2006 Ohio Student Research Forum

Abstract

Dopamine Influences Interactions between Coupled Neuronal Circuits
Ike Iloka and M. Papernov
Case Western Reserve University
Department of Biological Sciences
Mentor: Prof. D. Wood and M. Varrecchia

Rhythmic pattern generating networks underlie behaviors such as breathing or chewing in many different animals. Rhythmic circuits for related behaviors often must be coordinated. Circuits that drive related behaviors can produce motor patterns that are repetitive at very different speeds, despite their coordination. Circuits such as these can be modulated by a variety of neurotransmitters (Bucher and Marder, 2003). This neuromodulation imparts flexibility in rhythmic pattern generation in the neuronal circuit (Nusbaum and Beenhakker, 2002).

We use the stomatogastric nervous system (STNS) of adult crabs (Cancer borealis) to study the effects of modulatory transmitters on two discrete but interrelated circuits: the Pyloric (PR) and Gastric Mill rhythms (GMR). The PR is spontaneously active due to a group of pacemaker neurons that generate the motor patterns for the filtering of food in the crab’s digestive system (Harris-Warrick et al., 1992). The GMR is responsible for the chewing response in the crab’s internal digestive system. The STNS is the ideal model system to be used for the neural circuit study because of its ease of access, the complete identification of neurons located within these circuits, and the significant details known about its overall functions. The PR and GMR in crabs receive input from a known pair of projection neurons, the modulatory commissural neuron 1(MCN1) (Bartos et al., 1999). Activation of MCN1 allows unique GMR and PR speed motor patterns to be produced. During its activation, MCN1 initiates key filtering neurons that influence the rhythmic chewing circuit. But it is also believed that other hormones and transmitters may also play a role in the regulation of the rhythms. Using electrophysiological techniques we selectively released MCN1 transmitter and applied dopamine (DA) to the circuit to examine changes in the individual neurons and in circuit function.

Typically, the (PR) filtering circuit regulates the speed of the GMR chewing circuit. This allows the motor patterns to remain coordinated with one another. Our data shows that when DA is applied during MCN1 activation the chewing frequency of the system slowed considerably in relation to the filtering motor pattern. In conclusion, our data supports the hypothesis that DA modulates key chewing pattern generation neurons and decreases the influence from the filtering circuit (PR). This suggests a mechanism for change in the coordination between these two circuits that likely changes the overall behavior of the system.

Citations
Bartos M., Manor Y., Nadim F., Marder E., and Nusbaum M.P. (1999) Coordination of fast and slow rhythmic neuronal circuits. J. of Neuroscience. 19(15): 6650-6660.

Harris-Warrick R.M., Nagy F., and Nusbaum M.P. (1992) Neuromodulation of stomatogastric networks by identified neurons and transmitters. In: Dynamic Biological Networks: The Stomatogastric Nervous System (Harris-Warrick RM, Marder E, Selverston AI, Moulins M, eds), pp 87-138. Cambridge, MA: MIT Press.

Marder E. and Bucher D. (2001) Central pattern generators and the control of rhythmic movements. Current Biology. 11(23): R986-R996.

Nusbaum M.P. and Beenhakker M.P. (2002) A small-systems approach to motor pattern generation. Nature. 417: 343-350.