Abstract

Background & Objective: The parabrachial (PB) region definitely is a collection of neurons which are in the dorsolateral pons.  Considering strong nociceptive projection coming from the lamina I neurons of the spinal cord into the lateral parabrachial nucleus (LPBN) and the existence of GABA-A receptors in the LPBN in the present study we investigated the effects of the GABA-A receptor agonist (muscimol) and GABA-A receptor antagonist (bicuculline) into the PB nucleus on formalin-induced nociceptive behavior. Materials & Methods: Rats were anesthetized with sodium pentobarbital (55 mg/kg) and then special cannulas were inserted stereotaxically into the parabrachial nucleus. Different groups of seven rats were injected with formalin into the rear paw 5 min after administration of either GABA-A receptor agonist, muscimol (62.5, 125,250 ng/0.5μl) or GABA-A receptor antagonist, bicuculline (50, 100 and 200 ng/0.5μl) into the LPBN. The scores of nociceptive behaviors calculated immediately after formalin injection and continued for 60 min. Results: Intra- LPBN microinjection of muscimol (250 ng) caused a decrease in formalin-induced nociceptive behaviors in phases 2 compared to vehicle-treated rats (P = 0.002) but failed to show any significant change in interphase or phase 2 from AUC calculated value in the vehicle group. Microinjection of muscimol (62.5, 125 ng) had no effect on AUC in any phase of the formalin test compared with the vehicle-injected rats. Administration of 50, 100 and 200 ng of bicuculline into the LPBN resulted in similar pain behaviors compared to the vehicle group in three phase with similar AUC. Conclusion: The present study indicates that indicating GABA A receptors in the PB nucleus may be involved in the modulation of nociception in tonic pain (phase 2).