Visceral pain reflex after pretreatment with capsaicin and morphine.

1. Distension of the proximal jejunum by increasing the intraluminal pressure for short time periods causes a reflex response in blood pressure of anaesthetized rats which correlates with the magnitude of distension. The blood pressure response consists of a short initial rise followed by a marked decrease for the time of distension. 2. The absence of the depressor response in capsaicin desensitized rats indicates its mediation by C fibre afferents. These afferents are located within the periarterial mesenteric nerves. The depressor response was also elicited by stimulation of these nerves and abolished by local application of percain or capsaicin onto the mesenteric stalk. Vagal afferents were not involved in this depressor response as shown by bilateral vagotomy or by afferent vagus stimulation. 3. The depressor response is absent in spinal rats. Therefore, the location of the reflex centre is assumed to be supraspinal. Because it is augmented by naloxone and abolished by morphine in a naloxone reversible way it is regarded as a nociceptive reflex response. 4. The efferent side of the depressor response is unknown; cholinergic and alpha-adrenergic activation were excluded. 5. The initial pressor response to intestinal distension or to afferent periarterial mesenteric nerve stimulation persists in capsaicin desensitized rats excluding the involvement of C fibre afferents and in spinal rats indicating that the reflex centre is within the spinal cord. It is not diminished by morphine and therefore not a nociceptive response. Its inhibition by phentolamine suggests an alpha-adrenergic spinal response to intestinal distension. In control rats the pressor response is greatly overlapped by the much more pronounced depressor response.