Morphine alters the firing of cold-receptive neurons in the superficial dorsal horn of the medulla in the rat.
Effects of morphine (1-3 mg/kg, i.v.) were tested on the innocuous cold-receptive input in the superficial dorsal horn of the medulla. The static activity of most cold-receptive (cold-specific) neurons (12/16) was reduced, whereas an enhancement (4/16) was observed in the remaining neurons. Naloxone (200 micrograms/kg, i.v.) reversed, partially or completely, the effects of morphine in 9/12 cold-receptive neurons, and enhanced the static activity of some cold-receptive neurons. Static activity, at different adapting temperatures, during a warming (10 degrees C-->40 degrees C) and a cooling (40 degrees C-->10 degrees C) sequence at steps of 5 degrees C was reduced by morphine. The effects of morphine were also tested on the static as well as the dynamic responses of 9 cold-receptive neurons. The effects of morphine on the dynamic responses were not dependent on the static firing frequency. Morphine produced similar effects, excitatory or inhibitory, on the static as well as the dynamic responses of 7/9 neurons whether the static firing frequency was high (17-33 Hz) or low (< 12 Hz). However, morphine effects on static and dynamic responses were different in the remaining 2 neurons (high static firing frequency). We suggest that the predominantly inhibitory effect of morphine on the innocuous cold receptive input in the medullary dorsal horn may explain the inhibitory effect on the perception of cooling stimuli by systemic morphine in behavioral studies.
Mokha
S S
SS
Department of Physiology, Meharry Medical College, Nashville, TN 37208.
D000818:Animals; D003080:Cold Temperature; D008297:Male; D008526:Medulla Oblongata / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D009020:Morphine / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009475:Neurons, Afferent / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley
D009270:Naloxone; D009020:Morphine