S-Ketamine Rapidly Reverses Synaptic and Vascular Deficits of Hippocampus in Genetic Animal Model of Depression

Abstract

Background: The neurovascular plasticity of hippocampus is an important theory underlying major depression. Ketamine as a novel glutamatergic antidepressant drug can induce a rapid antidepressant effect within hours. In a mechanistic proof of this concept, we examined whether ketamine leads to an increase in synaptogenesis and vascularization within 24 hours after a single injection in a genetic rat model of depression.

Methods: Flinders Sensitive Line and Flinders Resistant Line rats were given a single intraperitoneal injection of ketamine (15 mg/kg) or saline. One day later, their behavior was evaluated by a modified forced swim test. Microvessel length was evaluated with global spatial sampling and optical microscopy, whereas the number of asymmetric synapses was quantified through serial section electron microscopy by using physical disector method in the CA1.stratum radiatum area of hippocampus.

Results: The immobility time in the forced swim test among Flinders Sensitive Line rats with ketamine treatment was significantly lower compared with Flinders Sensitive Line rats without treatment. The number of nonperforated and perforated synapses was significantly higher in the Flinders Sensitive Line-ketamine vs the Flinders Sensitive Line-vehicle group; however, ketamine did not induce a significant increase in the number of shaft synapses. Additionally, total length of microvessels was significantly increased 1 day after ketamine treatment in Flinders Sensitive Line rats in the hippocampal subregions, including the CA1.stratum radiatum.

Conclusion: Our findings indicate that hippocampal vascularization and synaptogenesis is co-regulated rapidly after ketamine, and microvascular elongation may be a supportive factor for synaptic plasticity and neuronal activity. These findings go hand-in-hand with the behavioral observations, where ketamine acts as a potent antidepressant.

Keywords: antidepressant; hippocampus; ketamine; synaptic plasticity; vascularization.

Figure 1.

Estimation of the length of microvessels in a 65-µm-thick vibratome section stained with thionin and visualized on a light microscope with a 60× objective oil immersion lens. Green test lines are superimposed on the live image by newCAST software, and they represent the intersection between isotropic virtual planes and the focal plane. Whenever the microvessels were in focus and virtual planes intersect them, they were counted. One microvessel is intersecting a virtual plane (arrow). Scale bar = 10 µm.

Figure 2.

Schematic illustration of tissue processing of sections for studying under electron microscopy. (A) Rat brain hemisphere, scale bar 5.5 = mm. (B) Isolated rat hippocampus, scale bar 5.5 = mm. (C) Epon block containing osmium tetroxide stained hippocampal section of thickness 65 µm, scale bar = 5.5 mm. (D) 65-µm-thick-Nissl stained vibratome section with trapezoid-shaped area of interest, scale bar = 0.55 mm. (E) Disector electron micrographs from CA1.SR hippocampal subregion with the 2D unbiased counting frame for counting the number of perforated spine synapses (arrow), nonperforated spine synapses (arrowhead). Scale bar = 2 µm.

Figure 3.

Perforated (arrow), nonperforated (arrowhead) spine, and shaft (star) synapses in electron micrograph from CA1 stratum radiatum subfield of hippocampus. Scale bar = 2 µm.

Figure 4.

Fast effect of a single i.p. ketamine injection on immobility behavior in forced swim test (FST) in Flinders Sensitive Line (FSL) and Flinders Resistant Line (FRL) rats. *P<.05 and ***P<.001.

Figure 5.

Total length of microvessels in CA1.SR area of hippocampus in male Flinders Sensitive Line (FSL) and Flinders Resistant Line (FRL) rats 1 day after a single ketamine injection, **P<.01.

Figure 6.

Total length of microvessels in molecular layer of dentate gyrus (MDG) area of hippocampus in male Flinders Sensitive Line (FSL) and Flinders Resistant Line (FRL) rats 1 day after a single ketamine injection, *P<.05 and ***P<.001.

Figure 7.

Volume of CA1.SR area of hippocampus in male Flinders Sensitive Line (FSL) and Flinders Resistant Line (FRL) rats 1 day after a single ketamine or saline administration, **P<.01, ***P<.001.

Figure 8.

Volume of the molecular layer of dentate gyrus (MDG) area of hippocampus in male Flinders Sensitive Line (FSL) and Flinders Resistant Line (FRL) rats 1 day after a single ketamine or saline administration, *P < .05.

Figure 9.

A significant negative correlation between the duration of immobility behavior in forced swim test (FST) and the volume of CA1.SR subregion of hippocampus (r=-0.52, P=.008).