Heroin is a powerful opioid drug derived from morphine, a naturally occurring substance found in the opium poppy plant. It is widely recognized for its profound and rapid impact on the brain’s chemistry. This substance acts as a central nervous system depressant, meaning it slows down various functions of the brain and nervous system, leading to its potent effects.
The Brain’s Chemical Messengers
Neurotransmitters are the brain’s chemical messengers, responsible for transmitting signals between nerve cells, known as neurons. These specialized chemicals are stored in small sacs called vesicles within the presynaptic neuron. When an electrical impulse, or action potential, arrives, neurotransmitters are released into the synaptic cleft, a tiny space between neurons.
Once released, neurotransmitters diffuse across this gap and bind to specific proteins called receptors on the surface of the receiving, or postsynaptic, neuron. This binding initiates a signal in the target cell, which can either excite or inhibit it. The effect of a neurotransmitter depends on the type of receptor it binds to.
Heroin’s Primary Neurotransmitter Targets
Heroin is an opioid that quickly crosses the blood-brain barrier after administration, whether injected, snorted, or smoked. Once in the brain, it is rapidly converted into morphine and 6-monoacetylmorphine (6-MAM). These substances then primarily bind to and activate mu-opioid receptors (MORs) located throughout the brain, spinal cord, and gut.
The activation of mu-opioid receptors by heroin mimics the action of the body’s natural opioids, such as endorphins, which normally modulate pain signals and contribute to feelings of well-being. This binding to MORs has an indirect effect on dopamine release in the brain’s reward pathway. Normally, gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, suppresses dopamine release.
Heroin’s binding to mu-opioid receptors on GABAergic neurons reduces GABA’s inhibitory activity. This disinhibition allows dopamine-producing neurons to release increased amounts of dopamine, particularly in the nucleus accumbens, a region associated with pleasure and reward. While mu-opioid receptors are the primary targets, heroin’s metabolites also bind to delta- and kappa-opioid receptors.
Immediate Brain Responses
The surge in dopamine in the reward pathway, triggered by heroin’s action on mu-opioid receptors, causes an intense feeling of euphoria, often described as a “rush.” This pleasurable sensation can vary in intensity based on the amount of heroin taken and how quickly it reaches the brain.
Beyond euphoria, heroin induces pain relief, or analgesia, by activating opioid receptors that reduce the transmission of pain signals. The drug also acts as a central nervous system depressant, leading to sedation and drowsiness.
Respiratory depression, where breathing becomes severely slowed, is an immediate effect. This occurs due to heroin’s influence on neurochemical activity in the brainstem, which controls automatic functions like breathing and heart rate. Other acute effects can include:
A warm flushing of the skin
Dry mouth
Heavy limbs
Nausea
Vomiting
Constricted pupils
Lasting Brain Adaptations
Repeated heroin use leads to neuroadaptations, changing the brain’s physical structure and physiology. The brain attempts to compensate for the continuous presence of heroin and altered neurotransmitter levels. These long-term imbalances in neuronal and hormonal systems are not easily reversed.
This brain adaptation contributes to the development of tolerance, a condition where increasing doses of heroin are needed to achieve the same initial effects. As the body adjusts to the drug’s presence, it becomes physically dependent, meaning it relies on heroin to function “normally.”
When heroin use is stopped abruptly, physical dependence leads to a withdrawal syndrome. This can manifest as:
Restlessness
Muscle and bone pain
Insomnia
Diarrhea
Vomiting
Cold flashes
Withdrawal symptoms are partly due to the rebound hyperactivity of neurotransmitter systems previously suppressed by heroin. Chronic heroin use affects the brain’s reward pathway, diminishing its ability to experience pleasure from natural rewards, a condition known as anhedonia. This persistent alteration in the dopamine system can contribute to compulsive drug-seeking behavior. Long-term use can also affect brain areas involved in decision-making and impulse control, leading to cognitive impairment and emotional instability.