SERT & Drug Addiction

When thinking about addictions, most people tend to think primarily of dopamine. This is not surprising as dopamine plays a very important role in reward and all known drugs of abuse increase dopamine release within the forebrain. However, while dopamine is certainly closely related to the acute reinforcing effect of addictive substances, other neurotransmitters are definitely involved as well. In this project we focus primarily on the role of serotonin.

Genetic studies have indicated that alterations in the SERT transporter might make individuals more susceptible to the rewarding properties of drugs of abuse and increase the likelihood of becoming substance abusers. However, from studies in humans it is difficult to determine the causal relationship between genetic changes in the SERT and addictive disorders.   

Therefore, in this overarching project, we investigate the effects of addictive substances in rats with a genetic reduction in the SERT. The project has both a behavioural and a biochemical, molecular component. Behaviourally we have already shown that rats with a genetic reduction in the SERT have an increased sensitivity to the rewarding effects of cocaine and MDMA (the active ingredient of ecstasy). However, they were not more sensitive to the rewarding effects of heroin. Currently we are looking at the rewarding properties of alcohol. This will be done both in SERT compromised animals as well as in their offspring to investigate whether drinking behaviour in the father affects the sensitivity of the children towards alcohol.

In addition to the behavioural changes induced by addictive substances we also investigate how such drugs affect the brain. Using an RNA sequencing technique, we found strong evidence that MDMA changes synaptic communication between cells, particularly glutamatergic neurotransmission which takes place on dendritic spines. Using a variety of techniques, such as Western Blot, quantitative PCR and RNAscope, we are now investigating this further. Additionally, we are looking at epigenetic changes (such as DNA methylation), particularly in the offspring of alcohol drinking rats.

These projects have in part been and are funded by grants from the Neurological Foundation and the Catalyst: Seeding fund and are performed in close collaboration with Drs Darren Day and Melanie McConnell from the School of Biological Sciences as well as Prof Tomoaki Shirao from Gunma University in Japan.

Long term biochemical changes in SERT compromised rats

When studying the role of serotonin in mood disorders there is an obvious paradox. We know that one of the most effective treatment for depression and anxiety disorders is blocking the SERT through selective serotonin reuptake inhibitors (SSRIs), leading to an increase in extracellular 5-HT. On the other hand, a genetic reduction in the SERT, which equally leads to increases in extracellular 5-HT, actually increased the risk of depression and anxiety disorder.

One possible explanation for this apparent paradox lies in the timing of the increases in extracellular 5-HT. Thus, in the case of a genetic reduction in SERT, 5-HT levels are increased already at a very early age. We know that 5-HT is critically involved in the development of the nervous system. It is therefore conceivable that the brain (and body) of genetically compromised SERT animals is fundamentally different from normal (so-called Wildtype) rats. However, since 5-HT plays such as broad role in development it is very hard to predict exactly what has changed in the SERT compromised animals.

In this project we therefore take a so-called “hypothesis-free” approach. So rather than setting a hypothesis a-priori about what may have changed, we aim to investigate as many changes as we can possible find. For that we take two different approaches: MALDI-MS and metabolomics. With MALDI-MS we carefully scan entire brain sections for regional changes in small molecules such as neurotransmitters and neurotransmitter metabolites. In metabolomics, we use brain or blood serum samples to investigate many different metabolites.

This project is in part supported by a grant from the Wellington Medical Research Foundation and is a collaboration with Drs Robert Keyzers and Bill Jordan.

The SERT & synaptic plasticity

While 5-HT is best known for its role in mood, cognition and reward, it also plays an important role in the development of the central nervous system. Several studies have found that different serotonin receptors can affect developmental processes such as axon and dendrite maturation, axon guidance and spine formation. This latter is very important, as dendritic spines are essential hubs for neuronal connections, especially excitatory connections that use glutamate as a neurotransmitter. Moreover, dendritic spine changes do not only occur during development, they are also a central element in adult synaptic plasticity.

Serotonin transporter (SERT) knock-out rats have much higher extracellular levels of 5-HT from very early on. Given its important role in neurodevelopment, we hypothesize these animals to show changes in neuronal connectivity. In this project we aim to investigate this with a variety of different techniques, such as Western Blot, quantitative PCR, immunohistochemistry and RNAscope. We will also grow neuronal cell cultures as it is easier to visualize dendritic spines in such culture than in normal brain tissues. To evaluate the changes over time we will study analyse the brain of young (first two weeks after birth) as well as adult rat brain tissues.

This project is a collaboration with Dr Darren Day from the School of Biological Sciences.

The SERT & Heart Disease

People with psychiatric disorders such as major depression and anxiety disorders are much more likely to also suffer from with heart problems than the general population. Likewise, individuals suffering from heart problems are more likely to develop major depression or anxiety disorders. In other words, there seems to a causal link between depression, anxiety and heart disease.

In this project we investigate the hypothesis that high levels of serotonin (5-HT) early in life may be this causal link. The reasoning behind this is that genetic reductions in the SERT are a vulnerability factor for major depression, anxiety disorders and heart disease and leads to high levels of 5-HT already very early on in life. From studies in rats and mice, we have learned that 5-HT, during development, plays an important role in shaping the structure and function of the brain as well as the heart.

For this project we will change the extracellular levels of 5-HT early in life through pharmacological means and subsequently investigate whether this leads to changes in the body and behaviour. Behaviourally, we will investigate depressive and anxiety-like symptoms. We will also assess changes in heart rate and especially heart rate variability. In addition, we will investigate changes in the structure and functioning of the brain and heart using immunohistochemistry.

This project is supported by a grant from the New Zealand Heart Foundation