Cell type-dependent trafficking of neuropeptide Y-containing dense core granules in CNS neurons.

Neuropeptide transmitters are synthesized throughout the CNS and play important modulatory roles. After synthesis in the neuronal cell body, it is generally assumed that peptides are transported to nonspecialized sites of release. However, apart from a few cases, this scenario has not been thoroughly examined. Using wild-type and NPY(GFP) transgenic mice, we have studied the subcellular distribution of neuropeptide Y (NPY), a prototypical and broadly expressed neuropeptide. NPY puncta were found in the dendrites and axons of hippocampal GABAergic interneurons in situ. In contrast in hypothalamic GABAergic interneurons, NPY was restricted to the axon. Surprisingly this differential trafficking was preserved when the neurons were maintained in vitro. When hippocampal and hypothalamic neurons were transfected with NPY-Venus, the distribution of the fluorescent puncta replicated the cell type-specific distribution of endogenous neuropeptide Y. The NPY puncta in the axons of hippocampal and hypothalamic neurons colocalized with the sites of classical transmitter release (identified by staining for synapsin and the vesicular GABAergic transporter, VGAT). In hippocampal neurons, most of the postsynaptic NPY puncta were clustered opposite synapsin-containing varicosities. When neurons were stained for a second neuropeptide, agouti-related protein, immunoreactivity was found in the axon and dendrites of hippocampal neurons but only in the axons of hypothalamic neurons, thus mimicking the polarized distribution of NPY. These results indicate that the trafficking of neuropeptide-containing dense core granules is markedly cell type specific and is not determined entirely by the characteristics of the particular peptide per se.