Engrailed protein is expressed in interneurons but not motor neurons of the dorsal unpaired median group in the adult grasshopper

Abstract

We report that the homeodomain protein Engrailed (En) is differentially expressed by neuronal type. Expression was examined within identified midline neurons in T3, A1, and A2 neuromeres of the adult grasshopper by using immunohistochemistry. All save a few neurons in the adult dorsal unpaired median (DUM) group arise embryonically from a single precursor, the median neuroblast. DUM neurons are efferent neurons, local interneurons, or intersegmental interneurons, recognizable as such by their distinct morphologies and neurotransmitter phenotypes. We show that interneurons are En-positive, whereas efferents are En-negative. In the T3 DUM group, the 70 or so interneurons contained cytoplasmic immunoreactivity for gamma-aminobutyric acid (GABA) and glutamate decarboxylase. In double-labeling experiments, all GABA-immunoreactive neurons were also En-positive, and all En-positive neurons contained GABA immunoreactivity. In complementary experiments, the 20 or so efferents in the T3 DUM group, which are octopaminergic, were selectively labeled with a histological marker and then processed to reveal En immunoreactivity. No efferents in the group were En-positive. The abdominal DUM groups contain fewer neurons, but the same dichotomy of labeling was found. The En pattern is established during embryogenesis, with the type-specific pattern apparent by stage 90% of development, the earliest stage examined here. The differential expression of En in the embryo and its continued expression in the adult nervous system suggest a role in the development and maintenance of neuronal phenotype. Morphological differences between efferents and interneurons are discussed in light of a hypothesis that En mediates differential expression of cell adhesion or cell-affinity molecules.

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

“…In the adult DUM group, En is expressed together with the neurotransmitter gammaaminobutyric acid (GABA). Coexpression occurs in the interneurons of the group, whereas neither En nor GABA is expressed in the efferent neurons of the DUM group (Siegler and Pankhaniya, 1997). However, we noted in passing that many neurons outside the DUM group also expressed En and GABA.…”

“…The DUM group in T3 contains about 70 En-positive neurons, in A1 about 50 -60, and in A2 about 20 (column B, Table 1). These are interneurons (Siegler and Pankhaniya, 1997). In the T3 DUM group, they are adjacent to or intermingled with about 20 larger En-negative somata of efferent neurons (Fig.…”

“…3d,e;Siegler and Pankhaniya, 1997). The Enpositive somata in the bilateral groups are also relatively small in diameter compared with somata of efferent DUM neurons.…”

“…In the DUM groups, all En-positive interneurons are immunoreactive for the neurotransmitter GABA (Siegler and Pankhaniya, 1997). To determine whether this is true also for other En-positive neurons, ganglia were doubled labeled using an anti-GABA antiserum along with the En antibody.…”

“…In the adult grasshopper, the homeodomain protein Engrailed (En) is expressed differentially in interneurons but not in efferents within the dorsal unpaired median (DUM) groups of neurons, which arise embryonically from the median neuroblasts (MNBs; Siegler and Pankhaniya, 1997). The interneurons and the efferents have somata in one tight cluster, but their primary neurites and axonal branches trace quite different pathways within the central nervous system (CNS; Siegler, 1991, 1993).…”