Direct inhibition of Ih by analgesic loperamide in rat DRG neurons.
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 microM 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 microM 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 microM 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.
Vasilyev
Dmitry V
DV
Discovery Neuroscience, Wyeth Research, CN 8000, Princeton, NJ 08543-8000, USA. vasylyd@wyeth.com
Shan
Qin
Q
Lee
Yan
Y
Mayer
Scott C
SC
Bowlby
Mark R
MR
Strassle
Brian W
BW
Kaftan
Edward J
EJ
Rogers
Kathryn E
KE
Dunlop
John
J
D000700:Analgesics / Q000494:pharmacology*; D000818:Animals; D002478:Cells, Cultured; D054815:Cyclic Nucleotide-Gated Cation Channels; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D004347:Drug Interactions; D004558:Electric Stimulation; D005727:Ganglia, Spinal / Q000166:cytology*; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D008139:Loperamide / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009433:Neural Inhibition / Q000187:drug effects*; D009474:Neurons / Q000187:drug effects*; D018408:Patch-Clamp Techniques / Q000379:methods; D015221:Potassium Channels / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D000700:Analgesics; D054815:Cyclic Nucleotide-Gated Cation Channels; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D009292:Narcotic Antagonists; D015221:Potassium Channels; D009270:Naloxone; D008139:Loperamide