Sensitization of group IV muscle receptors to bradykinin by 5-hydroxytryptamine and prostaglandin E2.

The aim of the study was to find out whether endogenous substances with a presumed sensitizing action on nociceptors alter the chemical excitability of muscle receptors with unmyelinated afferent fibres. In anaesthetized cats, the discharges of single group IV units in response to intra-arterial injections of bradykinin were evaluated quantitatively and the influence of 5-hydroxytryptamine (5-HT) and prostaglandin E2 (PGE2) on the response magnitude determined. Both 5-HT and PGE2 enhanced the bradykinin sensitivity of most of the muscle receptors, i.e. the receptors were sensitized to bradykinin by 5-HT and PGE2. Units that were activated by bradykinin before administration of the sensitizing chemicals showed an increase in response magnitude; in receptors not responding to the standard dose of bradykinin an activation often occurred after chemical sensitization. It is occurred that under the influence of elevated tissue levels of 5-HT and PGE2 the afferent impulse activity induced in group IV muscle receptors by bradykinin will be higher. As the substances used are released together from pathologically altered organs and since many of the group IV muscle receptors are considered to be nociceptive, the chemical sensitization of these receptors to the algesic agent bradykinin might play a role in the production of pain in an inflamed or injured muscle.