Brain Research, 557 (1991) 221-226 © 1991 Elsevier Science Publishers B.V. All rights reserved. 0006-8993/91/$03.50 ADONIS 000689939116915S BRES 16915 221 Glycinergic control of [LeuS]enkephalin levels in chicken retina Meeuwis K. Boelen 1, John Wellard 1, Mark Dowton 2, Ian W. Chubb 3 and Ian G. Morgan 4 1Centrefor Research on Ageing, La Trobe University College of Northern Victoria, Bendigo, Vic. (Australia), 2Department of Biology, University of Wollongong, N.S. W. (Australia), 3Division of Biochemistry and Molecular Biology, Faculty of Science, Australian National University, A.C.T. (Australia) and ¢Centre for Visual Sciences and Research School of Biological Sciences, Australian National University, A. C. T. (Australia) (Accepted 9 April 1991) Key words: Enkephalin; Glyeine; y-Aminobutyric acid; Amacrine cell; Retina; Chick; Release; Strychnine; Picrotoxin Retinal levels of [LeuS]enkephalin-like immunoreactivity (LE-LI) increase during the light and decrease during darkness, in vivo15. Intravitreal injection of the GABA antagonist picrotoxin had no effect on the accumulation of LE-LI during the light, suggesting the absence of significant GABAergic control over LE-LI cells. However, injection of the glycine antagonist strychnine, prevented the light-induced increase of retinal levels of LE-LI during 6 h exlx~ure to light, indicating the presence of glycinergic control over the LE-LI neurons. When applied during the dark, strychnine increased the depletion of LE-LI by 34% compared to vehicle-injected eyes, suggesting that the LE-LI neurons receive some glycinergic input during the dark as well. The release of LE-LI from retinas superfused in vitro is depressed by exposing the preparation to light 2. Superfusing isolated retinas with physiological buffer containing picrotoxin (100 ~M), GABA (50 mM), or the GABA agonists muscimol (100/~M), (+)-baclofen (200/~M), or 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyfidin-3-ol (THIP)I(100 mM), had no effect on the efflux of LE-LI. Strychnine (100 mM) however increased the efflux of LE-LI by 64%, compared to the spontaneous efflux during the light. Glycine (15 and 50 mM) decreased the spontaneous efilux of LE-LI from retinas superfused in darkness by 44-48% and by 31% at 5 mM. These data are consistent with the results from pharmacological manipulations in vivo. We conclude that the LE-LI amacrine cells are under inhibitory control from glycinergic but not from GABAergic neurons. A change in degree of glycinergic inhibition according ambient fighting may contribute to, but does not account entirely for, the fight-driven variation in activity of LE-LI neurons in chicken retina. INTRODUCTION The inner nuclear layer of the vertebrate retina contains enkephalin-immunoreactive amacrine cells (for reviews see ref. 4, 25). In avian retina, they are the only neurons which contain [LeuS]enkephalin-like immunore- activity (LE-LI) 5'9'10'15'19"26. In chicken, the retinal levels of LE-LI increase during the light and fall during the dark 15. These changes are not small; levels of LE-LI double during 12 h in the light. Furthermore, the changes appear to be light-driven rather than circadian, as pro- longed exposure to light or darkness results in further increases or decreases in the levels of LE-L115. On the basis of these results we suggested that the LE-LI cells might be inhibited by light, decreasing release of LE-LI and hence leading to accumulation of LE-LI. More recently we have demonstrated that the release of LE-LI from retinas superfused in vitro is indeed high in darkness and inhibited by exposure to figh t2. Electron microscopic studies of the LE-LI amacrine cells suggest that they receive substantial input from non-immunoreactive amacrine cells 1°,26 and some input from bipolar cells 26 (Millar, personal commu- nication). These inputs could provide light-activated inhib- itory controls over the LE-LI amacrine cells. y-Aminobutyric acid (GABA) and glycine are major inhibitory neurotransmitters in the retina (for reviews see refs. 12, 14, 27). In studies using [3H]GABA uptake followed by autoradiography and in studies using GABA- or glutamic acid decarboxylase- (GAD) immunoreactiv- ity, some horizontal cells, many amacrine cells and some ganglion cells have been shown to be potentially GABA- ergic (for detailed references see ref. 27). [3H]Glycine uptake and autoradiography suggest that in the avian retina many amacrine cells may be glycinergic, with only a few bipolar cells and horizontal cells also labeled 13'2°'21. Watt et al.21,23,24 have demonstrated that some LE-LI amacrine cells are able to take up GABA or glycine, which may indicate co-transmission. In the present study we have used GABAergic and glycinerglc agonists and antagonists, in vivo and in vitro, to determine the possible participation of these neuro- transmitter systems in pathways controlling the activity of the LE-LI amacrine cells. Parts of these results have been published in preliminary form 16. Correspondence: M.K. Boelen, Centre for Research on Ageing, La Trobe University of Northern Victoria, P.O. Box 199, Bendigo, Vic. 3550, Australia.