http://docs.joomla.org/Adding_custom_fields_to_the_article_component

$dispatcher = JDispatcher::getInstance();
JPluginHelper::importPlugin('content', 'plg_content_rating');
$results = $dispatcher->trigger('onContentPrepare',
    array('com_content.category', & $this->item, & $this->item->params, 0));

$results = $dispatcher->trigger('onContentPrepare',
    array ('com_content.category', &$item, &$this->params, 0));