Abstract
It is possible that nociceptive stimuli evoke neurotransmitter release not only from the primary afferent terminals in the dorsal horn but also from the peripheral terminals via axon reflex. Recent studies have shown the presence of glutamate receptors on peripheral terminals of nociceptors. Thus, evoked pain sensation and axon reflex was evaluated in order to investigate whether the subcutaneous injection of glutamate excites the nociceptors.
Sodium glutamate was injected subcutaneously into the forearms of healthy adults who gave their informed consent prior to participation. Glutamate caused severe pain immediately after the injection and the peak pain was observed within 5 minutes. Although the intensity of pain expressed by VAS has gradually decreased, slight pain was maintained for 30 minutes. The skin temperature monitored by thermography started to increase about 3 min after injection, reached a peak in 10-20 min and then decayed gradually. The area of the temperature-increase almost disappeared in about one hour.
Sodium glutamate injection into the hindpaw of the pentobarbital-anesthetized rats also produced a significant skin temperature increase. Subcutaneous glutamate also produced extravasation and paw edema. The volume of edema remained unchanged for more than one hour.
These data demonstrate that subcutaneous injection of glutamate evokes pain sensation and axon reflex. It is suggested that peripheral glutamate would create a vicious circle where pain is maintained if glutamate is released from peripheral terminals via axon reflex following nociceptive stimulus. [Jpn J Physiol 54 Suppl:S170 (2004)]
