Durk Pearson & Sandy Shaw’s®
Life Extension NewsTM
Volume 16 No. 3 • March 2013


Serotonin’s Role in Impulsivity

Serotonin in Certain Brain Neurons is Necessary for Waiting for Delayed Rewards

A recent study reveals a causal role for serotonin in the serotonin neurons in the midbrain dorsal raphe nucleus in the ability of rats to wait for a delayed reward. This is consistent with studies showing that reduced serotonin brain levels are associated with increased impulsivity, especially with respect to violent acts, including suicide.

In the new study,1 the authors explain that the dorsal raphe nucleus (DRN) is the major origin of serotonergic projections to the forebrain. They had previously reported that “DRN serotonergic neurons increase tonic firing while rats wait for delayed rewards and cease firing before rats give up waiting for long delayed rewards.”1 However, these results did not reveal whether the activation of serotonergic neurons is causal and necessary for waiting for delayed rewards. Hence, the researchers conducted this new study.

In the new study, the rats performed a task and could receive a prompt reward (waiting for 2 seconds) or a delayed reward (where they had to wait 7–11 seconds). The rats had to alternatively visit food and water sites to acquire rewards after either the rapid or delayed reward condition. The researchers found that “the suppression of 5-HT [serotonin] neural activity in the DRN increased premature exit from reward sites before the delivery of rewards, which indicated impaired patience for delayed rewards.”1

Interestingly, selective serotonin reuptake inhibitors (SSRIs) can have inconsistent results on the ability to wait for delayed rewards. “For example, some studies showed the administration of SSRIs increased the selection rate of a large, delayed reward over a small, immediate reward, indicating a decrease in impulsive choice. By contrast, a lack of effect has also been reported. Furthermore, opposite effects of different doses of citalopram in a probabilistic reversal learning have been reported. These inconsistent results may be due to opposing effects of SSRI on 5-HT neuron firing and postsynaptic 5-HT concentrations.”1 These inconsistencies may also relate to the occasional reports of impulsive violent acts carried out by adolescents treated with SSRIs, including schoolyard shootings and suicides. These unfortunate effects might be avoided by taking tryptophan or 5-hydroxytryptophan (5-HTP) supplements but the exact amounts needed will differ between individuals and require careful titration. Those individuals with lower levels of the enzyme required to convert tryptophan to 5-hydroxytryptophan (the first step in the conversion of tryptophan to serotonin) are likely to find taking 5-hydroxytryptophan rather than tryptophan a more effective way to increase their serotonin levels.

Reference

1. Miyazaki et al. Activation of dorsal raphe serotonin neurons is necessary for waiting for delayed rewards. J Neurosci. 32(31):10451-7 (2012).

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