused striata of m-AMPH-treated rats. Thus m-AMPH-induced DA overflow appears to be dissociated from the resulting DA terminal injury in the QA-infused striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

L-dopa pretreatment potentiates striatal dopamine overflow and produces dopamine terminal injury after a single methamphetamine injection.

Rats receiving L-dopa/carbidopa (70 mg/kg/17.5 mg/kg, i.p.) 1 h prior to a single methamphetamine (m-AMPH) (4 mg/kg, s.c.) pretreatment showed an extraordinary striatal dopamine (DA) overflow into the extracellular space (30-60 times basal overflow) as compared to the DA overflow elicited by m-AMPH alone (4-5 times basal). Animals treated with L-dopa/carbidopa plus m-AMPH, but not m-AMPH alone, had substantial (60%) decreases in striatal DA content 1 week later. These findings support the conclusion that the magnitude of m-AMPH-induced DA overflow contributes to the degree of nerve terminal damage and highlight the importance of extracellular DA in striatal terminal damage.

Methamphetamine-induced dopamine overflow and injury to striatal dopamine terminals: attenuation by dopamine D1 or D2 antagonists.

Pharmacological blockade of either D1 or D2 dopamine (DA) receptors prevents damage of striatal DA terminals by repeated doses of methamphetamine (m-AMPH). Because the substantial DA overflow produced by multiple m-AMPH treatments appears to contribute to the subsequent injury, we have investigated the effects of blockade of D1 or D2 receptors on m-AMPH-induced DA efflux using in vivo microdialysis. Four treatments with m-AMPH (4 mg/kg, s.c., 2-h intervals) produced large increases in striatal DA overflow, with particularly marked overflow (10 times the basal values) following the fourth injection. Administered by themselves, four injections of the D1 antagonist SCH 23390 or the D2 antagonist eticlopride (0.5 mg/kg, i.p., 2-h intervals) significantly increased striatal DA overflow. However, treatment with either SCH 23390 or eticlopride 15 min before each of four m-AMPH injections attenuated the marked DA peak otherwise seen after the fourth m-AMPH injection. These effects on DA overflow were related to subsequent DA depletions. Although our m-AMPH regimen produced a 54% reduction in striatal DA tissue content 1 week later, pretreatments with either the D1 or the D2 antagonist completely prevented subsequent DA content depletions. Furthermore, the DA content of striatal tissue remaining 1 week after m-AMPH treatment was significantly correlated with the magnitude of the cumulative DA overflow during the m-AMPH treatment (r = -0.69). Thus, the extensive DA overflow seen during neurotoxic regimens of m-AMPH appears critical to the subsequent neurotoxicity, and the neuroprotective action of DA receptor antagonists seems to result from their attenuation of stimulant-induced DA overflow.

Dopamine-glutamate interactions in methamphetamine-induced neurotoxicity.

Repeated administration of methamphetamine (m-AMPH) to rats induces dopamine (DA) terminal damage, and coadministration of antagonists of the N-methyl-D-aspartate (NMDA) or dopamine D1 or D2 receptors are protective. Striatal microdialysis of rats given a neurotoxic regimen of 4 x m-AMPH (4 mg/kg, s.c.) treatments revealed a dramatic and prolonged elevation of extracellular DA after the final m-AMPH administration. Neuroprotective regimens of MK-801, SCH 23390, or eticlopride greatly attenuated the overflow of DA resulting from the fourth m-AMPH treatment. By itself, MK-801 had no significant influence on striatal DA overflow, whereas either DA antagonist given alone elevated dialysate DA concentrations. A significant correlation was found between the magnitude of the m-AMPH-induced DA overflow of individual microdialyzed rats and their striatal DA content at sacrifice one week later. We conclude that the ability of non-competitive NMDA antagonists and of the D1 or D2 antagonists to protect against m-AMPH-induced striatal DA terminal injury can be accounted for by their attenuation of m-AMPH-evoked DA overflow. These findings underscore the important role played by elevated extracellular DA concentrations to the injurious effects of this stimulant drug.

Striatal subregions are differentially vulnerable to the neurotoxic effects of methamphetamine.

Methamphetamine (m-AMPH) or saline was repeatedly administered to rats. One week later, the caudate-putamen of the m-AMPH-treated rats revealed a decrease in both [3H]mazindol-labeled dopamine uptake sites and tissue dopamine content. Moreover, the resulting pattern of decline in these measures was regionally heterogeneous. The ventral caudate-putamen displayed the greatest decrease in both [3H]mazindol binding and dopamine content while the neighboring nucleus accumbens and the dorsal caudate-putamen remained relatively intact. These results indicate a regional difference in the susceptibility of striatal dopaminergic terminals to the neurotoxic effects of methamphetamine.

MK-801 protection against methamphetamine-induced striatal dopamine terminal injury is associated with attenuated dopamine overflow.

Repeated administrations of methamphetamine (m-AMPH) produce high extracellular levels of dopamine (DA) and subsequent striatal DA terminal damage. Pharmacological blockade of N-methyl-D-aspartate (NMDA) receptors has been shown previously to prevent m-AMPH-induced striatal DA terminal injury, but the mechanism for this protection is unclear. In the present study, in vivo microdialysis was used to determine the effects of blockade of NMDA receptors with the noncompetitive antagonist MK-801 on m-AMPH-induced striatal DA overflow. Four injections of MK-801 (0.5 mg/kg, ip) alone did not significantly change extracellular striatal DA concentrations from pretreatment values. Four treatments with m-AMPH (4.0 mg/kg, sc at 2-hr intervals) increased striatal DA overflow, and the overflow was particularly extensive following the fourth injection. This m-AMPH regimen produced a 40% reduction in striatal DA tissue content 1 week later. Treatment with MK-801 15 min before each of the four m-AMPH injections or prior to only the last two m-AMPH administrations attenuated the m-AMPH-induced increase in striatal DA overflow and protected completely against striatal DA depletions. Other MK-801 treatment regimens less effectively reduced the m-AMPH-induced striatal DA efflux and were ineffective in protecting against striatal DA depletions. Linear regression analysis indicated that cumulative DA overflow was strongly predictive (r = -.68) of striatal DA tissue levels measured one week later. These findings suggest that the extensive DA overflow seen during a neurotoxic regimen of m-AMPH is a crucial component of the subsequent neurotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple methamphetamine injections induce marked increases in extracellular striatal dopamine which correlate with subsequent neurotoxicity.

Acutely, methamphetamine (m-AMPH) is known to stimulate a net efflux of dopamine (DA) in the striatum while inhibiting DA uptake, thus producing high extracellular concentrations of DA. Repeated administration of m-AMPH has been shown to damage DA terminals in the striatum. However, little direct information exists about the relationship between m-AMPH-induced DA overflow and neurotoxicity. In the present study, we used in vivo microdialysis to explore this topic. Four, but not 3, injections of m-AMPH (4 mg/kg, sc, at 2 h intervals) damaged striatal DA terminals as measured by a 43-51% decrease in post mortem striatal DA content 1 week later. Striatal microdialysis in awake animals during the course of m-AMPH treatment showed that DA overflow increased after each m-AMPH injection, but that approximately 1.5 h after the fourth m-AMPH injection, a striking increase in DA overflow occurred that was significantly larger than that seen after any of the previous 3 injections. Additionally, in animals receiving 4 injections of m-AMPH, cumulative DA overflow was negatively correlated with striatal DA content 1 week later (r = -0.74, P less than 0.05), suggesting that the substantial DA overflow seen after the fourth m-AMPH injection is especially important in m-AMPH neurotoxicity.

MK-801 attenuates the dopamine-releasing but not the behavioral effects of methamphetamine: an in vivo microdialysis study.

Neuroanatomical and pharmacological evidence suggests that important modulatory relationships exist between mesostriatal dopaminergic terminals and corticostriatal inputs. The present study used in vivo microdialysis in awake animals to examine the results of pharmacological manipulations of these systems on net striatal dopamine (DA) efflux and behavioral activation. A single methamphetamine (m-AMPH) treatment induced a prolonged (greater than 6 h) increase (6-fold peak response) in extracellular striatal DA and increased stereotypic behavior. When given alone, the non-competitive N-methyl-D-aspartate (NMDA) antagonist MK-801 did not have a significant effect on extracellular striatal DA, but significantly increased stereotypic behaviors. Pretreatment with MK-801 markedly attenuated the m-AMPH-induced striatal DA overflow. In contrast to its effects on striatal DA overflow, MK-801 potentiated the locomotor effects of m-AMPH without reducing stereotypy rating scores. These findings suggest that the synaptic relationships between mesostriatal DA and corticostriatal excitatory amino acid terminals in the striatum are an important component in its behavioral output. Moreover, NMDA receptors appear to be capable of modulating striatal DA overflow.

Chronic treatment with clozapine or haloperidol differentially regulates dopamine and serotonin receptors in rat brain.

Long-term administration of the atypical neuroleptic clozapine (CLZ) poses a much lower risk of extrapyramidal side effects (EPS) than does the use of typical neuroleptics such as haloperidol (HAL). To investigate the neural mechanisms of the differing CNS activities of these two drugs, we used quantitative autoradiography to measure changes in dopamine and serotonin receptors in rats after injection with CLZ or HAL for 21 days at clinically relevant dose ratios. Levels of D1, D2, and 5-HT2 receptors were determined in frontal cortex, caudate-putamen, and nucleus accumbens. Rats that received CLZ chronically showed CNS receptor changes markedly different from those in chronic HAL-treated animals. Whereas rats treated chronically with HAL showed enhanced striatal D2 binding (average increase of 42%), those treated with CLZ did not. In contrast, chronic CLZ, but not chronic HAL, induced enhanced striatal D1 binding (average increase of 43%). Finally, CLZ treatment decreased 5-HT2 receptor binding by an average of 37%, while HAL had no significant effect. The effects of chronic HAL or CLZ treatment on receptors were similar in all forebrain areas examined. However, since D1 and 5-HT2 receptors are more abundant than D2 sites in limbic and neocortical areas, the preferential modulation of D1 and 5-HT2 receptors by CLZ suggests a greater impact of this atypical neuroleptic on activity of the limbic system than that achieved by the typical neuroleptic, HAL. These findings suggest that the clinical profile of atypical neuroleptics such as CLZ may be attributed to their effects on a receptor profile differing in pharmacological characteristics and anatomical distribution from that affected by typical neuroleptics.

Dopamine high-affinity transport site topography in rat brain: major differences between dorsal and ventral striatum.

Investigations were conducted to determine the topography of the high-affinity dopamine uptake process within the rat striatum. [3H]Dopamine uptake into crude synaptosomes prepared from micropunch samples was found to be two- to three-fold higher in dorsal caudate-putamen relative to nucleus accumbens septi. In contrast, the concentrations of dopamine in the two regions were equivalent. The recognition site associated with high-affinity dopamine uptake was labeled using [3H]mazindol, and the binding of this ligand was also found to be two- to three-fold higher in homogenates from dorsal caudate-putamen samples relative to nucleus accumbens septi. Regional differences in uptake of [3H]dopamine or binding of [3H]mazindol were shown to be due to variations in Vmax or Bmax, not to differences in apparent affinity. Autoradiography of [3H]mazindol binding in rat striatum revealed a decreasing density of the site along the dorsal-to-ventral axis, with the highest binding occurring in the dorsolateral caudate-putamen, lower binding in the ventral caudate-putamen, and lowest levels in the septal pole of the nucleus accumbens septi. Quantification showed that the extent of this gradient was two-fold. Further autoradiographic studies revealed less striatal heterogeneity in the pattern of binding of [3H]ketanserin, another radioligand associated with the striatal dopaminergic innervation but not linked to the dopamine uptake process of the plasma membrane. The findings suggest that the dopaminergic fibers of the ventral striatum, especially the medial nucleus accumbens septi, may be relatively lacking in their capacity for dopamine uptake following its release. This organization may result in regional differences in the time-course of of extraneuronal dopamine following transmitter release and may render the dopamine-containing terminals of the ventral striatum less susceptible to the degenerative influences of neurotoxins that are incorporated by the high-affinity dopamine uptake process.

Transport of [3H]mazindol binding sites in mesostriatal dopamine axons.

6-Hydroxydopamine injections along mesostriatal dopaminergic axons can be used to interrupt axonal transport from cell bodies in the substantia nigra pars compacta to terminal fields in the striatum. Such lesions produce accumulations of high-affinity dopamine uptake sites (as measured by [3H]mazindol binding) and acetylcholinesterase proximal to the injection, suggesting that at least a portion of the [3H]mazindol binding and acetylcholinesterase activity seen in the striatum is located presynaptically on the mesostriatal dopaminergic fibers.

Active Uptake of Amino Acids by Leaves of an Epiphytic Vascular Plant, Tillandsia paucifolia (Bromeliaceae).

Specialized epidermal trichomes on the leaves of the epiphyte, Tillandsia paucifolia (Bromeliaceae) accumulate amino acids from solution. Simultaneous net uptake of 17 amino acids was determined using high performance liquid chromatography. Uptake occurs against concentration gradients at least as high as 10(4).