Effects of Histamine on the Intensity-Response Function of the Electroretinographic b- and d-Waves in Dark Adapted Frog Eyes

It is known that histamine is neurotransmitter of the retinopetal axons that originate from the tuberomamillary nucleus of the posterior hypothalamus, but its role in visual information processing in the retina is not well understood. The aim of this study was to give insight into the significance that histamine has for the distal retina function revealed by electroretinogram (ERG). The effect of 5 M histamine on the intensity – response function of the b-wave (ON response) and d-wave (OFF response) of ERG was investigated in dark adapted perfused frog eyecup preparations. Perfusion with histamine caused a significant enhancement of the amplitude of both the ON and OFF responses over the entire intensity range studied in comparison with corresponding values obtained in the control experiments. The enhancing effect of histamine was more pronounced upon the OFF than ON response in the lower intensity range, where the responses were mediated by rods. The reverse was true for the higher intensity range, where the responses were cone-dominated. The b-wave V – log I function had a steeper slope and narrower dynamic range during histamine treatment. Histamine did not alter significantly the relative sensitivity of the ON response, while it significantly increased the relative sensitivity of the OFF response. The present results clearly demonstrate that histamine has a significant effect on the intensity-response function of frog ERG b- and d-waves. This effect shows some ON/OFF asymmetries in dependence of the photoreceptor input.