Altered ivermectin pharmacology and defective visual system in Drosophila mutants for histamine receptor HCLB

Abstract

The Drosophila gene hclB encodes a histamine-gated chloride channel, which can be activated by the neurotoxin ivermectin when expressed in vitro. We have identified two novel hclB mutants, carrying either a missense mutation (P293S, allele hclB (T1)) or a putative null mutation (W111*, allele hclB (T2)), as well as a novel splice form of the gene. In survival studies, hclB (T1) mutants were more sensitive to ivermectin than wild-type, whereas hclB (T2) were more resistant. Electroretinogram recordings from the two mutants exhibited enlarged peak amplitudes of the transient components, indicating altered synaptic transmission between retinal photoneurons and their target cells. Ivermectin treatment severely affected or completely suppressed these transient components in an allele-specific manner. This suppression of synaptic signals by ivermectin was dose-dependent. These results identify HCLB as an important in vivo target for ivermectin in Drosophila melanogaster, and demonstrate the involvement of this protein in the visual pathway.

Functional consequences of mutations in the Drosophila histamine receptor HCLB

“…The visual responses of several hclB mutants have been studied so far: two null mutants hisCl1 134 (Pantazis et al, 2008) and hclB T2 , as well as hclB T1 (Yusein et al, 2008). In the present study we demonstrated that the absolute sensitivity of both on- and off-transients, estimated by their 0.5 mV thresholds, was increased to almost the same degree.…”

“…The studies of Heisenberg (1971) and Coombe (1986) demonstrated that the lack of these transient components is the result of impaired synaptic transmission between the photoreceptors and their targets, the large monopolar cells in the lamina, which is consistent with the expression data. By contrast, ERG records from hclB mutants have been reported to contain both components; the on-transients only (Pantazis et al, 2008) or both on- and off-transients (Yusein et al, 2008) having higher amplitudes than those from control flies. At the same time, the LMCs response to low-intensity brief flashes, where the on- and off-components are not separated, were shown to be less sensitive than the corresponding responses of the controls (Pantazis et al, 2008).…”

“…Two strains with mutant hclB alleles having the genotype: st hclB T1 /TM3 , Sb and st hclB T2 /TM3 , Sb were described in Yusein et al (2008). The flies were kept on yeast-molasses medium at 25 °C.…”

“…Recordings were carried out as described by Yusein et al (2008). The electroretinograms were recorded using glass pipette microelectrodes with a tip diameter of 15–20 μm.…”

“…Hong et al (2006) also described the first hclB null mutant ( HisCl1 134 ) and demonstrated that it not only prefers a higher temperature than normal flies, but also has a reduced tolerance for high temperatures. Recently, we identified two new Drosophila mutants, hclB T1 (P293S) and hclB T2 (W111*, a null mutation), and showed that they have allele-specific visual system phenotypes and altered susceptibility to ivermectin (Yusein et al, 2008). Since the proline residue at position 293 affected in hclB T1 is highly conserved across the ligand-gated chloride channel family, we explore here the effect of its substitution on the tolerance of flies to high temperature.…”