5beta-reduced neuroactive steroids are novel voltage-dependent blockers of T-type Ca2+ channels in rat sensory neurons in vitro and potent peripheral analgesics in vivo.
T-type Ca(2+) channels are believed to play an important role in pain perception, and anesthetic steroids such as alphaxalone and allopregnanolone, which have a 5alpha-configuration at the steroid A, B ring fusion, are known to inhibit T-type Ca(2+) channels and cause analgesia in a thermal nociceptive model (Soc Neurosci Abstr 29:657.9, 2003). To define further the structure-activity relationships for steroid analgesia, we synthesized and examined a series of 5beta-reduced steroids for their ability to induce thermal antinociception in rats when injected locally into the peripheral receptive fields of the nociceptors and studied their effects on T-type Ca(2+) channel function in vitro. We found that most of the steroids completely blocked T-type Ca(2+) currents in vitro with IC(50) values at a holding potential of -90 mV ranging from 2.8 to 40 microM. T current blockade exhibited mild voltage-dependence, suggesting that 5beta-reduced neuroactive steroids stabilize inactive states of the channel. For the most potent steroids, we found that other voltage-gated currents were not significantly affected at concentrations that produce nearly maximal blockade of T currents. All tested compounds induced dose-dependent analgesia in thermal nociceptive testing; the most potent effect (ED(50), 30 ng/100 microl) obtained with a compound [(3beta,5beta,17beta)-3-hydroxyandrostane-17-carbonitrile] that was also the most effective blocker of T currents. Compared with previously studied 5alpha-reduced steroids, these 5beta-reduced steroids are more efficacious blockers of neuronal T-type Ca(2+) channels and are potentially useful as new experimental reagents for understanding the role of neuronal T-type Ca(2+) channels in peripheral pain pathways.
Todorovic
Slobodan M
SM
Department of Anesthesiology, University of Virginia School of Medicine, Charlottesville, VA, USA.
Pathirathna
Sriyani
S
Brimelow
Barbara C
BC
Jagodic
Miljen M
MM
Ko
Seong-Hoon
SH
Jiang
Xin
X
Nilsson
Kent R
KR
Zorumski
Charles F
CF
Covey
Douglas F
DF
Jevtovic-Todorovic
Vesna
V
D000700:Analgesics / Q000494:pharmacology* / Q000627:therapeutic use; D000818:Animals; D002121:Calcium Channel Blockers / Q000737:chemistry / Q000494:pharmacology* / Q000627:therapeutic use; D020747:Calcium Channels, T-Type / Q000378:metabolism*; D004195:Disease Models, Animal; D004594:Electrophysiology; D005260:Female; D005727:Ganglia, Spinal / Q000166:cytology; D008297:Male; D009475:Neurons, Afferent / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D010146:Pain / Q000188:drug therapy; D051381:Rats; D017207:Rats, Sprague-Dawley; D013256:Steroids / Q000737:chemistry / Q000494:pharmacology* / Q000627:therapeutic use; D013329:Structure-Activity Relationship
D000700:Analgesics; D002121:Calcium Channel Blockers; D020747:Calcium Channels, T-Type; D013256:Steroids