gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in insects and is widely distributed in the central nervous system (CNS). GABA acts on ion channel receptors (GABA(A)R) for fast inhibitory transmission and on G-protein-coupled ones (GABA(B)R) for slow and modulatory action. We used immunocytochemistry to map GABA(B)R sites in the Drosophila CNS and compared the distribution with that of the GABA(A)R subunit RDL. To identify GABAergic synapses, we raised an antiserum to the vesicular GABA transporter (vGAT). For general GABA distribution, we utilized an antiserum to glutamic acid decarboxylase (GAD1) and a gad1-GAL4 to drive green fluorescent protein. GABA(B)R-immunoreactive (IR) punctates were seen in specific patterns in all major neuropils of the brain. Most abundant labeling was seen in the mushroom body calyces, ellipsoid body, optic lobe neuropils, and antennal lobes. The RDL distribution is very similar to that of GABA(B)R-IR punctates. However, the mushroom body lobes displayed RDL-IR but not GABA(B)R-IR material, and there were subtle differences in other areas. The vGAT antiserum labeled punctates in the same areas as the GABA(B)R and appeared to display presynaptic sites of GABAergic neurons. Various GAL4 drivers were used to analyze the relation between GABA(B)R distribution and identified neurons in adults and larvae. Our findings suggest that slow GABA transmission is very widespread in the Drosophila CNS and that fast RDL-mediated transmission generally occurs at the same sites.

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 gamma-aminobutyric acid (GABA), glutamic acid decarboxylase, a GABA(B) 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 GABA(B) receptor immunoreactivity is seen on these neurons as well as the Cha-Tan neurons. Based on an rdl-Gal4 line, the ionotropic GABA(A) receptor subunit RDL may be expressed by L4 neurons and a type of tangential neuron (rdl-Tan). Strong vGluT immunoreactivity was detected in alpha-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.