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D. W. Marshak, L. Maybit, Jr., P. K. Rao, N. Kumar, R. D. Glickman; Histamine Inhibits Dopamine Release From Rat Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5792.
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The goal of these experiments was to learn how histaminergic input from the hypothalamus influences the processing of visual information in rat retina. Retinopetal axons containing histamine terminate in the inner plexiform layer, and histamine receptors are expressed by dopaminergic amacrine cells (Gastinger et al., 2001, 2006). Because histamine is released during the dark phase of the light cycle, it was expected to inhibit the release of dopamine, as it does in the guinea pig retina (Weber and Schlicker, 2001).
A superfused rat eyecup was maintained in vitro, under a constant, mesopic background light. All experiments were conducted in the daytime. The tissue was superfused with Ames medium containing 10µm pargyline and 1µm nomifensine equilibrated with 95%O2/5%CO2. Histamine was applied for 20 min followed by a 50 min washout; light stimulated release was studied using a 3 Hz, photopic flashing light. All samples were collected into1M HClO4, 200µm ascorbic acid, and 20 µM 3,4 dihydroxy benzylamine (internal standard). Samples were concentrated using a solid phase extraction method (Raggi et al., 1999), and dopamine was measured by HPLC with electrochemical detection at +0.46 V referenced to Ag/AgCl. The lower limit of quantitation for this method was approximately 1 ng/ml.
The sample preparation and solid phase extraction methods had at least 85% recovery with respect to dopamine. The background light did not induce detectable dopamine release under these conditions. Flashing light stimulated release to 2.49 ± 1.17 ng/ml, on average (n = 6). The levels of dopamine in the superfusate after 1µM histamine, 2.70 ± 1.73 (n = 4), were not significantly different from the control values. The light stimulated component of dopamine release was completely inhibited by either 5 µM (n = 4) or 10 µM (n = 4) histamine (p < 0.001).
At low micromolar concentrations, histamine clearly inhibited the release of endogenous dopamine from the rat retina. The histaminergic neurons fire tonically according to the sleep/wake cycle, with the highest rates at night in nocturnal animals such as rats (Saper et al., 2001). Because dopamine contributes to light adaptation (Witkovsky, 2004), we predict that histamine released from retinopetal axons promotes dark adaptation in rats.
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