Chronic repetitive transcranial magnetic stimulation increases hippocampal neurogenesis in rats

Authors


Satoshi Ukai, MD, PhD, Department of Neuropsychiatry, Wakayama Medical University, 811-1, Kimiidera, Wakayama 641-0012, Japan. Email: ukai@wakayama-med.ac.jp

Abstract

Aim:  While the underlying therapeutic mechanisms of repetitive transcranial magnetic stimulation (rTMS) treatment for depression remain unclear, recent animal studies have suggested that hippocampal neurogenesis might be required for the effects of antidepressant treatments including antidepressant drugs and electroconvulsive therapy. The aim of this study was to examine chronic rTMS effects on hippocampal neurogenesis in rats.

Methods:  Using a 70-mm figure-of-eight coil, the stimulating parameters were set to 25 Hz and 70% of the rTMS device's maximum power. For 14 consecutive days, bromodeoxyuridine (BrdU) and 1000 pulses of rTMS were administered daily. Cell proliferation in the dentate gyrus was examined with immunohistochemistry.

Results:  In the rTMS-treated group, BrdU-positive cells were significantly increased in the dentate gyrus.

Conclusion:  Our results suggest that hippocampal neurogenesis might be involved in the antidepressant effects of chronic rTMS.

REPETITIVE TRANSCRANIAL MAGNETIC STIMULATION (rTMS) is a technique to repeatedly induce electric currents in a small area of the brain non-invasively. Recently, this technique has been applied to the treatment of several psychiatric and neurological diseases. Many clinical trials of rTMS have been conducted, most of which are for patients with depression.1,2 Sachdev et al. showed antidepressant effects of chronic rTMS in a forced swim test in rodents,3 and while many studies have examined the neurobiological therapeutic mechanisms of rTMS, they remain unclear.4,5

Recent studies have suggested that hippocampal neurogenesis might be required for the effects of antidepressant treatments, although it may not be a major contributor to the development of depression.6 In mice, antidepressant drug effects were disturbed by X-ray ablation of hippocampal neurogenesis.7 As well as the chronic administration of several antidepressant drugs, electroconvulsive shock (ECS), analogous to human electroconvulsive therapy, increased hippocampal neurogenesis in rodents8–10 and non-human primates.11

The aforementioned studies suggest that chronic rTMS could increase hippocampal neurogenesis and that this increase might be related to its therapeutic mechanisms on depression. However, to date, only one study has examined the effects of chronic rTMS on hippocampal neurogenesis in rodents and it did not show any significant increase of neurogenesis.12 The lack of significant effects of rTMS in this study might be related to non-optimal rTMS conditions, considering that the optimal conditions for rTMS in the treatment of depression in humans and in experimental rodent models are still unknown. Hence, in this preliminary study, we examined chronic rTMS effects on hippocampal neurogenesis in rats using conditions similar to those of Sachdev et al., which showed the antidepressant effects of chronic rTMS in the forced swim test in rats.3

METHODS

Animals

Sixteen-week-old male Sprague–Dawley rats (SLC Japan, Shizuoka, Japan) were used for all experiments. Rats were kept under standard conditions with a controlled 12-h light/dark cycle and fed standard diet and tap water ad libitum. The experimental protocol was approved by the Committee for Animal Experimentation of Osaka University Medical School. All efforts were made to minimize the number of animals used and their suffering.

rTMS treatment

Rats were randomly assigned to the control group (n = 5) or the rTMS-treatment group (n = 5). rTMS was administered with a 70-mm figure-of-eight coil using a Magstim Super Rapid (Magstim, Whitland, UK). The rTMS parameters were as follows: stimulating frequency = 25 Hz, stimulating pulse intensity = 70% of the rTMS device's maximum power, train duration = 10 s. Four successive trains of rTMS (1000 pulses per day) were administered daily for 14 consecutive days (14 000 pulses in total). The coil was placed horizontally over the scalp and its handle was aligned parallel with the body of the rat. For sham stimulation of the control group, the coil was placed perpendicular to the scalp and all other conditions were identical to the conditions in the rTMS group. The real and sham rTMS treatments did not induce seizures or any apparent behavioral changes.

Administration of bromodeoxyuridine

Bromodeoxyuridine (BrdU) (40 mg/kg in saline, Sigma, St. Louis, MO, USA), a thymidine analog that labels DNA during the S phase, was intraperitoneally administered to the two groups following the rTMS treatments daily (Fig. 1).

Figure 1.

Experimental schema. Repetitive transcranial magnetic stimulation (rTMS) and bromodeoxyuridine (BrdU) were administered daily for 14 consecutive days. Rats were killed 24 h after the last BrdU administration.

Tissue preparation

Twenty-four hours after the last BrdU administration, the rats were deeply anesthetized with sodium pentobarbital and transcardially perfused with saline, followed by 4% paraformaldehyde in 0.1 M phosphate-buffered saline. The brains were removed and postfixed in the same fixative at 4°C overnight and consecutive hippocampal paraffin sections 5 µm thick were prepared.

Immunohistochemistry

After deparaffinizing, slide-mounted sections were incubated in 2 N HCl for 2 h and washed in Tris-buffered saline (TBS). Sections were blocked in TBS containing 10% normal rabbit serum at room temperature (RT) for 1 h and incubated overnight at 4°C with anti-BrdU antibody (1:100, OBT0030, Oxford Biotechnology, Oxford, UK) in TBS containing 10% normal rabbit serum. The next day, the sections were washed and incubated with biotinylated rabbit anti-rat immunoglobulin G (IgG) antibody (1:400, Vector Laboratories, Burlingame, CA, USA) at RT for 1 h. After washing, the sections were incubated with avidin-biotin peroxidase complex (Vectastain Elite ABC Kit, Vector Laboratories) at RT for 1 h. Peroxidase was visualized with 0.05% 3,3′-diaminobenzidine tetrahydrochloride (Sigma) in TBS containing 0.01% hydrogen peroxide. Counterstaining was performed with hematoxylin.

For double immunofluorescence staining, sections were preincubated in TBS containing 5% normal donkey serum and 0.1% Triton X-100 at RT for 1 h, and then incubated with primary antibodies in 3% bovine serum albumin (BSA) and 0.1% Triton X-100 overnight at 4°C. The primary antibodies used for immunofluorescence staining were as follows: anti-BrdU, and anti-neuron-specific class III β-tubulin (TuJ1, 1:500, MMS-435P, Covance, Berkeley, CA, USA). After washing, sections were incubated at RT for 1 h with biotinylated donkey anti-rat IgG (1:400, Jackson ImmunoResearch, West Grove, PA, USA) and Cy3-conjugated donkey anti-mouse IgG (1:400, Jackson ImmunoResearch), containing 1% BSA and 0.1% Triton X-100. After rinsing, the sections were incubated with Cy2-conjugated streptavidin (Jackson ImmunoResearch) at RT for 1 h.

Quantitative analysis

Images of immunostained sections were captured from a microscope (Eclipse E800, Nikon, Tokyo, Japan) equipped with a color 3CCD camera (C5810, Hamamatsu Photonics, Hamamatsu, Shizuoka, Japan). The number of BrdU-immunoreactive cells in the granule cell layer (GCL) and the subgranular zone (SGZ, defined as two cell widths below the GCL) of the dentate gyrus was counted in six representative sections (−2.8 mm to −4.5 mm, relative to bregma according to the coordinates of Paxinos and Watson13) per animal using Adobe Photoshop software (Adobe Systems, San Jose, CA, USA) in a blinded fashion. The area of the GCL and the SGZ was quantified using NIH Image to estimate the number of BrdU-positive cells per unit area of the dentate gyrus. Statistical analysis was performed on the average number of BrdU-positive cells per section.

For immunofluorescent double labeling, sections were photographed using a Nikon Eclipse E800 microscope equipped with a VFM epi-FL attachment (Kawasaki, Kanagawa, Japan). At least 50 BrdU-positive cells per animal were analyzed to determine the proportions of BrdU-positive cells co-labeling with TuJ1.

The results are expressed as mean ± SEM. Differences between groups were compared using the Student's t-test. Statistical significance was defined as P < 0.05.

RESULTS

Immunohistochemical staining showed that the majority of BrdU-positive cells were in the SGZ. There were significantly more BrdU-positive cells in the dentate gyrus of the rTMS-treated group as compared with the control group (Figs 2,3). Double immunofluorescence staining showed that most of the BrdU-positive cells were co-labeled with the neuronal marker TuJ1 (Fig. 4). The proportion of cells co-labeled with TuJ1 did not differ significantly between the rTMS-treated and control groups (TuJ1 co-labeled cells, 81.3 ± 2.5% and 80.4 ± 3.1%, respectively).

Figure 2.

Bromodeoxyuridine-positive cells in the hippocampal dentate gyrus of (a) sham-treated control and (b) repetitive-transcranial-magnetic-stimulation-treated rats. Scale bar: 100 µm. GCL, granule cell layer; H, hilus.

Figure 3.

Quantification of bromodeoxyuridine (BrdU)-positive cells in the dentate gyrus. Chronic repetitive transcranial magnetic stimulation (rTMS) treatment significantly increased BrdU-positive cells (Control, 63.1 ± 5.1 cells/mm2; rTMS, 102.3 ± 6.4 cells/mm2). Results are shown as mean ± SEM. *P < 0.05 vs control.

Figure 4.

Double labeling with bromodeoxyuridine (BrdU) and neuronal marker anti-neuron-specific class III β-tubulin (TuJ1) after chronic repetitive transcranial magnetic stimulation (rTMS) treatment. Co-localization of (a, green) nuclear BrdU staining and (b, red) cytoplasmic TuJ1 staining. (c) The merged image shows TuJ1-positive cytoplasm surrounding BrdU-labeled nuclei (arrows). Scale bar: 10 µm. GCL, granule cell layer; H, hilus.

DISCUSSION

In the present study, we examined the effects of chronic rTMS on neurogenesis in the dentate gyrus of adult rats. Our results showed that the number of subgranular progenitor cells was significantly increased in the dentate gyrus. To our knowledge, this is the first report that chronic rTMS increased hippocampal neurogenesis.

Our results are in line with previous studies that showed that chronic treatments with ECS or various antidepressant drugs increase hippocampal neurogenesis in rodents,6,8–10 and non-human primates.11 Hence, it appears that hippocampal neurogenesis might be involved in the antidepressant effects of chronic rTMS, although our study did not utilize an animal model of depression or behavioral assessment.

In the present study, we used the similar conditions of chronic rTMS to those described by Sachdev et al., who showed the antidepressant effects of rTMS in the forced swim test.3 We used the same rTMS device and the same figure-of-eight coil placed over the scalp with identical alignment. The rTMS parameters were also similar to them (25 Hz stimulating frequency, 70% of the rTMS device's maximum power, 1000 pulses per day). The stimulating frequency was set to 25 Hz because Sachdev et al. showed that this frequency was most effective among the four frequencies tested (1, 5, 15, and 25 Hz). However, while they assessed the effects of rTMS on the second day after the five daily rTMS treatments, we conducted rTMS treatment for 14 consecutive days according to the schedule of the most recent human clinical trials on depression.1,2

Only one study, reported by Czéh et al., has examined chronic rTMS effects on hippocampal neurogenesis in rats, and it showed that neurogenesis was not significantly increased.12 In contrast with this study, we used a faster stimulating frequency (25 Hz vs 20 Hz) and more total pulses (14 000 pulses vs 5400 pulses). Our use of more powerful chronic rTMS treatment seems to be more appropriate for increasing hippocampal neurogenesis in rats.

The set of the conditions that modulate the intensity and distribution of electric currents and fields induced by a single pulse in the rat brain (e.g. the shape, size, and location of the coil relative to the rodent small brain) is another important consideration.14 Czéh et al. used a smaller round coil over the left frontal brain region and theoretically estimated the characteristics of the intensity and distribution of electric currents and fields.12 Further studies are needed to evaluate their characteristics in the present study conditions and how they influence the effect of rTMS on hippocampal neurogenesis in rodents.

While most of the previous studies examined hippocampal neurogenesis roughly 1 month after a single or several injections of BrdU, our examinations were conducted on the next day after completion of 14 daily rTMS and BrdU treatments, and we assessed the overall proliferation during the daily treatments. Therefore, our results should be interpreted cautiously when comparisons are made with the results of the previous studies. In addition, the survival of nascent cells was not examined in our study. For more exact comparisons and discussions, further studies will be necessary to set the protocol of the BrdU treatment according to the previous studies under similar conditions of the chronic rTMS of our study.

In conclusion, the present study demonstrated an increase of hippocampal neurogenesis in rats using 14-day chronic rTMS, and it appeared that this increase might be related to the antidepressant effects of rTMS. To examine this relationship more exactly, further studies are needed using an animal model of depression and antidepressant drug-treated animal groups. While a standard rTMS protocol for the treatment of human depression has not been established, our results, even though not directly applicable to humans, could contribute to determining the optimal clinical rTMS conditions for such treatment.

ACKNOWLEDGMENTS

This research was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of the Japanese Government (13671001) and in part by a grant from Mitsubishi Pharma Research Foundation.

Ancillary