RESUMO
Ever since the seminal studies of Hans Selye, activation of hypothalamus-pituitary-adrenal (HPA) axis is emblematic of stress. Consequently, the lack of HPA axis responses following the undisputable psychological stress of a panic attack stands out as one of the most intriguing findings of contemporary psychiatry. On the other hand, the defensive behaviors and aversive emotions produced by stimulation of the dorsal periaqueductal gray matter (DPAG) have been proposed as a model of panic attacks. Therefore, we examined whether the plasma levels of 'stress hormones' corticotropin and prolactin show any change following the DPAG-evoked freezing and flight behaviors of the rat. Rats bearing an electrode into the DPAG and an intra-atrial catheter were stimulated at 9:00 a.m., 18-24 h after the catheter implantation. Blood samples were withdrawn just before 1-min stimulation of DPAG, immediately after (5 or 15 min) and throughout 3 to 27 h following stimulation. In another experiment, samples were withdrawn either before or following a prolonged stimulation (5 min) of the DPAG with flight threshold intensity. Hormones were measured by either chemiluminescent or double-antibody immunoassays. Hormone plasma levels following freezing and flight behaviors were compared to those of resting or restraint-stressed rats. Data show that stress hormones remain unaltered following the DPAG-evoked defensive behaviors. Not even the 5-min stimulation of DPAG with the flight threshold intensity changed corticotropin plasma levels significantly. As far as we known, this is the first demonstration of the lack of stress hormone responses following the intense emotional arousal and physical exertion of a fear-like behavior in rats. Data add new evidence of DPAG involvement in spontaneous panic attacks.