Opioid abuse and its effects to the brain reward system-How does the reward center in the brain work?

Life is the most precious and price less gift humanity has. This is only useful and meaning full with proper functioning brain. We all understand that the brain is certainly a busy little organ yet it is on duty tirelessly for life without taking any vacation, the moment it possess for whatever reason life ends or serenity is rearranged. Despite it busy roles in driving the body; it knows fun and pleasure when it sees it. When an external stimulus, such as a particular food or a potential mate, has been encountered and deemed a pleasurable sensation, the cerebral cortex signals the ventral tegmental area of the brain to release the chemical dopamine into the amygdala, the prefrontal cortex and the nucleus acumens. These latter regions of the brain make up the reward system. These areas work in conjunction to deliver a sense of pleasure and focus the attention of the individual so that he or she learns to repeat the behavior once more. Researchers theorize that this is how behaviors necessary for survival, such as reproduction and eating, are learned.

Interestingly, the reward center doesn’t kick into gear only when we eat something delicious or meet a potential new love interest. It turns out that generosity can be quite a kick too! MRI studies have revealed that when we perform an act of kindness, the brain’s reward center is aroused and we experience feelings of pleasure. The brain is flooded with happiness-inducing dopamine whenever we give a homeless person some money or help out someone in need. A study conducted in 2008 confirmed the belief that spending money on other people can result in elevated feelings of happiness for the giver.

It might not be all fun and games for the reward center, however, a recent study came to a startling conclusion that the brain’s reward center responds to bad experiences as well as good. Doing something scary or even merely thinking about it can trigger a release of dopamine. In essence, dopamine isn’t just triggered by fun and pleasurable events. Negative things can do the trick too.

Opioid abuse and its effects to the brain reward system-Reward pathway

The most important reward pathway in brain is the mesolimbic dopamine system. This circuit (VTA-NAc) is a key detector of a rewarding stimulus. Under normal conditions, the circuit controls an individual’s responses to natural rewards, such as food, sex, and social interactions, and is therefore an important determinant of motivation and incentive drive. In simplistic terms, activation of the pathway tells the individual to repeat what it just did to get that reward. It also tells the memory centers in the brain to pay particular attention to all features of that rewarding experience, so it can be repeated in the future. Not surprisingly, it is a very old pathway from an evolutionary point of view. The use of dopamine neurons to mediate behavioral responses to natural rewards is seen in worms and flies, which evolved 1-2 billion years ago.

The VTA-NAc pathway is part of a series of parallel, integrated circuits, which involve several other key brain regions.

The VTA is the site of dopaminergic neurons, which tell the organism whether an environmental stimulus (natural reward, drug of abuse, stress) is rewarding or aversive.

The NAc, also called ventral striatum, is a principle target of VTA dopamine neurons. This region mediates the rewarding effects of natural rewards and drugs of abuse.

The amygdala is particularly important for conditioned forms of learning. It helps an organism establish associations between environmental cues and whether or not that particular experience was rewarding or aversive, for example, remembering what accompanied finding food or fleeing a predator. It also interacts with the VTA-NAc pathway to determine the rewarding or aversive value of an environmental stimulus (natural reward, drug of abuse, stress).

The hippocampus is critical for declarative memory, the memory of persons, places, or things. Along with the amygdala, it establishes memories of drug experiences which are important mediators of relapse.

The hypothalamus is important for coordinating an individual’s interest in rewards with the body’s physiological state. This region integrates brain function with the physiological needs of the organism.

Probably the most important, but least understood, are frontal regions of cerebral cortex, such as medial prefrontal cortex, anterior cingulate cortex, and orbitofrontal cortex, which provide executive control over choices made in the environment (for example, whether to seek a reward).

The locus coeruleus is the primary site of noradrenergic neurons in the brain, which pervasively modulate brain function to regulate the state of activation and mood of the organism.

The dorsal raphe is the primary site of serotonergic neurons in the brain, which, like noradrenergic neurons, pervasively modulate brain function to regulate the state of activation and mood of the organism.

Of course, these various brain regions, and many more, do not function separately. Rather, they function in a highly inter-related manner and mediate an individual’s responses to a range of environmental stimuli.

Opioid abuse and its effects to the brain reward system-What are opioids?

Opioids are medications that relieve pain. They reduce the intensity of pain signals reaching the brain and affect those brain areas controlling emotion, which diminishes the effects of a painful stimulus. Medications that fall within this class include hydrocodone (e.g., Vicodin), oxycodone (e.g., OxyContin, Percocet), morphine (e.g., Kadian, Avinza), codeine, and related drugs. Hydrocodone products are the most commonly prescribed for a variety of painful conditions, including dental and injury-related pain. Morphine is often used before and after surgical procedures to alleviate severe pain. Codeine, on the other hand, is often prescribed for mild pain.

Symptoms of opioid abuse can be categorized by physical state.

Opioid abuse and its effects to the brain reward system-Intoxication state

Patients with opioid use disorders frequently relapse and present with intoxication. Symptoms vary according to level of intoxication. For mild to moderate intoxication, individuals may present with drowsiness, pupillary constriction, and slurred speech. For severe overdose, patients may experience respiratory depression, stupor, and coma. A severe overdose may be fatal.

Opioid abuse and its effects to the brain reward system-Withdrawal state

Symptoms of withdrawal include the following:

• Autonomic symptoms – diarrhea, rhinorrhea, diaphoresis, lacrimation, shivering, nausea, emesis, piloerection
• Central nervous system arousal – sleeplessness, restlessness, tremors
• Pain – abdominal cramping, bone pains, and diffuse muscle aching
• Craving – for the medication

Opioid abuse and its effects to the brain reward system-How does the reward center in the brain work?