Naloxone reversible inhibition of reticular neurones in the rat caudal medulla produced by electrical stimulation of the periaqueductal grey matter

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Abstract
Chronic dorsal periaqueductal gray matter electrodes were implanted into adult rats under pentobarbitone anesthesia. Stimulating these electrodes (25-300 .mu.A) produced behavioral analgesia in 23 of 44 rats tested. In rats given the opiate antagonist naloxone, attenuation of this analgesia was seen. In 14 rats displaying behavioral analgesia to periaqueductal gray matter stimulation acute electrophysiological experiments were performed under urethane anesthesia. Microelectrode recordings were made from neurons, excited by noxious heat or pinch applied to the limbs and tail, and located in the reticular formation of the caudal medulla. Stimulation of the periaqueductal gray matter at an intensity sufficient to produce analgesia in the conscious animal produced direct inhibition of the firing of 62% of neurons tested, excited 23%, had no effect on 14% and attenuated the nociceptive responses of 66%. The inhibitions were characteristically long. Local application of naloxone by microiontophoresis attenuated these long inhibitions in 11 out of 16 neurons tested. Immunohistochemical localization of .beta.-endorphin containing structures in the vicinity of stimulating and recording sites suggested that the naloxone sensitive inhibition of nociceptive neuronal responses in caudal medulla reticular formation may be due to activation of .beta.-endorphin fibers descending through the periaqueductal area to the caudal medulla.