Bidirectional variation in glutamate efflux in the medial prefrontal cortex induced by selective positive and negative allosteric mGluR5 modulators

Abstract

Dysregulation of prefrontal cortical glutamatergic signalling via NMDA receptor hypofunction has been implicated in cognitive dysfunction and impaired inhibitory control in such neuropsychiatric disorders as schizophrenia, attention-deficit hyperactivity disorder and drug addiction. Although NMDA receptors functionally interact with metabotropic glutamate receptor 5 (mGluR5), the consequence of this interaction for glutamate release in the prefrontal cortex (PFC) remains unknown. We therefore investigated the effects of positive and negative allosteric mGluR5 modulation on changes in extracellular glutamate efflux in the medial PFC (mPFC) induced by systemic administration of the non-competitive NMDA receptor antagonist dizocilpine (or MK801) in rats. Extracellular glutamate efflux was measured following systemic administration of the positive allosteric mGluR5 modulator [S-(4-Fluoro-phenyl)-{3-[3-(4-fluoro-phenyl)-[1,2,4]-oxadiazol-5-yl]-piperidin-1-yl}-methanone] (ADX47273; 100 mg/kg, p.o.) and negative allosteric mGluR5 modulator [2-chloro-4-{[1-(4-fluorophenyl)-2,5-dimethyl-1H-imidazol-4-yl]ethynyl}pyridine] (RO4917523; 0.3 mg/kg, p.o.), using a wireless glutamate biosensor in awake, freely moving rats. The effect of MK801 (0.03-0.06 mg/kg, s.c.) on mPFC glutamate efflux was also investigated in addition to the effects of MK801 (0.03 mg/kg, s.c.) following ADX47273 (100 mg/kg, p.o.) pre-treatment. ADX47273 produced a sustained increase in glutamate efflux and increased the effect of NMDA receptor antagonism on glutamate efflux in the mPFC. In contrast, negative allosteric mGluR5 modulation with RO4917523 decreased glutamate efflux in the mPFC. These findings indicate that positive and negative allosteric mGluR5 modulators produce long lasting and opposing actions on extracellular glutamate efflux in the mPFC. Positive and negative allosteric modulators of mGluR5 may therefore be viable therapeutic agents to correct abnormalities in glutamatergic signalling present in a range of neuropsychiatric disorders.

Keywords: NMDA receptors; glutamate; impulsivity; mGluR5; prefrontal cortex.

Figure 1

Schematic diagrams of coronal rat brain sections depicting reconstructed biosensor placements within the mPFC. Black lines represent the glutamate‐sensitive region of each sensor. Coordinates are mm forward of bregma. (a) Sensor placements for MK801 dose‐response experiment (n = 14 rats; two rats were excluded because of probe placements outside of the mPFC (shown in red). (b) Sensor placements for mGluR5 modulation experiments (n = 11 rats). Drawings adapted from Paxinos, George; Watson, Charles. (1998). The Rat Brain in Stereotaxic Coordinates. London: Academic Press.

Figure 2

Effect of NMDA receptor antagonism, by 0.03 or 0.06 mg/kg MK801 (s.c.), on extracellular glutamate efflux in the mPFC. (a) All rats received vehicle (n = 12 rats; s.c.; time bin zero), followed by vehicle (grey) (n = 2 rats; individual data shown) or MK801 (0.03 mg/kg (n = 4 rats) or 0.06 mg/kg (n = 6 rats) 1 h later. Data points represent mean ± 1SEM. Repeated‐measures anova (mixed design) (main effect of drug, *p < 0.05 vehicle versus MK801 (up to 1 h post‐injection). (b) area under the curve (AUC) presented, as a histogram, for vehicle and MK801 treatment, 1 h post‐injection. (c) Summary of experimental timeline (BL = baseline).

Figure 3

Effect of the positive allosteric mGluR5 modulator, S‐(4‐Fluoro‐phenyl)‐{3‐[3‐(4‐fluoro‐phenyl)‐[1,2,4]‐oxadiazol‐5‐yl]‐piperidin‐1‐yl}‐methanone (ADX47273), on extracellular glutamate efflux in the mPFC (n = 10 rats). (a) All rats received vehicle (p.o.; time bin zero). Three hours later, all rats received ADX47273 (100 mg/kg, p.o.; time bin zero). Data points represent mean ± 1 SEM. Repeated‐measures anova, Bonferroni post hoc, *p < 0.05, **p < 0.01, ADX47273 versus vehicle treatment. (b) area under the curve (AUC) presented, as a histogram, for vehicle and ADX47273 conditions. (c) Summary of experimental timeline (BL = baseline).

Figure 4

Effect of the negative allosteric mGluR5 modulator, RO4917523, on extracellular glutamate efflux in the mPFC (n = 8 rats). (a) All rats received vehicle (p.o.; time bin zero). Three hours later, rats received RO4917523 (0.3 mg/kg, p.o.; time bin zero). Data points represent mean ± 1SEM. Repeated‐measures anova, Bonferroni post hoc, *p < 0.05, **p < 0.01, RO4917523 versus vehicle condition. (b) area under the curve (AUC) presented, as a histogram, for vehicle and RO4917523 conditions. (c) Summary of experimental timeline (BL = baseline).

Figure 5

Effect of pre‐treatment with S‐(4‐Fluoro‐phenyl)‐{3‐[3‐(4‐fluoro‐phenyl)‐[1,2,4]‐oxadiazol‐5‐yl]‐piperidin‐1‐yl}‐methanone (ADX47273) on glutamate efflux in the mPFC evoked by the NMDA receptor antagonist MK801 (n = 10 rats). (a) Rats received vehicle (p.o.) or ADX47273 (100 mg/kg, p.o.; time bin zero), followed by vehicle (s.c.) or MK801 (0.03, s.c.) 80 min later. Data points represent mean ±1SEM. Repeated‐measures anova. (b) area under the curve (AUC) presented as a histogram, for vehicle or ADX47273 or MK801 or ADX47273 and MK801 treatment (2‐h post‐MK801/vehicle injection). (c) Summary of experimental timeline (BL = baseline).

Figure 6

Illustrative diagram showing putative mechanisms underlying the effects of NMDA receptor antagonism and allosteric mGluR5 modulation on glutamate efflux in the mPFC. (a) NMDA receptor antagonism inhibits GABA‐ergic neurons leading to a disinhibition of glutamatergic neurons and increased glutamate efflux. (b) Positive allosteric mGluR5 modulation activates GABA‐ergic neurons and glutamatergic neurons via the activation of pre‐ and post‐synaptic mGluR5 resulting in increased glutamate efflux. (c) NMDA receptor antagonism and positive allosteric mGluR5 modulation leads to a reversal of (i) NMDA receptor antagonist‐induced disinhibition; (ii) post‐synaptic mGluR5 activation of glutamatergic neurons; (iii) pre‐synaptic mGluR5 autoreceptor activation, leading to increased glutamate efflux. (d) Negative allosteric mGluR5 modulation inhibits GABA‐ergic neurons and glutamatergic neurons leading to decreased glutamate efflux. This is a hypothesized proposal since we did not assess GABA‐ergic mechanisms.