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.