H-89 Inhibits Serotonin Induced Short-Term Facilitation in the Crayfish Neuromuscular Junction

Abstract

Eric R. Kandel and his colleagues' composite research on serotonin (5-HT) in Aplysia emphasizes serotonins heterogeneous effects as a neurotransmitter on paired-pulse facilitation, since 5-HT not only opens up Ca²⁺ spikebroadening channels and Na-K channels to increase excitatory post-synaptic potentials (EPSPs), but also activates an independent protein kinase A (PKA) process of neurotransmitter regulation near the synaptic membrane. This PKA process increases the amount of neurotransmitter released (Byrne & Kandel, 1996). Since it had not yet been proven that this 5-HT-regulated process existed in more than a handful of other organisms outside of Aplysia, we set up an experiment to determine whether or not Kandel's PKA process is also present in the crayfish neuromuscular junction. We selectively inhibited the PKA process with N-[2-brommocinnamylamino] ethyl]-5 (H-89) and introduced 5-HT into the extracellular solution, measuring EPSPs to determine if the synaptic facilitation normally increased by 5-HT would be reduced. Our results show that the PKA process does in fact exist in the crayfish muscle neurons, since H-89 reduced the otherwise substantial 5-HT-triggered EPSP enhancement to insignificance. This suggests that Kandels discoveries in 5-HT-regulated stimulus-response relationships apply to organisms other than Aplysia and suggests that further 5-HT studies on crayfish are relevant in the broader field of research on serotonin and its synaptic effects.