Chronic mild stress induces anhedonic behavior and changes in glutamate release, BDNF trafficking and dendrite morphology only in stress vulnerable rats. The rapid restorative action of ketamine

Abstract

Depression is a debilitating mental disease, characterized by persistent low mood and anhedonia. Stress represents a major environmental risk factor for depression; the complex interaction of stress with genetic factors results in different individual vulnerability or resilience to the disorder. Dysfunctions of the glutamate system have a primary role in depression. Clinical neuroimaging studies have consistently reported alterations in volume and connectivity of cortico-limbic areas, where glutamate neurons and synapses predominate. This is confirmed by preclinical studies in rodents, showing that repeated stress induces morphological and functional maladaptive changes in the same brain regions altered in humans. Confirming the key role of glutamatergic transmission in depression, compelling evidence has shown that the non-competitive NMDA receptor antagonist, ketamine, induces, at sub-anesthetic dose, rapid and sustained antidepressant response in both humans and rodents. We show here that the Chronic Mild Stress model of depression induces, only in stress-vulnerable rats, depressed-like anhedonic behavior, together with impairment of glutamate/GABA presynaptic release, BDNF mRNA trafficking in dendrites and dendritic morphology in hippocampus. Moreover, we show that a single administration of ketamine restores, in 24 h, normal behavior and most of the cellular/molecular maladaptive changes in vulnerable rats. Interestingly, ketamine treatment did not restore BDNF mRNA levels reduced by chronic stress but rescued dendritic trafficking of BDNF mRNA. The present results are consistent with a mechanism of ketamine involving rapid restoration of synaptic homeostasis, through re-equilibration of glutamate/GABA release and dendritic BDNF for synaptic translation and reversal of synaptic and circuitry impairment.

Keywords: Antidepressant; BDNF; Chronic stress; Glutamate release; Ketamine; Stress vulnerability.

Fig. 1

(a) Experimental plan: animals were subjected to a variable sequence of mild stressors for five weeks. Sucrose preference test (SPT) was performed to evaluate anhedonic behavior. KET or vehicle (VEH) were acutely administered 24 h before sacrifice. (b) SPT of CNT and CMS rats at Day +22 of CMS. n = CNT 101; CMS 188. Unpaired t-test: **p < 0.001 vs CNT; (c) Separation of resilient and vulnerable animals applying a cut-off at 55% of sucrose preference at Day +22 of CMS. n = CNT 101; CMS-R 100; CMS-V 88. (d) SPT of CNT and CMS rats at Day +35 of CMS, 24 h after KET/VEH treatment n = CNT 59; CMS-R 53; CMS-V 27; CMS-V + KET 25. (e) Body weight gain; n = CNT 101; CMS-R 100; CMS-V 88. (f) Adrenal glands weight. n = CNT 54; CMS-R 48; CMS-V 25; CMS-V + KET 21. (g) CORT serum levels. n = CNT 30; CMS-R 27; CMS-V 15; CMS-V + KET 15. Data are shown as means ± standard error of the mean. TPHT: *p < 0.05 vs CNT; **p < 0.001 vs CNT; ^#p < 0.05 vs CMS-R; ^##p < 0.001 vs CMS-R; ^§§p < 0.001 vs CMS-V.

Fig. 2

(a) Basal glutamate release from HPC synaptosomes in superfusion. n = CNT 14; CMS-R 16; CMS-V 7; CMS-V + KET 9. (b) 15 mM KCl-evoked glutamate release from HPC synaptosomes in superfusion. n = CNT 11; CMS-R 16; CMS-V 6; CMS-V + KET 8. (c) Basal GABA release from HPC synaptosomes in superfusion. n = CNT 18; CMS-R 21; CMS-V 10; CMS-V + KET 9. (d) 15 mM KCl-evoked GABA release from HPC synaptosomes in superfusion. n = CNT 9; CMS-R 13; CMS-V 6; CMS-V + KET 6. TPHT: *p < 0.05 vs CNT; **p < 0.001 vs CNT; ^#p < 0.05 vs CMS-R; ^##p < 0.05 vs CMS-R; ^§p < 0.05 vs CMS-V.

Fig. 3

(a) Levels of total BDNF transcripts and of BDNF splice variant transcripts containing exon 1 (BDNF-1), 2 (BDNF-2), 4 (BDNF-4) or 6 (BDNF-6) in HPC homogenate. n = CNT 8; CMS-R 9; CMS-V 10; CMS-V + KET 10. (b) BDNF protein levels in HPC homogenate. n = CNT 13; CMS-R 13; CMS-V 14; CMS-V + KET 14. TPHT: *p < 0.05 vs CNT; **p < 0.001 vs CNT; ^#p < 0.05 vs CMS-R.

Fig. 4

(a) Representative images of in situ hybridization of total BDNF mRNA in CA1 and CA3 regions of HPC in CNT, CMS-R, CMS-V and CMS-V + KET rats. (b) Dendritic trafficking of total BDNF transcripts in CA1. (c) Dendritic trafficking of total BDNF transcripts in CA3. (d) Dendritic trafficking of BDNF splice variant transcripts containing exon 2 (BDNF-2) in CA1. (e) Dendritic trafficking of BDNF splice variant transcripts containing exon 2 (BDNF-2) in CA3. (f) Dendritic trafficking of BDNF splice variant transcripts containing exon 6 (BDNF-6) in CA1. (g) Dendritic trafficking of BDNF splice variant transcripts containing exon 6 (BDNF-6) in CA3. n = 3–4 rats/group, 2–3 slices/rat, 80–100 dendrites/slice. MCA: **p < 0.001 vs CNT; ^##p < 0.001 vs CMS-R; ^§§p < 0.001 vs CMS-V.

Fig. 5

(a) Total dendritic length of CA3 apical dendrites. (b) Branching number of CA3 apical dendrites. (c) Total dendritic length of CA3 basal dendrites. (d) Branching number of CA3 basal dendrites. (e) Sholl analysis of CA3 apical dendrites. (f) Sholl analysis of CA3 basal dendrites. n = CNT 11; CMS-R 11; CMS-V 10; CMS-V + KET 10. TPHT: *p < 0.05 vs CNT; ^#p < 0.05 vs CMS-R; ^§p < 0.05 vs CMS-V. (g) Representative drawings of CA3 pyramidal neurons reconstructed with Imaris software.