VISCERAL RECEPTORS AND VISCERAL PAIN

Possible role of central serotoninergic neurons in the development of dental pain and aspirin-induced analgesia in the monkey

The effects of aspirin or 5-hydroxytryptamine (5-HT)-related drugs on the dental pain induced by electrical stimulation of tooth pulp afferent fibers were assessed in conscious monkeys. The electrical current required for producing jaw opening is referred to as the pain threshold. Both systemic (25 to 75 mg/kg, i.p.) or central (0.5 to 1.5 mg, third cerebral ventricle) administration of aspirin produced analgesia in monkeys. In addition, activation of central 5-HT receptors with central injection of either 5-HT or its precursor, 5-hydroxytryptophan, also produced analgesia. On the other hand, inhibition of central 5-HT receptors with central administration of either cyproheptadine (a blocking agent of 5-HT receptors), p-chlorophenylalanine (PCPA, an inhibitor of 5-HT synthesis) or 5,7-dihydroxytryptamine (5,7-DHT, a depletor of central 5-HT nerve fibers) produced an enhancement in pain sensitivity (or a decrease in pain threshold). Furthermore, the analgesia induced by aspirin was antagonized by pretreatment of monkeys with either cyproheptadine, PCPA, or 5,7-DHT. The results indicate that increases in the activity of central 5-HT neurons are associated with reduced dental pain and enhanced aspirin-induced analgesia, whereas decreases in the activity of those neurons correlate with dental hyperalgesia and diminished aspirin-induced analgesia in monkeys.

Bradykinin as an algesic (pain producing) substance in the pulp

A rat uterine smooth muscle contracting substance was released into the superfusate of the dog's exposed canine pulp after noxious stimulation of the pulp by pricking, heat and electrical stimulation. This active substance was acid- and heat-resistant and was decomposed by carboxypeptidase B and alpha-chymotrypsin, but not by carboxypeptidase A and trypsin. This substance was also tested on several types of smooth muscle. Electrical activity of nerve cells in the reticular formation, which were sensitive to stimulation of the instep of the foot by pinching, was activated by the intrafemoral administration of the active substance. The algesic activity of this substance was examined in cantharidin blister base in man. This study conclusively demonstrated that the active substance of the pulp released by noxious stimulation produced pain and it was identified as bradykinin.

Responses of thoracic dorsal horn interneurons to cutaneous stimulation and to the administration of algogenic substances into the mesenteric artery in the spinal cat

The effects of the injection of algogenic substances (bradykinin, acetylcholine) into the inferior mesenteric artery were studied at the thoracic level on 47 dorsal horn interneurons responding to cutaneous stimulation. Each unit was characterized by its electrophysiological properties and carefully located within the cord by extracellular injection of pontamine sky blue. Twenty cells, driven only by non-noxious cutaneous stimulation and mainly located in lamina IV, were not affected by the administration of algogenic substances. The activity of 25/27 cells, excited by both non-noxious and noxious cutaneous stimulation and mainly located in lamina V, was strongly modified by nociceptive visceral stimulation, induced by bradykinin and acetylcholine: 8/27 cells were activated, 14/27 were inhibited, 3/27 had a mixed inhibitory-excitatory response. From our study it clearly appears that nociceptive visceral messages only project on dorsal horn cells receiving noxious cutaneous afferents. Thus viscerosomatic convergence seems only to concern nociceptive messages; the existence of this kind of convergence reinforces the hypothesis suggested by several authors to explain referred pain from a neurophysiological point of view.

Blockade of bradykinin-induced nociception in the rat as a test for analgesic drugs with particular reference to morphine antagonists

A rat test, recently outlined by Deffenu & others for the evaluation of analgesic drugs and using intra-arterially administered bradykinin as the nociceptive stimulus, has been applied to varied categories of analgesic drugs including, for the first time, morphine antagonists. For purposes of comparison, data obtained with the same drugs using established techniques are also given. The use of the bradykinin-antagonism test as a laboratory model with which to investigate drugs of potential therapeutic value in man is discussed and compared particularly with the mouse phenylquinone-induced writhing test. Attention is drawn to the possibility of correlating the effect of drugs on bradykinin-induced pseudo-affective responses in animals with human pain.