Naloxone elicits an excitatory response in the morphine-tolerant mouse vas deferens

Manifestations of acute opiate withdrawal contracture in rabbit jejunum after mu-, kappa- and delta-receptor agonist exposure

1. Following a 5 min in vitro exposure to morphine (1.3 x 10(-7) M), U-50,488H (2.5 x 10(-8) M) and deltorphin (1.6 x 10(-8)-6.5 x 10(-9) M), the rabbit isolated jejunum exhibited a precipitated contracture after the addition of naloxone (2.75 x 10(-7) M). 2. The precipitated responses to U-50,488H and deltorphin but not to morphine were reproducible in the same tissue. 3. The precipitated contractures were blocked completely by tetrodotoxin (3 x 10(-7) M), partially by atropine (1.5 x 10(-7) M) and not affected by hexamethonium (1.4 x 10(-5) M). 4. Naloxone administration (2.75 x 10(-7) M) before the agonist prevented the development of the adaptive response to morphine and U-50,488H but not to deltorphin. 5. The selective antagonists norbinaltorphimine (2.7 x 10(-8)-2.7 x 10(-9) M) and naltrindole (1.1 x 10(-7) M) prevented the adaptive response development only to the respective agonists. 6. The opioid agonists partially inhibited the spontaneous activity of the tissue. This study has shown that independent activation of mu-, kappa- and delta-opioid receptors can induce dependence in this isolated tissue. Rabbit jejunum is a suitable tissue for studying the acute effects of opioids on the adaptative processes determined by their administration.