VISCEROSOMATIC INTERACTIONS IN THE THALAMIC VENTRAL POSTEROLATERAL NUCLEUS (VPL) OF THE SQUIRREL-MONKEY

In anesthetized squirrel monkeys single cell recordings were performed using tungsten microelectrodes. The responses of 29 viscerosomatocept ive and somatoceptive VPL neurons to noxious distension of the urinary bladder, the lower esophagus and the distal colon and to innocuous an d noxious somatic stimuli were assessed when these stimuli were presen ted separately or together. Neuronal responses were defined as additiv e or interactive depending on the relative changes in responses to ind ividual somatic or visceral stimuli, and on their responses during con ditioning (somatic and visceral stimuli applied concurrently). In 13 n eurons interactions between the somatosensory and visceral inputs coul d be demonstrated. The dominant interactive effect was inhibition, alt hough facilitatory effects were seen as well (2 of 13). The magnitude or direction of the interactions seemed independent of the location of the somatic and visceral receptive fields. The mean population respon se of the neurons showing interactions was 4.66 spikes/s to somatic st imulation, and 0.07 spikes/s to visceral stimulation. During condition ing the mean interactive effect was -62% of the calculated additive ef fect. This implies that overall the somatic responses are halved durin g a coincident visceral stimulus. In a subgroup of the VPL neurons, wh ich were classified as pure somatic responsive (n = 14) due to their u nresponsiveness during visceral stimulation alone, a third (n = 5) sti ll exhibited visceral convergence during conditioning. The latter neur ons, therefore, receive visceral inputs, which function in a purely in teractive (modulatory) manner. It is concluded that part of the descri bed effects is due to competition (cross modality suppression) between the visceral and somatic inputs. We further conclude that the suppres sion of somatic information by noxious visceral stimuli may contribute to a more effective processing of the discriminatory aspects of nocic eptive visceral information previously demonstrated in VPL. (C) 1998 E lsevier Science B.V.