A novel metabotropic glutamate receptor 5 positive allosteric modulator acts at a unique site and confers stimulus bias to mGlu5 signaling.

Metabotropic glutamate receptor 5 (mGlu5) is a target for the treatment of central nervous system (CNS) disorders, such as schizophrenia and Alzheimer's disease. Furthermore, mGlu5 has been shown to play an important role in hippocampal synaptic plasticity, specifically in long-term depression (LTD) and long-term potentiation (LTP), which is thought to be involved in cognition. Multiple mGlu5-positive allosteric modulators (PAMs) have been developed from a variety of different scaffolds. Previous work has extensively characterized a common allosteric site on mGlu5, termed the MPEP (2-Methyl-6-(phenylethynyl)pyridine) binding site. However, one mGlu5 PAM, CPPHA (N-(4-chloro-2-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)methyl]phenyl)-2-hydroxybenzamide), interacts with a separate allosteric site on mGlu5. Using cell-based assays and brain slice preparations, we characterized the interaction of a potent and efficacious mGlu5 PAM from the CPPHA series termed NCFP (N-(4-chloro-2-((4-fluoro-1,3-dioxoisoindolin-2-yl)methyl)phenyl)picolinamide). NCFP binds to the CPPHA site on mGlu5 and potentiates mGlu5-mediated responses in both recombinant and native systems. However, NCFP provides greater mGlu5 subtype selectivity than does CPPHA, making it more suitable for studies of effects on mGlu5 in CNS preparations. Of interest, NCFP does not potentiate responses involved in hippocampal synaptic plasticity (LTD/LTP), setting it apart from other previously characterized MPEP site PAMs. This suggests that although mGlu5 PAMs may have similar responses in some systems, they can induce differential effects on mGlu5-mediated physiologic responses in the CNS. Such stimulus bias by mGlu5 PAMs may complicate drug discovery efforts but would also allow for specifically tailored therapies, if pharmacological biases can be attributed to different therapeutic outcomes.

Potentiation of mGlu5 receptors with the novel enhancer, VU0360172, reduces spontaneous absence seizures in WAG/Rij rats.

Absence epilepsy is generated by the cortico-thalamo-cortical network, which undergoes a finely tuned regulation by metabotropic glutamate (mGlu) receptors. We have shown previously that potentiation of mGlu1 receptors reduces spontaneous occurring spike and wave discharges (SWDs) in the WAG/Rij rat model of absence epilepsy, whereas activation of mGlu2/3 and mGlu4 receptors produces the opposite effect. Here, we have extended the study to mGlu5 receptors, which are known to be highly expressed within the cortico-thalamo-cortical network. We used presymptomatic and symptomatic WAG/Rij rats and aged-matched ACI rats. WAG/Rij rats showed a reduction in the mGlu5 receptor protein levels and in the mGlu5-receptor mediated stimulation of polyphosphoinositide hydrolysis in the ventrobasal thalamus, whereas the expression of mGlu5 receptors was increased in the somatosensory cortex. Interestingly, these changes preceded the onset of the epileptic phenotype, being already visible in pre-symptomatic WAG/Rij rats. SWDs in symptomatic WAG/Rij rats were not influenced by pharmacological blockade of mGlu5 receptors with MTEP (10 or 30 mg/kg, i.p.), but were significantly decreased by mGlu5 receptor potentiation with the novel enhancer, VU0360172 (3 or 10 mg/kg, s.c.), without affecting motor behaviour. The effect of VU0360172 was prevented by co-treatment with MTEP. These findings suggest that changes in mGlu5 receptors might lie at the core of the absence-seizure prone phenotype of WAG/Rij rats, and that mGlu5 receptor enhancers are potential candidates to the treatment of absence epilepsy. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Effect of neostigmine on concentration and extraction fraction of acetylcholine using quantitative microdialysis.

A quantitative microdialysis method was used to determine the effect of local perfusion of 0, 100, 200, and 300 nM neostigmine (NEO) on acetylcholine (ACh) extracellular concentration and microdialysis extraction fraction (E(d)) in the striatum of the rat. Because of the efficiency of AChE, the inhibition of this enzyme is expected to result in a substantial increase in ACh levels and a decrease in the E(d) of ACh. The extracellular concentration of ACh increased linearly with increasing concentrations of NEO. The control ACh concentration was determined to be 18.4 +/- 11.8 nM (n = 10; mean +/- S.E.M.) The ACh extracellular concentration for the remaining groups was determined to be 173 +/- 14 nM (n = 5), 329 +/- 52.5 nM (n = 13), and 581 +/- 109 nM (n = 10) for the 100, 200, and 300 nM NEO groups, respectively. Perfusion with 300 nM NEO resulted in a significant reduction in the E(d) of ACh (64.5 +/- 3.5% vs. 43.6 +/- 7.5%, P < 0.05). In contrast to ACh, perfusion with 0, 1, and 10 microM hemicholinium-3, an inhibitor of high-affinity choline uptake, increased choline levels but did not affect the E(d) of choline. The effects on E(d) are consistent with E(d) being influenced by rapid clearance mechanisms.