Modulation of Striatal Adenosinergic Function by HTL0041178, a Selective GPR52 Agonist
By Cliona MacSweeney | Apr 20, 2022
Cliona MacSweeney, a project director in our Translational Medicine department, recently presented this important pre-clinical work at SIRS2022 confirming the ability of the highly selective novel GPR52 agonist HTL00411718 to modulate centrally mediated locomotor activity in response to A2A receptor antagonists. This work highlights an important interplay between the adenosinergic and dopaminergic systems and suggests the potential for GPR52 agonists to control neuropsychiatric symptoms in conditions such as schizophrenia.
Caffeine, a non-selective adenosine receptor antagonist, is a psychostimulant which increases rodent locomotor activity principally via blockade of adenosine 2A receptors (A2AR). These receptors are densely expressed on the terminals of GABAergic striatopallidal neurons in the indirect pathway of the basal ganglia, in which dopamine D2 receptors (D2R) are co-expressed. Tonic activation of A2AR decreases the affinity of D2R to dopamine, while antagonism of A2AR facilitates dopaminergic signalling. A number of antipsychotic agents have been shown to block hyperlocomotion induced by caffeine and indeed the adenosine hypothesis of schizophrenia posits that hyperdopaminergia may be secondary to a loss of function of the adenosine system. GPR52, a constitutively active Gαs G protein-coupled orphan receptor, is predominantly expressed in the striatum on D2R striatopallidal neurons and a recent spatiomolecular mapping study showed a distinct overlap of GPR52 with A2AR in a subpopulation of striatal neurons. The aim of the present study was to explore whether HTL0041178, a selective GPR52 agonist, would modulate rat hyperlocomotor activity stimulated by caffeine or the selective A2AR antagonist istradefylline. An in vitro competition assay was also performed to determine whether a GPR52 agonist would alter the affinity of caffeine or istradefylline for A2AR.
Locomotor studies: after 1h habituation to the locomotor cages, male Sprague-Dawley rats (n=12 per group) were dosed with vehicle, risperidone (0.6 mg/kg, IP) or HTL0041178 (3, 10 and 30 mg/kg, PO). One hour later, they were dosed with vehicle/caffeine (15 mg/kg, SC) for the caffeine study, or vehicle/istradefylline (10 mg/kg, IP) for the istradefylline study, and locomotor activity was assessed for 2h. Data are back-transformed means, adjusted for differences between treatment groups in activity during the 30 minutes prior to treatment with test compound. They were analysed using a general linear model with treatment, cohort and cage rack as factors. HTL0041178 was compared to vehicle by Williams’ test. Samples were taken at the end of the study to confirm plasma and brain concentrations of HTL0041178.
In vitro competition assay: striatal membrane suspensions were incubated with HTL0041178 (1 µM) for 10 minutes and then with [3H]ZM241385 and either assay buffer (total binding), SCH442416 (non-specific binding), caffeine (10 concentrations 100-1E6 nM) or istradefylline (10 concentrations 0.01-1,000 nM) for 30 minutes. Log-transformed data were analysed by two-way analysis of variance with treatment and assays as factors.
Treatment with HTL0041178 resulted in a dose-dependent reduction of both the caffeine- and istradefylline-induced hyperlocomotor responses, reaching statistical significance (p<0.05) at all doses tested. Risperidone also significantly reduced (p<0.05) caffeine- and istradefylline-induced hyperlocomotion. The presence of HTL0041178 had no effect on the affinity of caffeine or istradefylline for A2AR as measured in the in vitro assay.
The present study demonstrates that a highly selective GPR52 agonist can modulate the behavioural response to A2AR antagonists without directly affecting A2AR binding. Interestingly, Nishiyama et al (2017) demonstrated that the locomotor response to istradefylline was significantly augmented in GPR52 KO mice compared to WT mice. Due to its localisation on D2R neurons, GPR52 has been proposed as a target for the treatment of psychosis but its specific co-expression with A2AR and a potential role in the interplay between the adenosinergic and dopaminergic systems warrants further investigation.
THE GPR52 AGONIST HTL0041178 ATTENUATES REVERSALLEARNING DEFICITS IN THE SUB-CHRONIC PHENCYCLIDINE RAT - (Presented at SIRS 2022, co-authored by Sosei Heptares and the University of Manchester)