Ionotropic glutamate receptors mediate juvenile hormone synthesis in the cockroach, Diploptera punctata

Abstract

By monitoring changes in the cytosolic [Ca 2+] i and rates of juvenile hormone (JH) synthesis in response to l-glutamate agonists and antagonists, we identified and characterized glutamate receptor subtypes in corpus allatum (CA) cells of the cockroach, Diploptera punctata. During the first ovarian cycle, corpora allata exhibited a cycle of changes in sensitivity to l-glutamate correlated to cyclic changes in rates of JH synthesis. When exposed to 60 μM l-glutamate in vitro, the active corpora allata of day-4 mated females produced 60% more JH, while inactive corpora allata at other ages showed 10–20% stimulatory response. Pharmacological characterization using various l-glutamate receptor agonists and antagonists indicated that several ionotropic subtypes of l-glutamate receptors were present in the CA. The CA showed an increase in rates of JH synthesis in response to NMDA, kainate, and quisqualate, but not to AMPA in both L-15 medium and minimum incubation medium. In contrast, applications of the metabotropic receptor-specific agonist trans-ACPD failed to elicit a change in the cytosolic [Ca 2+] i and JH production. An elevation of cytosolic calcium concentration, followed by 20–30% rise in JH production, was observed when active CA cells were exposed to 10–40 μM kainate. Kainate had no stimulatory effect on JH synthesis in calcium-free medium. The kainate-induced JH synthesis was blocked by 20 μM CNQX but was not affected by 20 μM NBQX. Kainate-stimulated JH production was not suppressed by MK-801 (a specific blocker of NMDA-receptor channel), nor was NMDA-stimulated JH production affected by CNQX (a specific antagonist of kainate receptor). These data suggest that active CA cells are stimulated to synthesize more JH by a glutamate-induced calcium rise via NMDA-, kainate- and/or quisqualate-sensitive subtypes of ionotropic l-glutamate receptors. The metabotropic-subtype and ionotropic AMPA-subtype l-glutamate receptors are unlikely to be present on active CA cells.