Withdrawal symptoms will mainly be down to the effects opiods have on the noradrenergic and dopamine system mainly via the mu-opiod receptors in some of the neural pathways in the brain. Once bound, opiods tend to alter the balance of circulating noadrenaline (norephinephrine in the USA) and dopamine levels. If removed suddenly, then it will feel as if the body has tried to overcompensate in it's desire to regain "normal" neurotransmitter levels. Call it a rebound effect if you will and this explains the rationale for a gradual tapering down of opiods (as well as oher drugs) so as to avoid hardcore withdrawal symptoms.
In the case of opiods, they tend to reduce noradrenaline release. If opiods are no longer present, then the result would be an increase in the activity of noradrenaline release which can lead to the appearance of withdrawal symptoms such as tremors, muscle cramps, piloerection, diarrhoea, anxiety etc.
Additionally, the mesolimbic system is also affected which is concerned with dopamine levels. Removal of opiods leads to a downshift of dopamine release causing dysphoria, depressed mood and drug craving.
Thank you and that concludes the lecture for today