The effect of epibatidine on spontaneous and evoked neurotransmitter release in the mouse and guinea pig isolated vas deferens

Background and purpose: Nicotinic agonists increase sympathetic field-stimulus-evoked contraction of the rodent vas deferens, presumably by increasing evoked neurotransmitter release. This presumption was tested in two species. Experimental approach: The effect of the nicotinic acetylcholine receptor (nAChR) agonist epibatidine on neurotransmitter release in mouse and guinea pig isolated vas deferens was investigated using contraction studies and conventional intracellular recording techniques. Key results: In 12 of 14 mouse vasa deferentia, slow bath application of epibatidine (100 nM) had no significant effect on excitatory junction potential (EJP) amplitude and spontaneous EJP (SEJP) frequency. However, rapid application of epibatidine to the mouse vas deferens caused an increase in SEJP frequency (by 530%), with no effect on EJP amplitude. Despite the absence of an effect on EJPs, electrically-evoked contractions of the mouse vas deferens were significantly increased in the presence of epibatidine (by 50%). A transient contraction was reliably induced by a higher epibatidine concentration (1 mu M). This contraction was significantly reduced in the presence of prazosin, tetrodotoxin, or alpha,beta-methyleneATP. Epibatidine did not induce a contraction in the presence of a combination of prazosin, alpha,beta-methyleneATP and cyclopentolate. In guinea pig vasa deferentia, bath-applied epibatidine potentiated EJP amplitude in a biphasic pattern, lasting for at least 30 minutes. Conclusion and implications: The nAChR-mediated augmentation of neurogenic contraction is indeed prejunctional, but in the mouse arises from an increase in spontaneous neurotransmitter release that primes smooth muscle for subsequent contraction, while in the guinea pig there is a direct augmentation of evoked neurotransmitter (ATP) release.