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In vivo perfusion of the subarachnoid space with an artificial cerebrospinal fluid (CSF) in paralyzed halothane-anesthetized rats allowed the collection of methionine-enkephalin (Met-Enk)-like material (MELM) released from the spinal cord. Bio-Gel P2 chromatography and high-performance liquid chromatography showed that 65% of this material corresponded to authentic Met-Enk. Under resting conditions, about 1 pg of MELM per minute was regularly released for at least 3 h; for Met-Enk, this value corresponded to a fractional rate constant of 0.002% (i.e. tissue content of the pentapeptide which was released per minute from the whole spinal cord). Perfusion with K+-enriched (40-60 mM) CSF resulted in a marked enhancement (+ 150-200%) of spinal MELM release. Similarly, calibrated pinches of the muzzle and i.p. administration of acetic acid, two strong noxious stimuli in awake animals, induced a significant increase (+ 75-150%) in spinal MELM release. In contrast, pinches applied to the tail did not enhance but instead slightly reduced (-35%) MELM release from the rat spinal cord. These data suggest that mechanisms other than segmental controls could be involved in the activation of spinal enkephalinergic neurons by some nociceptive stimuli.
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