Simultaneous electric activities of pain-excitation and pain-inhibition neurons in nucleus parafascicularis of thalamus in rats during acute morphine tolerance.

When acute morphine-tolerated rat was administered by ip morphine (10 mg/kg) which was effective before the acute tolerance to morphine, both the inhibitory effect of morphine on the electric discharges of pain-excitation neurons (PEN) in nucleus parafascicularis (PF) and the excitatory effect of morphine on the electric activities of pain-inhibition neurons (PIN) were simultaneously weakened, or even vanished. If a large dose of morphine (20 mg) was given ip, the modulating action of morphine on simultaneous electric discharges of PEN and PIN reappeared. It is obvious that the phenomenon of acute morphine tolerance and the antagonism to morphine tolerance can be explicitly expressed on the level of central neurons.