what is the mechanism of SSRI's according to the following paragraph?

GAD is considered a challenging disorder to treat. Although benzodiazepines have been a staple of treatment until recently, they do not alleviate all symptoms and are significantly correlated with increased costs due to things like falls and accidents while under the influence of the drugs (particularly in elderly clients) (Berger, Edelsberg, Treglia, Alvir, & Oster, 2012). Also, benzodiazepines carry the risk of tolerance and dependence whereas drugs like SSRIs and buspirone do not and have similar effi- cacy (Bandelow et al., 2013; Mokhber et al., 2010). Although most benzodiazepine users do not abuse their medication, some question whether it is worth the risk to use them long-term (Baldwin, 2012). Many researchers (such as Schatzberg, Cole, & DeBattista, 2010) thus believe buspirone is the ideal medication of first choice for GAD. As noted, Bandelow and colleagues (2013) found that buspirone/BuSpar compared favorably with the benzodiazepines. Unlike benzodiazepines, buspir- one/BuSpar cannot be used on a p.r.n. basis. Like the SSRI antidepressants, it seems to require daily dosing to exert its effects. Buspirone/BuSpar may also take two to four weeks for the full therapeutic benefit to be achieved. Recent research on the SNRI antidepressants (e.g., Effexor/venlafaxine; Prestiq/desvenlafaxine) are as useful for GAD as SSRIs and buspirone/BuSpar. One persisting prob- lem is that many clients discontinue treatment with SSRIs and SNRIs because of the side-effect profile and many state a preference for benzodiazepines (Table 6.6). There are a great number of unmet needs in the population of people suffering GAD and hopefully some of the newer anxiolytics in development will provide relief with fewer side effects (Chollet, Saragoussi, Clay, & Francois