A new technique for the study of vascular presynaptic receptors in freely moving rats.

A technique is described in which a silicon catheter is utilized for permanent catheterization of the portal vein, and curved platinum stimulation electrodes around this vein are used for the stimulation of the local nervous plexus. The method has a success ratio of over 70% during a period of more than 2 mo. Five animals were used in experiments over a period longer than 5 mo. Using this technique, we investigated the influence of variation of current density, pulse duration, and frequency of stimulation on the electrically evoked release of norepinephrine in portal blood. Enhancement of all stimulation parameters proportionally increased the evoked norepinephrine release. The alpha 2-adrenoceptor antagonist yohimbine was found to increase basal norepinephrine levels 4.43-fold and facilitated the evoked release additionally with a factor of 4.70. The results show that noradrenergic neurotransmission in the portal vein nervous plexus is controlled presynaptically by a strong local alpha 2-adrenergic autoinhibition. The method offers the means to explore locally the presynaptic modulation of endogenous neurotransmitter release in unrestrained unanesthetized animals.