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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. J. Comp. Neurol. 454:373–382, 2002. © 2002 Wiley‐Liss, Inc.

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Mutational inactivation of p53 is only one of the ways that tumors lose p53 function. An alternate route is through overexpression of HDM2, the negative regulator of p53. To further understand how excess HDM2 regulates p53-mediated functions, we generated H1299 cell clones that constitutively express both ectopic HDM2 and tetracycline-regulated inducible p53. We found that over a range of p53 concentrations constitutively expressed HDM2 did not affect the levels of p53 protein. Nevertheless, cells with excess HDM2 displayed numerous changes in their response to p53. After DNA damage, such cells had both increased p53-mediated G 2 arrest and reduced cell death. They also showed selective impairment of p53 target gene induction in that some p53 targets were unaffected whereas others were markedly less well induced in the presence of extra HDM2 protein. We also found that excess HDM2 was correlated with reduced p53 acetylation but did not affect p53 association with target promoters in vivo. Indeed, there was no significant difference in the amount of HDM2 associated with p53 at target promoters that differed in their expression depending on the presence of extra HDM2. Thus, HDM2 can selectively down-regulate the transcription function of p53 without either degrading p53 or affecting the interaction of p53 with target promoters.