Descending inhibition in the rabbit proximal colon, evoked by distension, was studied in vivo by recording extracellularly electrical activity from pressure electrodes placed on the serosa. 2. Distention produced, blow the level of the balloon, a brief hyperpolarization of smooth muscle fibres which could be recorded up to 20 cm from the point of distension. 3. This hyperpolarization like that produced by vagal stimulation (inhibitory junction potentials) persisted in the presence of sympathetic blocking agents and atropine, and was produced by non-adrenergic non-cholinergic intramural neurones. 4. In the presence of vagally evoked excitatory junction potentials (e.j.p.s), distension produced a transient inhibition of e.j.p.s, in addition to the hyperpolarization of smooth muscle. 5. The inhibition of these e.j.p.s persisted in the presence of sympathetic blocking agents, but in contrast to the hyperpolarization of smooth muscle produced by distension alone, was modulated by drugs interfering with 5-HT synthesis, re-uptake and activity. 6. The results indicate that descending inhibition in the rabbit proximal colon was produced by two distinct neuronal non-adrenergic inhibitory mechanisms exerted simultaneously on the smooth muscle and on the cholinergic excitatory pathways which innervate it.
