Objective: 

Histaminergic transmission between photoreceptors and second-order visual neurons in arthropods is mediated by ionotropic receptors, encoded by two genes, hclA and hclB. The lack of HCLA receptors, which are expressed in the second-order neurons, completely blocks histaminergic transmission. However, null mutations in hclB, which is expressed in the glial cells, result in increased amplitude of the second-order neuron components of the electroretinographic (ERG) responses. In this work, the HCLB-mediated influences on the ERG responses were studied in different conditions of ambient illumination.

Methods: 

The intensity-response functions (V (logI)) of the ERG responses of the hclB null mutant hclBT2 were studied under dark and light adaptation. The dynamics of the dark sensitivity recovery following short light adaptation was also characterized.

Results: 

The amplitudes of the ERG ON and OFF transients were increased in the hclBT2 mutant, the relative increase being independent on both stimulus intensity and state of adaptation. As a result, the amplitude scales and absolute sensitivities of the ERG transients were increased in a similar manner under both dark- and light-adaptation. The relative sensitivity of the transients and the adaptational shift of the V (logI) curves along the intensity axis were not altered. The hclBT2 mutant flies showed delayed dark sensitivity recovery with lacking postadaptational potentiation of the ERG transients.

Conclusions: 

Our results show that HCLB receptors are involved in visual sensitivity modulation under both dark and light adaptation conditions but they do not alter significantly the process of light adaptation. They modulate the characteristics and the dynamics of the dark sensitivity recovery following short light adaptation.