Anatomy and targets of Dorsal Unpaired Median neurones in the Terminal Abdominal Ganglion of the male cockroach Periplaneta americana L.

The morphology of the Dorsal Unpaired Median (DUM) neurones in the Terminal Abdominal Ganglion (TAG) of the adult male cockroach Periplaneta americana were described based on wholemount preparations and paraffin sections and by using anterograde and retrograde cobalt mapping, octopamine-like immunohistochemistry, and double immunofluorescence technique with both conjugated gamma-aminobutyric acid (GABA) and octopamine antisera. Among 60 +/- 6 neurones with large somata (diameter 40 to 60 microns) on the dorsal midline surface of the TAG that were stained with toluidine blue, about 36 efferent DUM neurones exhibited octopamine-like immunoreactivity. The DUM neurones were arranged in three clusters (anterior, median and posterior) corresponding to the 7th-11th abdominal ganglia of the fused TAG. Anterior efferent DUM neurones with one, two, and four pairs of lateral neurites entered segmental nerves VIIB; VIIB and phallic nerves; IXB and phallic nerves; VIIIA, IXA, X, and IX, respectively. Three octopamine-like immunoreactive DUM neurones innervating heart chambers via segmental nerves (VIIA, VIIIA, and IXA) in the last abdominal segments occurred within abdominal ganglia 7, 8, and 9. Together with octopamine-like immunoreactive efferent DUM neurones, GABA-like immunoreactive dorsal midline neurones with small somata (10 to 20 microns) also occurred within the median group. The spatial distribution of DUM neurones in the TAG suggested that they had their origins in the median neuroblast, as for DUM neurones in the grasshopper.

Neuritic GABAergic synapses in insect neurosecretory cells.

The localization of synaptic gamma-aminobutyric acid (GABA) receptors on cockroach dorsal unpaired median (DUM) neurons of the last abdominal ganglion was investigated. These neurosecretory cells, mainly octopaminergic, possess a soma located on the dorsal midline of the ganglion, from which emerges a short primary neurite dividing into two symmetrical lateral branches on both lateral edges of the ganglion. GABA pressure ejections onto the soma and onto the neuritic arborization elicited hyperpolarizations. Moreover, electrical stimulation of the anterior connectives evoked a postsynaptic potential, mainly inhibitory. This response and the GABA hyperpolarization of the neuritic field are antagonized by lateral application of picrotoxin while soma GABA hyperpolarization remained unchanged. This suggests that there are two kinds of GABA receptors located (i) onto the soma membrane of the DUM neurons and called extrasynaptic receptors, (ii) on the neuritic arborization, called synaptic receptors and implicated in the connection between neurons coming from the anterior part of the nervous system and the DUM cells. Immunohistological double staining technique reinforced the electrophysiological results by showing the presence of GABA-like immunoreactive processes next to octopamine-like immunoreactive ones.