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

Abstract

Previous studies examining behavioral responses to CO(2) revealed that high [CO(2)] acts as a natural repellent in a concentration dependent manner for crayfish. Physiologically, CO(2) 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 CO(2) 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 CO(2), 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 CO(2) 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.

The Effect of CO2, Intracellular pH and Extracellular pH on Mechanosensory Proprioceptor Responses in Crayfish and Crab

“…Exposure to CO2 did not significantly decrease the amplitude of action potentials in motor neurons of the crayfish, as measured with intracellular electrodes. 19 This suggests that the effects of CO2 on the proprioceptors may be primarily on mechanosensory transduction or on the initiation of an action potential. It was noted that crayfish axons did not have an alteration in voltage gated Na + channel activation with changes in pHo.…”

“…CO2 exposure was shown to block glutamate receptors at neuromuscular junctions whereas spontaneous events and evoked events are still present at neuromuscular junctions with low pH induced by other means. 18,19 Thus, any decrease in baseline response in muscle tension from a lack of responding to the spontaneously released glutamate might reduce the basal tension on the sensory endings. Low pHo does produce some depolarization in muscle for insects and crustaceans, 18,19 and thus could produce an increase in force production if the motor nerve was stimulated to produce muscle contraction.…”

“…With the use of ion sensitive electrodes, it has been found that the pHi in crab muscle and squid axons drops to around 6 and 5.7 respectively, when exposed to saline bubbled with CO2. [17][18][19] The pHo also decreases with CO2 bubbling, thus effects on cellular function may be due to extracellular or intracellular changes in pH or directly by CO2. Earlier studies indicate that molecular CO2, not the pHo or pHi associated change, is in part responsible for alterations in synaptic receptivity at the neuromuscular junctions of crayfish and Drosophila malanagaster.…”

“…Earlier studies indicate that molecular CO2, not the pHo or pHi associated change, is in part responsible for alterations in synaptic receptivity at the neuromuscular junctions of crayfish and Drosophila malanagaster. 18,19 On the other hand, the action of protons (H + ) themselves on cellular function is well established from effects on ion channels and pumps to enzymatic cascades. 4 Protons can even increase the sensitivity of sensory neurons by increasing the opening of stretch activated ion channels (SACs), which are commonly used for mechanosensory transduction.…”

Adverse Outcome Pathway on binding of agonists to ionotropic glutamate receptors in adult brain leading to excitotoxicity that mediates neuronal cell death, contributing to learning and memory impairment

“…However, in crayfish (Procambarus clarkia) the same concentration of DomA has no effect in spike activity (Bierbower and Cooper, 2013).…”