Neuropeptide Y (NPY) is a potent bioactive peptide that is widely expressed in the nervous system, including the retina. Here we show that specific NPY immunoreactivity was localized to amacrine and displaced amacrine cells in the rat retina. Immunoreactive cells had a regular distribution across the retina and an overall cell density of 280 cells/mm(2) in the inner nuclear layer (INL) and 90 cells/mm(2) in the ganglion cell layer (GCL). In the INL, most immunoreactive cells were characterized by small cell bodies and fine processes that appeared to ramify primarily in stratum 1 of the inner plexiform layer (IPL). A few cells in the INL also ramified in stratum 3 of the IPL. In the GCL, small to medium immunoreactive cells appeared to ramify primarily in stratum 5 of the IPL. A few immunoreactive processes, originating from somata in the INL and processes in the IPL, ramified in the OPL. NPY-immunoreactive cells contained GABA immunoreactivity, and some amacrine cells also contained tyrosine hydroxylase immunoreactivity. NPY-immunostained processes were most frequently presynaptic to nonimmunostained amacrine and ganglion cell processes and postsynaptic to nonimmunostained amacrine cell processes and cone bipolar cell axonal terminals. These findings indicate that NPY immunoreactivity is present in two populations of amacrine cells, one located in the INL and the other in the GCL, and that these cells mainly form synaptic contacts with other amacrine cells. These observations suggest that NPY-immunoreactive cells participate in multiple circuits mediating visual information processing in the inner retina.

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Neuropeptide Y (NPY), an inhibitory neuropeptide expressed by a moderately dense population of wide-field amacrine cells in the rat retina, acts through multiple (Y1-y6) G-protein-coupled receptors. This study determined the cellular localization of Y1 receptors and the synaptic connectivity of Y1 processes in the inner plexiform layer (IPL) of the rat retina. Specific Y1 immunoreactivity was localized to horizontal cell bodies in the distal inner nuclear layer and their processes in the outer plexiform layer. Immunoreactivity was also prominent in cell processes located in strata 2 and 4, and puncta in strata 4 and 5 of the IPL. Double-label immunohistochemical experiments with calbindin, a horizontal cell marker, confirmed Y1 immunostaining in all horizontal cells. Double-label immunohistochemical experiments, using antibodies to choline acetyltransferase and vesicular acetylcholine transporter to label cholinergic amacrine cell processes, demonstrated that Y1 immunoreactivity in strata 2 and 4 of the IPL was localized to cholinergic amacrine cell processes. Electron microscopic studies of the inner retina showed that Y1-immunostained amacrine cell processes and puncta received synaptic inputs from unlabeled amacrine cell processes (65.2%) and bipolar cell axon terminals (34.8%). Y1-immunoreactive amacrine cell processes most frequently formed synaptic outputs onto unlabeled amacrine cell processes (34.0%) and ganglion cell dendrites (54.1%). NPY immunoreactivity in the rat retina is distributed primarily to strata 1 and 5 of the IPL, and the present findings, thus, suggest that NPY acts in a paracrine manner on Y1 receptors to influence both horizontal and amacrine cells. Indexing termsamacrine cells; choline acetyltransferase; calcium binding protein; horizontal cells; neuropeptides Neuropeptide Y (NPY) has multiple physiologic actions in both the central and peripheral nervous system, including effects on blood flow, memory retention, food intake, epilepsy, and pain (Lundberg et al., 1996;Munglani et al., 1996;Balasubramaniam, 1997;Blomqvist and Herzog, 1997;Thorsell et al., 2000;Furtinger et al., 2001;Naveilhan et al., 2001). These NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript effects are mediated by means of at least five G-protein-coupled receptors, designated as Y1, Y2, Y4, Y5, and y6, which are coupled to pertussis toxin-sensitive inhibitory Gproteins (Lundberg et al., 1996;Munglani et al., 1996;Balasubramaniam, 1997;Blomqvist and Herzog, 1997;Michel et al., 1998). Y-receptors have been localized to specific regions of the central and peripheral nervous system by receptor binding autoradiography, immunohistochemistry, and in situ hybridization (Zhang et al., 1994;Bao et al., 1997;Jacques et al., 1997;Dumont et al., 1998;Hökfelt et al., 1998;Gackenheimer et al., 2001). Y1 receptor is the most studied Y-receptor and it plays key roles in many of the central and peripheral effects of NPY (Lundberg et al., 1996;Munglani et al., 1996;Balasubramaniam, 1997;Blomqvist and H...