Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated channels that contain combinations of α and β subunits (heteromeric) or only five α subunits (homomeric). To date, 12 subunits have been cloned. Although different types of neuronal nAChRs are expected to occur in vivo, most structure-activity relationship studies have been carried out for just a few neuronal subtypes. Clinical and basic research has shown that cholinergic receptors play a role in several disorders of the nervous system such as chronic pain, Alzheimer's disease and addiction to nicotine, alcohol and drugs. Unfortunately, the lack of selective modulators for each subtype of nAChR makes their pharmacological characterization difficult, which has slowed the development of therapeutic nAChR modulators with high selectivity and absence of off-target side effects.Animal venoms have proven to be an excellent natural source of bioactive molecules with activity against ion channels. Venoms from different snakes and in particular molluscs of the Conus genus have been an important source of modulators of cholinergic receptors. However, spider and scorpion venoms have been studied less extensively and there are only two examples of cholinergic receptor modulators reported from arachnid venom. Thus, the primary goal of this thesis was to identify novel modulators of the human ganglionic α3* nAChR (where the asterisk indicates that β subunits form part of the heteropentameric receptor) and the homomeric neuronal α7 nAChR using spider and scorpion venoms."Hit" venoms from a selection of "modern" and "primitive" spiders were identified via high-throughput screens against the α3* and α7 nAChRs endogenously expressed in SH-SY5Y cells. A total of 71 spider venoms were screened, with representatives from nine different taxonomic families. The resulting data showed that 91% of the venoms presented some activity against the nAChR subtypes tested and most of these were non-selective blockers of the α3* and α7 subtypes.Toxins directed against vertebrate nAChRs likely evolved through frequent interactions between spiders and small vertebrate predators. Our data revealed that nAChR agonists are likely to be present in venom of the "primitive" spiders 3 and therefore were probably basally recruited in spider venom. In summary, the screening of spider venoms against vertebrate cholinergic receptors provided information about the origin and evolution of toxins against vertebrate receptors.The search for nAChR modulators was extended to scorpion venoms that are recognized as sources of toxins that affect primarily the properties of voltage-gated sodium and potassium channels. In this work, venoms from Androctonus australis, Androctonus crassicauda, Leirurus quinquestriatus and Grosphus grandidieri (Buthidae) and Heterometus spinnifer (Scorpionidae) were screened against the α3* and α7 nAChRs using SH-SY5Y cells. The venom of Heterometrus spinnifer was found to contain an agonistic compound. Activity guided-fractionation allowed isolation of the "h...