Subtypes Y1 and Y2 of the Neuropeptide Y Receptor Are Respectively Expressed in Pro-Opiomelanocortin- and Neuropeptide-Y-Containing Neurons of the Rat Hypothalamic Arcuate Nucleus

The arcuate nucleus of the hypothalamus houses a number of neurochemically different cell populations. Among these, a dense cluster of small neuropeptide-Y (NPY)-expressing neurons is located in its ventromedial subdivision and a pro-opiomelanocortin (POMC)-expressing neuron population in its ventrolateral part. Furthermore, both neuropeptide Y Y1 and Y2 receptors (Y1-Rs and Y2-Rs) are expressed in the arcuate nucleus. Here we analyse the co-expression of NPY and POMC/adrenocorticotropic hormone with the Y1-R and Y2-R in arcuate neurons using immunohistochemistry and in situ hybridization. Many, but not all, POMC neurons expressed Y1-R mRNA and protein. Conversely, several Y1-R-positive, POMC-negative neurons were found. NPY-positive nerve terminals were found in close apposition to Y1-R-like immunoreactivity localized close to the dendritic and somatic cell membranes. Y2-R mRNA was found in almost all NPY mRNA-expressing neurons, but also in a group of NPY mRNA-negative cells. These results show that the POMC neurons are targets for NPY, which is presumably present in, and released from, fibres originating in the ventromedial arcuate nucleus and which may play a role in NPY-induced feeding. Release of NPY, and possible coexisting messengers, may be controlled by presynaptic Y2-R expressed in NPY neurons. Taken together, the findings support the division of Y1-Rs and Y2-Rs into post- and presynaptic receptors, respectively.