b-Endorphin Expression in the Mouse Retina

Evidence showing expression of endogenous opioids in the mammalian retina is sparse. In the present study we examined a transgenic mouse line expressing an obligate dimerized form of Discosoma red fluorescent protein (DsRed) under the control of the pro-opiomelanocortin promoter and distal upstream regulatory elements to assess whether pro-opiomelanocortin peptide (POMC), and its opioid cleavage product, b-endorphin, are expressed in the mouse retina. Using double label immunohistochemistry we found that DsRed fluorescence was restricted to a subset of GAD-67-positive cholinergic amacrine cells of both orthotopic and displaced subtypes. About 50% of cholinergic amacrine cells colocalized DsRed and a large fraction of DsRedexpressing amacrine cells was positive for b-endorphin immunostaining, whereas b-endorphin-immunoreactive neurons were absent in retinas of POMC null mice. Our findings contribute to a growing body of evidence demonstrating that opioid peptides are an integral component of vertebrate retinas, including those of mammals. J. Comp. Neurol. 518:3130–3148, 2010. VC 2010 Wiley-Liss, Inc. INDEXING TERMS: retina; b-endorphin; starburst amacrine cell; POMC In vertebrate retinas, neural processing of light signals is mediated primarily by the amino acid transmitters glutamate, gamma amino butyric acid (GABA), and glycine, with additional contributions from amines such as acetylcholine, dopamine and 5-hydroxytryptamine (reviewed in Ehinger, 1982). This short list, however, does not include a number of additional chemical messengers that influence retinal signal processing, for the retina also contains about 50 identified neuroactive peptides (reviewed in Brecha, 2003). In this study we focus on the retinal distribution of an opioid peptide, b-endorphin, and its precursor protein pro-opiomelanocortin (POMC). The great diversity of the retinal peptide population has made it difficult to formulate a general framework for the roles peptides play in retinal operation, but some generalizations can be made about peptide organization and function. Many of the identified retinal peptides have been shown to coexist with an amino acid or amine cotransmitter (Vaney et al., 1989; Casini and Brecha, 1992; Cuenca and Kolb, 1998; Hannibal et al., 2000), consistent with observations in other parts of the central nervous system (CNS) reporting co-release of classical neurotransmitters and peptides from the same neuron (Hökfelt et al., 2000). Moreover, almost all the retinal peptides are found in inner retinal neurons, particularly in subtypes of amacrine cell (Brecha, 2003). Endogenous opioid peptides possess a shared N-terminal tetrapeptide sequence Tyr-Gly-Gly-Phe and are divided into three families, originating from three large precursor proteins. Proenkephalin gives rise to two pentapeptide proteins, leu-enkephalin and met-enkephalin, the heptapeptide met-enkephalin-7, and the octapeptide met-enkephalin-8. Prodynorphin is cleaved to generate dynorphin A/B and a-neoendorphin (reviewed in Khalap et al., 2005). The alternative cleavage products of proopiomelanocortin include the opioid b-endorphin, the melanocortins adrenocorticotropic hormone (ACTH), Grant sponsor: Richard H. Chartrand Foundation (to P.W.); Grant sponsor: HFSP; Grant number: RGY0004/2003 (to M.J.R.); Grant sponsor: National Institutes of Health (NIH); Grant number: NIH 2R01DK066604 (to M.J.L.); Grant number: NIH R01DK078749 (to S.T.H.); Grant number: R01 EY019051 (to J.V.). *CORRESPONDENCE TO: Jozsef Vigh, Department of Biomedical Sciences, Colorado State University, Fort Collins, CO 80523. E-mail: Jozsef.Vigh@Colostate.edu VC 2010 Wiley-Liss, Inc. Received October 14, 2009; Revised January 27, 2010; Accepted March 3, 2010 DOI 10.1002/cne.22387 Published online March 23, 2010 in Wiley InterScience (www.interscience. wiley.com) 3130 The Journal of Comparative Neurology | Research in Systems Neuroscience 518:3130–3148 (2010) RESEARCH ARTICLE