Intra-BLA injection of orexin A counteracts the innate fear-induced analgesia

Background and Aim : The amygdala is in almond-shape nucleus that has a pivotal role in the expression and modulation of fear and anxiety. This complex structure has massive neural connections with dorsomedial and ventromedial hypothalamus (respectively DMH and VMH) nuclei and lateral hypothalamus (LH). DMH/VMH nuclei play a key role in the modulation of responses to harmful and fearful stimuli. Also, LH is rich in orexinergic neurons which modulate various brain functions such as appetite, sleep-wake cycle and pain. Moreover, neural projections of the descending pain modulatory pathway are originating from the amygdala and terminated in the dorsal horn of spinal cord. Therefore, the aim of this study was to induce the innate fear, to examine the innate fear-induced analgesia and to assess the effect of intra-BLA orexin A on the innate fear-induced analgesia in the thermal pain.Methods : In this study, male Wistar rats weighing 200–270 g (n=7 per group) were purchased 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 and BLA nuclei. After recovery period, 40 ng/300 nl bicuculline was injected into the DMH/VMH nuclei and then the innate fear-induced behaviors were studied by open field test. To confirm the innate fear induction, the following responses were assessed 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. After open field test, orexin A was immediately injected into the BLA nucleus. 10 Minutes later, the tail flick and hot plate tests were done by 60 seconds intervals. Every test was recorded for 70 minutes with 10 minutes intervals. At the end of the tests, animals were anesthetized and their brains were removed and examined the correct cannula implantation in the DMH/VMH and BLA nuclei.Results : : Our 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 findings confirm the innate fear induction. The innate fear induction reduced the sensitivity to thermal pain in the tail flick and hot plate tests. Intra-BLA injection of orexin A counteracted the innate fear-induced analgesia in the tail flick test. However, orexin A failed to prevent the innate fear-induced analgesia in the hot plate test. Conclusion : : In agreement with our results, it can be concluded that the inhibition of the GABAergic system of DMH/VMH nuclei can induce the innate fear and consequently stress-induced analgesia. The intra-BLA injection of orexin A counteracted the innate fear-induced analgesia.

کلیدواژه ها:

Orexin A ، innate fear induced analgesia ، Pain modulation