Effect of unilateral deafferentation on extracellular potassium concentration levels in rat thalamic nuclei

The autotomy performed by rats after unilateral section of the dorsal roots corresponding to the brachial plexus was frequently attributed to an abnormal painful sensation felt in the peripheral deafferented area. We studied the values of potassium (K(+)) extracellular concentration ([K(+)](e)) in thalamic nuclei of somatic projection in this animal model of deafferentation pain. Potassium concentrations were measured with microelectrodes sensitive to K(+) (K(+)-ISM). When such microelectrodes are gradually inserted into the thalamus, a sudden transient increase of K(+) concentration appears after each step lasting for about 30s. This is followed by a stabilization of K(+) values, which is present for several hours. We measured this stable concentration of K(+) (resting [K(+)](e)) for 3-5 min in different nuclei of the somatic projection to the lateral thalamus of controls and deafferented groups of adult rats under Equithesin anaesthesia. The following thalamic nuclei were explored: the ventral posterior medial and lateral (VPM and VPL) and the adjacent posterior oral (PO) as well as the lateral dorsal (LD) and posterior (LP) and ventral anterolateral (VAL). In the control group (12 rats), the mean values of resting [K(+)](e) expressed in mmol/l in these nuclei were between 3.26 and 3.62. All these values of the K(+) concentration were significantly higher than those in the cerebrospinal fluid for P<0.001. In the deafferented group (21 rats), a significant increase of the resting [K(+)](e) concentration was found in three nuclei only (VPL, VPM, PO) of the thalamus contralateral to the root section, when compared to the corresponding nuclei in the controls. No changes were observed in the other contralateral nuclei and in all nuclei of the thalamus ipsilateral to the deafferentation. The most statistically significant changes in mean values were found in five deafferented animals, which had recently performed autotomy. Localised variations of resting [K(+)](e) observed in each of VPL, VPM and PO nuclei were also demonstrated by curves traced for each control and deafferented animal. The greatest changes in resting [K(+)](e) were observed in the three nuclei mentioned above, where the value of [K(+)](e) attained 6 to 7 mmol/l in deafferented animals. The maximum amplitude peaks of resting [K(+)](e) were found in animals which had recently exhibited autotomy. These enhanced resting values of [K(+)](e) apparently reflect increased activity in the somatic lateral thalamus of deafferented animals. Their possible role in pain and autotomy are discussed.