Effects of capsaicin on spontaneous unit discharges in medial thalamic single neurons of cats.

Abstract

The effect of capsaicin was studied in gallamine triethiodide immobilized adult cats. Single neurons were recorded from the medial thalamus with a stainless steel microelectrode. Out of 21 neurons recorded in this experiment, 10 were responsive to both nociceptive (pinch) and non-nociceptive (hair bending and/or tapping) stimuli. Six neurons were responsive to only non-nociceptive stimuli and 5 were not responsive to these stimuli. Out of 10 neurons responding to both nociceptive and non-nociceptive stimuli, 9 were responsive to both bradykinin (3 .mu.g) and capsaicin (3 .mu.g). Out of 6 neurons responding to only non-nociceptive stimuli, 5 were not responsive to either bradykinin and capsaicin. All neurons responding to bradykinin were also responsive to capsaicin. The latency for bradykinin and capsaicin was 7.64 .+-. 1.12 s and 0.97 .+-. 0.07 s, respectively. The increase in firing frequency produced by capsaicin was depressed by morphine. These depressant effects of morphine on single unit activity were antagonized by naloxone.