Effects of Dopamine on Synaptic Strength of Heart Interneurons 4 and 7 on Heart Motor Neurons 8 and 12 in Segmental Ganglia of the Leech (Hirudo Species)

To better understand the neural mechanisms for human physiology, an invertebrate’s control on its cardiovascular system should be investigated. The leech’s heartbeat is controlled by segmental ganglia and has a phase delay in its peristaltic heartbeat pattern. This thesis hypothesized a neuromodulator, dopamine, can disrupt the phase delay by affecting inhibitory post synaptic potentials (IPSP). We dissected the leech, isolated a chain of ganglia onto a petri dish, and placed it onto a rig to record synaptic potentials. We recorded and analyzed the ganglia when submersed in saline, dopamine, and saline rinse. We looked at HN4/HN7 phase delay on HE8/HE12 of segmental ganglia. Our results showed dopamine had no effect on any HN/HE pair, except HN 7 and HE 8, HN 4 and HE 8, and HN 7 and HE 12. For HN 7 and HE 8, there was a significant difference in spike-triggered average, area under the IPSP curve, and resting potential between control and rinse. Moreover, there was a significant difference in area under the IPSP curve between control and saline rinse. HN4/HE8 and HN7/HE12 had a significant difference in IPSP size before dopamine and during saline rinse. Overall, the weak synapse between HN 7 and HE 8 might not be due to dopamine because the dopamine effect was not seen in the stronger synapses before and during dopamine (HN4/HE8 and HN7/HE12). We cannot fully conclude dopamine has a significant effect on the synaptic strength between HNs and HEs without more research done.