The synthesis of 2,4,5-trihydroxyphenylalanine (6-hydroxydopa). A centrally active norepinephrine-depleting agent

Neuroteratology and Animal Modeling of Brain Disorders

Over the past 60 years, a large number of selective neurotoxins were discovered and developed, making it possible to animal-model a broad range of human neuropsychiatric and neurodevelopmental disorders. In this paper, we highlight those neurotoxins that are most commonly used as neuroteratologic agents, to either produce lifelong destruction of neurons of a particular phenotype, or a group of neurons linked by a specific class of transporter proteins (i.e., dopamine transporter) or body of receptors for a specific neurotransmitter (i.e., NMDA class of glutamate receptors). Actions of a range of neurotoxins are described: 6-hydroxydopamine (6-OHDA), 6-hydroxydopa, DSP-4, MPTP, methamphetamine, IgG-saporin, domoate, NMDA receptor antagonists, and valproate. Their neuroteratologic features are outlined, as well as those of nerve growth factor, epidermal growth factor, and that of stress. The value of each of these neurotoxins in animal modeling of human neurologic, neurodegenerative, and neuropsychiatric disorders is discussed in terms of the respective value as well as limitations of the derived animal model. Neuroteratologic agents have proven to be of immense importance for understanding how associated neural systems in human neural disorders may be better targeted by new therapeutic agents.

Perinatal Lesioning and Lifelong Effects of the Noradrenergic Neurotoxin 6-Hydroxydopa

6-hydroxydopa (6-OHDOPA) was synthesized with the expectation that it would be able to cross the blood-brain barrier to be enzymatically decarboxylated to 6-hydroxydopamine (6-OHDA), the newly discovered neurotoxin for noradrenergic and dopaminergic neurons. In part, 6-OHDOPA fulfilled these criteria. When administered experimentally to rodents, 6-OHDOPA destroyed peripheral sympathetic noradrenergic nerves and did exert neurotoxicity to noradrenergic nerves in brain-in large part, from its conversion to 6-OHDA. However, the efficacy of 6-OHDOPA was less than that of 6-OHDA; also, 6-OHDOPA was relatively selective for noradrenergic neurons; near-lethal doses of 6-OHDOPA were required to damage dopaminergic nerves; and ultimately, 6-OHDOPA was found to be an agonist at AMPA receptors, thus accounting for more non-specificity. Nevertheless, 6-OHDOPA was found to be a particularly valuable tool in uncovering processes and mechanisms associated with noradrenergic nerve regeneration and sprouting, particularly when administered to perinatal rodents. Also, 6-OHDOPA was a good tool for selective mapping of noradrenergic nerve tracts in brain, since dopaminergic tracts were unaffected and did not interfere with the histofluorescent methodology used for this purpose in the early 1970s. As an experimental research tool, 6-OHDOPA was valuable in a short time-window, but its utility is largely limited because of newer research technologies that provide better means today for nerve tract mapping, and for experimental approaches engaged toward study of processes and mechanisms attending nerve regeneration. AMPA actions of 6-OHDOPA have not been extensively studied, so this avenue may enliven use of 6-OHDOPA in the future.

Effect and mechanisms underlying scorpion toxin action from Androctonus australis garzonii on atrial natriuretic peptide in rat atria: an in vitro study

Scorpion envenomation is considered public health problem in Northern African countries. The mechanisms of cardiac dysfunction following scorpion envenomation are not fully understood. This study examined the effect and mechanisms underlying scorpion toxin action from Androctonus australis garzonii on atrial natriuretic peptide (ANP) release from rat atrium using in vitro organ perifusion. Male Sprague Dawley rats were used in this study. Three experiments were conducted. In experiment 1, atrial tissues were exposed either to Krebs-bicarbonate buffer medium (control) or to scorpion toxin (10(-8) M to 10(-6) M). In experiment 2, animals were chemically sympathectomized with a single intraperitoneal injection of 6-hydroxydopamine (6-OHDOPA) at a dose of 40 microg/g 24 h before sacrifice. Vehicle-treated rats served as control. Atrial tissues were collected and perifused in the presence of 10(-6) M scorpion toxin. In experiment 3, atrial tissues were exposed to 10(-6) M scorpion toxin either in the absence or presence of 10(-6) M propranolol (a beta-adrenoceptor blocker), or 10(-6) M tetrodotoxin (a sodium channel blocker). ANP levels released in the perifusion medium were determined by radioimmunoassay. The scorpion toxin at 10(-6) M induced a significant (p<0.01) increase (106%) in ANP levels. This effect was decreased (20%) by 6-OHDOPA. Propranolol and tetrodotoxin significantly (p<0.01) inhibited 55% and 60%, respectively, the toxin-induced ANP release. The data show that the North African scorpion toxin from Androctonus australis garzonii increases the ANP release in rat atrium through stimulation of sympathetic cardiac nerves and sodium channels activation.

Selective neurotoxins, chemical tools to probe the mind: the first thirty years and beyond

For centuries, starting with the advent of the microscope, cytotoxins have been known to non-selectively destroy nerves and other tissue cells. However, neurotoxins restricted in effect to one kind of neuron are an invention of the 20th century. One might reasonably trace the origins of this field to 1960 when the Nobel Laureates, R. Levi- Montalcini and S Cohen, showed that an antibody to nerve growth factor effectively prevented development of sympathetic nerves in the absence of overt changes in dorsal root ganglia and other neural and non-neural tissues. The year 1967 marks discovery of 6-hydroxydopamine, the first of dozens of chemically-selective neurotoxins. As stated by the physiologist W.B. Cannon, neural function can be deduced by denoting absence-deficits. A wealth of knowledge in neuroscience has been realized through use of neurotoxins. In the 21st century we foresee neurotoxins for virtually all neurochemically-identifiable or receptor-specific neurons, acting at/via functional proteins or characteristic DNA sites. These tools will provide us with a better means to probe the mind and thereby lead to a fuller understanding of the intricate roles of identifiable neuronal systems in integrative neuroscience.

Destruction of catecholamine-containing neurons by 6-hydroxydopa, an endogenous amine oxidase cofactor

The amino acid, 6-hydroxydopa (6-OHDOPA), found at the active site of amine oxidases, exists as a keto-enol. Exogenously administered 6-OHDOPA is an excitotoxin like beta-N-oxalylamino-L-alanine (BOAA) and beta-N-methylamino-L-alanine (BMAA), acting at the non-N-methyl-D-aspartate (non-NMDA) alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor. BMAA and BOAA are causal factors of neurolathyrism in humans. Much exogenously administered 6-OHDOPA is biotransformed by aminoacid decarboxylase (AADC) to the highly potent and catecholamine-(CA) selective neurotoxin, 6-hydroxydopamine (6-OHDA). 6-OHDOPA destroys locus coeruleus noradrenergic perikarya and produces associated denervation of brain by norepinephrine-(NE) containing fibers. Opiopeptides and opioids enhance neurotoxic effects of 6-OHDOPA on noradrenergic nerves, by a naloxone-reversible process. An understanding of mechanisms underlying neurotoxic effects of 6-OHDOPA can be helpful in defining actions of known and newfound amino acids and for investigating their potential neurotoxic properties.

Effects of systemic administration of 6-hydroxydopamine, 6-hydroxydopa and 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP) on tuberoinfundibular dopaminergic neurons in the rat

Using systemic route of administration, the effects of several neurotoxins on hypothalamic tuberoinfundibular dopaminergic neurons were focused in this study. 6-Hydroxydopamine (6-OHDA, 10 or 100 mg/kg b.wt., i.v. or ip) produced a dose (37 vs. 50%)- and time (41 to 29% from day 4 to day 9)-dependent depletion of hypothalamic median eminence dopamine concentrations, and increases of serum prolactin levels in ovariectomized rats. Other central dopaminergic neurons, however, were not significantly affected. Similar treatments with 6-hydroxydopa (6-OHDOPA) were less effective. On the other hand, treatments of 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP, 10 mg/kg b.wt./day, ip) for 7 or 14 days produced significant decreases of dihydroxyphenylacetic acid (DOPAC) levels in the median eminence and periventricular regions, and increases in serum prolactin levels. Other central dopaminergic neurons were not significantly affected, though. These results suggest that systemic administration of 6-OHDA, 6-OHDOPA, or MPTP, can produce specific destructive effects on the tuberoinfundibular dopaminergic neurons.

Catecholamine uptake inhibitors elevate 6-hydroxydopamine in brain after administration of 6-hydroxydopa

Desipramine and nomifensine, two norepinephrine uptake blockers, elevated by 63-248% the levels of 6-hydroxydopamine found in the frontal cortex at 1-4 h after administration of 6-OH-DOPA. Nomifensine also elevated 6-hydroxydopamine by 83% in the striatum at 4 h. These results nullify prior suggestions in the literature that 6-OH-DOPA is decarboxylated to 6-hydroxydopamine mainly outside of catecholamine nerve terminals and that 6-hydroxydopamine subsequently gains access to nerve terminals via the axonal membrane transport system for catecholamines.

Labelled agents for PET studies of the dopaminergic system--some quality assurance methods, experience and issues

Practical methods are described for the quality assurance of three labelled agents (L-6-[18F]fluoro-DOPA, S-[N-methyl-11C]nomifensine and [O-methyl-11C]raclopride) now produced regularly for PET studies of the dopaminergic system in man. These include indirect methods for the initial determination of label position (e.g. 13C-NMR spectroscopy) and also direct methods for the assessment of chiral purity (TLC and HPLC) and the routine determination of radiochemical purity, chemical purity and specific activity (HPLC). Mass spectrometry has been used to identify some impurities. L-6-hydroxy-DOPA (a precursor in vivo of the neurotoxin, L-6-hydroxydopamine) has been detected by HPLC in some preparations of L-6-[18F]fluoro-DOPA. Formulated S-[N-methyl-11C]nomifensine has been found to decrease in radiochemical purity with storage, whereas formulated [O-methyl-11C]raclopride has been found to be stable. Some quality assurance issues are discussed in relation to experience in the application of the described methods and the obtained results.

Studies on the formation of 6-hydroxydopamine in mouse brain after administration of 2,4,5-trihydroxyphenylalanine (6-hydroxyDOPA)

2,4,5-Trihydroxyphenylalanine (6-OH-DOPA) destroys central and peripheral noradrenergic neurons, while sparing dopaminergic neurons. Previous studies indicate that 6-OH-DOPA toxicity is mediated by the formation of 6-hydroxydopamine. However, levels of 6-hydroxydopamine in brain following peripheral administration of 6-OH-DOPA have not been documented. In the current study, 6-OH-DOPA and 6-hydroxydopamine were measured in brain by HPLC with electrochemical detection after intraperitoneal injection of 6-OH-DOPA. When mice were injected with 100 mg 6-OH-DOPA/kg, 6-hydroxydopamine levels in the striatum were highest (1.9 microgram/g) at 15 min and fell slowly to 24% of the peak value at 4 h. Experiments with reserpine indicated that the relatively stability of 6-hydroxydopamine was largely dependent upon storage in synaptic vesicles. Reserpine (10 mg/kg) lowered striatal 6-hydroxydopamine levels to 21.6% of control (non-reserpine-treated) values at 1 h, and to 8.9% of control values at 4 h. Levels of 6-hydroxydopamine in the striatum at 1 h were increased 113% by pargyline (100 mg/kg), 145% by alpha-methyldopahydrazine (carbidopa; 25 mg/kg), and 261% by pargyline and carbidopa together. Levels of dopamine in the striatum were unchanged at 2.5 h after 200 mg 6-OH-DOPA/kg (with pargyline and 50 mg carbidopa/kg), whereas levels of norepinephrine in the frontal cortex fell by 77%. At the same time, 6-hydroxydopamine levels were 8.8-fold higher in the striatum (5.54 micrograms/g) than in the cortex (0.63 micrograms/g).(ABSTRACT TRUNCATED AT 250 WORDS)

Amygdalar noradrenergic and dopaminergic mechanisms in the regulation of hunger and thirst-motivated behavior

The feeding behavior of rats was studied after neurochemical damage of the amygdalar terminal fields of mesolimbic dopaminergic (DA) and coerular noradrenergic (NA) pathways. 6-Hydroxydopamine (6-OHDA) or 6-hydroxydopa (6-OHDOPA) were injected bilaterally into the central part of amygdala. 6-OHDA was also injected after desmethylimipramine (DMI) pretreatment in order to study the selective destruction of DA terminals. The body weight increased after 6-OHDA injection and a mild hyperphagia and hyperdipsia developed. The 6-OHDA plus DMI treatment resulted in body weight decrease, hypophagia and hypodipsia. These effects were dose-dependent. While a high dose of 6-OHDOPA (15 mug/mul) decreased the body weight, an increase of weight was observed after a low dose (4 mug/0.5 mul). After 6-OHDA, 6-OHDA plus DMI or the high dose of 6-OHDOPA the DA concentration dropped significantly in the amygdala while low-dose 6-OHDOPA resulted in DA increase. In every case there was a parallel change in striatal DA content. The amygdalar NA concentration decreased after both 6-OHDA and the high dose of 6-OHDOPA. There was no change in NA levels after 6-OHDA plus DMI treatment and the NA concentration increased after the injection of a low dose of 6-OHDOPA. When DA/NA ratio was calculated the results showed that body weight increases were accompanied by a relative deficit in NA while a relative deficit of DA was present if body weight decreased. Our results suggest that the amygdalar balance of these transmitters may play an important role in the regulation of body weight and the contradictions of results with electrolytic lesions in the amygdala can be resolved at transmitter level.

Loss of nerve cell bodies in caudal locus coeruleus following treatment of neonates with 6-hydroxydopa

The locus coeruleus is a well defined nucleus in cresylechtviolet preparations and the perikarya are easily distinguished. The coeruleus neurons are thought to be noradrenergic and during development can be selectively affected by the neurotoxin, 6-hydroxydopa (6-OHDOPA). In 6-month-old rats that were treated on day of birth with 6-OHDOPA (60 mg/kg, i.p.) there was a 32% loss of nerve cell bodies in the locus coeruleus. While it was apparent that loss of cell bodies occurred throughout the entire nucleus, the greatest loss of perikarya was from the caudal extent of the nucleus. It is known that sprouting of noradrenergic terminals occurs in the cerebellum of rats following treatment of newborns with 6-OHDOPA. That there are fewer cell bodies to contribute additional terminals further dramatizes this sprouting phenomenon.

6-hydroxydopa during development of central adrenergic neurons produces different long-term changes in rat brain noradrenaline

6-hydroxydopa (6-OH-DOPA) administered to rats during their early development produces long-term modifications in the content of brain noradrenaline (NA) which have regional differences. An increase in brain stem NA is observed when the rats are exposed to the drug between the day 14 of gestation and the 9th postnatal day. When 6-OH-DOPA is injected subcutaneously on the 13th postnatal day or later, there is a decrease in brain stem NA. On the other hand, the content of NA in the telediencephalon is depleted for the first time in rats exposed to the drug during the day 16 of gestation, the decrease is more evident when the injection is done on days 17 or 18 and the effect is also marked when the drug is administered in the period between the day of birth and the 20th day of age. These results indicate that 6-OH-DOPA exerts different effects during the process of development and that the increase in brain stem NA is not solely dependent on the depletion produced in the forebrain because both phenomena are temporally dissociated. The adrenergic neurons injured by the drug, most probably respond in such a way that leads to an increase in brain stem NA only during the period in which they are under the influence of the factors controlling their physiologic development.

Resistance of noradrenaline in blood vessels to depletion by 6-hydroxydopamine or immunosympathectomy

1. The degree of the decrease in the noradrenaline concentrations caused by 6-hydroxydopamine or immunosympathectomy was different in different areas of the cardiovascular system.2. In rats or guinea-pigs 6-hydroxydopamine depleted the noradrenaline content of the heart by 90%, of the mesenteric vein by 80% and of the mesenteric artery and aorta by 30-60%. Immunosympathectomy elicited a 70% reduction in the cardiac noradrenaline but only a 50% reduction in the noradrenaline of the blood vessels of the rat.3. The tyrosine hydroxylase activity of the heart, blood vessels, or adrenal glands was not significantly altered 2 weeks after 6-hydroxydopamine. Nor was the monoamine oxidase activity in heart or blood vessels changed.4. The inconsistent ability of both 6-hydroxydopamine and immunosympathectomy to abolish experimental hypertension may be due to the partial persistence of noradrenaline and functional sympathetic nervous system activity in the blood vessels.