Morphine injected into the periaqueductal gray attenuates brain stimulation-induced effects: an intensity discrimination study.

A discrimination situation was used to study the effects of morphine microinjected into the periaqueductal gray (PAG) on the aversive effects induced by PAG or medial hypothalamic (MH) electrical stimulation. Rats were trained in a T-maze to interrupt a high intensity (HI) stimulation inducing a short escape latency by pressing the lever (HI lever) located in one arm of the maze and a low intensity (LI) stimulation inducing a longer escape latency by pressing the lever (LI lever) located in the other arm. Microinjections of 15 nmol (5 micrograms) or 40 nmol (13 micrograms) of morphine both lengthened the escape latencies and shifted towards the LI lever the animal's choice in order to interrupt HI stimulations. This effect of morphine showed a similar time course as regards both escape latency and lever choice; it was more marked on PAG than on MH stimulation-induced aversive effects. The data are discussed in terms of morphine microinjections into PAG lengthening the escape latency by decreasing the aversiveness of PAG or MH stimulation rather than by affecting the animals' ability to respond to such stimulations.