Neurones responding to noxious stimulation in VB complex and caudal adjacent regions in the thalamus of the rat

(1) 163 cells responding to mechanical cutaneous stimulation have been recorded in VB complex and caudal adjacent region in rats anaesthetized with a mixture of 2/3 N2O--1/3 O2 and 0.5% halothane. (2) 51 cells were exclusively activated by non-noxious cutaneous stimuli applied to restricted and contralateral receptive fields (RF) and had the classical characteristics of "lemniscal" responses. 93 cells responded only to noxious mechanical stimuli (N cells) and had either uni- or bilateral receptive fields. 19 cells responded both to noxious and non-noxious stimuli (NnN cells). (3) When tested with intense electrical stimuli applied transcutaneously or on the sural nerve, N and NnN cells responded with a late irregular discharge. Poststimulus histograms obtained in one-third of these units revealed that the late component was consistent with a C fibre input; some of responses were consistent with A delta fibre input. NnN cells also had a short latency discharge probably due to A alpha fibre involvement. (4) When tested with other intense stimuli such as noxious radiant heat or noxious visceral stimulation induced by intraperitoneal injection of bradykinin, N and NnN cells were strongly activated. (5) The different kinds of cells were scattered in VB and PO and no significant differences were found between cells recorded in VB and caudal adjacent region (PO); however, a rostrocaudal organization of the cells, according to the location of their RF on the caudal or rostral part of the body, was clear not only for the Nn cells but also for N and NnN cells.