Science is always progressing and republishing updates. I would wager we can't yet truly understand or identify the stimuli correctly that would test this for sure, Everything, every species even populations in a species react differently to the same stimuli. Some animals may be more pain tolerant like an insect that is built to survive without limbs, or even regenerate them like a tarantula or even some reptiles but there is no deal breaker on me that pain of some sort is not felt. I think the reaction to pain is more the testing theory today and if there is no reaction it means nothing. I can stick myself with a pin and not show pain, doesn't mean I don't want to yell a few curses out loud.
Michael