EFFECT OF BRADYKININ AND PROSTAGLANDINS ON THE RELEASE OF CALCITONIN GENE-RELATED PEPTIDE-LIKE IMMUNOREACTIVITY FROM THE RAT SPINAL-CORD INVITRO

1 The release of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) from the dorsal horn of the rat spinal cord in vitro in response to dorsal root stimulation was measured by radioimmunoassay. 2 Stimulation of the dorsal roots (3 or 4 roots on each side) at 10 Hz for 5 min evoked a mean release (R1) of 134.3 +/- 17.5 (n = 10) fmol CGRP-LI; the release (R2) evoked by a second stimulation period 30 min later under control conditions was 77 +/- 10% (n = 10) of R1. Test compounds were applied to the preparation following release R1, and their effect calculated from the value of R2/R1. 3 Bradykinin (0.01-10 muM) had no significant effect on the basal release of CGRP-LI, but at 0.1-10 muM it increased 2-3 fold the release evoked by dorsal root stimulation. 4 This effect of bradykinin was prevented by indomethacin (10 muM), or by the B2-receptor antagonist, Hoe140 (1-10 muM). In the presence of Hoe140, bradykinin significantly reduced R2/R1; the explanation for this is not clear. 5 The B1-receptor agonist, Des-Arg9-bradykinin (10 muM), did not affect CGRP-LI release nor was the effect of bradykinin blocked by the B1-receptor antagonist, Des-Arg9-Leu8-bradykinin (10 muM). 6 Various prostaglandins were found to mimic the effect of bradykinin on CGRP-LI release. Their approximate order of potency was prostaglandin D2 (PGD2) = PGE1 > PGF2alpha = PGE2; PGI2 was ineffective at 10 muM. 7 Forskolin (30 muM) and 3-isobutyl 1-methylxanthine (IBMX; 10 muM) also increased the evoked release of CGRP-LI. 8 It is concluded that bradykinin acts on B2-receptors in the spinal cord, causing the formation of prostanoids, which in turn cause an enhancement of neuropeptide release from primary afferent nerve terminals in the dorsal horn. This effect may be secondary to activation of adenylate cyclase. Because B2-receptors are mainly associated with primary afferent nerve terminals, it is likely that prostanoid production is also a function of these structures. Whether this action of bradykinin has any physiological function in nociceptive transmission remains unclear.