GABAergic control of the release of diapause hormone from the suboesophageal ganglion of the silkworm, Bombyx mori

Abstract

Brain-suboesophageal ganglion complexes and suboesophageal ganglia were isolated from silkworm pupae destined to lay diapause eggs and pupae destined to lay non-diapause eggs and incubated in Grace's insect medium for 24 h. Diapause hormone released into the medium was determined by bioassay. Brain-suboesophageal ganglion complexes of diapause-egg-producing pupae secreted diapause hormone into the medium, whereas those of non-diapause-egg-producing pupae secreted little hormone. The suboesophageal ganglia of diapause-egg-producing and non-diapause-egg-producing pupae also released diapause hormone into the medium, and the amount of hormone was about 70% of that released by the brain-suboesophageal ganglion complexes of diapause-egg-producing pupae. Diapause hormone released from the brain-suboesophageal ganglion complexes of diapause-egg-producing was significantly reduced when the complexes were incubated in a medium containing more than 1 mM of γ-aminobutyric acid (GABA), an inhibitory neurotransmitter. The secretion of diapause hormone from the suboesophageal ganglia of both diapause-egg-producing and non-diapause-egg-producing pupae was also suppressed by GABA. Pictrotoxin, a specific blocker for GABA, induced diapause hormone release from the brain-suboesophageal ganglion complexes of non-diapause-egg-producing pupae in a dosedependent manner. When the concentration of picrotoxin was increased to 16.6 μM, these complexes secreted almost the same amount of diapause hormone as the complexes of diapause-egg-producing pupae. These lines of evidence showed the GABAergic neurones played a key role in the cerebral control of diapause hormone release from the suboesophageal ganglion.