ANTERIOR PRETECTAL NUCLEUS FACILITATION OF SUPERFICIAL DORSAL HORN NEURONS AND MODULATION OF DEAFFERENTATION PAIN IN THE RAT

1. Functional relationships between the anterior pretectal nucleus (AP TN) and nociceptive dorsal horn neurones were investigated electrophys iologically in the anaesthetized rat. The effects of APTN lesions were assessed behaviourally in a model of deafferentation pain. 2. Cells i n the dorsal and rostral parts of the APTN were excited orthodromicall y by electrical stimulation of the ipsilateral dorsolateral funiculus or the contralateral dorsal columns, and by noxious and innocuous cuta neous stimuli.3. Electrical stimulation of the APTN excited nociceptiv e lamina I spinal neurones. These cells all projected rostrally in the contralateral dorsolateral funiculus. Identical APTN stimulation also inhibited multireceptive spinal neurones which lay deep in the dorsal horn. These particular cells were shown to project to the brain in th e ventrolateral funiculus. 4. It is proposed that noxious stimuli exci te spinal lamina I projection neurones which send excitatory axons to the brain, including the APTN. The APTN inhibits deep multireceptive n eurones, to reduce the perception of noxious stimuli. The discharge of spinal lamina I neurones, however, will be sustained by the noxious s timulus and by facilitation from the APTN. A sustained descending inhi bition of this nature would reduce responses to prolonged injury. 5. T he involvement of the APTN in responses to a chronic pain state was ex amined by comparing the behaviour of animals with bilateral lesions of the APTN with normal controls. Lesions of the APTN strongly enhanced the autotomy behaviour triggered by sectioning of the dorsal roots. 6. These observations support the suggestion that the APTN reduces the d ebilitating effects of prolonged injury.