The roles of transcription and translation in mediating the effect of electrical afterdischarge on neurohormone synthesis in Aplysia bag cell neurons

The molecular links between membrane excitability and neurohormone synthesis were investigated using a simple model system: the bag cell neurons of Aplysia. We tested the hypothesis that the electrical afterdischarge, which leads to depletion of egg-laying hormone (ELH) by triggering secretion, rapidly stimulates ELH synthesis to replenish bag cell stores of hormone. Newly synthesized peptides were radiolabeled, and ELH peptide was immunoprecipitated. Within 4 h of afterdischarge, there was a 100% increase in ELH synthesis compared with that in unstimulated controls. Northern blot analysis showed that ELH messenger RNA (mRNA) levels were not altered for at least 8 h after the onset of afterdischarge. ELH mRNA levels were also not affected by 4-h treatments that inhibited either transcription (with actinomycin D) or translation (with anisomycin). Further work revealed that ELH mRNA is stable, with a half-life exceeding 32 h. Notably, the stimulatory effect of afterdischarge on ELH synthesis was blocked in response to treatment with the transcription inhibitor actinomycin D. These results suggest that the afterdischarge-induced increase in ELH synthesis is mediated by an increase in the rate of translation of already existing ELH mRNA, and that transcription of a non-ELH gene(s) is required for this effect to occur.