The brain’s reward circuit has always fascinated me. It’s the hugest player in substance abuse and it’s involved in the mood and anxiety disorders. Seems to me it merits attention on a Teachin’ Tuesday.
Our reward circuit is an incredibly complicated interplay of anatomical and physiological dynamics. There’s no way we could hope to capture all of it in an article, but we’ll hit the highlights.
Before we get into the nuts and bolts, it’s important to understand just why we have a reward system. Simple. Experiencing pleasure encourages survival behaviors – reproduction, eating, socializing; yes, even physical self-defense. Makes sense to me.
Anatomically, the limbic system looms large in our reward circuit. And if you’ve been a chipur reader for any length of time you’ll be very familiar with it. Need a refresher? Just click here. The limbic system has many components, and interacts with numerous brain structures.
Relevant to this discussion are the amygdala, hippocampus, hypothalamus, anterior cingulate gyrus, nucleus accumbens, the ventral caudate nucleus, and the putamen.
But of all the anatomical players in our reward circuit, the most prominent are the nucleus accumbens (NA), which is the target of drugs of abuse (DOA), and the ventral tegmental area (VTA).
The power-line, if you will, of our reward circuit is the medial forebrain bundle (MFB). The MFB transmits signals from the VTA to the NA. Appropriately, the MFB is often referred to as the hedonic highway. And when this highway is blocked, the longing for DOA is either greatly reduced or flat-out ceases.
Now, the signals that pass through the MFB are supported by what are known as dopaminergic neurons. That means the neurotransmitter they release is dopamine. And, generally speaking, drugs that are not abused have no effect on dopaminergic concentrations.
Also involved anatomically are the reticular activating system (RAS), the central gray around the aqueduct of Sylvius, the basal ganglia, thalamus, and the cerebellum.
Now to the physiological side of the fence. Again, known well to chipur readers are the endocrine system and autonomic nervous system (ANS). The ANS consists of the sympathetic nervous system (our fight/flight mechanism) and the parasympathetic (which pieces us back together after the rush). And the machinations of our old friend, the HPA axis, drives the action.
The neurotransmitter, gamma-aminobutyric acid (GABA), plays a part in the effects of DOA. GABA is an inhibitory neurotransmitter found throughout the brain. DOA act upon GABA receptors, which leads to neurons being inhibited from firing. And that results in the release of less GABA onto dopaminergic neurons.
That leads to a disinhibition of dopaminergic neurons, which makes them fire more rapidly – releasing more dopamine into our reward system. Higher dopamine concentrations result in feelings of well being – even euphoria.
It’s through GABAs interaction with limbic structures that we experience anxiety reduction, sedation, and behavioral disinhibition.
And the regions that generate these anxiolytic (anxiety reducing) effects also interact with our reward system.
By the way, GABAs cousin neurotransmitter, glutamate, is very much in the mix here. Glutamate is the most prevalent of all neurotransmitters, and its excitatory characteristic is the opposite of GABAs.
Finally, we need to talk about our endogenous opioid system. First of all, endogenous simply means the cause of something is internal in nature – the opposite being exogenous. Our endogenous opioid system is controls pain, reward, and addictive behaviors. Opioid receptors in the brain are activated by endogenous peptides such as enkephalins, dynorphins and endorphin. All are released by neurons and bring feelings of pleasure and well-being.
Opioid receptors can also be activated exogenously by opiates – most notably, morphine. The same applies to opioid medications (Vicodin, Oxycontin, etc.).
Now, DOA modify the workings of everything we’ve just discussed. And that equates to big-time changes in behavior. Simply, outside substances gain entry into our bodies and use our reward circuit to fulfill their mission. And it quickly becomes a reciprocating relationship. Our reward circuit ends up needing the substance, just as the substance needs our reward circuit.
And as is so often the case, DOA commandeer our reward circuit. And addiction is accomplished.
Phew! Had enough? I have – for now. Told you our reward system was complicated. But it’s information we need to know as we come to understand how we function and what we may have to look out for.
How ’bout some comments, chipur readers? Your input is always valuable!
image credit journals.prous.com