Dorsolateral PAG neurons: tonic immobility and morphine effect in freely moving rabbits.

The activity of 33 neurons of pontomesencephalic dorsolateral periaqueductal gray matter (PAG), not triggered by motor or sensory stimuli, has been recorded during tonic immobility (animal hypnosis) and after morphine injection (5 mg/kg IV). Several parameters of neural activity were chronically studied, including: frequency, variability and pattern of discharge. Tonic immobility affected the frequency and the variability of the firing rate of the majority of neurons. Morphine decreased frequency and increased variability of 73.3% of the neurons. Maximal morphine effect was observed with a wide range of latencies and was independent of frequency and variability. The negative relationship between frequency and variability recorded during the control period, wa abolished by morphine. Changes in the pattern of discharge were also observed after morphine injections. The effect of tonic immobility and morphine are discussed in the light of the relationships existing between PAG neurons and fear motivated behavior.