It is well known that electrical stimulation of the posterior limb of the internal capsule can relieve central pain resulting from neuronal damage in the central somatosensory pathways, though the mechanism of pain control is still unknown. Cats were employed in this study to investigate how internal capsule stimulation influences the unitary discharges of the nociceptive neurons of the thalamus. Electrical stimulation of the internal capsule was carried out with a bipolar stimulating electrode whose tip was introduced into the internal capsule (A:17.0, L:6.0, H:+5.0). A bipolar stimulating electrode was inserted into the tooth pulp of the left lower incisor, and microelectrodes for recording unitary discharges which were placed into the ventralis posteromedialis (VPM) (A:7.5-10.0, L:3.0-6.5, H:-1.0- +2.0), posterior nuclear group (PO) (A:3.5-7.0, L:4.0-6.5, H:+1.0- +4.0) and centre median (CM) (A:6.5-7.5, L:2.0-3.0, H:0- +1.5) of the right thalamus under local anesthesia. Unitary discharges responsive to the contralateral tooth pulp stimulation were found in the VPM, PO and CM of the thalamus. The unitary discharges in the VPM were suppressed for more than 15 seconds after electrical stimulation (2.0 V, 50 Hz, 0.5 msec., 2 seconds) of the internal capsule which contains sensori-motor fibers. However, stimulation of the internal capsule did not suppress the unitary discharges in the PO and CM of the thalamus. On the other hand, intraventricular administration of morphine (0.05 mg) suppressed the discharges of both PO and VPM neurons responsive to the pulp stimulation, and the suppression of PO discharges lasted for much longer periods. These results suggest that pain relief obtained by electrical stimulation of the internal capsule is not an analgesic effect related to the opioid system, but due to the activation of the non-opioid central pain-inhibiting system, resulting in the suppression of nociceptive neurons of the sensory thalamus.
thalamic unitary discharges
pain inhibitory system