Enhanced cocaine responsiveness and impaired motor coordination in metabotropic glutamate receptor subtype 2 knockout mice

Abstract

Extensive pharmacological studies have recently emerged indicating that group 2 metabotropic glutamate receptors (mGluRs) comprising mGluR2 and mGluR3 subtypes are associated with several neurological and psychiatric disorders. mGluR2 is widely distributed both presynaptically and postsynaptically in a variety of neuronal cells, but the physiological role of mGluR2 in brain function is poorly understood. This investigation involves a comprehensive behavioral analysis of mGluR2-/- knockout (KO) mice to explore the physiological role of mGluR2 in brain function. Although, under general observation, mGluR2-/- KO mice appeared to have no behavioral abnormalities, they exhibited several lines of behavioral alterations in the enforcing and defined behavioral tests. They showed a significant increase in locomotor sensitization and conditioned place preference in association with repeated cocaine administration, indicating that mGluR2 contributes to behavioral responses implicated in reinforcement and addiction of cocaine. Upon in vivo microdialysis analysis after cocaine administration, not only did extracellular levels of dopamine increase but also the response pattern of glutamate release markedly changed in the nucleus accumbens of mGluR2-/- KO mice. The mGluR2-/- KO mice also showed significant impairment in motor coordination in the accelerating rota-rod test and exhibited hyperlocomotion in novel environmental and stressful conditions, when assessed by the open-field and forced-swim tests. These results indicate that the inhibitory mGluR2 plays a pivotal role in synaptic regulation of glutamatergic transmission in the neural network.

Fig. 1.

Enhanced locomotion of mGluR2^–/– KO mice in novel environments. (A) Mice were placed in the open field and distance traveled was measured every 5 min for a 30-min period [(n = 19 (WT) and 17 (KO)]. There is a significant increase in locomotor activity in mGluR2^–/– KO mice as compared with WT mice [genotype effect, F(1, 34) = 4.55, P = 0.040] (*, P < 0.05). (B) Distances traveled were measured for a 10-min period in the indicated tests. Data are presented as percentages of average value of WT mice. Differences in distances traveled are not statistically significant between mGluR2^–/– KO and WT mice in these tests, but mGluR2^–/– KO mice displayed a tendency to increase locomotor activity in all three tests; the light-dark transition test, P = 0.36, n = 21 (WT) and 19 (KO); the elevated-plus maze test, P = 0.25, n = 21 (WT) and 19 (KO); the social interaction test, P = 0.21, n = 10 pairs (WT) and 9 pairs (KO). (C) Distances traveled of mGluR2^–/– KO (n = 16) and WT (n = 16) mice in their home cages exposed to a light-dark cycle of 12 h/12 h were measured at each 60-min point. There is no statistical difference between the two genotypes [genotype effect, F(1, 30) = 0.000, P = 0.999].

Fig. 2.

Decreased immobility of mGluR2^–/– KO mice in the forced-swim test. Mice were placed in a water-filled cylinder, and their immobility time was measured every 1 min for a 10- and a 6-min period on the first (A) and second (B) days, respectively (n = 9 for both WT and KO). The immobility time is not significantly different between the two genotypes on the first day [genotype effect, F(1, 16) = 0.041, P = 0.841] but significantly decreases in mGluR2^–/– KO mice as compared with WT mice on the second day [genotype effect, F(1, 16) = 9.255, P = 0.0078] (*, P < 0.05; **, P < 0.01).

Fig. 3.

Impairment of mGluR2^–/– KO mice in the rota-rod test. The latency to fall from an accelerating rota-rod (from 4 to 40 rpm) was measured by three trials per day [n = 21 (WT) and 19 (KO)]. mGluR2^–/– KO mice spend significantly less time on the rod than did WT mice [genotype effect, F(1, 38) = 12.53, P = 0.0011] (*, P < 0.05; **, P < 0.01), although their performance improves after repeated trials [trial effect, F(5, 90) = 11.35, P < 0.0001].

Fig. 4.

Enhanced locomotor sensitization to cocaine in mGluR2^–/– KO mice. After i.p. saline injection on day 0, saline (A) and 10 mg/kg (B) and 20 mg/kg (C) cocaine were i.p.-injected once a day from day 1 to day 5; n = 8 (WT) and 11 (KO) in A; n = 10 (WT) and 9 (KO) in B; n = 9 (WT) and 13 (KO) in C. Immediately after injection of saline or cocaine, locomotor activity was counted for a 10-min period. In A, there is no difference in locomotor activity between the two genotypes [genotype effect, F(1, 17) = 0.172, P = 0.682]. Daily administration of cocaine at doses of 10 (B) and 20 (C) mg/kg induces locomotor sensitization in both mGluR2^–/– KO and WT mice [10 mg in WT, F(5, 45) = 6.501, P < 0.0001; 10 mg in KO, F(5, 40) = 9.559, P < 0.0001; 20 mg in WT, F(5, 40) = 11.61, P < 0.0001; 20 mg in KO, F(5, 60) = 31.86, P < 0.0001]. In B, there is a significant increase in locomotor activity in mGluR2^–/– KO mice as compared with WT mice by repeated cocaine administration [genotype effect, F(1, 17) = 6.397, P = 0.022] (*, P < 0.05; **, P < 0.01). In C, there is a significant interaction between genotype and time by repeated cocaine administration [F(4, 80) = 4.712, P = 0.0018], and a significant increase in locomotor activity is noted in mGluR2^–/– KO mice as compared with WT mice at all time points after cocaine treatment (*, P < 0.05; **, P < 0.01; ***, P < 0.001).

Fig. 5.

Enhanced CPP to cocaine in mGluR2^–/– KO mice. Shown are results of before and after conditioning with indicated doses of i.p. cocaine injection for 3 days, and the time difference was calculated by subtracting time mice spent in the saline-paired side from the time they spent in the cocaine-paired side (for each group, n > 8). mGluR2^–/– KO mice spend significantly more time as compared with WT mice in the cocaine-paired side after conditioning with 10 and 20 mg/kg cocaine [genotype effect, F(1, 44) = 9.465, P = 0.0036] (*, P = 0.014; **, P = 0.006).

Fig. 6.

Enhanced release of DA and glutamate in the NAc of mGluR2^–/– KO mice after cocaine administration. In vivo microdialysis analysis of DA (A) and glutamate (B) were conducted for the NAc of freely moving WT and mGluR2^–/– KO mice. After collection of three baseline fractions, cocaine (10 mg/kg) was i.p.-administrated and six fractions were collected every 20 min. Data are normalized to percent changes by the average value of three baseline fractions. In A, basal levels of DA in the NAc show no difference between WT and mGluR2^–/– KO mice (WT, 0.60 ± 0.043 nM, n = 6; KO, 0.59 ± 0.081, n = 5). In both genotypes, cocaine significantly increases extracellular levels of DA as compared with basal levels at time points of 20, 40, and 60 min after cocaine administration (P < 0.01). There is a significant interaction between genotype and time [F(5, 45) = 2.808, P = 0.027] and a significant increase at the first fraction after cocaine administration in mGluR2^–/– KO mice as compared with WT mice (*, P = 0.016). In B, basal levels of glutamate in the NAc showed no difference between WT and mGluR2^–/– KO mice (WT, 82.4 ± 8.1 nM, n = 6; KO, 76.8 ± 11.6, n = 5). In mGluR2^–/– KO mice, extracellular levels of glutamate significantly increase at 20 min (P < 0.01) and 120 min (P < 0.05) after cocaine administration as compared with basal levels. There are a significant interaction between genotype and time [F(5, 45) = 2.726, P = 0.031] and a significant increase at the first fraction after cocaine administration in mGluR2^–/– KO mice as compared with WT mice (**, P = 0.002).