Fig. 1.

Scheme illustrating three issues addressed in this review: (1) axonal connectivity between brain nuclei, (2) neurochemical phenotyping; 3) microcircuits inside nuclei. This scheme is repeated as ‘template’ in Figs. 2, 5, 10, and 11 to illustrate which neurons can be expected to become labeled with various classical neuroanatomical tracers, viruses, toxins, and viral vector expressions. Nucleus ‘A’ has glutamatergic (Glu) and cholinergic (ACh) neurons (first-order) projecting to nucleus ‘B’, while the projection from ‘B’ to ‘A’ comes exclusively from glutamatergic (first-order) neurons. Nucleus ‘A’ contains (second-order) GABAergic and cholinergic interneurons; nucleus ‘B’ contains (second-order) GABAergic interneurons and third-order cholinergic neurons