Projections and Functional Implications of Labellar Neurons from Individual Sensilla of Drosophila Melanogaster

Abstract

Our previous studies using Golgi silver impregnations from labellar sensilla of adult Drosophila melanogaster had revealed seven distinct neuronal types projecting in the suboesophageal ganglion of the brain. These are: coiled fibre (type I); shrubby fibre (type II); ipsilateral ventral fibre (type III); ipsilateral dorsal fibre (type IV); contralateral ventral fibre (type V); contralateral dorsal fibre (type VI) and central fibre (type VII) (Nayak and Singh, 1985).

We have attempted to identify the neurons present in a single taste sensillum using the neuronal marker horse radish peroxidase (HRP). The success rate of single sensillum fillings with HRP is about 20%, but the fibre projections obtained could be identified without ambiguity. Although a single sensillum in question has five neurons, yet at a time, only one or at the most two neurons are labelled in any given experiment. The type of neuron labelled was found to be specific to the stimulus solute present in the HRP solution. (i) The presence of 0.1 M potassium chloride in HRP stains type II and type VI fibres separately or both together with an equal probability in any preparation. (ii) HRP dissolved in 0.1 M sucrose solution mainly showed type IV (60%) and less frequently type II (20%) fibres, while in the remaining 20% both types II and IV are labelled. (iii) When 0.1 M sodium chloride solution containing HRP is used, the type of fibres stained are very similar to those of HRP dissolved in 0.1 M sucrose, however, the frequency of fibres stained is different - type IV (35%), type II (35%) and types II and IV together (30%). (iv) HRP dissolved in distilled water alone, revealed type I (30%) and type II (20%) fibres separately or together (35%). Occasionally type V or type VII (<6% each) also get stained.

It is known from behavioural and electrophysiological studies in our laboratory that at lower concentrations flies are attracted by sodium chloride (≤0.1 M NaCl) but are not by potassium chloride (Arora 1985; Swati Joshi, personal communication).

In the present study type II fibres get stained irrespective of the stimulant present in HRP solution. However, when attractants are used as stimulant solute in HRP solution, only type IV fibres are labelled. Type VI fibres are stained when the stimulant is a repellant.