Intra-DMH/VMH injection of bicuculline can induce the innate fear that followed by analgesia

Background and Aim : The hypothalamus plays a key role in the expression of fear and defensive responses. The dorsomedial and ventromedial hypothalamic (respectively DMH and VMH) nuclei were considered to be essential for handling the threat and fearful situations. The blockade of GABAA receptors in the DMH and VMH nuclei by bicuculline induces augmented defensive behavioral responses which has been known as innate fear. Furthermore, the exposure to unconditioned or conditioned stressful stimuli can suppress the pain which universally known as stress-induced analgesia. Thus, induction of innate fear can produce the stress-induced analgesia by activating the descending inhibitory pain pathway. Therefore, the aim of this study was to investigate the possible involvement of the DMH and VMH nuclei in the elaboration of panic-like reactions and in the innate fear-induced antinociception.Methods : In this study, male Wistar rats weighing 200–270 g (n=7 per group) were obtained from the animal facility of Baqiyatallah University of Medical Sciences. Animals were anaesthetized with 60 mg/kg ketamine and 7.5 mg/kg xylazine and fixed in a stereotaxic apparatus. The stainless steel 23-gauge guide cannulas equipped with a 30-gauge stylet were unilaterally implanted in the right DMH/VMH nuclei. After recovery period, 40 ng/300 nl bicuculline was injected into the DMH/VMH nuclei and then the innate fear-induced behaviors were evaluated by open field test. To assess the innate fear induction, the following behaviors were studied over 10 min using a Sony Handycam camera: the crossings; the rearing (upright posture); the rapid defensive backward movements; the elaborated forward escape behavior; the defensive attention; the defensive immobility (‘freezing’) and the jumping oriented to the upper side of the arena. Once open field test was finished, 50 μl of 2.5% formalin was immediately injected subcutaneously (s.c.) into the plantar surface of left hind paw of rats. The formalin-induced flinching, flexing and licking behavior was recorded for 60 min. In this experiment, the formalin-evoked behaviors were divided into two phases including the first phase (0–7 min) and the second phase (16–60 min) separated with a relative inactivity interval. At the end of the tests, animals were killed and their brains were removed and examined for the correct cannula implantation in the DMH/VMH nuclei.Results : These results demonstrated that the intra-DMH/VMH injection of bicuculline significantly increased the frequency and duration of defensive attention, the frequency and duration of defensive immobility (‘freezing’), the frequency and duration of rearing (upright posture) and the frequency jumping behavior. These results approved the innate fear induction. The intra-DMH/VMH injection of bicuculline led to significant decrease in flinches (P < 0.001), exing (P < 0.001) and licking (P < 0.01) duration just during the first phase of the formalin test. Conclusion : According to our results, it can be concluded that the inhibition of the GABAergic system of DMH/VMH nuclei can induce the innate fear. Consequently, the innate fear induction can led to stress-induced analgesia.

Keywords:

DMH/VMH nuclei , Bicuculline , innate fear , Stress-induced analgesia