Sorting these poisons by their effect on breathing makes the answer clear. Opioids act on brainstem chemoreceptors and the respiratory rhythm generator to blunt the drive to breathe, which is why apnoea is the usual cause of death in opium poisoning. Barbiturates depress the entire central nervous system in a dose-related fashion, including the medullary respiratory centre, leading to shallow, slow respiration and coma. Datura, rich in atropine and scopolamine, causes an anticholinergic delirium that in severe cases progresses to central depression, coma, and failure of respiratory drive.
Strychnine behaves oppositely. By blocking glycine-mediated inhibition in the spinal cord, it unleashes motor neurons and produces explosive generalised convulsions with opisthotonus. It is a stimulant, not a depressant, and it does not suppress the respiratory centre; any breathing failure is mechanical, from tetanic spasm of the chest muscles. Therefore central respiratory depression is the one feature it does not share.
\[\boxed{\text{Strychnine}}\]