Moreover, transformants exhibit a large amount of measurable PLC activity in heads, whereas heads of norpAP24 mutants exhibit very little to none. Immunohistochemical staining of tissue sections using NorpA antiserum confirm that expression of NorpA protein in transformants localizes in the retina, more specifically in rhabdomeres of R1-6 photoreceptor cells, but not R7 or R8 photoreceptor cells. This study determines whether lack of function leads to structural repercussions in photoreceptor cells of the compound eye and their synapses. For this purpose, thin sections and freeze fracture replicas of norpA were examined using transmission electron microscopy. There are unusual membrane specializations on the plasmalemma of the retinula cell, dubbed 'zippers'.
Zippers appear to increase with age and can cause a distorted geometry of ommatidia.
Only a few retinula cells ultimately degenerate in norpA , and the proportion may not differ from that of wild type. Initial observations indicate losses in responsiveness at low light intensities and prolonged receptor potential waveforms. Subsequently, reductions in response amplitudes at higher light intensities occur, until no responses are obtained. On return to lower temperature the electrophysiological properties recover in reverse order. Based on these observations it is concluded that the primary defect of norpA affects the efficiency of the phototransduction process.