[Endomorphins and nociception]

Since the discovery of Met- and Leu-enkephalin (Hughes and Kosterlitz, in 1975) about 20 opioid peptides including beta-endorphin, enkephalins and dynorphins have been identified in the central nervous system. Multiple opiate receptors: mu, delta, kappa, etc, with distinct pharmacological characteristics and regional distributions are present in the CNS, which may correspond to the heterogeneity of opioid peptides. Although both endomorphins and opiate receptors have been found in all areas directly involved in nociception, pharmacological, electrophysiological and most biochemical investigations have not demonstrated so far that endomorphinergic neurones play a role in the control of pain. As emphasized in this review, only the measurement of endomorphin release directly in the CNS has allowed the demonstration that some endomorphinergic neurones, notably those containing Met-enkephalin in the spinal cord, can be activated by noxious stimuli. However, the characteristics of this activation depend on both the nature of the stimulus (chemical, mechanical or thermal) and the body area where it is applied. The functional significance of such "pain"--induced activation of spinal enkephalinergic neurones is still speculative.