Neuropeptide Y (NPY) is widely expressed in the brain and its biological effects are mediated through a variety of receptors. We examined, using immunohistochemistry, expression of the Y2 receptor (R) protein in the adult mouse brain and its association with NPY and the Y1R, as well as a range of additional neurotransmitters and signaling-related molecules, which previously have not been defined. Our main focus was on the hippocampal formation (HiFo), amygdaloid complex, and hypothalamus, considering the known functions of NPY and the wide expression of NPY, Y1R, and Y2R in these regions. Y2R-like immunoreactivity (-LI) was distributed in nerve fibers/terminal endings throughout the brain axis, without apparent colocalization with NPY or the Y1R. Occasional coexistence between NPY- and Y1R-LI was found in the HiFo. Following colchicine treatment, Y2R-LI accumulated in cell bodies that coexpressed γ-aminobutyric acid (GABA) in a population of cells in the amygdaloid complex and lateral septal nucleus, but not in the HiFo. Instead, Y2R-positive nerve terminals appeared to surround GABA-immunoreactive (ir) cells in the HiFo and other neuronal populations, e.g., NPY-ir cells in HiFo and tyrosine hydroxylase-ir cells in the hypothalamus. In the HiFo, Y2R-ir mossy fibers coexpressed GABA, glutamic acid decarboxylase 67 and calbindin, and Y2R-LI was found in the same fibers that contained the presynaptic metabotropic glutamate receptor 2, but not together with any of the three vesicular glutamate transporters. Our findings provide further support that Y2R is mostly presynaptic, and that Y2Rs thus have a modulatory role in mediating presynaptic neurotransmitter release.
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