Is the cutaneous silent period an opiate-sensitive nociceptive reflex?

In humans, high-intensity electrical stimuli delivered to the fingers induce an inhibitory effect on C7-T1 motoneurons. This inhibitory reflex, called the cutaneous silent period (CSP) is considered a defense response specific for the human upper limbs. It is not clear whether the CSP-like other defense responses such as the corneal reflex and the R III reflex-is an opiate-sensitive nociceptive reflex. Because opiates suppress some, but not all, nociceptive reflexes, we studied the effect of the narcotic-analgesic drug fentanyl on the CSP and the R III reflex. The CSP was recorded from the first dorsal interosseous (FDI) muscle in seven normal subjects during voluntary contraction, before and 10 and 20 min after fentanyl injection. To assess possible fentanyl-induced changes, we also tested the effect of finger stimulation on motor evoked potentials (MEPs) elicited in the FDI muscle by transcranial magnetic stimulation before and after fentanyl injection. Fentanyl-induced changes were also studied on the R III reflex recorded from the biceps femoris muscle. Fentanyl, as expected, suppressed the R III reflex but failed to change the inhibitory effect of finger stimulation on FDI motoneurons. Finger stimulation reduced the size of MEPs in the FDI, and fentanyl injection left this inhibitory effect unchanged. The differential fentanyl-induced modulation of the CSP and R III reflex provides evidence that the CSP circuit is devoid of mu-opiate receptors and is therefore an opiate-insensitive nociceptive reflex, which may be useful in the assessment of central-acting, non-opioid drugs.