Acute pharmacological modulation of mGluR8 reduces measures of anxiety

Abstract

Metabotropic glutamate receptors (mGluRs), which are coupled to second messenger pathways via G proteins, modulate glutamatergic and GABAergic neurotransmission. Because of their role in modulating neurotransmission, mGluRs are attractive therapeutic targets for anxiety disorders. Previously we showed that mGluR8(-/-) male mice showed higher measures of anxiety in the open field and elevated plus maze than age-matched wild-type mice. In this study, we assessed the potential effects of acute pharmacological modulation of mGluR8 on measures of avoidable and unavoidable anxiety. In addition to wild-type mice, we also tested apolipoprotein E-deficient (Apoe(-/-)) mice, as these mice show increased levels of anxiety-like behaviors and therefore might show an altered sensitivity to mGluR8 stimulation. mGluR8 stimulation with the specific agonist DCPG, or modulation with AZ12216052, a new, positive allosteric modulator of mGluR8 reduced measures of anxiety in both wild-type mice. The effects of mGluR8 positive allosteric modulators, which only affect neurotransmission in the presence of extracellular glutamate, seem particularly promising for patients with anxiety disorders showing benzodiazepine insensitivity.

Figure 1. Identification of AZ12216052, a novel positive allosteric modulator of mGluR8

A. Structure of AZ12216052. B. GTPγS binding assay in the presence of 300 nM glutamate relative to GTPγS binding in 1 mM glutamate alone. C. 30 µM AZ12216052 shifts the glutamate concentration-response curve to the left. D. The potentiator effect of AZ12216052 on mGluR8 is inhibited by group-II/III antagonists LY341495 and CPPG. In panels B–D,100% is the maximal GTPγS binding in the presence of 1 mM glutamate.

Figure 2. Effects of DCPG on anxiety-like behaviors

A. Effects of DCPG (3, 10, or 30 mg/kg) on measures of anxiety of 2-month-old Wt and Apoe^−/− male mice in the elevated zero maze. *p < 0.05 versus genotype-matched vehicle-treated control. There was an overall effect of genotype on time spent in the open areas (F = 58.3, p < 0.0001). DCPG did not affect velocity in Wt (vehicle: 3.0 ± 0.6 cm/sec; DCPG 3: 4.3 ± 0.3; DCPG 10: 4.1 ± 0.5; DCPG 30: 3.4 ± 0.2 cm/sec) or Apoe^−/− mice (vehicle: 2.2 ± 0.6 cm/sec; DCPG 3: 2.7 ± 0.6; DCPG 10: 2.0 ± 0.6; DCPG 30: 1.4 ± 0.4 cm/sec). There was no effect of genotype on velocity. n = 8 mice/genotype/dose. B. Effects of DCPG (3, 10, or 30 mg/kg) on baseline startle response in 3-month-old Wt and Apoe^−/− mice. DCPG increased the baseline startle response (F = 25.92, p = 0.0004). n = 8 mice/genotype/dose.

Figure 3. Effects of AZ12216052 on anxiety-like behaviors

A. Effects of AZ12216052 (10 mg/kg) on measures of anxiety of 2-month-old Wt mice. AZ12216052 increased the time spent in the anxiety-provoking open areas of the elevated zero maze (p = 0.03, n = 8 mice/treatment). B. Effects of AZ12216052 on acoustic startle response of 2-month-old mice (vehicle: 0.65 ± 0.04; AZ12216052: 0.52 ± 0.03 N; F = 15.32, p < 0.0001, n = 8 mice/treatment). AZ12216052 had no effects on the baseline response (vehicle: 0.22 ± 0.01; AZ12216052: 0.24 ± 0.01 N). *p < 0.05 versus vehicle. n = 8 mice/treatment. The thresholds for the acoustic startle response were for vehicle 100 dB and for AZ12216052 104 dB.

Figure 4. Effects of AZ12216052 (10 mg/kg) on measures of anxiety in 5-month-old Wt and Apoe^−/− mice

(A) Time spent in the open areas of the maze. (B) Distance moved in the open areas of the maze. *p < 0.05 versus vehicle-treated genotype-matched control. AZ12216052 did not affect velocity in wild-type (vehicle: 1.8 ± 0.3 cm/sec; AZ12216052: 2.1 ± 0.4 cm/sec) or Apoe^−/− (vehicle: 1.9 ± 0.4 cm/sec; AZ12216052: 1.4 ± 0.4 cm/sec) male mice. n = 7–8 wild-type mice; n = 5–6 Apoe^−/− mice.