Abstract
Synaptic connections of neurons in the Drosophila lamina, the most peripheral synaptic region of the visual system, have been comprehensively described. Although the lamina has been used extensively as a model for the development and plasticity of synaptic connections, the neurotransmitters in these circuits are still poorly known. Thus, to unravel possible neurotransmitter circuits in the lamina of Drosophila we combined Gal4 driven green fluorescent protein in specific lamina neurons with antisera to γ-aminobutyric acid (GABA), glutamic acid decarboxylase, a GABAB type of receptor, L-glutamate, a vesicular glutamate transporter (vGluT), ionotropic and metabotropic glutamate receptors, choline acetyltransferase and a vesicular acetylcholine transporter. We suggest that acetylcholine may be used as a neurotransmitter in both L4 monopolar neurons and a previously unreported type of wide-field tangential neuron (Cha-Tan). GABA is the likely transmitter of centrifugal neurons C2 and C3 and GABAB receptor immunoreactivity is seen on these neurons as well as the Cha-Tan neurons. Based on an rdl-Gal4 line, the ionotropic GABAA receptor subunit RDL may be expressed by L4 neurons and a type of tangential neuron (rdl-Tan). Strong vGluT immunoreactivity was detected in α-processes of amacrine neurons and possibly in the large monopolar neurons L1 and L2. These neurons also express glutamate-like immunoreactivity. However, antisera to ionotropic and metabotropic glutamate receptors did not produce distinct immunosignals in the lamina. In summary, this paper describes novel features of two distinct types of tangential neurons in the Drosophila lamina and assigns putative neurotransmitters and some receptors to a few identified neuron types.
Highlights
One of the most extensively investigated portions of the insect brain is the first synaptic neuropil in the optic lobe of flies, the lamina
The optic lobe neuropils are stratified (Figs. 1A,D, and 2B), the result of overlap between stratumspecific terminals of (1) columnar centripetal neurons, (2) columnar centrifugal neurons; and lateral arborizations of (3) various columnar neuronal elements and (4) tangentially oriented wide-field branches of non-columnar neurons
To confirm the failure of vesicular glutamate transporter (vGluT) immunolabeling to localize to processes of monopolar neurons, we investigated the relation between this label and OK371-driven green fluorescent protein (GFP) in terminals of monopolar neurons in the medulla (Fig. 7C3, D3 and E3)
Summary
One of the most extensively investigated portions of the insect brain is the first synaptic neuropil in the optic lobe of flies, the lamina This neuropil corresponds in its processing operations to the outer plexiform layer of the vertebrate retina, and since the seminal work of Cajal and Sanchez [1] insect visual interneurons and their synaptic populations have been explicitly compared with those in the retina of vertebrates [2,3]. Like the latter, photoreceptors of two functional classes innervate the fly’s optic lobe. The Drosophila lamina has become an excellent system for the analysis of the genetic regulation of many aspects of synaptic function, plasticity and synaptogenesis (see [14,15,16,17,18,19,20,21])
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