Controller:

public class ArticlePageController : PageController<ArticlePage>
{
    public ActionResult Index(ArticlePage currentPage)
    {
        ...
    }

    [HttpPost]
    public ActionResult MyDropdownAction(ArticlePage currentPage, string myDropdownValue)
    {
        // Do something with myDropdownValue...
    }
}

View:

@model ArticlePage

@using (Html.BeginForm("MyDropdownAction", "ArticlePage", new { node = Model.ContentLink }, FormMethod.Post))
{
    <select name="myDropDownValue" onchange="this.form.submit()">
        <option value=""></option>
        <option value="myvalue">My text</option>
    </select>
}