EFFECTS OF CALCIUM-CHANNEL BLOCKERS ON SPINO-BULBO-SPINAL REFLEXES EVERTED BY ACTIVATION OF THE HIGH-THRESHOLD AND LOW-THRESHOLD SOMATIC AFFERENTS

Effects of the calcium channel blockers nifedipine and verapamil on spino-bulbo-spinal (SBS) reflexes were studied in rats anesthetized with a mixture of urethane and chloralose; the reflexes evoked by activation of the high-threshold and low-threshold (tactile) somatic afferents were tested. The blockers were microinjected into the n. reticularis gigantocellularis, the main structure where the SBS reflexes are relayed. Nifedipine (10(-4) M) evoked fast (with a latency of 1-2 min) and significant (20%-60%) suppression of both the high-threshold and low-threshold SBS reflexes. The reflexes recovered within 40 to 90 min. Lower concentrations of nifedipine (10(-5) M) either did not modify both types of the SBS reflexes (in 25% of experiments) or facilitated these reflexes by 16%-40% within 15-20 min. Microinjections of verapamil affected the SBS reflexes in a similar manner, but the effects were shorter. Possible ways by which the L-type calcium channels are involved in the transmission of afferent impulsation of different modalities (nociceptive and non-nociceptive) are discussed.