Developmental changes and second messenger modulation of the electrical properties of cells of the cockroach corpora allata

Abstract

The duration of evoked action potentials in the cells of the corpora allata of recently emerged adult mated female cockroaches was dramatically increased following replacement of calcium ions by barium ions in the bathing saline. The magnitude of this increase was age-dependent. Furthermore, following exposure of corpora allata to barium, spontaneous action potentials (approx. 10–40 mV in amplitude) and small depolarizing potentials (approx. 2–5 mV in amplitude) were observed that were never seen in the absence of barium. This suggests that barium increases the excitability of the corpora allata. 8-bromo-cyclic AMP and the phorbol ester, phorbol 12,13-dibutyrate (PDBu), both increased the duration of barium action potentials in corpora allata from day-1 and day-6 females. However, 8-bromo-cyclic GMP, and the nonspecific protein kinase inhibitor H7 both reduced the barium action potential in day-1 corpora allata but had no significant effect in day-6 corpora allata. Furthemore, the inactive phorbol ester 4a-phorbol 12,13-didecanoate (PDD) did not significantly affect the barium action potential duration in day-1 or day-6 corpora allata, suggesting that PDBu is acting through phospholipid/calcium-dependent protein kinase C and not nonspecifically. 8-Br-cAMP and protein kinase C appear to modulate the electrophysiological properties of the cell membranes of the corpora allata, which suggests that they may modulate calcium influx. In addition, cAMP, protein kinase C and calcium have previously been implicated in the regulation of juvenile hormone biosynthesis. In view of the effects of cAMP and protein kinase C on both membrane properties and juvenile hormone biosynthesis, the cell membrane may be a site for integration of the calcium, cAMP and protein kinase C second messenger systems involved in the regulation of juvenile hormone biosynthesis. Furthermore, because juvenile hormone biosynthesis undergoes cyclical changes in adult mated female cockroaches and there are developmental changes in the electrical properties of the corpora allata, such changes may be important in the regulation of juvenile hormone biosynthesis, possibly by altering calcium influx.