Rotov, A. Y. | Search bioRxiv

bioRxiv2025

Unusual dopamine-mediated regulation of the phototransduction in lamprey compared to jawed vertebrates

The vertebrate retina uses neurotransmitters to regulate its various functions and adjust vision according to the day/night cycle. Dopamine is probably one of the most important of these neurotransmitters. It is released by dopaminergic amacrine cells in the retina and exerts its regulatory effects, in part, through the cAMP pathway. It has been demonstrated that dopamine affects the phototransduction cascade in isolated amphibian rods. Furthermore, elevated intracellular levels of cAMP increase the light sensitivity of vertebrate rods and modulate the response of vertebrate cones. These effects can be triggered by dopamine receptors and adjust vision to daily light variations. Therefore, the evolution of dopamine loops in the retina is of interest, and the lamprey, being the most primitive vertebrate, could be valuable in this regard. In the present study, we examined whether the photoresponse properties of long (cone-like) and short (rod-like) photoreceptors in the river lamprey could be regulated by dopamine or cAMP level modulation. Using suction pipette recording, we demonstrated that dopamine slightly increased short photoreceptors sensitivity and it slowed the rising and falling phases of photoresponses in long photoreceptors and increased the integration time, with no effect on the sensitivity to brief flashes. The second part of our study -- an immunohistochemical analysis of the lamprey retina -- revealed that both D1 and D2 dopamine receptors are expressed in both types of lamprey photoreceptors. Our results suggest that the regulation of photoreceptor functions by the neurotransmitter dopamine originated in the early stages of vertebrate evolution, specifically during the Cambrian period. SummaryThe lamprey, a primitive vertebrate, is a valuable object for studying the evolution of dopamine loops in the vertebrate retina. This study shows that photoresponse properties of lamprey photoreceptors are regulated by dopamine in a different way compared to gnathostomes.

physiology↗

bioRxiv2025

Morphology and connectivity of retinal horizontal cells in two avian species

In the outer vertebrate retina, the visual signal is separated into intensity and wavelength information. In birds, seven types of photoreceptors (one rod, four single cones, and two members of the double cone) mediate signals to >20 types of second-order neurons, the bipolar cells and horizontal cells. Horizontal cells contribute to color and contrast processing by providing feedback signals to photoreceptors and feedforward signals to bipolar cells. In fish, reptiles, and amphibians they either encode intensity or show color-opponent responses. Yet, for the bird retina, the number of horizontal cell types is not fully resolved and even more importantly, the synapses between photoreceptors and horizontal cells have never been quantified for any bird species. With a combination of light microscopy and serial EM reconstructions, we found four different types of horizontal cells in two distantly related species, the domestic chicken and the European robin. In agreement with some earlier studies, we confirmed two highly abundant cell types (H1, H2) and two rare cell types (H3, H4), of which H1 is an axon-bearing cell, whereas H2-H4 are axonless. Horizontal cell contacts to photoreceptors were type-specific and similar to the turtle retina, which confirms the high degree of evolutionary conservation in the vertebrate outer retina. Our data further suggests that H1 and potentially H2 cells may encode intensity, whereas H3 and H4 may represent color opponent horizontal cells which may contribute to the birds superb color and/or high acuity vision.

neuroscience↗