Cyclic Guanosine Monophosphate Modulates Locomotor Acceleration Induced by Nitric Oxide but not Serotonin in Clione limacina Central Pattern Generator Swim Interneurons.

Central pattern generators (CPGs) are networks of neurons that are capable of creating rhythmic movements (Delcomyn 1980; Marder and Bucher 2001; Clarac and Pearlstein 2007; Mullins et al. 2011). Studies of CPGs span the animal phyla and include nematodes (Karbowski et al. 2008), arthropods (Selverston 2005; Vierk et al. 2009, Marder 2010), annelids (Eisenhart et al. 2000; Kristan et al. 2005; Friesen 2010), mollusks (Satterlie 1985; Arshavsky et al. 1998; Straub et al. 2002; Thompson and Watson 2005; Arshavsky et al. 2010; Katz 2010), and chordates (Grillner and Wallen 2010; Horie et al. 2010). A common feature of many CPGs is that they must be pliant so as to transform an animal’s behavior in response to a large number of inevitable environmental conditions. The ability of CPGs to produce behavioral shifts requires neuromodulation that may consist of modification to network, synaptic, and cellular properties of CPG neurons (Dickinson 2006). Neuromodulation may occur through ionotropic receptors that have a direct effect on membrane potential or through signal transduction mechanisms, in which an extracellular neuromodulator binds to metabotropic receptors that are subsequently transduced into chemical signals, called second messengers, within the component neurons that comprise the CPG. Important second messengers include cyclic nucleotides (e.g., cyclic adenosine monophosphae [cAMP] and cyclic guanosine monophosphate [cGMP]), inositol triphosphate and diacylglycerol, and calcium ions (Worley et al. 1987; Kaczmarek and Levitan 1987a; Katz and Clemens 2001; Marder 2012). Some examples of second messenger systems in modulating CPGs include the role of cGMP in inhibiting swimming in Melibe (Newcomb and Watson 2002), the role of cAMP in neuron C2 of the escape swim CPG of Tritonia (Clemens et al. 2007), and the role of cAMP in modifying motor frequency produced by CPG neurons comprising the stomatogastric ganglion of decapod crustaceans (Flamm et al. 1987; Hempel et al. 1996). Second messengers produce their effect in excitable cells by modifying the ionic conductance of excitable cell membranes. Hence, second messengers can affect the membrane potential, the duration and amplitude of action potentials, and the timing of action potentials (Kaczmarek and Levitan 1987b; Hille 1992). Here, we describe the role of cGMP in mediating changes in locomotor speed in the pteropod mollusk, Clione. Clione is negatively buoyant and must maintain its position by an archetypal slow locomotor gait that involves the rhythmic dorsoventral movement of its wing-like parapodia. Swimming in Clione is typically slow and orients the long axis of the animal’s body perpendicular to the water surface. However, swimming speed may increase in response to stimuli applied to the tail during escape behavior or during hunting and feeding (Arshavsky et al. 1985a; Satterlie 1989). Previous experiments have shown that bath application of serotonin (5HT; 1–100 µM) to reduced preparations of Clione increased fictive swim frequency recorded from swim interneurons of the animal’s swim CPG (Panchin et al. 1996; Satterlie et al. 2000; Satterlie and Norekian 2001; Pirtle et al. 2010). Furthermore, 5HT also produces several cellular changes in swim interneurons that include baseline depolarization, reduction in action potential duration (spike narrowing), changes to postinhibitory rebound (PIR; increased amplitude of PIR and decreased latency to peak PIR), and enhanced sag potential amplitude (Satterlie 1991; Satterlie, et al. 2000; Pirtle and Satterlie 2007). In addition to 5HT, nitric oxide (NO) also increases fictive locomotor speed in Clione (Moroz et al. 2000). Here we show that cGMP produces many of the 5HT-induced cellular changes to Clione swim interneurons, and test the hypothesis that both 5HT and NO produce their effects through a cGMP signal transduction mechanism.