The effects of morphine and morphine-related compounds on the rate of afferent discharges from the pulmonary receptors in bullfrog.

We examined the effects of morphine and morphine-related compounds on the rate of afferent discharges from the pulmonary receptors in bullfrog. Morphine (1 X 10(-5) - 1 X 10(-3) M) decreased the rate of spontaneous afferent discharges in a concentration-dependent manner, but hardly affected the rate of afferent discharges synchronized with lung inflation. Dihydrocodeine, naloxone and dextrorphan had the same effects as morphine. Levallophan (1 X 10(-4) M), pentazocine (1 X 10(-4) M) and pethidine (1 X 10(-3) M) clearly decreased both rates of the spontaneous afferent discharges and the afferent discharges synchronized with lung inflation. In the presence of naloxone or levallorphan at the concentration of 1 X 10(-5) M, morphine (1 X 10(-5) and 1 X 10(-4) M) caused an additive decrease in the rate of afferent discharges. Dextromethorphan and apomorphine at the concentration of 1 X 10(-4) M caused an increase in the rate of spontaneous afferent discharges followed by decrease in both rates of the spontaneous afferent discharges and the afferent discharges synchronized with lung inflation. When dextromethorphan or apomorphine was washed out and the rates of afferent discharges were almost restored to the levels before application of each drug, reapplication of these drugs caused no increase in the rate of spontaneous afferent discharges, but the drugs inhibited the generation of afferent discharges. All of these drugs did not affect the flow rate of perfusion solution from the pulmonary vein.