Carbenoxolone induced depression of rhythmogenesis in the pre-Botzinger Complex

Background: Carbenoxolone (CBX), a gap junction uncoupler, alters the functioning of the pre-Botzinger Complex (preBotC), a central pattern generating neuronal network important for the production of respiratory rhythm in mammals. Even when isolated in a 1/2 mm-thick slice of medulla oblongata from neonatal mouse the preBotC continues producing periodic bursts of action potentials, termed population bursts that are thought to be important in generating various patterns of inspiration, in vivo. Whether gap junction communication contributes to preBotC rhythmogenesis remains unresolved, largely because existing gap junction uncouplers exert numerous non-specific effects (e. g., inhibition of active transport, alteration of membrane conductances). Here, we determined whether CBX alters preBotC rhythmogenesis by altering membrane properties including input resistance (R-in), voltage-gated Na+ current (I-Na), and/or voltage-gated K+ current (I-K), rather than by blocking gap junction communication. To do so we used a medullary slice preparation, network-level recordings, whole-cell voltage clamp, and glycyrrhizic acid (GZA; a substance used as a control for CBX, since it is similar in structure and does not block gap junctions). Results: Whereas neither of the control treatments [artificial cerebrospinal fluid (aCSF) or GZA (50 mu M)] noticeably affected preBotC rhythmogenesis, CBX (50 mu M) decreased the frequency, area and amplitude of population bursts, eventually terminating population burst production after 45 60 min. Both CBX and GZA decreased neuronal R-in and induced an outward holding current. Although neither agent altered the steady state component of I-K evoked by depolarizing voltage steps, CBX, but not GZA, increased peak I-Na. Conclusion: The data presented herein are consistent with the notion that gap junction communication is important for preBotC rhythmogenesis. By comparing the effects of CBX and GZA on membrane properties our data a) demonstrate that depression of preBotC rhythmogenesis by CBX results from actions on another variable or other variables; and b) show that this comparative approach can be used to evaluate the potential contribution of other nonspecific actions (e. g., Ca++ conductances or active transport) of CBX, or other uncouplers, in their alteration of preBotC rhythmogenesis, or the functioning of other networks.