Long-term sensitization training, or procedures that mimic the training, produces long-term facilitation of sensory-motor neuron synapses in Aplysia. The long-term effects of these procedures require mRNA and protein synthesis (Montarolo et al., 1986; Castellucci et al., 1989). Using the techniques of differential display reverse transcription PCR (DDRT-PCR) and ribonuclease protection assays (RPA), we identified a cDNA whose mRNA level was increased significantly in sensory neurons by treatments of isolated pleural-pedal ganglia with serotonin for 1.5 hr or by long-term behavioral training of Aplysia. The effects of serotonin and behavioral training on this mRNA were mimicked by treatments that elevate cAMP. The aplysia mRNA increased by serotonin and behavioral training was 41-45% identical to a developmentally regulated gene family which includes Drosophila tolloid and human bone morphogenetic protein-1 (BMP-1). Both tolloid and BMP-1 encode metalloproteases that might activate TGF-beta (transforming growth factor beta)-like molecules or process procollagens. Aplysia tolloid/BMP-1-like protein (apTBL-1) might regulate the morphology and efficacy of synaptic connections between sensory and motor neurons, which are associated with long-term sensitization.