To examine the effect of a central noradrenergic lesion on the reactivity of the 5-HT1B receptor we compared intact male rats with rats in which noradrenergic nerve terminals were largely destroyed with the neurotoxin DSP-4 (50 mg/kg × 2, on the 1st and 3rd days of postnatal life). When rats attained 10 weeks of age, control and DSP-4 rats were divided into two subgroups receiving either saline or the serotonin (5-HT) synthesis inhibitor (p-chlorophenylalanine; p-CPA; 100 mg/kg). Employing an elevated plus maze test, we demonstrated that CP 94,253 (5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine hydrochloride)(4.0 mg/kg; 5-HT1B agonist) induced an anxiogenic-like action in control rats; however, it failed to elicit this effect in the DSP-4 group. Surprisingly, in p-CPApretreated rats anxiogenic-like activity was observed both in control and DSP-4 treated rats. CP 94,253 significantly attenuated 5-HT synthesis in the medial prefrontal cortex (mPFC) of control rats, and SB 216641 (N-{3-[3-(dimethylamino) ethoxy]-4-methoxyphenyl}-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1'-biphenyl]-4-carboxamide hydrochloride) (4.0 mg/kg; 5-HT1B antagonist) was able to antagonize this effect. Conversely, CP 94,253 failed to significantly inhibit the 5-HT synthesis rate in DSP-4-treated rats. In the microdialysis study CP 94,253 induced long-lasting attenuation of 5-HT release in the mPFC of control rats but had no effect in DSP-4 rats. These data lead to the proposal that presynaptic 5-HT1B autoreceptors underwent desensitization in DSP-4 treated rats.

To assess the possible modulatory effects of noradrenergic and serotoninergic neurons on dopaminergic neuronal activity, the noradrenergic and serotoninergic neurotoxins DSP-4 N-(2-chlorethyl)-N-ethyl-2-bromobenzylamine (50.0 mg/kg, sc) and 5,7- dihydroxytryptamine (5,7-DHT) (37.5 μg icv, half in each lateral ventricle), respectively, were administered to Wistar rats on the first and third days of postnatal ontogeny, and dopamine (DA) agonist-induced behaviors were assessed in adulthood. At eight weeks, using an HPLC/ED technique, DSP-4 treatment was associated with a reduction in NE content of the corpus striatum (> 60%), hippocampus (95%), and frontal cortex (> 85%), while 5,7-DHT was associated with an 80-90% serotonin reduction in the same brain regions. DA content was unaltered in the striatum and the cortex. In the group lesioned with both DSP-4 and 5,7-DHT, quinpirole-induced (DA D2-agonist-agonist) yawning, 7-hydroxy-DPAT-induced (DA D3 agonist) yawning, and apomorphine-induced (non-selective DA agonist) stereotypies were enhanced. However, SKF 38393-induced (DA D1 agonist) oral activity was reduced in the DSP-4 + 5,7-DHT group. These findings demonstrate that DA D2- and D3-agonist-induced behaviors are enhanced while DA D1-agonist-induced behaviors are suppressed in adult rats in which brain noradrenergic and serotoninergic innervation of the brain has largely been destroyed. This study indicates that noradrenergic and serotoninergic neurons have a great impact on the development of DA receptor reactivity (sensitivity).

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.

To determine whether noradrenergic nerves might have a modulatory role on the sensitivity or reactivity of histaminergic receptor systems in brain, behavioral effects of the respective histamine H1, H2 and H3 antagonists S(+)chlorpheniramine, cimetidine and thioperimide in control adult rats were compared to the effects in adult rats that had been lesioned as neonates with the noradrenergic neurotoxin DSP-4. On the 1st and 3rd days after birth rat pups were treated with either saline or DSP-4 (50 mg/kg sc), then returned to their home cages with the dam. At 8 weeks when rats were tested, S(+)chlorpheniramine (10 mg/kg ip) was found to increase locomotor activity in intact and DSP-4 lesioned rats, while cimetidine (5 mg/kg, ip) and thioperimide (5 mg/kg, ip) increased activity severalfold solely in the DSP-4 group. Exploratory activity, nociceptive activity, and irritability were little altered by the histamine antagonists, although oral activity was increased by thioperimide in intact and lesioned rats, and by cimetidine or S(+)chlorpheniramine in DSP-4 rats. High performance liquid chromatography with electrochemical detection was used to determine that DSP-4 produced a 90% reduction in frontal cortex, hippocampus and hypothalamus, with a 90% elevation of NE in cerebellum - reflecting reactive sprouting of noradrenergic fibers consequent to lesion of noradrenergic tracts projecting to proximal brain regions. These findings indicate that perinatal noradrenergic fiber lesioning in rat brain is associated with an altered behavioral spectrum by histamine H1, H2 and H3 receptor antagonists, thereby implicating histaminergic systems as modulators of noradrenergic systems in brain.

Repeated treatments of rats with the dopamine (DA) D2 receptor agonist quinpirole, consistently produce long-lived DA D2 receptor supersensitization, by the process that has been termed priming. Rats so-primed in ontogeny behaviorally demonstrate adulthood enhancement of low-dose quinpirole-induced yawning. Because 1) dopaminergic neurons originate in midbrain nuclei (substantia nigra and ventral tegmental area), and 2) noradrenergic neurons originate in pontine (locus coeruleus) and medullary areas, it might be presumed that these two monoaminergic systems are independent, not interdependent. However, in the present study we demonstrate that there was an attenuation of quinpirole-enhanced yawning at 8 weeks in rats that were 1) primed by repeated neonatal quinpirole HCl treatments (50 μg/kg per day SC) during the first ten days of postnatal ontogeny, and 2) lesioned at 3 days after birth with DSP-4 (N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride, 50 mg/kg SC). Dose-effect curves indicated a 23-45% reduction in yawning by DSP-4 treatment of quinpirole-primed rats, acutely treated as adults with quinpirole (25, 50, or 100 μg/kg). Effectiveness of DSP-4 is reflected by the 95% and 99% reductions in norepinephrine contents of frontal cortex and hippocampus, respectively (HPLC/ED method). The findings are supportive of a modulatory role of noradrenergic fibers on dopamine receptor priming (supersensitization) in rat brain.

A selective neurotoxin takes many forms: as an antibody to a neurotrophin, as an alkylator, as an excitotoxin, as a blocker of requisite neuronal excitation during ontogenetic development, as a generator of oxidative stress, as an inhibitor of vital intraneuronal processes, and as an agent adversely affecting a host of multiple sites in neurons. Neurotoxins have been invaluable for elucidating cellular mechanisms attending or preventing neuronal necrosis and apoptosis, and for modeling and thereby discerning mechanisms invoked in neurological and psychiatric disorders. Neuroprotectants, endogenous and exogenous, are being explored as potentially useful agents to ward off diseases. Finally, hypothesized as posing a risk to humans as environmental constituents, neurotoxins are now being remodeled as adjuncts for therapeutic intervention in a variety of human medical 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.

The era of selective neurotoxins arose predominately in the 1960s with the discovery of the norepinephrine (NE) isomer 6-hydroxydopamine (6-OHDA), which selectively destroyed noradrenergic sympathetic nerves in rats. A series of similarly selective neurotoxins were later discovered, having high affinity for the transporter site on nerves and thus being accumulated and able to disrupt vital intraneuronal processes, to lead to cell death. The Trojan Horse botulinum neurotoxins (BoNT) and tetanus toxin bind to glycoproteins on the neuronal plasma membrane, then these stealth neurotoxins are taken inside respective cholinergic or glycinergic nerves, producing months-long functional inactivation but without overtly destroying those nerves. The mitochondrial complex I inhibitor rotenone, while lacking total specificity, still destroys dopaminergic nerves with some selectivity; and importantly, results in the neural accumulation of synuclein-to model Parkinson’s disease (PD) in animals. Other neurotoxins target specific subtypes of glutamate receptors and produce excitotoxicity in nerves with that receptor population. The dopamine D2 receptor agonist quinpirole, termed a selective neurotoxin, produces a behavioral state replicating some of the notable features of schizophrenia, but without overtly destroying nerves. These processes, mechanisms or treatment-outcomes account for the means by which neurotoxins are classified as such, and represent some of the means by which neurotoxins as a group are able to destroy or functionally inactivate nerves; or replicate an altered neurological state. Selective neurotoxins have proven to be important in gaining insight into biochemical processes and mechanisms responsible for survival or demise of a nerve. Selective neurotoxins are useful also for animal modeling of human neural disorders such as PD, Alzheimer disease, attention-deficit hyperactivity disorder (ADHD), Lesch-Nyhan disease, tardive dyskinesia, schizophrenia and others. The importance of neurotoxins in neuroscience will continue to be ever more important as even newer neurotoxins are discovered.

There has been an awareness of nerve poisons from ancient times. At the dawn of the twentieth century, the actions and mechanisms of these poisons were uncovered by modern physiological and biochemical experimentation. However, the era of selective neurotoxins began with the pioneering studies of R. Levi-Montalcini through her studies of the neurotrophin "nerve growth factor" (NGF), a protein promoting growth and development of sensory and sympathetic noradrenergic nerves. An antibody to NGF, namely, anti-NGF - developed in the 1950s in a collaboration with S. Cohen - was shown to produce an "immunosympathectomy" and virtual lifelong sympathetic denervation. These Nobel Laureates thus developed and characterized the first identifiable selective neurotoxin. Other selective neurotoxins were soon discovered, and the compendium of selective neurotoxins continues to grow, so that today there are numerous selective neurotoxins, with the potential to destroy or produce dysfunction of a variety of phenotypic nerves. Selective neurotoxins are of value because of their ability to selectively destroy or disable a common group of nerves possessing (1) a particular neural transporter, (2) a unique set of enzymes or vesicular transporter, (3) a specific type of receptor or (4) membranous protein, or (5) other uniqueness. The era of selective neurotoxins has developed to such an extent that the very definition of a "selective" neurotoxin has warped. For example, (1) N-methyl-D- aspartate receptor (NMDA-R) antagonists, considered to be neuroprotectants by virtue of their prevention of excitotoxicity from glutamate receptor agonists, actually lead to the demise of populations of neurons with NMDA receptors, when administered during ontogenetic development. The mere lack of natural excitation of this nerve population, consequent to NMDA-R block, sends a message that these nerves are redundant - and an apoptotic cascade is set in motion to eliminate these nerves. (2) The rodenticide rotenone, a global cytotoxin that acts mainly to inhibit complex I in the respiratory transport chain, is now used in low dose over a period of weeks to months to produce relatively selective destruction of substantia nigra dopaminergic nerves and promote alpha-synuclein deposition in brain to thus model Parkinson's disease. Similarly, (3) glial toxins, affecting oligodendrocytes or other satellite cells, can lead to the damage or dysfunction of identifiable groups of neurons. Consequently, these toxins might also be considered as "selective neurotoxins," despite the fact that the targeted cell is nonneuronal. Likewise, (4) the dopamine D2-receptor agonist quinpirole, administered daily for a week or more, leads to development of D2-receptor supersensitivity - exaggerated responses to the D2-receptor agonist, an effect persisting lifelong. Thus, neuroprotectants can become "selective" neurotoxins; nonspecific cytotoxins can become classified as "selective" neurotoxins; and receptor agonists, under defined dosing conditions, can supersensitize and thus be classified as "selective" neurotoxins. More examples will be uncovered as the area of selective neurotoxins expands. The description and characterization of selective neurotoxins, with unmasking of their mechanisms of action, have led to a level of understanding of neuronal activity and reactivity that could not be understood by conventional physiological observations. This chapter will be useful as an introduction to the scope of the field of selective neurotoxins and provide insight for in-depth analysis in later chapters with full descriptions of selective neurotoxins.