The effects of acute stress and acute corticosterone administration on the immobility response in rats

The immobility response is an innate antipredatory behavior in a broad variety of species. The immobility response varies in its postural components but in general is characterized by an absence of movement and a relative unresponsiveness to stimuli. Experimentally in rats, clamping the neck followed by body inversion and manual restrain elicits a response called “immobility by clamping the neck”. Stress reactions protect animals against predators and are characterized by activation of the sympathetic and hypothalamic–pituitary–adrenal systems. However, in mammals, the role of acute stress as a modulator of immobility response has been less studied. The aim of our study was to assess the effects of acute stress and the injection of corticosterone (5 mg/kg, ip) on immobility by clamping the neck in rats. We observed that either previous acute stress caused by forced exposure to elevated open platform or application of a heat-pain stimulus to the rat's tail during the immobility increased the duration of the immobility response caused by clamping the neck. Also, the corticosterone produced a rapid increase (15 min after injection) in the duration of this immobility response. Our results show that the acute stress, in rats, is a facilitator of the immobility response and suggest a possible nongenomic rapid action of corticosterone over brain structures that control this behavior.