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

Evolution of neurotoxins: from research modalities to clinical realities

In the 1950s, the discovery of anti-nerve growth factor, an immunotoxin stunting sympathetic neural development, signaled the advent of neurotoxins as research modalities. Other selective neurotoxins were discovered in rapid succession. In the 1960s, 6-hydroxydopamine and 6-hydroxydopa were shown to destroy noradrenergic and dopaminergic nerves. Excitotoxins (glutamate, aspartate, and analogs) were discovered in the 1970s. DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] proved to be selective for noradrenergic destruction, while 5,6- and 5,7-dihydroxytryptamines were relatively selective for serotonin neurons. Additional neurotoxins were discovered, but it was MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) that predominated neurotoxicity research in the 1980s. Eventually, Clostridium botulinum neurotoxin (BoNT), discovered as a "poisonous" principle in the late 1800s, resurfaced in purified and standardized forms as a clinically useful drug. Neurotoxins represent chemical tools, useful not only for discerning neuronal mechanisms and animal modeling of neurological disorders, but also for their use in medicine and potential as treatments for medical disorders. This unit reviews the early discovery of neurotoxins, describes categories of neurotoxins, and finally characterizes their usefulness--first as research tools, and eventually as clinical therapeutic agents.

Changes in open-field activity and novelty-seeking behavior in periadolescent rats neonatally treated with monosodium glutamate

Monosodium glutamate (MSG) treatment of neonatal rodents leads to degeneration of the neurons in the arcuate nucleus, inner retinal layers and various other brain areas. It also causes various changes in the motor activity, sensory performance and learning abilities. We have previously shown that MSG treatment delays the appearance of some reflexes during neurobehavioral development and leads to temporary changes in reflex performance and motor coordination. Investigation of novelty-seeking behavior is of growing importance for its relationship with sensitivity to psychomotor stimulants. Perinatal administration of numerous toxic agents has been shown to influence novelty-seeking behavior in rats, but little is known about the influence of neonatal MSG treatment on the novelty-seeking behavior. The aim of the present study was to compare changes in locomotor, spontaneous exploratory and novelty-seeking behavior in periadolescent rats neonatally treated with MSG. Newborn rats were treated with 4 mg/g MSG subcutaneously on postnatal days 1, 3, 5, 7 and 9. Open-field behavior was tested at 2, 3, 4, 6 and 8 weeks of age. We found that MSG administration led to only temporary increases in locomotor behavior, which was more pronounced during the first few postnatal weeks, followed by a subtle hypoactivity at 2 months of age. Novelty-seeking was tested in four 5-min trials at 3 weeks of age. Trial 1 was in an empty open-field, two identical objects were placed in the arena during trial 2 and 3, and one of them was replaced to a novel object during trial 4. We found that the behavioral pattern of MSG-treated rats was the opposite in all tested signs in the novelty exploration test compared to control pups. In summary, our present study shows that neonatal MSG treatment leads to early temporary changes in the locomotor activity followed by hypoactivity at 2 months of age. Furthermore, MSG-treated rats show a markedly disturbed novelty-seeking behavior represented by altered activity when subjected to a novel object.

Effect of PACAP in 6-OHDA-induced injury of the substantia nigra in intact young and ovariectomized female rats

Pituitary adenylate cyclase activating polypeptide (PACAP) has neuroprotective effects in various neuronal cultures and in models of brain pathologies in vivo. Among others, it protects dopaminergic neurons in vitro, against 6-OHDA- and rotenone-induced injury. Recently, we have shown that PACAP reduces dopaminergic cell loss and ameliorates behavioral outcome following unilateral 6-OHDA-induced injury of the substantia nigra in male rats. However, after castration, PACAP led only to a slight amelioration of the behavioral symptoms. The aim of the present study was to investigate the degree of neuroprotection exerted by PACAP in female rats, using the same model. It was found that PACAP had no effect on the dopaminergic cell loss in intact female rats, only caused amelioration of certain acute behavioral signs. In contrast, PACAP effectively increased dopaminergic cell survival and decreased behavioral deficits in ovariectomized females. These results indicate that the neuroprotective effect of PACAP in a rat model of Parkinson's disease is gender-specific.

Neurotoxins and neurotoxic species implicated in neurodegeneration

Neurotoxins, in the general sense, represent novel chemical structures which when administered in vivo or in vitro, are capable of producing neuronal damage or neurodegeneration--with some degree of specificity relating to neuronal phenotype or populations of neurons with specific characteristics (i.e., receptor type, ion channel type, astrocyte-dependence, etc.). The broader term 'neurotoxin' includes this categorization but extends the term to include intra- or extracellular mediators involved in the neurodegenerative event, including necrotic and apoptotic factors. Moreover, as it is recognized that astrocytes are essential supportive satellite cells for neurons, and because damage to these cells ultimately affects neuronal function, the term 'neurotoxin' might reasonably be extended to include those chemical species which also adversely affect astrocytes. This review is intended to highlight developments that have occurred in the field of 'neurotoxins' during the past 5 years, including MPTP/MPP+, 6-hydroxydopamine (6-OHDA), methamphetamine; salsolinol; leukoaminochrome-o-semiquinone; rotenone; iron; paraquat; HPP+; veratridine; soman; glutamate; kainate; 3-nitropropionic acid; peroxynitrite anion; and metals (copper, manganese, lead, mercury). Neurotoxins represent tools to help elucidate intra- and extra-cellular processes involved in neuronal necrosis and apoptosis, so that drugs can be developed towards targets that interrupt the processes leading towards neuronal death.

Dopamine- or L-DOPA-induced neurotoxicity: the role of dopamine quinone formation and tyrosinase in a model of Parkinson's disease

Dopamine (DA)- or L-dihydroxyphenylalanine-(L-DOPA-) induced neurotoxicity is thought to be involved not only in adverse reactions induced by long-term L-DOPA therapy but also in the pathogenesis of Parkinson's disease. Numerous in vitro and in vivo studies concerning DA- or L-DOPA-induced neurotoxicity have been reported in recent decades. The reactive oxygen or nitrogen species generated in the enzymatical oxidation or auto-oxidation of an excess amount of DA induce neuronal damage and/or apoptotic or non-apoptotic cell death; the DA-induced damage is prevented by various intrinsic and extrinsic antioxidants. DA and its metabolites containing two hydroxyl residues exert cytotoxicity in dopaminergic neuronal cells mainly due to the generation of highly reactive DA and DOPA quinones which are dopaminergic neuron-specific cytotoxic molecules. DA and DOPA quinones may irreversibly alter protein function through the formation of 5-cysteinyl-catechols on the proteins. For example, the formation of DA quinone-alpha-synuclein consequently increases cytotoxic protofibrils and the covalent modification of tyrosine hydroxylase by DA quinones. The melanin-synthetic enzyme tyrosinase in the brain may rapidly oxidize excess amounts of cytosolic DA and L-DOPA, thereby preventing slowly progressive cell damage by auto-oxidation of DA, thus maintainng DA levels. Since tyrosinase also possesses catecholamine-synthesizing activity in the absence of tyrosine hydroxylase (TH), the double-edged synthesizing and oxidizing functions of tyrosinase in the dopaminergic system suggest its potential for application in the synthesis of DA, instead of TH in the degeneration of dopaminergic neurons, and in the normalization of abnormal DA turnover in the long-term L-DOPA-treated Parkinson's disease patients.