# Effects of Lithium Chloride and Nitric Oxide Inhibitor on Orthodontic Tooth Movement in the Rat
## 1 Introduction  
Orthodontic tooth movement (OTM) is a consequence of the molecular and cellular events that are initiated when a force is applied, causing resorption of the alveolar bone on the pressure side and apposition of bone on the tension side [1, 2]. Thus, a biomechanical signal is translated into a biological signal that initiates a cascade of cellular events that finally allows the tooth to move within the mineralized alveolar bone. Similar to the physiological and even pathological systemic bone remodeling where the resorption and formation by osteoclasts and osteoblasts are coupled, this balance is affected by a large spectrum of factors including pharmacological compounds that target the effector cells [3]. Considering that osteoclasts and osteoblasts are also required for OTM, pharmacological compounds targeting remodeling usually also affect OTM [4, 5]. Thus, the hypothesis relevant for OTM arises from current knowledge on how pharmacological compoundschange bone remodeling. Among those pharmacological compounds is LiCl, a commonly prescribed medication for mental disorders [6], also affecting bone remodeling--in particular, LiCl can prevent osteopororotic bone loss in ovariectomized rats [7], suggesting that LiCl blocks bone resorption. Local release of lithium from biomaterials can also accelerate bone regeneration through activation of the Wingless (Wnt)/beta (\(\beta\))-catenin pathway [8]. However, with respect to OTM where bone resorption is blocked by LiCl, the enhanced bone formation is maybe not that relevant. Consistent with observations in ovariectomized rats, LiCl significantly reduced orthodontic tooth movement in rats [9] and attenuated force-induced root resorption [10]. The molecular mechanism remains unclear but may be attributed to a positive balance of bone turnover that also helped to compensate for bone loss and stimulates osseointegration in ovariectomized rats [11] involving the Wnt/\(\beta\)-catenin and other signaling pathways [12]. Importantly, biomechanical activation of the Wnt/\(\beta\)-catenin pathway in osteocytes involves nitric oxide production [13] and in OTM, these are the osteocytes that become the major promotors to mediate osteoclastogenesis and to regulate bone formation. For example, osteocyte-derived receptor activator of nuclear factor kappa-B ligand (RANKL) is required for OTM in mice [14]. This question arises if the way LiCl reduces orthodontic tooth movement requires nitric oxide production. The support for this hypothesis comes from observations that administration of L-NAME, a general inhibitor of nitric oxide synthases (NOS) activity, significantly reduced tooth movement in a rat model [15; 16; 17]. Similar to LiCl, also L-NAME prevented ovariectomy-induced bone loss [18]. Moreover, co-administration of LiCl and L-NAME had additive effects [19]. If, however, LiCl requires the NOS activity to reduce tooth movement was yet unclear. That LiCl-reduced OTM in rats [9; 10] that may be related to lithium ability to cause hypothyroidism [20], indicated by an increased thyroid-stimulating hormone (TSH) and consequently, a decrease in triiodothyronine (T3) and thyroxine (T4) production. Hypothyroidism lowers bone turnover by reducing both osteoclonic bone resorption and osteoblastic activity [21], and decreases bone regeneration [22]. One hypothesis is that LiCl causes hypothyroidism which in turn reduces bone remodeling, and thus also OTM [9; 10]. Experimental thyroxine gavage consequently stimulates OTM [23; 24]. Indirect support for this hypothesis comes from observations that hypothyroidism affects the development of teeth and craniofacial structure in children and animal models [25; 26]. We assume that LiCl mediates at least part of its own activity on OTM via significant lowering of thyroid hormones, while L-NAME and thus nitric oxide have no dramatic impacts on thyroid hormones. Based on this assumption, it is, unlikely that the LiCl-induced reduction of OTM is modulated mainly by L-NAME.