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First published online December 16, 2008
Journal of Experimental Biology 212, 126-136 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.020412

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Source, topography and excitatory effects of GABAergic innervation in cockroach salivary glands

Cathleen Rotte, Jeannine Witte, Wolfgang Blenau, Otto Baumann and Bernd Walz^*

Institute of Biochemistry and Biology, Department of Animal Physiology, University of Potsdam Karl-Liebknecht-Str. 24–25, 14476 Potsdam, Germany

^* Author for correspondence (e-mail: walz{at}uni-potsdam.de)

Accepted 3 November 2008

Cockroach salivary glands are innervated by dopaminergic and serotonergic neurons. Both transmitters elicit saliva secretion. We studied the distribution pattern of neurons containing -aminobutyric acid (GABA) and their physiological role. Immunofluorescence revealed a GABA-immunoreactive axon that originates within the subesophageal ganglion at the salivary neuron 2 (SN2) and this extends within the salivary duct nerve towards the salivary gland. GABA-positive fibers form a network on most acinar lobules and a dense plexus in the interior of a minor fraction of acinar lobules. Co-staining with anti-synapsin revealed that some putative GABAergic terminals seem to make pre-synaptic contacts with GABA-negative release sites. Many putative GABAergic release sites are at some distance from other synapses and at distance from the acinar tissue. Intracellular recordings from isolated salivary glands have revealed that GABA does not affect the basolateral membrane potential of the acinar cells directly. When applied during salivary duct nerve stimulation, GABA enhances the electrical response of the acinar cells and increases the rates of fluid and protein secretion. The effect on electrical cell responses is mimicked by the GABA_B receptor agonists baclofen and SKF97541, and blocked by the GABA_B receptor antagonists CGP52432 and CGP54626. These findings indicate that GABA has a modulatory role in the control of salivation, acting presynaptically on serotonergic and/or dopaminergic neurotransmission.

Key words: GABA, salivary gland, innervation, insect, cockroach, Periplaneta americana

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