The Ventral Tegmental Area - Reward and Arousal
During day 4, one of my favorite poster sessions took place (Sleep systems, and sleep regulators). Here, a poster that grabbed my attention was titled “GABA and glutamate networks in the VTA regulate sleep and wakefulness” from Xiao Yu, a member of William Wisden 1’s lab at Imperial College London.
Dopamine (green) and GABA (red) expressing neurons in the mouse ventral tegmental area (VTA; outlined) studies by Xiao Yu and colleagues demonstrates that these neurons bidirectionally regulate sleep and wakefulness (Credit: Jeremy C Borniger, PhD; Stanford University)
The ventral tegmental area 1 (VTA) is largely known as the seat of the brain’s ‘reward’ system. This is because neurons in this area are the primary source of all the brain’s dopamine, a ‘feel good molecule’ that is responsible for the rewarding effects of drugs, sex, and all things fun. Neurons in this area signal reward by calculating the so called ‘reward prediction error’. This is the difference between an expected and unexpected reward. For example, if you expect to get one piece of candy from your mom, but then she gives you 100 pieces of your favorite treat, neurons in the VTA calculate the difference, fire, and release a large surge of dopamine proportional to the reward ‘error’. This signal acts to reinforce the behaviors that led to the unexpected reward. A ‘good’ error like this is a called a ‘positive prediction error’ while the opposite, where a reward is omitted when it is expected, is called a ‘negative prediction error’. Negative prediction errors result in less dopamine release, and therefore aversion to the behaviors that led to this unexpected ‘disappointment’. As you may well predict, drugs of abuse like cocaine, alcohol, heroin, and others elicit a strong positive prediction error, resulting in a lot of dopamine release and reinforcement of drug seeking behavior.
In addition to dopamine neurons in the VTA, there exists two other primary populations, one that expresses the inhibitory neurotransmitter GABA, and another that produce primarily glutamate, an excitatory neurotransmitter. Recent research has demonstrated that in addition to their roles in reward signaling, VTA-dopamine neurons strongly promote wakefulness, likely through their projections to the nucleus accumbens (NAc) (see image below). How other VTA populations relate to wake/sleep states remains unknown.
Activation of VTA-dopamine neurons (TH-positive) strongly promotes wakefulness. You can see that when these neurons are stimulated (by light sensitive ChR2 activation), the mice rapidly wake up (panels c,d,e)(Credit: Eban-Rothschild et al., 2016; Nature Neuroscience)
To investigate these other populations, Xiao Yu and colleagues used optogenetics, chemogenetics, fiber photometry (Ca2+), and neuropharmacology to untangle the roles GABA and glutamate neurons in the VTA play in sleep/wake states.
First, they identified that most glutamate neurons in the VTA also express NOS1 (nitric oxide synthase 1), and therefore used NOS1 and vglut2-cre mice to specifically target these neurons for manipulation. VGLUT2 stands for ‘vesicular glutamate transporter 2’, and is expressed on virtually all subcortical neurons that signal via glutamate. Using viral vectors to specifically express the stimulatory (hM3Dq) or inhibitory (hM4Di) DREADDs, they demonstrated that stimulation of VTA-glutamate neurons strongly promotes wakefulness while inhibition of this population strongly promotes sleep. To investigate how these neurons promoted arousal, they stimulated their projections in different brain regions using optogenetics. They focused on two primary output regions, the lateral hypothalamus (which contains many sleep-related neural populations) ,and the nucleus accumbens. Stimulation of glutamate nerve terminals arriving from the VTA to the lateral hypothalamus strongly promoted wakefulness, while stimulation of similar fibers arriving at the NAc had a less pronounced effect. This suggests that VTA-glutamate neurons likely promote wakefulness via dual projections to the lateral hypothalamus and NAc. Importantly, the natural activity of these neurons (examined via fiber photometry) was shown to be highest during wakefulness and REM sleep compared to NREM sleep. This suggests that they normally change their firing rates during distinct vigilance states.
Example of a fiber photometry trace showing the activity of GABA neurons across sleep-wake states. As you can see, these neurons are mostly active during wakefulness and REM sleep compared to NREM sleep (wake = white background, NREM = blue, REM = red) (Credit: Jeremy C Borniger, PhD, Stanford University)
Similar experiments were done to examine the VTA-GABA population. Activation of these neurons (via DREADDs or optogenetics) strongly promoted sleep, while inhibition of this population powerfully promoted wakefulness. Activation of GABA nerve terminals from the VTA to the LH strongly promoted sleep, an opposite effect to that of glutamate stimulation in LH. This effect was partially inhibited when stimulations occurred in combination with a drug (gabazine) that inhibits GABA signaling. This suggests that it is GABA (and not other molecules) released by these neurons that is largely responsible for their effects of sleep/wake states. Finally, they hypothesized that this effect could be driven by GABA’s inhibitory influence over VTA-dopamine populations. By inhibiting VTA-GABA neurons in combination with dopamine blockade, they were able to (mostly) eliminate the effect of VTA-GABA silencing on wakefulness. This supports a model in which VTA-GABA neurons inhibit neighboring VTA-dopamine neurons in order to promote sleep.
This is an exciting research area as a major problems for drug abuse victims are insomnia and chronic fatigue, which inevitably lead to the reinstatement of drug seeking behavior. Sleep drugs targeting the VTA could really help rectify general sleep problems and specifically those related to drug abuse.
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