Effects of capsaicin on spontaneous unit discharges in amygdala single neurons of cats

Abstract

Effects of capsaicin on spontaneous unit discharges in the amygdala single neurons were studied in 26 gallamine immobilized adult cats. Single units were recorded from the amygdala using a stainless steel microelectrode. Of 63 amygdala neurons, 34 were responsive to both noxious (pinching the skin with toothed forceps) and innoxious (tap and/or hair bending) stimuli, 11 were responsive to only innoxious stimuli and the others did not respond to these stimuli. Of 34 neurons responsive to both noxious and innoxious stimuli, 20 were responsive to both bradykinin and capsaicin, and 7 were responsive to only capsaicin. All neurons responsive to bradykinin were also responsive to capsaicin. All neurons responsive to only innoxious stimuli responded to neither bradykinin nor capsaicin. The latency for bradykinin and capsaicin was 8.05 +/- 0.92 sec and 1.21 +/- 0.33 sec, respectively. The duration for bradykinin and capsaicin was 14.72 +/- 1.78 sec and 26.22 +/- 4.16 sec, respectively. The increase in firing frequency produced by capsaicin was depressed by morphine (1 mg/kg, i.v.). These depressant effects of morphine on the amygdala neurons were antagonized by naloxone (0.2 mg/kg, i.v.).