Identification of opioid receptors in the sympathetic and parasympathetic nerves of guinea‐pig atria

The opioid receptor subtypes of autonomic nerves of guinea-pig atria were elucidated by monitoring the effects of selective opioid receptor agonists on the negative and positive inotropic responses associated with the stimulation of the parasympathetic and sympathetic nerves, respectively. The positive inotropic effect, evoked by electrical field stimulation (2 Hz) was strongly reduced by the selective OP2-opioid receptor agonists U-50488 and U-69593, but partly by the OP3-opioid receptor agonist morphine. This effect of U-50488 and U-69593 were reversed by the selective OP2-opioid receptor antagonist nor-BNI. The effect of morphine was partly reversed by naloxone, whereas OP1-opioid receptor agonists, BW373 U86 and DPDPE, were ineffective. On the other hand, the negative inotropic response to electrical field stimulation was not affected by opioid receptor agonists. These results suggest that the noradrenaline release from cardiac sympathetic nerves of guinea-pig could be modulated, mainly by the OP2-opioid receptor, however, the acetylcholine release from cardiac parasympathetic nerves is not modulated by opioid receptors.