Nociceptive responses in medial thalamus of the normal and arthritic rat

Recordings were obtained from 773 neurons located in the medial thalamus of rats. 23 of the 46 rats studied had been rendered arthritic by prior inoculation with Freund's adjuvant. 262 of the neurons could be activated by peripheral stimulation. In all cases but one, only stimuli considered to be nociceptive were effective in producing responses. Most of the responses were excitatory. The majority of the responsive neurons were located in the submedius (SM), mediodorsal (MD), centrolateral, paracentral, ventromedial (VM) nuclei and medial parts of the ventrolateral (VL) nucleus. A few nociceptive neurons were also recorded in anteromedial (AM), reuniens and a few other nearby regions of thalamus. Most neurons could be activated by stimuli applied bilaterally and frequently to large regions of the body. In almost all cases the responses were maintained for the entire duration of the 15 sec stimuli used and in some cases continued after cessation of the stimuli. No marked differences in incidence of responsive neurons were found between the normal and arthritic rats or between different regions. There were also no marked differences in the spontaneous rates, magnitudes of responses, or incidence of after-discharges of neurons in the various regions of medial thalamus. These findings indicate the existence of neurons responding to nociceptive stimuli in MD, AM, VM, and VL in addition to the intralaminar nuclei and SM and suggest that all these regions may be involved in mediating various aspects of nociception.