The Mechanistic Action of Carbon Dioxide on a Neural Circuit and NMJ Communication

Previous studies examining behavioral responses to CO2 revealed that high [CO2] acts as a natural repellent in a concentration dependent manner for crayfish. Physiologically, CO2 can rapidly block the autonomic responses in heart rate, as well as, inhibit an escape tail flip reflex in crayfish. Here, we demonstrate that the behavioral observations can be mechanistically explained by CO2 blocking glutamate receptors at the neuromuscular junction and through inhibition of recruiting motor neurons within the CNS. The effects are not mimicked with a lower pH in the bathing solution. Since spontaneous and sensory-evoked activities in the sensory root and motor neurons are reduced by CO2, this is an anesthetic effect. We propose this is due to blockage of electrical synapses, as well as, some of the central glutamatergic-drive. We used agonists and antagonists (glutamate, nicotine, domoic acid, cadmium, heptanol) to various synaptic inputs, which are possibly present in the ventral nerve cord (VNC). Results from these chemicals supported the idea that there is electrical as well as chemical drive within the circuit that can modulate intrinsic as well as sensory evoked activity in the motor neurons. We have documented that CO2 has actions in the periphery as well as in the CNS, to account for the behavioral responses previously shown. Furthermore, we document that gap junctions as well as glutamatergic synapses are potential targets. This study also aids in the dissection of a neural circuitry within the VNC that drives spontaneous and sensory evoked activity of the superficial flexor motor neurons. J. Exp. Zool. 319A:340-354, 2013. (c) 2013 (c) 2013 Wiley Periodicals, Inc.