Facilitation of TRKB Activation by the Angiotensin II Receptor Type-2 (AT2R) Agonist C21

Abstract

Blockers of angiotensin II type 1 receptor (AT1R) exert antidepressant-like effects by indirectly facilitating the activation of the angiotensin II type 2 receptor (AT2R), which leads to increased surface expression and transactivation of tropomyosin-related kinase B receptors (TRKB). Compound 21 (C21) is a non-peptide AT2R agonist that produces neuroprotective effects. However, the behavioral effects of C21 and its involvement with the brain-derived neurotrophic factor (BDNF)-TRKB system still need further investigation. The aim of the present study was to assess the effect of C21 on the activation of TRKB and its consequences on conditioned fear. The administration of C21 (0.1–10 μM/15 min) increased the surface levels of TRKB but was not sufficient to increase the levels of phosphorylated TRKB (pTRKB) in cultured cortical neurons from rat embryos. Consistent with increased TRKB surface expression, C21 (10 μM/15 min or 3 days) facilitated the effect of BDNF (0.1 ng/mL/15 min) on pTRKB in these cells. In contextual fear conditioning, the freezing time of C21-treated (administered intranasally) wild-type mice was decreased compared to the vehicle-treated group, but no effect of C21 was observed in BDNF.het animals. We observed no effect of C21 in the elevated plus-maze test for anxiety. Taken together, our results indicate that C21 facilitated BDNF effect by increasing the levels of TRKB on the cell surface and reduced the freezing time of mice in a BDNF-dependent manner, but not through a general anxiolytic-like effect.

Mutation in the TRKB cholesterol recognition site that blocks antidepressant binding does not influence the basal or BDNF-stimulated activation of TRKB

“…No genotype effect of Y433F mutation was observed in the ability to discriminate between conditioned and unconditioned contexts [interaction: F(1,15)=0.6472; P=0.4337; trial: F(1,15)=45.87; P < 0.0001; genotype: F(1,15)=0.5711, P=0.4615; N=8,9], figure 4B. For comparison, BDNF.het mice have compromised performance in this test [interaction F(1,10)=6.745, P=0.0266; trial: F(1,10)=15.64, P=0.0027; genotype: F(1,10)=12.57, P=0.0053; N=6/group], figure 4C, as observed previously (Laukkanen et al, 2021), as well as in the extinction of conditioned fear (Karpova et al, 2011).…”

“…Context Discrimination Task -CDT. Male TRKB.Y433F mice and their TRKB.wt littermates were submitted to context discrimination task as described previously (Michels et al, 2018;Laukkanen et al, 2021). Briefly, the animals were acclimated to the experimental room for 1h, following 5 min habituation to the experimental chamber, the animals received 3 scrambled shocks (0.6mA/2s, intervals 30s-1min) in context A (conditioning, transparent walls, LxWxH: 23×23×35cm, with metal grid bottom) followed by a 2min period without any shocks.…”

“…Elevated Plus Maze -EPM. Male and female TRKB.Y433F mice and their TRKB.wt littermates were submitted to the elevated plus maze as described previously (Laukkanen et al, 2021). Briefly, the animals were acclimated to the experimental room for 1h, and submitted (5 min session) to an acrylic-built elevated plus-maze.…”

Sex-dependent effects of angiotensin type 2 receptor expressing medial prefrontal cortex (mPFC) interneurons in fear extinction learning

“…Recent studies from our lab and others have demonstrated AT2R's role in fear extinction (11,18,70). This, as well as its direct translation to exposure-based PTSD therapies in humans (33,44,45), showed extinction learning as an ideal model for assessing the functional effect of AT2R on fear memory.…”

“…The behavioral effects that we see with mPFC AT2R deletion may additionally be dependent on a TrkB/BDNF signaling pathway, as there is emerging evidence that cortical AT2R is dependent on co-expression with TrkB to influence learned fear (18,70,72). Further studies will directly interrogate the electrophysiological impact of AT2R activation or deletion on somatostatin interneuron activity and mPFC projections, as well as whether chemogenetic activation of AT2Rexpressing mPFC interneurons can rescue or facilitate fear extinction.…”