Direct Inhibition of Ih by Analgesic Loperamide in Rat DRG Neurons

• Published in: Journal of neurophysiology Vol. 97; no. 5; p. 3713
• Main Authors: Vasilyev, Dmitry V, Shan, Qin, Lee, Yan, Mayer, Scott C, Bowlby, Mark R, Strassle, Brian W, Kaftan, Edward J, Rogers, Kathryn E, Dunlop, John
• Format: Journal Article
• Language: English
• Published: United States Am Phys Soc 01.05.2007
• Subjects: Analgesics - pharmacology; Animals; Cells, Cultured; Cyclic Nucleotide-Gated Cation Channels; Dose-Response Relationship, Drug; Dose-Response Relationship, Radiation; Drug Interactions; Electric Stimulation; Ganglia, Spinal - cytology; Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; Loperamide - pharmacology; Membrane Potentials - drug effects; Membrane Potentials - physiology; Membrane Potentials - radiation effects; Naloxone - pharmacology; Narcotic Antagonists - pharmacology; Neural Inhibition - drug effects; Neurons - drug effects; Patch-Clamp Techniques - methods; Potassium Channels - metabolism; Rats; Rats, Wistar
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.00841.2006

1 Discovery Neuroscience and 2 Chemical and Screening Sciences, Wyeth Research, Princeton, New Jersey Submitted 10 August 2006; accepted in final form 18 March 2007 Hyperpolarization-activated cyclic nucleotide–gated (HCN) channels are responsible for the functional hyperpolarization-activated current ( I h ) in dorsal root ganglion (DRG) neurons, playing an important role in pain processing. We found that the known analgesic loperamide inhibited I h channels in rat DRG neurons. Loperamide blocked I h in a concentration-dependent manner, with an IC 50 = 4.9 ± 0.6 and 11.0 ± 0.5 µM for large- and small-diameter neurons, respectively. Loperamide-induced I h inhibition was unrelated to the activation of opioid receptors and was reversible, voltage-dependent, use-independent, and was associated with a negative shift of V 1/2 for I h steady-state activation. Loperamide block of I h was voltage-dependent, gradually decreasing at more hyperpolarized membrane voltages from 89% at –60 mV to 4% at –120 mV in the presence of 3.7 µM loperamide. The voltage sensitivity of block can be explained by a loperamide-induced shift in the steady-state activation of I h . Inclusion of 10 µM loperamide into the recording pipette did not affect I h voltage for half-maximal activation, activation kinetics, and the peak current amplitude, whereas concurrent application of equimolar external loperamide produced a rapid, reversible I h inhibition. The observed loperamide-induced I h inhibition was not caused by the activation of peripheral opioid receptors because the broad-spectrum opioid receptor antagonist naloxone did not reverse I h inhibition. Therefore we suggest that loperamide inhibits I h by direct binding to the extracellular region of the channel. Because I h channels are involved in pain processing, loperamide-induced inhibition of I h channels could provide an additional molecular mechanism for its analgesic action. Address for reprint requests and other correspondence: D. Vasilyev, Discovery Neuroscience, Wyeth Research, CN 8000, Princeton, NJ 08543-8000 (E-mail: vasylyd{at}wyeth.com )