Die Wirkung von zwei endogen neuroprotektiven Substanzen, Melatonin und 17 beta-Estradiol wurde an drei Caspase-abhängigen, apoptotischen, aber Exzitotoxin-unabhängigen Schadensmodellen an neuronalen Primärkulturen untersucht und mit der bei vorwiegend nekrotischen Schadensmodellen verglichen. Es zeigten sich eine Abhängigkeit des neuroprotektiven Potentials von der Art des Zelluntergangs sowie unterschiedliche Mechanismen der Neuroprotektion. Melatonin wirkte in allen drei apoptischen Modellen nicht neuroprotektiv, sondern verstärkte die Schädigung der Neurone noch, während partiell gegen die OGD-induzierte Nekrose (OGD, engl. Oxygen glucose deprivation, kombinierter Sauerstoff- und Glukoseentzug) kortikaler Neurone Schutz erzielt wurde. Der Einsatz des endogenen neuroprotektiven Faktors Melatonin als Therapeutikum ist möglicherweise nur bei neurodegenerativen Erkrankungen mit exzitotoxischer Schädigung durch Glutamat oder oxidativem Stress wie bei Epilepsie oder dem Schlaganfall durch Ischämie sinnvoll. Die fehlende bzw. potenzierenden Wirkung von Melatonin bei neuronaler Apoptose in vitro, stellt jedoch einen therapeutischen Erfolg bei der Behandlung der mit apoptotischer Schädigung einhergehenden Alzheimer'schen Erkrankung in Frage. Bei klinischer Anwendung ist auch der von uns erhobene Befund zu beachten, dass in vitro native neuronale Zellen durch Melatonin geschädigt werden. 17 beta-Estradiol wirkte sowohl bei nekrotischer als auch bei apoptotischer Zellschädigung. Dabei zeigten sich wesentliche Unterschiede in den Mechanismen der Neuroprotektion und in der Ansprechbarkeit verschiedener Regionen des Gehirns. Schutz vor Apoptose konnte nur durch eine Langzeitvorbehandlung (20 h) in septalen und hippokampalen Kulturen, nicht jedoch in kortikalen Kulturen beobachtet werden. Dieser Effekt liess sich durch Rezeptorantagonisten, Proteinsynthesehemmung sowie durch Hemmung der Phosphoinositol-3-Kinase blockieren. Eine Kurzzeitbehandlung war gegen Apoptose nicht wirksam, zeigte gegen OGD und Glutamattoxizität jedoch neuroprotektives Potential. Dieser Effekt liess sich nicht antagonisieren, so dass hier ein direkter antioxidativer Mechanismus wahrscheinlich erscheint. Die antiapoptotische Wirkung in septalen und hippokampalen Kulturen korrelierte mit einer höheren Dichte des Estrogenrezeptors-alpha und einer erhöhten Expression antiapoptotischer Proteine in diesen Regionen. Da bei der Alzheimer'schen Erkrankung der Kortex betroffen ist, könnte der fehlende Effekt von 17 beta-Estradiol in kortikalen Neuronen sowohl auf die neuronale Apoptose als auch auf die Proteinexpression von Bcl-2 und Bcl-xL möglicherweise auf experimenteller Basis erklären, warum eine langfristige Estrogentherapie bei Frauen mit milder bis moderater Alzeimer'scher Erkrankung den Progress der Erkrankung nicht aufhalten konnte (Mulnard et al. 2000). / The neuroprotective effect of melatonin and 17 beta-estradiol has been evaluated in several in vitro models of neuronal apoptosis and necrosis. Melatonin was not neuroprotective in three models of apoptosis but showed a pro-apoptotic effect in primary cortical neurons. Melatonin revealed to damage naïve neurons, too. Partial protection was observed against necrotic neurodegeneration after oxygen-glucose deprivation (OGD). The use of melatonin as a therapeutic agent might be of interest in neurodegenerative diseases with excitotoxic damage like epilepsia or ischemia, but is questioned in case of apoptotic neurodegeneration. 17 beta-estradiol was neuroprotectiv in both necrotic and apoptotic neurodegeneration. Differences in the mechanism of neuroprotetion and in the efficacy in different regions of the brain were observed. A neuroprotective effect was visible only in hippocampal and septal cultures if 17 beta-estradiol was applied 20 h prior (long term pre-treatment) but not in cortical neurons. This effect correlates with an increased density of estrogen receptor-alpha and an increased expression of anti-apoptotic proteins like Bcl-2 and Bcl-xL in these regions. These effect could be blocked with receptor antagonists, protein synthesis inhibitors and an inhibitor of the phosphatidylinositol 3-kinase. A short term pre-treatment revealed a receptor independent neuroprotective potential against OGD and glutamate toxicity. The failure of 17 beta-estradiol to protect cortical neurons against apoptosis could be an experimental basis to understand, why a long lasting treatment with estrogens of women with mild to moderate Alzheimer´s disease failed to inhibit the progress of the illness (Mulnard et al., 2000)

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.