Mentor: Peter Brodfuehrer
In all organisms, including humans, rhythmic/repetitive behaviors such as walking are generated by central pattern generators (CPG), or networks of neurons (nerve cells). In the medicinal leech, swimming, a rhythmic behavior, is produced by a CPG. Swimming involves episodic sinusoidal movement caused by the alternating contraction of longitudinal muscles (muscles that run the length of the leech) on both the dorsal and ventral surfaces. When the central nervous system is isolated (i.e. the body wall has been removed), the fictive motor pattern for swimming can be produced by electrical stimulation of a peripheral nerve. Interestingly, the length of an elicited swim bout can differ from one bout to the next. The Brodfuehrer lab is currently investigating the neuronal mechanisms that influence the length of these swim episodes.
It has been hypothesized that a positive feedback loop drives the swim CPG and that cell 204, a swim-maintenance interneuron that directly excites several other CPG interneurons (known as oscillators) is a component of this positive feedback loop. The existence of a positive feedback loop indicates that as the activity of cell 204 increases, it activates oscillators to which it is coupled; as the activity of the oscillators increases, the activity of cell 204 also increases, and so on. This raises an interesting question: if a positive feedback loop is involved in generating swimming, why does swimming stop? Previous results have shown that blocking excitatory inputs (those signals that induce behavior) to a portion of the leech central nervous system, specifically segmental ganglion (a mass of cells) 10, decreases the length of the swim bouts elicited by peripheral nerve stimulation (the stimulation of nerves on another ganglion), supporting the hypothesis that a positive feedback loop is at work. My research this summer focuses on determining what factors affect swim bout duration and whether a positive feedback system exists in ganglia other than ganglion 10.
In an attempt to identify cells involved in controlling swimming, we will use electrical stimulation of both peripheral nerves (known as DP nerves) and individual cells in conjunction with an activity-dependent dye, Neurobiotin, tagged with fluorescently tagged anti-bodies. Theoretically, those cells active in the neural pathway for swimming will fluoresce because they will have taken-up the tagged Neurobiotin after being activated by the stimulation of a cell or nerve involved in the pathway. We will apply DNQX (6,7-Dinitroquinoxaline-2,3-dione), a non-NMDA receptor antagonist, to determine whether a positive feedback loop is present in a ganglion. The excitation of neurons vital to leech swimming is mediated by the binding of glutamate—a neurotransmitter (a chemical known to relay signals from one neuron to the next)—to non-NMDA receptors. DNQX binds to the non-NMDA receptors, inhibiting the binding of glutamate. If there is no decrease in swim bout length in the ganglion to which DNQX was applied, then that ganglion does not contain a positive feedback loop for swimming.
The nervous systems of vertebrates and invertebrates are more alike than most expect. Hopefully, with the exploration of central pattern generators vital to the everyday functionality of the leech, researchers can gain insight into the networks of neurons that affect behavior in humans.