Substantia gelatinosa neurones hyperpolarized in vitro by enkephalin

Abstract

The sensation of pain is carried to the central nervous system by small-diameter fibres which terminate in the superficial layers of the spinal grey matter, particularly the substantia gelatinosa¹. Opiate drugs administered into the substantia gelatinosa inhibit the excitation of deeper dorsal horn neurones by noxious peripheral stimuli while sparing the responses to other sensations such as touch². It has been suggested that opiates impair the release of the transmitter from the small-diameter nociceptive fibres^3,4 but it is also possible that opiates directly depress the responses of cells in the substantia gelatinosa which serve as interneurones in the pain pathway^5–7. We have made intracellular recordings from substantia gelatinosa neurones in a slice of rat spinal cord, and report here that morphine and enkephalin directly hyperpolarize substantia gelatinosa cells by opening membrane K⁺ channels. This inhibitory action of exogenous morphine provides a likely cellular mechanism for its analgesic properties. Moreover, enkephalin is an endogenous constituent of some cells of the substantia gelatinosa^8,9, where it is concentrated within terminals making synaptic contacts with other cell bodies and dendrites^10,11. The present findings thus strongly suggest that enkephalin serves as an inhibitory neurotransmitter within the substantia gelatinosa.