Inhibition of presynaptic alpha-2-adrenoceptor and opioid receptor agonist responses in the rat vas deferens by chronic imipramine treatment.

The effects of chronic imipramine administration on agonist responses in rat isolated smooth muscle preparations were investigated. The administration of 20 mg/kg imipramine two times a day for 4 and 11 days resulted in an equivalent subsensitivity (approximately 8-fold) of clonidine-induced inhibition of electrically evoked contractions in the rat vas deferens (presynaptic alpha 2-adrenoceptor response). Imipramine (4 days) resulted in a marked inhibition of the ability of [D-Ala2, D-Leu5] enkephalin to decrease electrically evoked contractions of the vas deferens (presynaptic opioid receptor response) but did not significantly affect the carbachol-induced increase in electrically evoked contractions (muscarinic receptor response). In the absence of cocaine the contractile effects of norepinephrine and tyramine in the vas deferens were, respectively, potentiated and inhibited, following imipramine (4 days), suggesting a decrease in the activity of the neuronal uptake mechanism. When determined in the presence of cocaine, the potency of the postsynaptic effects of norepinephrine in the vas deferens (alpha 1-adrenoceptor response) was not significantly altered by imipramine (4 days). With regard to other postsynaptic receptors, imipramine (4 days) decreased slightly the potency of phenylephrine in the aorta (alpha 1-adrenoceptor response) and increased slightly the potency of carbachol in the trachea (muscarinic receptor response) and the potency of serotonin in the rat aorta (5HT2-receptor response). Thus, chronic imipramine administration decreased the potency of presynaptic alpha 2- and opioid agonist responses in the vas deferens but caused very little or no changes in the potencies of agonists at postsynaptic sites.