Received for publication May 5, 2004.
Revised July 23, 2004.
Accepted for publication July 26, 2004.

5-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

Abstract

T-type Ca²⁺ channels are believed to play an important role in pain perception, and anesthetic steroids like alphaxalone and allopregnanolone, which have a 5-configuration at the steroid A, B ring fusion, are known to inhibit T-type Ca²⁺ channels and cause analgesia in a thermal nociceptive model (Todorovic et al., 2003a). To define further the structure-activity relationships for steroid analgesia, we synthesized and examined a series of 5-reduced steroids for their ability to induce thermal anti-nociception in rats when injected locally into the peripheral receptive fields of the nociceptors and studied their effects on T-type Ca²⁺ channel function in vitro. We found that most of the steroids completely blocked T-type Ca²⁺ currents in vitro with IC₅₀s at a holding potential of -90 mV ranging from 2.8 to 40 µM. T current blockade exhibited mild voltage-dependence suggesting that 5-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 near maximal blockade of T currents. All tested compounds induced dose-dependent analgesia in thermal nociceptive testing with the most potent effect (ED₅₀ 30 ng/ 100 µL) obtained with a compound [(3, 5, 17)-3-hydroxyandrostane-17-carbonitrile, 3OH] that was also the most effective blocker of T currents. In comparison with previously studied 5-reduced steroids, these 5-reduced steroids are more efficacious blockers of neuronal T-type Ca²⁺ channels and are potentially useful as new experimental reagents for understanding the role of neuronal T-type Ca²⁺ channels in peripheral pain pathways.

Key words: Ion channel regulation, Calcium (Votage-Gated Channels), Structure-activity relationships and modeling

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