function filter_next_post_sort($sort) {
  $sort = "ORDER BY p.post_title ASC LIMIT 1";
  return $sort;
}
function filter_next_post_where($where) {
  global $post, $wpdb;

  return $wpdb->prepare("WHERE p.post_title > '%s' AND p.post_type = 'post' AND p.post_status = 'publish'",$post->post_title);
}

function filter_previous_post_sort($sort) {
  $sort = "ORDER BY p.post_title DESC LIMIT 1";
  return $sort;
}
function filter_previous_post_where($where) {
  global $post, $wpdb;

  return $wpdb->prepare("WHERE p.post_title < '%s' AND p.post_type = 'post' AND p.post_status = 'publish'",$post->post_title);
}

add_filter('get_next_post_sort',   'filter_next_post_sort');
add_filter('get_next_post_where',  'filter_next_post_where');

add_filter('get_previous_post_sort',  'filter_previous_post_sort');
add_filter('get_previous_post_where', 'filter_previous_post_where');

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