A neuropeptide, I suppose, or some pharmacological activity that facilitates the transmission of pain or stress signals?
Endorphins are neuropeptides that can bind to opioid receptors, thus exerting an analgesic effect in the brain. You mention a counterpart, which is a badly defined concept in this context. There are other substances that can bind to these opioid receptors (opiods, for one :-)) and substances that can block opioid receptors without triggering them, like naloxone and naltrexone. Those are called opioid antagonist and for example used for treating substance dependence. They don't cause pain, though.
Pain is transmitted in the body through neurons the same way other signals are transmitted. The difference to other signals is in how the brain interprets these signals. If they are coming from pain receptors (nociceptors), they are interpreted as pain. There is no special neurotransmitter for pain, though as all signal transmission, there isn't just one. Pain transmission is also relatively slow compared to some other neuronal activity, having to do with the mixture of neurotransmitter and the way nocireceptors are structured.
For example, for pain generated by heat exposure:
The nociceptive axons, (...) begin to discharge only when the strength of the stimulus (a thermal one in the example in Figure 10.1) reaches high levels; at this same stimulus intensity, other thermoreceptors discharge at a rate no different from the maximum rate already achieved within the nonpainful temperature range, indicating that there are both nociceptive and nonnociceptive thermoreceptors.
Basically, in this case there are two signals being transmitted, one normal temperature signal and one pain signal.
Stress is not a signal that is transmitted through any pathway in the human body. With high stress levels come changes in brain chemistry, for example in cortisol and adrenaline levels, but again, these aren't exactly counterparts of endorphins.