Great Danes are a guardian breed, and in the distant past they were also used for hunting. Here is a link:http://www.5stardog.com/dog-breeds-great-dane.asp
I would not try to train prey drive out of a dog. I don't think its fair to the dog to punish them for something that is instinct. I have a small dog who is quite prey driven, and I am not going to try to teach him that the cats are Friends not Food because he is a terrier, hunting is what they do. I just keep him seperated from them. You can't train out prey drive anyway; you can manage it so that your dog doesn't drag you after squirrels on walks, but you can't make it go away completely.
I always wonder why people ask how to train a dog not to hunt. Nobody asks how to train a cat not to kill a mouse.
No you can't 'train prey drive' out of a dog, but you can teach him to follow your cues and commands inspite of what may have his attention; not only is that essential with an animal that wants to chase other pets, but it can also save his life should he see another 'prey animal' if he happens to be off lead, and want to chase it across a street...yes some dogs are more loving to cats, but others, even with training should never be left alone with them; however when we are with them we should be able to have voice control over them...either that, don't have both species, because what happens if a cat gets out of his 'area' and dog sees it first?