The effects of antidepressant treatment in prenatally stressed rats support the glutamatergic hypothesis of stress-related disorders

Abstract

Abnormalities of synaptic transmission in the hippocampus represent an integral part of the altered programming triggered by early life stress, which enhances the vulnerability to stress-related disorders in the adult life. Rats exposed to prenatal restraint stress (PRS) develop enduring biochemical and behavioral changes characteristic of an anxious/depressive-like phenotype. Most neurochemical abnormalities in PRS rats are found in the ventral hippocampus, a region that encodes memories related to stress and emotions. We have recently demonstrated a causal link between the reduction of glutamate release in the ventral hippocampus and anxiety-like behavior in PRS rats. To confer pharmacological validity to the glutamatergic hypothesis of stress-related disorders, we examined whether chronic treatment with two antidepressants with different mechanisms of action could correct the defect in glutamate release and associated behavioral abnormalities in PRS rats. Adult unstressed or PRS rats were treated daily with either agomelatine (40 mg/kg, i.p.) or fluoxetine (5 mg/kg, i.p.) for 21 d. Both treatments reversed the reduction in depolarization-evoked glutamate release and in the expression of synaptic vesicle-associated proteins in the ventral hippocampus of PRS rats. Antidepressant treatment also corrected abnormalities in anxiety-/depression-like behavior and social memory performance in PRS rats. The effect on glutamate release was strongly correlated with the improvement of anxiety-like behavior and social memory. These data offer the pharmacological demonstration that glutamatergic hypofunction in the ventral hippocampus lies at the core of the pathological phenotype caused by early life stress and represents an attractive pharmacological target for novel therapeutic strategies.

Figure 1.

Chronic treatment with agomelatine or fluoxetine corrects abnormalities in the expression of synaptic vesicle-associated proteins in the ventral hippocampus of PRS rats. Control unstressed rats (CONT) and PRS rats were treated intraperitoneally with vehicle, agomelatine (40 mg/kg), or fluoxetine (5 mg/kg) for 21 d. Representative continuous, uncropped images of 12 sample immunoblots (2 samples per group per treatment) are shown on the left side (C, control unstressed rats; P, PRS rats). Values are given as the mean ± SEM (n = 4 rats per group). *p < 0.05 or **p < 0.01 vs the respective CONT rats; #p < 0.05 or ##p < 0.01 vs vehicle-treated CONT or PRS rats.

Figure 2.

Chronic treatment with agomelatine or fluoxetine largely restores glutamate release in synaptosomes prepared from the ventral hippocampus of PRS rats. A, B, Depolarization-evoked glutamate (A) or GABA (B) release was assessed in superfused synaptosomes prepared from control unstressed rats (CONT) and PRS rats treated with vehicle, agomelatine (40 mg/kg), or fluoxetine (5 mg/kg) for 21 d. Values are given as the mean ± SEM (n = 5 rats per group). *p < 0.05 or **p < 0.01 vs the respective CONT rats; ##p < 0.01 vs vehicle-treated CONT or PRS rats.

Figure 3.

Chronic treatment with agomelatine or fluoxetine corrects anxiety- and depression-like behaviors in PRS rats. A, B, The same groups of rats used for the assessment of glutamate release were examined in the light-dark box (A), and in the forced swim test (B), as indicated in the Methods session. Different groups of rats were tested in the splash test (C). Control unstressed rats (CONT) and PRS rats were treated intraperitoneally with vehicle, agomelatine (40 mg/kg), or fluoxetine (5 mg/kg) for 21 d. Values are given as the mean ± SEM (n = 9 rats per group). *p < 0.05 or **p < 0.01 vs the respective CONT rats; #p < 0.05 or ##p < 0.01 vs vehicle-treated CONT or PRS rats.

Figure 4.

Chronic treatment with agomelatine or fluoxetine improves social memory in PRS rats. Control unstressed rats (CONT) and PRS rats were treated intraperitoneally with vehicle, agomelatine (40 mg/kg), or fluoxetine (5 mg/kg) for 21 d. Data on sniffing behavior at the first (T1), second (T2), and third (T3) exposure to the juvenile challenger are shown in A. The percentage of reduction at the second exposure respect to the first is shown in B. Values are means ± SEM (n = 9 rats per group). **p < 0.01 vs the respective CONT rats; #p < 0.05 vs vehicle-treated CONT or PRS rats.

Figure 5.

Correlation analysis of depolarization evoked glutamate release in the ventral hippocampus toward anxiety-like behavior in the light-dark box, depression-like behavior in the forced swim test, and social memory performance. Pearson's correlation coefficient (r) values and related p values are reported in Table 1 (n = 5 rats per group). CONT, Control unstressed rats.