As aced said; and a result of this is the feelings of withdrawal - the symptoms are caused by lowered endogenous opioid activity. It makes sense, too, because withdrawal is for the most part a reversal of the symptoms of being high.
Does the amount of neurotransmitters stay the same or decrease initially (during use) as a part of feedback response? Isn't GABA synthesis inhibited in the case of overstimulated GABAergic system, for example?