## 3 Brain Stimulation Techniques
### tDCS and tMS in Animal Models for Depression  
The most common animal models for studying depression are based on the induction of anxiety-like and despair-like behaviors in response to stress. The Forced Swim Test, where rats are placed for a few minutes in a water-filled cylinder from which they cannot escape, tests withdrawal behavior. Induction of chronic stress through repeated exposure to a stressor for a long period induces long-lasting neurochemical, neuroimmune, and neuroendocrine changes, as well as behavioral changes manifested by increased stress and anhedonia [67]. In recent years, there has been increasing awareness and interest in tDCS and depression, thus appearing in various animal models (Table 2), specifically for rats [68] and mice [69]. These animals were experimentally subjected to classic depression paradigms such as the Open Field Test, Forced Swim Test, and Sucrose Preference Test, with tDCS (at the prefrontal, frontal, or neocortical level) and drugs as the intervention. Additionally, studies have included anxiety measures, such as the Elevated Plus Maze Test, as an additional investigation of comorbidities related to depression, reporting positive effects on both depression and anxiety symptoms [70]. These findings may be important for generalizing tDCS to human clinical settings and personal home use. [PERSON] et al. [73] emphasized that animal-based tDCS research must carefully consider how electrode current density, electrode placement, and the specific animal model interact. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Therapeutic Approach** & **Type of Depression** & **Depression Rating Scales** & **Outcome** & **References** \\ \hline
13 sessions; & Major depressive episode (on & MADRS & HAD depression effect-size 0.86, \(\backslash\)\(\circ\) & \\
10 Hz for 4-6 weeks & antidepressants) & HAD & (4 th and 6 th week) & [66] \\ (left frontal rTMS) & & major depressive disorder & & \\ \hline
20 sessions & & & & \\
20 Hz for 2 weeks & & & & \\ (DLPFC rTMS) & & & & \\ \hline
20 sessions & & & & \\
20 Hz for 2 weeks & & & & \\
60 Hz for 2 weeks & & & & \\
20 sessions & & & & \\
20 Hz for 5  
Factors like the animal's characteristics (e.g., weight, sex), electrode positioning, stimulation polarity, and exposure time all influence results. They also stressed that metrics like charge density alone are not sufficient--other variables, including duration of exposure and electrode size, are crucial. Building on these findings, [PERSON] et al. [74] outlined the main parameters for defining a tDCS dose, including current (mA), duration (minutes), and electrode size (cm\({}^{2}\)), as well as derived factors like charge (C), current density (A/m\({}^{2}\)), and charge density (kC/m\({}^{2}\)). Although no strict consensus exists on what constitutes a dangerous dose capable of causing brain lesions, the researchers identified a current density threshold around 142.9 A/m\({}^{2}\) (0.5 mA with a 3.5 mm\({}^{2}\) electrode). Within a range of 142.9 A/m\({}^{2}\) to 285.7 A/m\({}^{2}\), lesion size increases in proportion to the charge density. Studies performed on animals have shown the importance of astrocytes, indicating an astrocyte-neuron model involved in the complex neurobiological mechanisms of certain clinical diseases such as depression, Parkinson's disease, chronic pain, stroke, and improved cognitive abilities [75]. Thus, the study realized by [PERSON] and [PERSON] [76] proposes the notion that activation of astrocytic Ca\({}^{2+}\) signaling could be a potent remedy for depression in a clinical setting. [PERSON] et al. [77] investigated the effects of tDCS on spontaneous spiking activity and concluded that at the current densities typically used in human or animal tDCS studies, the observed effects of applying electrical stimulation are most likely generated by mechanisms other than neuronal depolarization.