Many different neurons in different parts of the brain release endorphins or enkephalins, but normally each neuron is simply doing its job, firing away only if stimulated by other neurons. So, activation of all the endogenous opioid neurons virtually never happens. Taking heroin is like every endogenous opioid neuron in the brain firing all at once. Heroin causes constipation because it impairs the functioning of muscles in the digestive system that would normally move food along the digestive tract. This is not the cumulative effect of years of misuse, but can happen the first time you take the drug.
Breathing simply slows to the point that it ceases.
Heroin overdoses are most common with indictable forms of the drug, but can occur with any dosage form if enough is taken. Medical attention is critical. The OD scene in the movie 'Pulp Fiction' was NOT accurate. The effects on breathing that were causing the woman to OD were mediated in the brain, not the heart. Heroin overdose is further unpredictable because it can occur even when the user has not upped their usual dose.