Decreased Formalin Induced Nociceptive Behaviors by Morphine Microinjection into the Nucleus Reticularis Paragigantocellularis Lateralis

Systemic injections of morphine have been shown to elicit analgesic responses in formalin test. However, the locations of central sites that may mediate these effects have not been clearly elucidated. This study assessed the antinociceptive action of morphine after microinjection into the nucleus reticularis paragigantocellularis lateralis (LPGI) in the formalin test in rats. Formalin-induced behaviour is characterized by two phases relevant to acute and tonic. In this behavioral study, Inflammatory Pain Model by formalin was used in white rats with weight range of 200-300g. Using a stereotaxic apparatus, canulla was inserted into the LPGI. After 1 week recovery, animals were initially submitted to the injection of 50 ml of 2% formalin solution in hind paw. Intra-LPGI microinjection of morphine produced robust inhibition of formalin-evoked behaviour in phase 1, interphase and phase 2 (P<0.001); however, exogenous injection of morphine in the LPGI had no effect on reducing formalin induced nociceptive behaviors The results suggested that morphine plays an anti-nociceptive role in LPGI in phase 1, interphase and late phase of formalin test in rats, which suggests that opioid receptors in the LPGI may be involved in the LPGI -mediated depression of formalin test as inflammatory pain.