Reduced Tyrosine Hydroxylase Immunoreactivity Research Articles

Nitration and Increased α‐Synuclein Expression Associated With Dopaminergic Neurodegeneration In Equine Pituitary Pars Intermedia Dysfunction

Equine pituitary pars intermedia dysfunction (PPID) is a spontaneously occurring progressive disease affecting aged horses and ponies. The pathogenesis of PPID is poorly understood, but the available evidence supports a loss of dopaminergic inhibition of the melanotropes of the pars intermedia. Horses with PPID have increased plasma concentrations of pars intermedia pro-opiomelanocortin-derived peptides that decrease in response to dopamine or dopamine agonist administration. Dopamine and dopamine metabolite concentrations are decreased in the pars intermedia of affected horses compared to age-matched control horses. Horses with disease that are treated with the dopamine agonist pergolide show improvement in clinical signs and normalisation of diagnostic test results. In the present study, immunohistochemical evaluation of pituitary and hypothalamic tissue demonstrated reduced tyrosine hydroxylase immunoreactivity in affected horses compared to age-matched and young controls, supporting the role of dopaminergic neurodegeneration in PPID. In addition, immunohistochemical evaluation revealed an increase in the oxidative stress marker, 3-nitrotyrosine and in nerve terminal protein, alpha-synuclein that colocalised in the pars intermedia of horses with disease. These findings suggest a role for nitration of overexpressed alpha-synuclein in the pathogenesis of neurodegeneration in PPID.

Rotenone, Deguelin, Their Metabolites, and the Rat Model of Parkinson's Disease

Rotenone and deguelin are the major active ingredients and principal components of cuberesin from Lonchocarpus utilis used as a botanical insecticide and piscicide. They are also potent complex I (NADH:ubiquinone oxidoreductase) inhibitors. Rotenone was known earlier, and deguelin is shown here to induce a Parkinson's disease (PD)-like syndrome after subcutaneous treatment of rats by osmotic minipump. Rotenone at 3 mg/kg/day or deguelin at 6 but not 3 mg/kg/day induces degeneration of the nigrostriatal dopaminergic pathway, as shown by reduced tyrosine hydroxylase immunoreactivity with treatments for 5 or 6 days. The neuropathological lesions are associated with a brain level of parent rotenoid of 0.4-1.3 ppm but not with the much smaller brain level of 12abeta-hydroxyrotenoids or other metabolites analyzed by HPLC and LC/MS. We previously established that the hydroxylated metabolites and derivatives of rotenone and deguelin are all less active (i.e., detoxified) as complex I inhibitors relative to the parent rotenoids. The PD-like syndrome induced in rats by rotenone and deguelin is therefore due to the parent compounds rather than metabolites. Deguelin is about half as active as rotenone in inducing the PD-like syndrome in rats and in acute ip LD50 in mice. Rotenone and deguelin are metabolized by human recombinant 3A4 and 2C19 but not five other P450 enzymes. 2C19 is more selective than 3A4 in forming the 12abeta-hydroxyrotenoids. Identified sites of metabolic attack individually or in combination are as follows: 12abeta hydroxylation and 2-O-demethylation of both compounds, oxidation of the rotenone isopropenyl substituent to mono and diol derivatives, and probable oxidation of the deguelin dimethylchromene double bond. These toxicological features must be considered in using rotenone, deguelin, and their analogues as pesticides, candidate radioimaging and cancer chemopreventive agents, and models of PD.

Selective microglial activation in the rat rotenone model of Parkinson's disease

Chronic rotenone exposure reproduces features of Parkinson's disease (PD) (Nat. Neurosci. 3 (2000) 1301; Exp. Neurol. 179 (2003) 9). We investigated the role of glial activation in rotenone toxicity in vivo. Male Lewis rats received 2–3 mg/kg rotenone per day for up to 4 weeks. In 50% of surviving rotenone-treated animals, there was nigrostriatal dopaminergic degeneration, marked by reduced tyrosine hydroxylase immunoreactivity). Extensive microglial activation, determined by OX-42-ir, occurred in striatum and nigra of rotenone-treated animals, and was prominent before anatomical evidence of dopaminergic lesions. Microglia enlarged and developed short, stubby processes in rotenone-treated animals. Rotenone-induced microglial activation was less pronounced in cortex. Reactive astrocytosis was minimal and limited to a thin rim around the lesion. Marked microglial activation with minimal astrocytosis is another pathological feature of PD reproduced by rotenone treatment.

The retina as a novel in vivo model for studying the role of molecules of the Bcl-2 family in relation to MPTP neurotoxicity.

To determine the roles of different members of the family of B cell lymphoma protooncogene (Bcl-2) in relation to neurotoxin-induced neuronal degeneration, the pattern of the expression of a number of molecules of the Bcl-2 family was studied immunocytochemically in the retinas of C57BL/6J mice after intraperitoneal (IP) injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Three days to 12 weeks after MPTP treatment, a detectable reduction of tyrosine hydroxylase immunoreactivity in the amacrine cells was observed, with an increase of Bcl-2 expression in the Müller glial cells, and a de novo expression of Bad and Bax in the retinal ganglion cells, optic nerve fibers and plexiform layers. In contrast, a slight decrease of Bcl-x(L) immunoreactivity in the retinal ganglion cells was observed, whereas Bcl-x(S/L) immunoreactivity was increased slightly in the retinas of MPTP-treated mice compared with that of the controls. In animals that received MPTP injection, an increase in immunostaining of GFAP, glutamine synthetase, and Mac-1 (CD11b) in astrocytes, Müller cells, and microglia was invariably observed, indicating an activation or dysfunction of retinal glial cells. These findings are consistent with the current view that glial dysfunction is important in mediating the cytotoxic effect of a variety of neurotoxic molecules, including MPTP, and that different members of Bcl-2 family may have different roles as far as neuronal degeneration or neuroprotection is concerned.

Activation of tyrosine hydroxylase in the lateral efferent terminals by sound conditioning

Preconditioning to sound is a well-documented strategy to provide protections against a subsequent acoustic trauma. In the present study, preconditioning (1.0 kHz tone at 81 dB sound pressure level (SPL) for 24 h) protected ABR thresholds by 17–28 dB from an acoustic trauma (2.7 kHz, 103 dB SPL, 30 min) that resulted in a temporary threshold shift. The protection afforded by sound conditioning was shown to be blocked by the administration of 6-hydroxydopamine which disrupts tyrosine hydroxylase in the nerve terminals of the lateral efferent fibers. Furthermore, tyrosine hydroxylase immunoreactivity was up-regulated both by sound conditioning alone, and by the combined treatment of sound conditioning and acoustic trauma. In contrast, acoustic trauma alone resulted in a reduction in tyrosine hydroxylase immunoreactivity compared to unexposed controls. These findings are the first demonstration that tyrosine hydroxylase in the lateral efferents are up-regulated during sound conditioning and suggests a role for the lateral efferent system in protecting against acoustic trauma by sound conditioning.

Rotenone destroys dopaminergic neurons and induces parkinsonian symptoms in rats

Rotenone (an inhibitor of mitochondrial NADH dehydrogenase, a naturally occurring toxin and a commonly used pesticide) appears to reproduce the neurochemical, neuropathological and behavioural feature of Parkinson's disease (PD) in the rat. In this study, rotenone was administrated on a daily basis systemically by intraperitoneal injection of two different doses: 1.5 mg/kg (low dose) and 2.5 mg/kg (moderate dose), over a period of 2 months. This treatment caused depletion of dopamine in the posterior striatum (CPu) and prefrontal cortex and also reduced tyrosine hydroxylase-immunoreactivity in CPu. Behavioural experiments showed dose-dependent catalepsy in the two treatment groups of rats. Data from this study indicate that in rats rotenone is capable of causing degeneration of dopaminergic neurons and induction of parkinsonian symptoms. It is concluded that the causal mechanisms of neuronal degeneration implicate a complex I deficiency in the aetiology of rotenone-induced and perhaps in some cases of sporadic PD.

Catecholamines and serotonin are differently regulated by tetrahydrobiopterin. A study from 6-pyruvoyltetrahydropterin synthase knockout mice.

(6R)-L-erythro-5,6,7,8-Tetrahydrobiopterin (BH4) is an essential cofactor for tyrosine hydroxylase (TH), tryptophan hydroxylase, phenylalanine hydroxylase, and nitric-oxide synthase. These enzymes synthesize neurotransmitters, e.g. catecholamines, serotonin, and nitric oxide (NO). We established mice unable to synthesize BH4 by disruption of the 6-pyruvoyltetrahydropterin synthase gene, the encoded protein of which catalyzes the second step of BH4 biosynthesis. Homozygous mice were born at the almost expected Mendelian ratio, but died within 48 h after birth. In the brain of homozygous mutant neonates, levels of biopterin, catecholamines, and serotonin were extremely low. The number of TH molecules was highly dependent on the intracellular concentration of BH4 at nerve terminals. Alteration of the TH protein level by modulation of the BH4 content is a novel regulatory mechanism. Our data showing that catecholaminergic, serotonergic, and NO systems were differently affected by BH4 starvation suggest the possible involvement of BH4 synthesis in the etiology of monoamine-based neurological and neuropsychiatric disorders.

Extinction Training Regulates Tyrosine Hydroxylase during Withdrawal from Cocaine Self-Administration

Chronic exposure to drugs of abuse is known to modulate tyrosine hydroxylase (TH) levels in the mesolimbic dopamine system. In this study, 12 d of cocaine self-administration in rats (4 hr/d) reduced TH immunoreactivity by 29% in the nucleus accumbens (NAc) shell, but not core, after a 1 week withdrawal period. In contrast, TH immunoreactivity in the NAc was completely restored in animals that experienced extinction training (4 hr/d) during the same withdrawal period. Extinction training also increased TH levels in the ventral tegmental area (VTA) by 45%, whereas TH was not altered in the VTA by cocaine withdrawal alone. Thus, extinction-induced normalization of NAc TH levels could involve increased TH synthesis, stability, and/or transport from the VTA to the NAc. A similar extinction training regimen failed to alter TH levels in the NAc or VTA of rats trained to self-administer sucrose pellets, indicating that TH regulation in cocaine-trained animals is not a generalized effect of extinction learning per se. Rather, these data suggest that neuroadaptative responses during cocaine withdrawal ultimately are determined by a complex interaction between chronic drug exposure and drug-seeking experience. The ability of extinction training to restore NAc TH levels is hypothesized to accelerate recovery from dopamine depletion and anhedonia during cocaine withdrawal.

Role of oxidative changes in the degeneration of dopamine terminals after injection of neurotoxic levels of dopamine

Dopamine may contribute to the loss of dopamine neurons in Parkinson’s disease by generating reactive oxygen species and quinones. A previous report from this laboratory showed that intrastriatal injection of dopamine resulted in the selective reduction of tyrosine hydroxylase immunoreactivity, accompanied by an increase in indices of dopamine oxidation. However, conclusive proof that decreased tyrosine hydroxylase immunoreactivity represented a loss of dopamine terminals was lacking. In this paper, we demonstrate that injection of dopamine results in a selective loss of dopamine terminals by (i) showing that immunoreactivity for another selective marker for dopamine terminals, the dopamine transporter, is also reduced; and (ii) that amino-cupric-silver stain reveals terminal degeneration within the area of selective loss of dopamine terminals. To determine the dopamine concentration that is selectively toxic to dopamine terminals, we examined changes in extracellular dopamine and 3,4-dihydroxyphenylacetic acid in the area of selective terminal loss following intrastriatal dopamine. Dopamine and 3,4-dihydroxyphenylacetic acid in this region reached peak levels 1–2 h after the injection, and then returned towards baseline. The peak level of dopamine in the area of selective dopamine terminal damage was 10 2–10 3-fold lower than the injected concentration. Changes in striatal tissue levels of cysteinyl-catechols and glutathione were examined at 2, 4, 8, and 24 h after intrastriatal dopamine. Levels of protein cysteinyl-dopamine and cysteinyl-3,4-dihydroxyphenylacetic acid were increased at all time-points following the dopamine injection. High levels of free cysteinyl-catechols and glutathione-dopamine were detected within 2 h after the dopamine injection. Glutathione levels were decreased significantly at 4 and 8 h after the injection of dopamine, and returned to control levels by 24 h. These data indicate that dopamine terminals actively degenerate following a single intrastriatal injection of dopamine, and furthermore that oxidative stress plays a key role in this process. As oxidative stress is thought to play an active role in the pathobiology of Parkinson’s disease, these data may be relevant to our understanding of the disorder.

Systemic administration of the propargylamine CGP 3466B prevents behavioural and morphological deficits in rats with 6‐hydroxydopamine‐induced lesions in the substantia nigra

The ability of CGP 3466B to attenuate the behavioural and morphological consequences of experimentally induced cell death was investigated in a recently updated animal model of Parkinson's disease. 6-Hydroxydopamine was infused bilaterally into the substantia nigra pars compacta of rats that were pretreated with desimipramine. Treatment with CGP 3466B (0.0014-1.4 mg/kg, injected subcutaneously) or its solvent was begun 2 h after the 6-OHDA injection, and maintained twice daily for 14 days. After a washout period of 14 days, changes in motor behaviour were evaluated, using the open field test (analysis of normal and abnormal stepping, e.g.) and the paw test (analysis of retraction time of limbs). Changes in learning and memory were evaluated with the help of the Morris water maze task. Following immunocytochemical staining of tyrosine hydroxylase, the extent of the lesion was quantified using a computerized system. CGP 3466B prevented all deficits produced by 6-hydroxydopamine (6-OHDA), though at different doses. It prevented: abnormal stepping (0.0014-0.014 mg/kg); increased forelimb and hindlimb retraction time (0.014-0.14 mg/kg and 0.0014-0.14 mg/kg, respectively); delayed learning (1.4 mg/kg); and reduced tyrosine hydroxylase immunoreactivity in the substantia nigra (0.0014-0.014 mg/kg). CGP 3466B (0.0014-0.14 mg/kg) induced no deficits in sham-treated rats. CGP 3466B (1.4 mg/kg), however, did not show any benefit on motor deficits in 6-OHDA-lesioned rats, and induced abnormal movements and decreased the tyrosine hydroxylase immunoreactivity in the substantia nigra pars compacta and the ventral tegmental area of sham-lesioned animals. It is concluded that CGP 3466B prevents all 6-OHDA-induced behavioural and immunocytochemical deficits, though at different doses. CGP 3466B is suggested to be a valuable agent for inhibiting the dopaminergic degeneration in patients with Parkinson's disease.

Chronic prenatal exposure to carbon monoxide results in a reduction in tyrosine hydroxylase-immunoreactivity and an increase in choline acetyltransferase-immunoreactivity in the fetal medulla: implications for Sudden Infant Death Syndrome.

Maternal cigarette smoking during pregnancy is associated with a significantly increased risk of Sudden Infant Death Syndrome (SIDS). This study investigated the effects of prenatal exposure to carbon monoxide (CO), a major component of cigarette smoke, on the neuroglial and neurochemical development of the medulla in the fetal guinea pig. Pregnant guinea pigs were exposed to 200 p.p.m CO for 10 h per day from day 23-25 of gestation (term = 68 days) until day 61-63, at which time fetuses were removed and brains collected for analysis. Using immunohistochemistry and quantitative image analysis, examination of the medulla of CO-exposed fetuses revealed a significant decrease in tyrosine hydroxylase-immunoreactivity (TH-IR) in the nucleus tractus solitarius, dorsal motor nucleus of the vagus (DMV), area postrema, intermediate reticular nucleus, and the ventrolateral medulla (VLM), and a significant increase in choline acetyltransferase-immunoreactivity (ChAT-IR) in the DMV and hypoglossal nucleus compared with controls. There was no difference between groups in immunoreactivity for the m2 muscarinic acetylcholine receptor, substance P- or met-enkephalin in any of the medullary nuclei examined, nor was there evidence of reactive astrogliosis. The results show that prenatal exposure to CO affects cholinergic and catecholaminergic pathways in the medulla of the guinea pig fetus, particularly in cardiorespiratory centers, regions thought to be compromised in SIDS.

Enhanced N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity in mice deficient in CuZn-superoxide dismutase or glutathione peroxidase.

Administration of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to mammals causes damage to the nigrostriatal dopaminergic pathway similar to that observed in Parkinson disease (PD). Reactive oxygen species (ROS) are thought to be involved in the pathogenesis of MPTP-mediated dopaminergic neurodegeneration. To further clarify the role of superoxide anion radical (*O2-) and to study the possible involvement of hydroperoxides in MPTP-mediated neurodegeneration, MPTP neurotoxicity was induced in mice deficient in either CuZn superoxide dismutase (SOD), a scavenger enzyme for *O2-, or cellular glutathione peroxidase (GSHPx-1), a scavenger enzyme for hydroperoxides. Littermate control and homozygous deficient mice were injected intraperitoneally with a total cumulative dose of 0, 75, or 150 mg/kg of MPTP delivered over 5 d. All mice were killed 5 d after the last injection and the brains were processed for immunohistological analysis for tyrosine hydroxylase (TH) in the striatum and the substantia nigra pars compacta (SNc), as well as for direct measurements of dopamine concentrations in the striatum. The intensity of TH immunoreactivity in the striatum was evaluated by measuring the relative optical density (OD) with NIH IMAGE, and expressed as Log (OD of striatum)/Log (OD of white matter). Degeneration of TH-containing neurons was assessed by counting TH-positive neurons in the SNc. We found that this MPTP exposure protocol produced dose-dependent depletion of TH immunoreactivity and dopamine in the striatum in littermate control mice and both strains of knockout mice; however. reduction in TH immunoreactivity and dopamine content were significantly greater in CuZn-SOD or GSHPx-1 deficient mice compared with littermate controls. MPTP exposure did not significantly alter the number of TH-positive neurons in the SNc in littermate control or knockout mice. These data suggest that some of the deleterious effects of MPTP on striatal dopaminergic nerve terminals are mediated by both *O2- and hydroperoxides, and that they occur prior to dopaminergic neurodegeneration in the SNc. The similarity between the MPTP model and PD raises the possibility that both types of ROS may play a significant role in the early pathogenesis of dopaminergic neurodegeneration in PD.

Subgroups of parkinsonian patients differentiated by peptidergic immunostaining of caudate nucleus biopsies

In this study, Met-enkephalin (Met-enk), substance P (SP) and tyrosine hydroxylase (TH) immunostaining was assessed in caudate nucleus biopsies from 15 Parkinson’s disease patients who were treated surgically. According to the combination of changes in Met-enk, SP and TH immunostaining, several subgroups of parkinsonian patients were disclosed. Group I: Patients showing low SP and normal Met-enk immunostaining, and variably reduced TH immunoreactivity. Group II: both SP and Met-enk immunostaining were apparently of normal intensity in these PD patients, but they showed the greatest decrease in TH labeling. Group III: PD patients that showed normal SP, very low Met-enk and variably reduced TH immunostaining. Low Met-enk immunostaining tended to correlate with the severity of the disease as judged by higher Unified Parkinson’s disease Rating Scale and gait scores. These results suggest that different neurochemical phenotypes may exist among Parkinson’s disease patients. Peptidergic deficits should be taken into account for therapeutic intervention.

Specific A10 Dopaminergic Nuclei in the Midbrain Degenerate in Parkinson's Disease

Using unbiased quantitative techniques, we evaluated the effect of Parkinson's disease on the regional size and the number of tyrosine hydroxylase-producing neurons and all neurons in the midbrain A8 and A10 dopaminergic cell groups located adjacent to the substantia nigra. Seven patients with Lewy body Parkinson's disease were evaluated and compared with five controls. Four of the patients with Parkinson's disease had additional neuropathology, and the effect of concomitant pathology on A10 populations was also determined. Degeneration was not observed in the A8 regions of any patient, and only certain A10 nuclei were affected by the disease. The parabrachial pigmented nucleus situated dorsal to the substantial nigra, and the parapeduncular nucleus located rostromedially were significantly reduced by 40–50% in patients with Parkinson's disease. Few differences were found between patients with or without additional pathology, suggesting a similar pathogenic mechanism to that observed in the substantia nigra of these patients. However, patients with additional pathology also had serotonergic cell loss in the caudal linear nucleus. There was a reduction in tyrosine hydroxylase immunoreactivity but no overt neurodegeneration in other A10 regions, suggesting the disease may also influence the production of dopamine in some surviving neurons.

Striatal Infarction in the Rat Causes a Transient Reduction of Tyrosine Hydroxylase Immunoreactivity in the Ipsilateral Substantia Nigra

Dopaminergic neurons of the substantia nigra pars compacta were examined in the rat brain following striatal infarction subsequent to transient focal cerebral ischemia. Rats had the middle cerebral artery occluded for 2 h or were sham-operated, and tyrosine hydroxylase immunoreactivity was evaluated by Western blot and immunohistochemistry at different times ranging from 1 to 60 days after ischemia. The number of tyrosine hydroxylase-immunoreactive cells in the substantia nigra pars compacta was counted under the light microscope and compared to that in the contralateral side and controls. No changes of tyrosine hydroxylase immunoreactivity were detected in the ipsilateral versus the contralateral substantia nigra of sham-operated rats or 1 day after ischemia. However, a statistically significant reduction of tyrosine hydroxylase-immunoreactive cells became apparent in the ipsilateral compared with the contralateral substantia nigra at 7 and 14 days after ischemia. This reduction showed a clear recovery at 30 days after ischemia, and no signs of difference between the ipsilateral and the contralateral side were apparent by 60 days. Therefore, the reduction of tyrosine hydroxylase immunoreactivity in the ipsilateral substantia nigra was only transiently seen from 1 to 2 weeks following ischemia. The observed loss of tyrosine hydroxylase was not accompanied by signs of cell death or gliosis in the ipsilateral pars compacta. The present results show a transitory reduction of tyrosine hydroxylase immunoreactivity in the ipsilateral substantia nigra pars compacta after focal ischemia and suggest that striatal infarction causes a transient deficit of dopaminergic function.

Monoamine oxidase B inhibitor selegiline protects young and aged rat peripheral sympathetic neurons against 6-hydroxydopamine-induced neurotoxicity.

Selegiline is a selective and irreversible monoamine B inhibitor with the capacity to increase the level of several antioxidative enzymes in rat brain. It can protect adrenergic neurons against injury induced by neurotoxins such as MPTP, DSP-4 and AF64A in animal studies. In addition, the protective action is not limited to catecholaminergic cells, as selegiline can also minimize the loss of developing motoneurons after axotomy. The aim of this study was to determine whether selegiline can protect peripheral catecholaminergic neurons against the neurotoxic effect of 6-OHDA. This kind of protective effect against 6-OHDA neurotoxicity has not been reported before. Wistar albino male rats aged 4 or 24 months were treated with selegiline or saline solution 1 h before 6-OHDA injection. At 2 weeks after the 6-OHDA injection, the superior cervical ganglia (SCG) and submandibular glands (SMG) were studied using catecholamine histofluorescence and immunohistochemistry for tyrosine hydroxylase (TH). The number of TH-positive cells in the SCG and the length and number of adrenergic nerve fibers in the SMG were quantified. Our findings showed that 6-OHDA caused a reduction of TH immunoreactivity and catecholamine histofluorescence in neuronal somata, as well as a decrease in the number and length of adrenergic nerve fibers in the submandibular gland. Selegiline pretreatment protected SCG neurons and their postganglionic nerve fibers in SMG against these changes in a dose-dependent manner. The mechanism through which selegiline exerts its neuroprotective effect is as yet unknown.

Role of oxidation in the neurotoxic effects of intrastriatal dopamine injections.

We have examined the biochemical and histological effects of high concentrations of dopamine (0.05-1.0 micromol) injected into the rat striatum. Twenty-four hours after such injections, the oxidation products of dopamine and dihydroxyphenylacetic acid were detected as both free and protein-bound cysteinyl dopamine and cysteinyl dihydroxyphenylacetic acid. Protein-bound cysteinyl catechols were increased 7- to 20-fold above control tissue levels. By 7 days postinjection, the protein-bound cysteinyl catechols were still detectable, although reduced in concentration, whereas the free forms could no longer be measured. Histological examination of striatum at 7 days revealed a central core of nonspecific damage including neuronal loss and gliosis. This core was surrounded by a region containing a marked reduction in tyrosine hydroxylase immunoreactivity but no apparent loss of serotonin or synaptophysin immunoreactivity. When dopamine was injected with an equimolar concentration of either ascorbic acid or glutathione, the formation of protein-bound cysteinyl catechols was greatly reduced. Moreover, the specific loss of tyrosine hydroxylase immunoreactivity associated with dopamine injections was no longer detectable, although the nonspecific changes in cytoarchitecture were still apparent. Thus, following its oxidation, dopamine in high concentrations binds to protein in the striatum, an event that is correlated with the specific loss of dopaminergic terminals. We suggest that the selective degeneration of dopamine neurons in Parkinson's disease may be caused by an imbalance between the oxidation of dopamine and the availability of antioxidant defenses.

The D2 antagonist spiperone mimics the effects of olfactory deprivation on mitral/tufted cell odor response patterns

Wistar rats had a single nare occluded on postnatal day 30, depriving the ipsilateral olfactory bulb of odor stimulation. The deprivation lasted for either 1-2 months (short-term) or 12 months (long-term). As previously reported, deprivation greatly reduced tyrosine hydroxylase immunoreactivity (the rate limiting enzyme for dopamine synthesis) in the glomerular layer of the ipsilateral olfactory bulb. The nare was then reopened and odor response patterns of mitral/tufted cells were examined. The proportion of mitral/tufted cell single-units responding to a single odor was enhanced by deprivation. Furthermore, the proportion of mitral/tufted cells responding to more than one odor was increased by deprivation, suggesting a decrease in discrimination. Finally, in undeprived bulbs, the dopamine D2 receptor antagonist spiperone mimicked the effects of deprivation on mitral/tufted cell odor response patterns. The results are interpreted as an activity-dependent dopamine modulation of lateral and feedback inhibition in the olfactory bulb, and are compared with similar events in the dark-adapted retina.

2-methyl-1, 2, 3, 4-tetrahydroisoquinoline does dependently reduce the number of tyrosine hydroxylase-immunoreactive cells in the substantia nigra and locus ceruleus of C57BL/6J mice

2-methyl-1, 2, 3, 4-tetrahydroisoquinoline (2-MTIQ) was administered chronically to C57BL/6J mice. Light and electron microscopic examinations revealed atrophic neurons restricted to the central portion of the substantia nigra pars compacta (SNc) and pars lateralis as well as a reduction in tyrosine hydroxylase immunoreactivity, which showed a significant inverse correlation with the dose administered in the SNc. 2-MTIQ may be a candidate endogenous or environmental factor that induces parkinsonism.

Dopamine depletion in nigrostriatal neurons in the genetically diabetic rat

Employing tyrosine hydroxylase (TH) immunohistochemistry, we have revealed that the enzyme is depleted in nigrostriatal neurons in the genetically diabetic BB Wistar (BBWd) rat. A similar, but much less marked reduction in TH immunoreactivity occurred in the mesolimbic dopamine system, which arises from the ventral tegmental area and terminates in the nucleus accumbens. The projection to the olfactory tubercle appeared unaffected. Since the decreased levels of dopamine in the mesencephalic dopamine systems could not be restored under insulin-controlled conditions, the impairment of dopamine biosynthesis in the BBWd rat may be genetically programmed.