Angiotensinergic Neurotransmissions in the Medial Amygdala Nucleus Modulate Behavioral Changes in the Forced Swimming Test Evoked by Acute Restraint Stress in Rats

Abstract

We investigated the role of angiotensin II type 1 (AT₁ receptor) and type 2 (AT₂ receptor) and MAS receptors present in the medial amygdaloid nucleus (MeA) in behavioral changes in the forced swimming test (FST) evoked by acute restraint stress in male rats. For this, rats received bilateral microinjection of either the selective AT₁ receptor antagonist losartan, the selective AT₂ receptor antagonist PD123319, the selective MAS receptor antagonist A-779, or vehicle 10 min before a 60 min restraint session. Then, behavior in the FST was evaluated immediately after the restraint (15 min session) and 24 h later (5 min session). The behavior in the FST of a non-stressed group was also evaluated. We observed that acute restraint stress decreased immobility during both sessions of the FST in animals treated with vehicle in the MeA. The decreased immobility during the first session was inhibited by intra-MeA administration of PD123319, whereas the effect during the second session was not identified in animals treated with A-779 into the MeA. Microinjection of PD123319 into the MeA also affected the pattern of active behaviors (i.e., swimming and climbing) during the second session of the FST. Taken together, these results indicate an involvement of angiotensinergic neurotransmissions within the MeA in behavioral changes in the FST evoked by stress.

Keywords: amygdala; angiotensin; depression; rodents; stress.

Figure 1

Microinjection sites into the MeA. (Left) Photomicrograph of a coronal brain section illustrating bilateral injection sites in the medial amygdaloid nucleus (MeA) of a representative animal. (Right) Diagrammatic representation based on Paxinos & Watson [40] showing the microinjection sites in the medial amygdaloid nucleus (MeA) of the selective AT₁ receptor antagonist losartan (red circles), the selective MAS receptor antagonist A-779 (blue circles), the selective AT₂ receptor antagonist PD123319 (green circles) and vehicle (white circles). The number shown in the figure can be smaller than the total number of animals used in this experiment due to the overlap of some injection sites. Opt—optical tract.

Figure 2

Immobility time and latency to the first bout of immobility in the forced swimming test (FST) of animals submitted to an acute session of restraint stress and that received bilateral microinjection into the medial amygdaloid nucleus (MeA) of the selective AT₁ receptor antagonist losartan (1 nmol/100 nL, n = 9), the selective MAS receptor antagonist A-779 (0.1 nmol/100 nL, n = 10), the selective AT₂ receptor antagonist PD123319 (5 nmol/100 nL, n = 8) or the vehicle (saline, 100 nL, n = 21); as well as animals who were not submitted to restraint stress (non-stressed, n = 15). (Top) Immobility time each 5 min during the pre-test session (first session, left), which was performed immediately after the end of the acute restraint session, and during the entire test session (second session, right) that occurred 24 h after the pre-test session of the FST. The bars and circles represent the mean ± SEM. Results of the first session were analyzed using two-way ANOVA followed by Bonferroni post-hoc test, whereas one-way ANOVA was used in the results of the second session. * p < 0.05 vs. non-stressed group (during the entire period for the results of the first session); # p < 0.05 vs. vehicle group (subjected to restraint stress) during the entire period. (Bottom) Latency to the first bout of immobility during the first and second sessions of the FST. The bars represent the mean ± SEM. One-way ANOVA followed by Bonferroni post-hoc test.

Figure 3

Swimming and climbing time in the forced swimming test (FST) of animals submitted to an acute session of restraint stress and that received bilateral microinjection in the medial amygdaloid nucleus (MeA) of the selective AT₁ receptor antagonist losartan (1 nmol/100 nL, n = 9), the selective MAS receptor antagonist A-779 (0.1 nmol/100 nL, n = 10), the selective AT₂ receptor antagonist PD123319 (5 nmol/100 nL, n = 8) or the vehicle (saline, 100 nL, n = 21); as well as animals who were not submitted to restraint stress (non-stressed, n = 15). (Top) Swimming time each 5 min during the pre-test session (first session, left), which was performed immediately after the end of the acute restraint session, and during the entire test session (second session, right) that occurred 24 h after the pre-test session of the FST. (Bottom) Climbing time each 5 min during the pre-test session (first session, left), and during the entire test session (second session, right). The bars represent the mean ± SEM. Results of the first session were analyzed using two-way ANOVA, whereas one-way ANOVA followed by Bonferroni post-hoc test was used in the results of the second session. # p < 0.05 vs. vehicle group (subjected to restraint stress).