4.1 GPR6
GPR6 is a constitutively active Class C GPCR that couples to a
stimulatory G-protein (GS) leading to increased cAMP
levels at a similar amplitude of fully activated GPCRs
(Tanaka et al., 2007; Uhlenbrock et
al., 2002) and enhanced neurite outgrowth in vitro (Tanaka et
al., 2007). GPR6 mRNA is predominantly expressed in neurons in the
brain, particularly in the striatum (caudate, putamen, nucleus
accumbens, and olfactory tubercle) and to a lesser extent the frontal
cortex, retrosplenial cortex, hippocampus, amygdala, and hypothalamus
(Heiber et al., 1995; Marchese et al., 1994; Song et al., 1994). Within
the striatum GPR6 is localised on dopamine D2 receptor-expressing
striatopallidal medium spiny neurons (MSNs; Heiman et al., 2008; Lobo et
al., 2007). Controversy about the endogenous ligand for GPR6 still
persist. Initial studies showed agonism of GPR6 by
sphingosine-1-phosphate (S1P) but these results were not replicated
(Ignatov et al., 2003; Yin et al., 2009), leaving
GPR6 as an orphan receptor (Alexander
et al., 2017).
GPR6 expression is most dense within the striatum, a brain region
important for reward behaviours, decision making and motor control (Lobo
et al., 2007). Two intermingled but distinct populations of MSNs,
differing in dopamine receptor subtype expression control behavioural
output from the striatum. Dopamine D1receptor-expressing MSNs in the dorsal striatum project into and inhibit
the substantia nigra pars reticulata (SNr; direct, or striatonigral
pathway), releasing inhibition of thalamic activity and therefore
prompting behaviour. In contrast, D2 receptor-expressing
MSNs project to and inhibit the external globus pallidus (indirect or
striatopallidal pathway), disinhibiting the subthalamic nucleus (STN)
and exciting the downstream SNr, which ultimately inhibits the thalamus
and supresses behaviour (Kreitzer & Malenka, 2008). These pathways are
postulated to antagonise each other to allow a balanced striatal output
(Albin et al., 1989).
Dopamine is released in the dorsal striatum by neurons located in the
substantia nigra pars compacta (SNpc) and acts upon D1dopamine receptors (which depolarise the cell in response to dopamine)
and D2 dopamine receptors (which hyperpolarise the cell
in response to dopamine). Drugs of abuse acutely increase dopamine
release in the striatum, and thus have the dual effect of exciting the
direct pathway while simultaneously inhibiting the indirect pathway
(Kreitzer & Malenka, 2008). GPR6 is specifically expressed in
striatopallidal, D2 receptor-positive neurons (Heiman et
al., 2008; Lobo et al., 2007), and their deletion leads to adaptive
changes in both striatopallidal-specific genes (Drd2 andAdora2a ) and a striatonigral-specific gene (Tac1; Lobo et
al., 2007). Further, GPR6 deficient mice have increased in dopamine and
metabolite levels in the striatum (Oeckl et al., 2014) and enhanced
instrumental responding for a sucrose reward, without altered motor
co-ordination (Lobo et al., 2007).
Genome-wide RNA sequencing has recently shown GPR6 is
downregulated in the dorsal striatum of individuals with alcohol use
disorder (Walker et al., 2020). In the prefrontal cortex (PFC) however,
sequencing revealed Gpr6 was upregulated in both food and
cocaine- “addicted” compared to “non-addicted” mice (Navandar et
al., 2021). However, the exact role of GPR6 in driving alcohol and
substance use is not known; whether dysregulation of GPR6 signalling is
causal or a consequence of substance use is not established and whether
targeting GPR6 may have potential to reduce alcohol and substance use
requires elucidation.
Recently several small molecule inverse agonists have been identified
and developed to interact with GPR6. Phylogenetically GPR6 is closely
related to the cannabinoid receptors, and cannabidiol (CBD), several
synthetic cannabinoids and endocannabinoid-like N-acylamides act as
inverse agonists at GPR6 (Laun et al., 2019; Laun & Song, 2017; Shrader
& Song, 2020). Preclinical studies have shown CBD may be effective to
reduce opioid, psychostimulant, nicotine and alcohol use (Nona et al.,
2019; Prud’homme et al., 2015). However, the mechanisms that CBD acts
through are widespread, and whether any actions are mediated via GPR6
would require further examination. A novel compound CVN424 has recently
been developed and shown to be a potent, orally active, and
brain-penetrant selective inverse agonist for GPR6, which is effective
in reducing Parkinson’s-like symptoms in a rodent model (Brice et al.,
2021). This compound has successfully undergone Phase I safety trials
and is currently in Phase II trials (Margolin et al., 2022),
highlighting a potential future opportunity of repurposing for other
indications. However, given the opposing regulation of GPR6 in the
striatum and PFC in response to drugs of abuse, whether inverse agonism
would further exacerbate symptoms is a possibility that may limit
development in this regard.