Agonist-induced glycogenolysis in rabbit retinal slices and cultures

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Abstract
1 The effects of different putative retinal transmitters and/or modulators on glycogenolysis in rabbit retinal slices and in retinal Müller cell cultures were examined. 2 Incubation of rabbit retinal slices or primary retinal cultures (either 3–5 day-old or 25–30 day-old) in a buffer solution containing [3H]-glucose resulted in the accumulation of newly synthesized [3H]-glycogen. 3 Noradrenaline (NA), isoprenaline, vasoactive intestinal peptide (VIP), 5-hydroxytryptamine (5-HT) and 8-hydroxy-dipropylaminetetralin (8-OH-DPAT) stimulated the hydrolysis of this newly formed 3H-polymer. The potency order of maximal stimulations was: VIP > NA > isoprenaline > 5-HT > 8-OH-DPAT. 4 The putative retinal transmitters, dopamine, γ-aminobutyric acid (GABA), glycine and taurine and the muscarinic agonist carbachol (CCh) had no effect on [3H]-glycogen content. 5 The glycogenolytic effects of NA/isoprenaline and 5-HT/8-OH-DPAT appear to be mediated by β-adrenoceptors and 5-HT1 receptors (possibly 5-HT1A), respectively while the VIP-induced response involved another receptor subtype. 6 Agonists which mediated [3H]-glycogen hydrolysis also stimulated an increase in adenosine 3′:5′-cyclic monophosphate (cyclic AMP) formation. Both responses are blocked to a similar extent by the same antagonists and so are probably mediated via the same receptor subtypes. Moreover, dibutyryl cyclic AMP (db cyclic AMP) promoted tritiated glycogen breakdown in the three retinal preparations. 7 Not all receptors linked to cyclic AMP production however promote glycogenolysis. Dopamine and apomorphine stimulated cyclic AMP formation via D1-receptors without influencing glycogenolysis. These receptors are exclusively associated with neurones.