Selective Blockers of Persistent Na Current

Low concentrations of local anesthetics block persistent current selectively and thus are far more effective against ectopic activity than in blocking normal action potential transmission (e.g., Devor et al., 1992). This selectivity arises because of channel-state-dependent drug binding. Block by charged local anesthetics and other anticonvulsants, such as carbamazepine, have an open-channel requirement, and hence show "use-dependence." Individual channels that are predisposed to open, and subsequently reopen, near the resting potential are affected by local anesthetic at concentrations an order of magnitude less than that effective against the much larger transient current present in the same neuron (Baker, 2000b). The uncharged local anesthetic, benzocaine, appeared to be able to block closed persistent channels in large diameter DRG neurons, whereas the transient channels, even in the continuous presence of drug, had to be activated before block became evident (Baker, 2000b). Block of persistent Na+ current, as predicted from its negative voltage dependence, has a major effect on neuronal excitability (Fig. 4).

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Figure 4 Low-threshold persistent Na+ current in a large DRG neuron contributes to excitability and is selectively blocked by the local anesthetic benzocaine. (A) large diameter neuron exhibits subthreshold and suprathreshold responses to incrementing applied currents (left hand panel). Superfusion of 250 |M benzocaine (that blocks persistent Na+ current by more than 90%) reversibly increases the current threshold, but fails to prevent action potential generation (center two panels). A matched concentration of TTX, expected to block persistent Na+ current to a closely similar degree, does not select between transient and persistent Na+ current and anesthetizes the neuron (right hand panel). The neuron is inexcitable even when the stimulus current is increased to 8.25 nA. (B) In another large-diameter neuron 250 |M benzocaine eliminates the contribution of persistent Na+ current to the subthreshold electrotonic responses to applied currents, thus reducing them in amplitude. Left hand panel control recordings, right hand panel in the presence of benzocaine. (Reproduced, with permission, from Baker, 2000b.)

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Figure 4 Low-threshold persistent Na+ current in a large DRG neuron contributes to excitability and is selectively blocked by the local anesthetic benzocaine. (A) large diameter neuron exhibits subthreshold and suprathreshold responses to incrementing applied currents (left hand panel). Superfusion of 250 |M benzocaine (that blocks persistent Na+ current by more than 90%) reversibly increases the current threshold, but fails to prevent action potential generation (center two panels). A matched concentration of TTX, expected to block persistent Na+ current to a closely similar degree, does not select between transient and persistent Na+ current and anesthetizes the neuron (right hand panel). The neuron is inexcitable even when the stimulus current is increased to 8.25 nA. (B) In another large-diameter neuron 250 |M benzocaine eliminates the contribution of persistent Na+ current to the subthreshold electrotonic responses to applied currents, thus reducing them in amplitude. Left hand panel control recordings, right hand panel in the presence of benzocaine. (Reproduced, with permission, from Baker, 2000b.)

In small-diameter DRG neurons, |-conotoxin PIIIA selectively blocks TTX-s currents and can be used to discriminate different subtypes of Na+ channel (Safo et al., 2000). The toxin also blocks transient Na+ current in isolated hippocampal neurons (Nielsen et al., 2002). In experiments in which hippocampal neurons were exposed to a truncated analog of PIIA (PIIIA-(2-22)), the persistent Na+ current found in these neurons was preferentially blocked by modified toxin (by 70 % with 1 |M), implying a submicromolar IC50 for the persistent current (Nielsen et al., 2002). When activated from a negative holding potential of -80 mV, the transient current amplitude in the same neurons was unaffected at 1 |M, but reduced close to 80% by 10 |M. This selectivity for persistent current is intriguing because it shows that the truncated toxin is able to discriminate between persistent and transient Na+ channels. For this reason conotoxins or their derivatives may represent a novel pharmacological approach to the control of ectopic activity that is dependent on TTX-s persistent Na+ current.

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