Because they have similar chemical structures, morphine and other opiates are able to lock into receptor sites for

gaba

serotonin

dopamine

endorphins

cholinergics

Asubstitute is standing at the halfway line waiting to enter the match at the next stoppage of play the ball has gone over the touchline for throwing and referee allows for the substitution to substitute wants to take the throw-in does the substitute first have to enter the field of play before being allowed to take the throw-in