CHARACTERISTICS OF PROPRIOSPINAL MODULATION OF NOCICEPTIVE LUMBAR SPINAL DORSAL HORN NEURONS IN THE CAT

The segmental and laminar origin of propriospinal antinociceptive syst ems in the cat spinal cord and the modes to activate them are characte rized. The experiments were performed on pentobarbital-anesthetized ca ts with a high cervical spinalization. Recordings were made from singl e lumbar spinal dorsal horn neurons responding to noxious radiant skin heating and to innocuous mechanical skin stimuli. The segemental and laminar origin of heterosegmental, propriospinal neurons modulating ba ckground activity and nociceptive responses were identified and the co nditions to activate them were characterized. Conditioning noxious fro nt paw stimulation and superfusion of the cervical enlargement with L- glutamate, but not with substance P, reduced noxious heat-evoked respo nses of about 50% of all lumbar neurons tested. Glutamate superfusions of the lower thoracic or upper sacral spinal cord enhanced background activity and reduced nociceptive responses of most lumbar spinal dors al horn neurons. Superfusions with substance P or somatostatin were in effective. Glutamate microinjections into the superficial layers of th e thoracic, upper lumbar or sacral dorsal horn ipsi- or contralateral to the recording sites or into lamina VIII of the ipsilateral thoracic or upper lumbar cord reduced noxious heat-evoked responses with or wi thout changes in the level of background activity. It is concluded tha t propriospinal neurons originating from circumscribed areas of the ce rvical, thoracic, lumbar or sacral spinal cord independently modulate background activity and noxious heat-evoked responses of multireceptiv e lumbar spinal dorsal horn neurons. The incidence and efficacy of pro priospinal antinociceptive stimulation sites was found to be as high a s for the classical region of endogenous antinociception, the midbrain periaqueductal gray.