Responses of lateral thalamic neurons to algesic chemical stimulation of the cat knee joint.

In order to gain insight into the representation of articular pain of the knee at the supraspinal level, recordings were made from lateral thalamic neurons receiving input from afferent fibres of the knee joint in chloralose-anaesthetized cats. Dorsoventral penetrations were made through the ventral posterior lateral nucleus (VPL) using high intensity electrical stimulation of the medial articular nerve (MAN), which contains a high proportion (80%) of A delta and C afferent fibres. All recording sites were verified histologically. Close retrograde injections (300 microliters over 6 s) into geniculate artery of KCl (2 x isotonic), bradykinin (BK, 2.6 or 26 micrograms) and capsaicin (200 microM) were used to test the response properties of thalamic neurons. Of the 50 MAN-positive units tested, 20 showed a response to intra-arterial KCl; of these 20, 12 had a response to BK; 8 of these 12 units were additionally tested with capsaicin and all responded. KCl and capsaicin injections had similar mean response latencies (4.5 and 6.8 s), whereas BK had a longer mean latency (18.6 s). The mean peak response was greatest for capsaicin (168 impulses/s), then KCl (87.5 imp/s) and least with BK (36.4 imp/s). The mean response duration was longest with capsaicin (118 s), followed by BK (67.5 s) and least with KCl (27.9 s). Most of these were convergent wide dynamic range (WDR) neurons with a deep receptive field in the knee joint and hindlimb muscle and/or cutaneous distal hind limb digit, located to the dorsal or ventral periphery of the lateral division of the VPL, the VPL1. In addition, 8 neurons showed inhibitory responses to KCl and/or BK injections. The background activity of VPL1 neurons activated by saphenous nerve stimulation was inhibited by the nociceptive articular stimulus with a magnitude and time course which mirrored the excitatory responses in the periphery of VPL1. These results support the concept that the lateral thalamus plays an important role in mediating discriminative aspects of joint pain.