In alpha-chloralose-anesthetized cats the region surrounding the ventral posterolateral nucleus (VPL) of the thalamus was investigated to locate foci with input from the urinary bladder stimulated by application of intravesical pressure. The locations of the recording sites were verified in Nissl-stained histological sections with reference to electrolytic lesions. 2. Of the 23 visceroceptive thalamic neurons identified, 4 (17%) were located in the periphery of the VPL (VPLp) and 19 (83%) in the lateral and dorsal aspects of the posterior complex (POl and POd, respectively) adjoining VPLp. 3. The neurons responded to noxious intensities of intravesical pressure in the range of 50-100 mmHg. Excitatory responses were elicited in 8 (35%) neurons, "inhibitory" responses in 13 (57%) neurons, and 2 (9%) neurons responded with an increase and a decrease of their discharge to subsequent stimuli. 4. Of the 22 visceroceptive thalamic neurons tested for this parameter, 73% had low-threshold cutaneous receptive fields (RFs). These were located in the region of the lower back, the hip, the thigh, and the proximal tail (12 PO neurons), or covered the entire postcranial contralateral part of the body (3 PO neurons). For only one of the VPLp neurons, a somatic RF was found and this was located on the distal tail. The neurons responded to tap stimuli applied at a low repetition rate. None of the 11 neurons tested with noxious pinching of the skin was activated by this kind of stimulus. 5. It is concluded that the cat's lateral thalamic region, around but not within VPL proper, contains neurons that play a role in the processing of information about noxious events in the urinary bladder. A comparison with results from experiments in the monkey indicates differences in the organization of the visceroceptive systems between both species, regarding the thalamic localization of visceroceptive neurons, the occurrence of convergent low-threshold somatic RFs, and the association of excitatory and inhibitory effects of urinary bladder stimulation with the location of somatic RFs.
