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Department of Physiology, University of Wales, College of Cardiff.

1. The effects of stimulating A fibres in the dorsal columns on the responses of dorsal horn neurones to intense cutaneous stimuli were studied in the rat anaesthetized with urethane. 2. Multireceptive cells deep in the lumbar dorsal horn were excited for 5-10 ms by dorsal column stimulation and subsequently responses to noxiously hot water placed on the cutaneous receptive field were reduced for the following 4-5 min. Seven of the cells studied projected to the brain via the contralateral anterolateral funiculus. 3. If the discharge of the multireceptive neurones was raised by ionophoretic application of DL-homocysteic acid, a brief period of inhibition lasting for 100-150 ms was seen following a single stimulus to the dorsal columns. Studies were conducted to determine if this brief inhibition could account for the long-lasting inhibition of responses to high-threshold stimuli. 4. Dorsal columns were transected at cervical levels. Stimulation caudal to the transection evoked only the brief excitation and subsequent inhibition for 100-150 ms. No long-lasting inhibition of high-threshold cutaneous afferent input was seen. 5. Stimulation of the dorsal columns rostral to transection did not evoke the brief excitation or inhibition of multireceptive dorsal horn neurones. However, the 4-5 min inhibition of responses to high-threshold cutaneous stimuli was present. 6. The long-lasting inhibition of responses to high-threshold stimuli by dorsal column stimulation was blocked by microinjection of gamma-aminobutyric acid into the anterior pretectal nucleus (APTN) but not by microinjections into adjacent areas of the brain. 7. Ipsilateral lesions of the dorsolateral funiculus at the cervical level also blocked the long-lasting inhibitory effects of dorsal column stimulation. 8. It is concluded that the brief excitation and inhibition of multireceptive dorsal horn neurones is due to antidromic action potentials passing caudally in the dorsal columns to activate spinal segmental mechanisms. The longer-lasting inhibition of responses to high-threshold cutaneous stimuli is due to action potentials ascending in the dorsal columns to activate cells in the APTN which in turn activate a descending inhibition mediated by the dorsolateral funiculus.

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