Visceral nociceptive input into the ventral posterolateral nucleus of the thalamus: a new function for the dorsal column pathway.

1. Extracellular recordings were made from single neurons in the ventral posterolateral (VPL) nucleus of the thalamus in anesthetized male rats. VPL cells that responded to colorectal distension (CRD) were further tested with cutaneous and visceral stimuli before and after sequential lesions of the dorsal column (DC) and of the ventrolateral column (VLC). 2. A total of 105 VPL viscerosensitive cells was isolated. In this study, we report results from 27 cells that were tested for visceral responses before and after lesions of the DC and the VLC. 3. Ten VPL cells, isolated in 10 different rats, that responded to CRD and to activation of their cutaneous receptive fields with innocuous and noxious stimuli were tested before and after a lesion of the DC and again after a lesion of the VLC. Ten other VPL cells were isolated in 10 different rats. These were also tested for responses to CRD and to cutaneous stimuli spanning the innocuous and the noxious range before and after a lesion of the VLC and again after a lesion of the DC. 4. A lesion of the DC dramatically reduced the responses of VPL cells to CRD as well as to innocuous cutaneous stimuli. A lesion of the VLC reduced and sometimes abolished the responses of VPL cells to noxious cutaneous stimuli; however, its effect on the responses to visceral or innocuous cutaneous stimuli was minor when compared with that of a DC lesion. 5. The activity of seven VPL cells that responded to CRD and to cutaneous stimuli was observed after an injection of a chemical irritant, mustard oil (MO), into the colon. These cells showed an increase in background activity that started approximately 25-30 min after the MO injection. A lesion of the DC reduced the background activity of the cells to levels near initial values. 6. We conclude that the DC in the rat contains a pathway that is more important for transmitting visceral nociceptive signals to the VPL nucleus than are pathways in the VLC, including the spinothalamic tract. Conversely, the VLC is more important for transmitting nociceptive information of cutaneous origin.