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Morphine microinjected into the periaqueductal gray has differential effects on 3 classes of medullary neurons

Z F Cheng, H L Fields, M M Heinricher

    Brain Research
    |June 4, 1986
    PubMed

    Insights

    Morphine microinjections into the periaqueductal gray (PAG) alter rostral ventral medulla (RVM) neuron activity, affecting pain reflexes. Off-cells and on-cells in the RVM mediate these morphine-induced changes in spinal nociceptive reflexes.

    Area of Science:

    • Neuroscience
    • Pharmacology
    • Pain Research

    Background:

    • The periaqueductal gray (PAG) plays a crucial role in pain modulation.
    • Neurons in the rostral ventral medulla (RVM) are involved in processing nociceptive information.

    Purpose of the Study:

    • To investigate the effects of morphine microinjections into the PAG on RVM neuronal activity.
    • To determine the role of RVM neurons in mediating morphine's analgesic effects.

    Main Methods:

    • Microinjection of morphine into the PAG of anesthetized rats.
    • Recording neuronal activity in the RVM.
    • Correlation of neuronal activity with the tail-flick reflex (TF).
    • Administration of naloxone to assess opioid receptor involvement.

    Main Results:

    • Morphine in the PAG inhibited the tail-flick reflex.
    • Spontaneous activity of RVM off-cells increased, while on-cells decreased, preceding TF inhibition.
    • Morphine reduced TF-related responses of off- and on-cells.
    • Naloxone reversed these effects, and non-inhibitory microinjections had no effect.

    Conclusions:

    • RVM off- and on-cells are critical mediators of morphine's effects on spinal nociceptive reflexes.
    • These findings support a model where PAG-mediated analgesia involves modulation of RVM neuronal activity.

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