A little story about dopamine & adolescent brains

Dopamine is a neurotransmitter in the brain and it's important for a lot of stuff - motivation, learning about reward, movement - as examples. Since we want to know about dog brains and how they work, let's talk about dopamine in the context of reward and how it works in the adolescent brain!

A few things first - I'm going to summarize research that's been done in the laboratory. These studies typically use rats, and there's also some data collected from humans that's relevant here. If you're skeptical that this can apply to dogs...well I can assure you it translates. I'll write about that specifically in the future. And I know you're thinking about the quadrants here - so when I say 'reward' here I'm talking about a food reward in most cases.

Back to adolescent brains... Of course, adolescent brains are still developing. And as they develop, we can expect behavior to reflect that. Adolescence is a time in development when behavior is characterized at 'impulsive', and when 'risk taking' is more prevalent.

The important brain area for this story is an area known as the striatum. It has two different compartments that are known to do different things. These compartments are known as the dorsal striatum and the ventral striatum. The ventral striatum has an important area of its own, called the nucleus accumbens, which is essential for learning about reward. Let's refer to it from now on as the NA so I don't have to type so much.

The NA gets a boatload of dopamine from another brain area known as the ventral tegmental area (VTA for short). It's the pathway between the VTA and the NA that is often studied when we're trying to figure out how the brain responds to reward.

Of course the brain is complicated, and there IS another dopamine pathway. A brain area known as the substantia nigra (SN for short) also has neurons that make dopamine, and this dopamine is sent to the dorsal striatum. Traditionally, this pathway between the SN and the dorsal striatum has been thought of as regulating movement. In part, this is because the death of the dopamine neurons in the SN is what underlies Parkinson's disease, a disorder with prominent changes in movement. It is also important for developing habits.

Now that we have some background, let's talk about a REALLY cool study by McCane and colleagues that was published in 2021 in the Journal of Neuroscience (there's a link below). They studied how dopamine neurons in the VTA and SN respond (which means become active and release dopamine) in adult brains versus adolescent rat brains. And one of the REALLY cool things they did is use two different learning paradigms - a Pavlovian task and an operant task to see if there's a difference.

The behavioral tasks that were used are standard in many laboratories around the world. For the Pavlovian task, rats were placed in an operant chamber (essentially, a box) with a light on the wall and a food cup where a sugar pellet is delivered. The sugar pellets are the reward in this scenario. The light inside the chamber was the predictive cue - when the light went on, it meant that a sugar pellet would be delivered in the food cup. For the operant task, the light went on in the chamber, but the rat had to DO something to get the sugar pellet - in this case, poke his nose in a little port. The nose poke behavior resulted in the sugar pellet being released into the food cup.

Easy peasy, right? Both adult rats and adolescent rats learned how to do the operant task, and both groups quickly learned what the light meant in the Pavlovian task. But here's where it starts to get interesting: the adolescent rats were slower to nose poke than the adult rats in the operant task. They nose poked as often as the adult rats, but they took their time. Ha!

So what does dopamine have to do with it? When adult rats do either the Pavlovian or operant task, neurons in BOTH the VTA and the SN respond to the reward by increasing their activity. Presumably this means they are releasing dopamine into the NA and the dorsal striatum. For adolescent rats, a different picture emerges. When adolescent rats are doing the Pavlovian task, dopamine neurons in the VTA and the SN respond EVEN MORE than neurons in the adult brains. That could mean even MORE dopamine is being released in their brains compared to adult brains. Not only that (!) - the dopamine neurons in the SN, which release dopamine in the dorsal striatum, are more active than the dopamine neurons in the VTA (which release dopamine in the NA). Most surprisingly, dopamine neurons in the adolescent brain don't give hoot about the reward in the operant task! They don't respond to the reward if an action (nose poke) is required to get the reward.

What does it all mean? One interpretation of these research findings is that adolescents find the 'easy' reward in the Pavlovian task more rewarding. When effort is required to get the reward, it's no longer rewarding. Or at least, maybe less rewarding. As I noted above, the adolescent rats where also slower to do the nose poke behavior in the operant task, and this adolescent-specific dopamine response to reward in Pavlovian versus operant tasks may be part of the reason why. Note that the reward the rats got in both tasks was the same - a sugar pellet. Really, the only difference is the effort required to get it! This could mean that adolescent rats are less motivated than adult rats when EFFORT is required to get a reward. And there's certainly lots of evidence that dopamine plays a role in regulating motivation.

If we think about this from a practical perspective, adolescent brains may be more sensitive or attuned to learning about the cues in their environment that indicate a reward is available. They may even find the reward-predicting cues themselves rewarding. When there is an action-outcome contingency - in other words 'do this' to 'get that', adolescents lack the motivation (because of lower dopamine in this scenario) to do the behavior required to get the reward. Research by others (e.g., Marshall & colleagues) has also shown that adolescents are more sensitive to cues that predict reward and that these cues can have a powerful effect on behavior, essentially motivating very strong, even excessive reward seeking behaviors, even when the cue is relatively weak. I wonder if that's why many adolescent dogs seem so quick to show frustration (e.g., barking, jumping up, mouthiness) under certain conditions. Maybe there's a cue present that they view as a reward predictor, and when the reward doesn't occur, the frustration is a reflection of this excessive reward seeking behavior.

That's my take, anyway.

It's a fascinating story, and one that may be able to help us understand how to set up training scenarios with the developing adolescent in mind. So often, I hear from people who are frustrated with their adolescent dogs. Knowing and appreciating that their brains are not like adult brains may not solve any specific problems, but I do think it can add a little empathy to the situation.

What do you think?

The science - so you can read about it further if you like:

This is the study I described above: Adolescent Dopamine Neurons Represent Reward Differently during Action and State Guided Learning. McCane & colleagues. Published in 2021 in the Journal of Neuroscience. Linked here.

This is a commentary on this study. Differential Dopamine Dynamics in Adolescents and Adults. By Toth. Published in 2022 in the Journal of Neuroscience. Linked here.

I also mentioned this study: Reward-predictive cues elicit excessive reward seeking in adolescent rats. By Marshall & colleagues. Published in 2020 in the journal Developmental Cognitive Neuroscience. Linked here.

There are MANY studies that support some of the background information I wrote about. Just so you know I didn't just make it up, here's a few:

The adolescent brain and age-related behavioral manifestations. By Spear. Published in 2000 in the journal Neuroscience and Biobehavioural Reviews. Linked here.

New perspectives on adolescent motivated behavior: attention and conditioning. By Ernst & colleagues. Published in 2011 the journal Developmental Cognitive Neuroscience. Linked here.

Dissociable roles of ventral and dorsal striatum in instrumental conditioning. By O'Doherty & colleagues. Published in 2004 in the journal Science. Linked here.