Dopamine effects on inhibitory heart interneurons 4 and 7 in segmental ganglia of Hirudo species

The ability to understand the neurophysiological mechanisms that influence any behavior to occur in an organism is the ultimate goal of neurobiology. Neuroscience researchers want to create a neuronal model of the human brain to know how we can produce physiological functions such as walking, throwing, and jumping. Vertebrates have neural structures that are too complex to map neuronal models, so we must start at the invertebrate level. Leeches are simple invertebrates who possess the necessary characteristics to study neuronal regulation. Easily identifiable neurons in the leech allow researchers to record action potential burst activity and evaluate the mechanisms behind physiological functions. Central pattern generators (CPGs) are the “brain” of the leech and are known to control the rhythmic action potential bursting activity that cause certain behaviors to occur. The focus of this investigation was the CPGs that regulate the heart contractions in leeches. Inhibitory heart interneurons 4 (HN(4)) and 7 (HN(7)) within this system of CPGs influence burst activity on heart motor neurons. These heart motor neurons regulate contractions of the heart vessels. Dopamine is a known neuromodulator that either excite or inhibit the bursting of action potentials to produce a variety of behaviors in organisms. The overall goal in this experiment was to assess the effects of dopamine on HN(4) and HN(7) to further establish a model for neuronal regulation in leeches. Response variables of timing and strength of the action potential bursts were calculated before and after different concentrations of dopamine treatment were applied on HN(4) and HN(7). The treatment of dopamine significantly increased the number of spikes in an action potential burst in HN(4) and HN(7), and altered the rhythmic pattern timing of bursting in HN(7). Future experiments will enable researchers to distinguish if these specific inhibitory interneurons regulate the heart contraction by themselves or if the heart motor neurons play the dominate role with a concentration of 1x10-6M dopamine.