Molecular Psychiatry (2006) 11, 361–371 & 2006 Nature Publishing Group All rights reserved 1359-4184/06 $30.00 www.nature.com/mp ORIGINAL ARTICLE Modulation of adult hippocampal neurogenesis by thyroid hormones: implications in depressive-like behavior A Montero-Pedrazuela1, C Venero2, R Lavado-Autric1, I Ferna´ndez-Lamo1, JM Garcı´a-Verdugo3, J Bernal1 and A Guadan˜o-Ferraz1 1Department of Molecular Endocrinology, Instituto de Investigaciones Biome´dicas Alberto Sols, Consejo Superior de Investigaciones Cientı´ficas-Universidad Auto´noma de Madrid, Madrid, Spain; 2Department of Psychobiology, Universidad Nacional de Educacio´n a Distancia, Madrid, Spain and 3Laboratorio de Morfologia Celular, Unidad Mixta CIPF-UVEG, Valencia, Spain Hormonal imbalances are involved in many of the age-related pathologies, as neurodegen- erative and psychiatric diseases. Specifically, thyroid state alterations in the adult are related to psychological changes and mood disorders as depression. The dentate gyrus of the hippocampal formation undergoes neurogenesis in adult mammals including humans. Recent evidence suggests that depressive disorders and their treatment are tightly related to the number of newly born neurons in the dentate gyrus. We have studied the effect of thyroid hormones (TH) on hippocampal neurogenesis in adult rats in vivo. A short period of adult- onset hypothyroidism impaired normal neurogenesis in the subgranular zone of the dentate gyrus with a 30% reduction in the number of proliferating cells. Hypothyroidism also reduced the number of newborn neuroblasts and immature neurons (doublecortin (DCX) immunopo- sitive cells) which had a severely hypoplastic dendritic arborization. To correlate these changes with hippocampal function, we subjected the rats to the forced swimming and novel object recognition tests. Hypothyroid rats showed normal memory in object recognition, but displayed abnormal behavior in the forced swimming test, indicating a depressive-like disorder. Chronic treatment of hypothyroid rats with TH not only normalized the abnormal behavior but also restored the number of proliferative and DCX-positive cells, and induced growth of their dendritic trees. Therefore, hypothyroidism induced a reversible depressive-like disorder, which correlated to changes in neurogenesis. Our results indicate that TH are essential for adult hippocampal neurogenesis and suggest that mood disorders related to adult-onset hypothyroidism in humans could be due, in part, to impaired neurogenesis. Molecular Psychiatry (2006) 11, 361–371. doi:10.1038/sj.mp.4001802; published online 31 January 2006 Keywords: thyroid hormones; adult neurogenesis; dentate gyrus; proliferation; doublecortin; depressive disorder Introduction serotonergic, and GABAergic systems.3 Mood disor- ders have a complex etiology, and the understanding Hormonal imbalances are involved in many age- of the biological processes affected is of clinical related pathologies, such as neurodegenerative and relevance. Recent evidence suggests that depressive psychiatric disorders. Hypothyroidism is a prevalent disorders and their response to treatment are tightly condition in humans with an incidence of 8% in the related to the number of newly born neurons in the adult population.1 The clinical picture includes dentate gyrus (DG).4,5 cognitive, attention, and mood disorders such as Adult neurogenesis takes place in two main depression, in many cases suggesting hippocampal neurogenic areas: the subventricular zone (SVZ) alterations. Most symptoms usually remit after TH adjacent to the lateral ventricles which generates replacement, but some may persist, especially after olfactory bulb interneurons, and the subgranular zone prolonged hypothyroidism.2 The mechanisms respon- (SGZ) which gives rise to granular neurons in the sible for these alterations have not been clarified, hippocampal DG.6–9 although many studies point out to disturbances in In the DG, granular neurons are generated through- neurotransmission mainly affecting noradrenergic, out adulthood from progenitor cells within tight proliferative clusters located around small capillaries Correspondence: Dr A Guadan˜ o-Ferraz, Instituto de Investiga- in the SGZ.10 The neuroblasts then migrate a short ciones Biome´dicas, CSIC-UAM, Arturo Duperier 4, E-28029 distance to the granular layer and differentiate Madrid, Spain. to mature granular neurons. Newly generated E-mail: [email protected] Received 25 October 2005; revised 19 December 2005; accepted neuroblasts and immature neurons in the adult 3 January 2006; published online 31 January 2006 DG specifically express doublecortin (DCX),11 a Thyroid hormones: adult neurogenesis and depression A Montero-Pedrazuela et al 362 microtubule-associated phosphoprotein also ex- P120 (P120R) and compared with untreated hypothyr- pressed in migrating neuroblasts and immature oid (P120H) and euthyroid controls (P120E). neurons during development. Most importantly, Hypothyroidism was induced in all cases at P75 by newly born granular neurons are incorporated into surgical thyroidectomy. We followed a method used the neural circuitry and mature into functional through the years in our laboratory,29,30 based on the neurons.12 The specific function of these neurons is original procedure by Zarrow et al.31 but leaving the not yet clear,13 but some hippocampus-dependent parathyroids intact (see Supplementary data). Thyr- learning and memory tasks have been related to an oidectomized rats, both untreated and treated, were increased neurogenesis,14,15 suggesting that they fed a low iodine diet starting the day of surgery.21 At might contribute to cognitive processes. the time of killing, both P95 and P120 thyroidecto- Cell proliferation in the adult hippocampus is mized rats had greatly decreased T4 and T3 in serum decreased in several animal models of stress and and liver (serum T4 < 1.1 ng/ml and T3 < 0.08 ng/ml; depression.5,16,17 The mechanisms controlling pro- liver T4 < 2.1 ng/g and T3 < 0.70 ng/g), a significant genitor cell and neuroblast proliferation have not reduction in body weight (BW; P < 0.001), and a 66% been clarified but circulating factors, including reduction of liver type 1 deiodinase (D1) mRNA hormones, may play a role. As an example, IGF-I is expression32 with respect to sham-operated controls. needed to induce hippocampal proliferation by TH treatment consisted in the administration of a physical exercise,18 and stress-elevated circulating physiological combination of T4 and T3 in the levels of adrenal steroids inhibit the production of drinking water (0.18 mg T4/ml and 0.03 mg T3/ml). new granular neurons.16 This procedure was based on previous data.33 The Thyroid hormones (TH: thyroxine (T4) and 3,5, solution was prepared daily in sterile 0.01% bovine 30-triiodothyronine (T3)) are essential for brain devel- serum albumin and kept in the dark to avoid hormone opment and function.19 In particular, in the hippo- degradation. The hormonal concentration in the campus, TH deficiency causes reduced growth, drinking water was calculated, on the basis of fluid reduced number of cells in the DG, and abnormal intake, to provide 2.4 mg T4 and 0.4 mg T3 /100 g BW/ neuronal migration and maturation.20–22 TH action is day. TH treatment resulted in a significant increased mediated through T3 nuclear receptors, and T3 in BW and in serum and liver T4 and T3 (P < 0.001 receptor deficiency or mutations also alter adult versus P120H). D1 mRNA was fourfold increased by hippocampal structure and hippocampus-dependent TH treatment. Thyroidal status affected neither brain behavior.23,24 Very recently, TH have been implicated size (not shown) nor DG volume (in mm3, P95E = 0.75; in adult neurogenesis,25–28 but little is known on the P95H = 0.69; P120E = 0.90; P120H = 0.76; P120R = functional consequences of TH actions on prolifera- 0.80; P > 0.05 in all cases) (see Supplementary tion of neuronal progenitors. Figure 1). In this paper, we show that short-term adult-onset To evaluate the acute effects of TH on the reversal of hypothyroidism significantly reduces SGZ prolifera- depressive-like behavior, we analyzed two additional tive capacity and impairs dendritic arborization of groups of hypothyroid-treated animals with their immature neurons. In addition, hypothyroid animals corresponding E and H counterparts. One group display a depressive-like behavior. All these altera- (P120I) received one single i.p. injection of 24 mgT4 tions are reversed by chronic TH treatment. Our plus 4 mg T3 /100 g BW, that is, 10-fold the daily dose findings suggest that in humans, the mood disorders used for the recovery group under chronic treatment. secondary to adult hypothyroidism could be related, The hormone combination was administered to each at least in part, to the impairment of hippocampal animal just after the training session of the Porsolt proliferation. These results are relevant for the swimming test (see below) and, therefore, 24 h before handling of hypothyroid patients and, in particular, the test. The second group (P120W) received the to the design of clinical protocols that include TH hormone combination in the drinking water at the withdrawal. same dosage as the chronic-treated (P120R) animals starting 24 h before the training, that is, 48 h before the behavioral test session. To compare the results of both Materials and methods conditions, the animals of this group also received a Animals and treatments single injection of vehicle just after the training. Adult male Wistar rats were housed