Research reportBlock of sodium currents in rat dorsal root ganglion neurons by diphenhydramine
Introduction
A variety of adverse reactions are associated with local anesthetics, some of which are thought to be allergic. Local anesthetics are classified as ester or amide types according to their chemical structures. Esters are associated with a higher incidence of allergic reactions. Patients who are allergic to one type of local anesthetics can be treated with the other. In the instance that hypersensitivity to both ester and amide local anesthetics occurs, then alternative therapies are required [8].
Diphenhydramine is one of the first-generation histamine H1 receptor antagonists. It is widely used in various allergic conditions such as rhinitis, urticaria and conjunctivitis, and in motion sickness. In addition diphenhydramine has been recommended as an alternative local anesthetic among others for patients claiming allergy to lidocaine and its chemical analogues. In clinical studies diphenhydramine has been proven to be as effective as lidocaine for achieving local anesthesia without clinically noticeable complications, although injection is more painful [10], [12], [19].
Local anesthetics exert their effect by preventing the generation and the conduction of the sensory nerve impulse. The nerve conduction block by local anesthetics is mainly caused by their inhibition of voltage-gated sodium channels in the nerve membrane [6]. Likewise the local anesthetic activity of diphenhydramine may arise from sodium channel inhibition. Indeed the interaction of diphenhydramine with sodium channels has been reported. Batrachotoxin (BTX) is a depolarizing agent that causes persistent activation of sodium channels at the resting membrane potential by altering the voltage sensitivity of both activation and inactivation of the sodium channels [5]. Diphenhydramine inhibited BTX-elicited sodium influx, BTX-induced depolarization and the binding of BTX to sodium channels in guinea pig cerebral cortical synaptoneurosomes [7], [17]. Recently, it was demonstrated that diphenhydramine inhibited sodium currents in hippocampal neurons by binding preferentially to the inactivated channels [16].
Dorsal root ganglion (DRG) neurons are primary sensory neurons and transmit sensory information from peripheral regions to the central nervous system. Two distinct sodium currents are expressed in DRG neurons on the basis of differential sensitivity to a neurotoxin, tetrodotoxin (TTX) [1], [4], [9], [15], [22], [23]. One is a rapidly activating and inactivating TTX-sensitive (TTX-S) sodium current, and the other is a slowly activating and inactivating TTX-resistant (TTX-R) sodium current. TTX-R sodium currents are predominantly expressed in small capsaicin-sensitive C- and Aδ-neurons, which are important in nociceptive mechanisms [2], [18].
Even though the local anesthetic property of diphenhydramine is well known, the electrophysiological characteristics of its action on sodium currents in sensory neurons have not been established. The present study characterized the effects of diphenhydramine on the electrophysiological properties of TTX-S and TTX-R sodium currents in acutely dissociated rat DRG neurons. It was found that diphenhydramine inhibited both types of sodium currents in a dose-and use-dependent manner.
Section snippets
Cell preparation
Cells were prepared from DRG of rats (2–6 days postnatal) as described previously [21]. Rats were anesthetized with isoflurane and the vertebral column was removed and cut longitudinally. Ganglia were plucked from between the vertebrae of the spinal column. The ganglia were incubated at 36°C first in a solution of collagenase (1.25 mg/ml, type II-S, Sigma Chemical Co., St. Louis, MO) for 15 min, and then in a solution of trypsin (2.5 mg/ml, Type IX, Sigma) for 10 min in Ca2+- and Mg2+-free
Block of sodium currents by diphenhydramine
Typical TTX-S and TTX-R sodium currents are shown in Fig. 1. The currents were evoked by step depolarizations to 0 mV from a holding potential of −80 mV. TTX-S sodium currents exhibited much faster time courses of activation and inactivation than TTX-R sodium currents. Both types of sodium currents were blocked after bath application of diphenhydramine. Fig. 1A and B show the time course of diphenhydramine block of TTX-S and TTX-R sodium currents, respectively.
The current amplitude decreased
Discussion
Diphenhydramine caused a reversible blockade of TTX-S and TTX-R sodium currents in rat DRG neurons. The Kd values for the current block were calculated to be 48 and 86 μM for TTX-S and TTX-R sodium currents, respectively, at a holding potential of −80 mV. Thus TTX-S sodium channels appear to be more sensitive to diphenhydramine than TTX-R sodium channels. However, when the holding potential was lowered to remove the effect of diphenhydramine on the steady-state inactivation of the channels, the
Acknowledgements
This work was supported by the Research Grant of Chung-Ang University College of Medicine Alumni Association in 1999 to J.-H.S.
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