Inhibitory neurons in the Ventral Medial Midbrain/Pons (VMP) dealing with GABA, called GABAergic neurons, suppress dopamine systems to induce sleep.

            Neurotransmitters are chemicals released by a neuron that are targeted to another,  carrying a message of what the next neuron is supposed to do. They have been, referred to as “the body’s chemical messengers.” They can be classified into excitatory and inhibitory neurotransmitters. Excitatory neurotransmitters encourage action potentials in the  receiving  neuron, while an inhibitory neurotransmitter inhibits the reciever.¹ Gamma aminobutyric acid (GABA), an inhibitory transmitter, reduces neuronal activity.² Writing in The Journal Of Neuroscience, Takata et al. studied the effects of GABA on sleep, and its relationship with dopamine, in the ventral medial midbrain/pons area (VMP) of mice.³ The VMP is an area of the brain on the border between the pons and midbrain.

The question of why, and how, we sleep has perplexed scientists for a very long time. Some theories suggest that sleep is instrumental when it comes to the memory’s ability to  retain information.⁴ There is also a traditional theory of sleep being essential for restoration of our body and mind.⁵ Due to sleep being such a vulnerable state, it has been hypothesized that sleep has survived evolutionarily because it serves a purpose that we have yet to determine.⁶ Even after years of study, we still do not know all the answers of why we sleep or the mechanisms for how it works. Indirect pathway neurons have been identified in the nucleus accumbens, a part of the brain in the ventral section of the midbrain that is crucial for reward and reinforcement behavior, as sleep controlling neurons.⁷

Since sleep controlling neurons were found, Takata et al., decided to study what neurotransmitter specific neurons control sleep. They had hypothesized that GABAergic neurons, or neurons dealing with GABA, control sleep and wake behavior. They also found that GABAergic neurons induce sleep through the suppression of dopaminergic systems, which deal with the excitatory neurotransmitter dopamine. They began with general ablation of neurons through the use of diphtheria toxin fragment a (DTA) and found that wakefulness levels had increased throughout a 24-hour period, measured in minutes per hour. They also had longer duration of wakeful episodes while experiencing fewer episodes. The fewer episodes of wakefulness makes sense since the episodes were lasting longer in each period.

The increased wakefulness seen in ablation of VMP neurons brings about the question; what kind of neurons are controlling sleep wake behavior? They decided to study whether GABAergic neurons or dopaminergic neurons are in control of sleep wake behavior. To test whether dopaminergic neurons control sleep wake behavior the used DAT-Cre transgenic mice, mice with a dopamine transporter protein. These mice were microinjected with DTA to ablate dopaminergic neurons. They found that lack of dopaminergic neurons had little to no effect on levels of wakefulness with selective dopaminergic neuron ablation. Then, they repeated the same style microinjection on transgenic VGAT-Cre mice, a vesicular GABA transporter. They found that mice with selective GABAergic neuron ablation had significantly increased levels of wakefulness. The mice experienced more minutes per hour of wakefulness at every measured point throughout the day, much longer durations of wakefulness, and decreased episodes. This led the researchers to wonder the relationship between GABAergic neurons and dopaminergic neurons in this application.

Takata et al. studied the relationship between dopamine and GABA, finding that dopamine mediates the increased wakefulness that was seen in the mice with GABAergic neuron deficiency. Mice with GABAergic neuron deficiency and the control group were injected with either SCH23390, a Dopamine D1 receptor inhibitor, Raclopride, a Dopamine D2 and D3 receptor inhibitor, or a combination of both. It was seen that neither on their own had a significant effect on the level of wakefulness on mice with GABAergic neuron deficiency and those without, but that when given both, the mice experienced decreased wakefulness. This prompted them to the conclusion that in both normal mice and those with GABAergic neuron deficiency, dopamine receptors mediate wakefulness.

The researchers also set out to determine whether  inhibiting GABAergic neurons would produce the same reactions as ablating them. They found that through the inhibition of GABAergic neurons by using inhibitory designer receptors exclusively activated by designer drugs (DREADD) hM4Di used with CNO, that the mice with inhibited GABAergic neurons experienced increased wakefulness. This further supports that GABA is at work when inducing sleep because when it is inhibited as well as when the neurons involved with it are ablated, wakefulness is increased.  

Takata and their colleagues give an insightful look into some of the neuronal mechanisms dealing with sleep wake behavior. They demonstrated that ablation GABAergic neurons strongly increase wakefulness levels (Fig. 1). This has certainly furthered the study of the field on the neuroscience of sleep, showing us that GABA induces sleep and that dopamine receptors mediate wakefulness in normal conditions, as well as when GABAergic neurons are inhibited. This has added some fine detail to the big picture we have so far about the mystery that is sleep.
            While many questions have been answered through this study, it also brings some new questions into this field of study. “GABAergic neurons in the ventral tegmental area are a key regulator for non-rapid eye movement sleep.”⁸ This is helpful information for Takata et al. bringing up that the circuit between the VMP GABAergic neurons and dopaminergic system are still unknown, which could be a new question worth studying. “It was found that activation of dopamine neurons in the ventral tegmental area strongly induce wakefulness.”⁹ Takata et al. then found that GABAergic neurons regulate sleep and wakefulness as well. This leads them to  state that future studies are required to understand how the levels of sleep and wakefulness are regulated by GABAergic neurons.

Through the research of Takata and their colleagues we now have a much better understanding of the neuronal mechanisms involved in sleep. We now know the role of GABAergic neurons in controlling sleep and wake behavior, as well as having a better understanding of the relationship between GABA and dopamine. While many questions have been answered through Takata and their colleagues work, the question of the  circuitry between GABAergic neurons and dopamine receptors has yet to be answered.

Science is always advancing, now faster than ever, thanks in part to modern technology and such dedicated researchers. Once one question is answered, more become asked, going deeper into the study. The study of sleep is a hot topic. Studies such as this will certainly help us toward understanding all the fine details of how sleep works, and it could be possible that the circuitry involved with GABA and dopamine will be found soon. This will all benefit our society and hopefully help us when it comes to treating sleep disorders and understanding them better.

Figure 1 | Transgenic mice with selective neuron ablation show different sleep behavior.

Shown above is the method and result of Takata and their colleagues work. A wild type mouse was made transgenic for either GABA transport (VGAT-Cre) or Dopamine Transport (DAT-Cre). Then cells were specifically ablated (DTA/VMP) or not (hrGFP/VMP). Mice with ablated GABAergic neurons showed increased levels of wakefulness, while all other groups showed normal wakefulness.

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