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VOL. 66, (2) 2000-05-17 RECEPTORES NICOTÍNICOS NEURONALES

epibatidine. They contain a larger variety of a and ß subunits and the combination a4/ß2
is very frequent.

Both neural nicotinic receptors when stimulated allow the Ca2+ and Na+
entrance, originating a membrane depolarisation and subsequently the exocytotic release
of neurotransmitters. Nicotinic receptors are widely distributed in mammalian brain, and
their localisation in nerve terminals is mainly presynaptic. There, they facilitate,
potentiate or induce the neurotransmitter release of the acetylcholine itself, or other
neurotransmitters, such as glutamate, noradrenaline or GABA. Special mention deserves
the facilitation of dopamine release from striatum/nucleus accumbens that provide a
plausible explanation on tobacco smoking addiction. This wide effect on secretion
potentiation carried out by acetylcholine via nicotinic receptors could explain the fatal
consequences derived from the cholinergic neurones lost, as it is the case in Alzheimer
disease.

The presence of nicotinic receptors in isolated nerve terminals was studied by
microfluorescence coupled to video imaging, measuring the Ca2+ entrance with a
fluorescent die, as it was also done for the ATP and ApnA ionotropic receptors. At the
same terminals by immunohistochemical studies, the presence of the P2X3, subtype of
ATP receptors was shown. ATP and Ap5A were able to induce the release of
acetilcholine from rat midbrain synaptic terminals. These results corroborate the idea of
a certainly complex cross-talk between nucleotide and nicotinic receptors at the same
presynaptic terminals, with relevant consequences for neural functioning and future
pharmacology.

INTRODUCCIÓN

Era difícil de imaginar que las hojas traídas por Colón del Nuevo
Mundo, conocidas como “tabaco”, y con los mismos usos que en la
actualidad, alcanzarían tan amplia difusión y consumo. En la primera
mitad del siglo XVI su cultivo se extiende por los países del Sur de
Europa, llegando las semillas de tabaco a Francia, a la corte de la reina
regente Catalina de Médicis a través de Jean Nicot, a la sazón embajador
en Lisboa, y origen del nombre de Nicotinana tabacum dado a la planta.

Los efectos del tabaco despiertan el interés por conocer sus
principios activos, y es a finales de este mismo siglo XVI, cuando se
obtienen y emplean, extractos crudos de la planta. No es de extrañar que
sea uno de sus principales componentes, la nicotina, purificado en fecha
tan temprana como 1828. Este alcaloide, uno de los pocos que es líquido
y fácilmente volátil demuestra ser un poderoso neurotóxico. La fórmula
molecular se establece en 1843, y la síntesis química en 1904, pero serían

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