Glial Gap Junctions Boost Modafinil Action on Arousal

Modafinil is a widely used drug for the treatment of sleepiness, narcolepsy, and cataplexy. Unlike other dopamine transporter inhibitors like methamphetamine, modafinil does not appear to be addictive. The mechanism of action of modafinil's wake-promoting effects is unclear, but modafinil has been proposed to act on several circuits, including through increases in dopa-mine levels or via GABAergic neurons or even astrocytes.

In this issue of SLEEP, Duchêne and colleagues¹ reveal a surprising twist to the possibility of modafinil acting through astrocytes. They found that astrocyte gap junctions are involved in the arousal and working memory mechanisms of modafinil in mice. The gap junction connexin CX30 inhibitor flecainide—but not a CX40 inhibitor—enhances the wake-promoting effects of modafinil in wild type mice and reduces transitions from wake to REM sleep (cataplexy) in orexin knockout mice. These effects are accomplished by co-injection of lower dosages of flecainide and modafinil that by themselves have little effect on waking or memory. Furthermore, they found that flecainide acts not by altering modafinil levels in plasma or the brain but by reducing the modafinil-enhanced astrocyte coupling. These observations indicate that astrocyte coupling inhibition is involved in the regulation of the arousal-promoting mechanisms of modafinil. The authors propose that modafinil disorganizes astrocyte network and function which, in turn, reduces the efficacy of modafinil. Gap junction inhibition by flecainide re-establishes the glial network and enhances the wake-promoting and pro-cognitive effects of modafinil. These observations open many avenues of future research into the role of glial-glial and neuro-glial interactions and pave the way for novel pharmacological interventions for sleep disorders.

Gap junctions directly regulate astrocyte coupling by allowing electrical signals and small molecules such as lactate, calcium, and potassium to pass from cell to cell, regulating neuronal activity via electrical and chemical signaling. To understand how Cx30 regulates the effects of modafinil, we have to first to understand the mechanism of how modafinil regulates arousal and memory. Gap junctions themselves, specifically connexin Cx30, is highly expressed in astrocytes virtually everywhere in the brain.² The interaction between modafinil and Cx30 is dependent on the neural substrate of modafinil's action.

Sleep studies by Wisor and colleagues linked the dopa-mine system to the control of arousal by methamphetamine and modafinil.³ The arousal effects of methamphetamine and modafinil were lost in dopamine transporter (DAT) knockout mice, suggesting that dopamine was necessary for the arousal promoting effect. Interestingly, the arousal effects of caffeine remain intact in DAT knockout mice. Using a dopamine D2 knockout in combination with a D1 antagonist, Huang and Qui and colleagues established that both D1 and D2 receptors are involved in the arousal induced by methamphetamine and modafinil.⁴

As both D1 and D2 receptors are involved in modafinil-induced arousal, the next question is which of the many dopa-mine groups provide the dopaminergic input that is modulated by modafinil. There are several major dopamine cell groups that are thought to regulate arousal, all concentrated in the midbrain and pons: midbrain ventral tegmental area (VTA, A10 group), substantia nigra pars compacta (SNc, A9 group), retrorubral field (RRF, A8 group), and pontine ventral periaqueductal grey matter (vPAG). The VTA mostly projects to the nucleus of accumbens (NAc), while SNc and RRF mostly project to the dorsal striatum (or caudoputamen, CPu). The VTA dopamine group also projects to the hippocampus and medial prefrontal cortex and moderately also projects to the central amygdala nucleus (CeA) and bed nucleus of stria terminalis (BST). The vPAG dopamine group mostly projects to the CeA and BST, posterior lateral hypothalamus and medial pre-frontal cortex. Our series of recent studies identifies the neural circuitry and role of the SNc dopamine group in sleep promotion,^5–7 which eliminates SNc dopamine group as the potential site for wake promotion.

By means of selective lesion method, c-Fos markers of neuronal activation, and projection mapping, we revealed that the vPAG dopamine group is wake-active; loss of this group reduces wake by 20%.⁸ Interestingly, the VTA dopa-mine group does not show a sleep-wake state dependent pattern, and lesions of VTA do not alter sleep-wake amounts significantly.^9,10 We still lack the direct evidence that the VTA and vPAG dopamine system are critically involved in the actions of modafinil or methamphetamine. Lesions of the NAc, the main target of the VTA, increases overall total wake time and blocks the arousal effects of modafinil. However, saline injections in rats with NAc lesions also increases wakefulness than normal control.¹¹ One possibility is that rats with NAc lesions become susceptible to any irritant, including both saline and modafinil injections. Nevertheless, the VTA-NAc pathway may at least partially mediate the arousal effects of modafinil. The VTA dopamine group also projects to the medial prefrontal cortex and hippocampus, where dopamine and modafinil may promote working memory. Work from Scammell's lab suggests that the medial prefrontal cortex and CeA, by possibly acting on a pontine REM-off site, regulate cataplexy (direct REM sleep transition into wake).^12,13 Thus the VTA is an attractive candidate neural substrate of the interaction of Cx30 and modafinil in promoting wakefulness and working memory, although other potential sites may also mediate the observed effects, including the posterior lateral hypothalamus (pLH).

There are many mechanisms by which astrocytes regulate neurons, including lactate regulation. Lactate from astrocytes can be used by neurons. Cx30 itself is upregulated by prior wakefulness and modafinil,¹⁴ and lactate can directly inhibit cortical neuronal activity via Gi coupled receptors.¹⁵ Thus Cx30 inhibitors like flecainide reduce astrocyte coupling, which may reduce lactate release, resulting in disinhibition of neuronal activity in dopaminergic targets (Figure 1). What remains to be discovered is the exact effect of Cx30 and the overall regulatory role of astrocytes on specific dopaminergic groups or targets. If, for example, the VTA is a common substrate of the arousal, anti-cataplexy, and memory-promoting effects of modafinil, we may assume that astrocyte coupling and gap junctions regulate neuronal activity in these sites. It would be interesting to test the specific effect of Cx30 on not just somatosensory neurons but also astrocytes extracted from the dopaminergic circuits of interest, as Cx30's interaction with modafinil may have different effects on different astrocytic populations.

Figure 1.

Proposed neural circuitry regulating modafinil action on arousal. Two dopamine groups in the VTA and vPAG may underlie arousal and anti-cataplexy effects of modafinil that elevates dopamine level by inhibiting dopamine reuptake at the VTA and vPAG projection fields: the nucleus of accumbens (NAc), posterior lateral hypothalamus (pLH), medial prefrontal cortex (mPFC), and central amygdala (CeA). In these projection sites, gap junctions may regulate neuronal activity by modulating lactate transporting between astrocytes and from astrocytes to neurons.

CITATION

Lu J, Chen M. Glial gap junctions boost modafinil action on arousal. SLEEP 2016;39(6):1175–1177.

DISCLOSURE STATEMENT

The authors have indicated no financial conflicts of interest.