Role of opioidergic and GABAergic neurotransmission of the nucleus raphe magnus in the modulation of tonic immobility in guinea pigs

• Published in: Brain research bulletin Vol. 72; no. 1; pp. 25 - 31
• Main Authors: da Silva, Luis Felipe Souza, Menescal-de-Oliveira, Leda
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 02.04.2007
• Subjects: Analgesics, Opioid - metabolism; Analgesics, Opioid - pharmacology; Analysis of Variance; Animals; Behavior, Animal - drug effects; Bicuculline - pharmacology; Defensive behavior; GABA Agonists - pharmacology; GABA Antagonists - pharmacology; GABAergic system; gamma-Aminobutyric Acid - metabolism; Guinea pig; Guinea Pigs; Immobility Response, Tonic - drug effects; Immobility Response, Tonic - physiology; Male; Microinjections - methods; Models, Neurological; Morphine - pharmacology; Muscimol - pharmacology; Naloxone - pharmacology; Narcotic Antagonists - pharmacology; Neurology; Nucleus raphe magnus; Opioidergic system; Raphe Nuclei - drug effects; Raphe Nuclei - physiology; Tonic immobility; Nucleus raphe magnus; Opioidergic system; Guinea pig; GABAergic system; Tonic immobility; Defensive behavior
• ISSN: 0361-9230; 1873-2747
• DOI: 10.1016/j.brainresbull.2006.12.005

Abstract Tonic immobility (TI) is an inborn defensive behavior characterized by a temporary state of profound and reversible motor inhibition elicited by some forms of physical restraint. Previous results from our laboratory have demonstrated that nucleus raphe magnus (NRM) is also a structure involved in the modulation of TI behavior, as chemical stimulation through carbachol decreases the duration of TI in guinea pigs. In view of the fact that GABAergic and opioidergic circuits participate in the regulation of neuronal activity in the NRM and since these neurotransmitters are also involved in the modulation of TI, the objective of the present study was to evaluate the role of these circuits of the NRM in the modulation of the behavioral TI response. Microinjection of morphine (4.4 nmol/0.2 μl) or bicuculline (0.4 nmol/0.2 μl) into the NRM increased the duration of TI episodes while muscimol (0.5 nmol/0.2 μl) decreased it. The effect of morphine injection into the NRM was blocked by previous microinjection of naloxone (2.7 nmol/0.2 μl). Muscimol at 0.25 nmol did not produce any change in TI duration; however, it blocked the increased response induced by morphine. Our results indicate a facilitatory role of opioidergic neurotransmission in the modulation of the TI response within the NRM, whereas GABAergic activity plays an inhibitory role. In addition, in the present study the modulation of TI in the NRM possibly occurred via an interaction between opioidergic and GABAergic systems, where the opioidergic effect might be due to inhibition of tonically active GABAergic interneurons.