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A New Target for Treatment of Schizophrenia

 

By: Carol A. Rouzer, VICB Communications
Published: September 10, 2018

 

Activation of the mGlu1 receptor has promising anti-psychotic effects without the side effects of traditional drugs used to treat schizophrenia.

 

Although current drugs used to treat schizophrenia are highly effective at combatting the positive symptoms of the disease, such as hallucinations and delusions, they offer little relief against the negative symptoms such as lethargy, apathy, and blunted emotions, and they cause significant adverse effects. Thus, better drugs to treat this debilitating neuropsychiatric illness are sorely needed. One of the most exciting new targets in this search is the M4 muscarinic acetylcholine receptor. Highly selective positive allosteric modulators (PAMs) of M4 demonstrate impressive antipsychotic effects in animal models, and a dual M1/M4 agonist yielded promising results in a recent clinical trial. Furthermore, single nucleotide polymorphisms in the gene encoding M4 are associated with schizophrenia. Now, however, Vanderbilt Institute of Chemical Biology members Jeff Conn and Craig Lindsley, working with colleagues at the Vanderbilt Center for Neuroscience Drug Discovery, show that M4 agonists exert their beneficial effects via activation of the mGlu1 metabotropic glutamate receptor, suggesting that mGlu1 may prove to be an even more effective target than M4 for the treatment of schizophrenia [S. E. Yohn, et al. Mol. Psychiatry, (2018) published online Aug. 16, DOI: 10.1038/s41380-018-0206-2].

 

Many symptoms of schizophrenia result from excessive dopamine signaling in the dorsal striatum and insufficient dopamine signaling in extrastriatal regions. Activation of M4 receptors expressed on spiny projection neurons (SPNs) in the striatum results in their release of the endocannabinoid 2-arachidonoylglycerol (2-AG), which travels to neighboring dopamine-releasing neurons. There, the 2-AG binds to CB2 receptors, and the resulting signaling leads to a reduction in dopamine release (Figure 1). This suppression of dopamine release in the striatum is the basis for the beneficial effects of M4 agonists in schizophrenia. However, recent studies have also demonstrated that M4 signaling in SPNs suppresses the activity of D1 dopamine receptors that are also  expressed by these cells. This has led to the concern that M4 agonists may lead to an imbalance between D1 and D2 receptor signaling with the potential for unwanted side effects (Figure 1).

 

 

FIGURE 1. Mechanism of suppression of dopamine release by M4-dependent signaling. Spiny projection neurons expressing both M4 and D1 receptors (green) project to neighboring neurons that release dopamine (blue). Activation of M4 in the spiny projection neurons leads to the release of 2-AG, which then suppresses dopamine release. However, M4 activation also blocks signaling by D1 receptors.

 

 

In their search for a way to address this possible problem, the investigators became intrigued that M4 stimulation leads to 2-AG release despite the fact that M4 acts via the Gαq/11 G protein, whereas 2-AG release usually requires signaling mediated by Gαi/o. These considerations led the researchers to hypothesize that signaling by an intermediate receptor is required to trigger 2-AG release by M4 agonists, and they soon focused on a possible role for group 1 metabotropic glutamate receptors (mGlu1 and mGlu5), which are coupled to Gαi/o. These receptors are widely expressed in SPNs, and single nucleotide polymorphisms in the gene for mGlu1 are associated with schizophrenia. Thus, the researchers proposed that activation of M4 leads to subsequent activation of mGlu1 and/or mGlu5, which in turn, leads to 2-AG release (Figure 2).

 

 


FIGURE 2. Role of mGlu1 in suppression of dopamine release by M4-dependent signaling. The mechanism is the same as in Figure 1, except that here activation of M4 in the spiny projection neurons leads to activation of mGlu1, which then stimulates the release of 2-AG. 2-AG suppresses dopamine release. However, M4 activation also blocks signaling by D1 receptors.

 

 

To test their hypothesis, the researchers used cyclic voltametry to measure dopamine release in striatal brain slices from mice. They treated the slices with oxotremorine-M, a nonselective muscarinic acetylcholine receptor agonist known to elicit a decrease in dopamine release via activation of M4. Inclusion of an mGlu1 negative allosteric modulator (NAM) suppressed the effect of oxotremorine-M on dopamine release, while an mGlu5 NAM had no effect, supporting the hypothesis that mGlu1 mediates the suppression of dopamine release that results from M4 activation.
     

The investigators next turned to two animal models of schizophrenia symptoms. The first was amphetamine-induced hyperlocomotion (AHL) in which administration of amphetamine induces increased activity in mice. The second was amphetamine-induced suppression of inhibition of the startle response to a loud noise by a prior noise, known as prepulse inhibition (PPI). Note that each of these models depends on amphetamine's ability to increase dopamine signaling in the brain. In both cases, an M4 PAM reduced the effects of amphetamine, and in both cases, prior treatment with an mGlu1 NAM reversed the effects of the M4 PAM. These findings further support a role for mGlu1 in M4-mediated signaling.
     

Returning to studies in brain slices, the researchers next demonstrated that a dual agonist of mGlu1 and mGlu5 inhibited dopamine release in the absence of M4 signaling (Figure 3). This effect was blocked by an mGlu1 NAM but not an mGlu5 NAM, indicating once again that the responsible receptor was mGlu1. The effect was also blocked by an antagonist of CB2, confirming a role for 2-AG in the response.


 


     

FIGURE 3. Direct suppression of dopamine release by mGlu1-dependent signaling. The mechanism is the same as in Figure 2, except that here mGlu1 is activated directly, leading to stimulation of 2-AG release and suppression of dopamine signaling. Bypass of M4 avoids suppression of signaling by D1.

 

 

To identify the neuronal pathways involved in physiological modulation of dopamine release by mGlu1 signaling, the researchers expressed channelrhodopsin-2 in neurons of the motor cortex and paraventricular nucleus of the thalamus, both of which send projections to the striatum. Expression of channelrhodopsin-2 enabled the investigators to selectively activate the neurons by shining light on them. They found that activation of neurons in the thalamus led to an inhibition of dopamine release in the striatum, and this inhibition was blocked by an mGlu1 NAM. They also observed inhibition of dopamine release upon stimulation of the cortical neurons, but in this case the mGlu1 NAM was ineffective, suggesting that receptors other than mGlu1 mediated this response. Thus, the researchers concluded that thalamostriatal pathways control dopamine release via mGlu1-mediated signaling in the brain.

 

Finally, the investigators returned to their animal models. They showed that systemic administration of an mGlu1 PAM reversed the effects of amphetamine in both the AHL and PPI models. In each case, the effects of the mGlu1 PAM were blocked by a CB2 receptor antagonist. Further studies showed that the mGlu1 PAM had no effect on signaling by the D1 receptor. In addition, unlike M4 PAMs and traditional antipsychotic medications, the mGlu1 PAM had no effects in an animal model of motivation.
     

The researchers concluded that M4-dependent suppression of dopamine signaling in the striatum is mediated by mGlu1 activation. Thus, the same effects can be achieved by direct stimulation of mGlu1 without disruption of D1 signaling observed with agonists of M4 (Figure 3).These findings identify mGlu1 as a promising new target for the treatment of schizophrenia, and more importantly, they suggest that drugs directed against this target may lack some of the more serious side effects of currently available antipsychotic medications.

 

 

View Mol Psychiatry article: Activation of the mGlu1 metabotropic glutamate receptor has antipsychotic-like effects and is required for efficacy of M4 muscarinic receptor allosteric modulators

 

 

 

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