Subcutaneous formalin induces a segmental release of Met-enkephalin-like material from the rat spinal cord

The possible influence of a noxious chemical stimulus on the activity of spinal enkephalinergic neurones was examined by measuring the outflow of Met-enkephalin-like material (MELM) in CSF perifusates from the lumbar and cervico-trigeminal zones in halothane-anaesthetized rats. Following subcutaneous injection of 50 microliters of a 10% formalin solution in saline into the muzzle, MELM outflow increased at the cervico-trigeminal level but not at the lumbar level. Conversely, a significant enhancement in MELM outflow occurred at the lumbar but not at the cervico-trigeminal level when formalin was injected into a hind paw. In both cases, the increase in MELM release took place 5-10 min after the treatment and was of short duration (5-10 min). It is concluded that noxious chemical stimuli can induce a transient activation of enkephalinergic neurones within spinal zones receiving the nociceptive inputs. The marked differences in the characteristics of spinal MELM overflow due to noxious chemical, thermal and mechanical stimuli further support the notion that each type of nociceptive stimulus probably activates spinal enkephalinergic systems by triggering distinct neuronal mechanisms.