Modulation of tonic immobility by GABAA and GABAB receptors of the medial amygdala.

Tonic immobility (TI) is a temporary state of profound motor inhibition associated with great danger as the attack of a predator. Previous studies carried out in our laboratory evidenced high Fos-IR in the posteroventral region of the medial nucleus of the amygdala (MEA) after induction of the TI response. Here, we investigated the effects of GABA and GABA of the MEA on TI duration. Intra-MEA injections of the GABA agonist muscimol and GABA agonist baclofen reduced TI response, while intra-MEA injections of the GABA antagonist bicuculline and GABA antagonist phaclofen increased the TI response. Moreover, the effects observed with muscimol and baclofen administrations into MEA were blocked by pretreatment with bicuculline and phaclofen (at ineffective doses per se). Finally, the activation of GABA and GABA receptors in the MEA did not alter the spontaneous motor activity in the open field test. These data support the role of the GABAergic system of the MEA in the modulation of innate fear.