Alpha-adrenoceptors and facilitation at a sympathetic neuroeffector junction

Excitatory junction potentials were recorded from the mouse vas deferens following 5 stimuli at 1Hz and a single stimulus at times from 100 msec to 9 sec later. E.j.ps facilitate and facilitation decayed over a biexponential time course, with time constants of 120 msec and 4.3 sec. Clonidine (10(-8)-10(-7) M) depressed e.j.p. amplitude, and also accelerated both phases of the decay of facilitation. Yohimbine (10(-7) M) and piperoxan (10(-7)-10(-6) M) increased the amplitude of all but the first e.j.p. in a train, and slowed both phases of the decay of facilitation. One way in which prejunctional modulation by alpha-adrenoceptors may occur is therefore by modifying the carry-over of facilitation from one stimulus to the next.