**3.5 Brain distribution of nAChRs**

The nAChRs are distributed in various brain regions including the ventral tegmental area, hippocampus, prefrontal cortex, amygdala, and nucleus accumbens. The two most abundant nAChRs, α7 nAChRs (α-bungarotoxin sensitive) and α4β2 (α-bungarotoxin insensitive), are localized in the brainstem, cerebellum, mesencephalic structures, limbic system and cortex. The α4β2, the first to be pharmacologically characterized, constitutes the principal nAChR subtype in the cortex, striatum, superior colliculus, lateral geniculate nucleus and cerebellum. The second abundant nAChR subtype, α7 subunit-containing receptors, have high expression in the cortex, hippocampus and subcortical limbic regions, and are expressed at low levels in the thalamic regions and basal ganglia. Although α7 nAChRs anatomical distribution differ markedly from α4β2, the two co-localize in some areas such as in the superficial layer of the superior colliculus. nAChRs are located in pre- and postsynaptic regions well as in extra-synaptic locations. Specially, receptors containing the β2 subunit are located diffusely throughout the membrane of the neuron.
