Distribution of GABA-like immunoreactive neurons in insects suggests lineage homology

Abstract

Gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in vertebrates and invertebrates (Sattelle [1990] Adv. Insect Physiol. 22:1-113). The GABA phenotype is lineally determined in postembryonic neurons in the tobacco hawkmoth, Manduca sexta (Witten and Truman, [1991] J. Neurosci. 11:1980-1989) and is restricted to six identifiable postembryonic lineages in the moth's thoracic hemiganglia. We used a comparative approach to determine whether this distinct clustering of GABAergic neurons is conserved in Insecta. In the nine orders of insects surveyed (Thysanura, Odonata, Orthoptera, Isoptera, Hemiptera, Coleoptera, Diptera, Lepidoptera, and Hymenoptera), GABA-like immunoreactive neurons within a thoracic hemiganglion were clustered into six distinct groups that occupied positions similar to the six postembryonic lineages in Manduca. On the basis of cell body position and axon trajectories, we suggest that these are indeed homologous lineage groups and that the lineal origins of the GABAergic cells have been very conservative through insect evolution. The distinctive clustering of GABA-positive cells is shared with crustaceans (Mulloney and Hall [1990] J. Comp. Neurol. 291:383-394; Homberg et al. [1993] Cell Tissue Res. 271:279-288) but is not found in the centipede Lithobius forficulatus. There is a two- to threefold increase in numbers of thoracic neurons between the flightless Thysanura and the most advanced orders of insects. Using the GABA clusters as indicators of specific lineages, we find that only selected lineages have significantly contributed to this increase in neuronal numbers.

Pattern of expression of engrailed in relation to gamma‐aminobutyric acid immunoreactivity in the central nervous system of the adult grasshopper

“…The more lateral cluster of GABA-positive and En-negative neurons may originate from NB 7-4. GABA-positive clusters also have been described in Manduca, grasshopper, and several other insects using nomenclature from Manduca (Witten and Truman, 1998). Based on soma position, we suggest that the posterior portions of the N and M groups of Manduca are equivalent to the more medial clusters delineated in this study (putative NB 7-1 and NB 7-2 progeny), whereas part of the T group corresponds to putative NB 7-4 progeny.…”

“…Also in Drosophila, subsets of En-positive interneurons are similar in soma position and morphology to the NB 6-1 and NB 6-2 progeny. The B group and a part of the heterogeneous A group in grasshopper may also be comparable to the anterior portions of the N and M GABA-positive groups of Manduca and other insects (Witten and Truman, 1998). We identify two other distinct groups of interneuronal somata that are more lateral, and that are also GABApositive but En-negative.…”

“…Although the abdominal neuromeres lack NB 5-5, they do contain at least two and perhaps three peripheral inhibitors (Watson and Pflü ger, 1987). By contrast, the holometabolous insects, including Drosophila and Manduca, lack peripheral inhibitors in any neuromere (Witten and Truman, 1998). Drosophila also lacks NB 6-3 (Bossing et al, 1996;Schmidt et al, 1997).…”

Dopamine-synthesizing neurons include the putative H-cell homologue in the mothManduca sexta

“…Progeny of the MNB include the well‐studied efferent dorsal unpaired medial (DUM) or VUM cells, which are octopaminergic (Hoyle, 1978; Goodman et al, 1979; Stevenson et al, 1992; for review see Roeder, 1999). They also include the smaller interneurons containing γ‐aminobutyric acid (GABA) (Thompson and Siegler, 1991; Stevenson et al, 1992; Witten and Truman, 1998). Median neurons expressing octopamine immunoreactivity in Manduca include the two large VUM neurons (Pflüger et al, 1993) and two smaller midline neurons that we located more dorsally (Lehman et al, 2000).…”

“…In addition, the H‐cells are probably not GABAergic. The typical anterior location of the H‐cell soma and its relatively large size (>20 μm) contrast with the posterior locations of midline GABA‐ir cells, which are much smaller (<10 μm; Witten and Truman, 1998).…”