What specifically makes it so the pain neurons cause pain and the pleasure neurons cause pleasure? Supposing I invented a sort of hybrid neuron, with some features of a pain neuron and some features of a pleasure neuron -- is there any way a neuroscientist could look at its structure and predict whether it will produce pleasures vs pain?
It is likely that it only matters where they are connected in the sensory paths that carry the information about sensations towards the central nervous system. Probably any signal coming into the central nervous system on those paths dedicated for pain is interpreted as pain, like a signal coming through the optical nerves would be interpreted as light, even when it would be caused by an impact on the head.
The sensation of pain is provided by dedicated sensory neurons, like other sensory neurons are specialized for sensing light, sound, smell, taste, temperature, tactile pressure, gravity, force in the muscles/tendons, electric currents, magnetic fields, radiant heat a.k.a. infrared light and so on (some of these sensors exist only in some non-human animals).
The pain-sensing neurons, a.k.a. nociceptors, can be identified anatomically in some of the better studied animals, including humans, but it is likely that they also exist in most other animals, with the possible exception of some parasitic or sedentary animals, where all the sense organs are strongly reduced.
So all animals with such sensory neurons that cause pain are certain to suffer.
The nociceptors are activated by various stimuli, e.g. either by otherwise normal stimuli that exceed some pain threshold, e.g. too intense light or noise, or by substances generated by damaged cells from their neighborhood.