The effect of Morphine injection in the periaqueductal gray matter on the neuronal responsiveness of nucleus reticularis paragigantocellularis to Formalin

Abstract Background: Nucleus reticularis paragigantocellularis (PGi) has a very effective role in the supraspinal pain modulation. PGi as a rostroventromedial medulla (RVM) structure receives a major input from periaqueductal gray matter (PAG). Formalin as a peripheral noxious stimulus has biphasic nociception and behavioral manifestations. Objective: To assess morphine injection in the periaqueductal gray matter (PAG) on the neuronal responsiveness of nucleus reticularis paragigantocellularis (PGi) to formalin. Methods: The experimental subjects were male NMRI rats. Diluted formalin (2.5%) as a chemical noxious stimulus and morphine as an analgesic drug were used. Using single unit recording (An extracelluar recording electrophysiologic method) the research was done. Findings: Findings indicate that the responses of the PGi inhibitory neurons evoked by formalin were disinhibited by morphine microinjection in the PAG. In the excitatory neurons, morphine decreased the firing rate to the baseline activity. Conclusion: PGi inhibitory neurons are affected by the PAG’s GABAergic interneurons. Morphine blocks GABA released from these interneurons and results to off-cell disinhibition. On-cells suppressions are probably related to morphine effects directly.